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

Abstract

Treating an equimolar mixture of R¹P(SiMe₃)₂ (R¹ = ^tBu, Ad, Mes, Me₃Si; Ad = 1-adamantyl, Mes = 2,4,6-trimethylphenyl) and R²C(O)R³ (R²:R³ = ^tBu:H, Ph:Ph, ^tBu:^tBu, ^tBu:Ph, ^tBu:Me) with AlCl₃ (1 equiv.) affords the corresponding phosphaalkenes R¹PCR²R³ as monitored by ³¹P NMR spectroscopy. This new method was applied to the multigram synthesis of R¹PCR²R³ [1a: R¹ = ^tBu, R² = ^tBu, R³ = H; 2a: R¹ = Ad, R² = ^tBu, R³ = H; 3a: R¹ = Mes, R² = ^tBu, R³ = H; 3b: R¹ = Mes, R² = R³ = 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·AlCl₃, 1a·GaCl₃ and 2a·GaCl₃ reveal that the ^tBu substituents are configured in a trans arrangement and the PC bond lengths are as expected (avg. 1.64 Å).