Gauging theIntramolecular <i>C</i>_Ar-I Activation from TwoParallel Sets of MCl (M = Cu, Ag,Au) Adducts of an Iodobenzyl CAAC and Its NHC Variant

A cyclic(alkyl)(amino)carbene (CAAC; <b>1</b>) with an iodobenzyl side arm and an analogous N-heterocyclic carbene (NHC; <b>2</b>) are isolated by deprotonating their corresponding pyrrolinium and imidazolium salt precursors <b>[1</b><i>H</i><b>]Br</b> and <b>[2</b><i>H</i><b>]Br</b>, respectively. Two parallel series of MCl (M = Cu, Ag, Au) complexes of <b>1</b> and <b>2</b> are then considered for the potential intramolecular C_Ar–I oxidative addition (OA) at the metal center. CuCl not only carries out the OA with both the carbenes but also further undergoes a <i>C</i>_Ar–<i>C</i>_carbene coupling by reductive elimination (RE) to give the fused tricyclic pyrrolinium (<b>3</b>) and imidazolium (<b>9</b>) cations, respectively, with [ICuCl]⁻ as the counteranion. Though the Cu^(III) intermediate is not detected in either case, a starting <b>2</b>-CuCl adduct (<b>6</b>) is isolated by its insolubility in THF. In contrast, AgCl and AuCl do not proceed beyond the carbene adducts (<b>1</b>-AgCl (<b>4</b>); <b>1</b>-AuCl (<b>5</b>); <b>2</b>-AgCl (<b>7</b>); and <b>2</b>-AuCl (<b>8</b>)). 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 C_Ar–Br bond also follows a similar pathway as the C_Ar–I with both the carbene types and CuCl.