Iridium Complex-Catalyzed Allylic Alkylation of Allylic Esters and Allylic Alcohols: Unique Regio- and Stereoselectivity
journal contributionposted on 15.08.1998, 00:00 by Ryo Takeuchi, Mikihiro Kashio
An iridium complex was found to be an efficient catalyst for allylic alkylation of allylic esters with a stabilized carbon nucleophile. Highly regioselective alkylation at the substituted allylic terminus was achieved. The catalytic activity and regioselectivity were affected by the ligand used. The reaction of (E)-2-alkenyl acetates or 1-substituted 2-propeny acetates with dialkyl sodiomalonate in the presence of a catalytic amount of [Ir(COD)Cl]2/P(OPh)3 (P/Ir = 1−2) gave a product alkylated at the substituted allylic terminus in 95−99% selectivity. Construction of a quaternary carbon center is possible by this methodology. The reaction of 1,1-dialkyl-2-propenyl acetates gave a product alkylated at the disubstituted allylic terminus exclusively. (E)-2-Alken-1-ol could be successfully used as a substrate. The products alkylated at the substituted allylic terminus were obtained in 93−99% selectivity. A 31P NMR study of the reaction of [Ir(COD)Cl]2 with P(OPh)3 revealed that a catalytically active species is a monophosphite species. The π-acceptor property of P(OPh)3 promotes a carbonium ion character at the substituted allylic terminus and directs the nucleophilic attack to this position. The stereochemistry of the allyl system affected the regioselectivity. In contrast to the reaction of (E)-2-alkenyl acetates, the reaction of (Z)-2-alkenyl acetates gave a product alkylated at the unsubstituted allylic terminus predominantly. This shows that the regioselectivity of the alkylation of the syn π-allyl iridium intermediate is different from that of the anti π-allyl iridium intermediate. (Z)-Selective allylic alkylation of (Z)-2-alkenyl esters is also possible by iridium catalysis.