Approach to the Synthesis of Five-Membered Organophosphorus Compounds via Alumoles and Alumolanes †

: This work summarizes the results of a new approach to the synthesis of previously undescribed, hard-to-obtain ﬁve-membered cyclic organophosphorus compounds: 3-alkyl(aryl)- substituted phospholanes, α , ω -bisphospholanes, polycyclic phospholanes, 4,5-dialkyl(diaryl)- dis-ubstituted 2,3-dihydrophospholes, as well as their oxides and sulﬁdes. Alumoles and alumolanes synthesized by the reaction of cycloalumination of available unsaturated compounds (terminal alkenes, α , ω -alkadienes, norbornene derivatives, symmetrical internal alkynes) with Et 3 Al in the presence of a Cp 2 ZrCl 2 catalyst were used as precursors. The substitution of aluminum atoms in cyclic organoaluminum compounds for phosphorus atoms takes place using alkyl(aryl)phosphorus (III) dichlorides. The developed one-pot method gives high yields of products under mild conditions.


Introduction
Phosphorus-containing heterocycles, due to their unique properties, are widely used as intermediates, ligands for organometallic chemistry and catalysis, monomers for electronics, and effective drugs for medicine and agriculture [1][2][3][4][5][6]. Therefore, the development of original methods for obtaining hard-to-obtain and previously undescribed five-membered organophosphorus compounds (OPCs) is a highly demanded, relevant task and is of great practical importance.
A number of fairly effective approaches to the synthesis of cyclic OPCs include methods based on the direct conversion of five-membered metallacarbocycles based on transition metals into phosphocarbocycles using phosphorus dihalides. A few examples of the synthesis of phospholenes and phospholes from zirconacyclopentenes [7,8], zirconacyclopentadienes [9][10][11], and titanocyclopentadienes [12] are known in the literature. The direct conversion of aluminacarbocycles into cyclic OPCs has not been practically studied. We assumed that the replacement of the aluminum atom in aluminacarbocycles by a phosphorus atom using organic phosphorus dihalides would allow us to develop promising methods for practical application to obtain a wide range of previously inaccessible and new classes of cyclic and acyclic organophosphorus compounds of a given structure.

Results and Discussion
Presently, we have developed effective one-pot methods for the synthesis of phospholanes and phospholes of various structures, based on the reaction of catalytic cycloalumination of unsaturated compounds (alkenes, alkynes, α,ω-diolefins, norbornenes) through the formation of alumoles (aluminacyclopentadienes) and alumolanes (aluminacyclopentanes). The alumoles and alumolanes obtained in situ were involved in the substitution of aluminum atoms in substituted alumolanes for phosphorus atoms with alkyl(aryl)phosphorus dichlorides.
In 2012, 3-substituted phospholanes were obtained [13,14]. It was found that aluminocyclopentanes 1 obtained by the reaction of alkenes with Et 3 Al in the presence of 5 mol. % Cp 2 ZrCl 2 (20 • C, 6-8 h), reacted in situ with R'PCl 2 (R' = Me, Bu, Ph) in toluene for 30 min, with the replacement of the Al atom by the P atom to form the corresponding phospholanes 2 in yields 79-84% (Scheme 1). Compounds 2 are mixtures of diastereomers 3:2, formed due to the presence of two centers of asymmetry in the molecule at C-3 and P-1. The latter exists due to the high configuration inversion barrier at the phosphorus atom [15].

Results and Discussion
Presently, we have developed effective one-pot methods for the synthesis of phospholanes and phospholes of various structures, based on the reaction of catalytic cycloalumination of unsaturated compounds (alkenes, alkynes, α,ω-diolefins, norbornenes) through the formation of alumoles (aluminacyclopentadienes) and alumolanes (aluminacyclopentanes). The alumoles and alumolanes obtained in situ were involved in the substitution of aluminum atoms in substituted alumolanes for phosphorus atoms with alkyl(aryl)phosphorus dichlorides.
In 2012, 3-substituted phospholanes were obtained [13,14]. It was found that aluminocyclopentanes 1 obtained by the reaction of alkenes with Et3Al in the presence of 5 mol. % Cp2ZrCl2 (20 °С, 6-8 h), reacted in situ with R'PCl2 (R' = Me, Bu, Ph) in toluene for 30 min, with the replacement of the Al atom by the P atom to form the corresponding phospholanes 2 in yields 79-84% (Scheme 1). Compounds 2 are mixtures of diastereomers 3:2, formed due to the presence of two centers of asymmetry in the molecule at С-3 and Р-1. The latter exists due to the high configuration inversion barrier at the phosphorus atom [15]. Phospholanes 2 easily react with H2O2 in chloroform due to the presence of a lone electron pair in phosphorus with quantitative yields of phospholane 1-oxides 3. The reaction of 2 with S8 leads to phospholane-1-sulfides 4 also in quantitative yields (Scheme 2). Scheme 2. Synthesis of 3-substituted phospholane 1-oxides and 1-sulfides.

Scheme 1. Synthesis of 3-substituted phospholanes 2.
Phospholanes 2 easily react with H 2 O 2 in chloroform due to the presence of a lone electron pair in phosphorus with quantitative yields of phospholane 1-oxides 3. The reaction of 2 with S 8 leads to phospholane-1-sulfides 4 also in quantitative yields (Scheme 2).

Results and Discussion
Presently, we have developed effective one-pot methods for the synthesis of phospholanes and phospholes of various structures, based on the reaction of catalytic cycloalumination of unsaturated compounds (alkenes, alkynes, α,ω-diolefins, norbornenes) through the formation of alumoles (aluminacyclopentadienes) and alumolanes (aluminacyclopentanes). The alumoles and alumolanes obtained in situ were involved in the substitution of aluminum atoms in substituted alumolanes for phosphorus atoms with alkyl(aryl)phosphorus dichlorides.
In 2012, 3-substituted phospholanes were obtained [13,14]. It was found that aluminocyclopentanes 1 obtained by the reaction of alkenes with Et3Al in the presence of 5 mol. % Cp2ZrCl2 (20 °С, 6-8 h), reacted in situ with R'PCl2 (R' = Me, Bu, Ph) in toluene for 30 min, with the replacement of the Al atom by the P atom to form the corresponding phospholanes 2 in yields 79-84% (Scheme 1). Compounds 2 are mixtures of diastereomers 3:2, formed due to the presence of two centers of asymmetry in the molecule at С-3 and Р-1. The latter exists due to the high configuration inversion barrier at the phosphorus atom [15]. Phospholanes 2 easily react with H2O2 in chloroform due to the presence of a lone electron pair in phosphorus with quantitative yields of phospholane 1-oxides 3. The reaction of 2 with S8 leads to phospholane-1-sulfides 4 also in quantitative yields (Scheme 2). Scheme 2. Synthesis of 3-substituted phospholane 1-oxides and 1-sulfides.
An efficient one-pot method was developed for the synthesis of polycyclic phospholane oxides 18, 20, 22, and 24 by the in situ reaction of dichlorophosphines with norbornane-annelated aluminacyclopentanes obtained by the catalytic cycloalumination of norbornenes in 81-92% yields (Schemes 7 and 8) [17]. It was found that compounds with a bulkier hydrocarbon framework, such as 19 and 21, predominantly form syn-phenyl-substituted phospholane 3-oxides 20а and 22а; the proportion of anti-isomers in these experiments does not exceed 15% and 10%, respectively. In the case of 23, the formation of a single syn-isomer 24 is observed (Scheme 8). Thus, the ratio of polycyclic syn-and anti-3-phenyl-phospholane 3-oxides depends on the structure of the polycyclic hydrocarbon substituent annelated to aluminacyclopentane. It was found that compounds with a bulkier hydrocarbon framework, such as 19 and 21, predominantly form syn-phenyl-substituted phospholane 3-oxides 20a and 22a; the proportion of anti-isomers in these experiments does not exceed 15% and 10%, respectively. In the case of 23, the formation of a single syn-isomer 24 is observed (Scheme 8). Thus, the ratio of polycyclic synand anti-3-phenyl-phospholane 3-oxides depends on the structure of the polycyclic hydrocarbon substituent annelated to aluminacyclopentane.

Conclusions
Thus, in our research, we have developed efficient one-pot methods for the synthesis of a wide range of previously undescribed and hard to obtain five-membered cyclic organic molecules containing a phosphorus atom. The studied reaction of the replacement of an aluminum atom by a phosphorus atom in five-membered aluminacarbocycles-alumolanes and alumols-is an effective tool for designing cyclic organophosphorus compounds in one preparative step.

Conclusions
Thus, in our research, we have developed efficient one-pot methods for the synthesis of a wide range of previously undescribed and hard to obtain five-membered cyclic organic molecules containing a phosphorus atom. The studied reaction of the replacement of an aluminum atom by a phosphorus atom in five-membered aluminacarbocycles-alumolanes and alumols-is an effective tool for designing cyclic organophosphorus compounds in one preparative step.