Phenyl cyclopropyl ring opening with tritium and deuterium in palladium catalyzed hydrogenation: Evidence for a metallocyclobutane in equilibrium with a Π-allyl palladium complex

摘要

Phenyl cyclopropyl compounds were subjected to catalytic reduction using palladium on carbon and either tritium or deuterium gas. Tritium and deuterium NMR showed the isotopic labeling at the C-1 and C-3 positions of ring opened product, as expected, but also to varying degrees labeling was found at the C-2 methylene, away from the carbon-carbon bond that was broken. This is in contrast to a study of cyclopropyl ring opening reported in 1970 where deuterium and substrate in the gas phase were passed over a silica-supported palladium catalyst at elevated temperatures. In that study, the beta methylene was found to obtain no deuterium while each of the alpha and gamma carbons got about one deuterium. The discrepancy may be due to different mechanisms at work in the gas phase vs. the liquid phase or that the equilibrium is not operative in the gas phase. The present observation can be explained by a 4-membered metallocyclic intermediate in equilibrium with a π-allyl species that is stable enough to undergo tritium/hydrogen exchange with metal hydride.