Ir/Sn dual reagent catalysis for the alkylation of arenes with benzyl alcohols: kinetic evidence for an electrophilic pathway and a guide towards electronic tuning of catalyst efficiency

Abstract

Presented herein is a detailed account of the [Ir(COD)(μ-Cl)]₂/SnCl₄ dual reagent catalyzed alkylation of arene/heteroarene with benzyl alcohols [J. Choudhury, S. Podder, S. Roy, J. Am. Chem. Soc. 127 (2005) 6162]. A high-valent heterobimetallic complex namely [IrI^II(COD)(μ-Cl)(SnCl₃)Cl]₂ (isolated from [Ir(COD)(μ-Cl)]₂ and SnCl₄) could also promote the alkylation. A working model on substrate binding, activation and coupling across Ir/Sn catalyst is proposed considering the hard-soft nature of the two metals (Ir and Sn) and the organic substrates (arene and alcohol). The "tin-alcohol" hard-hard interaction is indicated by the alkylation rate for PhCH₂-Y which varied with the HSAB donor strength of Y in the order OH > OAc > OMe > OC(O)H > Cl. Hammett studies with respect to arene and alcohol indicate an electrophilic mechanism. A small secondary kinetic isotope effect ruled out a C-H activation pathway. Competitive and non-competitive rate studies (isomer distribution and k_T/k_B ratio) showed a similarity with Friedel-Crafts like kinetics. Temperature dependent kinetics showed that the reaction is characterized by a positive enthalpy of activation, and small negative entropy of activation. The stereoelectronic influence of the two metal centers on the catalyst efficiency is also studied. It is shown that the coordinated ligand at the iridium center and the Lewis acidity at the tin center control the efficiency of the Ir/Sn catalysts towards aromatic alkylation.