Disilenes stability

The overall procedures of the methodology are shown in Scheme 11.1. The first reaction depicts the synthesis of masked disilenes of representative structures and the second is a conventional anionic polymerization such as might be applied in the synthesis of polystyrene. Masked disilenes are so-called as they are to all intents and purposes disilenes stabilized by a biphenyl bridge. During the polymerization reaction the bridge is released. Product polymers have been reported to have molecular weights up to 50,000 with... 

The chemistry of the disilenes (disilaethenes) has developed very rapidly since the discovery of stable compounds. It was an obvious challenge to explore also the possibility of a n-coordination of disilenes to transition metals. According to the Dewar-Chatt-Duncason bonding model, a high stability for a disilene complex should result. 

However, it was about 8 years after the first synthesis of tetramesityldisilene before stable coordination compounds became known. The main reason for this is the kinetic stabilization of the known disilenes by bulky substituents, which effectively prevents the coordination of the double bond to a metal fragment. Thus, a direct coordination of stable disilenes appeared to be reasonable only if metals with very low coordination numbers were used. 

Photolytic [4+2] cycloreversion of a disilabicyclo[2.2.2]octadiene precursor is considered to be a general method for the synthesis of disilenes of varying stability. Several examples of this method have been reported (Eq. 9).27-31 First used in reactions producing an unstable disilene, Me2 Si=SiMe2,27 this method is also successful for synthesizing the marginally stable -Bu2Si=Si(t-Bu)2,28 However, it has not yet been applied to the synthesis of disilenes that are fully stable at room temperature, probably because the appropriate precursors are inaccessible. 

It is an intriguing idea to stabilize low-valent silicon species, such as silyl cations, silylenes, silenes, and disilenes using intramolecularly coordinating ligands. Corriu et al. succeeded in the preparation of the first hypervalent silyl cation [(8-Me2NCioH6)2SiH]+l/2[l8]z 782 by the reaction of the hexacoordinated diorganosilane... 

Thus, with an increase in the number of silicon atoms in a cyclic structure, analysis, in terms of the aromaticity concept, of the structural types and stability loses its prognostic value. A striking example of this is given by the pyramidalization of the Si atoms, typical of disilene, in hexasilaben-... 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

Thermal E-Z isomerization was also observed for kinetically stabilized disilenes 5-9 (equation l)19-22. The n bond strength was estimated to range from 24.7 to 30.6 kcalmol-1. These data are in good agreement with those of 1,2-dimethyl-1,2-diphenyldisilene and those predicted by ab initio calculations for H2Si=SiH2 (22-28 kcalmol-1)23,24. 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

The most spectacular properties of silylenes such as 83-90 are their thermostability and their stability toward dimerization reactions leading to disilenes. 

The [2+2] cycloaddition of the Si=Si double bond of disilenes across a hetero double bond belongs to the most typical reactions for the preparation of disiletanes. Reaction of the supersilyl stabilized disilene 90 with PhHC=0 and Ph2C=S gave oxa- and thiadisiletanes 91 and 92, respectively (Scheme 15). The use of heterocumulenes 0=C=0 and 0=C=S in a similar cycloaddition reaction yielded oxa- and thiadisiletanes 44 and 31. The isolated disiletanes are colorless and oxygen, water, and thermostable compounds <2002CEJ2730>. 

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