C- and N-Phosphorylated Enamines—An Avenue to Heterocycles: NMR Spectroscopy

Abstract

The review presents extensive data (from the works of the author and literature) on the structure of C- and N-chlorophosphorylated enamines and the related heterocycles obtained by multipulse multinuclear ¹H, ¹³C, and ³¹P NMR spectroscopy. The use of phosphorus pentachloride as a phosphorylating agent for functional enamines enables the synthesis of various C- and N-phosphorylated products that are heterocyclized to form various promising nitrogen- and phosphorus-containing heterocyclic systems. ³¹P NMR spectroscopy is the most convenient, reliable and unambiguous method for the study and identification of organophosphorus compounds with different coordination numbers of the phosphorus atom, as well as for the determination of their Z- and E-isomeric forms. An alteration of the coordination number of the phosphorus atom in the phosphorylated compounds from 3 to 6 leads to a drastic screening of the ³¹P nucleus from about +200 to −300 ppm. The unique structural features of nitrogen–phosphorus-containing heterocyclic compounds are discussed.

1. Introduction

The chemistry of organophosphorus compounds (OPCs) is one of the rapidly developing areas of organic chemistry. Phosphorus compounds have attracted much interest in the past decades due to their wide applications ranging from synthetic chemistry to materials science and life sciences. Interest in organophosphorus compounds is due to their importance for the chemistry of macromolecular materials, complexones, chelating agents, extragents, antipyrenes, drugs, phosphorus-containing pesticides and other materials [1,2,3,4,5,6,7,8,9]. Due to a wide range of physiological activity and easy degradation into the simplest non-toxic products, organophosphorus compounds rank among the best plant protection chemicals. Although organophosphorus chemistry is known as a mature and specialized field, researchers would like to develop new methods for the synthesis of OPCs to improve the safety and sustainability of chemical processes [10,11,12,13].

A well-known method for obtaining unsaturated OPCs having phosphorus chloride groups at the double bond is the reaction of phosphorus pentachloride with various organic nucleophiles—alkenes, alkadienes, alkynes, ethers and esters, tertiary amines and other compounds [14,15,16,17,18,19,20]. The availability of the starting reagents, mild reaction conditions and the ability to vary the structure of the final OPCs are advantages of this method. This reaction has not exhausted its synthetic capabilities in terms of the covering of new nucleophiles. Therefore, research in this area still remains a challenge.

Organic derivatives of phosphorus pentachloride in various coordination states, including the products of phosphorylation of alkenes and heterocycles based on them, can be conveniently studied by ³¹P NMR spectroscopy. As is known, phosphorus pentachloride, a common electrophilic phosphorylating agent, dissociates into ions both in the crystalline state and in polar solvents and exists in a pentacoordinated state in nonpolar solvents:

$$2\mathrm{P}\mathrm{C}{\mathrm{l}}_5\rightleftharpoons \mathrm{P}\mathrm{C}{\mathrm{l}}_4^{+}+\mathrm{P}\mathrm{C}{\mathrm{l}}_6^{-}$$

A variation in the coordination number of phosphorus from 3 to 6 significantly changes the shielding of the ³¹P nucleus from about +200 to −300 ppm:

Phosphorous chloride	${\mathrm{PCl}}_3$	${\mathrm{PCI}}_4^{+}$	${\mathrm{PCl}}_5$	${\mathrm{PCl}}_6^{-}$
Coordination number	3	4	5	6
31P NMR chemical shift, pm	+200	+80	−80	−300

The tetra-, penta- and hexacoordinated states of phosphorus pentachloride can be very easily characterized by multinuclear multipulse NMR spectroscopy and, in particular, by ³¹P NMR spectroscopy [9].

The phosphorylation reaction of a number of nitrogen-containing organic nucleophiles involves the electrophilic addition of phosphorus pentachloride to the double bond of enamines introduced into the reaction or formed during the phosphorylation of enamides, amides, ureides, tertiary amines and other nitrogen-containing organic nucleophiles. Phosphorus-containing enamines and enamides successfully combine a double bond and phosphorus and nitrogen organic fragments in a molecule. Due to the presence of several reaction centers, they are key compounds in the synthesis of promising functionalized heterocycles. In addition, organic compounds containing phosphorus and nitrogen at the same time are interesting objects for studying conjugation, stereochemistry and geometric isomerism.

The stereochemistry of functionalized enamines incorporating phosphorus atoms is a challenging topic in heteroatom chemistry because the correct interpretation of their chemical behavior and biological activity depends on understanding the factors that determine the stereochemical features and relative stability of their isomers.

The structural aspects of phosphorylated unsaturated compounds, such as N-vinylazoles and their derivatives, are considered in detail in the review [9]. It has been shown that many N-vinylazoles react with phosphorus pentachloride to form organophosphorus compounds containing tetra-, penta- and hexacoordinated phosphorus atoms.

2. The Phosphorylated Tertiary Amines

Tertiary amines are known to react intensely with phosphorus chlorides. This often leads to the resinification of the reaction mixture and does not allow the reaction products to be isolated as individual compounds. The reactions of tertiary amines with phosphorus chlorides deliver donor–acceptor complexes followed by the chlorination of alkyl groups, which in most cases leads to iminium salts with the N–C bond cleavage [21,22,23]. In the presence of excess tertiary amine, iminium salts are transformed into enamines, which can undergo C-phosphorylation with phosphorus pentachloride.

The phosphorylation of tertiary N-ethylamines with phosphorus pentachloride has been studied in detail by two-dimensional (2D) and multinuclear ¹H, ¹³C and ³¹P NMR spectroscopy [14,24,25]. The tertiary amines, containing at least two ethyl groups (triethylamine, diethylaniline, diethylbenzylamine), react with phosphorus pentachloride under mild conditions (15–20 °C) to form hexachlorophosphorates of 2-aminoethenyltrichlorophosphonium (1–3) and hexachlorophosphorates of 2-amino-1-chloroethenyltrichlorophosphorates (4–6). The following sequence of transformations was suggested to explain the formation of compounds 1–6 (Scheme 1).

The reaction is initiated by the formation of donor–acceptor complexes of tertiary amines with phosphorus pentachlorides, followed by chlorination of the complex compounds at the methylene group in the α-position to the nitrogen atom. Due to the instability of tertiary chloramines, they lose chlorine in the form of an anion, which leads to the formation of iminium salts. In the presence of excess tertiary amine, iminium salts are dehydrochlorinated into enamines, the latter being phosphorylated with phosphorus pentachloride. Phosphorylated enamines (1–3), obtained from tertiary amines due to the high basicity of dialkyl- and alkyl(aryl)amino groups, have an increased nucleophilicity of the double bond and are easily chlorinated by phosphorus pentachloride at the double bond, in contrast to phosphorylated derivatives of N-vinylheterocycles, for example, N-vinylbenzotriazole and N-vinylcarbazole [9,26,27]. The chlorination products are easily dehydrochlorinated due to the high C-H acidity of protons (P–CH=) with the formation of hexachlorophosphorates 4–6 (Scheme 1,

Table 1. The ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of phosphorylated enamines 1–13.

No.	Structure	1H a	13C	31P b
1		8.00 dd, 1H, N–CH= 3JPH = 25.9, 3JHH = 13.7 5.65 dd, 1H, P–CH= 2JPH = 33.7, 3JHH = 13.7	153.25 d, N–CH=, 2JPC = 47.3 81.47 d, P–CH=, 1JPC = 195.4	89.9 dd ${\mathrm{PCl}}_3^{+}$ 2JPH = 33.7 3JPH = 25.9
2		7.81 dd, 1H, N–CH= 3JPH = 23.8, 3JHH = 13.9 5.50 dd, 1H, P–CH= 2JPH = 34.0, 3JHH = 13.9	155.12 d, N–CH=, 2JPC = 48.1 80.92 d, P–CH=, 1JPC = 200.2	80.1 dd ${\mathrm{PCl}}_3^{+}$ 2JPH = 34.0 3JPH = 23.8
3		7.60 dd, 1H, N–CH= 3JPH = 21.4, 3JHH = 13.9 4.55 dd, 1H, P–CH= 2JPH = 34.3, 3JHH = 13.9	150.40 d, N–CH=, 2JPC = 52.7 80.29 d, P–CH=, 1JPC = 200.8	79.8 dd ${\mathrm{PCl}}_3^{+}$ 2JPH = 34.3 3JPH = 21.4
4		7.50 d, 1H, N–CH= 3JPH = 14.1	158.11 d, N–CH=, 2JPC = 37.5 77.26 d, P–CH=, 1JPC = 187.1	80.7 d ${\mathrm{PCl}}_3^{+}$ 3JPH = 14.1
5		7.65 d, 1H, N–CH= 3JPH = 13.9	157.20 d, N–CH=, 2JPC = 35.4 77.82 d, P–CH=, 1JPC = 179.5	82.0 d ${\mathrm{PCl}}_3^{+}$ 3JPH = 13.9
6		7.50 d, 1H, N–CH= 3JPH = 13.7	159.21 d, N–CH=, 2JPC = 30.9 75.64 d, P–CH=, 1JPC = 159.6	72.0 d ${\mathrm{PCl}}_3^{+}$ 3JPH = 13.7
7		7.25 dd, 1H, N–CH= 3JPH = 18.3, 3JHH = 13.1 5.36 dd, 1H, P–CH= 2JPH = 9.0, 3JHH = 13.1 3.27 m 4H, CH2 , 1.29 m 3HCH3	156.80 d, N–CH=, 2JPC = 31.0 85.11 d, P–CH=, 1JPC = 201.2 50.67 s, CH2 14.86 s, CH3	160.0 dd PCl2 2JPH = 9.0 3JPH = 18.3
8		7.06 dd, 1H, N–CH= 3JPH = 21.5, 3JHH = 13.7 5.12 dd, 1H, P–CH= 2JPH = 25.8, 3JHH = 13.7 3.51 m 4H, CH2, 1.27 m 6H, CH3	150.88 d, N–CH=, 2JPC = 38.9 85.45 d, P–CH=, 1JPC = 208.9 43.51 s, CH2 13.84 s, CH3	35.0 dd POCl2 2JPH = 25.8 3JPH = 21.5
9		7.25 dd, 1H, N–CH= 3JPH = 21.2, 3JHH = 14.2 7.1–7.3 m, 5H, Ph 5.20 dd, 1H, P–CH= 2JPH = 25.5, 3JHH = 14.2 4.62 s, 2H, CH2-Ph 3.54 m, 2H, CH2 1.28 m, 3H, CH3	149.27 d, N–CH=, 2JPC = 37.1 86.23 d, P–CH=, 1JPC = 208.9 125.20 m, Ph 51.60 s, CH2-Ph 49.70 s, CH2 15.23 s, CH3	36.2 dd POCl2 2JPH = 25.5 3JPH = 21.2
10		7.20 dd, 1H, N–CH= 3JPH = 21.8, 3JHH = 14.1 7.1–7.3 m, 5H, Ph 4.95 dd, 1H, P–CH= 2JPH = 26.4, 3JHH = 14.1 3.73 m, 2H, CH2 1.25 m, 3H, CH3	150.10 d, N–CH=, 2JPC = 32.8 85.32 d, P–CH=, 1JPC = 201.4 120.40 m, Ph 48.34 s, CH2 15.00 s, CH3	37.0 dd POCl2 2JPH = 26.4 3JPH = 21.8
11		7.15 d, 1H, N–CH= 3JPH = 11.6 3.45 m, 4H, CH2 1.18 m, 6H, CH3	145.19 d, N–CH=, 2JPC = 39.4 1JCH=165.7 88.64 d, P–CH=, 1JPC = 212.5 46.91 s, CH2 14.84 s, CH3	36.1 d POCl2 3JPH = 11.6
12		7.34 d, 1H, N–CH= 3JPH = 12.1 7.2–7.4 m, 5H, Ph 5.37 s, 2H, CH2-Ph 3.53 m, 2H, CH2 1.23 m, 3H, CH3	148.95 d, N–CH=, 2JPC = 39.8 86.02 d, P–CH=, 1JPC = 210.3 124.24 m, Ph 54.26 s, CH2 -Ph 49.15 s, CH2 14.65 s, CH3	35.9 d POCl2 3JPH = 12.1
13		7.51 d, 1H, N–CH= 3JPH = 11.1 7.1–7.3 m, 5H, Ph 3.55 m, 2H, CH2 1.20 m, 3H, CH3	151.44 d, N–CH=, 2JPC = 33.5 84.57 d, P–CH=, 1JPC = 200.4 127.40 m, Ph 51.34 s, CH2 15.07 s, CH3	34.8 d POCl2 3JPH = 11.1

^a It is impossible to identify the signals of protons of both ethyl and phenyl groups in compounds 1–6 (hexachlorophosphorates), since they and the tertiary amine hydrochlorides present in the crystalline reaction product have similar ethyl and phenyl substituents (δ¹H: 1.20–1.28, 3.2–3.5 and 7.2–7.5 ppm, respectively, CH₃, CH₂, Ph). ^b The phosphorus signal of ${\mathrm{PCl}}_6^{-}$ anion in the ³¹P NMR spectra of organyltrichlorophosphonium hexachlorophosphorates 1–6 appears in the region −(297–298) ppm. For (13a) Ph₂N—CH=C(Cl)-POCl₂, the value of δ ³¹P is 34.6 ppm with constant ³J_PH = 11.6 Hz.

).

This was confirmed by studies of the reaction mixture by ³¹P NMR spectroscopy [24]. For example, two signals are observed in ³¹P NMR spectrum of the reaction products of phosphorus pentachloride with triethylamine: a doublet of doublets at 89.9 ppm ²J_PH 35.0 Hz and ³J_PH 25.9 Hz and a doublet at 80.7 ppm ³J_PH 14.1 Hz assigned to organyltrichlorophosphonium cations 1 and 4 (Table 1).

The amount of hexachlorophosphorate 1 in the reaction mixture in the ³¹P NMR data decreases in time, while that of chlorenamine 4 grows. This testifies that enamine 4 is formed from hexachlorophosphate 1. The phosphorylation process is completed after the formation of enamine (4).

Phosphorylation of triethylamine in the presence of excess amine led to the arising in the reaction mixture of dichloroanhydride of 2-(diethylamino)-ethenylphosphonius acid (7), which was indicated by the presence of the signal at 160 ppm in the ³¹P NMR spectrum. Apparently, compound 7 is obtained through the reduction of 2-aminoethenyl-1-trichlorophosphonium hexachlorophosphate (1) with excess amine (Scheme 2).

It is not possible to separate crystalline hexachlorophosphates (1–6) from triorganylammonium chlorides presented in the reaction mixture, since the solubility of compounds 1–6 in organic solvents is close to that of triorganylammonium salts. This poses a serious preparative problem in the studies of the phosphorylation of tertiary amines with phosphorus pentachloride. Phosphorylation products of tertiary amines can be isolated in pure form by converting enaminotrichlorophosphonium hexachlorophosphates into phosphonic dichlorides 8–13, which is achieved by treating hexachlorophosphates with sulfur dioxide or dry acetone (Scheme 2). As hydrochlorides of tertiary amines are insoluble in diethyl ether, and chloroanhydrides of phosphonous acids are soluble, compounds 8–13 have been isolated in pure form by extraction with diethyl ether. The structure of phosphorus-containing enamines (1–6, 8–13) has been characterized by ¹H, ¹³C and ³¹P NMR spectroscopy (Table 1).

The values of coupling constants ³J_HH 13–14 Hz for the phosphorylated enamines having both vinyl protons indicate the trans-orientation of protons in the molecule. The ³¹P NMR spectra of phosphorylated chlorenamines are characterized by the coupling constant ³J_PH values being decreased by 7–10 Hz as compared to phosphorylated enamines, containing no chlorine atom at the double bond. This fact is widely used to identify chlorophosphorylated compounds in organic synthesis.

The presence of a chlorine atom exactly at the carbon atom, nearest to the phosphorus-containing substituent in compounds 4–6 and 11–13, is unambiguously confirmed by ¹³C NMR spectral data (Table 1). The high value of coupling constants ¹J_CH 164–168 Hz (N–CH=) in the ¹³C NMR spectra of these compounds can testify that the carbon atom is bound with the electronegative nitrogen atom of the amino group. The values of coupling constant ²J_CP 35–40 Hz indicate the trans-(PN)-configuration (E-isomer) of hexachlorophosphorates of N,N-diorganylamino-1-chlorethenyl-phosphonium (4–6) and dichloroanhydride of N,N-diorganylamino-1-chloroethenylphosphonous acids (11–13). A similar pattern (regularity) was observed in the phosphorylation products of N-vinylazoles and a number of other enamines [28]. A low value of ³J_PH (11–26 Hz) (Table 1) indicates the cis-orientation of the phosphorus atom relative to the vinyl proton. In the case of the trans-orientation, ³J_PH values reach 54–68 Hz [28].

The phosphorylation of N-methyl-N-ethylamine leads to the substitution of two chlorine atoms by the alkenyl groups in the PCl₅ molecule to form a derivative of enaminophosphonium acids. Along with the reaction route leading to phosphorylated chloramine (14), the second direction is realized to afford chloroanhydride of bis-[2(N-methyl-N-phenylamino)ethyl] phosphonous acids (15) (Scheme 3).

In the ³¹P NMR spectrum, besides the doublet signal at δ 34.2 ppm with the value ³J_PH = 11.8 Hz characteristic of chloranhydrides of 2-diorganylamino-1-chlorethenyl-phosphonous acid and related to compound 14, a signal at 35.6 ppm is present (

Table 2. The ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of phosphorylated enamines 14–18.

No.	Structure	1H	13C	31P
14		7.47 d, 1H, N–CH= 3JPH = 11.8 7.3 m, 5H, Ph 3.02 s, 3H, CH3	148.40 d, N–CH=, 2JPC = 30.5 123.4 m Ph 81.37 d, P–CH=, 1JPC = 201.0 18.07 CH3	34.2 d POCl2 3JPH = 11.8
15		7.63 dd, 1H, N–CH= 3JPH = 16.4, 3JHH = 12.0 7.18 m, 5H, Ph 6.68 dd, 1H, P–CH= 3JHH = 12.0, 2JPH = 5.1 3.11 s, 3H, CH3	150.20 d, N–CH=, 2JPC = 36.9 123.40 m Ph 88.11 d, P–CH=, 1JPC = 140.9 15.12 CH3	35.6 tt POCl 3JPH = 16.4 2JPH = 5.1
16		7.59 dd, 1H, O–CH= 3JPH = 31.4, 3JHH = 6.4 5.68 dd, 1H, P–CH= 3JHH = 6.4, 2JPH = 1.5	149.27 d, O–CH = 120.23 d, P–CH= 1JPC = 120.9	10.7 tt POCl 3JPH = 31.4 2JPH = 1.5
17		7.28 dd, 1H, N–CH= 3JPH = 22.9, 3JHH = 13.8 7.30 m, 4H, Ph 4.90 dd, 1H, P–CH= 2JPH = 33.6, 3JHH = 13.8 3.53 m, 2H, CH2 1.21 m, 3H, CH3	156.10 d, N–CH=, 2JPC = 32.8 129.24 m Ph 85.32 d, P–CH=, 1JPC = 201.4 48.34 CH2 15.00 CH3	82.7 dd ${\mathrm{PCl}}_3^{+}$ 2JPH = 33.6 3JPH = 22.9 −297.9 br s ${\mathrm{PCl}}_6^{-}$
18		7.45 d, 1H, N–CH= 3JPH = 14.5 7.24 m, 4H, Ph 3.41 m, 2H, CH2 1.16 m, 3H, CH3	145.19 d, N–CH=, 2JPC = 39.6 86.64 d, P–CH=, 1JPC = 200.5 46.91 CH2 14.84 CH3	74.0 d PCl3 3JPH = 14.5 −298.0 br s ${\mathrm{PCl}}_6^{-}$

). The splitting of this signal in a triplet of triplets unequivocally indicates the binding of the phosphorus atom with two vinyl groups in compound 15. NMR spectral data of oxaphosphinine (16) are given for comparison (Table 2). The chlorophosphoryl group (POCl) in 16 also is bound to two vinyl groups, but because of the heterocyclic nature of compound 16, vinyl protons are oriented in cis-position towards each other [29]. Coupling constant values ³J_PH for compound 16 exceed twice those of compound 15, which correlates well with various geometry of structural fragments of these molecules containing a chlorophosphoryl group and vinyl proton. Similar conclusions follow from the comparison of ³J_HH values for compounds 15 and 16 (12 Hz and 6.4 Hz, respectively).

The phosphorylation of N,N-diethylaniline in tetrachloromethane is accompanied (apart from phosphorylation) by the chlorination of organophosphorus compounds into the ring. In the ³¹P NMR spectrum of organyltrichlorophosphonium cations, four signals are observed. A signal at 79.8 ppm (dd) with ²J_PH 34.3 and ³J_PH 21.4 Hz is assigned to compound 3. Apparently, a high-frequency signal of smaller intensity (82.7 ppm), also split into a doublet of doublets with very close constants ²J_PH 33.6 and ³J_PH 22.9 Hz, belongs to compound 17 (Scheme 4). Chlorination of the ethyl group in hexachlorophosphorate (3) is improbable, as the nitrogen atom is bound to the acceptor ethyltrichlorophosphonium group. Electrophilic hydrogen substitution into the para-position of the phenyl ring is more likely because it is sterically less shielded.

Two doublets of different intensities are observed in the high field of the spectrum. The major signal at δ 72.0 ppm with ³J_PH = 13.7 Hz is attributed to compound 6 and completely coincides with the characteristics of hexachlorophosphorate 6, which has been synthesized in benzene (Table 1). The minor doublet at δ 74.0 ppm with ³J_PH = 14.5 Hz is assigned to compound 18. These parameters of the ³¹P NMR spectrum indicate close similarity of compounds 6 and 18.

3. The Structure of the Reaction Products of Triethylamine with Organyltrichlorophosphonium Hexachlorophosphorates

The most abundant phosphorylating agents are alkyl-, aryl- and tetrachloro(styryl)phosphoranes [14,30]. The interaction of triethylamine with organic derivatives of phosphorus pentachloride, organyltrichlorophosphonium hexachlorophosphorates, in particular, 2-ethoxyethenyltrichlorophosphonium hexachlorophosphorate, has been studied by NMR spectroscopy [31]. The doublet signal at 40.4 ppm (³J_PH = 18.3 Hz) in the ³¹P NMR spectrum refers to the phosphorus atom in 1-chloro-2-ethoxyethenylphosphonyl dichloride (19). The appearance of a doublet at 36–40 ppm in the ³¹P NMR spectrum instead of a doublet of doublets is unexpected. Evidently, one vinyl hydrogen atom is replaced by the chlorine atom. The chlorination of the double bond followed by dehydrochlorination in the presence of triethylamine indicates the high electrophilicity of the chlorine atom in the donor–acceptor complexes formed between phosphorus pentachloride and triethylamine. It is most likely that these complexes are able to dissociate partially to ions (Scheme 5). The chlorine atom in the formed cation is electrophilic enough for the chlorination of a multiple bond. 1-Chloro-2-ethoxyethenylphosphonyl dichloride (19) is formed, most likely, according to Scheme 5.

The appearance of the dark orange color of the reaction mixture can be due to the partial dehydrochlorination of compound 19 with triethylamine to 2-ethoxyethynylphosphonyl chloride (20), which is unstable and undergoes easy resinification (Scheme 5) (

Table 3. The ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of phosphorylated compounds 19–24.

No.	Structure	1H	13C	31P
19		7.34 d, 1H, CH= 3JPH = 18.3 4.50 m, 2H, CH2 1.52 m, 3H, CH3	135.30 s, CH= 125.76 d, CCl 1JPC = 92.2 42.54 s, OCH2 15.32 s, CH3	80.1 d 3JPH = 18.3
20		4.42 m, 2H, CH2 1.50 m, 3H, CH3	102.30 CO 94.30 CP 44.50 OCH2 13.40 CH3	6.0 s
21		8.15 ddd, 1H, O–CH= 3JP1H = 57.2, 5JP2H = 19.1 3JHH = 11.0 7.59 ddd, 1H, P–CH= 4JP2H = 40.4, 2JP1H = 27.0 3JHH = 11.0 4.30 m, 2H, CH2 1.55 m, 3H, CH3	138.5 d, C-2 1JP1C2 = 101.2 106.4 dd, C-1 1JP1C1 = 157.7 1JP2C1 = 67.5	28.3 ddd, P1 3JP1H = 57.2, 2JPP = 56.5, 2JP1H = 27.0 21.4 ddd P2 2JPP = 56.5, 4JP2H = 40.4 5JP2H = 19.1
22		8.20 ddd, 1H, O–CH= 3JP1H = 62.0, 5JP2H = 16.8 3JHH = 11.0 7.50 d, 1H, P–CH= 4JP2H = 36.6, 2JP1H = 27.5 3JHH = 11.0	139.1 d, C-2 1JP1C2 = 100.0 108.7 dd, C-1 1JP1C1 = 150.0 1JP2C1 = 68.0	25.6 ddd P1 3JP1H = 62.0 2JPP = 48.8, 2JP1H = 27.5 21.9 ddd P2 2JPP = 48.8, 4JP2H = 36.6 5JP2H = 16.8
23		7.58 dd, 1H, P–CH= 3JHH = 15.3, 2JPH = 9.2 7.50 m, 5H, Ph 7.30 dd, 1H, Ph–CH= 3JPH = 19.1, 3JHH = 15.3	142.2 d, C-P 1JPC = 98.0 112.4 d, C-Ph 2JPC = 35.0	162.8 dd PCl2 3JPH = 19.1 2JPH = 9.2
24		7.50 m, 5H, Ph 7.40 d, 1H, Ph–CH= 3JPH = 22.1	142.2 d, C-P 1JPC = 99.0 118.3 d, C-Ph 2JPC = 33.0	157.8 d PCl2 3JPH = 22.1

). The intense singlet (dichlorophosphoryl substituent bound to the C≡C group) is observed at 6 ppm in the ³¹P NMR spectrum. This is consistent with the data of [32]. The ³¹P NMR spectrum also contains four doublets with pairwise coinciding coupling constants J_PP observed in the ³¹P NMR spectrum of a mixture of the products of the reaction of trichloro(2-ethoxyethenyl)phosphonium hexachlorophosphorate with triethylamine. These data indicate that the reaction affords two compounds, each of which contains two magnetically nonequivalent phosphorus atoms in the P=O groups. The doublets at 28.3 and 21.4 ppm (²J_PP = 56.5 Hz) belong to compound 21, while the other compound, 22, is characterized by the doublets at 25.6 and 21.9 ppm (²J_PP = 48.8 Hz) (Table 3).

The spectral pattern of the obtained mixture of the reaction products changes over time. The concentration of 1-chloro-2-ethoxyethenylphosphonyl dichloride (19) decreases in the mixture, and the amount of two compounds (each containing two phosphorus atoms) noticeably increases. The signals at 6 ppm disappear from the spectrum. These spectral data show that compound 19 and phosphorus-containing acetylene 20 participate in the formation of the compounds containing two phosphorus atoms in the molecules. Since compound 19 is stable in the absence of triethylamine, we assume that triethylamine is also involved in the formation of diphosphorylated compounds. Based on these data and ¹H and ¹³C NMR results, we propose the scheme for the synthesis of diphosphorylated heterocyclic compounds 21 and 22, i.e., 1,4-oxaphosphinines containing the dichlorophosphoryl substituent in position 2 (Scheme 6).

The formation of 1,4-oxaphosphinine 21 starts from the conversion of 1-chloro-2-ethoxyethenylphosphonyl dichloride (19) with triethylamine to phosphorus-containing acetylene, 2-ethoxyethynylphosphonyl dichloride (20). Then, the attack of the trichlorophosphonium group of trichloro(2-ethoxyethenyl)phosphonium hexachlorophosphorate on the triple bond of compound 20 leads to diphosphorylated ketene A. The dichlorophosphonium group of diphosphorylated ketene A is transformed into the —POCl— group (ketene B) due to the phosphorylated derivatives of ethyl vinyl ether that are present in the reaction mixture. Then, heterocyclization occurs, preceded by the intramolecular E,Z-isomerization of the ethoxyvinyl fragment of the diphosphorylated ketene molecule (B,C). The formed oxaphosphinine 21 is acetal; hence, a portion of this product is transformed into 2-chlorosubstituted oxaphosphinine 22 in the presence of phosphorus pentachloride. Heterocyclization permits the rationalization of the rather high coupling constants ³J_P1H = 57.2 Hz for compound 21 and 62.0 Hz for compound 22. The high values of coupling constant ⁴J_P2H = 40.4 Hz for compound 21 and 36.6 Hz for compound 22 can be explained by the W-shaped configuration of the fragment of the oxaphosphinine molecule binding the P₂ phosphorus atom to the vinyl proton (Table 3).

The ³¹P NMR chemical shift values of the tetracoordinated phosphorus atom of POCl₂ in compounds 19 and 20 differ several times: 80.1 and 6.0 ppm, respectively (Table 3). The presence of a triple bond (20) has a great shielding influence on the position of the resonance signal of phosphorus in the ³¹P NMR spectra.

The ¹³C NMR spectra (with proton decoupling) of a mixture of compounds 21 and 22 exhibit two groups of signals characterizing the C₁ and C₂ atoms directly bound to the phosphorus atoms. Two doublets of doublets at 106.4–108.7 ppm shifted relatively to each other by 8.5 Hz relate to the C₁ atoms in compounds 21 and 22. The high values of coupling constants ¹J_P1C1 = 157.7 and 150.0 Hz and ¹J_P2C1 = 67.5 and 68.0 Hz indicate that two different phosphorus atoms are directly bonded to the C-1 carbon atom. The significant value of ¹J_P1C2 = 101.2 and 100.0 Hz in compounds 21 and 22 indicates that one phosphorus atom is bonded to the C-2 carbon atom. Thus, the ¹³C NMR spectral data are consistent with the structures of oxaphosphinines 21 and 22.

Trichloro(styryl)phosphonium hexachlorophosphorate was used as such a model compound (Scheme 7).

Its reaction with triethylamine is accompanied by a weak coloration of the reaction mixture. The ³¹P NMR spectrum exhibits a singlet at 219.6 ppm (PCl₃). A specific feature of the spectrum is the presence of two intense signals in the range characteristic of organic chloride of trivalent phosphorus. It is most likely that styrylphosphonyl (23) and 1-chloro-2-phenylethynylphosphonyl (24) dichlorides are formed (Scheme 7). Compound 23 is characterized by a doublet of doublets at 162.8 ppm (²J_PH = 9.2 Hz, ³J_PH = 19.1 Hz), and a doublet at 157.8 ppm (³J_PH = 22.1 Hz) belongs to compound 24. A reason for the formation of compound 24 is the chlorination at the multiple bond. It should be mentioned that compound 24 was obtained [30] by heating styryltetrachlorophosphorane under much harsher conditions. The replacement of the oxygen-containing ethoxy group in hexachlorophosphorate by the phenyl group containing no oxygen atoms results in the absence of styrylphosphonyl chloride in the reaction mixture. Thus, the ³¹P NMR chemical shifts for PCl₂ in compounds 23 and 24 in the region of 160 ppm are characteristic of a tricoordinated (trivalent) phosphorus atom.

Thus, the NMR studies of chlorophosphorylated enamines on the basis of accessible tertiary amines and a wide range of other nitrogen-containing organic nucleophiles contributes essentially to the chemistry of unsaturated organophosphorus compounds. ³¹P NMR spectroscopy provides the most reliable and unambiguous method for the investigation of E–Z-isomeric structures of phosphorylated enamines.

4. The Structural Aspects of the Phosphorylation Products of Enamides

Phosphorus-containing enamines and enamides attract the attention of researchers due to their synthetic potential and biological properties. Organophosphorus complexes are widely used in biological, pharmaceutical and material fields. Great demands on their diversity and availability encourage chemists to intensify research efforts to find effective and general routes to the synthesis of their derivatives and to study the structure of the reaction products [33,34,35,36,37]. Moreover, various enamides are suitable reactants and show high reactivity.

The low stability of alkyl- and aryl-substituted tertiary enamines in acidic media hinders the application of phosphorus pentachloride as a phosphorylating agent. So, N, N-diphenylethenamine was reacted with PCl₅ in benzene with instant resinification, and in the reaction mixture after its treatment with sulfurous anhydride, only ³¹P NMR was used to identify 2-diphenylaminoethene-1chloro-1-phosphonic acid dichloride—E-(PN) isomer (Scheme 8) (see footnote in Table 1).

Enamines, containing strong electron-withdrawing substituents at the nitrogen atom and having a less nucleophilic character, behave in the phosphorylation reaction like vinyl ethers; therefore, it is possible to obtain organophosphorus compounds not contaminated with by-products in high yield.

The phosphorylation of N-vinyl-substituted tertiary amides, in which nitrogen nucleophilicity is significantly weakened, has been studied [14,38,39,40] (Scheme 9,

Table 4. The ¹H and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of phosphorylated enamides 25–33 (CH₃NO₂) and 34–42 (CDCl₃).

R	No.	31P a, ${\mathbf{PCl}}_3^{+}$	No.	1H	31P, POCl2
	25	94.1 dd 2JPH = 40.2 3JPH = 25.9	34	8.50 dd, 1H, N–CH= 3JPH = 21.8, 3JHH = 15.0 7.32 m, 5H, Ph 4.75 dd, 1H, P–CH= 2JPH = 27.8, 3JHH = 15.0 1.91 3H, CH3	34.7 dd 2JPH = 27.8 3JPH = 21.8
	26	93.7 dd 2JPH = 40.0 3JPH = 26.0	35	8.54 dd, 1H, N–CH= 3JPH = 21.9, 3JHH = 15.0 7.35 m, 10H, Ph 5.12 dd, 1H, P–CH= 2JPH = 27.8, 3JHH = 15.0	33.7 dd 2JPH = 27.8 3JPH = 21.9
	27	94.0 dd 2JPH = 39.6 3JPH = 26.6	36	7.81 dd, 1H, N–CH= 3JPH = 21.9, 3JHH = 15.2 7.20 m, 4H, Ph 5.00 dd, 1H, P–CH= 2JPH = 27.8, 3JHH = 15.2 2.15, 3H, CH3 1.98 3H, CH3	34.7 dd 2JPH = 27.8 3JPH = 21.9
	28	94.0 dd 2JPH = 40.0 3JPH = 26.1	37	8.32 dd, 1H, N–CH= 3JPH = 21.5, 3JHH = 15.1 7.33 m, 4H, Ph 4.88 dd, 1H, P–CH= 2JPH = 28.5, 3JHH = 15.1 4.15 3H, OMe; 1.96 3H, CH3	34.1 dd 2JPH = 28.5 3JPH = 21.5
	29	93.5 dd 2JPH = 40.0 3JPH = 26.0	38	8.77 dd, 1H, N–CH= 3JPH = 21.6, 3JHH = 14.6 8.1–7.4 m, 7H, Napht 4.72 dd, 1H, P–CH= 2JPH = 27.9, 3JHH = 14.6 1.88 3H, CH3	33.7 dd 2JPH = 27.9 3JPH = 21.6
	30	92.1 dd 2JPH = 39.7 3JPH = 26.8	39	7.84 dd, 1H, N–CH= 3JPH = 21.7, 3JHH = 15.1 5.30 dd, 1H, P–CH= 2JPH = 26.9, 3JHH = 15.1 3.53 m, 2H, NCH2 2.49 m, 2H, CH2CO 2.1 m, 2H, CH2	33.9 dd 2JPH = 26.9 3JPH = 21.7
	31	92.5 dd 2JPH = 40.4 3JPH = 26.3	40	8.12 dd, 1H, N–CH= 3JPH = 24.0, 3JHH = 15.2 5.41 dd, 1H, P–CH= 2JPH = 26.5, 3JHH = 15.2 3.55 m, 2H, NCH2 2.58 m, 2H, CH2CO 1.6 m, 6H, CH2	37.3 dd 2JPH = 26.5 3JPH = 24.0
	32	98.9 dd 2JPH = 40.0 3JPH = 29.4	41	7.53 dd, 1H, N–CH= 3JPH = 26.2, 3JHH = 15.6 7.31 dd, 1H, P–CH= 2JPH = 30.5, 3JHH = 15.6 2.84 m, 4H, CH2CO	33.1 dd 2JPH = 30.5 3JPH = 26.2
	33	94.0 dd 2JPH = 39.6 3JPH = 26.6	42	7.64 dd, 1H, N–CH= 3JPH = 25.4, 3JHH = 15.6 7.85 m, 4H, Ph 7.12 dd, 1H, P–CH= 2JPH = 30.1, 3JHH = 15.6	34.4 dd 2JPH = 30.1 3JPH = 25.4

^a The phosphorus signal of ${\mathrm{PCl}}_6^{-}$ anion in the ³¹P NMR spectra of hexachlorophosphorates 25–33 appears in the region −(296–298) ppm; the ³J_HH value varies within 15–16 Hz.

).

Tertiary enamides can be attacked by phosphorus pentachloride at two reaction centers, namely at the double bond of the vinyl fragment and at the carbonyl group, and the reactivity of the carbonyl group should increase upon passing from N-aryl-substituted to N-alkyl-substituted enamides.

It was found [39,40] that the studied enamides react with phosphorus pentachloride in the same way as with alkenes, i.e., at the vinyl group. Hexachlorophosphates 25–33 are isolated as finely crystalline precipitates that are easily hydrolyzed in air. The N-benzyl derivative of hexachlorophosphorate PhCH₂N(COMe)CH=CH-${\mathrm{PCl}}_3^{+}$ ${\mathrm{PCl}}_6^{-}$ (33a) is formed as a slowly crystallizing oil.

Phosphorylation products of the most reactive N-vinyl-N-alkylacetamides undergo such a rapid transformation with the participation of the carbonyl group that compounds of the 25–33 type cannot be detected. The replacement of the alkyl substituent at the nitrogen atom by the more electron-withdrawing benzyl group somewhat slows down the attack of phosphorus on carbonyl oxygen, and compound 33a is stable at room temperature for 1 h. Aryl substituents at nitrogen, competing for its lone electron pair with a phosphorus-containing double bond and a carbonyl group, further reduce the nucleophilicity of the latter. This leads to a significant increase in the stability of compounds 25–29. Phosphorylation products of N-vinyl lactams and N-vinylimides are stable for several days, with compound 33 being the most stable, apparently due to efficient conjugation in the condensed phthalimide system.

The ³J_HH value (15–16 Hz) of vinyl protons for the studied compounds 25–42 indicates E-(PN)-isomers; i.e., when going from hexachlorophosphates (25–33) to the corresponding enaminophosphoric acid dichlorides (34–42), the configuration of the molecule does not change. The ³¹P values of chemical shifts for compounds 34–42 are typical for acid chlorides of alkenylphosphonic acids and vary in the region of 33–37 ppm.

The values of the constants ³J_PH = 25–27 and ³J_PH = 21–25 Hz are typical for E-isomers, for hexachlorophosphates (34–42) and for acid dichlorides (34–42), respectively. Compounds 34–42 are more stable than complex compounds 25–33. The most stable are 2-(N-phenyl-N-benzoylamido)ethene-1-phosphonic acid dichloride (35) and 2-(N-phthalimido)ethene-1-phosphonic acid dichloride (42). 2-(N-phthalimido)ethene-1-trichlorophosphonium hexachlorophosphate (33) is stable at room temperature and under weak heating; at 100°C, it turns into 2-(N-3,3-dichloro-1-hydroxyisoindolinyl)-ethenephosphonic acid dichloride (43) (Scheme 10) [14,40].

The intramolecular mechanism of the formation of the phosphoryl group is evidenced by the Z-configuration of dichloride 43, which inevitably arises during the intramolecular electrophilic attack of the dichlorophosphonium cation at one of the carbonyl oxygen atoms of compound 33. The trans-cis-isomerization of the enaminotrichlorophosphonium group, which is necessary for the formation of the transition state (A), occurs either under the catalytic effect of hydrogen chloride present in the reaction medium or due to high-temperature rotation around the multiple bond without opening it. At a temperature above 100 °C, the partial resinification of the reaction mixture occurs, and the amount of chlorination products and various organophosphorus compounds of unknown structure, resulting from thermal degradation, increases.

The ¹H, ¹³C and ³¹P NMR chemical shifts and coupling constants of phosphonic dichloride 43 are presented in

Table 5. The ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of phosphonic dichlorides 43 and 45 (CDCl₃) and hexachlorophosphorate (44) (CH₃NO₂).

No.	R	1H	13C	31P
43		7.80 m, 4H, H-(4–7) 7.10 dd, 1H, N–CH= 3JPH = 57.1, 3JHH = 10.5 6.28 dd, 1H, P–CH= 2JPH = 26.2, 3JHH = 10.5	157.70 C-1 140.98 C-9 131.14 C-4 128.20 d, N–CH= 2JPC = 8.5 127.53 C-7 121.05 C-8 119.64 C-6 118.57 C-5 114.55 d, P–CH= 1JPC = 151.4 86.77 C-3	23.3 dd 2JPH = 26.2 3JPH = 57.1
44		7.78 dd, 1H, N–CH= 3JPH = 40.8, 3JHH = 10.0 6.44 dd, 1H, P–CH= 2JPH = 20.0, 3JHH = 10.0 6.22 d, 1H, O–CH= 3JHH = 6.0	142.60 dd, C-9 2JPC = 23.3 d, 3JPC = 10.0 133.40 d, C-4, 2JPC = 6.0 117.84 d, C-6, 2JPC = 16.2 113.75 d, C-7, 1JPC = 185.3 101.50 dd, C-8 2JPC = 16.6 d, 3JPC = 5.5	83.5 dd ${\mathrm{PCl}}_3^{+}$ 3JPH = 6.0 5JPP = 2.8 11.1 ddd POCl 3JPH = 40.8 2JPH = 20.0 4JPH = 2.8 −297.7 br s ${\mathrm{PCl}}_6^{-}$
45		7.67 dd, 1H, N–CH= 3JPH = 40.5, 3JHH = 10.5 6.17 dd, 1H, P–CH= 2JPH = 18.6, 3JHH = 10.5 6.23 d, 1H, O–CH= 3JHH = 6.1	139.82 dd, C-9 2JPC = 25.6 d, 3JPC = 11.0 135.29 d, C-4 2JPC = 6.1 115.95 d, C-6, 2JPC = 18.3 114.80 d, C-7, 1JPC = 194.7 96.50 dd, C-8 2JPC = 14.6 d, 3JPC = 4.5	18.6 dd POCl2 3JPH = 6.1 5JPP = 2.8 10.4 ddd POCl 3JPH = 40.5 2JPH = 18.6 4JPH = 2.8

. The ³¹P NMR spectrum of compound 43 contains a single signal at 23.3 ppm in the form of a doublet of doublets. The Z-geometry of 43 follows from the values of the constants ³J_HH = 10.5 and ³J_PH = 57.1 Hz. A small value of the constant ²J_PC = 8.5 Hz also corresponds to the Z-(PN)-configuration. The ³⁵Cl NQR spectrum of acid dichloride 43 consists of four signals (ν⁷⁷ 26.030, 26.766, 38.072, 38.228 MHz). Two low-frequency signals refer to chlorine atoms in the POCl₂ group since they are in the frequency range characteristic of compounds of the RPOCl₂ series. The other two signals belong to the chlorine atoms of the CCl₂ group (Table 5) [40].

The quantitative transition of the E-(PN)-isomer of hexachlorophosphate (33) to the Z-(PN)-isomer 43, as well as the stability of the latter even at 100 °C, is unusual for the chemistry of phosphorus-containing enamines [39,41].

2-(N-succinimido)ethenyltrichlorophosphonium hexachlorophosphorate (32), in contrast to hexachlorophosphorate 33, is already unstable at room temperature and gradually undergoes deep transformations, especially in the presence of phosphorus pentachloride, culminating in the formation of a bicyclic heterocycle: 2-oxo-2,6-dichloropyrrolo[1,2-e]1,5,2-oxazaphosphorin-7-trichlorophosphonium (44) (Scheme 11) [14,41].

The E,Z-isomerization appears to be catalyzed by the hydrogen chloride present in the reaction medium. Intermediates A and B cannot be fixed in the reaction medium due to their rapid conversion to the Z-isomer of 2-(N-5,5-dichloropyrrolidone)ethenephosphonic acid dichloride C. The fixation of the geometry of intermediate C with the maximum distance of the dichlorophosphoryl group from the dichloromethylene group is due to steric factors; i.e., general regularity of intramolecular transformations of phosphorylated enimides containing phthalimide and succinimide groups is observed. Intermediate C was not isolated individually, but the signal at 21.6 ppm (dd) with ³J_PH = 57.1 and ²J_PH = 26.2 Hz in the ³¹P NMR spectrum corresponds to this compound. The values of the chemical shift and the coupling constants are close to those of dichloride 43 formed upon heating 33 (Scheme 10). The intermediate C undergoes rapid heterocyclization to bicyclic heterocycle D, followed by the elimination of hydrogen chloride and the formation of heterocyclic dienamine E. For intermediates D and E, the ³¹P NMR spectra show close values of chemical shifts of phosphorus atoms and coupling constants: 13.6 ppm (dd) with ³J_PH = 20.0, ²J_PH = 42.0 Hz, and 13.0 ppm (dd) with ³J_PH = 22.0, ²J_PH = 40.0 Hz, respectively. The compound E is phosphorylated at the double bond of the Δ₂–pyrroline ring with excess phosphorus pentachloride to form 2-oxo-2,6-dichloropyrrolo[1,2-e]1,5,2-oxazaphosphorine-7-trichlorophosphonium hexachlorophosphorate (44). The ³¹P NMR spectrum of hexachlorophosphorate (44) shows three signals: 83.5 ppm, dd (${\mathrm{PCl}}_3^{+}$); 11.1 ppm, ddd (POCl); and a singlet of −297.7 ppm (${\mathrm{PCl}}_6^{-}$) (Table 5). In the ¹H NMR spectrum, doublets of doublets at 6.44 and 7.78 ppm with proton–proton constant ³J_HH = 10.0 Hz correspond to vinyl protons, which indicates the cis-orientation of vinyl protons.

The action of SO₂ on compound 44 gave 2-oxo-2,6-dichloro-7-dichlorophosphorylpyrrolo[1,2-e]1,5,2-oxazaphosphorine (45). Heterocycles 44 and 45, despite the presence of a chloropyrrole fragment in the molecule, are stable in the absence of moisture. Two signals are observed in the ³¹P NMR spectrum of oxazaphosphorine 45. The signal at 18.6 ppm corresponds to the exocyclic phosphorus atom, and the signal at 10.4 ppm belongs to the P(O)Cl group. The cis-configuration of the N-vinylchlorophosphoryl group is confirmed by the constants ³J_HH = 10.5 and ³J_PH = 40.5 Hz. It follows from the ¹³C NMR spectrum data that each phosphorus atom is bonded directly to one carbon atom. The spin–spin interaction of C-9 and C-8 carbon nuclei with the nuclei of both phosphorus atoms is consistent with the bicyclic structure of compound 45 (Table 5).

It was established [14,42] that 1-(N-carbazolyl)-1-butene is easily phosphorylated by phosphorus pentachloride to form 1-ethyl-2-(N-carbazolyl)ethene-1-trichlorophosphonium hexachlorophosphorate (46) and 1-ethyldichloride-2-(N-carbazolyl)ethene-1-phosphonic acid (47) (Scheme 12). In the ³¹P NMR spectrum of hexachlorophosphorate 46, two signals are observed: 96.6 ppm, td (${\mathrm{PCl}}_3^{+}$), ³J_PH = 33.0 Hz (CH₂, t), ³J_PH = 26.0 Hz (d) and 11.1 ppm singlet −297.7 (${\mathrm{PCl}}_6^{-}$). The value ³J_PH = 26.0 Hz clearly indicates the E-isomer (P,N) of the alkenyltrichlorophosphonium cation 46. The ³¹P NMR spectrum of compound 47 shows 39.1 ppm, td (POCl₂), ³J_PH = 27.0 Hz (CH₂, t), ³J_PH = 18.5 Hz (=CH, d).

N-Acetylcarbazole reacts with PCl₅ (Scheme 12) with the formation of the corresponding complex salt, which, after the elimination of SO₂, turns into 2-carbazolyl-2-chlorovinyl-1-phosphonic acid dichloride (49) [14,43]. The ³¹P NMR spectrum of compound 49 contains a single doublet at 22.4 ppm with ²J_PH = 21.6 Hz.

The attack of PCl₅ in N-phenyl-substituted acetamides occurs at the oxygen of the C=O group; the intermediate unstable compound cleaves POCl₃, turning into α-chloro-substituted tertiary enamine, which is further phosphorylated by excess phosphorus pentachloride at the carbon–carbon double bond to afford unsaturated organophosphorus compounds 50 and 51 (Scheme 13) [43,44]. Hexachlorophosphorate 51 is easily converted by the action of ${\mathrm{Alk}}_4{\mathrm{N}}^{+}{\mathrm{I}}^{\text{–}}$ into dichlorophosphine 51a, and upon treatment with SO₂ into phosphonic acid dichloride 52.

The ³¹P NMR spectrum of hexachlorophosphorate 50 has an intense narrow signal of −298.1 ppm (${\mathrm{PCl}}_6^{-}$) and a doublet at 71.4 ppm, with ²J_PH = 32.8 Hz (${\mathrm{PCl}}_3^{+}$). The ³¹P NMR spectrum of complex salt 51 contained two signals: a doublet at 75.7 ppm with ²J_PH = 36.5 Hz, related to the organyltrichlorophosphonium cation (${\mathrm{PCl}}_3^{+}$), and −298.0 ppm, a broadened singlet characteristic of the ${\mathrm{PCl}}_6^{-}$ anion. The ³¹P NMR spectrum of compound 51a contains two doublets, 161.9 ppm, ²J_PH = 2.5 Hz, and 152.9 ppm, ²J_PH = 4.4 Hz, in a 6:1 ratio, related to the E- and Z-isomers. In the ¹H NMR spectrum, the doublets at 5.58 ppm with ²J_PH = 2.5 Hz and 6.14 ppm with ²J_PH = 4.4 Hz correspond to the vinyl protons of these isomers. The proton spectrum of 52 contains a multiplet in the region of 7.2 ppm (Ph) and a doublet at 5.14 ppm, ²J_PH = 19.5 Hz. The ³¹P NMR spectrum has a single signal at 24.6 ppm with ²J_PH = 19.5 Hz.

The acetyl group of enamides of enamidotrichlorophosphonium hexachlorophosphorates, initially formed during phosphorylation of enamides, participates in further transformations of organophosphorus compounds, which involve phosphorus chloride substituents at the double bond. It is established that enamidotrichlorophosphonium hexachlorophosphorates containing an acetamide group in the molecule in the presence of PCl₅ are rapidly converted upon heating and more slowly at room temperature into 2-N-alkyl(aryl)-N-2-(dichlorophosphoryl)-alkenylamino-2-chloroethenylphosphonium hexachlorophosphorates (53–57) (Scheme 14) [14,40,41,42,45,46,47].

The formation of compounds 53–57 is the result of an intramolecular attack of the carbonyl group by the organyl trichlorophosphonium oxygen cation, leading to the appearance of a vinyl chloride group, which then reacts with PCl₅ similarly to the phosphorylation of enamines. Hexachlorophosphorates 53–57 cannot be isolated individually without impurities, since the reaction mixture, along with them, always contains products of further transformations. However, after treatment of compounds 53–57 with sulfur dioxide, stable diphosphonic acid chlorides are formed (58–62) (

Table 6. The ¹H and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of phosphorylated acetamides 53–57 (CH₃NO₂) and 58–62 (CDCl₃).

No.	R1	R	31P, POCl2	31P a, ${\mathbf{PCl}}_3^{+}$	No.	1H	31P, P1OCl2	31P, POCl2
53	H	Pr	31.2 dd 2JPH = 23.4 3JPH = 23.4	82.7 d 2JPH = 32.2	58	7.50 dd, 1H, N–CH= 3JPH = 23.0, 3JHH = 15.0 5.75 dd, 1H, P–CH= 2JPH = 17.0 5.48 dd, 1H, P1–CH= 2JPH = 23.0, 3JHH = 15.0 3.64 t, NCH2, 3JHH = 6.7 1.49 m, 2H, CH2 0.98 t, 3H, CH3, 3JHH = 6.9	33.7 dd 2JPH = 23.0 3JPH = 23.0	25.4 d 2JPH = 17.0
54	H	Bu	30.7 dd 2JPH = 22.4 3JPH = 22.4	81.5 d 2JPH = 30.3	59	7.73 dd, 1H, N–CH= 3JPH = 21.8, 3JHH = 14.8 5.78 d, 1H, P–CH= 2JPH = 17.1 5.40 dd, 1H, P1–CH= 2JPH = 23.9, 3JHH = 14.8 3.62 t, NCH2, 3JHH = 6.8 1.44 m, 4H, (CH2)2 0.92 t, 3H, CH3, 3JHH = 6.7	32.5 dd 2JPH = 23.9 3JPH = 21.8	24.1 d 2JPH = 17.1
55	H	PhCH2	30.2 dd 2JPH = 24.2 3JPH = 23.4	81.0 d 2JPH = 30.6	60	7.54 dd, 1H, N–CH= 3JPH = 26.0, 3JHH = 15.2 7.28 m, 5H, Ph 5.69 d, 1H, P–CH= 2JPH = 16.2 5.48 dd, 1H, P1–CH= 2JPH = 26.0, 3JHH = 15.2 4.68 s, 2H, NCH2	33.0 dd 2JPH = 26.0 3JPH = 26.0	24.7 d 2JPH = 16.2
56	H	Ph	30.0 dd 2JPH = 22.5 3JPH = 22.5	83.1 d 2JPH = 31.8	61	7.70 dd, 1H, N–CH= 3JPH = 20.3, 3JHH = 14.8 7.22 m, 5H, Ph 5.70 d, 1H, P–CH= 2JPH = 18.0 5.47 dd, 1H, P1–CH= 2JPH = 25.0, 3JHH = 14.8	33.4 dd 2JPH = 25.0 3JPH = 20.3	26.0 d 2JPH = 18.0
57	Et	PhCH2	31.3 dd 3JPH = 28.6 CH2 (Et) 3JPH = 19.7	75.5 d 2JPH = 30.5	62	7.49 d, 1H, N–CH= 3JPH = 21.6 7.20 m, 5H, Ph 5.62 d, 1H, P–CH= 2JPH = 18.7 4.23 s, 2H, NCH2 3.28 d, 2H, CH2 3JPH = 28.6 1.70 3H, CH3	38.7 dd 3JPH = 28.6 CH2 (Et) 3JPH = 21.6	24.5 d 2JPH = 18.7

^a The phosphorus signal of ${\mathrm{PCl}}_6^{-}$ anion in the ³¹P NMR spectra of hexachlorophosphorates 53–57 appears in the region −(297–298) ppm.

).

The value of the constant ³J_HH = 14–16 Hz, as well as ³J_PH = 20–26 Hz, indicates E-isomerization of (P,N)-organophosphorus compounds 53–62. For compounds 53, 54, 56, 58 and 60, the constants ²J_PH and ³J_PH in the ³¹P NMR spectra are almost equal in magnitude.

The ³⁵Cl NQR spectrum of compound 59 consists of four signals: ν⁷⁷ 26.608, 26.734, 26.933 and 36.775 MHz. The high-frequency signal (36.775 MHz) relates to the chlorine atom bonded to the carbon atom, and the three low-frequency signals belong to the chlorine atoms bonded to the phosphorus group POCl₂.

5. The Structure of Enamide-Derived Azaphosphorines

It has been established that heating or long-term room-temperature storage of bisphosphorylated divinylamines (53–56, 56a) delivers cyclic hexachlorophosphorates of 1,4-azaphosphoniarines (63–67). The formation of compounds 63–67 is the result of an intramolecular attack by the ethenyltrichlorophosphonium cation at the π-bond of the ethenyldichlorophosphoryl group. The transformation of disphosphorylated dialkenyls into heterocycles (63–67) is noteworthy in that the authors of [14,47] using the trichlorophosphonium cation managed to phosphorylate a carbon–carbon double bond having a strong electron-withdrawing substituent. Compound 57 is unable to form the 1,4-azaphosphorine ring, apparently due to steric hindrance created by three substituents at the double bond for intramolecular cation attack of 1,4-azaphosphorine derivatives (68–72) (Scheme 15) (

Table 7. The ¹H and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of hexachlorophosphorates 63–67 (CH₃NO₂) and 1,4-azaphosphorines 68–72 (CDCl₃).

No.	R	1H	31P, POCl2	31P a, ${\mathbf{PCl}}_{\mathbf{2}}^{\mathbf{+}}$/POCl	2JPP
63	Pr	9.65 dd, 1H, N–CH= 3JPH = 45.1, 3JPH = 22.9 8.25 dd, 1H, P–CH=, 4JPH = 10.4 3.64 t, NCH2, 3JHH = 6.6 1.49 m, 2H, CH2 0.98 t, 3H, CH3, 3JHH = 6.8	19.8 ddd 2JPP = 25.9 3JPH = 22.9 4JPH = 10.4	46.8 dd 3JPH = 45.1 2JPP = 25.9	25.9
64	Bu	9.90 dd, 1H, N–CH= 3JPH = 45.5, 3JPH = 22.3 8.28 d, 1H, P–CH=, 4JPH = 10.1 4.18 t, NCH2, 3JHH = 6.6 1.44 m, 1.72 m, 4H, (CH2)2 0.90 t, 3H, CH3, 3JHH = 6.7	20.3 ddd 2JPP = 25.5 3JPH = 22.3 4JPH = 10.1	46.2 dd 3JPH = 45.5 2JPP = 25.5	25.5
65	PhCH2	9.85 dd, 1H, N–CH= 3JPH = 45.0, 3JPH = 22.3 8.02 d, 1H, P–CH=, 4JPH = 9.9 7.27 m, 5H, Ph 4.47 s, 2H, NCH2	19.5 ddd 2JPP = 25.6 3JPH = 22.3 4JPH = 9.9	46.4 dd 3JPH = 45.0 2JPP = 25.6	25.6
66	Ph	8.77 dd, 1H, N–CH= 3JPH = 44.6, 3JPH = 24.3 7.21 d, 1H, P–CH=, 4JPH = 9.6 7.34 m, 5H, Ph	20.0 ddd 2JPP = 25.0 3JPH = 24.8 4JPH = 9.6	48.1 dd 3JPH = 44.6 2JPP = 25.0	25.0
67	s-Bu	8.85 dd, 1H, N–CH= 3JPH = 45.0, 3JPH = 26.0 7.40 d, 1H, P–CH=, 4JPH = 10.5 3.34 m, 1H, NCH 1.5 m, 2H, CH2 1.15 m, 6H, CH3	20.6 ddd 2JPP = 26.0 3JPH = 26.0 4JPH = 10.5	45.5 dd 3JPH = 45.0 2JPP = 26.0	26.0
68	Pr	8.12 dd, 1H, N–CH= 3JPH = 33.0, 3JPH = 23.9 6.68 d, 1H, P–CH=, 4JPH = 10.9 3.40 t, NCH2, 3JHH = 6.4 1.53 m, 2H, CH2 1.04 t, 3H, CH3, 3JHH = 6.7	22.6 ddd 2JPP = 25.9 3JPH = 23.9 4JPH = 10.9	10.9 dd 3JPH = 33.0 2JPP = 25.9	25.9
69	Bu	8.07 dd, 1H, N–CH= 3JPH = 32.9, 3JPH = 23.5 6.22 d, 1H, P–CH=, 4JPH = 10.3 4.05 t, NCH2, 3JHH = 6.5 1.40 m, 1.75 m, 4H, (CH2)2 0.92 t, 3H, CH3, 3JHH = 6.4	22.1 ddd 2JPP = 24.3 3JPH = 23.5 4JPH = 10.3	10.2 dd 3JPH = 32.9 2JPP = 24.3	24.3
70	PhCH2	8.07 dd, 1H, N–CH= 3JPH = 32.4, 3JPH = 22.9 7.48 m, 5H, Ph 6.24 d, 1H, P–CH=, 4JPH = 10.2 4.24 s, 2H, NCH2	25.5 ddd 2JPP = 24.4 3JPH = 22.9 4JPH = 10.2	13.5 dd 2JPP = 24.4 3JPH = 32.4	24.4
71	Ph	7.95 dd, 1H, N–CH= 3JPH = 32.6, 3JPH = 22.8 7.44 m, 5H, Ph 6.18 d, 1H, P–CH=, 4JPH = 9.9	25.8 ddd 2JPP = 24.1 3JPH = 22.8 4JPH = 9.9	13.9 dd 2JPP = 24.1 3JPH = 32.6	24.1
72	s-Bu	7.90 dd, 1H, N–CH= 3JPH = 32.2, 3JPH = 23.4 6.47 d, 1H, P–CH=, 4JPH = 10.8 3.44 m, 1H, NCH 1.5 m, 2H, CH2 1.13 m, 6H, CH3	24.0 ddd 2JPP = 24.6 3JPH = 23.4 4JPH = 10.8	13.1 dd 2JPP = 24.6 3JPH = 32.2	24.6

^a The ${\mathrm{PCl}}_6^{-}$ anion in organyltrichlorophosphonium hexachlorophosphorates 63–67 has a broad signal in the ³¹P NMR spectra in the region −(296–298) ppm.

) [46,47].

The transformation of compounds 63–67 under the action of SO₂ into 1,4-azaphosphorines (68–72) occurs much more slowly than similar transformations of acyclic alkenyltrichlorophosphonium hexachlorophosphonates.

The coupling constant ²J_PH of the proton of the P–CH= group (${\mathrm{PCl}}_3^{+}$) is almost not observed in the ³¹P NMR spectra, and only for some compounds is it possible to fix the constant in the range of 0.6 Hz. The insignificant value of the geminal constant is characteristic of 1,4-heterophosphorine cycles. The unusually large coupling constant across four bonds (⁴J_PH) is due to the W-shaped planar structure of the molecular fragment of these compounds, which includes both phosphorus atoms and a proton. The value of the coupling constant ²J_PP (15–18 Hz) is typical for geminal bonded phosphorus atoms. The existence of the C-Cl bond in the molecule of compounds 63–72 is confirmed by the presence of signals in the ³⁵Cl NQR spectra in the frequency range of 38.7–39.4 MHz.

Similarly, the reaction of N-methyl-N,N-diacetamide with phosphorus pentachloride leads to 1,4-azaphosphorine containing an exocyclic phosphoryl group [41,48], while the phosphorylation of N,N-diacetamide proceeds differently [49] (Scheme 16).

The reaction, carried out under moderate vacuum conditions, furnishes 1H-2,4,4,6-tetrachloro-1,4-dihydro-1,4-azaphosphorinonium hexachlorophosphorate (75). The formation of compound 75 is probably preceded by imine–enamine rearrangement, which converts imidoyl chloride A to dialkenylamine B. Intramolecular phosphorylation of the latter leads to the formation of heterocycle 75, which is transformed by formic acid to 1H-4-oxo-2,4,6-trichloro-1,4-dihydro-1,4-azaphosphorine (76) (

Table 8. The ¹H and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of hexachlorophosphorates 73 and 75 (CH₃NO₂) and 1,4-azaphosphorines 74 and 76 (CDCl₃).

No.	1H/13C	31P, POCl2	31P, ${\mathbf{PCl}}_2^{+}/\mathbf{POCl}$
73	7.34 d, 1H, CH=, 4JPH = 9.3 4.32 s, CH3 140.8 d, CCl=, 2JPC = 14.0 108.47 dd, P-C-P, 1JPC = 152.6 (POCl2) 1JPC = 101.3 (POCl) 106.20 dd, CH=, 1JPC = 124.4, 3JPC = 8.5 42.15 s, CH3	21.8 dd 2JPP = 21.5 4JPH = 9.3	49.2 d 2JPP = 21.5
74	6.24 d, 1H, CH=, 4JPH = 8.5 3.80 s, CH3 148.8 d, CCl=, 2JPC = 12.0 110.24 dd, P-C-P, 1JPC = 149.6 (POCl2) 1JPC = 102.5 (POCl) 104.34 ddd, CH=, 1JCH = 180.6, 1JPC = 118.2, 3JPC = 9.2 43.67 s, CH3	17.2 dd 2JPP = 21.2 4JPH = 8.5	9.0 d 2JPP = 21.2
75	11.37 br s, 1H, NH 5.98 d, 2H, CH=, 2JPH = 2.2 145.20 d, CCl=, 2JPC = 16.1 82.31 dd, CH=, 1JCH = 187.0, 1JPC = 110.8	-	49.9 t 2JPH = 2.2
76	10.60 br s, 1H, NH 5.72 d, 2H, CH=, 2JPH = 1.1 143.70 d, CCl=, 2JPC = 14.2 80.44 dd, CH=, 1JCH = 180.0, 1JPC = 103.6	-	25.8 t 2JPH = 1.1

).

The phosphorus signal of ${\mathrm{PCl}}_6^{-}$in the ³¹P NMR spectra of compounds 73 and 75 appears in the region of −297.2 and −298.9 ppm, respectively. The large value of the direct coupling constant ¹J_CH=180.0–187.0 Hz may indicate the influence of the lone electron pair (LEP) of the oxygen atom on the C-H bond, and in addition, the proximity of the carbon atom to the electronegative phosphorus (or nitrogen) atom in the cycle [50,51,52,53]. NQR spectroscopy data also indicate the formation of heterocyclic compounds. It should be noted that ³⁵Cl NQR spectroscopy is successfully used to prove the structure of chlorine-containing compounds [54].

So, diacetamide reacts with phosphorus pentachloride to give the cyclic phosphonium salt (75) which is converted to azaphosphorinone (76) upon treatment with formic acid. The ³¹P NMR chemical shift values of POCl in 1,4-oxaphosphinines 21 and 22, azaphosphorines 68–72 and azaphosphorinone 76 appear in the region 24–28 ppm and practically do not depend on the nature (structure) of the heterocycle.

The phosphorylation of a representative of cyclic N-acylated hydrazones, 1-acetyl-3-methyl-6-oxo-1,4,5,6-tetrahydropyridazine, was carried out (Scheme 17) [55].

The process proceeds similarly to the phosphorylation of N,N-disubstituted acetamides [43]. The carbonyl group of the heterocycle lowers the nucleophilicity of the N-acetyl nitrogen atom, directing the reaction to the formation of α-chlorenamine A. The intermediate A is further phosphorylated by phosphorus pentachloride to afford complex salt 77, from which dichloride 2-(3-methyl-6- oxo-1,4,5,6-tetrahydropyridazin-1-yl)-2-chloroethenylphosphonic acid (78) is easily obtained. The ³¹P NMR spectrum of dichloride 78 contains a doublet at 33.1 ppm with ²J_PH = 24.8 Hz.

Enamino ketones as well as ethyl ethers of phenylaminocrotonic acids are convenient starting compounds for the synthesis of nitrogen–phosphorus-containing heterocycles via phosphorylation [56,57,58]. 4-Arylamino-3-penten-2-ones react with PCl₅ to give 4-(N-aryl-N′-dichlorophosphorylamino)-2-chloro-1,3-dienyltrichlorophosphonium hexachlorophosphorates 79–81. Compounds 79–81 and diphosphonic acid chloride 82 readily formed from 79 undergo gradual heterocyclization to 1,2-dihydro-1,2-azaphosphorines 85–87 and compound 88 with the elimination of phosphorus oxychloride (Scheme 18) (

Table 9. The ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of compounds 88 and 89 (CDCl₃).

No.	Structure	1H	13C	31P
88		7.07–7.20 m, 5H, Ph 6.44 dd, 1H, H-6 3JPH = 23.9, 3JHH = 8.1 5.85 dd, 1H, H-3, 3JPH = 11.4, 4JHH = 2.2 5.36 ddd, 1H, H-5, 3JHH = 8.1, 4JHH = 2.2, 4JPH = 2.2	147.5, 129.8, 120.0, 127.2 Ph 139.5 d, C-6, 2JPC = 2.0 138.3 s, C-Cl, 107, d, C-3, 1JPC = 149.6 105.6 d, C-5, 3JPC = 11.6	21.6 dd 3JPH = 23.9 2JPH = 11.4
89		13.39 s, 1H, NH 8.22 d, 1H, H-5, 3JHH = 8.2 7.89 dd, 1H, H-7, 3JHH = 8.2, 3JHH = 7.6 7.78 d, 1H, H-8, 3JHH = 8.2 7.59 dd, 1H, H-6, 3JHH = 8.2, 3JHH = 7.6 3.8 br s, 2H, OH 2.83 s, 3H, CH3	176.03 d, C-4, 2JPC = 7.7 157.24 d, C-2, 2JPC = 13.8 138.66 s, C-9, 133.91 s, C-7, 126.05 s, C-5, 124.87 s, C-6, 121.17 d, C-10, 3JPC = 11.1 118.86 s, C-8, 109.39 d, C-3, 1JPC = 161.0 20.42 s, CH3	11.6 s

) [56,57].

The ¹H, ¹³C and ³¹P NMR chemical shifts and coupling constants of phosphonic chloride 88 (CDCl₃) are presented in Table 9. The ³¹P NMR chemical shifts of 85, 86, and 87 have the following values (ppm): 56.2 dd, ³J_PH = 34.4 and ²J_PH = 16.5 Hz; 57.3 d, ²J_PH = 17.8 Hz; and 60.4 d, ²J_PH = 20.1 Hz, respectively.

It has been found that a longer (for several hours) treatment of ethyl phenylaminocrotonoate with phosphorus pentachloride leads to 1,4-quinolone 89 containing a trichlorophosphonium group in position 3 (Scheme 19) (Table 9) [58]. The heterocyclization is preceded by the formation of phosphorus-containing phenylaminocrotonic acid chloride. Its carbonyl group exhibits increased electrophilicity.

We found that 2-methyl-1,4-quinolone is not phosphorylated by phosphorus pentachloride, which confirms the proposed scheme for the reaction of crotonate with PCl₅, according to which the phosphorylation at the double bond is preceded by acylation of the benzene ring. The phosphorylation at the NH nitrogen atom of the quinolone is also possible, but the NH bond in the resulting phosphonic acid is regenerated in the course of hydrolysis.

The nitrogen chemical shift in the ¹⁵N NMR spectrum of compound 89 is at −220.1 ppm, which is characteristic of a pyrrole-type nitrogen atom [52,53].

It should be noted that unsaturated ketones behave similarly to enamino ketones when phosphorylated with phosphorus pentachloride [59,60]. C-phosphorylation products dienyltrichlorophosphonium hexachlorophosphorates have been studied by means of 2D and multinuclear ¹H, ¹³C and ³¹P NMR spectroscopy. The reaction most probably starts with the attack of phosphorus pentachloride at the carbonyl group to form O-phosphorylated product A (Scheme 20) [59].

In the presence of excess phosphorus pentachloride, tetrachlorophosphorane A undergoes ionization to give intermediate B, which decomposes to afford diene C. The latter is phosphorylated by excess phosphorus pentachloride similarly to the known alkenes with the formation of hexachlorophosphorates 90–94. The ³¹P NMR spectra of compounds 90–94 have signals at 84–89 ppm belonging to the ${\mathrm{PCl}}_3^{+}$ cation; a broadened signal at −297 to −298 ppm corresponds to the ${\mathrm{PCl}}_6^{-}$ anion (

Table 10. The ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of compounds 90–94.

No.	R1	R	1H	13C	31P
90	H	Me	7.80 dd, 1H, H-3 3JHH = 15.2, 4JPH = 2.9 7.21 d, 1H, H-4, 3JHH = 15.2 5.30 d, 1H, H-1, 2JPH = 56.4 2.10 s, 3H, CH3	143.50 d, C-2, 2JPC = 27.8 133.85 s, C-4, 128.67, d, C-3, 3JPC = 9.8 118.20 d, C-1, 1JPC = 116.3 21.28 s CH3	84.3 dd 2JPH = 56.4 4JPH = 2.9
91	Me	Me	7.73 d, 1H, H-3, 4JPH = 2.7 5.61 d, 1H, H-1, 2JPH = 56.2 2.08 s, 3H, CH3 1.98 s, 3H, CH3	142.05 d, C-2, 2JPC = 26.7 139.75 s, C-4, 124.51, d, C-3, 3JPC = 7.2 116.50 d, C-1, 1JPC = 114.9 29.27 s CH3 21.66 s CH3	85.7 dd 2JPH = 56.2 4JPH = 2.7
92	H	Ph	7.75 dd, 1H, H-3 3JHH = 14.8, 4JPH = 3.1 7.55–7.40 m, 5H, Ph 7.15 d, 1H, H-4, 3JHH = 14.8 5.87 d, 1H, H-1, 2JPH = 41.0	144.65 d, C-2, 2JPC = 26.0 140.90 s, i-Ph 136.86 s, C-4 128.98 s, m-Ph, p-Ph 127.69 s, o-Ph 127.48 d, C-3, 3JPC = 8.9 117.27 d, C-1, 1JPC = 115.5	89.1 dd 2JPH = 41.0 4JPH = 3.1
93	H	p-MeO-Ph	7.80 dd, 1H, H-3 3JHH = 14.9, 4JPH = 3.0 7.60, d 2H, m-Ph, 3JHH = 8.7 7.19 d, 1H, H-4, 3JHH = 14.9 6.92, d 2H, o-Ph, 3JHH = 8.7 5.00 d, 1H, H-1, 2JPH = 42.7 4.23 s, OMe	159.27 s, C-OMe 144.25 d, C-2, 2JPC = 26.9 136.95 s, C-4 133.10 s, i-Ph 129.36 s, o-Ph 128.44 d, C-3, 3JPC = 8.2 119.54 d, C-1, 1JPC = 117.1 114.32 s, m-Ph 49.36 s, OMe	84.6 dd 2JPH = 42.7 4JPH = 3.0
94	H	p-Cl-Ph	7.77 dd, 1H, H-3 3JHH = 14.5, 4JPH = 2.7 7.63, d 2H, m-Ph, 3JHH = 8.3 7.16 d, 1H, H-4. 3JHH = 14.5 6.76, d 2H, o-Ph, 3JHH = 8.3 5.63 d, 1H, H-1, 2JPH = 41.2	143.09 d, C-2, 2JPC = 27.6 139.00 s, i-Ph 136.29 s, C-4 134.60 s, C-Cl 129.24 s, m-Ph 129.18 s, o-Ph 128.51 d, C-3, 3JPC = 7.8 119.34 d, C-1, 1JPC = 116.9	88.7 dd 2JPH = 41.2 4JPH = 2.7

).

The ¹³C NMR spectra of hexachlorophosphorates 90–94 contain doublet signals in the region of 116–119 ppm, which are characteristic of the carbon atom directly bonded to the phosphorus atom (Table 10). The coupling constant ¹J_PC lies in the range of 115–117 Hz, but the spin–spin coupling constant ²J_PH between the phosphorus atom and vinyl protons turns out to be quite sensitive to the effect of substituents (41–56 Hz).

Chlorophosphorylated 1,3-dienes are also formed by the action of PCl₅ on diacetone alcohol [60]. It has been established that two pentadienyltrichlorophosphonium hexachlorophosphates (95, 96) are produced in approximately equal amounts via phosphorylation. Under the action of dimethylacetamide, hexachlorophosphorates (95, 96) are converted into a mixture of two isomers: 2-chloro-2,4-pentadiene-4-methyl-5-phosphonic (97) and 2-methyl-2,4-pentadiene-4-chloro- 5-phosphonic (98) acids (Scheme 21).

The process starts from the attack by the hydroxy oxygen atom on the phosphorus atom of phosphorus pentachloride, which is evidenced by a vigorous evolution of hydrogen chloride. The formed intermediate A readily undergoes cyclization to trichlorophosphorane B as a result of the intramolecular attack by the carbonyl oxygen atom of intermediate A on the phosphorus atom of tetrachlorophosphorane group. The attack by the carbonyl group of compound A at phosphorus pentachloride present in the reaction mixture in excess is unlikely because the intramolecular process is preferable. The intermediate B readily dissociates to ions, and the formed quasiphosphonium salt C, according to the second step of the Arbuzov reaction, is transformed to compound D. The subsequent conversion of D to dienes E and F proceeds in the presence of phosphorus pentachloride with the elimination of trichlorophosphorus oxide and hydrogen chloride, similarly to the process of the formation of alkenes in the phosphorylation of tertiary alcohols containing at least two methyl groups next to the hydroxy group with phosphorus pentachloride. The process is finalized by the phosphorylation of pentadienes with phosphorus pentachloride (Scheme 21).

The structure of the obtained products has been established by two-dimensional and multinuclear ¹H, ¹³C and ³¹P NMR spectroscopy. In the ³¹P NMR spectra of dienyltrichlorophosphonium hexachlorophosphorates (95, 96) recorded both in nitrobenzene and in nitromethane, two doublets of approximately the same intensity and a broadened singlet in the region of −296 ppm, characteristic of the ${\mathrm{PCl}}_6^{-}$ anion, are observed in the region 90.5 ppm (d, ²J_PH 52.8 Hz) and 85.7 ppm (d, ²J_PH 56.0 Hz). The ¹³C NMR spectrum of a mixture of compounds 95 and 96 shows two doublets at 116.5 ppm (C-1, ¹J_PC 114.9 Hz) and at 114.8 ppm (C-1, ¹J_PC 111.2 Hz) characteristic of carbon atoms directly bonded to the phosphorus atom. In the ³¹P NMR spectrum of a mixture of isomers 97 and 98, signals detected in the region 26.2 ppm (d, POCl₂, ²J_PH¹ 35.3 Hz) and 25.6 ppm (d, POCl₂, ²J_PH¹ 36.1 Hz) relate to compounds 97 and 98, respectively. The ¹H NMR spectrum of compound 97 contains signals at 2.36 ppm (d, C(2)Me, ⁴J_PH 3.5 Hz) and 2.29 ppm (s, C(4)Me). The ¹H NMR spectroscopy data of diene 97 indicate the cis-orientation of the methyl group towards the POCl₂ moiety in the propenyl fragment. This is evidenced by the value of the constant ⁴J_PH = 3.5 Hz.

It was established [61] that acetaldoxime reacts with PCl₅ to form the stable phosphorus-containing enamine 99. The trichlorophosphase group formed in compound 99 reduces the nucleophilicity of the nitrogen atom, which ensures the stability of this compound (Scheme 22).

2-(Trichlorophosphazo)-2-chloroethenyltrichlorophosphonium hexachlorophosphorate (99) is isolated as a white powder. The ³¹P NMR spectrum of compound 99 shows δ 80.5 ppm, d (${\mathrm{PCl}}_3^{+}$), ²J_PH = 44.2 Hz; δ 15.9 ppm, d (P=N), ⁴J_PH = 15.4 Hz; δ –297.8 ppm, br s (${\mathrm{PCl}}_6^{-}$). In the ¹H NMR spectrum in the region of vinyl protons, there is a single signal at 5.67 ppm, dd, which indicates that both PCl₃ groups belong to the same molecule.

Oxime derivatives containing a vinyl group at the oxygen atom react with PCl₅ in the same way as vinyl esters [62]. O-vinyloximes are phosphorylated at the O-vinyl group at room temperature, forming alkenyltrichlorophosphonium hexachlorophosphorates (Scheme 23).

Under the action of sulfur dioxide, hexachlorophosphorates are converted into E-ethenylphosphonic acid dichlorides (100, 101) (³J_HH = 13.0 Hz). The structure of the compounds has been established by 2D and multinuclear ¹H, ¹³C and ³¹P NMR spectroscopy [62].

6. The Structural Features of Acetylurea-Derived Diazaphosphorines

Organophosphorus compounds obtained by phosphorylation of urea derivatives and related compounds are widely used in the life sciences, medicine chemistry, pharmaceuticals, organic catalysis and materials science [63,64,65,66,67,68,69,70,71,72]. Efficient P-C or P-N bond formation using the various available phosphorus sources plays a critical role in the construction of organophosphorus compounds. Among them, PCl₅ acts as an essential source of phosphorus; therefore, its use to form P-C or P-N covalent bonds is being actively developed.

The problem of obtaining phosphorylated nitrogen-containing heterocyclic compounds, which play an important role in medicine, agrochemistry and modern metal complex catalysis, deserves special attention. The need to select conditions for each particular type of heterocycles, harsh reaction conditions (high temperature, the presence of strong bases and acids) and the high cost of the catalysts used significantly limit the application of this approach in practice. An alternative devoid of these disadvantages can be the construction of a heterocyclic ring using universal phosphorus pentachloride. With a rational approach, the “multiple” reactivity of such a reagent will provide an arsenal of substrate-controlled transformations leading to structurally diverse products.

Due to the wide practical application, the chemistry of organophosphorus compounds is developing rapidly. Among numerous series, phosphorus-containing heterocycles have been synthesized, which can be represented as structural analogues of azabenzenes: pyridine, pyrimidine, s-triazine. The interaction of urea derivatives with phosphorus pentachloride and the phosphorylation products have been successfully studied [73,74,75,76,77,78,79,80,81,82,83,84]. It is known that the results of phosphorylation of urea derivatives depend on the conditions of the process.

The direction of phosphorylation of N-acetylurea under the action of phosphorus pentachloride significantly depends on the ratio of reagents (Scheme 24) [73,76,77,78,79]. Further transformations of intermediate A are determined by the presence or absence of PCl₅ in the reaction medium. At a molar ratio of urea and PCl₅ of 1:5, hexachlorophosphorate 102 is obtained. However, in the absence of an excess of PCl₅ in the reaction medium, intramolecular phosphorylation of the C=C double bond of azabutadiene A by the trichlorophosphazol group occurs to deliver 2,2,4,6-tetrachloro-2,2-dihydro-1,5,2-diazaphosphorine 103 (Scheme 24) (

Table 11. ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of 1,5,2-diazaphosphorines 103–107.

No.	Structure	1H	13C	31P
103		5.93 d, H-3 2JPH 14.0	163.15 d, C-4, JPC 21.2 160.90 d, C-6, 2JPC 6.2 88.02 d, C-3, JPC 112.0	53.4 d 2JPH 14.0
104		-	160.15 d, C-4, JPC 37.8 157.73 d, C-6, 2JPC 4.9 97.27 d, C-3, JPC 132.8	44.7 s
105		5.88 d, H-3 2JPH 42.2 3.45 d, CH3 3JPH 6.9	158.15 C-4, 2JPC 23.4 156.90 C-6, 2JPC 6.8 82.20 C-3, 1JPC 116.0	64.4 dq 2JPH 42.2 3JPH 6.9 –297.5 br s ${\mathrm{PCl}}_6^{-}$
106		6.08 d, H-3 2JPH 12.8 3.38 d, CH3 3JPH 6.6	166.45 C-4, 2JPC 24.2 159.70 C-6, 2JPC 6.2 85.50 C-3, 1JPC 112.0	21.5 dq 2JPH 12.8 3JPH 6.6
107		6.84 d, CHCl2 4JPH 1.0 3.10 d, CH3 3JPH 24.4	147.7 d, C=O, 2JPC 17.5 95.8 d, C-Cl, 2JPC 6.1 76.5 d, CHCl, 3JPC 7.3 28.9 d, CH3, 2JPC 2.5	−69.1 qd 3JPH 24.4 4JPH 1.0

). The action of formic acid on complex compound 102 gives N¹-dichlorophosphoryl-N²-(1-chloro-2-dichlorophosphorylethenyl)-C-chloroformamidine 102a [73,79].

The ³¹P NMR spectrum of compound 102 is represented by three signals: δ, ppm, 82.8 d (${\mathrm{PCl}}_3^{+}$), ²J_PH = 43.2 Hz; 23.7 s (${\mathrm{PCl}}_3$); −297.2 s (${\mathrm{PCl}}_6^{-}$). A single signal at 6.55 ppm (=CH) is observed in the proton spectrum. The ¹³C NMR spectrum of complex compound 102 shows the following: δ, ppm, 156.00 dd (ClC=CH), ²J_PC = 10.2 and ⁴J_PC = 2.0 Hz; 144.12 dd (ClC=N), ²J_PC = 6.8 and ⁴J_PC = 2.0 Hz; 95.62 (=CH). The presence of a spin–spin interaction of each of the carbon atoms simultaneously with two phosphorus atoms proves the presence of two phosphorus atoms in one molecule.

The ³¹P NMR spectrum of chloroformamidine 102a has two signals, δ, ppm: 25.3 d (C-POCl₂), ²J_PH = 19.8 Hz, and 3.3 s (N-POCl₂). In the ¹H spectrum, the signal =CH of the proton is in the region of 6.2 ppm. Two high-frequency signals in the ³⁵Cl NQR spectrum of compound 102a at 36.210 and 37.243 MHz refer to the chlorine atoms of the C–Cl bond, the latter being related to the chlorine atom in the C(Cl)=CH group. Two low-frequency signals at 27.638 and 27.088 MHz refer to chlorine atoms in the POCl₂ group.

Heating of the reaction mixture obtained by the interaction of PCI₅ and chloroacetylurea is accompanied by the release of hydrogen chloride and leads to the formation of 2,2,3,4,6-pentachloro-2,2-dihydro-1,5,2-diazaphosphorine (104) (Scheme 25) [76].

The ³¹P NMR spectrum of diazaphosphorine (104) contains a singlet at 44.7 ppm, located in the region characteristic of azaphosphorines containing a PCl₂ group in the heterocycle (Table 11). The structure of heterocycle 104 is also confirmed by the data of ³⁵Cl NQR spectroscopy [76].

Upon transition of diazaphosphorine (DAP) (103) to the crystalline state, the conjugation of the P=N, C=C and C=N bonds is disturbed or significantly weakened, as indicated by a significant change in the characteristic vibrations in the IR spectra of these structures [78]. The violation of conjugation occurs, most likely, as a result of the dimerization of diazaphosphorine (103) at the P=N bond, and the resulting dimer (103a) is stable only in the crystalline state, and upon melting or dissolution, it dissociates extremely easily to give the monomeric form (103) (Scheme 26).

In the ³¹P NMR spectra of solutions of compound 103 at room and low (−80 °C) temperatures, no signals are observed in the region characteristic of the pentacoordinated phosphorus atom, which confirms the conclusion that only the monomeric form is present in the solution. The dimeric structure of DAP (103a) in the crystalline state is confirmed by NQR and photoelectron spectroscopy data, as well as by quantum chemical calculations [78,85]. A weighty argument in favor of the dimeric structure (103a) is the significant splitting of the resonant doublet signal (ν⁷⁷ 27.788 and 29.308 Hz), which characterizes the PCl₂ fragment in the NQR ³⁵Cl spectrum of compound 103a. This indicates that the electronic distribution (state) of chlorine atoms differs significantly. Upon dimerization of 103 to 103a, the phosphorus atom becomes pentacoordinated, thereby acquiring a trigonal bipyramidal configuration. As a result, the nitrogen atoms of the four-membered cycle can occupy axial (one) and equatorial (second) positions. Accordingly, one chlorine atom becomes axial, and the other becomes equatorial, which causes the observed splitting of the ν⁷⁷ (P-Cl) signal. In the NQR ³⁵Cl spectra of chlorophosphoranes, the signals of chlorine nuclei occupying the equatorial position in the trigonal bipyramid are observed at higher frequencies than the signals of axial chlorine atoms.

The interaction of PCl₅ and N-methyl-N′-acetylurea unexpectedly affords 2-(trichlorophosphazo)-2-chloroethenyltrichlorophosphonium hexachlorophosphorate (99) (see Scheme 22), and a minor amount of 1-methyl-2,2,4,6-tetrachloro-1,2-dihydro-1,5,2-diazaphosphorinonium hexachlorophosphorate (105) is formed (Scheme 27). Under the action of SO₂, compound 105 is readily converted to 1-methyl-2,4,6-trichloro-1,2-dihydro-1,5,2-diazaphosphorine (106) (Table 11). Both heterocycles are easily hydrolyzable crystals, stable without moisture access [14,75].

The structures of compound 99 and heterocycles 105 and 106 were proved by ¹H, ¹³C and ³¹P NMR methods, as well as ³⁵Cl NQR spectroscopy data. The ³¹P NMR spectrum of compound 105 has a doublet at 64.4 ppm corresponding to the phosphorus atom of the PCl₂ group and a broadened singlet in the region of −297.5 ppm (Table 11).

Under the action of PCl₅ on N-methyl-N′-chloroacetylurea, the chloroacetyl group is not phosphorylated; the reaction furnishes a compound with a four-membered diazaphosphetidine ring—1-methyl-2,2,2-trichloro-3-(1,1,2,2-tetrachloroethyl)-1,3-diaza-2-phosphetidinone (107) (Scheme 28) [84].

The structure of compound 107 has been established by NMR spectroscopy data. In the ³¹P NMR spectrum of phosphetidinone 107, a single signal refers to the pentacoordinated phosphorus atom: −69.1 ppm, qd, ³J_PH = 24.4 and ⁴J_PH = 1.0 Hz (Table 11).

The phosphorylation of N-acyl-N,N′-dimethylurea derivatives with phosphorus pentachloride gives rise to six-membered heterocycles, 1,5-dimethyl-2,2,4-trichloro-6-oxo-3-alkyl-1,5,2-diazaphosphoniarin hexachlorophosphorates (108–110) (Scheme 29) [74].

The phosphorylation begins with the attack of phosphorus pentachloride at the oxygen atom of the acetyl group to produce α-chloramine. The enurea is further phosphorylated at the double bond with the formation of acyclic hexachlorophosphorate 2-(N,N′-dimethylurea)-2-chloro-1-alkylethenyltrichlorophosphonium A. The subsequent rapid intramolecular attack of the chlorophosphonium cation at the secondary nitrogen atom of urea ends with cyclization to deliver diazaphosphorines (108–110). The action of SO₂ on compounds 108–110 results in 1,5-dimethyl-2,4-dichloro-2,6-dioxo-3-alkyl-1,2,5,6-tetrahydro-1,5,2-diazaphosphoniarins (111–113) (

Table 12. ¹H and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of 1,5,2-diazaphosphorines 108–114.

No.	Structure	1H	31P
108		6.83 d, H-3, 2JPH = 10.6 3.90 s, 5-CH3 3.78 d, 1-CH3 3JPH = 9.9	58.3 dq 2JPH = 10.6, 3JPH = 9.9 −294.7 br s, ${\mathrm{PCl}}_6^{-}$
109		3.68 s, 5-CH3 3.37 d, 1-CH3 3JPH = 9.6 1.85 d, 3-CH3 3JPH = 6.7	61.2 qq 3JPH = 9.6, 3JPH = 6.7 −297.6 br s, ${\mathrm{PCl}}_6^{-}$
110		3.57 s, 5-CH3 3.30 d, 1-CH3 3JPH = 12.2 2.85 dd, 3-CH2 3JPH = 10.0, 3JHH = 8.2 1.75 d, 3-CH3 3JHH = 8.2	60.8 qt 3JPH = 12.2, 3JPH = 10.0 −297.9 br s, ${\mathrm{PCl}}_6^{-}$
111		5.71 d, H-3 2JPH = 6.8 3.50 s, 5-CH3 3.19 d, 1-CH3 3JPH = 7.3	17.1 qd 3JPH = 7.3, 2JPH = 6.8
112		3.35 s, 5-CH3 3.04 d, 1-CH3 3JPH = 7.6 1.96 d, 3-CH3 3JPH = 15.1	18.8 qq 3JPH = 15.1, 3JPH = 7.6
113		3.48 s, 5-CH3 3.32 d, 1-CH3 3JPH = 10.0 2.85 dd, 3-CH2 3JPH = 9.0, 3JHH = 7.8 1.75 d, 3-CH3 3JHH = 7.8	19.2 qt 3JPH = 10.0, 3JPH = 9.0
114		5.70 d, H-3 2JPH = 61.4 3.42 s, CH3 3.14 d, CH3 3JPH = 14.2	100.5 dq 2JPH = 61.4, 3JPH = 14.2

).

The signals of the tetracoordinated phosphorus atom of the ${\mathrm{PCl}}_2^{+}$ group in compounds 63–67 (Table 8), 73 and 75 (Table 8), 85–87 and 105 (Table 11), and 108–110 (Table 12) in the ³¹P NMR spectra are observed in a higher frequency field (45–64 ppm) than those of the POCl moiety in products 111–113 (17–19 ppm) (Table 12), which is apparently more associated with the existence of a molecule in a charged form.

Reduction of complex compounds 108–110 with tetraethylammonium iodide gives 1,5-dimethyl-2,4-dichloro-6-oxo-3-alkyl-1,2,5,6-tetrahydro-1,5,2-diazaphosphoniarines (114, 115) (Scheme 29) (Table 12) [74]. In the ³¹P NMR spectrum of compound 115, a multiplet signal is observed in the region of 99.3 ppm, which corresponds to the phosphorus atom of the PCl group. It should be noted that the ³¹P NMR chemical shift of the tricoordinated (trivalent) phosphorus atom of the PCl group (114, 115) is quite large, as expected, and lies in the region of approximately 100 ppm (Table 12).

N-acetyl-N,N′-ethyleneurea (1-acetylimidazolidin-2-one) reacts with PCl₅ to form 2-(2-oxo-1-imidazolidinyl)-2-chloroethenyltrichlorophosphonium hexachlorophosphorate (116) (Scheme 30) [81].

Phosphorus pentachloride attacks the oxygen atom of the acetyl group of acetylethyleneurea, but steric and conformational hindrances in cyclic ureide do not allow intramolecular attack of the nitrogen atom N-H by the organyltrichlorophosphonium cation (116) with the formation of a heterocycle, as observed in the phosphorylation of acyclic trisubstituted ureides [74]. Compound 116, upon treatment with SO₂, is converted into 2-(2-oxo-1-imidazolidinyl)-2-chloroethenylphosphonic dichloride (117); its hydrolysis yields 2-oxo-2-(2-oxo-1-imidazolidinyl) ethylphosphonic acid (118) (

Table 13. ¹H, ¹³C and ³¹P NMR chemical shifts (δ, ppm) and coupling constants (J, Hz) of 1,5,2-diazaphosphorines 117 and 118.

No.	Structure	1H	31P	13C
117		7.14 d, = CH 2JPH = 21.1 6.38 br s, NH 4.10 t, 5-CH2 3JHH = 8.4 3.52 t, 4-CH2 3JHH = 8.4	27.9 d 2JPH = 21.1	155.98 s, C = O 143.89 d, C-Cl, 2JPC = 6.5 103.28 d, = CH 1JPC = 176.4 45.75 s, 5-CH2 36.38 s, 4-CH2
118		7.64 br s, NH 3.75 t, 5-CH2 3JHH = 8.2 3.57 d, CH2 2JPH = 21.8 3.28 t, 4-CH2 3JHH = 8.2	16.0 t 2JPH = 21.8	165.70 d, C = O 2JPC = 6.9 155.80 s, 2-C = O 42.21 s, 5-CH2 35.33 s, 4-CH2 35.28 d CH2 1JPC = 126.3

).

The ³¹P NMR chemical shifts of 116 have the following values (ppm): 80.2 d, ²J_PH = 38.9 Hz (${\mathrm{PCl}}_3^{+}$) and −295.5 br s (${\mathrm{PCl}}_6^{-}$).

A generalization of the analysis of ³¹P NMR chemical shifts of nitrogen–phosphorus-containing enamines and the corresponding heterocycles shows that a change in the coordination number of the phosphorus atom from 3 to 6 significantly changes δ³¹P from about + 200 to −300 ppm. As regards the spin–spin coupling constants, the Karplus equation is applicable to the saturated systems of the H–C(sp³)–C(sp³)–H type. The vicinal interaction in such systems strongly (more precisely, mainly) depends on the value of the dihedral angle φ. If one of the carbon atoms is in the sp² hybridization state, the Karplus equation becomes inapplicable. In our case, we deal with the unsaturated compounds (enamines), in which carbon atoms are in the state of sp² hybridization.

7. Conclusions

The reactions of phosphorus pentachloride with enamines, tertiary amines, enamides, diacetamides, N-vinyl-substituted cyclic imides and N-acetylureas can be employed as a platform for the simple and convenient synthesis of hard-to-reach nitrogen-containing organophosphorus compounds and new types of heterocycles based on them. The development of efficient and reliable methods for the formation of carbon–phosphorus bonds is of great importance in connection with the widespread use of organophosphorus compounds in chemistry, materials science and biology. In recent years, interest in functionalized organophosphorus compounds has been steadily increasing, as evidenced by a large number of publications in the field of their chemical and structural studies [86,87,88,89,90,91,92]. It should be noted that the skillful use of phosphorus pentachloride as a phosphorylating agent for derivatives of enamines, enamides, tertiary amines, acetyl ureas and other nucleophiles leads to the production of various C- and N-chlorophosphorylated compounds, which, under certain conditions, are transformed to form promising nitrogen- and phosphorus-containing heterocyclic systems of diverse structures.

NMR spectroscopy studies of chlorophosphorylated enamines based on available tertiary amines and a wide range of other nitrogen-containing organic nucleophiles make a significant contribution to the development of the chemistry of unsaturated organophosphorus compounds. The use (involvement) of multipulse and multinuclear NMR spectroscopy, in particular ³¹P NMR, simplifies the tasks associated with establishing the structure of phosphorylation products and stereochemical behavior (e.g., for E,Z-isomeric compounds). Analysis of the ³¹P NMR chemical shifts of phosphorus–nitrogen-containing enamines and the corresponding heterocycles indicates that a variation in the coordination number of the phosphorus atom from 3 to 6 leads to a dramatic change in the screening of the ³¹P nucleus from about +200 to −300 ppm.

³¹P NMR spectroscopy is the most convenient, powerful and simple express method for recognizing the coordination number of the phosphorus atom in organophosphorus compounds, as well as for determining (identifying) the structure of isomeric forms of organophosphorus products. A certain contribution to the determination of the structure of chlorophosphorylated compounds is made by ³⁵Cl NQR spectroscopy.

This review article can be considered as the first step towards a further deeper analysis of NMR spectroscopy and quantum-chemical data for a wide range of organic compounds, in particular, saturated and unsaturated compounds and heterocycles based on them. In this regard, it will be possible to apply the Karplus equation, which works mainly for saturated systems of the H–C(sp³)–C(sp³)–H type.