Rh-Catalyzed Asymmetric Allylic Arylation of Racemic Halides
The achievement of stereocontrolled Csp2–Csp3 cross-coupling reactions between allylic halides and arylboronic acids continues to represent a challenging problem in organic synthesis, despite the enormous potential of this transformation. In 2015, Professor Steve Fletcher and his group at the University of Oxford (UK) reported that non-stabilized alkyl nucleophiles generated through the hydrometalation of alkenes could be used in Cu-catalyzed asymmetric allylic alkylation (AAA) reactions with cyclic racemic allyl halides which is complementary to the widely used Pd-catalyzed AAAs using stabilized nucleophiles in dynamic kinetic asymmetric transformations (DYKAT). Mechanistic work suggested that the Cu-catalyzed DYKATs occur because the starting materials are being racemized by the same Cu catalyst that selects one of the enantiomers of the starting material for a highly enantioselective AAA. In follow-up work, these methods were extended to sp2-hybridized alkenylzirconium nucleophiles. Now, the Fletcher group was able to develop an elegant and efficient alternative method involving arylboronic acids as the non-stabilized sp2-hybridized nucleophile and Rh(I) catalysts in addition to racemic cyclic allyl chlorides.
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