Phenylated Cyclosilanes

The oldest known cyclosilanes are the phenyl derivatives. Perphenylated cyclosilanes (Si ) with n = 4,5 and 6 are known. Their high molecular weight, chemical stability and low solubility led to some mistaken conclusions regarding ring sizes and structures206- 274- 682 , but today the picture seems clearer. 

These organometallic compounds split the four-membered ring and form more stable ring systems, especially the five-membered ring. 

During the reaction, smaller chains may be formed since cleavage does not stop after the initial splitting of the ring. The transitory presence of silylenes is also possible313.  

The almost strain-free five-membered ring (compound B in Kipping s designation) is more stable than the four-membered ring. The yield is nearly quantitative if one uses an excess of lithium and a long reaction-time. Under these conditions, the formation of the thermodynamically more stable five-membered ring is preferred. 

Gilman found that the a, w-dihydroxy-decaphenylpentasilane formed by a normal cleavage reaction with bromine followed by hydrolysis reacts to form a trisilane when the solution is run through a column of aluminium oxide8 The terminal Si atoms are apparently cleaved by the basic OH groups of the A1203  

---

The octa- and the nonahalocyclopentasilanes 1128(5X3 and IISisXg with X = Cl, Br, I are accessible by dearylation of the corresponding phenylated cyclosilanes SisPhs and HSi5Ph9 with HX/AIX362. 

Together with the solid phenylated cyclosilanes isolated from these reactions, there were also obtained rather large quantities of viscous liquid materials which have not as yet been fully characterized. Partial separation of these oils was achieved by fractionation from acetone. Some less soluble portions appeared to be materials made up of only diphenylsilylene units. Cryoscopic determinations of various fractions indicated multiples of Si(C6H5)2 ranging from seven to nine, and chemical evidence seemed to indicate that the materials were cyclic polysilanes. ... 

The methyl(chloro)cyclosilanes were achieved by the reaction of the corresponding phenyl-(methyl)cyclosilanes with hydrogen chloride (Eqs 7-8). 

The purpose of this chapter is to discuss the structure, properties, preparation, and reactions of the cyclosilanes in the light of recent investigations of the phenyl and methyl series, and to correct some misconceptions concerning the perphenylated cyclosilanes. Because reactions of these cyclosilanes invariably lead to polysilanes, a section has been included to list these derivatives, many of them new compounds, in tabular form. 

---