Hexasila-Dewar benzene

The hexasila-Dewar benzene 13 is thermally stable at —150 °C, but it gradually reverted to the hexasilaprismane 1243. The half-life is 11/2 = 0.52 min at 0 °C in 3-methylpentane. The activation parameters for the isomerization of 13 to 12 are a = 13.7 kcalmol-1, A= 13.2 kcalmol-1 and A= — 17.8 cal K-1 mol-1. The small Ea value is consistent with the high reactivity of Si=Si double bonds. Most probably, the small HOMO-LUMO gap of 13 makes it possible that the Si=Si double bonds undergo a formally symmetry forbidden [2 + 2] thermal reaction. Hexasila-Dewar benzene is a key... 

FIGURE 9. Energy diagram of Dewar benzene and of hexasila-Dewar benzene calculated at the HF/6-31G level... 

Fig. 15. UV-Vis difference spectra showing the formation of hexasila-Dewar benzene (29) and subsequent photochemistry (2-MeTHF, 100 K).

For reasons of symmetry, the bridged Si-Si bond interacts with tts and TTs MOs, and thus the former is destabilized and the latter is stabilized (Fig. 16). The splittings of ita/tts and Tr lTr s can be estimated to be 0.25 and 1.22 eV, respectively. Due to the reason of the symmetry, the photochemical excitation of the hexasila-Dewar benzene 29 readily occurs to give the hexasilaprismane 25 (Scheme 20). 

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