BEHAVIOR OF 1,1-DIPHENYL-2,5-DIHYDROPHOSPHOLIUM SALTS TOWARD BASES - YLIDE FORMATION OR RING-OPENING

Efficient procedures ave described leading to pure 3, 4-R(1), R(2)-sub stituted 1,1-diphenyl-2 5-dihydrophospholium salts (R(1) = R(2) = CH3, H; R(1) = CH3, R(2) = H). Their behaviors reward bases such as nBuLi and t-BuOK in THF or DMSO have been examined. According to the nature of the substituents R(1) and R(2), the complete monodeprotonation of t hese salts leads either to the corresponding pure five-membered cyclic ylide (and, in some cases, its prototropic isomer) or to a dienylphos phine resulting from a ring opening. The reactivity of the 3,4-dimethy l-disubstituted salt was especially studied. The corresponding monoyli de 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 unde r an open dienylphosphine form giving rise to P-alkylated phosphonium salts. Nevertheless, this monoylide does not undergo further deprotona tion into the corresponding cyclic diylide. Most of the synthetisized derivatives are original. (C) 1996 John Wiley & Sons, Inc.