Disilabenzenes, aromaticity

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. 

Of the (276)-(277) structures, the 1,3-isomer (276a) is the most stable. The calculation of ISE also shows that the 1,3-isomer possesses a greater value for the energy of aromatic stabilization (see the values of R). The preparation of hexamethyl-l,4-disilabenzene has been reported (82-JA6884). According to ab initio calculations on 1,4-disilabenzene (277) and its isomers (278)-(280) (85JOM51), structure (277) is more stable than its valence isomers (278) (by 5.9 kcal/mol, 3-21G //STO-3G) and (280) (by... 

Using the appropriate bond-separation reactions, the UF/3-21G aromatic stabilization energies are calculated to be 47.2, 36.4 and 22.5 kcal mol-1 for 22,11 and 21, respectively, compared to 59.0 kcal mol-1 for benzene503 thus the meta-, para- and ortho-isomers have 80, 62 and 38% of the aromaticity of benzene. The different orders of the thermodynamic stability of the three isomeric disilabenzene and of their aromatic stabilization energies... 

Early work in the area is summarized in Reference 1. A survey of successful approaches to transient neutral aromatics is given in Chart 10 most of them apply to silabenzene and its derivatives but a few of the methods apply also to 1,4-disilabenzenes (see below). 

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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