Phosphaallyl cations

The only method reported so far, for the preparation of 3,5-unsubstituted [1,2,4] diazaphopholes, is the condensation of 2-phosphaallyl cation with hydrazines. This route also provides a good method for obtaining 3,5-homo substituted [ 1,2,4]diaza-phopholes [39,40], New [l,2,4]diazaphosphole representatives have been prepared by this route for extensive X-ray structure investigations [41],... 

Phosphaallyl cation was formed from 1 and tetramethylchloroformamidinium chloride25. When 1 was treated with sterically hindered (2,4,6-tri-f-butylphenyl)phos-phonous dichloride (Mes PCl2), signals due to a symmetrical Mes P=PMes and an unsymmetrical diphosphene (probably Mes P=PSiMe3) were observed by 31P NMR spectroscopy26. 

With tetramethylchloroformamidinum chloride, the reaction with tris(trimethyl-silyl)phosphine (Me3Si)3P provides an elegant route to the air-stable 2-phosphaallyl cation (equation 19)27. 

Tris(TMS)phosphine can undergo a related reaction with isocyanates to furnish carbamoylphosphines and their imidate tautomers in a ratio of 9 1. Such compounds are used in the synthesis of 1,2,4-azadiphospholes and 1,3-azaphos-phinines (eq 11). Reaction with imidoyl chlorides can give 2-phosphaallylic cations,which can be condensed with hydroxylamine to give 1,2,4-oxazaphospholes and 1,2,4-diazaphospholes. " Selected examples are given in eq 12. 

Two separate ab initio studies have considered many aspects of the singlet and triplet [C2,H4,P] potential energy surfaces . The stabilities of five isomers at the MP4(SDTQ)/6-31+G(d,p)//RHF/6-31 +G(d,p) level of theory (including zero-point energy corrections) are shown below (isomers 10). The phosphiranyl cation 5 is more stable than the phosphaallyl cation 6, with a barrier to disrotatory ring opening of 5 to 6 of 11.2 kcal moT (ref. 59a). 

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