Behavior of 1,1-diphenyl-2,5-dihydrophospholium salts toward bases: Ylide formation or ring opening

Efficient procedures are described leading to pure 3,4-R¹, R²-substituted 1,1-diphenyl-2,5-dihydrophospholium salts (R¹ = R² = CH₃, H; R¹ = CH₃, R² = H). Their behaviors toward bases such as nBuLi and tBuOK in THF or DMSO have been examined. According to the nature of the substituents R¹ and R², the complete monodeprotonation of these salts leads either to the corresponding pure five-membered cyclic ylide (and, in some cases, its prototropic isomer) or to a dienylphosphine resulting from a ring opening. The reactivity of the 3,4-dimethyl-disubstituted salt was especially studied. The corresponding monoylide functions as a good Wittig reagent, allowing stereoselective access to interesting alkadienylphosphine oxides and subsequently to trienes. However, in the presence of alkylating electrophiles, it reacts under an open dienylphosphine form giving rise to P-alkylated phosphonium salts. Nevertheless, this monoylide does not undergo further deprotonation into the corresponding cyclic diylide. Most of the synthetisized derivatives are original. © 1996 John Wiley & Sons, Inc.