Unexpected Pd/C-catalyzed room temperature and atmospheric pressure hydrogenation of 2-methylenecyclobutanones

Pd/C-catalyzed hydrogenation reaction of 2-methylenecyclobutanones (2-MCBones) occurs at their exocyclic C=C sites regio-specifically to produce 2-substituted cyclobutanones, which are useful building blocks for the synthesis of drug and pesticide intermediates and are key intermediates in the total synthesis of many natural products. Surprisingly, atmospheric pressure H-2 in balloon can be used as the cheap, clean and safe reductant and the reaction occurs smoothly at room temperature (25 degrees C). Quantitative calculation demonstrates that the release of the intramolecular ring strain caused by the combined effect of the four-membered ring and exocyclic C=C bond in 2-MCBones provides additional dynamic driving forces for the reaction. The higher hydrogenation energy releasing of the exocyclic C=C than C=O was also found to be the causation for good regio-selectivity of the reaction. These new findings may broaden the understandings on the catalytic hydrogenation reactions of strained organic molecules.