Disilabenzenes

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... 

The stabilization of a silicon-containing arene has not as yet been achieved. Nevertheless, silabenzene (7), 1-silatoluene (8) and hexamethyl-l,4-disilabenzene (9) have all been generated as transients.42"47 Both silabenzene and 1-silatoluene are stable when condensed in argon matrices. The UV spectrum exhibits three bands at 212, 272 and 320 nm expected for a jr-perturbed benzene. In its photoelectron spectrum, the lowest ionization energies are at 8.0, 9.3 and 11.3 eV (cfcalculated values derived by SCF methods of 8.2, 9.2 and 11.5 eV).48... 

FIGURE 2. (a) B3LYP/6-311+G optimized structure of silabenzene (8) (b-d) B3LYP/3-21G opti-mized geometries of ortho-, meta- and para- disilabenzenes, respectively values in parentheses are at MP2/6-31G. All data are from Y. Apeloig, M. Bendikov and S. Sklenak, unpublished results... 

Of the three possible disilabenzene isomers, ortho-21, meta-22 and para-11, only 11 is known experimentally and was observed in a matrix36. The published calculations on disilabenzenes are all at relatively low levels of theory (i.e. they used a relatively... 

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... 

FIGURE 4. The -molecular orbitals of disilabenzenes (a) 1,2-disilabenzene (21) (b) 1,3-disilabenzene (22) (c) 1,4-disilabenzene (11). The orbital coefficients given are of the outer valence -atomic orbitals (at HF/3-21G). Reproduced by permission of Elsevier Sequoia S.A., from Reference 50a... 

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... 

After the successful preparation of silabenzene 24 by hydrogen elimination from 27, it was tempting to try to generate 1,4-disilabenzene 31 by pyrolysis of the easily accessible disilacyclohexadiene 3041 (equation 9). 

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. 

West and coworkers39 used a 9,10-disilacyclohexadienyl-bridged anthracene 74 to synthesize a permethyl-l,4-disilabenzene, 75, as shown in equation 10, and to study its... 

Dipole length formula, 23, S6-Slt Dipole moment, 465. See also Electric dipole moment Magnetic dipole moment excipicx, 282 excited-slate. 47-48. 132 induced.130 permanent, 130-31 Dipole strength, IS8 Dipole velocity formula, 2, 56-57 Direct reaction. See Reaction 1,4-Disilabenzene, 105-7 Disproportionation, 228-29, 380, 390, 433 Disrotatory. See Elecirocyclic reaction ... 

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