Theoretical and experimental investigation of the basicity of phosphino(silyl)carbenes.
[reaction: see text] The reactivity of phosphino(trimethylsilyl)carbenes 1 with several organic acids has been examined in order to evaluate the pKa values of the conjugate acids. Carbenes 1 react efficiently with C-organic acids such as 1,3-dimesitylimidazolium chloride, phenylacetylene, acetonitrile, and acetyltrimethylsilane, which have pKa's in DMSO in the range 18-31. However, the reaction of the conjugate acids 1H+ with the anion perturbs the determination of the genuine basicity of 1. Theoretical calculations have been performed in order to quantify the basicity of phosphino(trimethylsilyl)carbenes 1 and to compare them with that of N-heterocyclic carbenes 2. The pKa of 1H+ in DMSO has been computed to be in the 23.0-23.4 range, so that 1 is not strong enough as a base to spontaneously deprotonate organic acids such as phenylacetylene, acetonitrile, or acetyltrimethylsilane. However, its conjugate acid 1H+ is a strong electrophile and easily reacts with the nucleophilic conjugate bases of these acids leading to the formation of the corresponding phosphorus ylides.
Laboratoire d'Hétérochimie Fondamentale et Appliquée, UMR CNRS 5069, Université Paul Sabatier, 118, route de Narbonne, F-31062 Toulouse Cedex 04, France.
David
D
Martin
Ona
O
Illa
Antoine
A
Baceiredo
Guy
G
Bertrand
Rosa M
RM
Ortuño
Vicenç
V
Branchadell