RING-OPENING OF SUBSTITUTED CYCLOPROPYLIDENES TO CYCLIC ALLENES

Density functional theory and ab initio quantum mechanical computation s elucidated the ring opening of trans- and cis-2,3-dimethylcyclopropy lidene (1b and 1c, respectively), bicyclo[4.1.0]hept-7-ylidene (3), an d bicyclo[3.1.0]hex-6-ylidene (7). The B3LYP geometry optimizations em ployed 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 intera ctions in the ring-opening transition structure TS2. While isomerizati on of bicyclo[3.1.0.]hex-6-ylidene (7) to 1,2-cyclohexadiene (8) proce eds 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 f rom unfavorable conformational changes in the cyclohexane moiety of 3 during the reaction. Intramolecular carbene CH insertions to give tric yclo[4.1.0.0(2,7)]heptane and tricyclo[4.1.0.0(3,7)]heptane are charac terized by lower barriers, 6.4 and 9.1 kcal mol(-1), respectively, and these are the products observed experimentally. The geometries and vi brational frequencies of cyclic allenes 4 and 8 were computed with B3L YP, with second-order Moller-Plesset perturbation theory (MP2), and wi th the coupled-cluster method involving single and double excitations using the 6-31G and DZP basis sets. Both Runge and Sander (nu(as) = 1 829 cm(-1)) (Tetrahedron Lett. 1986, 27, 5835) as well as Wentrup et a l. (nu(as) = 1886 cm(-1)) (Angew. Chem., Int. Ed. Engl. 1983, 22, 542) claimed to have spectroscopic evidence for 1,2-cyclohexadiene (8). Th e calculated values for nu(as)(C=C) (1718-1838 cm-L) favor the experim ental data of Runge and Sander.