Ring Opening of Substituted Cyclopropylidenes to Cyclic Allenes

Density functional theory and ab initio quantum mechanical computations elucidated the ring opening of trans - and cis -2,3-dimethylcyclopropylidene ( 1b and 1c, respectively), bicyclo[4.1.0]hept-7-ylidene ( 3 ), and bicyclo[3.1.0]hex-6-ylidene ( 7 ). The B3LYP geometry optimizations employed a DZP basis set. Single-point energies were evaluated at B3LYP/TZP. The ring-opening barrier leading to allene, around 5 kcal mol - 1 for the parent cyclopropylidene ( 1a ), is lowered by 2,3- cis -dimethyl substitution to almost zero for 1c . The larger barrier, 4.2 kcal mol - 1, for the 2,3- trans compound ( 1b ) is due to repulsive H···H interactions in the ring-opening transition structure TS2 . While isomerization of bicyclo[3.1.0.]hex-6-ylidene ( 7 ) to 1,2-cyclohexadiene ( 8 ) proceeds almost spontaneously, the analogous cyclopropylidene ring opening of bicyclo[4.1.0.]hept-7-ylidene ( 3 ) to 1,2-cycloheptadiene ( 4 ) has an unusually high activation energy of 14.6 kcal mol - 1 . This results from unfavorable conformational changes in the cyclohexane moiety of 3 during the reaction. Intramolecular carbene CH insertions to give tricyclo[4.1.0.0 2,7 ]heptane and tricyclo[4.1.0.0 3,7 ]heptane are characterized by lower barriers, 6.4 and 9.1 kcal mol - 1, respectively, and these are the products observed experimentally. The geometries and vibrational frequencies of cyclic allenes 4 and 8 were computed with B3LYP, with second-order Møller−Plesset perturbation theory (MP2), and with the coupled-cluster method involving single and double excitations using the 6-31G* and DZP basis sets. Both Runge and Sander (ν as = 1829 cm - 1 ) ( Tetrahedron Lett. 1986, 27, 5835) as well as Wentrup et al. (ν as = 1886 cm - 1 ) ( Angew. Chem., Int. Ed. Engl. 1983, 22, 542) claimed to have spectroscopic evidence for 1,2-cyclohexadiene ( 8 ). The calculated values for ν as (C C) (1718−1838 cm - 1 ) favor the experimental data of Runge and Sander.