Rhodium ortho-metalation reaction

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

The reactivity of iridium is often associated with that of rhodium however, as shown in Scheme 36, iridium is more favorable toward activating the C-H bond and so this metal has been widely used for this reaction. Reaction of [Ir(COD)Cl]2 with one equivalent of the pyridine-fimctionalizedNHC (232) gave in quantitative yield the complex (233) by cleavage of the metallic precursor dimer and orthometallation of the pyridine moiety (Scheme 36). In contrast, the same reaction carried ont with [Rh(COD)Cl]2 as the metal sonrce afforded (234), for which a strong interaction between the rhodinm center and the ortho hydrogen of the pyridine was observed. Treatment of (234) with a sodinm salt did not lead to a C-H activation, bnt led to the coordination of the pyridine onto the metal center yielding the air stable (235). 

Due to the high cost of rhodium, it would be advantageous to develop a process that utilized less expensive metals. To this end, a cobalt-catalyzed ortho-directed halogenation was devised (Scheme 7.85 and Example 7.25) [144]. Similar to the rhodium-catalyzed reactions, the process was successful with NBS and NIS. The cobalt chemistry required elevated temperatures and longer reactions times at a higher catalyst loading (10% vs. 1% for Rh). The yields were not as high as those obtained with rhodium catalysts, and the substrate scope of the reaction was not as broad. 

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