Synthesis, Structure, and Reactivities of cis-Dimethyl(η1-allyl)gold(III) Complex. Diastereoselective Allylation of Aromatic Aldehyde

Article abstract of DOI:10.1246/bcsj.68.1523

cis-Dimethyl(η¹-allyl)(triphenylphosphine)gold(III), cis-1-allyl)(PPh3)> (η¹-allyl = allyl (1), crotyl (2), methallyl (3)) has been synthesized by reaction of cis- (X = I, NO3) with corresponding Grignard reagents.X-Ray structure analysis of 2 shows that 2 has a typical square planar cis configuration and the crotyl ligand binds to Au by η¹-mode: orthorhombic, space group P2₁2₁2₁, a = 9.816(2), b = 23.208(4), c = 9.5290(9) Angstroem, Z = 4, R(R_w) = 0.037(0.041) with use of 2551 reflections.Reactions of 2 with HCl and Br2 selectively give 1-butene and 3-bromo-1-butene, respectively, indicating γ-regioselectivity of the reactions.Compounds 1-3 react with aldehydes and ketones to give the corresponding homoallyl alcohols.C-C bond formation also takes place at the γ-position of η¹-allyl moiety selectively.The rate of the allylation reaction of carbonyl compounds decreases in the order of methallyl (3) > allyl (1) > crotyl (2) gold(III) complexes.The homoallyl alcohol formed by the reaction of 2 with benzaldehyde is a diastereoisomeric mixture of 2-methyl-1-phenyl-3-buten-1-ol, with high anti selectivity.Both the reaction rate and anti selectivity in the allylation reaction were reduced by addition of tertiary phosphines such as PPh3 and PCy3, and a 6-membered transition state has been proposed, which indicates coordination of the carbonyl group to a T-shape Au intermediate produced by dissociation of the PPh3 ligand.