Addressing the Poly- to Oligo-ketone Selectivity in Styrene Carbonylation Catalyzed by Palladium/bpy Complexes. Effect of the 6-Alkyl Substitution

Abstract

Two series of organometallic Pd complexes, namely, (i) [Pd(CH3)(CH3CN)(N−N′)][PF6] and (ii) [Pd(CH3)(N−N′)2][PF6], with a range of 6-alkyl-substituted-2,2′-bipyridine ligands, including the new 6-(1-methoxyethyl)-2,2′-bipyridine, in both racemic and enantiopure form, and 2-(methoxymethyl)-6-(1H-1,2,3-triazol-1-yl)pyridine, have been studied. 6-(1-Methoxyethyl)-2,2′-bipyridine was synthesized both in racemic and in the opposite homochiral enantiomeric forms by two stereocomplementary chemoenzymatic procedures. The characterization of the new complexes, both in solid state and in solution, provides evidence for the formation of a unique isomer featuring the methyl ligand trans to the Pd−N bond of the substituted pyridine ring. For the complex with the bpy ligand having the sec-butyl substituent a cyclometalation reaction with the release of methane occurs, leading the substituted bpy to act as a terdentate N−N′−C ligand. Complexes of series ii feature one chelate N−N′ ligand, while the other one is coordinated to Pd in a monodentate fashion. In solution a fluxional process that makes equivalent the two N−N′ ligands is present, and the static 1H NMR spectra correspond to an averaged structure where palladium is a stereogenic center. All these complexes behave as catalysts for styrene carbonylation, yielding CO/styrene oligoketones, which are optically active when catalysts containing chiral, enantiomerically pure, ligands are applied. For both series of complexes the reactivity with labeled CO has been investigated, leading to the formation of the corresponding Pd−acetyl species, with that for complexes of series ii featuring both N−N′ molecules bonded to the same metal center.