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

Abstract

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.

Enantioselective SN1-type Reaction via Electrochemically Generated Chiral α-Imino Carbocation Intermediate

“…The extension to an aminocatalytic version is easily conceived; however, this was not further pursued for half a century even in the golden-rush of aminocatalysis. Recently, Jørgensen reported oxidative coupling of catalytic secondary enamine with oxygen-centered nucleophiles such as alcohols in the presence of chemical oxidants such as silver carbonate or quinones [23][24][25] . An α-imino carbocation intermediate was proposed 23 , while this proposal with 2e-oxidation via the intermediacy of α-imino radical 26 was disproved in the presence of quinones 27 coupling pathway, attesting the challenges for in-situ generation of catalytic carbocation intermediate 25,27 .…”