Lithiated anions derived from (alkenyl)pentamethyl phosphoric triamides: an accurate study of the carbanion formation mechanism

The mechanism of the formation of lithiated carbanions derived from (alkenyl) pentamethyl phosphoric triamides has been elucidated. Whereas a few initial ambident allylphosphoramide anion was formed with n-BuLi, both reversible α-reprotonation and not reversible γ-reprotonation simultaneously occurred as a result of the reaction between the ambident carbanion formed and the starting enephosphoramide. A such autocatalytic process led partially to the transposed allylphosphoramide isomer. In the case of α-phenyl substituent the transposed phosphoramide was not a difficulty because it was further γ-deprotonated in situ with the n-BuLi still present, provided finally the expected ambident anion. With α-methyl and α-propyl substituent the transposed enephosphoramide formed in the autocatalytic process was not γ-deprotonated and consequently was prejudicial to the preparation of the target ambident carbanion. In these last cases, adapted experimental conditions avoided the autocatalytic process and allowed the preparation of the corresponding anions.