Deprotonated phosphorus betaines react with D, alkyl, or halogen donors to form a-substituted derivs. which decomp. either spontaneously or after \"activation\" to give Ph3PO and olefins with the substitution occurring on one of the olefinic carbons. This represents a versatile preparative scheme for new compounds. The reaction is stereoselective, giving mainly the trans isomer except when the original phosphonium salt contains a Me group, e.g. MePh3PX. A suspension of 30 millimoles PrPh3PBr in 60 ml tetrahydrofuran was stirred under N 30 min at -20 Deg with 30 millimoles 0.9M ethereal PhLi contg. LiBr, 30 millimoles BzH added at -78 Deg, 30 millimoles PhLi added to decolorize the soln., the temp. rose to -30 Deg, 50 millimoles MeI was added, after rapid stirring the temp. rose to 0 Deg, and 45 millimoles KOCMe3 was added. Removal of the solvent gave 55% mixt. of cis- and trans-b-ethylstyren e, (cis-trans = 3:97), b. 91-3 Deg. In a similar manner the following compds. were prepd. cis- and trans-styre ne-w-d, Me2C:CHPh, trans-EtMeC:CHPh, trans-4-fluoro-3-heptene, cis-MeFC:CHPh, 4-fluoro-1-phenyl-1,3-butadiene, 3-tetrahy-dropyranyloxy-17-[3-fluoro-cis-2-pentene-2-yl]androst-5-ene, cis-MeClC:CHPh, and cis-MeBrC:CHPh. [on SciFinder (R)]