Gauging the Intramolecular CAr-I Activation from Two Parallel Sets of MCl (M = Cu, Ag, Au) Adducts of an Iodobenzyl CAAC and Its NHC Variant.

Abstract

A cyclic(alkyl)(amino)carbene (CAAC; 1) with an iodobenzyl side arm and an analogous N-heterocyclic carbene (NHC; 2) are isolated by deprotonating their corresponding pyrrolinium and imidazolium salt precursors [1H]Br and [2H]Br, respectively. Two parallel series of MCl (M = Cu, Ag, Au) complexes of 1 and 2 are then considered for the potential intramolecular CAr-I oxidative addition (OA) at the metal center. CuCl not only carries out the OA with both the carbenes but also further undergoes a CAr-Ccarbene coupling by reductive elimination (RE) to give the fused tricyclic pyrrolinium (3) and imidazolium (9) cations, respectively, with [ICuCl]- as the counteranion. Though the Cu(III) intermediate is not detected in either case, a starting 2-CuCl adduct (6) is isolated by its insolubility in THF. In contrast, AgCl and AuCl do not proceed beyond the carbene adducts (1-AgCl (4); 1-AuCl (5); 2-AgCl (7); and 2-AuCl (8)). The experimental observations agree well with DFT analyses, which also show the transformation to be more facile in the NHC case than the CAAC due to a higher conformational flexibility offered by the former. Lastly, an intramolecular CAr-Br bond also follows a similar pathway as the CAr-I with both the carbene types and CuCl.