Stereo and regioselectivity in the phenylation of cationic allylpalladium(II) α-diimine complexes by tetraphenylborate anion

Article abstract of DOI:10.1016/0022-328X(93)80131-T

The reaction of the cationic complex 3-cyclohexenyl>(py-2-CH=NC6H4OMe-4)>⁺ (1) with BPh4^- in the presence of fumaronitrile yields trans-3-methoxy-6-phenylcyclohexene (2a) and trans-4-methoxy-3-phenyl-cycohexene (2b), in ca. 1:1 molar ratio.The trans stereochemistry of these products implies that the phenylation of the allyl ligand involves prior transfer of a phenyl group from BPh4^- to the metal, followed by reductive coupling of the organic moieties.In the reaction of 3-1,1-R₁,R₂-C3H3)(N-N')>⁺ (3) 1 = H, R₂ = Ph, Me; R₁ = R₂ = Me> with BPh4^- in the presence of activated olefins, both regioisomers PhCH2-CH=CR₁R₂ (4a) and CH2=CH-CR₁R₂Ph (4b) are formed with a relative ratio which depends essentially on the allylic substituents: R₁ = H, R₂ = Ph, 4a above 98percent; R₁ = H, R₂ = Me, 4a 75-67percent; R₁ = R₂ = Me, 4a 64-58percent.The regioisomer distribution is very little affected by the nature of the α-diimine, of the activated olefin, and of the solvent.For R₁ = H and R₂ = Ph, Me, the olefinic product 4a has a trans (E) geometry.These results have been interpreted in terms of reductive elimination occurring in the intermediate 3-1,1,R₁,R₂C3H3)(N-N')> with a ?-N monodentate α-diimine ligand.