Mechanistic study of acetate-assisted c-h activation of 2-substituted pyridines with [MCl2Cp*]2 (M = Rh, Ir) and [RuCl2(p-cymene)]2

Abstract

Reactions of 2-substituted pyridines HL with [MCl2Cp*] 2 (M = Ir, Rh) and [RuCl2(p-cymene)]2 have been carried out in the presence and absence of sodium acetate. 2-Phenylpyridine (HL1) is cyclometalated easily to form [MCl(Ll)(ring)] la-c (M = Rh, Ir, ring = Cp*; M = Ru, ring = p-cymene). However, in the case of 2-acetylpyridine (HL2) sp3 C-H activation occurs cleanly with rhodium to form N,C chelate complex [RhCl(L2)Cp*] 2b, but the reactions with iridium and ruthenium give unseparable mixtures of products. The N,C cyclometalated products [MCl(L2)(ring)] 2a-c (M = Ir, Rh, ring = Cp*; M = Ru, ring = p-cymene) have been independently prepared from the lithium enolates of 2-acetylpyridine. Notably, in the absence of acetate, [RhCl2Cp*]2 shows no reaction with 2-acetylpyridine, whereas [IrCl2Cp*] 2 and [RuCl2(p-cymene)]2 react to form equilibrium mixtures of the starting materials and N,0 chelate complexes 4a,c, respectively. In the presence of KPF6 the N,0 chelate complexes [MCl(HL2)(ring)][PF6] 4a,c.d (M = Ir, ring = Cp*; M = Ru, ring = p-cymene, mesitylene) can be isolated. These are not intermediates en route to the N,C cyclometalated products. These results suggest that for C-H activation to occur under these mild conditions acetate must coordinate to the metal prior to coordination of the ligand. © 2009 American Chemical Society.