Tetrahedrane, derivatives

Tetrahedron. Sn2Bi22- and Pb2Sb22 have (2)(2) + (2)(3) + 2 = 12 skeletal electrons for tetrahedra analogous to P4 and organic C4R4 tetrahedrane derivatives (see Section 1.1.3.4). 

Despite the highly unusual structure of tetrahedrane, 3, the problem of its synthesis had been formulated more than 70 years ago (96). The first review article on tetrahedrane (97) appeared almost simultaneously with the paper describing the first synthesis of its tetra-t-butyl derivative 77 (5), which, until recently (98), remained the only tetrahedrane derivative known. On the basis of MM calculations using different force fields Hounshell and Mislow predicted T symmetry for 77 (99). Minkin etal. (17c) discussed the synthesis, reactions, and stability of 3 and 77 pointing out that high kinetic stability of the latter molecule is partly due to unfavorable steric repulsions in the... 

Last year saw the first report of a synthesis of a tetrahedrane derivative, tetra-t-butyltetrahedrane. Zefirov and his co-workers now claim to have accomplished a very short synthesis of tetramethyltetrahedrane (17), which proceeds via irradiation of dilithio-acetylene followed by treatment with iodomethane. ... 

Cyclobuladiene HeterocycUc Four-Membered Rings Tetrahedrane Derivatives and Related Compounds. 15-18... 

Two recent theoretical studies, one of ffie C4H4 isomers and the other of tetramethyl- and tetra-t-butyl-substituted cyclobutadiene and tetrahedrane, give further insights into ffie stabilities of tetrahedrane derivatives. 

THE cvcLOBUTADENE-TETRAHEDRANE SYSTEM. A related reaction is the photoisomerization of cyclobutadiene (CBD). It was found that unsubstituted CBD does not react in an argon matrix upon irradiation, while the tri-butyl substituted derivative forms the corresponding tetrahedrane [86,87]. These results may be understood on the basis of a conical intersection enclosed by the loop shown in Figure 37. The analogy with the butadiene loop (Fig. 13) is obvious. The two CBDs and the biradical shown in the figure are the three anchors in this system. With small substituents, the two lobes containing the lone electrons can be far... 

The irradiation of tetra-/-butylcyclopentadienone with 254 nm light at 77 K produced a tricyclopentanone which, upon extended irradiation, lost carbon monoxide. Tetra-f-butyltetrahedrane was formed. This derivative of the second fundamental hydrocarbon of molecular formula (CH), namely tetrahedrane, is stable at room temperature and could be isolated after chromatography on silica gel in crystalline form (G. Maier, 1978). 

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. 

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... 

Tetrahedrane (CR)4 with bulky tert. butyl substituents R has been synthesized by G. Maier and coworkers in 1978 (28). In their already classic investigations, an interesting valence isomerization has been observed on melting, the C4 cluster rearranges to the thermodynamically more stable cyclobutadiene derivative, which on photochemical energy uptake forms again the tetrahedrane (28) ... 

Also, there is an esthetic element mixed into this motivation, which probably is derived from the tradition of stripped-down test molecules in physical organic chemistry, in which studies of the parent member of a series tend to be valued more highly than those of its derivatives. One thinks, for example, of the emphasis placed on studies of the unsubstimted molecules methylene, norbomyl cation, cyclobutadiene, tetrahedrane, benzyne, and so on. The higher valuation also is associated in some cases with the formidable difficulties experienced by experimentalists in the synthesis and observation of these species. 

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. 

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. 

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],... 

The (non-combustion calorimetrically) experimentally determined enthalpies of formation for benzvalene [31], semibullvalene [32], and triquinacene [33] are 363, 308, and 224 kJ mol1 respectively. Assuming linear dependence of enthalpy of formation on any of these counts (r2 = 0.98), we derive a value for the enthalpy of formation of tetrahedrane of 437 kJ mol1. The equivalent non-statistical derivation is to assume thermoneutrality for the reaction... 

Most of the other products (16-22), can be made directly from (14) and aUcylammonium or phosphonium salts, R4EX (E = N, P), under appropriate thermal and stoichiometric conditions. Mixed-halide derivatives have also been made. The preparation of the derivatives of the [ReX(CO)3] fragment, (19), (20), (21), and the tetrahedrane (22), require higher temperatures. 

Tetrahedrane (11) is the ruthenium analog of the much-studied tricobaltnonacarbonyl clusters Co3(CO)9CR see Cobalt Organometallic Chemistty). The substitution chemistry of (11) has been studied. A starting material is prepared from (11) by reaction with BX3 (equation 2), which gives the chloro and bromo compounds (12). In addition, (11) can also be treated directly with compounds such as diynes to yield interesting substitution products. For example, when (11) is refluxed in THF with diphenylbutadiyne, cis- and trans-alkene isomers of two alkyne insertion regioisomers are formed (equation 3). The product seems to arise from dehydrogenation of one end of the diyne to yield cis and trans enynes and an imsaturated monohydride cluster intermediate, which then reacts with the enynes to yield the allylic derivative products... 

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. ... 

Matrix irradiation (240nm) of tricyclopentanone (111) yields cyclobutadiene, CO, cyclopentadienone, and acetylene.No evidence for the formation of tetrahedrane (112a) was obtained, in contrast with the previously reported behaviour of the tetra-t-butyl derivative of (111), which gave the tetrahedrane... 

Since the appearance of the theoretical consideration that the formation of tetrahedrane from two acetylenes would be a photochemically allowed process, a number of papers dealing with attempted syntheses of tetrahedranes have been published. It was reported that 2,2 -bis(phenylethynyl)biphenyl (26) was transformed thermally and photochemically into an isomeric hydrocarbon, for which a tetrahedrane (27) or a cyclobutadiene (28) structure would be possible . Actually, the structure of the hydrocarbon was confirmed to be 9-phenyl-l,2 3,4-dibenzo-anthracene (29) derived from transannular bond formation . ... 

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