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Factors That Control C-C Cleavage versus C-H Bond Hydroxylation in Copper-Catalyzed Oxidations of Ketones with O2
Althea Tsang S.K., , Franziska Schoenebeck
Published in
Volume: 138
Issue: 2
Pages: 518 - 526
The Cu-catalyzed oxidation of ketones with O2 has recently been extensively utilized to cleave the $\alpha$-C-C bond. This report examines the selective aerobic hydroxylation of tertiary $\alpha$-C-H bonds in ketones without C-C cleavage. We set out to understand the underlying mechanisms of these two possible reactivity modes. Using experimental, in situ IR spectroscopic and computational studies, we investigated several mechanisms. Our data suggest that both C-C cleavage and C-H hydroxylation pathways proceed via a common key intermediate, i.e., an $\alpha$-peroxo ketone. The fate of this peroxide dictates the ultimate product selectivity. Specifically, we uncovered the role of hppH [=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine] to act not only as a base in the transformation but also as a reductant of the peroxide to the corresponding $\alpha$-hydroxy ketone. This reduction may also be accomplished through exogenous phosphine additives, therefore allowing the tuning of reduction efficiency toward higher driving forces, if required (e.g., for more-activated substrates). The likely competitive pathway is the cleavage of peroxide to the $\alpha$-oxy radical (likely catalyzed by Cu), which is computationally predicted to spontaneously trigger C-C bond cleavage. Increasing the susceptibility of this deperoxidation step via (i) the removal of reductant (use of different base, e.g., DBU) or the modulation of (ii) the substitution pattern toward greater activation (substrate control) and (iii) the nature of Cu catalyst (counterion and solvent dependence) will favor the C-C cleavage product.
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