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.