Alkylidenetriphenylphosphoranes (I) tend in general towards cis olefination. The proportion of cis olefin in the isomer mixt. is highest when the ylide used is salt-free. Li salts favor the formation of the trans isomers. Salt-free I and BzH in 4:1 C6H6-petr. ether at 0 Deg gave a 96:4 mixt. of cis- and trans-PhCH:CHEt; the isomer ratio became 90:10, 86:14, 83:17, and 52:48 when the reactions were carried out in the presence of equiv. amts. LiCl, LiBr, LiI, and tetraphenylboronate, resp. NaNH2 (2 g.) in 200 cc. dry NH3 treated with stirring with 0.050 mol appropriate alkyltriphenylphosphonium bromide, refluxed 1 h., and evapd., the residue boiled 1 h. with 200 cc. dry C6H6, and the cooled C6H6 soln. added with stirring at 0 Deg to 0.020 mol suitable aldehyde and 50 cc. 1:1 C6H6-petr. ether, and stirred 1 h. gave the corresponding cis,trans-RCH:CHR' (R, R', % yield of cis,trans-mixt., and % cis isomer content of the mixt. given): Me, Am, 66, 91; Me, trans-PhCH:CH, 77, 87; Me, p-MeOC6H4, 78, 90; Me, p-MeC6H4, 88, 89; Me, Ph, 98, 87; Me, p-ClC6H4, 77, 82; Et, Et, 70, 97; Et, Pr, 49, 95; Et, p-MeOC6H4, 95, 92; Et, p-MeC6H4, 59, 95; Et, Ph, 88, 96; Et, p-ClC6H4, 54, >99; Pr, Pr, 78, 95; Pr, p-MeOC6H4, 80, >90; Pr, p-MeC6H4, 71, 92; Pr, Ph, 66, 94; Pr, p-ClC6H4, 64, >99; Ph, Et, 61, 25; Ph, p-MeOC6H4, 76, 19; Ph, p-MeC6H4, 88, 36; Ph, Ph, 100, 44; Ph, p-ClC6H4, 72, <30. [on SciFinder (R)]