Novel, CO2-promoted synthesis of anhydrous alkylammonium tetraphenylborates: a study on their reactivity as intra- and inter-molecular proton-transfer agents

Anhydrous alkylammonium tetraphenylborates (RR'R"NH)BPh 4 [R' = R"= H, R = allyl (1), cyclohexyl (2), or benzyl (3); R" = H, R = R' = ethyl (4); R" = H, RR' = -CH2CH2OCH2CH 2- (5); R = R' = R" = Bu (6)] have been prepared with high yield in mild conditions from the corresponding amines and NaBPh 4 in the presence of carbon dioxide. The synthesis of trialkylammonium salts, (R3NH)BPh4, also requires a suitable reagent that can act as a proton source. The method has been extended to the direct synthesis of [RNH 3. (18-crown-6)]BPh 4 salts [R = allyl (la), cyclohexyl (2a), or benzyl (3a)]. The behaviour of 1-6 in several solvents has been investigated. In CHzC1 ~ or THF, 1-6 salts can undergo intramolecular protolytic cleavage of a B-C bond to form benzene and RR'R"N-BPh 3 adducts. In acetone, both mono- and di-alkylammonium tetraphenylborates easily convert in high yield into the corresponding iminium-BPh 4 salts, (RR'N = CMe2)BPh 4 (R = alkyl; R'= H, or alkyl) through intermolecular transfer of a proton to a solvent molecule. The reactivity of alkylammonium tetraphenylborates can be modified markedly by a complexing agent that firmly coordinatcs the cation and acts as a proton-transfer inhibitor