Silylenes matrix isolation

Oxidation of Silenes and Silylenes Matrix Isolation of Unusual Silicon Species... 

W. Sander, M. Trommer, A. Patyk, Oxidation of Silenes and Silylenes Matrix Isolation of Unusual Silicon Species, Organosilicon Chem. Ill, N. Auner, J. Weis, Wiley-VCH 1998, 86-94. 

In the course of this development, knowledge about low valent (in the sense of formal low oxidation states) reactive intermediates has significantly increased [26-30]. On the basis of numerous direct observations of silylenes (silanediyles), e.g., by matrix isolation techniques, the physical data and reactivities of these intermediates are now precisely known [31], The number of kinetic studies and theoretical articles on reactive intermediates of silicon is still continuously growing... 

Recently, a variety of silylenes were generated and characterized by matrix isolation techniques. The observed loose donor adducts between silylenes and the matrix molecules (THF, CO) are only stable at very low temperatures. Melting of the matrix induces polymerization of the silylenes which proceeds through disilenes. However, 0->Si transfer reactions do not occur only in the case of 1-methyl-THF has an insertion of the silylene into the C —O bond been observed [155-158],... 

In view of these facts it is no surprise that matrix isolation studies of silylenes have been carried out with the same intensity as was applied to carbenes. A comprehensive survey on these efforts has recently been published.164 Therefore only one example is given in order to illustrate the close correlation between carbenes and their silicon analogs. Similar to the carbon series S-2 T-36... 

There is much evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as a Lewis acid are now well established (Fig. 14.3, Table 14.2). These complexes are also described as silaylides, R2Si —X+. Formation of silylene complexes with Lewis bases is conhrmed by a strong blue shift of the n-p transition. Matrix isolated dimesitylsilylene reacts with carbon monoxide to form the complex shown in Eq. The complex absorbs at 354 nm. 

Whereas UV spectroscopic data are available for base complexes of a variety of matrix-isolated silylenes (Table IV), solution data are reported almost exclusively for complexes of dimethylsilylene (13) (Table V). Again, the complexation of this silylene is accompanied by a significant blue shift of the absorption maximum. Probably due to matrix effects of unknown nature, the Amax values of dimethylsilylene complexes in solution are shifted to wavelengths shorter than those of the corresponding matrix-isolated complexes. 

Nefedov, Michl and coworkers reported the successful spectroscopic characterization of the l-silacyclopenta-2,4-diene 287 and its photochemical transformation into the isomeric species 288-290 (equation 71)159. Caspar and coworkers160 had suggested that silole 287 might be formed by a rearrangement of the cyclic silylene 288 via the 1-silacyclopenta-1,3-diene 289. This proposal was confirmed in an elegant UV-visible, IR-matrix isolation... 

The parent silaethylene 2 could be matrix-isolated by our group in 198117 by pyrolysis of the bicyclic system 1 (equation 1). Later, a new entry into this system was found by photolysis of methyldiazidosilane 318 (equation 1). This simplest representative 2 shows already most aspects of the chemistry of the whole class of silenes, e.g. the photoisomer-izability into the corresponding silylene (see Section VUI). 

Whereas the analogous carbenes easily isomerize wherever possible to compounds containing doubly bonded carbon atoms even under the conditions of matrix isolation, silylenes are almost as stable as the corresponding substances with doubly bonded silicon atoms. For example, methyl- and silylsilylene lie just 4 and 8 kcalmol-1 above silaethene and disilene, whereas the difference between ethene and methylcarbene is as high as 70 kcalmol-1 149-151 As a consequence, silylenes are often key intermediates on the way to other highly reactive silicon compounds discussed above. 

A relatively long known access to silylenes is the thermolysis of disilanes13, used for preparative purposes180 as well as for matrix-isolation studies of silylenes107. In our group this method was recently used in the generation of C2H2Si isomers. Trimethylsilane... 

The observed product distributions reflect the influence of both factors. In the parent compound 102 and in the silyl substituted 104, the initially formed open-chain silylenes 103 and 146 are relatively sensitive and the migration tendency of the group R is quite large, and therefore the silacyclopropenylidenes 104 and 145 are found. In contrast, the methyl and the ethynyl compound 138 and 150 are stable enough to pass the pyrolysis tube and both can be matrix-isolated upon pyrolysis of the appropriate precursors 137 and 149. Due to the small migration tendencies of the methyl and the ethynyl group, the rearrangement product 135 is not found at all and 151 is detected only in relatively minor amounts. 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

The members of this inorganic subclass of silylenes were matrix-isolated much earlier than their organic counterparts. Dihalosilylenes are accessible in several ways. 

Some of the reactions given in the sections above are important routes to reactive organosilicon intermediates such as silenes and silylenes, and because of their tendency to dimerize readily, to disilenes, the latter being formed when matrix-isolated silylenes are warmed up. It appears worthwhile to summarize some of the more useful reactions leading to silenes and silylenes, and their subsequent behaviors. The topics of silylenes97,142 and disilenes97,142,143 have recently been reviewed. 

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