Bis silole

Reductive cleavage of the Si-Si bond of bis(siloles) with alkali metals resulted in the formation of silole monoanions (Scheme 2.49). Silole monoanions were found to be aromatic on the basis of NMR spectral data and calculations. 

The 2,2-bis(silyl)tolanes 138 are converted to the fused bis(siloles) 139 by treatment with naphthalene lithium in THF at room temperature, followed by oxidation with I2 <2003JA13662>. The analogous reaction of the diyne 140 affords 141 in high yield. 

Silole dianions, which are of current interest as aromatic species [1 - 3], behave as nucleophiles in simple displacement reactions, generally yielding simple trapping products [4 - 6]. However, the reaction of l,l-dilithio-2,3,4,5-tetraphenylsilole (1) with l,l-dichloro-2,3-diphenylcyclopropene (2) takes a most unexpected course, leading to ring-opening of the cyclopropene ring and ultimate formation of a tetracyclic disilapentalene and a stable bis-silole diradical [7]. 

The l,l-bis(diethylamino) silols 304 and 305 are excellent precursors for the synthesis of other 1,1-difunctionalized silols (Scheme 43).374 The reaction of 304 and 305 with EtOH/AlCl3 and HC1 produces the silols 306 and 307 containing ethoxy groups or chlorine atoms attached to the silicon. The 1,1-dichloro silols 308 and 309 have been converted into the corresponding 1,1-difluoro silols 310 and 311 with ZnF2. Hydrolysis of 309 and 311 yields the fluorosilanol 312 and the silanediol 313, respectively (Scheme 43). 

The reaction of a silirene with an alkyne in the presence of a palladium catalyst allows cyclization of two molecules of the alkyne with the silylene, as in equation 9 above. For example, Seyferth and coworkers have prepared the silole 33 in 80% yield from 1,1-dimethyl-2,3-bis (trimethylsilyl) silirene and phenylacetylene (equation 10)45. Without catalyst, this reaction yielded the silole 34 and the ene-yne 35, resulting respectively from ring expansion and cleavage by PhC=CH of the silirene. Under UV irradiation, 35 alone was formed. 

Bis(diethylamino) silol complexes, preparation, 3, 435 Bis-dihydropyran, via ring-rearrangement reactions, 11, 260 1,3-Bis(dimethylchlorosilyl)cyclodisilazane, preparation,... 

While some of the reactions mentioned above can be used for the synthesis of 1,1-difunctionalized siloles30, none of them can afford 2,5-difunctionalized siloles. 2,5-Difunctionalized siloles can be directly prepared only by route B. Wrackmeyer has reported that a reaction of bis(stannylethynyl)silanes 8 with BR3 (R = Me, Et, i-Pr, and n-Bu) affords 2,5-distannyl-3-borylsiloles 9 (equation 3)31. We have recently reported a general synthesis of 2,5-difunctionalized siloles by the intramolecular reductive cyclization of diethynylsilanes (equation 4)32. Thus, the reaction of bis(phenylethynyl)silanes 10 with lithium naphthalenide affords 2,5-dilithiosiloles 11, which can be further transformed into a variety of 2,5-difunctionalized siloles 12. In addition, starting from bis(phenylethynyl)diaminosilanes 13, various 1,1-difunctionalized siloles 15 are also accessible through 1,1-diaminosiloles 14 (equation 5)33. With these functionalized siloles in hand, various silole-based <7- and jr-con jugated compounds have been prepared as described below. 

A similar reaction is observed, when the siloles 23 (R = H) are treated with sodium bis(trimethylsilyl)amide <19990M3813, 19970M1445>. The silicate 24 formed in the first step either eliminates NaH to give 26 or rearranges to the carbanion 25 which reacts further with Mel to the heterocycles 27. 

Closely related to the Birch reduction, benzene, and other aromatic compounds afforded reduction-silylation products (equation 10). Electron-transfer reactions of bis(phenylethynyl)dimethylsilane gives 2,5-dianion of 3,4-diphenylsilole that is a useful intermediate to variously substituted siloles (equation 11). ... 

The reaction of disubstituted acetylenes with 1,1,2,2-tetramethyldisilane catalyzed by bis(triethylphosphine)nickel(II) or palladium(II) dichloride provides a convenient source of siloles... 

A key role of orbital interaction in the main group element-containing Ti-electron systems (the systems were synthesized using silole or bis-silicon-bridged stilbene as building units) 05CL2. 

TTie first general synthetic route to 2,5-difunctionaUzed siloles is the intramolecular reductive cyclization of diethynylsilanes [10]. Thus, the reduction of bis(phenylethynyl)silane 1 using an... 

Scheme 1. Synthesis of 2,5-difunctionalized siloles 4 by reduction of bis(phenylethynyl)silane 1...

In addition, 3,4-dialkyl-2,5-bis(trimethylsilyl)siloles 13, prepared from 12 by alkylation on the silicon atom, are further transformed into 2,5-dihalosiloles 14 (Scheme 5) [16], which would be useful precursors for new silole n-conjugated compounds. 

Lee, Y, et al. 2001. A new narrow band gap electroactive polymer Poly[2,5-bis 2-(3,4-ethylene-dioxy)thienyl silole]. Chem Mater 13 2234. 

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