Tetrahedranes structure

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... 

Co2(CO)8 in 34% yield, adopts a tetrahedrane structure with three fused SnCo2 rings (280). 

The typical Nazarov strategy 51 followed by 53 disconnects the rest of the ring from the double bond, but you cannot be quite sure where the double bond will end up in the ring. Another special method, the Pauson-Khand reaction,24,25 differs in both respects. It adds the enone portion of the ring to the rest of the molecule and you can be quite certain where the new double bond will be. The reaction is between an alkene, say cyclopentene 97, an alkyne, and Co2(CO)8 to form a cyclopentenone26 98 in one step. A complex 99 is first formed between the alkyne and the cobalt atoms with the two n-bonds replacing two CO molecules (both are two-electron donors). You may see this complex drawn as 99a but it really has a tricyclic tetrahedrane structure 99b composed of three-membered rings and with a Co-Co bond. 

As far as the mechanism for those processes is concerned, taking into account (i) Viehe s work on t-butylacetylene (Viehe et al., 1964), (ii) the mildness of the trimerization conditions, (iii) the symmetry restrictions for the relevant conversions, (iv) the steric repulsions among perchlorotriphenyl groups and (v) the presumed electronic stability of the intermediates, it is postulated that a dimeric head-to-head diradical is formed first. This then cyclizes to give two bicyclic diradicaloid structures possessing minimal steric repulsions among the substituents (cyclobutadiene and quasi-tetrahedrane structures). By addition of a third molecule of perchlorophenylacetylene, only the 1,2,3- and the 1,2,4-isomers would result (107) (Ballester et al.. 

B4 Bu4 has been prepared by treating BuBp2 with Na/K alloy, and shown by X-ray crystallography to be a tetrahedrane structure. The ion reacts with a... 

Tetrahedranes —One of the more intriguing papers of 1978 describes the isolation of a hydrocarbon, C20H36, m.p. 135 C ( ), from low-temperature irradiation of the dienone (22). This new hydrocarbon is assigned the so-far unknown tetrahedrane structure (23). The spectroscopic evidence is consistent with this structure, and an X-ray structure analysis is awaited. More remarkably, it is suggested that the cyclobutadiene (24) is formed from the tetrahedrane (23) at 130 C. Further reports in this area are awaited with interest. 

Figure 25 Synthesis and structures of supersilyl-substituted gallanyls 396 and 400, gallanide 398 and tetrahedrane 401.

Similarly, the classical tetrahedrane D with six 2c2e bonds may be classified as a nido-cluster having 2n + 4 SE. It is derived from the doso-structure (trigonal-bipyramid), with one apical vertex missing. 

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). 

For the practical design of hypersurfaces, i.e. cuts through the (3n-6)dimensional hyperspace, some hints are outlined. The main purpose, however, is to illustrate the usefulness of hypersurface calculations especially for the detection, identification and characterization of unstable molecules. Examples chosen comprise the structure of RS-C=C-SR, the relative stability of thioacroleine isomers C,H S, the structural changes accompanying the oxidation of hydrazine and some of its derivatives, the isomerization of tetrahedrane to cyclobutadiene both thermally as well as on oxidation, the predicted existence of F SS and nonexistence of CI2SS or H2SS, and, finally, some aspects of the thermal decomposition of methyl and vinyl azides. 

Example IV The Thermal and Oxidative Isomerization of Tetraalkyl Substituted Tetrahedrane Clusters to Cyclobutadiene Derivatives. Other remarkable structural changes during redox reactions, i.e. charge redistributions enforced by the respective energy differences, are observed for cluster compounds (1). On... 

In order to further substantiate the spectroscopic result and to gain more insight into both the thermal rearrangement of the neutral tetrahedrane and the structural change accompanying the one-electron oxidation to its radical cation, MNDO closed and open shell hypersurface calculations have been performed (30). 

The polycyclic cage compounds of Group 14 elements heavier than carbon (i.e. tetrahedranes, cubanes, prismanes, etc.) have fascinated chemists for a long time because of their unique structures and expected unusual physico-chemical properties and reactivity1. It was quite reasonable to assume that such exotic compounds could possess properties... 

From time to time throughout the book we have spread before your eyes some wonderful structures. Some have been very large and complicated (such as palytoxin, p. 19) and some small but difficult to believe (such as tetra-f-butyl tetrahedrane, p. 373). They all have one thing in common. Their structures were determined by spectroscopic methods and everyone believes them to be true. Among the most important organic molecules today is Taxol, an anticancer compound from yew trees. Though it is a modern compound, in that chemists became interested in it only in the 1990s, its structure was actually determined in 1971. 

This molecule now has three-, four-, and five-membered rings fused together in a tricyclic cage structure. This is nowhere near the limit for cage molecules. You saw tetra-t-butyl tetrahedrane in Chapter 15, and you will see in Chapter 37 how even molecules such as cubane can be made. 

Examination of the vertical column entitled Platonic hydrocarbon in Fig. 6 illustrates a dimensional progression of exoskeleton structures beginning with tetrahedrane -> cubane -> dodecahedrane -> buckminsterfullerene. This progression shows how space may be incarcerated from the sub-nanoscopic to the nanoscopic level by geometric closure with exoskeleton structures. 

It reduces to ct + e. Again, the ordering is different, but the similarity between their electronic structure is obvious. A series of molecules and their similarities are illustrated in Figure 7-30. The first molecule is tetrahedrane and the last one is a cluster with metal-metal bonds which can be considered as being the inorganic analog of tetrahedrane. 

Finally, in analogy with organic chemistry, "polymerization" of metal-metal multiple bonds may lead to clusters as illustrated in eqs. 12 and 13. To date, the only definitely characterized oligomerization reaction of this type has been reported by McCarley et al. (8j(eq. 14), although Chisholm et al. ( ) have observed that Mo2(0Et)6 dimerizes to a tetranuclear complex of unknown structure and that the 0-H bond of isopropanol oxidatively adds to the WeW bond of W2(i-Pr0)6 to give a tetranuclear complex with an "open" as opposed to a closed, or cluster, structure (10). Also, some evidence has been presented that Cp2Mo2(C0)it may form unstable tetrahedrane intermediates (eq. 12) (. 

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