Tetrahedrane, tetra-tert-butyl

It has to be pointed out, however, that additional valence isomers (CR)4 such as cyclopropenyl carbene, are found in local energy minima, if other cuts through the 54 dimensional hyperspace are calculated (30) - in agreement with compounds isolated recently e.g. from the reactions of both isomers, tetra (tert. butyl) tetrahedrane and cyclobutadiene, with tetracyanoethylene (32). 

Tetra(tert-butyl)tetrahedrane converts into tetra(tert-butyl)cyclobutadiene only when heated up to 140°C in vacuum. A barrier of 170 kJ mol separates these two isomers (Heilbronner et al. 1980). In the cation-radical state, the tetrahedrane structure converts into the cyclobutadiene structure without heating (Bock et al. 1980, Fox et al. 1982). From Scheme 6.34 it can be seen that by the action of aluminum chloride on methylene chloride, tetrahedrane forms the cation-radical of its isomer—the cyclobutadiene cation-radical and not the cation-radical of the same skeleton. The latter is more stable than the former because of more effective delocalization of the unpaired electron and positive... 

While tetrahedrane 1 remains an elusive goal, some of its tetrasubstituted derivatives, namely tetra-tert-butyl- and (trimethylsilyl)-tri-t Tt-butyltetrahe-dranes (4 and 5, respectively, vide infra) have been synthesized by Maier s group. ... 

Bald, M., McKee, M.L., and Schleyer, R von R., Theoretical study of tetramethyl- and tetra-tert-butyl-substituted cyclobutadiene and tetrahedrane, J.Phys. Chem. A, 104, 1246, 2000 J.Phys. Chem. A, 104, 6338, 2000. 

Since the structure of tetra-tert-butyltetrahedrane (19) has not been experimentally determined (128), one needs some courage to make predictions about it. Nevertheless, it seems likely that C—C, bond will be twisted from a gauche value (0(CCC,jMe) = 60°, point group) because of severe crowding among the tert-butyl groups at the tetrahedrane nucleus. MM calculations predict a structure with a 0 value near 45° T symmetry) to be 4 kcal/mol more stable than the gauche conformation (130). 

In some cases, steric interactions can prevent unimolecular reactions. Tetrahe-drane (18) has been the subject of a number of studies, and the conclusion is that, if formed, it would rapidly decompose to form two molecules of acetylene. However, tetra-tert-butyltetrahedrane (19) is a quite stable substance, and on heating rearranges to tetra-tert-butylcyclobutadiene. An orbital symmetry " analysis of the cleavage of tetrahedrane to acetylene indicates that it involves a torsional motion that in the case of the tert-butyl substituted derivative would bring the tert-butyl groups very close to each other. As a result, this mode of reaction is not possible, and the compound is relatively stable. 

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