Carboxylation of Acetophenone with Zinc(II) Alkoxides/CO2 Systems: A Mechanistic Study

Abstract

The crystal structures of the homoleptic dimer zinc(II) 2,6-di-tert-butylphenoxide (1b) and of the corresponding monomer 1b/(DMSO)2 are presented. The structural motif of 1b is a Zn−O−Zn−O four-membered ring. The bulky phenoxide ligands led to a coordination number of only three for the zinc atom. Monomer 1b/(DMSO)2 crystallized as a four-coordinated monomer with a highly distorted tetrahedral geometry around the zinc center. 13C NMR investigations reveal details for the reaction of CO2 with a series of alkoxide zinc(II)/solvent systems [2,6-dimethylphenoxide zinc(II) (1a), zinc(II) 2,6-di-tert-butylphenoxide (1b), zinc(II) 2,6-diphenylphenoxide (1c), zinc(II) 4-tert-butylphenoxide (1d), and zinc(II) 2-pyridylcarbinolato (2)]. The interpretation of the 13C NMR signals agree with ab initio calculations of a model zinc(II) phenoxide/CO2 system. Experimentally, the carboxylation of acetophenone as a representative for CH-acidic substrates was carried out with specific zinc(II) phenoxide/CO2 systems. The reactions resulted in yields of ca. 40% of benzoyl acetic acid. The role of the soluble zinc(II) phenylcarbonate intermediate is discussed with respect to the subsequent CO2 transfer reaction.