Disilenes properties

Finally, the chemical properties of the new structures available from disilene addition reactions have hardly been touched. In future developments, theoretical and experimental studies are likely to proceed together and complement one another, as they have from the beginning days of this research. 

While disilene 5 does not undergo Diels-Alder reactions with 1,3-dienes, the [4+2]-cycloaddition products are formed with heterodienes, e.g. 1,4-diazabutadienes [17] or a-ketoimines [19]. It can be deduced that the electron deficient properties of such dienes cause them to readily take part in hetero-Diels-Alder reactions, which have inverse electron demands. This is corroborated by theoretical calculations which predict an inverse electron demand of the Si-Si double bond it is strongly electron donating rather than electron accepting towards butadienes and other compounds [24,25]. 

In 1981, West et al. synthesized the first stable disilene 1 via the dimerization of the corresponding silylene generated by the photolysis of a trisilane and characterized the structure by conventional spectroscopies [Eq. (2)].5 Availability of 1 and other stable disilenes has stimulated theoretical and experimental studies of various aspects of disilenes such as their bonding and structure, spectroscopic properties, reactivities, applications to the synthesis of novel types of organosilicon compounds, etc. 

Using these methods, 27 acyclic disilenes with the substitution types of A2Si = SiA2, A2Si = SiB2, ( )- and (Z)-ABSi SiAB, and A2Si = SiAB as shown in Chart 1 had been synthesized, at the time when review OW was written. In this article, the numbers for labeling these disilenes are kept to be identical with those used in review OW for convenience. The properties of 47 stable disilenes and two disilynes newly synthesized since then are summarized in Tables I IV. 

Properties of Radicals and Ions Including Disilenes and Disilynes... 

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

Irrespective of whether the bonding situation in the disiliranes is best represented by a partial jr-character98,99, as indicated in 80a, or rather by an extremely arched Si=Si bond100, the disiliranes — mostly derived from disilenes—constitute an interesting class of compounds with unusual properties. 

This chapter reviews the body of literature (to August 2000) that deals specifically with kinetic studies of the reactions of silenes and disilenes and the mechanistic information that has been derived from them. The spectroscopic properties, structures, methods of synthesis and qualitative aspects of the reactivity of silenes and disilenes have been covered comprehensively in the preceding two volumes in this series and elsewhere, and so will not be treated extensively here. 

Many of the results reviewed here suggest that a replacement of the usual alkyl or aryl substituents by silyl substituents in unsaturated silicon and germanium compounds may be rewarding. As we noted, silyl substituents do tilt the properties of silylenes, silyl radicals, and sequential BDE trends toward those in carbon chemistry. They have already been shown to stabilize disilenes with respect to dissociation to two silylenes, and this may be crucial to the further development of digermene and distannene chemistry. 

Summary Molecular orbital ab initio calculations were used to reproduce and to predict various properties of silylenes and disilenes ... 

This has provided computational chemistry with a unique opportunity to influence the development of silicon chemistry not only by providing interpretations to experimental findings but more importantly by making predictions and directing future experiments. Indeed in the last decade theoretical calculations played a major and sometimes even a crucial role in the development of silicon chemistry [5]. In this paper we hope to demonstrate the importance of theory for understanding and predicting the properties and chemistry of silylenes and of disilenes. 

Since the spectacular isolation of the first stable disilene by West, Fink, and Michl in 1981 [19] this field has enjoyed a surge of activity [3]. Yet, despite the many important discoveries that have been made, the study of compounds containing double bonds to silicon is still in its infancy. In particular, relatively little is known on the effect of substituents, especially heteroatom substituents, on the properties of disilenes as only aryl-, alkyl-, (Me3Si)2N-, and MeaSi-substituted disilenes have been isolated to date [3]. With experimental progress being relatively slow, ab initio calculations provide a reliable, fast and economical method to extend our knowledge on these novel compounds. 

Calculations (mostly at the correlated MP3/6-31G level) of mono-substituted disilenes, RSiH=SiH2, and disubstituted disilenes, RHSi=SiHR, where R spans over the entire range of first-row substituents, i.e., Li to F (H3Si was also included), reveal many interesting and unique properties of the Si=Si bond... 

Since the first isolation of tetramesityldisilene by West et al. in 1981, a number of stable disilenes have been synthesized, and their structures and properties have been extensively investigated up to date [1,2]. However, known types of stable disilenes had been limited to simple acyclic derivatives with one Si=Si double bond until 1996, when the first stable cyclotetrasilene was synthesized [3]. Recently, the number of known skeletal types of disilenes has increased remarkably, and studies of their structures and reactions have made astonishing progress. The chemistry of disilenes is now approaching a stage to be compared with the extensive and profound chemistry of olefins. Here, we discuss the synthesis, structure, and reactions of novel cyclic and bicyclic disilenes and a trisilaallene, the first compound with a formally sp-hybridized silicon atom. 

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