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.