Pyramidalization in Derivatives of Bicyclo[5.1.0]oct-1(7)-enes and 2,2,5,5-Tetramethylbicyclo[4.1.0]hept-1(6)-enes

Bicyclo[4.1.0]hept-1(6)-ene and bicyclo[5.1.0]oct-1(7)-ene systems have been investigated by X-ray diffraction and ab initio methods. The syntheses of 3b−3e, 4a, 4b, 6a, 7a, 8b, 9b, 12, 13, and 16 have been described and the structures of 1, 3a, 4b, 5a, 7a, 10, 11, and 13−16 have been determined by X-ray analysis. To complement these experimental data, several model compounds have been studied theoretically. For the cyclopropenes 14 and 15, substantial pyramidalization has been observed, and the six-membered ring species is considerably more strongly deformed than the seven-membered ring system [14: Φ = 14.5(6)°; 15: Φ = 30.2(5)°]. Our analysis, with the aid of the model compounds M-3.OH, M-63.H, M-63.OH, M-73.H, and M-73.OH, clearly shows that the decisive factors for pyramidalization are ring size, conformation, and repulsion between the occupied π bond and the oxygen lone pairs. The last factor contributes to the pyramidalization stemming from ring strain in 14 and is the dominant source of deformation in 15. In the cyclopropenylidene systems 4b [Φ = 0.7(4)°], 11 [Φ = 1.1(3)°], 13 [Φ = 4.7(5)°], and 1 [Φ = 7.9(5)°], we have studied the electronic influence on pyramidalization. A high degree of pyramidalization is favored by weak cyclopropenylium character, and cyclopropenylidenes exhibit, in general, smaller deformations than do analogous cyclopropene systems.