Chromium mediated arene transformations : cyclohexenones, fused ring systems and cyclopropanations

Abstract

Dearomatization of [Cr(arene)(CO)3] complexes by sequential regio-selective and diastereo-selective addition of a nucleophile followed by an electrophile gave substituted cyclohexenones. The presence of protons ortho to the methoxy group in [Cr(anisole)(CO)3] is shown to be potential source of problem. Therefore we sought to introduce a synthetic equivalent of the proton (trimethyl silyl) at the two ortho positions to prevent ortho-metalation. The use of [Cr(CO)3(di-o-TMS-anisole)] complex as starting complex allow to add different nucleophiles and electrophiles followed by hydrolysis gave α,β-unsaturated disubstituted cyclohexenones with improved yield. Trans-cyclohexenones with unsaturated side chains, in the dearomatization of [Cr(CO)3(di-o-TMS-anisole)], are used in ring closing metathesis (RCM) reactions. The obtained cyclohexenones in combination with RCM gave rapid access to fused and bridged carbocycles. Finally, an attempt has been made to form nitrogen ylides at the benzylic position starting from [Cr(benzyl chloride)(CO)3]. Trapping of this ylide with t-butyl acrylates affords the corresponding cyclopropane.