BF₃-mediated addition of lithium phenylacetylide to an imine: Correlations of structures and reactivities. BF₃·R₃N derivatives as substitutes for BF₃·Et₂O

BF₃-mediated additions of lithium phenylacetylide (PhCCLi) to the N-(n-butyl)imine of cyclohexane carboxaldehyde were investigated. IR spectroscopic investigations reveal dramatic aging effects on the addition rates. ⁶Li, ¹¹B, and ¹³C NMR spectroscopic studies correlate the loss in reactivity with the condensation of PhCCLi and BF₃ and the consequent formation of a complex mixture of PhCCLi-BF₃ adducts. Employing BF₃·R₃N complexes eliminates the aging effects by retarding the formation of borates. Kinetic studies implicate a mechanism in which rate-limiting associative substitution of n-Bu₃N on the BF₃ by the imine is followed by a rapid 1,2-addition of PhCCLi, BF₃·R₃N complexes are potentially useful substitutes for BF₃·Et₂O.