Silicon aromatic rings

Data reported on the low-temperature crystallization (at —90 C) of (PDMS), by. Y-ray and proton n.m.r., show that crystallization is a bulk phenomenon and not surface induced. Dielectric studies on amorphous and crystalline polymers have been reported. Dynamic Kerr-effects and dielectric relaxation studies of a poly(methylphenyl siloxane) have been reported in the region of its relaxation progress, and it is suggested that a substantial contribution to the process is due to motions about the silicon-aromatic ring bond. ... 

Unsolvated organomagnesium compounds have been recommended for the synthesis of organometallic derivatives of mercury, boron, aluminum, silicon, germanium, tin, phosphorus, arsenic, and antimony6-8 and have been used in procedures for the alkylation of aromatic rings and for the production of various polymerization catalysts.4 9... 

The remarkably stable silaallene 133 described recently by West213,214 showed related behavior. Photolysis of 133 is reported to result in the C—H bond addition, across the Si=C bond, of a methyl group on the aromatic ring attached to the carbon end of the silaallene, resulting in the polycyclic compound 134. Alternatively, treatment of the silaallene with acid yields compound 135 by the nominal addition that occurs when one of the C—H bonds of an ortho r-Bu group of the supermesityl group attached to silicon adds to the ends of the Si=C bond with the opposite regiochemistry. 

Aromatic rings containing, s/j2-hybridized silicon atoms also constitute members of the silene family. Interest in these compounds largely focuses on the possible delocalization of electron density in the rings, the extent to which they display aromatic character, and their relative stabilities. Raabe and Michl6 have reported much data, which will not be repeated here other computational studies are reported in Section III.D. A few interesting additions to our knowledge of these systems have been reported in recent years. 

Several reports have been made of a successful catalyzed addition/ substitution reaction resulting in direct attachment of phosphorus to aromatic rings. The preparation of mixed triarylphosphines has been accomplished by the reaction of tin- or silicon-substituted diphe-nylphosphines with aryl halides catalyzed by palladium reagents.74 A similar transformation has also been reported using nickel catalysis.75 The addition/substitution of diphenylphosphine to triflate functionalized phenolic linkages has been of use for the preparation of substances as analogues of tyrosine-related amino acid derivatives, accomplished with catalysis by palladium acetate (Equation 4.29).76... 

Usually, silicon polymers have extremely low Tgs owing to the highly flexible Si—0 linkages in the main chain. Their low Tg s are very advantageous for low-temperature applications, whereas some other applications are limited. One approach to raise the Tg of silicon polymers is the introduction of aromatic rings into the polymer backbone. 

We have concluded that in order to obtain the stable polysilane containing a phenol group our synthesis requires that a small alkyl group such as methyl must be a companion substituent on silicon, the phenolic group must be separated from Si by two carbons or more, and the position of the OH group on the aromatic ring must be preferably meta.(Figure 5). 

Careful studies by C. Eabom have shown that electrophilic aromatic substitution of silicon is faster than substitution of hydrogen. Thus a silicon in an aromatic ring directs substitution with hardly any rearrangement. This technique is particularly useful for preparation of specifically deuterated arenes as protolysis (deuterolysis) or aryl silanes is rapid. 

The presence of a tnalkylsilyl group in a fluonnated organic compound may be useful to direct further transformations of that material Yet m some instances it is the fluonnated substituent that controls the reactions of the tnalkylsdyl group Contrary to predictions, treatment of rm-butyl 3-tnfluoromethyl-6-tnmethylsilyl-phenyl carbamate with rei f-butyllithium results in metallation of one of the methyl groups attached to silicon rather than that of the aromatic ring [90] (equation 75)... 

Hydration of the benzene ring in 1-phenylsilatrane leads to a sharp decrease of its toxicity (the LDS0 value of 1-cyclohexylsilatrane is 150 mg/kg). The toxicity of 1-arylsilatranes drops even more on separation of the aromatic ring from the silicon atom by the methylene group (LDS0 of 1-benzylsilatrane is 1115 mg/kg) (Table 1). 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

---