Copper-Mediated Oxygenation of Aldehydes and Internal Cannizzaro-like Rearrangement of Phenylglyoxal

Article abstract of DOI:10.1021/jo00297a013

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 independent O2-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 interp reted in terms of preferential C-C cleavage of an α-hydroxyperoxide intermediate initially formed from PhCH2CH2CHO.The alternative pathway for this intermediate, dehydration to α-keto aldehyde PhCH2COCHO, is barely competitive, because the independently prepared α-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-MeOH-O2 system.Product comparisons using either an O2 atmosphere or a N2 atmosphere (with varying equivalents of Cu^II) permit a distinction between stoichiometric Cu(II) oxidations and O2-dependent reactions.Mechanism are proposed for the observed transformations.