NOVEL MICHAEL ADDITIONS TO PHENOLS PROMOTED BY OSMIUM(II) - CONVENIENT STEREOSELECTIVE SYNTHESES OF 2,4-CYCLOHEXADIENONE AND 2,5-CYCLOHEXADIENONE

A series of alkylated phenols (phenol,p- and o-cresol, 4-methoxy- and 3,4-dimethoxyphenol, 5,6,7,8-tetrahydro-2-naphthol, and beta-estradiol) are complexed with pentaammineosmium(II) and treated with a variety of Michael accepters (e.g., MVK, methyl acrylate, acrylonitrile, N-methylmaleimide, and 3-butyn-2-one). Under both acidic and basic reaction conditions, a single conjugate addition takes place either ortho or para to the hydroxy group to generate stable 2,4- or 2,5-cyclohexadien-1-one complexes, respectively. For phenol the conjugate addition occurs exclusively at C(4) for all Michael accepters investigated. For cases where C(4) is substituted, the regioselectivity depends on reaction conditions and the nature of the electrophile, but examples of selective addition at both C(4) and C(6) are reported. When the sp(3) carbon of the resulting cyclohexadienone is a methine, the ligand may be rearomatized and removed from the metal by moderate heating (70-80 degrees C). When the sp(3) carbon is quaternary, stable dienone products may be obtained by oxidative removal of the metal with Ce(IV) or DDQ, and a number of substituted arene and dienone products are synthesized by this methodology in good overall yield (54-83%).