Silylene

No attempt is made to include every known reaction of the three silylenes rather, we hope to describe in this chapter, in a systematic and critical manner, our understanding of how inorganic silylenes participate in reactions of fundamental interest. 

In view of the fact that silylene, SiH2, is the simplest divalent silicon species, the variety of reactions that have been studied is surprisingly small. Effort has so far been concentrated on the study of its reaction mechanisms. 

Although the existence of SiH2 was first postulated in pyrolysis studies on silane and higher silanes long ago (50), the evidence for the formation of silylene has been controversial (50, 81, 84). A balanced view on this matter is that while the possibility of a primary process leading to the formation of silyl radicals cannot be ruled out (84), the involvement of SiH2 in these pyrolysis reactions is generally accepted (50, 81). 

The controversy of silylene vs. silyl radical exists in almost every experimental preparation of silylene. For example, in the fast-neutron 

The most studied reaction of silylene is the insertion reaction into silicon-hydrogen bonds. It was first postulated on the basis that higher silanes were generally formed in pyrolysis of silane, disilane, and trisilane (74, 81, 98). If silylene is the major intermediate, the higher silanes are formed by insertion reactions of the type 

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However, Bu3SnSiMe2H (154) reacts with terminal aikynes to give the 3,4-disubs tituted silacyclopenta-2,4-diene 156 at room temperature. The dimetliyl-silylene 155 is an intermediate[80]. (Me3Sn)2 undergoes facile addition to aikynes to give the 1,2-distannylalkene 157[8I-83],... 

Atomic carbon deoxygenates oxiranes (Scheme 37) (B-71MI50501, 76JA3820). Silylenes can also deoxygenate oxiranes (Scheme 38) (80JA1451). 

Three different silylene derivatives were used to achieve selective protection of a rnore hindered diol during a taxol synthesis. Treatment of the silylene with MeLi opens the ring to afford the more hindered silyl ether. 

In the vast majority of its compounds Si is tetrahedrally coordinated but sixfold coordination also occurs, and occasional examples of other coordination geometries are known as indicated in Table 9.2 (p. 335). Unstable 2-coordinate Si has been known for many years but in 1994 the stable, colourless, crystalline silylene [ SiNBu CH=CHNBu j, structure (1), p. 336, was... 

Direct spectroscopic studies and calculations of cyclic silylene-to-silene and germylene-to-germene interconversions 99IZV2027. 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

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... 

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

Donor free silylene complexes are reactive intermediates in a variety of chemical reactions. In many cases, evidence for the coordinated silylenes involved has been obtained indirectly by means of trapping experiments [49-60]. 

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

A variety of further reactions are known in which silylenes are transferred to a substrate in the presence of a transition-metal catalyst. In most cases, silylene complexes can now be identified as reactive intermediates. For a detailed discussion refer to Sects. 2.5.3 and 2.5.4. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

Does a Silylene-Complex exist This rhetorical question is the title of a theoretical paper published in 1983 [84], As a result of an ab-initio calculation, the authors came to the conclusion that a moderately positive answer can be given. However, silylene complexes are thermodynamically less stable than carbene complexes (the MSi bond energy for the hypothetical complex (OC)5Cr = Si(OH)H is 29.6 kcal/mol, the bond energy of the MC bond in (OC)5Cr = C(OH)H is 44.4 kcal/mol) [85], and therefore silylene complexes should be difficult to isolate. 

In 1987 a major breakthrough was achieved when two research groups independently succeeded in the synthesis of monomeric silylene complexes in the form of stable base adducts [35-38]. 

The synthetic approach to silylene complexes (Eq. (2)) is versatile and allows a high variability of both the metals and the substituents at the silicon. A whole series of compounds with bulky substituents like 1 -adamantyloxy, 2-adamantyloxy, neopentyloxy, triphenylmethoxy or f-butylthio could be prepared (Table 1). Compounds with sulfur at silicon are particularly interesting however, their synthesis proved to be very difficult. 

Silylene complexes are not only stable with donor substituents but also with simple alkyl residues at silicon. These alkyl complexes still have a sufficient thermodynamic stability, but otherwise are reactive enough to allow a rich and diverse chemistry. Particularly the chlorocompounds 7 and 11 are valuable starting materials for further functionalization reactions the details of these reactions will be discussed in the forthcoming sections. The data for the known compounds are summarized in Table 1. 

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

Considering the interesting bissilylene complexes, the question arises how many silylenes can be coordinated to a transition metal. In the series of stannylene complexes, a maximum coordination number of at least 3 could be established. 

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

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