NOVEL, CO2-PROMOTED SYNTHESIS OF ANHYDROUS ALKYLAMMONIUM TETRAPHENYLBORATES - A STUDY ON THEIR REACTIVITY AS INTRAMOLECULAR AND INTERMOLECULAR 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' = -CH2CH2OCH2CH2- (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, (R(3)NH)BPh(4), 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 (1a), cyclohexyl (2a), or benzyl (3a)]. The behaviour of 1-6 in several solvents has been investigated. In CH2Cl2 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 imimium-BPh(4) salts, (RR'N = CMe(2))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 coordinates the cation and acts as a proton-transfer inhibitor.