Disrotatory Ring-Opening of Furans Gives Stereocontrol

The ring-opening of 2-methylfuran and 2,3-dihydro-5-methylfuran catalyzed by the Lewis acid catalyst tris­(pentafluorophenyl)­borane in the presence of hydrosilanes was studied using quantum chemical methods. In a previous study, it was suggested that the stereoselective formation of the product is due to a nucleophilic vinylic substitution (SNV) during the reaction. Our calculations show that the pathway involving the SNV reaction is energetically not accessible. Instead, the intramolecular C–O bond cleavage is found to be much more favorable in energy for the ring opening reaction. The experimentally observed excellent stereoselectivity toward the Z-isomer product originates from an intrinsic preference of the furan ring to couple the C–O bond cleavage with a disrotatory motion of the oxygen and carbon fragments. This stereoselective feature is naturally programmed into the furan ring manifold and should be generally exploitable for engineering stereoselective ring-opening processes of bioderived furans.