Acetate-assisted C-H activation: Mechanism, Scope and Applications

This thesis describes mechanistic investigations of acetate assisted C-H activation, the synthesis of cyclometallated complexes containing nitrogen donor ligands via this method and the applications of cyclometallated complexes in terms of insertion reactions. Chapter one introduces the synthesis, by C-H activation, of cyclometallated complexes containing C,N bidentate ligands of palladium, ruthenium, rhodium and iridium. The introduction also gives an overview on the mechanisms of C-H activation and the applications of C-H activation in catalysis, particularly in direct arylation. Chapter two provides an introduction to the synthesis of arene ruthenium and Cp*M (M = Ir, Rh) half sandwich cyclometallated complexes). The scope of cyclometallation via acetate- assisted C-H activation with different directing groups (pyrazole, pyridine, imines, imidazole, oxazoline and triazole) is discussed. The methodology is extended to six membered rings, non aromatic sp2 and sp3 C-H bonds. Mechanistic investigations using bidentate ligands showed that chelating ligands can prevent the C-H activation process. Chapter three describes a joint computational and experimental study of the cyclometallation reactions of dimethylbenzylamine (DMBA) with [IrC12Cp*]2 using a range of chelating bases. DFT calculations show that facile C-H bond cleavage occurs via 'ambiphilic metal ligand activation' (AMLA) and the ease of C-H activation is governed by the accessibility of the K2-xl base displacement step; thus, more weakly coordinating bases promote C-H activation. Chapter four reports the reactivity of cyclometallated half-sandwich complexes (synthesised in chapter two). Alkynes are shown to insert into the M-C bond. In some cases C-N bond formation occurs to form a heterocycle. The product formed depends on the ease of the reductive elimination step. The relevance of these results to the catalytic synthesis of hetero-and carbocycles is discussed. Throughout the thesis all new compounds are fully characterised spectroscopically and by elemental analysis and several compounds have been structurally characterised by X-ray crystallography

Triazoles from N-Alkynylheterocycles and Their Coordination to Iridium

N-alkynylheterocycles (benzimidazole and indazole) are converted to triazoles by click chemistry, and the resulting triazoles react with [IrCl₂Cp*]₂. The benzimidazole-triazole coordinates in a monodentate fashion through the benzimidazole, whereas the indazole-triazole is bidentate through coordination of both heterocycles. Reaction of the benzimidazole-triazole with methyliodide gives a benzimidazolium salt that deprotonates on coordination to afford a rare example of a bidentate NHC–triazole

Mechanistic Study of Acetate-Assisted C−H Activation of 2-Substituted Pyridines with [MCl₂Cp*]₂ (M = Rh, Ir) and [RuCl₂(<i>p</i>-cymene)]₂

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(L1)(ring)] 1a−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,O chelate complexes 4a,c, respectively. In the presence of KPF6 the N,O 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

Mechanistic Study of Acetate-Assisted C−H Activation of 2-Substituted Pyridines with [MCl₂Cp*]₂ (M = Rh, Ir) and [RuCl₂(<i>p</i>-cymene)]₂

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(L1)(ring)] 1a−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,O chelate complexes 4a,c, respectively. In the presence of KPF6 the N,O 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