Homodimerization of 2H-chromenes catalyzed by Brønsted-acid derived UiO-66 MOFs

Two UiO-66 derived BrOnsted acid metal-organic framework (MOF) catalysts were prepared through a facile post-synthetic modification method, characterized by SEM, XRD, H-1 NMR, FTIR, BET and TGA and applied as heterogeneous catalysts in the homodimerization of 2H-chromenes. Several substituted tetrahydrochromeno heterocycles were generated in high yields, at reduced reaction times and with good diastereoselectivities, taking advantage of the strong BrOnsted-acidic sites. A detailed mechanism was proposed and a tandem rearrangement/hetero-Diels-Alder reaction sequence was revealed. Deuterium labeling and various dienophiles were studied to verify the hypothesis of a pathway based on a hydride shift followed by ortho-quinone methide cycloaddition.