The cubane cage - A sensitive probe for assessing substituent effects on afour-membered ring, part II

The crystal structures of methyl C-methoxycubane-1-carboxylate (1), 1-aceta mido-4-fluorocubane (2), methyl 4-acetoxycubane-1-carboxylate (3), 1,8-difl uorocubane (4), 1,4-dichlorocubane (5), and N,N-diisopropylcubane-1,4-dicar boxamide (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 pi-a cceptor influence of this group become longer than the CH-CH bonds. Further more, the influence on bond length with respect to the orientation of this group relative to bonds within the cubane skeleton has been investigated ex perimentally. The effect of the methoxy group has also been found to depend on the orientation. The cage bond antiperiplanar to the methyl group is sh ortened, while the cage bonds in gauche orientation to this group are lengt hened. As seen in the case of the halogen-substituted derivatives, the bond s hearing the acetoxy substituent are shortened due to the sigma-acceptor p roperty of this group. Ab initio calculations on compounds 1, 2, 4, and 5 p erformed at the 6-31G* level confirm the experimental results.