A Lewis acid-mediated synthesis of P-alkyl-substituted phosphaalkenes

Treating an equimolar mixture of (RP)-P-1(SiMe3)(2) (R-1 = Bu-t, Ad, Mes, Me3Si; Ad = 1-adamantyl, Mes = 2,4,6-trimethylphenyl) and (RC)-C-2(O)R-3 (R-2:R-3 = Bu-t:H, Ph:Ph, Bu-t:Bu-t, Bu-t:Ph, Bu-t:Me) with AlCl3 (1 equiv.) affords the corresponding phosphaalkenes (RP)-P-1=(CRR3)-R-2 as monitored by P-31 NMR spectroscopy. This new method was applied to the multigram synthesis of (RP)-P-1=(CRR3)-R-2 [1a: R-1 = Bu-t, R-2 = Bu-t, R-3 = H; 2a: R-1 = Ad, R-2 = Bu-t, R-3 = H; 3a: R-1 = Mes, R-2 = Bu-t, R-3 = H; 3b: R-1 = Mes, R-2 = R-3 = Ph] in good isolated yields (1a: 80%; 2a: 57%; 3a: 63%; 3b: 76%). Previously unknown 2a has been fully characterized. The reactivity of 1a and 2a with group 13 Lewis acids was performed in an effort to probe their reactivity and provide a means to structurally characterize these P-alkyl phosphaalkenes. The X-ray crystal structures of 1a center dot AlCl3, 1a center dot GaCl3 and 2a center dot GaCl3 reveal that the Bu-t substituents are configured in a trans arrangement and the P = C bond lengths are as expected (avg. 1.64 angstrom).