Novel Type of Aromatic Silicon Compound, Experiment & Theory

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The replacement of two carbon atoms by two silicon atoms reduces the energy gap by influence all three factors. First, silasubstituted benzenes are estimated to be less aromatic than benzene. Second, silicon can easily adopt angles of around 90ﾟ while C prefers essentially tetrahedral angles. Third, Si prefers to form Si-C and Si-Si single bonds over Si=C or Si=Si pi bonds. The calculations reveal that 1,4-disilabenzene is more stable than its Dewar form but the enthalpy difference is much small than for benzene. The B3LYP calculations show that 9,10-disilaanthracene (SA) is less stable than the Dewar isomer by -l2.1 kcal/mol. The change in relative stability of the aromatic vs.the Dewar isomer is similar on going from benzene to anthracene in the all carbon system (20kca1/mol) and in the disila-systems (26 kcal/mol). SA possesses C2h-symmetry, in which the central disilabenzene ring adopts a slightly nonplanar chair-type structure (with a Si-C-C-Si dihedral angle of 5.5ﾟ). This is con … More sistent with the smaller degree of aromaticity in this ring. The planar form of SA (i.e., possessing D2h -symmetry) is a transition state that connects the two isomeric C2h-minima, but it is by only 0.02kcal/mol higher in energy, suggesting that SA is essentially a planar molecule. We underlined the similarities between silicon and carbon. The formation of disilabenzene as well as Dewar-silabenzene, silabenzvalene, and silaprismane could be obtained from corresponding bissilayclopropenes. Thermolysis of 3,3'-biscyclopropenyl (BC) at l50ﾟC afforded the rearranged product benzvalene (82%). It could be considered the initial formation of an intermediate either Dewar benzene and 1,4-disilabenezene and their subsequent isomerization to benzvalene.To evaluate the stabilities of BC,the relative energies of the corresponding valence isomers were determined by 6-3lG^<**> molecular orbital calculations. As a result, the benzvalene and the Dewar benzene have similar energy levels and are slightly less stable than 1,4-disilabenzene. Since large basis sets and inclusion of electron correlation generally tend to favor tricyclic over bicyclic or monocyclic geometries may have quite similar energies ; the stability order may even be reversed. Actually, methyl substituted BC arranged to Dewar-silabenzene. However, disilabenzene was not isolated under the reaction conditions. SA dimer prepared from the reaction of dihydro-9,10-disilaanthracene with lithium was treated with excess lithium or potassium resulted in formation of the 9,10-alkali-9,10-disilaanthracene. On treatment with a, omega-dichloropolysilanes, the dianion was converted into the corresponding cyclic silicon compounds. Oxidation of dianion with electron acceptors gave the SA dimer and in the presence of diphenylacetylene gave [2+4] addition product. The results show that SA could be generated from dianion oxidation process. Less