Stereoselective tandem 1,4-addition reactions for benzenes: a comparison of Os(II), Re(I), and W(0) systems.

The arene ligand in the complex TpRe(CO)(MeIm)(eta2-benzene) (Tp = hydridotris(pyrazolyl)borate; MeIm = N-methylimidazole) undergoes tandem electrophile/nucleophile 1,4-addition reactions. Subsequent oxidative demetalation affords cis-3,6-disubstituted 1,4-cyclohexadienes (46-84%). Common organic electrophiles such as acetals and Michael acceptors were successfully added to the bound benzene to generate eta3-benzenium complexes, which then were treated with a silyl ketene acetal, silyl vinyl ether, phenyllithium, or malonate ester to afford 1,4-dialkylated dihydrobenzene complexes. The d6 transition metal analogues TpW(NO)(PMe3)(eta2-benzene) and [Os(NH3)5(eta2-benzene)]2+ also undergo 1,4-dialkylation reactions, and the relative ability of all three metals to activate arenes is compared. Copyright 2004 American Chemical Society