1, 4-disilabenzene

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... 

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... 

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. 

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 ... 

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). 

The pyrolysis of ds-hexamethyl-l,4-dihydrodisilabenzene yields hexamethyl-1,4-disilabenzene, which was characterized by trapping experiments but attempts at its matrix isolation failed319. The trans isomer is unreactive under the same conditions. In the absence of trapping agents, two products were obtained, a disilanorbornadiene and a bismethylenesilacyclopentene (equation 147). A complicated scheme for their formation... 

A survey of the reactions observed for silabenzenes is given in Chart 11. Those of 1,4-disilabenzenes have been studied less but appear to be similar. 

Flash vapour pyrolysis of the 1,4-disilabenzene (149), generated from the anthracene adduct (147) or the dihydro derivative (148), provides the unusual silole (150) accompanied by minor amounts of (151) <87JOM(325)105>. 

Fig. 13. Approximate energy hypersurface for the thermal intramolecular H2 elimination from 1,4-disilacyclohexa-2,5-diene to 1,4-disilabenzene in the gasphase (of. text).

Several studies reported on silabenzenes in which more than one carbon was snbsti-tuted by silicon. Maier and coworkers observed 1,4-disilabenzene (11) in a matrix and identified it by its IR absorbtion at 1273 cm and its electronic absorptions at 408, 340 and 275 nm . Evidence for the formation in the gas phase of 1,3,5-trisilabenzene (12) as a ligand in the dehydrogenation reaction of 1,3,5-trisilacyclohexane with [Cp,Fe]+, [Cp,Co]+ and [Cp,Ni]+ was recently reported by Bjamason and Amason. The strncture of 12 was assigned on the basis of the coUision-indnced dissociation spectrum and from deuterium-labelling experiments. ... 

From Bis-silylene to 1,4-Disilabenzene An Efficient Method to Generate Silicon Analog of Benzene 240... 

FROM BIS-SILYLENE TO 1,4-DISILABENZENE AN EFFICIENT METHOD TO GENERATE SILICON ANALOG OF BENZENE... 

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