PdII-Catalyzed Oxidative Aldehyde-sp2C-H Functionalization and Cyclization Using NHC with Mild Oxidant DMSO for the Selective Synthesis of Esters, Sugar-Based Analogues, and β-Hydroxy Chromanones: An 18O-Labeling Study.

Abstract

We assume formation of acyl-PdII–N-heterocyclic-carbene (NHC) organometalics for diverse C–O/O–C and C–C/C–O coupling catalysis of direct functionalization and cyclization reactions. We report the first use of dimethyl sulfoxide (DMSO) as an oxidant under an inert atmosphere to O2-sensitive NHC for oxidative transformations. In situ generated imidazolium halides are utilized as a precursor of NHC and as a source of alkyl group for the sp2C–H functionalization of aldehydes to esters under mild conditions. In contrast to the reported NHC-catalyzed esterification strategies, the outstanding substrate scope of this mild catalysis approach is established through synthesis of thermally labile sugar-based chiral esters. Our competition experiments using various unsymmetrical imidazolium halides revealed an ascending rate of migratory aptitude among methyl ≪ allyl < crotyl < cinnamyl < benzyl moiety. DMSO is used as an oxidant for both esterification and cyclization reactions, and the transfer of the DMSO-oxygen to ester is confirmed using an 18O-labeling experiment. The diverse activity using DMSO-oxygen to acyl-PdII–NHC is verified by developing a unique C–C-coupled cyclization with side-chain hydroxylation of olefin to achieve valuable β-hydroxy chromanones.