Selective, radical-free activation of benzylic C-H bonds in methylarenes

Selective ortho-C-H Activation in Arenes without Functional Groups

Aromatic C-H activation in alkylarenes is a key step for the synthesis of functionalized organic molecules from simple hydrocarbon precursors. Known examples of such C-H activations often yield mixtures of products resulting from activation of the least hindered C-H bonds. Here we report highly selective ortho-C-H activation in alkylarenes by simple iridium complexes. We demonstrate that the capacity of the alkyl substituent to override the typical preference of metal-mediated C-H activation for the least hindered aromatic C-H bonds results from transient insertion of iridium into the benzylic C-H bond. This enables fast iridium insertion into the ortho-C-H bond, followed by regeneration of the benzylic C-H bond by reductive elimination. Bulkier alkyl substituents increase the ortho selectivity. The described chemistry represents a conceptually new alternative to existing approaches for aromatic C-H bond activation.

Selective benzylic Csp3-H bond activations mediated by a phosphorus-nitrogen PN3P-nickel complex

In contrast to the typical C_sp2-H activation, a PN³P-Nickel complex chemoselectively cleaved the benzylic C_sp3-H bond of toluene in the presence of KHMDS, presumably via an in situ generated potassium benzyl intermediate. Under similar conditions, CO underwent deoxygenation to afford the corresponding nickel cyano complex, and ethylbenzene was dehydrogenated to give styrene and a nickel hydride compound. 2,6-Xylyl isocyanide was transformed into an unprecedented indolyl complex, likely by trapping the activated benzyl species with an isocyanide moiety.