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)(C6H4CH2NMe2–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.