Directing power of cyclobutenoid annelations on the double bonds of planar cyclooctatetraenes

Ab initio and hybrid density functional quantum mechanical computations are applied to the structure and energetics of a series of two-atom-bridge annelated cyclooctatetraenes. The contribution of each annelation to the exo/endo relative energy is estimated. Key directing factors for a given type of annelation, such as strain, electronegativity, or cyclic electron count, can be sorted out by comparison of various bridge compositions. Overall, electron count and the essential components of the Clar/Robinson rule work well to predict the exo/endo preferences. Specifically, three 4-e(-) Hückel systems (CH-CH, NH-BH and NH-C(O)) display dominant exo forms whereas the three 4n + 2 Hückel counterparts (C(O)-C(O), BH-BH, and planar NH-NH) display a common preference for endo. These endo systems act like four independent four-membered "aromatic" rings linked by "single" bonds. An analysis based on the effective hybridization of carbon atoms in the annulene (Bent's rule) provides a rationale for subtle trends in their specific annulene geometry.