COPPER-MEDIATED OXYGENATION OF ALDEHYDES AND INTERNAL CANNIZZARO-LIKE REARRANGEMENT OF PHENYLGLYOXAL

Under the influence of Cu(II) in MeOH containing py and Et3N, PhCH2CHO undergoes competitive O2-dependent conversions to PhCHO and phenylglyoxal. The latter, as the MeOH hemiacetal, undergoes a Cu-(II)-catalyzed rearrangement to PhCHOHCOOMe and a Cu(II) oxidation to PhCOCOOMe, and there appears to be an independent 02-mediated production of PhCOCOOH. Phenylglyoxal also undergoes oxidative cleavage to PhCOOH, but does not give rise to PhCHO. The homologous aldehyde PhCH2CH2CHO is converted mainly via PhCH2CHO to a product mixture derived from the latter. This result is interpreted in terms of preferential C-C cleavage of an a-hydroxyperoxide intermediate initially formed from PhCH2CH2CHO. The alternative pathway for this intermediate, dehydration to a-keto aldehyde PhCH2COCHO, is barely competitive, because the independently prepared a-keto aldehyde gives a distinct set of products under the reaction conditions. The preference for cleavage over dehydration explains the previously published finding of a stepwise degradation of long-chain aldehydes to formate units by the Cu(II)-py-Et3N-Me0H-02 system. Product comparisons using either an 02 atmosphere or a N2 atmosphere (with varying equivalents of Cu11) permit a distinction between stoichiometric Cu(II) oxidations and 02-dependent reactions. Mechanisms are proposed for the observed transformations. © 1990, American Chemical Society. All rights reserved.