C-H alkylation of pyridines with olefins catalyzed by imidazolin-2-iminato-ligated rare-earth alkyl complexes

An array of rare-earth bis(aminobenzyl) complexes supported by imidazolin-2-iminato ligands were synthesized and structurally characterized. These complexes showed high activity towards ortho-Csp(2)-H alkylation of 2-alkylpyridines and benzylic Csp(3)-H alkylation of 2,6-dialkylpyridines with alkenes. A wide range of alkyl or aryl substituted olefin substrates are compatible, providing an atom-economical route to linear or branched alkylated pyridine derivatives in moderate to high yields (45 examples, up to 99% yield). The primary study suggested that the asymmetric version of Csp(3)-H alkylation of 2,4,6-collidine with allylbenzene was feasible with chiral imidazolin-2-iminato-ligated scandium(III) complexes, and good yield with moderate enantioselectivity was obtained (84% yield, 84:16 er). Mechanistic investigations including kinetic isotope effect (KIE) experiments and density functional theory (DFT) calculations shed light on the catalytic cycle and the origin of enantiocontrol.