Sodium chlorosilanes reaction

Similar reactions can also be written for the alkoxysilanes but in commercial practice the chlorosilanes are favoured. These materials may be prepared by many routes, of which four appear to be of commercial value, the Grignard process, the direct process, the olefin addition method and the sodium condensation method. 

Variables in the condensation reaction include the temperature, nature of the solvent, order of addition (either chlorosilane to excess sodium or "inverse" addition, sodium to excess chlorosilane), and the rate of addition. A careful study of reaction variables by the Sandia group of Dr. John Zeigler(15) will be described in detail elsewhere in this volume. 

The silylmercury compounds are most conveniently available by reacting a chlorosilane with sodium amalgam. In contrast to the two methods mentioned above, which in most cases require polar, aprotic solvents such as THF, DME or HMPA, transmetalation reactions can be performed in nonpolar solvents and often result in better yields than when performed in ethereal solvents. 

The mechanism in Scheme 5.1 can be elaborated further, as shown in Scheme 5.2.17 The reaction of chlorosilanes with sodium probably proceeds by an initial single electron transfer to form an anion radical, which loses chloride rapidly to form a silyl radical. 

Wurtz coupling reactions of chlorosilanes are the main route to the silicon-silicon bonded compounds. For example, hexamethyldisilane can be prepared by refluxing trimethylchlorosilane with lithium sand in THF (97%). Lithium may be substituted by sodium by using a mixture of HMPA-THF as the solvent. Linear and branched oligosilanes can be prepared by the same method (equations 55-57). 

An important end group in silicone chemistry is the acetoxy group the familiar silicone sealants release acetic acid during moisture cure of these acetoxy-stopped polymers. Acetoxysilanes hydrolyze more readily than alkoxy groups. Acylation of a chlorosilane can he accomplished by the addition of sodium acetate or by reaction with acetic anhydride. Other reactions that permit formation of oiganofunctional sflicones ate shown in Figure 3. 

Reaction of a chlorosilane with sodium or potassium. (More severe reaction conditions are often required for silylsodium and silylpotas-sium than for silyllithium.)... 

The fluoro compounds could be obtained by halogen exchange reactions from the corresponding chloro compounds with zinc fluoride in diethylether The acetylene-bridged compound 17 was prepared by reaction of di(/er/-butyl)chlorosilane with sodium acetylide in THF (Eq 2). Remarkably, during such reactions gaseous acetylene was formed, so that the acetylene bridged compounds arose in a one-step synthesis. 

PS-fc-poly(4-f-butylstyrene)]n, (PS-fi-PfBuS) star-block copolymers were prepared by anionic polymerization and sequential addition of monomers with DVB as the linking agent for the formation of the star structure [156]. The functionality of the stars ranged between 10 and 20. Selective sulfonation of PS blocks was subsequently performed using the sulphur trioxide and triethyl phosphate complex in 1,2-dichloroethane, followed by neutralization with sodium methoxide. For this reason DVB was used for the linking reaction instead of chlorosilanes, where a better control could be achieved. DVB stars are more robust and the sulfonation reaction proceeds without cleavage of the arms from the star structure. 

Chloride is determined by mercurimetric titration to the sodium nitroprusside end-point. Carbonates, acetates and borates do not interfere in the titration but large amounts of ammonium salts do interfere. The method has been applied to methyltrichlorosilane, dimethyldichlorosilane, diethyldichlorosilane, phenylmethyldichlorosilane and phenyltrichlorosilane. Methods have been described76,77 based on reaction with amines for the determination of chlorine directly linked to silicon in alkyl aryl chlorosilanes. One method76 is based on the formation of aniline hydrochloride according to the equation ... 

These compounds are best prepared from the chlorosilane and sodium thiolate, or thiol and a tertiary amine. Well-dried yellow lead dithiolates Pb(SR)2 proved to be convenient synthetic intermediates3 but the product has to be washed from the resulting white lead chloride as the reaction reverses on heating (equation 1). 

The reductive coupling of carbonyl compounds with active metals (Na, Mg, Al) yields pinacols. An electron transfer from the metal surface to the carbonyl oxygen (ketyl formation), a soft-soft interaction, is undoubtedly involved. The conversion of esters to acyloins (22, 23) on the surface of metallic sodium is well known. Here the enediolate products can be trapped in situ by Me3SiCl (24). The chlorosilane does not interfere with the coupling, yet it effectively removes the alkoxide ions and neutralizes the enediolate ions immediately on formation. The elimination of RO is imperative, for otherwise Claisen or Dieckmann condensations would compete with the normal course of reaction. These complicating processes require a hard base (e.g. RO ) to abstract a proton from the starting esters, whereas the desired coupling is accomplished by a soft base which is the electrons on the metal surface. 

The general procedure for the synthesis of 9-15 is shown in Scheme 1. Bromobenzene 1 was prepared accotding to Bickelhaupt et al. [3], and the novel bromobenzene 2 was synthesized by the reaction of two equivalents of sodium methane thiolate with one equivalent of 2,6-bis(bromomethyl)bromobenzene. Compounds 1 and 2 were then converted with butyllithium to the corresponding litfaialed species which reacted with one equivalent of either methyidichlorosilane, phenyldichlorosilane or dimethyldichlorosilane to afford the chlorosilanes 3-8. The chlorosilanes were then treated with one equivalent of trimethylsilyl triflate to yield the silyl triflates 9-14. Silyl triflate 10 could also be prepared by the reaction of chlorosilane 4 with one equivalent of triflic add. The silyl tetralds(pentafluorphenyl)borate 15 was synthesized by the reaction of chlorosilane 4 with one equivalent of lithium tetrakis(pentafluorphenyl)borate. Compounds 9-15 are dissodated in dichloromethane solution. 

An advance by Mackenzie has made nickel-ji-aUyl complexes accessible from enals and enones.P In a reaction that is mechanistically analogous to Method 1 in Section 1.1.2.1, enals, when treated with bis(q" -cycloocta-l,5-diene)nickel(0) (2) in the presence of chlorosilanes, afford chloro-bridged dimeric q -aUylnickel complexes such as 25 (Scheme 14). Enones are less reactive in the process and require p5n idine to facilitate the oxidative addition. Rather than using bis(ti -cycloocta-l,5-diene)nickel(0) (2), a more convenient and less expensive alternative involves the in situ reduction of dichlorotetra-lds(p3n idine)nickel(II) (26) with sodium metal in the presence of cyclooctadiene to give enone-derived q -allylnickel complexes (e.g., 27). 

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