Syntheses and Reactions of Pyrroline, Piperidine Nitroxide Phosphonates

Organophosphorus compounds occupy a significant position among the plethora of organic compounds, but a limited number of paramagnetic phosphorus compounds have been reported, including paramagnetic phosphonates. This paper describes the syntheses and further transformations of pyrroline and piperidine nitroxide phosphonates by well-established methods, such as the Pudovik, Arbuzov and Horner-Wadsworth-Emmons (HWE) reactions. The reaction of paramagnetic α-bromoketone produced a vinylphosphonate in the Perkow reaction. Paramagnetic α-hydroxyphosphonates could be subjected to oxidation, elimination and substitution reactions to produce various paramagnetic phosphonates. The synthesized paramagnetic phosphonates proved to be useful synthetic building blocks for carbon-carbon bond-forming reactions in the Horner-Wadsworth-Emmons olefination reactions. The unsaturated compounds achieved could be transformed into various substituted pyrroline nitroxides, proxyl nitroxides and paramagnetic polyaromatics. The Trolox® equivalent antioxidant capacity (TEAC) of new phosphonates was also screened, and tertiary α-hydroxyphosphonatate nitroxides exhibited remarkable antioxidant activity.


Introduction
Functionalized phosphonates are fascinating organophosphorus compounds used in biology, pharmacology, agriculture and organic chemistry [1][2][3]. The main interest in preparation of these compounds originated from their application in the Horner-Wadsworth-Emmons (HWE) olefination reaction to produce various unsaturated compounds [4]. Despite the simplicity of the syntheses of phosphonates or α-hydroxyphosphonates or trialkylphosphates by the Arbuzov [5], Pudovik [6] or Perkow reactions [7], these reactions were applied limitedly to access paramagnetic phosphorus compounds, although many phosphorus containing nitroxides have been published [8][9][10][11]. Remarkable part of these materials are mainly 2-substituted βor γ-phosphorylated five-membered nitroxides exhibiting a second notably large hyperfine splitting with the one-half spin nucleus of the phosphorus atom [12][13][14][15][16] (Figure 1). However, no further transformations of these paramagnetic phosphonates were reported beyond phosphonate hydrolysis [8] or transesterification [16]. In this paper, we report the syntheses of new pyrroline and piperidine nitroxide phosphonates starting from nitroxide halogenides, acetylenes, aldehydes and ketones. Our purpose was to evaluate the scope and limitations of the reactions of the newly synthesized paramagnetic phosphonates or α-hydroxyphosphonates as potential

Use of Arbusov Reaction
Treatment of five-and six-membered allylic bromides 1a-c [19][20][21] with triethyl phosphite at 120 °C with stirring in an open vessel resulted in the formation of phosphonates 2a-c in 65-81% yield (monitored by thin layer chromatography). As expected in the case of compound 1b, only the more reactive allylic bromide was converted to a phosphonate, and the vinyl bromine atom was not substituted. Under these conditions, we did not observe the reduction of nitroxide function. The same reaction could be performed with dibromo compound 3 [22] to furnish bisphosphonate ester 4 (Scheme 1). Scheme 1. Synthesis of paramagnetic phosphonates by the Arbusov reaction.

Use of HWE and Perkow Reaction
Because the synthesis of compound 1c is a long multistep procedure from the readily available 4-oxo-TEMPO (1-oxyl-4-oxo-2,2,6,6-tetramethyplpiperidine radical) (5b) [21,23,24], we are pleased to report a simpler and more direct method that heats the sodium salt of tetraethyl methylenediphosphonate with compound 5b in toluene at reflux temperature to produce compound

Use of Arbusov Reaction
Treatment of five-and six-membered allylic bromides 1a-c [19][20][21] with triethyl phosphite at 120 • C with stirring in an open vessel resulted in the formation of phosphonates 2a-c in 65-81% yield (monitored by thin layer chromatography). As expected in the case of compound 1b, only the more reactive allylic bromide was converted to a phosphonate, and the vinyl bromine atom was not substituted. Under these conditions, we did not observe the reduction of nitroxide function. The same reaction could be performed with dibromo compound 3 [22] to furnish bisphosphonate ester 4 (Scheme 1).
Molecules 2020, 25, x FOR PEER REVIEW 2 of 16 C=C bond-forming reactions have been published [17], considering the advantages of use of phosphonates [18] over phosphonium ylides (e.g., avoiding the formation of non-water-soluble triphenylphosphine oxide), paramagnetic phosphonates can be more appropriate building blocks for synthetic chemists working in this field.

Use of Arbusov Reaction
Treatment of five-and six-membered allylic bromides 1a-c [19][20][21] with triethyl phosphite at 120 °C with stirring in an open vessel resulted in the formation of phosphonates 2a-c in 65-81% yield (monitored by thin layer chromatography). As expected in the case of compound 1b, only the more reactive allylic bromide was converted to a phosphonate, and the vinyl bromine atom was not substituted. Under these conditions, we did not observe the reduction of nitroxide function. The same reaction could be performed with dibromo compound 3 [22] to furnish bisphosphonate ester 4 (Scheme 1). Scheme 1. Synthesis of paramagnetic phosphonates by the Arbusov reaction.

Use of HWE and Perkow Reaction
Because the synthesis of compound 1c is a long multistep procedure from the readily available 4-oxo-TEMPO (1-oxyl-4-oxo-2,2,6,6-tetramethyplpiperidine radical) (5b) [21,23,24], we are pleased to report a simpler and more direct method that heats the sodium salt of tetraethyl methylenediphosphonate with compound 5b in toluene at reflux temperature to produce compound Scheme 1. Synthesis of paramagnetic phosphonates by the Arbusov reaction.
The formation of ketophosphonate in an Arbusov reaction can be excluded because the appearance of the vinyl proton at 5.43 ppm and the 31 P-NMR shift at −6.22 ppm verify the formation of diethylvinyl phosphate 7. The latter 31 P-NMR data show good correlation with the reported values [26].

Pudovik Hydroxyphosphonate Synthesis and Transformations
The above results drove our decision to study the reactions of paramagnetic aldehydes and ketones with diethyl phosphite to produce -hydroxyphosphonates because these derivatives have biological importance, i.e., herbicidal, antibacterial, antifungal and antioxidant effects, to mention but a few [27][28][29]. To access paramagnetic -hydroxyphosphonates among the possible reaction conditions [30,31] tested, we choose the methodology of Kulkarni et al. [32], e.g., solvent-free conditions in the presence of 0.05 eq. K3PO4. Therefore, treatment of ketones 5a [33] or 5b [23] or fiveor six-membered nitroxide aldehydes 9a [34], 9b [20], or 9c [21] with diethyl phosphite in the presence of 0.05 eq. K3PO4 offered the -hydroxyphosphonates 8a or 8b or 10a or 10b or 10c, respectively, in 78-92% yield (Scheme 3). The structure of these compounds is proven by the appearance of hydroxyl band of OH groups at ~ 3200 cm −1 compared with compounds 2a-c. We attributed the conversion of -hydroxyphosphonates 8a or 8b to the corresponding vinyl phosphonate by water elimination. By treatment of compound 8a or 8b with POCl3 in anhydr. pyridine [23] after 48 h at room temperature, 11 vinylphosphonate could be isolated from 8b in 29% yield, but the expected five-membered vinylphosphonate was not formed under these conditions. The structure of vinylphosphonate 11 is proven by the split vinyl proton at 6.62 ppm with J = 21.5 Hz and the upfield shift of the 31 P-NMR signal at 19.3 ppm compared with that of the compound 2c 31 P signal at 27.1 ppm (see Supplementary Materials). Further attempts to eliminate the water from compound 8a with sulfuric acid [35] or FeCl3/silica gel microwave heating [36] did not produce the required vinyl phosphonate. Our efforts to substitute the tertiary alcohols 8a or 8b with various nucleophiles via mesylate did not succeed, Scheme 2. Synthesis of paramagnetic phosphonate (2c) by a HWE reaction and phosphate 7 by a Perkow reaction from 4-oxo-TEMPO (5b).
The formation of ketophosphonate in an Arbusov reaction can be excluded because the appearance of the vinyl proton at 5.43 ppm and the 31 P-NMR shift at −6.22 ppm verify the formation of diethylvinyl phosphate 7. The latter 31 P-NMR data show good correlation with the reported values [26].

Pudovik Hydroxyphosphonate Synthesis and Transformations
The above results drove our decision to study the reactions of paramagnetic aldehydes and ketones with diethyl phosphite to produce α-hydroxyphosphonates because these derivatives have biological importance, i.e., herbicidal, antibacterial, antifungal and antioxidant effects, to mention but a few [27][28][29]. To access paramagnetic α-hydroxyphosphonates among the possible reaction conditions [30,31] tested, we choose the methodology of Kulkarni et al. [32], e.g., solvent-free conditions in the presence of 0.05 eq. K 3 PO 4 . Therefore, treatment of ketones 5a [33] or 5b [23] or five-or six-membered nitroxide aldehydes 9a [34], 9b [20], or 9c [21] with diethyl phosphite in the presence of 0.05 eq. K 3 PO 4 offered the α-hydroxyphosphonates 8a or 8b or 10a or 10b or 10c, respectively, in 78-92% yield (Scheme 3). The structure of these compounds is proven by the appearance of hydroxyl band of OH groups at 3200 cm −1 compared with compounds 2a-c. We attributed the conversion of α-hydroxyphosphonates 8a or 8b to the corresponding vinyl phosphonate by water elimination. By treatment of compound 8a or 8b with POCl 3 in anhydr. pyridine [23] after 48 h at room temperature, 11 vinylphosphonate could be isolated from 8b in 29% yield, but the expected five-membered vinylphosphonate was not formed under these conditions. The structure of vinylphosphonate 11 is proven by the split vinyl proton at 6.62 ppm with J = 21.5 Hz and the upfield shift of the 31 P-NMR signal at 19.3 ppm compared with that of the compound 2c 31 P signal at 27.1 ppm (see Supplementary Materials). Further attempts to eliminate the water from compound 8a with sulfuric acid [35] or FeCl 3 /silica gel microwave heating [36] did not produce the required vinyl phosphonate. Our efforts to substitute the tertiary alcohols 8a or 8b with various nucleophiles via mesylate did not succeed, similar to the same experiments with the secondary alcohols 10a-c. For further possible transformations, we focused on compound 10a conversions, which could be smoothly oxidized to α-ketophosphonate 12 with 3.0 eq. Dess-Martin periodinane (1,1,1-tris(acetyloxy)-1,1-dihydro-1,2-benziodoxol-3-(1H)-one) [37] in CH 2 Cl 2 at room temperature.
With the reaction of compound 10a with DEAD (diethyl azodicarboxylate) and PPh 3 in the presence of HN 3 under Mitsonubu reaction conditions [38], we created paramagnetic α-azidophosphonate 13.

Scheme 3. Synthesis of -hydroxyphosphonates.
Under similar conditions and using methyl iodide as a source for the I − nucleophile [39], we obtained iodo compound 14, which was rather inert for attempts at further nucleophilic substitution conditions (Scheme 4). The limited success of these transformations is attributed to the sterically hindered allylic position, which is surrounded by a bulky phosphonate group and a densely substituted pyrroline nitroxide ring.  Under similar conditions and using methyl iodide as a source for the I − nucleophile [39], we obtained iodo compound 14, which was rather inert for attempts at further nucleophilic substitution conditions (Scheme 4). The limited success of these transformations is attributed to the sterically hindered allylic position, which is surrounded by a bulky phosphonate group and a densely substituted pyrroline nitroxide ring.

Scheme 3. Synthesis of -hydroxyphosphonates.
Under similar conditions and using methyl iodide as a source for the I − nucleophile [39], we obtained iodo compound 14, which was rather inert for attempts at further nucleophilic substitution conditions (Scheme 4). The limited success of these transformations is attributed to the sterically hindered allylic position, which is surrounded by a bulky phosphonate group and a densely substituted pyrroline nitroxide ring.

Phosphonate Synthesis with Lithiation
To obtain the five-membered vinylphosphonate, we attempted heating of compound 15 [40] with diethylphosphite in the presence of a catalytic amount of NiCl 2 [41], but no conversion was observed. Our efforts to construct a P-C bond with diethylphospite via the Pd-catalyzed Hirao reaction with the conventional or microwave-assisted method [42] also failed. As a result, we finally decided to lithiate [43] the O-methyl derivative 16, as achieved via Fenton reaction in a dimethylsulfoxide/H 2 O 2 /Fe 2+ system [44], followed by treatment with 1.0 eq. BuLi (buthyl lithium) and addition of diethylchlorophosphate to produce the diamagnetic vinyl phosphonate, which was not isolated but the crude product was treated with meta-chloroperoxybenzoic acid [45]. Thus we obtained compound 17, fortunately without epoxidation of the double bond. The paramagnetic acetylene phosphonate can be prepared by deprotonating acetylene 18 [46] at a terminal acetylene carbon with lithium hexamethyldisilazane (LiHMDS) followed by treatment with diethylchlorophosphate to give compound 19 (Scheme 5). The formation of acetylenephosphonate is proven by the shielded 31

Phosphonate Synthesis with Lithiation
To obtain the five-membered vinylphosphonate, we attempted heating of compound 15 [40] with diethylphosphite in the presence of a catalytic amount of NiCl2 [41], but no conversion was observed. Our efforts to construct a P-C bond with diethylphospite via the Pd-catalyzed Hirao reaction with the conventional or microwave-assisted method [42] also failed. As a result, we finally decided to lithiate [43] the O-methyl derivative 16, as achieved via Fenton reaction in a dimethylsulfoxide/H2O2/Fe 2+ system [44], followed by treatment with 1.0 eq. BuLi (buthyl lithium) and addition of diethylchlorophosphate to produce the diamagnetic vinyl phosphonate, which was not isolated but the crude product was treated with meta-chloroperoxybenzoic acid [45]. Thus we obtained compound 17, fortunately without epoxidation of the double bond. The paramagnetic acetylene phosphonate can be prepared by deprotonating acetylene 18 [46] at a terminal acetylene carbon with lithium hexamethyldisilazane (LiHMDS) followed by treatment with diethylchlorophosphate to give compound 19 (Scheme 5). The formation of acetylenephosphonate is proven by the shielded 31 P signal at −6.4 ppm (see Supplementary Materials). Scheme 5. Synthesis of paramagnetic phosphonate esters by lithiation.

General Methods and Reagents
Mass spectra were recorded with a Thermoquest Automass Multi system (ThermoQuest, CE, Instruments, Milan, Italy), a GCMS-2020 (Shimadzu, Tokyo, Japan) both operated in EI mode (70 eV) and a Thermo Q-Exactive HPLC/MS/MS (Thermo Scientific, Waltham, MA, USA) with ESI(+) ionization. Elemental analyses were obtained with a Fisons EA 1110 CHNS elemental analyzer (Fisons Instruments, Milan, Italy). The melting points were determined with a Boetius micromelting point apparatus (Franz Küstner Nachf. K. G., Dresden, Germany). The 1 H-NMR spectra were recorded with a Bruker Avance 3 Ascend 500 system (Bruker BioSpin Corp., Karslruhe, Germany) operated at 500 MHz, and the 13 C-NMR spectra were obtained at 125 MHz and 31 P-NMR 202 MHz in CDCl3 or DMSO-d6 at 298 K. The "in situ" reduction of the nitroxides was achieved by addition of five equivalents of hydrazobenzene ((PhNH)2/radical). The O-acetyl derivative of compound 22 for NMR measurement was prepared as described previously [49]. The EPR (electron paramagnetic resonance) spectra were recorded on MiniScope MS 200 (Magnettech GMBH, Berlin, Germany) instrument in CHCl3 solution, and the concentrations were 1.0 × 10 −4 M. All radicals gave a 3-line Scheme 6. HWE reactions of phosphonates to various alkenes and aromatic compounds including a reduction of 20b compound to a substituted 21 proxyl nitroxide.

General Procedure for Arbusov Reactions (2a-c, 4)
In a well-ventilated hood, a mixture of compound 1a or 1b or 1c or 3 (10.0 mmol) and triethylphosphite (2.5 g, 15.0 mmol, or 5.0 g, 30.0 mmol, for compound 3) was stirred in an open vessel at 120 • C in an oil bath. The ethylbromide byproduct was allowed to escape. The reaction mixture was monitored by TLC, and after consumption of the starting material (~2 h), the mixture was allowed to cool spontaneously with stirring. After cooling, the resulting mixture was purified by flash column chromatography to give the allylic phosphonates.

Conclusions
In conclusion, the Arbusov, Pudovik, Perkow and HWE reactions were adopted to access paramagnetic allylic-, vinyl-, acetylene-and α-hydroxyphosphonates or vinyl phosphates, giving the desired products with moderate to good yields. α-hydroxyphosphonates could be further transformed by oxidation, substitution or elimination reactions. We demonstrated that allylic phosphonates are good building blocks in olefination reactions for the introduction of pyrroline nitroxide rings in various scaffolds. Additionally, paramagnetic saturated α-hydroxyphosphonates exhibited remarkable antioxidant (proton and electron donor) activity against the ABTS•+ radical. Further synthetic, biological and biophysical applications of the newly synthesized nitroxide phosphonates are in progress.