Construction of All-Carbon Quaternary Stereocenters by Scandium-Catalyzed Intramolecular C-H Alkylation of Imidazoles with 1,1-Disubstituted Alkenes.

The exo-selective C-H cycloaddition of imidazoles to 1,1-disubstituted alkenes has been achieved for the first time by using half-sandwich scandium catalysts. A wide range of imidazole compounds bearing various 1,1-disubstituted aliphatic alkenes, styrenes, dienes, and enynes have been selectively converted in high yields to the corresponding bicyclic imidazole derivatives bearing β-all-carbon-substituted quaternary stereocenters. By using a chiral half-sandwich scandium catalyst, the asymmetric exo-selective cyclization has also been achieved with a high level of enantioselectivity.