Brethomé AV, Paton RS, Fletcher SP. Retooling Asymmetric Conjugate Additions for Sterically Demanding Substrates with an Iterative Data-Driven Approach.
ACS Catal 2019;
9:7179-7187. [PMID:
32064147 PMCID:
PMC7011729 DOI:
10.1021/acscatal.9b01814]
[Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2019] [Revised: 06/27/2019] [Indexed: 12/13/2022]
Abstract
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The
development of catalytic enantioselective methods is routinely
carried out using easily accessible and prototypical substrates. This
approach to reaction development often yields asymmetric methods that
perform poorly using substrates that are sterically or electronically
dissimilar to those used during the reaction optimization campaign.
Consequently, expanding the scope of previously optimized catalytic
asymmetric reactions to include more challenging substrates is decidedly
nontrivial. Here, we address this challenge through the development
of a systematic workflow to broaden the applicability and reliability
of asymmetric conjugate additions to substrates conventionally regarded
as sterically and electronically demanding. The copper-catalyzed asymmetric
conjugate addition of alkylzirconium nucleophiles to form tertiary
centers, although successful for linear alkyl chains, fails for more
sterically demanding linear α,β-unsaturated ketones. Key
to adapting this method to obtain high enantioselectivity was the
synthesis of modified phosphoramidite ligands, designed using quantitative
structure–selectivity relationships (QSSRs). Iterative rounds
of model construction and ligand synthesis were executed in parallel
to evaluate the performance of 20 chiral ligands. The copper-catalyzed
asymmetric addition is now more broadly applicable, even tolerating
linear enones bearing tert-butyl β-substituents.
The presence of common functional groups is tolerated in both nucleophiles
and electrophiles, giving up to 99% yield and 95% ee across 20 examples.
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