Silylene chemistry, review

In what follows, emphasis will be placed on developments of the past decade, with sufficient (but brief) summaries of previous work and references to earlier reviews and seminal papers to allow this chapter to function as a stand-alone unit. We attempt to place silylene chemistry in a broader context and to interpret as well as describe important discoveries. 

Reviews have been presented of inorganic silylenes6, silylene chemistry in the gas phase7 and sterically encumbered silylenes8. 

In writing this chapter, the authors have been much impressed by the tremendous strides made since the silylenes were last comprehensively reviewed, more than a decade ago. Here we shall attempt to look ahead, and predict some of the discoveries concerning silylenes which may take place during the next ten years, foolhardy as this exercise of imagination may seem. We ask then where is silylene chemistry likely to advance in the future ... 

Isoelectronic with monovalent group 13 molecules are divalent molecules of group 14 elements see Germanium Inorganic Chemistry Silicon Inorganic Chemistry and Silicon Organosilicon Chemistry). The most populous class of these species are the silylenes, divalent compounds of silicon, ft is not appropriate here to provide a detailed review of silicon chemistry since such material is more expertly and comprehensively covered elsewhere. However, it is pertinent to this discussion to make some mention of silylene chemistry. ... 

Gaspar, P. P. In Reactive Intermediates Jones, M. Moss, R., Eds. Wiley New York. Keeping up with silylene chemistry is made easy by these critical comprehensive reviews every two or three years by Gaspar. 

The chemistry of the polysilane (polysilylene) high polymers is undergoing rapid development. These polymers have unique photophysical properties of intrinsic scientific interest and are acquiring technological applications in several areas. Poly(silylenes) were reviewed in 198610, and several useful chapters on polysilanes appear in a recent monograph83. 

Silicon and germanium also exhibit carbene-like chemistry. A review comparing carbene and silylene chemistry was published. All the examples of germylene chemistry are from theoretical studies of their structure and their interaction with alkenes. A practical demonstration of silylene chemistry was reported. Ab initio calculations have been performed on the addition of silylenes to alkenes and they shed new light on the presence of intermediates, the structure of the transition state and the effect of substituents. ... 

Many of the fundamental reaction patterns of disilenes, except for thermal dissociation into silylenes, have been described in the previous reviews,2-5 but ongoing studies on the reactivity of disilenes continue to reveal a rich and diverse chemistry. 

The chemistry of silylene-metal complexes has developed in quite another direction, however, from reactions of disilyl-metal complexes, leading to complexes of otherwise unstable disilenes such as Me2Si=SiMe2. Molybdenum and tungsten complexes have been particularly well investigated by Berry and co-workers,103 and platinum complexes have also been isolated.104 Readers interested in this field are directed to a 1992 review of silylene, silene, and disilene-metal complexes.105... 

Review on silylenes P. Caspar, R. West, in The Chemistry of Organosilicon Compounds Z. Rappoport, Y. Apeloig, Eds. John Wiley and Sons. Chichester, 1998 Vol.2, Part 3, pp 2463-2567. 

The chemistry of bis(amino)silylenes, particularly of 2 and 3 and to a lesser extent 4, has been studied in some detail and has relatively recently been comprehensively reviewed " hence only an outline of these researches will be featured in Sections... 

Silylenes are, in a certain sense, the younger but bigger siblings of car-benes, and their chemistry has been reviewed several times.1 In 1978, Gaspar asked the question Are we to conclude from our present knowledge that nature has designed silylenes to humbly mimic carbenes, or is the situation different 13 Today, we know that silylenes and carbenes have a number of properties in common, but also show significant differences. 

The past ten years have seen extensive development of the chemistry of silylene-transition metal complexes. Since this topic has been thoroughly treated in several recent reviews by Zybill and coworkers330-332 and by Tilley333,334, who have carried out much of the fundamental work, this active area will only be summarized here. 

The results described above clearly reflect the diversity of the possible reactions of acyclic disilenes. The 1,2-aryl migrations in tetraaryldisilenes have not been mentioned as they have already been reviewed in depth6. The same is true for the thermally induced decomposition of the disilene 35 to furnish the silylene molecule Tbt(Mes)Si , the unusual chemistry of which is reviewed elsewhere in this series129. 

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. 

The chemistry of sUylene complexes has become much more developed in the past 10 years as silylene complexes have been found to be important intermediates in reactions snch as the dehydrogenative conphng of hydrosilanes, redistribntion reactions, and the Direct Process for the prodnction of simple chlorosilanes. For reviews of this work, see References 43, 44 and 294. Several complexes have now been prepared in which there is a sUene or disilene ligand (R2C=SiR2 or R2Si=SiR2 species) bnt, since all these contain several Si-C bonds, they will not be discnssed further here. 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored. ... 

Silylenes have attracted strong interest of both experimentalists and theoreticians and their chemistry has been reviewed1 3,262. The theoretical studies of silylenes have been reviewed most recently by Gordon and coworkers22. 

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