Abstract
Herein is reported the asymmetric allylic benzylation of Morita–Baylis–Hillman (MBH) carbonates with 2-methylbenzophenone (MBP) derivatives as nonstabilized photogenerated C-nucleophiles. The dual activation of both reaction partners, chiral Lewis-base activation of the electrophile and light activation of the nucleophile, enables the stereoselective installation of benzyl groups at the allylic position to forge tertiary and quaternary carbon centers.
Highlights
The asymmetric allylic alkylation (AAA) reaction lies among the most powerful strategies for the catalytic construction of C−C bonds in a stereoselective manner
No traces of product could be detected in the absence of either catalyst or light irradiation, confirming the proposed dual activation strategy
The dual activation of both reaction partners enables the utilization of 2-methyl benzophenone derivatives (1) as nonstabilized, photogenerated C-pronucleophiles
Summary
The asymmetric allylic alkylation (AAA) reaction lies among the most powerful strategies for the catalytic construction of C−C bonds in a stereoselective manner. Extensive efforts have been devoted to expanding their generality and harnessing their asymmetric potential,[5−7] these transformations are mainly restricted to the use of stabilized or acidic C-nucleophiles, such as alkali-metal salts,[5] active methylene compounds,[6] or enolates[7] (Scheme 1a). Its synthetic potential[11] as nucleophile has only been successfully implemented in asymmetric catalysis very recently.[12] In 2016, the [4 + 2]-cycloaddition between I and maleimides via H-bonding catalysis was reported.12b Subsequently, the asymmetric Mannich-type reaction with cyclic imines12c and the desymmetrization of 1,3-diketones through aldol addition were accomplished under the same activation strategy.12d Interestingly, the stereoselective trapping of the photoenol intermediate (I) is restricted to H-bond donor activation. In 2017, the photoenol I was implemented in iminium-ion catalysis for its asymmetric conjugate additions to enals12e and enones.12f
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