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Ring Opening of Substituted Cyclopropylidenes to Cyclic Allenes

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journal contribution
posted on 26.12.1997 by Holger F. Bettinger, Paul v. R. Schleyer, Peter R. Schreiner, Henry F. Schaefer
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[,7]heptane and tricyclo[,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 (νas = 1829 cm-1) (Tetrahedron Lett. 1986, 27, 5835) as well as Wentrup et = 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 νas(CC) (1718−1838 cm-1) favor the experimental data of Runge and Sander.