The stereochemistry of organometallic compounds. Part VII. Acetylation of some tricarbonyl(alkylbenzene)chromium compounds

Abstract

The isomer distributions (o : m : p) have been determined for the Friedel–Crafts acetylation of some tricarbonyl-(alkylbenzene)chromium compounds. The ratios are very different for those determined for the free ligands under similar conditions, e.g. the normally strong para-directing t-butyl group is meta-directing in tricarbonyl-(t-butyl-benzene)chromium. The acetylation rates, relative to tricarbonyl(benzene)chromium, have been established and show that the alkyl and methoxy-groups do not appear to activate the para-position towards electrophiles in the tricarbonylchromium compounds. The unusual isomer ratios in the complexes are interpreted in terms of the ring metal bonding in the transition states and the effects of the preferred conformations of the tricarbonylchromium moieties.

The conformational influence of the tricarbonylchromium group has been demonstrated by the stereoselectivity shown in the acetylation of tricarbonyl-(trans-1,3-dimethylindane)chromium.

In the n.m.r. spectrum of tricarbonyl-(o-ethylacetophenone)chromium the methylene protons are anisochronous by an unusually large value.