Synthesis of a Brominated Biphenyl Chromium Tricarbonyl Complex

Hydroxycarbene complexes have been proposed as intermediates in industrial processes such as the Fischer-Tropsch process and hydroformylation. As part of the overall synthesis of an aryltethered version of a chromium hydroxycarbene complex we have examined three synthetic routes toward the brominated biphenyl chromium tricarbonyl compound (η-2’-bromobiphenyl)Cr(CO)3, which is an immediate precursor of our target hydroxycarbene. A direct nucleophilic attack of 1,2-dibromobenzene by phenyllithium-chromium-tricarbonyl failed, and a successful Suzuki coupling route was abandoned due to difficulties in purification of the product. Surprisingly, simply refluxing 2-bromobiphenyl with chromium hexacarbonyl led to isolation of the desired intermediate in high purity, where only the non-brominated biphenyl ring had coordinated to the metal with no evidence of coordination of the brominated ring. This is a crucial intermediate, since metal-halogen exchange would convert it to a “locked” anion that, upon protonation, would provide the desired aryl-tethered hydroxycarbene complex.