Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 131, Issue 7, Pages 2514-2520Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja808857w
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Funding
- Merck
- Robert A. Welch Foundation
- NIH-NIGMS [RO1-GM069445]
- Korea Research Foundation [KRF-2007-356-E00037]
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Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium catalyst generated in situ from [Ir(cod)Cl](2), 4-cyano-3-nitrobenzoic acid and the chiral phosphine ligand (S)-SEGPHOS, alpha-methyl allyl acetate couples to alcohols 1a-1j with complete levels of branched regioselectivity to furnish products of carbonyl crotylation 3a-3j, which are formed with good levels of anti-diastereo-selectivity and exceptional levels of enantioselectivity. An identical set of optically enriched carbonyl crotylation products 3a-3j is accessible from the corresponding aldehydes 2a-2j under the same conditions, but employing isopropanol as the terminal reductant. Experiments aimed at probing the origins of stereoselection establish a matched mode of ionization for the (R)-acetate and the iridium catalyst modified by (S)-SEGPHOS, as well as reversible ionization of the allylic acetate with rapid pi-facial interconversion of the resulting pi-crotyl intermediate in advance of C-C bond formation. Additionally, rapid alcohol-aldehyde redox equilibration in advance of carbonyl addition is demonstrated. Thus, anti-diastereo- and enantio-selective carbonyl crotylation from the alcohol or aldehyde oxidation level is achieved in the absence of any stoichiometric metallic reagents or stoichiometric metallic byproducts.
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