Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 131, Issue 20, Pages 6916-+Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja902437k
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Funding
- Robert A. Welch Foundation
- NIH-NIGMS [RO1-GM069445]
- University of Texas Center for Green Chemistry and Catalysis
- ACS-GCI Pharmaceutical Roundtable
- Korea Research Foundation [KRF-2007-356-E00037]
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Enantioselective transfer hydrogenation of 1,1-dimethylallene 1 a in the presence of aromatic, alpha,beta-unsaturated, or aliphatic aldehydes 2a-i mediated by 2-propanol and employing a cyclometalated iridium C,O-benzoate derived from allyl acetate, m-nitrobenzoic acid, and (S)-SEGPHOS delivers reverse-prenylation products 4a-i in good to excellent isolated yields (65-96%) and enantioselectivities (87-93% ee). In the absence of 2-propanol, enantioselective carbonyl reverse prenylation is achieved directly from the alcohol oxidation level to furnish an equivalent set of adducts 4a-i in good to excellent isolated yields (68-94%) and enantioselectivities (86-91% ee). Competition and isotopic labeling experiments suggest rapid alcohol-aldehyde redox equilibration in advance of carbonyl addition along with capture of the kinetically formed pi-allyl complex at a higher rate than reversible beta-hydride elimination-hydrometalation. This protocol represents an alternative to the use of allylmetal reagents in enantioselective carbonyl reverse prenylation and represents the first use of allenes in enantioselective C-C bond-forming transfer hydrogenation.
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