Nucleofugality of diphenylphosphinate and kinetic stabilities of secondary and tertiary benzylic diphenylphosphinates in aqueous solvents

Abstract

First‐order rate constants for solvolyses of variously substituted benzhydryl diphenylphosphinates in four aqueous solvents have been measured conductometrically in order to investigate leaving group ability of diphenylphosphinate (DPP), that is, to determine corresponding nucleofuge‐specific parameters (Nf and sf) according to the correlation equation log k (25°C) = sf(Ef + Nf). DPP is the first of phosphorus leaving groups included on the Nf nucleofugality scale which presently covers a reactivity range of 15 orders of magnitude. Application of log k (25°C) versus Ef plots has unequivocally shown that benzhydryl diphenylphosphinates solvolyze in hydroxylic solvents by the SN1 mechanism. As the abovementioned equation and corresponding reactivity parameters can also be employed for predicting heterolytic reactivity of substrates in various solvents, the range of kinetic stability of secondary and tertiary benzylic diphenylphosphinates toward heterolysis has been examined in terms of reaction half‐lives. Further, in order to investigate the origin of stability of the DPP anion and, consequently, the ability of the DPP leaving group, quantum mechanical calculations and the Natural Bond Orbital (NBO) analysis were performed at the M06‐2X/6‐311+G(3df,3pd) level of theory using the SMD solvation model. The NBO analysis has revealed that negative hyperconjugation plays a dominant role in delocalizing the anionic charge in disubstituted phosphinate leaving groups.