Diphosphonio dihydrophosphetide and 1,2-diphospholide cations

Condensation of a 1,3-diphosphoniopropenide cation at its reactive 1,3 positions with a dichlorophosphine in the presence of triethylamine provides a route to 2,4-diphosphoniodihydrophosphetide cations. An excess of dichlorophosphine in the presence of an additional reducing agent results in a ring expansion and yields 3,5-diphosphoniodihydro-1,2-diphospholide cations. The chlorosubstituted cation derived from PCl3 can be further reduced to the hydrolytically stable 3,5-diphosphonio-1,2-diphospholide cation. It adds halogen to the P = P bond and can easily be regained from the halogen adduct. Structural comparison of the 1,2-diphenyl- and 1,2-dichlorodihydro-1,2-diphospholide cation with the 1,2-diphospholide cation shows three stages of interaction of the C-3 and the P-2 entities of the ring: no conjugation in the first case, hyperconjugative extension of the allylic system to include the phosphorus atoms in the second case and cyclic pi conjugation in the third.