Electrocyclic Ring Openings of 2-Furylcarbene and Related Carbenes:  A Comparison between Pseudopericyclic and Coarctate Reactions

Ring openings of 2-furylcarbene (5a), 2-pyrrolylcarbene (5b), 1-cyclopenta-1,3-dienylcarbene (5c), and of 11, as well as of the 3- and five-membered ring vinylogs (7a, 7c, 9a, 9c) were investigated at the B3LYP/6-31G(d,p) level of theory. The reactions of 5a, 5c, and 11 were also studied at the G2(MP2) level. This work provides the first comparison of pseudopericyclic and coarctate orbital topologies in concerted reactions. The ring openings of 5a, 5b, 11 and their vinylogs have pseudopericyclic (but not coarctate) topologies, while the ring openings of 5c and vinylogs are pericyclic and can be described as coarctate. The transition structure geometry for the ring opening of 5c is not well described by MP2 theory. The G2(MP2) barrier for this ring opening is estimated to be 6.4 kcal/mol, significantly higher than that calculated by Sun and Wong. The ring opening is an eight-electron, Möbius, conrotatory process, not the six-electron, Huckel process described by Herges in his original work describing coarctate orbital topology. The vinylogous reactions of 7c and 9c are disrotatory. The triplet photochemistry of 3a and 3b, as studied by Nakatani et al., was modeled by 6a and 12, respectively. The difference between the two is attributed to the greater stability of the triplet carbene ³11 as compared to ³syn-5.