Alkylsilanes cyclic—

Similarly to the low chemical reactivity of (simple) alkylsilanes devoid of functional groups, the electrochemical reactivity of simple alkylsilanes is quite low. Klingler and Kochi measured the oxidation potentials of tetraalkyl derivatives of group-14-metal compounds by using cyclic voltammetry3. These compounds exhibit an irreversible anodic peak in acetonitrile. The oxidation potential (7 p) decreases in the order of Si>Ge>Sn>Pb as illustrated in Table 1. This order is the same as that of the gas-phase ionization potentials (7p). The absence of steric effects on the correlation of Ev with 7p indicates that the electron transfer should take place by an outer-sphere mechanism. Since tetraalkylsilane has an extremely high oxidation potential (>2.5 V), it is generally difficult to oxidize such alkylsilanes anodically. 

Benzylsilanes and allylsilanes are easily oxidized anodically compared with alkylsilanes and arylsilanes. Benzylsilanes exhibit irreversible cyclic voltammetric waves. It is notable that their oxidation potentials (Ep) are markedly less positive than those of the unsilylated parent compounds owing to the a-jr interaction (Table 3)10a. It is interesting that a-trimethylsilylation of xylenes markedly decreases their oxidation potential while additional a -trimethylsilylation makes a little change (Table 3). It has also been reported that a a, a-interacting system (the neighboring C—Si bonds) in addition to a a-ir interaction caused a significant decrease of the oxidation potentials1013. 

Electrochemical synthesis of various cyclic alkylsilanes has been performed similarly113. It should be noted that 5-silaspiro[4,4]nonane is formed despite the high probability of polymer formation due to the high functionality of the silicon. Such high selectivity in the electrochemical ring closure seems to be due to the orientating effect of an electrode in the course of an irreversible reduction of a carbon-halogen bond in the monosilylated intermediate (equations 87 and 88). 

Many investigations of alkyl and aryl chlorosilanes have been reported (125, 220, 225, 254). The probability of cleavage of the R—Si bond is found to decrease with increase in the size of R (220). Methylsilanes (143) and hydrocarbons with trialkylsilyl groups (145) are discussed and extensive studies of linear and cyclic silicon-methylene compounds are also available (22-24, 105-107, 225). Russian workers have looked at alkylsilanes (68), silacycloalkanes (69), and silylvinylacetylenes (144, 195). Silicon-carbon... 

A facile synthesis of cyclic alkylsilanes consisting in the electrochemical reduction of aliphatic dibromides in the presence of polychlorosilanes of the formula R SiCl4 ( = 0, 2) affords heterocyclic compounds in good yields <1995JOM213>. According to the procedure described in Equation (8), the compound 26 was obtained in 57% yield. In contrast to nonelectrochemical methods, which are based on the ring closure of terminal unsaturated compounds, the electrochemical route is claimed to be more efficient and selective. 

Pyrolysis of the formed polysilazane at 750° C generates several alkylsilanes such as dimethylsilane, trimethylsilane, ethyldimethylsilane, tetramethyidisiloxane, pentamethyidisiloxane, methylenebisdimethylsilane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, etc. The presence of the cyclic compounds similar to those from poly(dimethylsiloxane) was an indication that some polysiloxane sequences may be present in the polymer. Thermal degradation studied between 350° C and 650° C showed the formation of some hydrogen, methane, ethane, and propene. 

The disproportionation (or alkyl exchange) and the alkylation reactions of alkylsilanes have been carried out in a closed recirculation reactor at 373 - 623 K and 373 - 473 K, respectively, by using 100 - 200 mg of catalysts. For the disproportionation reaction, 30 Torr of diethylsilane (E2), diethyidimethylsilane (E2M2), and triethylsilane (E3) were used. For the alkylation reaction, 30 Torr of E2 and 30 Torr of alkylating reagents (propene, 1- and c/s-2-butene, 2-methyl-1-butene, 1,3-butadiene, methylacetylene, ethylacetylene) were used. Cyclic olefins, nitriles, benzene and carbonyl compounds were also tested. 

The six-membered ring (Me2Si)g, originally obtained only in low yieldS is the first of > 100 alkylcyclosilanes synthesized. Among cyclic alkylsilanes the compounds in the permethyl series (Me2Si) are best known. 

The addition of an Si-H bond across a C-C double bond is called hydrosilylation, and this reaction is presented in Chapter 16. The alkylsilane products of hydrosilylation can be converted to alcohols upon oxidation of the newly formed Si-C bond. Thus, desymmetrization of divinyl carbinols by hydrosilylation can generate enantioenriched 1,3-diols. In the presence of a (R,R)-DIOP-based rhodium catalyst, intramolecular hydrosilylation of a 3,5-dimethylphenyl-substituted silane derivative formed the cyclic product in Figure 14.34 in 93% ee. ... 

The hydrosilylation of alkenes produces terminal alkylsilane products. Several examples of these reactions described in Speier s original paper are shown in Equations 16.18-16.22. These examples first show that the terminal anti-Markovnikov products are formed from a-olefins (Equation 16.18). These results also show that linear products are formed from the hydrosilylation of a,S-unsaturated esters with Speier s catalyst (Equation 16.19). Reactions of internal olefins are more complex. Reactions of imsubstituted cyclic alkenes form a single symmetrical product (Equation 16.20). However, as shown in Equations 16.21a and 16.21b, reactions of internal olefins form the same major product as reactions of terminal olefins. This result was corifusing at the time, but the now weU-known isomerization of secondary alkyl complexes to primary alkyl complexes accounts for this result. More details about this isomerization process are given in Section 16.3.5 that covers the mechanism of hydrosilylation. Finally, the silane can affect regioselectivity of the hydrosilylation of alkenes catalyzed by Speier s catalyst. Reaction of dichlorosilane with 2-hexene formed the 2- and 3-alkylsilanes without formation of the terminal alkylsilane (Equation 16.22). ... 

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