Rollover Cyclometalation Reactions

Generally, 2,2 -bipyridines are representative -chelating ligands that are similar to ethylenediamine and 1,10-phenanthroline as inorganic chelate ligands [30]. Only recently has a new coordinating behavior called a rollover cyclometalation reaction appeared in the literature. A rollover reaction involves y-C-H bond activation and formation of a y-C-M bond, as shown in Eq. (7.7) [31]. 

7 Applications of Cyclometalation Reactions and Five-Membered Ring Products. 

The reaction is not restricted to 2,2 -bipyridines but is also applicable to (5S,7S)-5,7-methane-6,6-dimethyl-2-(pyridin-2-yl)-5,6,7,8-tetrahydroquinoline [31], iV-(2 -pyridyl)-7-azaindole [32] and 2-phenylpyrines [33-36], as shown in Fig. 7.1. 

A platinum complex with a bipyridine as the most widely used ligand is easily prepared in high yields by reaction with Pt(Me)2(DMSO)2 in toluene to give an air- and moisture-stable product. The ligand exchange reactions proceed easily with other ligands such as phosphine, CO, and pyridine derivative, as shown in Eq. (7.8) [48]. 

Treatment of the platinum species [Pt(bipy-H)(Me)(DMSO)] 7 6 with acids containing poorly coordinating anions, such as [18-crown-6-H30][BF4], forms a mesoionic complex [Pt(bipy )(Me)(DMSO)]+ 7.8 derived from a rollover cyclometalation reaction as shown in Eq. (7.9) [38]. 

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This monograph presents recent types of new cyclometalation reactions in the hrst category. Reactions with new types of substrates, cyclometalation reactions via mild oxidation, and rollover cyclometalation reactions are presented, for example. 

Second, cyclometalation reactions with substrates that are formed by mild oxidation Third, rollover cyclometalation reactions... 

In 0-xylene, 1-phenylpyrazoles 34a and 40 couple with 2c in a 1 1 manner [7b], accompanied by rollover cyclometallation [19] to produee pyrazolo[l,5-a]quinolines 41 (Scheme 25.21). This reaction proceeds smoothly even in the absence of ligand 21. The addition of a base such as NaaCOs is essential for the reaction. 

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