Disilanes cleavage

Silyl Anions (RsSE"). For years aryl substitution was thought to be necessary for the formation of silyl alkali compounds. This is not the case, and trialkylsilyl alkali metal compounds are now readily available. The most general and convenient method of generation is disilane cleavage. For example ... 

Methylchlorodisilanes are by-products of the dkect-process residue, commonly called high boiling point residue or simply residue, and are formed in about 4% of the total (CH2)2SiCl2 produced, which in 1994 was about 30,000 tons per year. Disilanes are key constituents of the residue, and novel reactions forming Si—Cl bonds have been described (44,45). Some chemical reactions of dkect-process disilanes are shown in Figure 2. Cleavage chemistry of Si—Si compounds with HCl practiced industrially has also been described (47). 

Early work on the chemistry of organosilyl anions/anionoids has been thoroughly reviewed (/). The most frequently employed preparative routes involve either cleavage of a disilane, when HMPA (CAUTION—CANCER SUSPECT AGENT) is normally required as solvent, or reaction of bulky silyl chlorides with lithium metal. 

The photochemistry of benzyl-substituted digermanes is thus basically the same as that observed for vinyl- and styryl-substituted digermanes reported in Section in. In comparison, the equivalent disilane compounds exhibit only Si—Si bond homolysis, with no Si—C cleavage or silylene formation39. 

Cleavage of the Si—Si bond of a disilane with nucleophiles such as organolithium compounds or alkali metal alkoxides or hydrides (equation 1) ... 

A silyl compound with two different alkyl substituents, (t-Bu)2MeSiK, was prepared by Ftirstner and Weidmann in low yield by the cleavage of the Si—Si bond of (t-Bu)2MeSi—SiMe(t-Bu)2 with CsK20. This reaction is quite exceptional, as usually the Si—Si bond of peralkylated disilanes is not susceptible to reductive cleavage by alkali metals. 

Reductive cleavage of disilanes by alkali metals is the most versatile method to prepare metalated arylsilanes, which carry, in addition to the aromatic group, also an aliphatic, benzylic or allylic substituent. Table 1 reports all the metalated arylsilanes which have been prepared by reductive cleavage of disilanes with alkali metals. 

TABLE 1. Metalated arylsilanes prepared by reductive cleavage of disilanes with alkali metals... 

It may be concluded from the results of Scheme 5 that steric effects are responsible for the removal of the peripheral Me3Si group. However, the cleavage of unsymmetrical disilanes such as MesSi—SiPh3, Me3Si—SiMePh2 or MesSi—SiMe2Ph by potassium... 

The first metalated silole, 48, which was characterized unambiguously by means of NMR spectroscopy, has been obtained by Boudjouk and coworkers via reductive cleavage of the Si—Si bond of disilane 47 with lithium or sodium under ultrasonic activation (equation 54)110a. 

In contrast to these experimental and computational results, lithiated 1-silafluorenide 50, which was prepared by the reductive cleavage of the central Si—Si bond of disilane 49 with lithium under ultrasonic activation, provides some evidence for the existence of localized metalated siloles (equation 55)111. Thus, upon metalation of 49 to form 50, a highfield shift of the 29Si nucleus (AS = —47.9 ppm) is observed. In addition, the chemical shifts of the phenyl carbons indicate that there is no accumulation of tt electron density, which would be expected for a delocalized lithium silafluorenide111. 

A number of disilanes have been investigated for their suitability as potential silene precursors. The formation of silenes by the 1,3-silyl migration pathway competes with dehydrosilylation, which also gives silenes, and with homolytic Si—Si bond cleavage to give silyl radicals. 

The 1,3-silyl shift in aryl disilanes is suppressed when the aromatic ring is ortho-substituted144. An attempted silylene synthesis from 1,3-dimesitylhexamethyltrisilane 259, however, led to low yields of silylene trapping products (ca 30% generation of Me2S ). The major pathway is the homolytic cleavage of the trisilane, followed by disproportionation of the radicals 260 and 261 to the silene 262 and the disilane 263 (equation 65). 

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. 

Homolytic cleavage of the Si—Si bond, followed by homolytic aromatic substitution, was also invoked to explain the photochemical reactions of 1,l-di( 1 -naphthyl)- 117, and l,2-di(l-naphthyl)-tetramethyldisilane 11858 which yielded 119 and 120, respectively, as the main reaction products (Scheme 17). A minor reaction pathway of the latter disilane involved dimethylsilylene expulsion. 

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

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