Directed Hydroarylation of Olefins

Other pathways are available for hydroarylation of heterocycles. A mechanistic study by Bergman and Elhnan was conducted on intramolecular reactions of imidazole-type heterocycles. These mechanistic studies have shown that the C-H activation of the imidazole is followed by isomerization to generate an N-heterocyclic carbene ligand (Equation 18.60). Following this isomerization, the olefin appears to couple with the carbene by a [2+2] process to generate the carbon-carbon bond, followed by proton transfer to generate a rhodium hydride and reductive elimination to form the C-H bond. 

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In parallel with the directed hydroarylation of olefins, a series of papers described the formation of ketones from heteroarenes, carbon monoxide, and an alkene. Moore first reported the reaction of CO and ethylene with pyridine at the position a to nitrogen to form a ketone (Equation 18.28). Related reactions at the less-hindered C-H bond in the 4-position of an A/-benzyl imidazole were also reported (Equation 18.29). - Reaction of CO and ethylene to form a ketone at the ortho C-H bond of a 2-arylpyridine or an N-Bu aromatic aldimine has also been reported (Equations 18.30 and 18.31). Reaction at an sp C-H bond of an N-2-pyridylpiperazine results in both alkylative carbonylation and dehydrogenation of the piperazine to form an a,p-unsaturated ketone (Equation 18.32). The proposed mechanism of the alkylative carbonylation reaction is shown in Scheme 18.6. This process is believed to occur by oxidative addition of the C-H bond, insertion of CO into the metal-heteroaryl linkage, insertion of olefin into the metal-acyl bond, and reductive elimination to form the new C-H bond in the product. 

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