Regioselective Double Oxidative Addition of Bis-NHC Precursors

Abstract

The 4,4’-dimethylenebiphenyl linked symmetric bis-(2-chlorobenzimidazole) 1 and the unsymmetric 2-chlorobenzimidazolium/2-chlorobenzimidazole 2(BF4) have been reacted with one equivalent [Pd(PPh3)4] in oxidative addition reactions. For the symmetric bis-NHC precursor 1, double metallation of both 2-chlorobenzimidazole sites to give the dinuclear bis-pNHC complex [3](BF4)2 was observed. The unsymmetric compound 2(BF4) was only metallated at the 2-chlorobenz-imidazolium site with formation of the mononuclear NHC complex [4](BF4). The remaining 2-chlorobenzimidazole moiety was subsequently metallated in a second oxidative addition reaction with [M(PPh3)4] (M=Pd, Pt) to give the homobimetallic NHC/pNHC complex [5](BF4)2 (M=Pd) and the heterobimetallic complex [6](BF4)2 demonstrating the unique site-selective metallation of the bis-NHC precursor 2(BF4) by two consecutive oxidative additions.