Insertion reactions of unsymmetrical ester-activated alkynes with o-benzylamine palladacycles: A regioselectivity study

The regioselectivities of the insertion reactions of RC≡CCO₂Et (R=Ph, CF₃) into the aryl-palladium bond of several five-membered, ortho-palladated dimethylbenzylamine complexes, [PdCl(C₆H₄CH₂NMe₂-κC,N)] ₂ (1), [PdCl(C₆H₄CH₂NMe₂-κC,N)(L)] [L=PEt₃ (2), DMPP (3)], [Pd(C₆H₄CH₂NMe₂-κC,N) (NCMe)(L)]PF₆ [L=MeCN (4), PEt₃ (9), DMPP (5)], [Pd(OSO₂CF₃)(C₆H₄CH ₂NMe₂-κC,N)(DMPP)] (6), [Pd(C₆H₄CH₂NMe₂-κC,N) (solvent)]PF₆ (7) and [Pd(C₆H₄CH₂NMe₂-κC,N) (DMPP)(solvent)]PF₆ (8), have been compared with the help of multinuclear NMR spectroscopy. In general, the carboxylate group in the resulting seven-membered palladacycles is preferentially located next to the phenyl group of the benzylamine moiety, but this substitution pattern can be reversed by use of complexes containing electron-deficient and/or coordinatively-unsaturated palladium centres. A mechanism, based both on the experimental results described in this paper and on DFT computations, is proposed.