Substituent Effects on Ring Opening of 2-Furylcarbenes:  An ab Initio Study

The ring-opening reactions of a series of substituted 2-furylcarbenes were investigated by G2(MP2,SVP) theory. The substitutents examined include CH 3, SiH 3, NH 2, OH, F, Cl, Br, and CH CH 2 . The furan ring-opening reaction is predicted to proceed via a six-membered ring transition state, which was described by Herges as a coarctate transition state. The calculated activation barrier depends strongly on the nature of the substituent group, ranging from 8 to 83 kJ mol -1 . Substituent effects on the carbene reactant and ring-opening transition state were analyzed using isodesmic reactions. Methyl and vinyl substituents stabilize the reactant and transition state to a similar extent. On the other hand, NH 2, OH, F, Cl, and Br substituents raise the energy barrier due to the greater stabilizing effect on the reactant than the transition state. The predicted aptitude of ring opening in 2-furylcarbenes is in the order SiH 3 > H > CH CH 2 > CH 3 > Br > Cl > F > NH 2 > OH. This calculated trend is consistent with experimental observations. The amino-, hydroxyl-, and fluoro-substituted 2-furylcarbenes are predicted to be experimentally accessible species in the gas phase. The effects of substitution on furan oxygen of 2-furylcarbene are also reported.