Silabenzenes matrix isolation

UV irradiation (A>320 nm) of matrix-isolated silabenzene [137a] led to the disappearance of its absorptions and the appearance of bands of other unstable molecules, which were assigned to Dewar silabenzene [138]. This assumption is reasonable because sp hybridization of the silicon atom is preferable to sp hybridization and favourable to the stability of [138]. Besides, according to calculations the energy differences between mono-cyclic [137a] and bicyclic [138] structures decrease from CeHe (314 kJ moP ) to silabenzene CsSiHe (125 kJ moP ). 

The parent silabenzene 24 was first matrix-isolated by our group in 198035 by pyrolysis of precursors 25 and 26, which yield the expected silabenzene by retro-ene fragmentation. Later, it could be shown that in analogy to carbon chemistry the hydrogen elimination from silacyclohexadiene 27 also gives the silaaromatic 2436. This reaction is allowed by the Woodward-Hoffmann rules. In accordance with the Woodward-Hoffmann rules, it could be demonstrated that silabenzene 24 is not accessible by pyrolysis of the conjugated silacyclohexadiene 28 (equation 8). 

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. 

Our attempts to matrix-isolate silacyclobutadiene (4) in analogy to silabenzene (2) were unsuccessful. While the aromatic silabenzene (2) could be generated in a retro-ene reaction from silacyclohexadiene 1 [3], the analogous silacyclobutadiene precursor 3 did not react in the same way [4]. 

The first to generate and trap a silaaromatic compound were Barton and Banasink2413, who reported in 1977 on silatoluene, 85241. The discovery was soon followed by a matrix isolation and IR, UV and PE spectral characterization of both 85241b,c and silabenzene 86242. 

The simplest and apparently also the best gas-phase pyrolytic precursors for silaaromatics are their 1,4-dihydro derivatives, which have been very successful in the production of matrix-isolated silabenzene316 and 1,4-disilabenzene317. On the other hand, pyrolysis of l-sila-2,4-cyclohexadiene under identical conditions only gave traces of silabenzene (equation 149). This result can be easily understood in terms of Woodward-Hoffmann rules. 

All attempts to matrix-isolate silabenzene generated from chlorosilacyclohexadiene failed318. 

Irradiation of matrix-isolated silabenzene at 320 nm causes a transformation to the Dewar isomer318,324. This conversion is readily followed by the shift of the Si-H stretching frequency from that characteristic of an sp2 to that characteristic of an sp3 hybridized silicon. Irradiation of the Dewar isomer with 240 nm light318 causes partial reversion to silabenzene. A similar photostationary state can be reached in the case of 1-phenyl-1-silabenzene315. Since there are no wavelengths at which the Dewar isomer absorbs while the silaaromatic itself does not, it is not possible to achieve complete conversion of the Dewar form into the silaaromatic form. 

In summary, the benzoannelation of the central silabenzene ring is not enough to stabilize compounds of this type to a high extent. They still proved to be very thermolabile. Even phenyl substitution as in 34 (R = Ph) does not furnish sufficient steric protection for isolation outside of an argon matrix. 

T. J. Barton had succeeded earlier (in 1977) in isolating the first representative of this class, 1-methyl-silabenzene (silatoluene), and one year later the unsubstituted silbenzene [222-226], Both compounds could only be studied in solid argon matrices at 10-23 K. Maier and coworkers using a different synthetic approach also studied the very unstable unsubstituted silabenzene, which was shown to react photochemically in the argon matrix [227-235],... 

Markel and Schlosser reported the synthesis of the substituted silabenzene 3 which was stable in solution up to 170 K . Kinetic stabilization by bulky substituents led to the isolation in an argon matrix of 9, R = MesSi or -PrMe2Si by Jutzi, Maier and coworkers 9, R = -PrMe2Si was stable up to 90 K even without an argon cage . In 1991 Maia-and coworkers isolated and characterized spectroscopically in the gas phase and in an argon matrix at 12 K the 9-silaanthracenes, 10, R = H, Ph . Most recently, Okazaki and coworkers reported the synthesis and isolation of 2-silanaphthalene (1), the first silaben-zenoid compound which is a stable crystalline material even at room temperature. ... 

Silabenzene [37] can also be isolated in an argon matrix at 10 K. It is a product of flash pyrolysis of some cyclohexadiene derivatives, namely l-acetoxy-l-sila-2,4-cyclohexadiene (a), l-allyl-l-sila-2,4-cyclohexadiene (b), and l-siIa-2,5-cyclohexadiene (c) (Scheme 1.3) ... 

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