Much Less Strained Cubane Analogues with Si, Ge, Sn, and Pb Skeletons

Virtually strain‐free four‐membered rings are formed by the higher homologues of carbon, namely silicon, germanium, tin, and lead. This followed from ab initio calculations of the corresponding hetero‐cubanes, ‐prismanes, and ‐tetrahedranes. In the case of carbon the cubane skeleton exhibits the strongest strain, whereas in the case of the other four elements it is always the least strained skeleton. Thus the strain energy of perplumbacubane is ca. 100 kcal−1 smaller than that of cubane. The XX bonds of the cubanes are only ca. 0.02–0.04 Å longer than those of the cyclobutanes, and also the strain energies per ring are very similar in the case of the cubanes and cyclobutanes.