NickelⅡ-catalyzed asymmetric photoenolization/Mannich reaction of (2-alkylphenyl) ketones

Abstract

A diasteteo- and enantioselective photoenolization/Mannich (PEM) reaction of ortho-alkyl aromatic ketones with benzosulfonimides was established by utilizing a chiral N,Nʹ-dioxide/Ni(OTf)₂ complex as the Lewis acid catalyst. It afforded a series of benzosulfonamides and the corresponding ring-closure products, and a reversal of diastereoselectivity was observed through epimerization of the benzosulfonamide products under continuous irradiation. On the basis of the control experiments, the role of the additive LiNTf₂ in achieving high stereoselectivity was elucidated. This PEM reaction was proposed to undergo a direct nucleophilic addition mechanism rather than hetero-Diels–Alder/ring-opening sequence. A possible transition state model with photoenolization process was proposed to explain the origin of high level of stereoinduction.