Tetrahedrane parent

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. 

The second is the existence of the dication of variously substituted cyclobutadienes Ab initio quantum chemical calculations indicate that at least the parent dication, 42, is significantly non-planar (43). This is suggestive of some 1,3-bonding although one would hesitate to describe this ion as bicyclobutane-1,3-diium (44) vide infra). Perhaps even more reluctantly, one may describe this species as the dication of tetrahedrane (45) where both electrons have been removed from the same C-C bond. Indeed, the closed shell (or singlet) dication of tetrahedrane will lack strict tetrahedral symmetry because of unequal orbital occupancies and resulting Jahn-Teller distortion It is presumably this C-C bond... 

The neutral chloride B4CLt, with four skeletal bond pairs, and tetrahedrane C4H4 or tetraphosphorus P4 and related molecules, with six skeletal bond pairs, are systems with regular tetrahedral structures. Tetrahedral molecules held together by six skeletal bond pairs can of course be accommodated in the carborane cluster systematics as nido clusters with n atoms and n -1- 2 skeletal bond pairs, if a low connectivity (axial) vertex of the parent trigonal bipyramid is left vacant, instead of the expected high-connectivity (equatorial) vertex. 

As with the isomerization of tetra-t-butyltetrahedrane to tetra-/-butylcyclobuta-diene, the isomerization of 12 to 18, 19 and 20 likely begins with opening to a diradical (discussed below for the parent) 21 which isomerizes to a (hradical 22 the second diradical isomerizes to the cyclobutadiene 13, but unlike the case in the tetra-t-buty system, the cyclobutadiene is of higher energy than the tetrahedrane, and furthermore can drain away to the low-energy aUg ne 18. A control experiment showed that 19 and 20 do not arise from the cyclobutadiene 19 was suggested to... 

The compound is sensitive to air and moisture, but thermally stable. Lithiated tetra-hedranes are synthetically important because of the ease with which it should be possible to replace the lithium by other groups. In particular, tetralithiotetrahedrane is potentially a precursor of the parent tetrahedrane, by protonation. In fact, protonation of 13 by the relatively acidic cyclopentadiene gave tris(trimethylsilyl)tetrahe-drane (14), and reaction with dimethyl sulfate gave methyltris((trimethylsilyl) tetrahedrane (15). 

Calculations on tetra(f-butyl)tetrahedrane, tetrakis(trimetltylsilyl)tetrahedrane, and the parent tetrahedrane. 

Let ns apply these principles to tetra-i-butyltetrahedrane, tetrakis(trimethylsilyl) tetrahedrane, and tetrahedrane itself. Unreferenced calcrrlations are by the author, and some of these results are strmmarized in Table 6.1. Note that calcrrlations designed to address kinetic stability directly, by examining transition states for interconversion of diradicals (below), seem to be limited to the parent molecule. 

Table 6.1 Energies from B3LYP/6-31G calculations on tetrahedranes and cyclobutadienes (by the author, using Gaussian 03). Note that for the tetra-t-buty and the tetrakis(trimethylsilyl) systems the tetrahedrane is calculated to lie below the cyclobutadiene in energy, but for the parent molecules the tetrahedrane is calculated to lie above the eyelobutadiene...

The synthesis of the parent tetrahedrane must be considered an enormous challenge, in view of the tremendous but thwarted effort expended by the Maier group to this end, but their great success in obtaining several substituted tetrahedranes indicates the quest is by no means hopeless (e.g. Note 4). It is the kinetic stability of tetrahedrane that concerns ns here, for no one would expect it to be the global minimum on the C4H4 potential energy surface. The indication from computational... 

Fusion of a cyclopropane ring onto tetrahedrane may render 4 kinetically and thermodynamically stabler than tetrahedrane (unfortunately we do not know how stable the parent tetrahedrane will be). Recognizing 4 as a bicyclobutane with an extra cyclopropane ring evokes the possibihty of its synthesis from a... 

Txi-tert-hvXy trimethylsilyl tetrahedrane " and tetra-trimethylsilyltetrahedrane have also been prepared. The former gives the cyclobutadiene at 160°C, and the latter is stable up to 300°C. Interestingly, B3LYP calculations indicate that the latter tetrahedrane is 9 kcal/mol more stable than the corresponding cyclobutadiene. These same calculations provide an energy difference of 24.3 kcal/mol in the parent case favoring the cyclobutadiene which is close to the G2 value so the value with the tetra TMS derivative is probably believable. 

Whereas the PE spectrum of the parent hydrocarbon tetrahedrane is still... 

Tetrahedrane has not been synthesized, but the compound tetra-f-butyltetrcihe-drane has been prepared. Suggest reasons why the substituted compound has been made but the parent compound has not. 

Convenient new routes to tricyclo[4,1,0,0 ]hept-3-ene and its derivatives have been disclosed. Acetone-sensitized irradiation of bicyclo[3,2,0]hept-6-en-2-one affords the ketone (624), whose enol phosphate is reduced by lithium-ammonia to give the parent alkene (625). " A second route is also described, starting from the 7,7-di-bromonorcarane derivatives (626) and (627). Reaction of (626) with methyl-lithium in ether at 0°C afforded a 3 1 mixture of the tricycloheptenes (628) and (629) similar reaction of (627) gave (630 40%), but reactions of the parent dibromide were unsuccessful. Catalytic Ag ion causes the rearrangement of (628) and (629) to, respectively, 3-methyl- and 1-methyl-cycloheptatriene. Initial Ag" attack at the least hindered edge bond is implicated. Attempted preparation of the tetrahedrane dimer (631) by the addition of dibromocarbene to homobenzvalene followed by treatment of the adduct so obtained with excess methyl-lithium in ether at 0°C afforded instead 5-ethynyl-cyclohexa-1,3-diene. 

The parent compound has not yet been synthesized but the tetra-t-butyl derivative is known. Molecular orbital calculations estimate that the breaking of a C—C bond in tetrahedrane would require only about 10 kcal/mol, indicating that the molecule would have only a short existence even at quite low temperatures. The tetra-t-butyl derivative, however, is stable up to 100 C. ... 

We needn t stop here. More rings can be attached in fused or bridged fashion to produce wondrously complex stmctures.The naming protocols are complicated, if ultimately logical, and won t be covered here. Many of the compounds have common names that are meant to be evocative of their shapes. Prismane and cubane are examples.The molecular versions of the Platonic solids, tetrahedrane, cubane, and dodecahedrane are all known, although the parent, unsubstituted tetrahedrane stih evades synthesis (Fig. 5.63). 

---