Ring Opening of Substituted Cyclopropylidenes to Cyclic Allenes

Abstract

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[4.1.0.02.7]heptane and tricyclo[4.1.0.03.7]-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 (vas = 1829 cm-1) (Tetrahedron Lett. 1986, 27, 5835) as well as Wentrup et al. (vas = 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 vas(C=C) (1718-1838 cm-1) favor the experimental data of Runge and Sander.