Enantioselective α‐Etherification of Branched Aldehydes via an Oxidative Umpolung Strategy

Saturated carbonyl compounds are, via their enolate analogues, inherently nucleophilic at the α‐position. In the presence of a benzoquinone oxidant, the polarity of the α‐position of racemic α‐branched aldehydes is inverted, allowing for an enantioselective etherification using readily available oxygen‐based nucleophiles and an amino acid‐derived primary amine catalyst. A survey of benzoquinone oxidants identified p‐fluoranil and DDQ as suitable reaction partners. p‐Fluoranil enables the preparation of α‐aryloxylated aldehydes using phenol nucleophiles in up to 91 % ee, following either a one‐step or a two‐step, one‐pot protocol. DDQ allows for a more general etherification protocol in combination with a broader range of alcohol nucleophiles with enantioselectivities up to 95 % ee. Control experiments and isolation of a key quinol intermediate supports a mechanism proceeding via an SN2 dynamic‐kinetic resolution. These studies provide the basis for an aminocatalytic umpolung concept that allows for the asymmetric construction of tertiary ethers in the α‐position of aldehydes. An aminocatalytic oxidative umpolung strategy for asymmetric etherification of the α‐position of branched aldehydes is presented. The protocol avoids pre‐functionalization and employs commercially available oxygen nucleophiles. Polarity inversion proceeds via an aminocatalytic SN2 dynamic‐kinetic resolution of in situ generated racemic quinone adducts.