Dewar disilabenzene

Comparison of the energy of para-disilabcnzcne (11) with that of the isomeric Dewar disilabenzene (23) and the silylenes 24 and 25 follows the trends discussed above for the isomers of silabenzene. At IIF/3-21 G( )//I H7STO-3G the most stable isomer is the... 

The dehydrogenation of 30 turned out to be more difficult than the analogous conversion of 27 to 24. The IR spectrum of the matrix-isolated products showed only one absorption at 1273 cm-1, which disappeared upon irradiation with X = 405 nm probably yielding Dewar disilabenzene 32. The UV spectrum proved to be of higher diagnostic value. 1,4-Disilabenzene 31 shows a typical heterobenzene electronic spectrum with absorptions at X = 408, 340 and 275 nm displaying another bathochromic shift compared to silabenzene 24. 

Ab initio (3-21G( )//STO-3G) calculations by Chandrasekhar and Schleyer163 on 1,4-disilabenzene 58, its Dewar benzene isomer 59, and a silylene isomer 60 showed that all three species exhibited approximately similar stabilities, the silylene 60 being 9.9 kcal mol-1 more stable than the planar aromatic form 58, which was 5.9 kcal mol-1 more stable than the Dewar benzene form 59. 

Following an earlier qualitative prediction that para-disilabenzene 93 and the corresponding Dewar structure 94 should be close in energy245, Chandrasekhar and Schleyer250 studied this system using the 3-21G( V/STO-3G level of theory. In addition to 94 the isomeric silylenes 95 and 96 were also studied. 

It is very likely that the thermal interconversion between silaaromatics and their Dewar isomers is facile. The ultimate transformation products from pyrolytically produced hexamethyl-l,4-disilabenzene have been accounted for by a scheme in which isomerization to the Dewar form represents the first step319. 

All three of the isomers of disilabenzene have been studied by ab initio methods. Using the 3-21G/STO-2G level of theory all of the isomers show some delocalization with the 1,3-isomer being the most stable <84JOM(27i)369>. Further ab initio calculations on the 1,4-isomer show that the disilabenzene, its Dewar form, and one of its silylene isomers all have similar thermodynamic stabilities <85JOM(289)5i>. Calculations on aromaticity in disilabenzenes and other group 14 containing heterocycles are also discussed in a review <93AHC(56)303>. 

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