Silicon double bond

The classic problem of the silicon-silicon double bond dates back at least to the early part of this century, when F. S. Kipping and his students attempted unsuccessfully to synthesize disilenes. Evidence for the probable transient existence of disilenes began to appear in the 1970s, but it was the isolation of the stable disilene 1 in 19811 that opened up modem disilene chemistry. The early history of this discovery has been recounted in a review2 several other reviews covering Si=Si double bonds have been published.2 5... 

Information on the E-Z isomerization of the disilenes sheds light on the nature of the chemical bonding of the silicon-silicon double bond. Kinetic parameters, which have been reported for cis-trans isomerization of five disilenes, are listed in Table IV. 

To a large extent the chemical shifts of carbon and silicon run parallel, but the chemistry of the two elements is somewhat different. Thus silicon can have extend its valence shell beyond the coordination number of 4. A few stable or-ganosilicon compounds in which silicon is divalent are known (the silylenes), and compounds with a silicon-silicon double bond also exist (the disilenes). 

Apeloig and Kami (13) have also studied the effects of substituents on the reactivity of silenes by the frontier molecular orbital (FMO) approach. They have concluded that, concerning electronic factors, the polarity of the carbon-silicon double bond, and thus the coefficients of the frontier orbitals, play a more important role than the energies of these orbitals in controlling the reactivity of silenes. 

Successes in producing reactive intermediates like o-xylylene and carbene and in preparing bimetal lies in high yields using ultrasound led us to attempt to generate West s novel compound, tetramesityldisilene the first example of a stable species with a silicon-silicon double bond(32). We prepared this species in one step and trapped it with methanol(33). The disilene is reactive towards lithium, however, and we have found it very difficult to obtain consistent results. Most often, hexamesitylcyclotrisilane is isolated in very good yield(34). 

The first evidence for the formation of compounds having a silicon-silicon double bond first appeared in 1969 when it was formed that disilebicyclo [2.2.2] octadienes on heating extruded tetramethyldisilene. 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

An organosilicon compound (tetramesityldisilene) containing a silicon to silicon double bond has been synthesized. It is a crystalline solid, mp 176C, and has reactive properties similar to olefins. Compounds of the type are silylenes. 

West, R, Isolable Compounds Containing a Silicon-Silicon Double Bond, Science, 225, 1109-1114 (1984). 

For the successful synthesis of silicon-silicon double bonds, see ... 

It is clear that introduction of the functional groups to the silicon-silicon double bond causes a large red shift of the absorption maxima of disilenes. Similar substituent effects have been known for a variety of alkenes31. Qualitatively, the reason for the red shift should arise from destabilization of the HOMO of the disilene by an appreciable interaction between the n-orbital of the heteroatoms and the tr-orbital of the disilene, although the effect of their distorted structure should also be considered as discussed later. 

The formation of a metal-silicon double bond is most effective by the salt elimination route . It is accomplished in a one-step procedure reacting the supernucleophilic metal-late dianions with dihalosilanes in polar solvents as HMPA, as shown for chromium in... 

Robert West was bom in New Jersey and educated at Cornell University (B.A.) and Harvard University (A.M., Ph.D.). For the past 45 years he has been a faculty member in the chemistry department at the University of Wisconsin, where he is now E. G. Rochow Professor and Director of the Organosilicon Research Center. His many awards include the Frederick Stanley Kipping Award, the Wacker silicone prize, the Alexander von Humboldt Award, and the main group chemistry medal. He has published more than 600 scientific papers, mostly in the area of silicon chemistry. Major discoveries in his laboratories include the first soluble polysilanes (1978), the silicon-silicon double bond (1981), the first stable silylenes (1994), and electrically conducting organosilanes for high energy density batteries (2000). He is an airplane pilot and a mountaineer, with numerous first ascents in Canada and Alaska. 

The reduction of tri(ferf-butyl)cyclopropenyl-substituted tribromosilane 104 with KCg in 2-methyltetrahydrofuran gives a quite unique disilene 63 having a lattice framework [Eq. (28)].35 Disilene 63 constitutes an A2Si = SiA2 type disilene but has C2-symmetric chirality through the silicon-silicon double bond. 

The authors postulate an intermediate containing a silicon-silicon double bond. Stable products include various disilacyclobutanes. A possible mechanism of formation from the postulated me2Si=Sime2 is suggested by the authors. 

Fig. 3. Some examples of postulated concerted [2 + 2] and [4 + 2] cycloaddition reactions of alkenes with silicon double bonds, as well as alternate descriptions in terms of a [1 + 2 + 1] reaction with silicon free radicals that would be expected to proceed in a non-concerted fashion through the intermediate [1 + 1] adduct shown.

Since the isolation of the first, thermally stable molecular compound containing a silicon-silicon double bond in 19811, the chemistry of the disilenes has experienced an explosive development as reflected in the many review articles published in the meantime2-6. Several novel classes of compounds that are only accessible with difficulty by other routes have become readily available with the use of disilenes as starting materials. 

Preparations of phosphasilenes have been mentioned in Sections II and V. Driess recently reviewed the synthesis and properties of phosphasilenes137. In most of the reported reactions63,130,132,137 of phosphasilenes, cleavage of phosphorus-silicon double bonds occur to give compounds with a P—Si single bond, reflecting the high reactivity of the —P=Si< bond thus they lie beyond the scope of this Chapter. 

Systems which contain cr(E-E)-7r conjugated systems (E = Si or Ge) have been the subject of many photochemical investigations. For the most part investigations have focussed on silicon systems, that first identified carbon-silicon double-bonded species as photoproducts . Later studies also indicated that arylsilanes undergo 1,3-sigmatropic shift reactions as well as extrusion processes yielding silylenes . ESR studies have... 

Notable advances in the last decade are isolation of kinetically stabilized silicon-containing donble-bonded species. As we have seen already, a variety of sterically bulky substituents have been utilized to stabihze silicon-centered reactive intermediates. This kinetic stabilization is the key issue also to isolate silicon double-bonded species. For example, the synthesis of isolable silicon analog of benzene and other aromatic compounds has been a target for a long time. One example is shown below for the generation of transient silabenezene. ... 

In the last years transition metal-silyl complexes have received special attention for several reasons [1, 2], On the one hand, they are assumed to be important intermediates in catalytic processes [2] (transition metal-catalyzed hydrosilylation reaction, dehydrogenative coupling of silanes to polysilanes, etc.), on the other metal-substituted silanes show special properties, which can be tuned systematically by judicious choice of the metal and its ligands [3] Furthermore, silylenes (silanediyls) are stabilized by unsaturated transition metal fragments leading to metal-silicon double-bonds [4]. In the light of a possible application in MOCVD processes some of these complexes are of interest as potential single-source precursors for the manufacture of thin silicide films [5]. 

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