Titanium cyclometallation

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. 

Amongst some specific reactions used in regioselective syntheses, we note the cyclometallation processes [Sec. 2.2.5.1, reaction (2.8) Sec. 3.1.1.2, reactions (3.40)-(3.44) Sec. 3.3.2.3, reactions (3.226)-(3.228)]. In this respect, we note that C,N-donor ligands form, depending on the nature of Lewis acids, two types of complexes. In the case of immediate interaction (4.35) of azomethines 859 with titanium or tin tetrachlorides (MC14), the molecular complexes with M—N bond 860 [101] were obtained (route A), while the cyclometallation reaction with the use of palladium diacetate leads to binuclear chelates 861, in which the Pd — N, C metal-cycles are formed (4.35) [11,114-116] ... 

The pyridine adduct of the early transition metal titanium undergoes a unique cyclometallation with formation of a three-membered ring adduct ... 

Hagadom JR, Arnold J. Low-valent chemistry of titanium benzamidinates leading to New Ti P-N2, p-O, alkyl derivatives, and the cyclometalation of TMEDA. J Am Chem Soc. 1996 118 893-894. 

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