In the presence of phenylsilane and 5 mol % cobalt(II) bis(2,2,6,6-tetramethylheptane-3,5-dionate),aryl-substituted monoenone monoaldehydes and bis(enones) undergo reductive cyclization to afford
syn-aldol and
anti-Michael products, respectively. For both aldol and Michael cycloreductions, five- and six-membered ring formation occurs in good yield with high levels of diastereoselectivity. Cycloreduction ofmonoenone monoaldehyde
1a in the presence of d
3-phenylsilane reveals incorporation of a single deuteriumat the enone
-position as an equimolar mixture of epimers, inferring rapid isomerization of the kineticallyformed cobalt enolate prior to cyclization. The deuterated product was characterized by single-crystal neutrondiffraction analysis. For bis(enone) substrates, modulation of the silane source enables partitioning of thecompetitive Michael cycloreduction and [2 + 2] cycloaddition manifolds. A study of
para-substitutedacetophenone-derived bis(enones) reveals that substrate electronic features also direct partitioning ofcycloreduction and cycloaddition manifolds. Further mechanistic insight is obtained through examinationof the effects of enone geometry on product stereochemistry and electrochemical studies involving cathodicreduction of bis(enone) substrates. The collective experiments reveal competitive enone reduction pathways.Enone hydrometalation produces metallo-enolates en route to aldol and Michael cycloreduction products,that is, products derived from coupling at the
-position of the enone. Electron-transfer-mediated enonereduction produces metallo-oxy-
-allyls en route to [2 + 2] cycloadducts and, under Ni catalysis, homoaldolcycloreduction products, that is, products derived from coupling at the
-position of the enone. Theconvergent outcome of the metal-catalyzed and electrochemically induced transformations suggests theproposed oxy-
-allyl intermediates embody character consistent with the mesomeric metal-complexed anionradicals.