Abstract: A series of alkylated phenols (phenol, p- and o-cresol, 4-methoxy- and 3,4-dimethoxyphenol, 5,6,7,8-tetrahydro-
2-naphthol, and ¡8-estradiol) are complexed with pentaammineosmium(II) and treated with a variety of Michael acceptors
(e.g., MVK, methyl acrylate, acrylonitrile, jV-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 acceptors 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*123 carbon of the resulting cyclohexadienone is a methine, the ligand may be rearomatized and removed from
the metal by moderate heating (70-80 °C). When the sp3 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%).