Isolation and reactivity of sodium benzyl cations.

Abstract

Carbanions are an important class of reactive species used as nucleophiles in C-C bond-forming reactions. Such reactive carbanions are generated by deprotonating relatively acidic C-H bonds using strong bases like sodium/potassium hydroxides or amides. Lochmann-Schlosser base, consisting of a mixture of alkyl lithium reagents and heavier alkali metal alkoxides, is widely used for the metalation of organic compounds via the deprotonation of weakly acidic C-H bond. The aggregation of alkoxides makes it challenging to understand the course of the reaction, demanding the consideration of molecular compounds as alternatives to sodium/potassium alkoxides. Here, we demonstrate the development of new organosodium reagents using molecular compounds as alternatives to oligomeric sodium/potassium alkoxides. Sodium tris(3,5-dimethylpyrazolyl)borate, along with mesityllithium in the presence of tris[2-(dimethylamino)ethyl]amine (Me₆TREN) deprotonates benzylic C-H bonds. The resulting carbanions generated in the reaction mixtures are trapped as [(Me₆TRENNa)₂CH₂C₆H₃(3,5-CH₃)]⁺, [(Me₆TRENNa)₂CH₂Ph]⁺ and [(Me₆TRENNa)₂CH₂C₆H₄(3-CH₃)]⁺, which represent distinct examples of structurally characterised sodium benzyl cations. Preliminary reactivity studies on [(Me₆TRENNa)₂CH₂Ph]⁺ indicate that it functions as an efficient alkylating reagent. Further, [(Me₆TRENNa)₂CH₂Ph]⁺ is used to investigate the catalytic reduction of olefins in the presence of PhSiH₃. Computational investigations on the catalytic hydrosilylative reduction infer that the reaction is mediated by a hypercoordinate hydridosilyl anion stabilised across two sodium centres. These investigations lead to new prospects in organoalkali metal chemistry.