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. 2014 Sep 5;16(17):4658-61.
doi: 10.1021/ol5022417. Epub 2014 Aug 25.

Catalytic enantioselective protoboration of disubstituted allenes. Access to alkenylboron compounds in high enantiomeric purity

Hwanjong Jang  1 , Byunghyuck Jung, Amir H Hoveyda
Affiliations

Catalytic enantioselective protoboration of disubstituted allenes. Access to alkenylboron compounds in high enantiomeric purity

Hwanjong Jang et al. Org Lett. .

Abstract

Proto-boryl additions to 1,1-disubstituted allenes in the presence of 1.0-5.0 mol % of chiral NHC-Cu complexes, B2(pin)2, and t-BuOH proceed to afford alkenyl-B(pin) products in up to 98% yield, >98:2 site selectivity, and 98:2 er. The enantiomerically enriched alkenylboron products can be converted to otherwise difficult-to-access alkenyl bromides, methyl ketones or carboxylic acids. What's more, the corresponding boronic acids may be used in highly stereoselective NHC-Cu-catalyzed allylic substitution reactions.

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Figures

Scheme 1
Scheme 1. Proposed Catalytic Cycle
B(pin) = (pinacolato)boron.
Scheme 2
Scheme 2. Preparation of Allene Substrates
See the Supporting Information for details.
Scheme 3
Scheme 3. Stereochemical Models
Scheme 4
Scheme 4. Enantioselective Protoboration of Aryl-Substituted Allenes
Same conditions as those in Table 1; all conv = >98%, except otherwise noted. Yields are of purified products (2 and 3; ± 5%). Site selectivities determined by analysis of 1H NMR spectra of the unpurified mixtures. Enantioselectivities determined by GC analysis. b Conv = 88%. c Conv = 91%.
Scheme 5
Scheme 5
Scheme 6
Scheme 6. Representative Functionalization Procedures,b
See the Supporting Information for experimental and analytical details. b Yield values correspond to conversion of alkyl-B(pin) to ketones or alkenyl bromides; es = enantiospecificity (product enantiomeric excess/substrate enantiomeric excess) × 100.
Scheme 7
Scheme 7. Catalytic Allylic Substitution Reactions
Ar = 2,4,6-(i-Pr)3C6H2.
See this image and copyright information in PMC

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