We report a mechanistic study of C(sp)-H bond activation mediated by nickel. Cyclometalated Ni(II) ureate [(PEt)Ni(κ- C,N,N-(CH)N(Cy)(CO)N((N)-quinolin-8-yl))] was synthesized and isolated from the urea precursor, (Me)(Cy)N(CO)N(H)(quinolin-8-yl), via C(sp)-H activation. We investigated the effects of solvents and base additives on the rate of C-H activation. Kinetic isotope effect experiments showed that C-H activation is rate determining. Through deuterium labeling and protonation studies, we also showed that C-H activation can be reversible. We extended this reaction to a range of ureas with primary and secondary C(sp)-H bonds, which activate readily to form analogous nickelated products. Finally, we showed that carboxylate additives assist with both ligand dissociation and initial N-H bond activation, consistent with a concerted metalation-deprotonation mechanism.
The palladium-catalyzed arylation and alkenylation of N-substituted methyl imidazole-4-carboxylates are described through inter- and intramolecular pathways. Both direct C2–H and C5–H arylation and alkenylation proceed under Pd(0)/Cu(I) cooperative catalysis and Pd(0) catalysis, respectively, in low-polarity 1,4-dioxane solvent. The methodology gives access to C2 (hetero)aryl or alkenyl imidazoles as well as innovative C2- and C5-arylated fused imidazoles tricycles with a five- to seven-membered middle ring.
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