Cyclometalation reactions

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]. 

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. 

Reactions that involve significant bond formation in the ratedetermining step are in general expected to exhibit large and negative volumes of activation. This was for instance found for a series of cyclometallation reactions of benzylidenebenzylamines, -anilines, and -propylamine with palladium acetate in toluene and acetic acid solution (171, 172). The cyclometallated compounds are formed via C-H electrophilic bond activation to produce different types of metalla-... 

Interestingly, the triphenylphosphine analogue underwent mainly the normal cyclometallation reaction (Figure 19.4). 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Although cyclometallation reactions have seldom been observed with d° metal compounds, intramolecular metallation involving early transition metal dialkylamides with formation of azametallacyclopropane, which occurs by /3 H abstraction, has been reported.281... 

Attempts to prepare Ta(NEt2)5 by metathetic exchange reactions were found to lead to a complex in which activation of a )3-CH bond of an NEt2 ligand occurs with the elimination of diethylamine (equation 92), 244 345 Presumably steric pressure at the tantalum center promotes the cyclometallation reaction to generate the iminomethyl group. 

Stereopopulation Control on Cyclometallation Reactions and the Stability of Very Large Chelate Rings... 

A wide number of possibilities for using heteroatomic nitrogen-containing organic molecules as electron-donor carbon center have been opened with the advent of the cyclometallation reactions, for example (2.8) [8,13,672-689] ... 

A wide range of cyclometallation reactions have been attempted [685]. Depending on the nature of the metal, these reactions were separated into three categories nucleophilic (Co, Fe, Ru, Rh, Pd, Os, Ir, Pt), electrophilic (Ti, Zr, Hf), and multicentered (Th) cyclometallation [685]. 

Complex compounds with halide bridges are prepared by immediate interaction of unsaturated hydrocarbons with metal salts (Sec. 2.2.4.1). Their examples are classic 7i-complexes of the type 123 which are characteristic for fi 8-metals [75]. Such complexes are also formed by the method of ligand exchange. The bridges of this type are widespread in products of cyclometallation reactions with the use of metal halides (Sec. 2.2.5.1) 371-374, 381, 382 [41,46,48]. An example of such a synthesis is the reaction of arylhydrazones of 2-oxopropionic aldehyde and benzoylformalde-hyde, as well as butadiene-2,3-dione 635, with palladium dichloride, leading (3.71) to dipalladium complexes 636 [88] ... 

Ligand exchange has found a wide application in the syntheses of metal chelates. Metal-cycles containing C,N-donor atoms are formed as a result of cyclometallation reactions using ligand exchange. For example, the complex [Pt(DMSO)2Cl2], which is a universal cycloplatination reactant, takes part in such a process (3.104) [276] ... 

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