Me. Kopach et Wd. Harman, NOVEL MICHAEL ADDITIONS TO PHENOLS PROMOTED BY OSMIUM(II) - CONVENIENT STEREOSELECTIVE SYNTHESES OF 2,4-CYCLOHEXADIENONE AND 2,5-CYCLOHEXADIENONE, Journal of the American Chemical Society, 116(15), 1994, pp. 6581-6592
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-met
hylmaleimide, and 3-butyn-2-one). Under both acidic and basic reaction
conditions, a single conjugate addition takes place either ortho or p
ara to the hydroxy group to generate stable 2,4- or 2,5-cyclohexadien-
1-one complexes, respectively. For phenol the conjugate addition occur
s exclusively at C(4) for all Michael accepters investigated. For case
s where C(4) is substituted, the regioselectivity depends on reaction
conditions and the nature of the electrophile, but examples of selecti
ve 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 rear
omatized 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%).