Tetrahedrane acidity

Irradiation of the closely fixed diacetylene (30), which seemed to provide a greater possibility of formation of a tetrahedrane (31), gave unchanged 30 in almost quantitative yield , whereas 2,2 -diethynylbiphenyl (32) afforded the dibromide 34 as the result of an unexpected intramolecular carbon-carbon bond formation when hydrogen bromide in acetic acid was added. ... 

Alkorta and Elguero found that there is good correlation between PAs calculated by B3LYP/6-311-I--I-G computations and available gas phase experimental measurements. There was also good correlation with solution pA values. Based on these correlations, they made the interesting prediction that the as yet unknown hydrocarbon tetrahedrane would be comparable to water in acidity. 

Exotic chemistry the concatenation of strained rings in tetrahedrane may not ordy thwart conventional routes to its realization, but could also confer unusual properties, such as anomalous reactivity, acidity or basicity, and ionization energy, to name some possibilities. 

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). 

Other than its kinetic and thermodynamic stability (above), several properties of tetrahedrane are of interest, and can be readily calculated by current computational chemical techniques. Here we will look at four strain, ionization energy, acidity, and basicity. 

Basicity. Computational studies indicate that protonation of tetrahedrane would lead to the homocyclopropenyl cation, the stability of which, taken with the release of the great strain in tetrahedrane, results in a big enthalpy drop. This was calculated (B3LYP) to correspond to a proton affinity of 1053 kJ mol (298 K enthalpy of reaction) [80]. A CBS-4 M (cf Tetrahedrane Acidity above) calculation by the author gave a value of 1004 kJ mol. The gas-phase basicity... 

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