anti-Diastereo- and enantioselective carbonyl crotylation from the alcohol or aldehyde oxidation level employing a cyclometallated iridium catalyst: alpha-methyl allyl acetate as a surrogate to preformed crotylmetal reagents.
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-diastereoselectivity 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 enantioselective carbonyl crotylation from the alcohol or aldehyde oxidation level is achieved in the absence of any stoichiometric metallic reagents or stoichiometric metallic byproducts.
Kim
In Su
IS
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712, USA.
Han
Soo Bong
SB
Krische
Michael J
MJ
D000085:Acetates / Q000737:chemistry*; D000438:Alcohols / Q000138:chemical synthesis / Q000737:chemistry*; D000447:Aldehydes / Q000737:chemistry*; D000498:Allyl Compounds / Q000737:chemistry*; D002384:Catalysis; D006865:Hydrogenation; D007495:Iridium / Q000737:chemistry*; D009579:Nitrobenzoates / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D010720:Phosphines / Q000737:chemistry; D013237:Stereoisomerism
D000085:Acetates; D000438:Alcohols; D000447:Aldehydes; D000498:Allyl Compounds; D009579:Nitrobenzoates; D009942:Organometallic Compounds; D010720:Phosphines; D007495:Iridium; C498551:allyl acetate