The Cubane Cage – a Sensible Probe for Substituent Effects on a Four‐membered Ring

The crystals of methyl 4‐fluoro‐1‐cubanecarboxylate (1), methyl 4‐chloro‐1‐cubanecarboxylate (2) and methyl cubanecarboxylate (5) are isomorphous to each other as are the crystals of methyl 4‐bromo‐1‐cubanecarboxylate (3) and methyl 4‐iodo‐1‐cubanecarboxylate (4). As a result of the space groups P21/m and Pnma, respectively, all molecules lie in a crystallographic mirror plane. Therefore, the methoxycarbonyl group is exactly planar, and the carbonyl fragment is in an eclipsed position to the cubane skeletal bond C2–C7. The electronic effects of the halogen atoms, in particular fluorine and chlorine, give rise to a shortening of the vicinal skeletal bonds. In contrast, the methoxycarbonyl groups causes a lengthening of the vicinal skeletal bonds C2–C6 and C2–C6A which are not in an eclipsed conformation. Ab initio calculations at the 6–31G level of compounds 1, 2, and 5 confirm the experimental results.