The Cubane Cage – A Sensitive Probe for Assessing Substituent Effects on a Four-Membered Ring, Part II

Abstract

The crystal structures of methyl 4-methoxycubane-1-carboxylate (1), 1-acetamido-4-fluorocubane (2), methyl 4-acetoxycubane-1-carboxylate (3), 1,4-difluorocubane (4), 1,4-dichlorocubane (5), and N,N-diisopropylcubane-1,4-dicarboxamide (6) have been investigated by means of X-ray diffraction analysis. Fluorine and chlorine substituents cause a shortening of the vicinal bonds, as is seen in the 4-halocubane-1-carboxylates. The cage bonds vicinal to the ester substituent, with a favorable orientation with regard to the π-acceptor influence of this group become longer than the CH–CH bonds. Furthermore, the influence on bond length with respect to the orientation of this group relative to bonds within the cubane skeleton has been investigated experimentally. The effect of the methoxy group has also been found to depend on the orientation. The cage bond antiperiplanar to the methyl group is shortened, while the cage bonds in gauche orientation to this group are lengthened. As seen in the case of the halogen-substituted derivatives, the bonds bearing the acetoxy substituent are shortened due to the σ-acceptor property of this group. Ab initio calculations on compounds 1, 2, 4, and 5 performed at the 6-31G* level confirm the experimental results.