Involvement of an Oxidation‐Reduction Equilibrium in Chromium‐Mediated Enantioselective Nozaki–Hiyama Reactions

Abstract

Ligand induced enantioselective versions of the chromium(II)-mediated Nozaki±Hiyama reaction to homoallyl alcohols proved to be very difficult to achieve, especially if any other nucleophile than the parent allylchromium(III) species was applied. Also, the reaction is frequently accompanied by the formation of oxidation side products, predominantly allyl ketones. This can be explained by an Oppenauer ±(Meerwein±Ponndorf À Verley) type mechanism (OMPV reaction). The addition of an enantiopure ligand to racemic chromium homoallyl alcoholate intermediates produced enantiomerically enriched homoallyl alcohols with an enantiomeric excess of up to 32%. This observation not only supports that the proposed OMPV oxidation-reduction equilibrium plays a crucial role in Nozaki±Hiyama reactions, but also proves its involvement in enantioselective versions.

Cr-catalyzed chiral allenone synthesisviasequential radical–polar crossover and Oppenauer oxidation

“…19 In 2002, Wessjohann and co-workers reported the reductive coupling of allyl bromides and aryl aldehydes to afford allyl ketones with stoichiometric amounts of chromium reagents via the chromium-mediated Oppenauer oxidation. 20 Inspired by this seminal work, 20 a we envisioned the possibility of achieving chiral ketone synthesis in the NHK reaction via precise control of reaction parameters. Herein, we describe the Cr-catalyzed enantioconvergent allenylation reaction from racemic propargylic halides and aldehydes to afford chiral 2,3-allenones (Scheme 1c).…”

The Catalyzed Desulfinylative Allylation of Carbonyl Compounds with Alk‐2‐enesulfonyl Chlorides and Silyl Alk‐2‐enesulfinates

“…[19] Alternative mechanisms implying radical intermediates [20] and oxidation/reduction equilibria (OppenauerMeerwein-Ponndorf-Verley-type mechanism) have been proposed. [21] These alternative mechanisms suggested to us that CrCl 2 might induce desulfinylation of alk-2-enesulfonyl chlorides generating allylchromiumA C H T U N G T R E N N U N G (III) intermediates that could be used as nucleophilic partners in carbonyl-compound allylation, thus creating umpolung [22] reactivity by converting allyl-SO 2 Cl compounds (C-electrophile) into allyl-metal compounds (C-nucleophile; Scheme 2).…”