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. 2012 Jan 25;134(3):1490-3.
doi: 10.1021/ja211269w. Epub 2012 Jan 3.

Site- and enantioselective formation of allene-bearing tertiary or quaternary carbon stereogenic centers through NHC-Cu-catalyzed allylic substitution

Byunghyuck Jung  1 , Amir H Hoveyda
Affiliations

Site- and enantioselective formation of allene-bearing tertiary or quaternary carbon stereogenic centers through NHC-Cu-catalyzed allylic substitution

Byunghyuck Jung et al. J Am Chem Soc. .

Abstract

Catalytic enantioselective allylic substitutions that result in addition of an allenyl group (<2% propargyl addition) and formation of tertiary or quaternary C-C bonds are described. Commercially available allenylboronic acid pinacol ester is used. Reactions are promoted by a 5.0-10 mol % loading of sulfonate-bearing chiral bidentate N-heterocyclic carbene (NHC) complexes of copper, which exhibit the unique ability to furnish chiral products arising from the S(N)2' mode of addition. Allenyl-containing products are generated in up to 95% yield, >98% S(N)2' selectivity, and 99:1 enantiomeric ratio (er). Site-selective NHC-Cu-catalyzed hydroboration of enantiomerically enriched allenes and conversion to the corresponding β-vinyl ketones demonstrates the method's utility.

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Figures

Scheme 1
Scheme 1
Efficient Group-, Site- and Enantioselective Catalytic Allylic Substitution with an Allenylboron Reagenta
Scheme 2
Scheme 2
Proposed Modes of Reaction with Organometallic Reagents
Scheme 3
Scheme 3
Modes of Catalyst–Substrate Association with Organoboron Reagents (Chelate-Based Activation and Organization Absent)
Scheme 4
Scheme 4
NHC–Cu-Catalyzed Allenyl Additions to Alkyl- and Si-Containing Disubstituted Allylic Phosphatesa aReactions performed under conditions in Table 2. Relatively low yields of 13a and 13c are due to volatility.
Scheme 5
Scheme 5
Representative Functionalization: Chemo- and Site-Selective Catalytic Hydroboration of Enantiomerically Enriched Allenes
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References

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