Tetrahedrane strain

FIGURE 1. The strain energies of the tetrahedrane and [n]prismane system (M2 H2 ) calculated at the HF/6-31G level for M = C and Si and the HF/DZ(d) level for M = Ge and Sn. Reprinted with permission from Reference 10. Copyright 1995 American Chemical Society... 

The tetrahedron is another of Plato s perfect solids. The hydrocarbon having this shape is known as tetrahedrane. Because of its three-membered rings, it has considerably more strain than cubane and has, so far, resisted many attempts to prepare it. However, tetrahedrane substituted with fert-butyl groups at its vertices was prepared in 1981. It is a stable solid at room temperature. 

Tetrahedrane, (CH)4, would be the simplest regular polyhedral polycyclic hydrocarbon (Figure 3-21a). However, since it has such a high strain energy, it has not (yet ) been prepared in spite of considerable efforts [46], By now, over 10 different derivatives of tetrahedrane have been prepared, for example, tetra-/c/7-butyltetrahedrane (Figure 3-21b) [47], It is amazingly stable, perhaps because the substituents help clasp the molecule together. 

Let us now consider tetrahedrane 6 and [l.l.l]propellane 7. Both species have numerous 3MRs or cyclopropane rings. We thus expect both compounds to be highly strained. Do these hydrocarbons belong to anv homologous series Are they to be understood as avatars for the study of yet more wondrous strained species Are they, as it were, alicycles resplendent in glorious isolation despite the considerable attention paid to both tetrahedrane [27] and [l.l.l]propellane [28] by computationally oriented theoretical chemists ... 

Tetrahedrane is certainly to be found amongst any collection of consummately strained polycyclic hydrocarbons. While the 2-dimensional cyclopropane is composed of three carbons and a single strained 3-membered ring, the 3-dimensional tetrahedrane is made of four carbons and a number of strained cyclopropanes as faces. As befits relative availability and seemingly stability as well, there is yet only one thermochemically characterized tetrahedrane, the tetra-t-butyl derivative [29], for which both condensed and gas phase enthalpies of formation are available [30],... 

Tetrahedranes belong to the most strained molecules. The molecular structure of tetra- r butyltetrahedrane (209) has been determined in its low-temperature modification at 213 K. The molecule lies on a mirror plane in the hexagonal P /m crystal, and is close to symmetry. The C—C bonds in the tetrahedron, 1.485 (4) A, are shorter than in... 

Thermochemical data substantiated the highly strained character of 4. In fact, the strain energy per C-C bond of the tetrahedrane skeleton in 4 turned out to be 21.5 kcal mol , the highest value known (c/ data for cubane, vide supra). Why then is it so stable Obviously this is a purely kinetic stability, as was the case with cubane, but the origin of this stability of 4 is rather unusual. It is speculated that the presence of four terf-butyl groups creates a kind of corset ... 

Tetrahedrane (tricyclo[l. 1.0.0 Jbutane) is a highly strained compound that has yet to be directly detected (although the tetra-t-butyl derivative has been isolated Nevertheless, some ingenious labeling experiments have suggested its intermediacy in certain carbene reactions. 

Relief of steric strain is also an important factor in the oxidation of tetra- cr/-butyl-tetrahedrane to the radical cation of tetra-tc/7-butylcyclobutadiene [113] ... 

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