Silole Rings

It is noted here that the related poly(l,l-silole)s, polysilanes, in which each CRLJ is a silole ring catenating through the silicon atom, have been prepared by Wurtz-type coupling of dichlorosilole monomer54,55 and are described above in the section on Wurtz-type coupling (Section 3.11.4.1.1). 

The remarkable unreactivity of this diradical has been explained by steric hindrance, as well as delocalization of the unpaired electrons over the silole ring. 

Table 3 lists PAEs containing Si, Fe, and B. Silole containing Si in the five-mem-bered ring [157] has a low transition energy because of its low LUMO level [158], and its homopolymers have been prepared [159, 160]. 2,5-Dili-thiosiloles, which can be prepared by the ring-closing reaction of diethynyl-silane, serve as starting materials for various 2,5-disubstituted siloles such as 2,5-dibromosiloles and 2,5-di(trialkylstannyl)siloles [161,162]. PAEs with the silole ring have been reported as shown in Nos. 1-4 in Table 3. PAE-type polymers with Si atoms in the main chain have also been prepared (Nos. 12-16), and their optical properties including photoconductivity have been revealed [155,156].

NMR studies showed that the only anionic intermediate which is present in solution is dilithiosilole 59115. The marked downfield shift in the 29 Si NMR spectrum (S = +68.4 ppm) of 59 as well as the shielding of the 0 and Cp nuclei indicate, in agreement with calculations113, an aromatic character of this compound. This conclusion is supported by the solid state structure of 59 (Figure 10)116. The silole ring in 59 is planar and exhibits nearly equal C—C distances, as one would expect for a delocalized system. However, in contrast to the computational results113,116, the lithium atoms in 59 are not... 

D. Polymers Involving only the Silicon Atom of the Silole Ring in the... 

Iodine reacts differently with the C-methylated silole ring. Cleavage of the ring and formation of a complex mixture instead of the expected l,4-diiodo-2,3-dimethylbutadiene is observed12. 

The solid-state structure of the silolyl dianion is substantially affected by the nature of the substituents attached to the silole ring and of the metal alkali counterions. Stirring C4Me4SiBr2 with four equivalents of potassium in the presence of 18-crown-6 gives... 

A soluble silole-containing polyplatinayne 51 was prepared.47 As compared to 2,5-dibromo-l,l-diethylsilole (Xmax = 326 nm), the positions of the low-lying shoulder bands (Xmax = 504 nm in CH2C12) are remarkably red-shifted by 178 nm for 51 after the inclusion of heavy-metal chromophores. This is possibly due to the intramolecular D-A interaction between the electron-rich metal ethynyl unit and the electron-poor silole ring. The Ee value is impressive at 2.10 eV for 51, and it is significantly lowered by 1.0 eV relative to 53 (3.10 eV).48 Compound 51 is photoluminescent with the singlet emission band at 537 nm. No room temperature emission from the Ti state was detected over the measured spectral window. 

This section is concerned with the structure-properties relationships of 2,5-diarylsiloles and related compounds, including the substituent effects of 2,5-aryl groups, 3,4-substituents and 1,1-substituents on the silole rings. This fundamental study provides valuable information for the molecular design of new silole materials applicable to organic electronic devices. 

Figure 8 Bibenzothiophene (BBT), dithienosilole (DTS), and BBTS, compounds having a silole ring condensed with benzothiophene.

Silole rings have been constructed by treating 3,3 -dibromo-2,2 -bithienyl with BuLi and reacting the dilithio derivative so obtained with a dialkyldichlorosilane <2006JA9034>. The aryl-aryl bond formation by electron-transfer oxidation of Lipshutz cuprates has been utilized for the construction of macrocycles involving thiophene rings <2006JOC6110>. 

The stability and inertness of diradical 4 are quite unprecedented for silicon radicals. 4 does not react at room temperature with water, alcohols, or chloroform, and only slowly with oxygen. The diradical is not indefinitely stable, but the half-time for its disappearance at room temperature is more than one year. The surprising, indeed amazing, inertness of 4 can be explained partly by steric effects, but probably also by specific stabilization due to the silole ring. The results of this study suggest that silole free radicals may form a previously unrecognized class of stable radical species. 

---