THE CUBANE CAGE - A SENSIBLE PROBE FOR SUBSTITUENT EFFECTS ON A 4-MEMBERED RING

The crystals of methyl 4-fluoro-1-cubanecarboxylate (1), methyl 4-chlo ro-1-cubanecarboxylate (2) and methyl cubanecarboxylate (5) are isomor phous to each other as are the crystals of methyl 4-bromo-1-cubanecarb oxylate (3) and methyl 4-iodo-1-cubanecarboxylate (4). As a result of the space groups P2(1)/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 positio n to the cubane skeletal bond C2-C7. The electronic effects of the hal ogen atoms, in particular fluorine and chlorine, give rise to a shorte ning of the vicinal skeletal bonds. In contrast, the methoxycarbonyl g roups 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.