Silabenzenes synthesis

In 1988, Markl and Schlosser51 reported the synthesis of the substituted silabenzene 40 by irradiation of the diazo compound 39, which was found to be stable in solution up to 170 K (equation 12). 

Nevertheless, the synthesis of the first silabenzene stable up to room temperature remains a challenging problem. 

Previous attempts by Markl for kinetic stabilization had led to the synthesis of (i) 1,4-di-tert-butyl-l-silabenzene, which dimerized at 0 °C by a [2 + 21-cycloaddition (ii) l,4-di-tert-butyl-2,6-bis(trimethylsilyl)-l-silabenzene, which was studied by NMR at low temperatures in special solvents and (iii) l-tert-butyl-2,6-bis-(di-methylisopropylsilyl)-4-trimethylsilyl-l-silabenzene, which was stable also only at very low temperatures [236-239], Similar germabenzene derivatives were also obtained, and they were also unstable under normal conditions [240],... 

In the same year, Tokitoh and coworkers have succeeded in the synthesis and isolation of the first stable 2-silanaphthalene 97 by taking advantage of the steric protection afforded by the Tbt group (Scheme 36) °. These two compounds 94 and 97 should be noted as the first examples which showed that double bonds containing a silicon can make a conjugated system as well as the parent carbon analogues. In 2000, a stable silabenzene 98, a much simpler sUaaromatic system than 97, was also synthesized and isolated using a similar kinetic stabilization method (Scheme 36). ... 

More recently, Scheschkewitz and coworkers reported the synthesis of an exotic tricyclic isomer stabilized with six 2,4,6-triisopropylphenyl (named Tip) substituents, which is an attempt toward production of the first hexa-silabenzene (Chart 13.19). In the crystalline form, the tricyclic ring adopted a cyclohexane-like puckered geometry, which is significantly distorted from a planar benzene-like geometry. The isomer showcased a unique aromaticity, called dismutational aromaticity, supported by three Si resonances and high-level density functional theory calculations. This result can lead to the first isolation of an ideal hexasilabenzene with higher molecular symmetry (six- and three-fold) by designing more elaborate substituents and reaction routes. 

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. ... 

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