Copper-Catalyzed Oxidative Dehydrogenative Carboxylation of Unactivated Alkanes to Allylic Esters via Alkenes
We report copper-catalyzed oxidative dehydrogenative carboxylation (ODC) of unactivated alkanes with various substituted benzoic acids to produce the corresponding allylic esters. Spectroscopic studies (EPR, UV–vis) revealed that the resting state of the catalyst is [(BPI)Cu(O2CPh)] (1-O2CPh), formed from [(BPI)Cu(PPh3)2], oxidant, and benzoic acid. Catalytic and stoichiometric reactions of 1-O2CPh with alkyl radicals and radical probes imply that C–H bond cleavage occurs by a tert-butoxy radical. In addition, the deuterium kinetic isotope effect from reactions of cyclohexane and d12-cyclohexane in separate vessels showed that the turnover-limiting step for the ODC of cyclohexane is C–H bond cleavage. To understand the origin of the difference in products formed from copper-catalyzed amidation and copper-catalyzed ODC, reactions of an alkyl radical with a series of copper–carboxylate, copper–amidate, and copper–imidate complexes were performed. The results of competition experiments revealed that the relative rate of reaction of alkyl radicals with the copper complexes follows the trend Cu(II)–amidate > Cu(II)–imidate > Cu(II)–benzoate. Consistent with this trend, Cu(II)–amidates and Cu(II)–benzoates containing more electron-rich aryl groups on the benzamidate and benzoate react faster with the alkyl radical than do those with more electron-poor aryl groups on these ligands to produce the corresponding products. These data on the ODC of cyclohexane led to preliminary investigation of copper-catalyzed oxidative dehydrogenative amination of cyclohexane to generate a mixture of N-alkyl and N-allylic products.
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