Carbene insertion reactions, group 4 metal direction

On the way to further extension of the scope of Cp Co "-catalyzed C-H functionalization, Glorius and coworkers developed a condensation reaction of 2-arylpyridine derivatives and diazoesters to form unique polycyclic heteroaromatics having 6//-pyrido[2,l-a]isoquinolin-6-one skeletons (Scheme 10.15) [38]. The reaction is achieved by the combination of a bench-stable Co precatalyst [Cp Co(CO)l2] [39], a silver salt (AgSbFg), and an acetate source (KOAc) in trifluoroethanol. The in situ-formed Cp Co catalyst is proposed to play a dual role in this condensation reaction. First, it promotes formal carbene insertion into the ortho C-H bond through pyridine-directed C-H metalation, cobalt-carbene formation, carbene insertion into the aryl-Co bond, and protodemetalation. Second, it acts as a Lewis acid to facilitate nucleophilic attack of the pyridine moiety to the ester group, which eventually leads to the product through aromatization and elimination of methanol. The thus-synthesized polycyclic heteroarenes exhibit bright and color-tunable fluorescence in solution and in the solid state. 

The second example is a rhodium(II)-catalysed decomposition of an a-diazo ketone (71). This interaction generated a metal-bound carbene that appears to undergo CH-insertion when tethered to thiophenes the product (72) was an aromatic tricycle. The direct CH-insertion mechanism is probably excluded by the reaction of (73). Treatment of (73) with a rhodium catalyst gave rise to a bicyclic system (75) with a pendant ketene group. The ketene appears to have migrated the authors suggested a mechanism via ketene (74), being formed by nucleophilic attack of the enol ether on the electrophilic carbene. ... 

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