Sustainable Pd(OAc)(2)/Hydroquinone Cocatalyst System for Cis-Selective Dibenzoyloxylation of 1,3-Cyclohexadiene

摘要

The 1,4-diacyloxylation of 1,3-cyclohexadiene (CHD) affords valuable stereochemically defined scaffolds for natural product and pharmaceutical synthesis. Existing cis-selective diacyloxylation protocols require superstoichiometric quantities of benzoquinone (BQ) or MnO2, which limit process sustainability and large-scale application. In this report, reaction development and mechanistic studies are described that overcome these limitations by pairing catalytic BQ with tert-butyl hydroperoxide as the stoichiometric oxidant. Catalytic quantities of bromide enable a switch from trans to cis diastereoselectivity. A catalyst with a 1:2 Pd:Br ratio supports high cis selectivity while retaining good rate and product yield. Further studies enable replacement of BQ with hydroquinone (HQ) as a source of cocatalyst, avoiding the handling of volatile and toxic BQ in large-scale applications.