Cyclometallation reaction

Cyclometalation/ one of the earliest reactions involving CH oxidative addition, has been reviewed recently. Apart from the Murai reaction, cyclometalation has also been incorporated into a number of other catalytic reactions of interest in organic synthesis." ... 

Just as this Nobel Prize winning research may be considered to provide a basis for synthetic organic reactions, cyclometalation reactions may also be considered as a basis for synthetic organic reactions. Many organometalhc intramolecular-coordination five-membered ring compounds are used, moreover, as catalysts for synthetic organic reactions. 

The reversible reaction of tri-n-butylstannylfuran with the cyclometallated palladium complex 24 yields the ti C) coordinated 2-furyl complex 25 (98JA11016). 

Phenylpyrazole upon reaction with [MeMn(CO)5] yields 42 (75AJC1259). This product is cyclometallated and contains a five-membered ring. 

Reaction of the cyclometallated complexes 244-246 with pyrazole and excess sodium hydride affords cyclic dimers 247 (990M3991), where C N denotes the corresponding cyclometallated ligand in accordance with structures 244-246. The [Pt2(thienylpyridine)2(/j.-pz)](C104)3] is known as well. 

Diphenylimidazole with palladium acetate forms the cyclometallated complex 80 (X = OAc) (97AOC491). The acetate group is replaced by chloride or bromide when 80 (X = OAc) reacts with sodium chloride or lithium bromide, respectively, to give 80 (X = C1, Br). Bromide with diethyl sulfide forms the mononuclear complex 81. Similar reactions are known for 1 -acetyl-2-phenylimidazole (96JOM(522)97). 1,5-Bis(A -methylimidazol-2-yl)pen-tane with palladium(II) acetate gives the cyclometallated complex 82 (OOJOM (607)194). 

When the phosphonium ylide 81 is reacted with zinc amide, the corresponding a-zincated phosphorus yUde is formed. Thermally unstable, it evolves almost quantitatively to zincatacyclobutane 82 which in presence of pyridine leads to the formation of the zincataphosphoniaindane 83. In order to explain this unprecedented cyclometallation reaction, a mechanism is proposed involving a low coordinated zinc center. The new product, reacted with benzaldehyde leads to the diphenylallene 84 (Scheme 27) [106-108]. 

Treatment of the cyclometallated complexes [Au(N,N,C)Cl][PF6] [N,N,CH = 6-methylbenzyl- (a) or 6-(l,l-dimethylbenzyl)-2,2 -bipyridine (b)] [20] with KOH or Ag20 in aqueous media affords the hydroxo complexes [Au(N,N,C)(OH)][PF6] (36) in fairly good yields [45b, 101] these are air-stable white solids, quite soluble in water and in many organic solvents. When refluxed in anhydrous THF they condense to give the oxo-bridged complexes [Au2(N,N,C)2( J--0)] (37) (Equation 2.10 in Scheme 2.5) which, in turn, can be obtained by a different route [102] (see Section 3.2) the reaction can be reversed by refluxing the 0x0 complex in water. 

Constable, E.C., Henney, R.P.G., Leese, T.A. and Tocher, D.A. (1990) Cyclometallation reactions of 6-phenyl-2,2 -bipyridine a potential C,N,N-donor analogue of 2,2 6, 2"-terpyridine. Crystal and molecular structure of dichlorobis (6-phenyl-2,2 -bipyridine)ruthenium(II). Journal of the Chemical Society, Dalton Transactions, (2), 443. 

Cyclometallation reactions of 6-(thienyl)-2,2 -bipyridine with d transition metal ions. Journal of the Chemical Society, Dalton Transactions, (14), 2251. 

Kitadai, K., Takahashi, M., Takeda, M., Bhargava, S.K., Priver, S.Fl. and Bennett, M.A. (2006) Synthesis, structures and reactions of cyclometalated gold complexes containing (2-diphenylarsino-n-methyl)phenyl (n = 5, 6). Dalton Transactions, (21), 2560—2571. 

The reaction between [PdCl2(MeCN)2] and ethylene diamine-A, 3-ethylguanine yields the analogous C8-cyclometalated compound [PdCl(en-Et-A-C< )]+ 28 (Pd-C bond length 1.974 A) (56) (Fig. 29). Again, proton transfer to the adjacent N7 accompanies C8-metalation and this is observed downfield in the NMR spectrum... 

N7H = 11.57 ppm). While these reactions are rather unusual with regard to metal-nucleobase chemistry, they are in fact examples of classical cyclometalation (97). Such reactions often involve a pre-coordination step, as observed in the formation of 25. 

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