Optically Active Transition-Metal Complexes. 9. A General Stereoselective Route to α-Chiral (R)-Tricarbonyl(η⁶-ethylbenzene)chromium Complexes. Novel Organometallic Phosphine Catalysts for the Asymmetric Hydrovinylation Reaction

Treatment of (R)-[{α-(dimethylamino)ethyl}-η⁶-benzene]Cr(CO)₃ with esters of chloroformic acid leads to stereoselective substitution of the dimethylamino group for a chloro substituent. The reaction can be extended to systems in which the chromium arene complex, after metalation, is diastereoselectively substituted in the ortho position with carbon and silicon electrophiles to generate planar chirality. The chloro group in turn can be replaced stereoselectively for various phosphorus, nitrogen, and oxygen nucleophiles. Both substitution reactions in the benzylic position proceed via retention of configuration. The addition of cyanide is not stereospecific. The phosphine derivatives are efficient catalysts for the enantioselective hydrovinylation of styrene to 3-phenyl-1-butene. X-ray crystal structures establish the absolute configuration of (R)-[(α-chloroethyl)η⁶-benzene]Cr(CO)₃, (R)-[{α-(diphenylphosphanyl)ethyl}-η⁶-benzene]Cr(CO)₃, and (pS,S)-[1-(α-cyanoethyl)-2-methyl-η⁶-benzene]Cr(CO)₃.