Formation of Silylenes

Polysilanes with alkoxy groups are more light sensitive than conventional polysilanes. They degrade rapidly in the presence of light in agreement with the facile formation of silylene from dialkoxydisilanes. Properties of these polymers are currently being investigated. 

Decomposition with the elimination of arsenic or phosphorus ylide and formation of silylene (germylene, stannylene) Me2E14, which represents the process inverse to the synthesis of these betaines. 

In addition, no formation of silylenes has been observed to date39. The differences in photochemistry are attributed to the lower energy of Ge—Ge and Ge—C bonds relative to Si—Si and Si—C bonds. 

The product 33 (Amax = 345 nm) was formed as a film (estimated by the optical density of the UV spectrum to be several tens of nanometres thick) on a target quartz plate, the surface of which had been treated with 10 wt.% NaOH solution, followed by dichlorodimethylsilane and finally LiAlH4 to afford a surface terminated with -OSiMe2H groups. On the basis of trapping experiments (to confirm the formation of silylenes) and photo-CVD in the presence of styrene (to confirm the absence of radical species), a mechanism based on silylene formation and insertion into the surface Si-H bond was shown to be likely, as outlined in Scheme 19. 

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. 

Only few theoretical studies have been devoted exclusively to the coordination of Lewis bases to silylenes. Most calculational evidence for the formation of silylene-Lewis base complexes was obtained from the computational investigation of the insertion reaction of HiSi into various H-X a bonds, where X is an heteroatom center possessing one or more free... 

This section presents reactions that proceed via the formation of silylene-Lewis base complexes. The material is organized according to the method used for silylene generation. 

Tamao et al,83 found that a higher coordinated silylene 119 can be formed from penta-coordinated silane 118 (Scheme 31). Warming a solution of 118 in toluene or dimethylformamide in the presence of diphenylacetylene or 2,3-dimethyl-l,3-butadiene resulted in the formation of silylene-trapping products 120 and 121. Interestingly, no 1 1 reaction product between the silylene and the acetylene was isolated. Thus, it must be concluded that the insertion of silylene 119 into a Si-C bond of initially formed silacyclopro-pene is faster than the addition to the triple bond of the acetylene so that the silacyclopropene cannot be isolated under the reaction conditions. 

A key report investigated a variety of substrates in their reaction with silicon in an effort to find evidence for silylene intermediates during the silicon direct process reaction. When silicon, copper and methanol were reacted as described above but in the presence of alkenes, alkyldimethoxysilanes and (MeO SiH were formed95-97. The use of allyl propyl ether instead of alkenes gave allyldimethoxysilane, with 38% selectivity. These results and the reaction of silicon with MeCl in the presence of butadiene to give silacyclopent-3-enes indicates intermediate formation of silylenes. 

Insertion, addition and abstraction reactions of free silicon atoms can lead to the formation of silylenes (equation 9)1-5. Silylene formation for reaction, spectroscopic and... 

As discussed in the previous section, thermal dissociation of disilenes into the corresponding silylenes may occur if the BDE of the disilenes is small. As shown in review OW, a facile thermal dissociation of disilene 27 into silylene 127 occurs at 50 °C [Eq. (49)],61,91 The formation of silylene 127 is evidenced by its trapping by methanol, triethylsilane, and 2,3-dimethyl-1,3-butadiene. The activation enthalpy and entropy for the dissociation of (Z)-27 to 127 are 25.5kcalmol-1 and 7.8 cal mol-1 K-1 respectively.91 The activation free energy for the dissociation at 323 K (22.9 kcal mol-1) is much smaller than that for the Z-to-E isomerization of 26 (27.8 kcal mol-1), indicating that the E,Z-isomerization of 27 should occur via the pathway (2) in Eq. (47) rather than pathway (1) in Eq. (48). 

If the activation energy for the formation of silylenes is smaller than the activation energy for the formation of radicals, a silylene mechanism occurs ... 

TABLE 4. Binding energies (kcalmol-1) for H2E—AH adduct formation of silylene, germylene and stannylene with various AH3 and AH2 units3... 

They are very reactive an important synthetic reaction for which they are used is the formation of silylenes and disilenes ... 

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). 

It is tempting to assume, that the facile formation of silylene 2 from cyclotrisilane 1 is due to the effective stabilization of 2 by intramolecular coordination of the dimethylamino group to the silicon centre [10], which should lower the activation energy of a dissociation process from 1 to 2. Reaction of 1 with benzylvinylether resulted in a complex reaction mixture, from which 12 % of vinylsilane 4 was isolated 4 is presumably formed by rearrangement of the unstable oxy-substituted silacyclopropane 3c. 

Silylenes Divalent, dicoordinate silicon compounds, are the silicon coimterparts to the carbenes well known in organic chemistry. Since silylenes are frequent intermediates in both thermal and photochemical reactions, their importance in organosilicon chemistry is great [1]. There is recent evidence that even the direct reaction of methyl chloride with silicon, the foundation stone of the worldwide silicone industry, may proceed through the formation of silylene intermediates [2]. 

The formation of silylenes and silaethenes by dehalogenation of dichloro- and chloromethyl-chlorosilanes was previously described using the reaction of Na/K vapor in the gas-phase at 300 °C [1]. In the presence of butadiene the reaction with RR SiCl2 leads to silacyclopentenes. 

The presence of very reactive intermediates is obvious from the formation of a variety of non identified by-products. However, the reactions with DMB as the diene substrate provide evidence for the formation of silylene and silaethene intermediates. Further investigations are necessary to elucidate the reaction pathways. 

The lower heats of formation of silylenes compared with carbenes translate into somewhat lower, albeit similar, reactivity. There is a general lesson here that slowing down the reactions of carbene-like species makes possible processes like dimerization that are more or less impossible for carbenes. We have already seen that slowing down their rearrangements allows the observation of intermolecular reactions of alkylsilylenes not observed for alkylcarbenes. The dimerization of silylenes to disilenes, discovered in the gas phase [36], was employed to brilliant effect by West and coworkers in the condensed phase synthesis of isolable disilenes [37]. While these were not the first disilenes, silylene dimerization opened the way for the preparation of many new molecules, including digermenes from the dimerization of germylenes [38]. 

Another type of reaction allowed us to study the philicity of silylene 4. As we have shown earlier, 4 adds smoothly to a variety of alkynes giving way to the silacyclopropene framework (Eq. 4) [10], Varying the / ara-substituents of diphenylacetylene offers us the possibility to tune the electron density of the triple bond, and we now studied the rates of the reaction of 3 with diphenylacetylenes lOa-c. The absolute reaction rates of these three first-order reactions are identical (Ai = 6.3 0.2 10 4s-l) this result is in accordance with a mechanism, in which the formation of silylene 4 from cyclotrisilane 3 is the rate determining step [6], However, the relative reaction rates of the addition of 4 to the triple bond of lOa-c, which were determined by competition experiments, turned out to differ appreciably from each other. Electron withdrawing substituents favor the addition of 4 to the alkyne, whereas electron donating substituents, such as a methyl group, slow down the reaction rate. As shown in Fig. 5, the relative reactions rates correlate well with the [Pg.62]

For some recent reviews on the formation of silylene-transition metal complexes, see T. D. 

In Sect. 2.3, generation of silylene complexes of transition metals was discussed on the basis of the reactivity of disilanyl-transition-metal complexes. The formation of silylene species in the presence of a catalytic amount of transition metals is also involved in the reactions of hydrodisilanes, which may readily form disilanyl complexes through oxidative addition of the Si-H bond prior to the activation of the Si-Si bond. Platinum-catalyzed disproportionation of hydrodisilanes affords a mixture of oligosilanes 89 up to hexasilane (Eq.45) [83]. The involvement of silylene-platinum intermediate was proven by the formation of a l,4-disila-2,5-cyclohexadiene derivative in the reaction of the hydrodisilane in the presence of diphenylacetylene. 

On the other hand, it is unlikely that methylene is formed from methane or other hydrocarbons in the pyrolysis of coke. In contrast to methylene, its analog silylene is a product of the pyrolysis of silane and disilane (8-17 Purnell and Walsh, 1966 Bowrey and Purnell, 1970). Laser-induced fluorescence was used to study the formation of silylene and its reactivity (see, e.g., Baggott et al., 1988 Jasinski and Chu, 1988), but silylene is not within the scope of this book as there are no diazo compounds involved in its chemistry. Literature in which the reactivity of silylene is compared with that of methylene is reviewed briefly in a publication of Skancke (1993, p. 640). 

Obviously, the first step of the reaction of R SiX3 with NaR in THF consists — according to Scheme 1 and Ref. [7] — in an exchange of halogen against sodium. The monoanions formed are metastable at -78 C, but eliminate NaX at about -50 °C with the formation of silylenes R SiX (8), which may then insert into the Si-Na bond of their precursors several times to build a silicon chain. The di-, tri-, or tetrasilanides obtained may transform, by elimination of NaX, into dihalodisilenes R XSi=SiXR (9), cyclotrisilanes (R XSi)3 (10) and cyclotetrasilanes (R XSi)4 (11). 

The redistribution reactions mentioned above indicate that the opening of a coordination site not only promotes oxidative addition and possibly a-bond metathesis reactions, but — difG ndy from carbon compounds — also induces migrations of silicon substituents with the concomitant formation of silylene complex intermediates. It was pointed out by Tilley and co-workers that the transfer of a silyl substituent from silicon to platinum is easier in three- than in four-coordinated... 

Besides the nuclear recoil methods, the formation of silylenes from the reactions of elemental silicon with various halides as described in Section III. 6 can also be viewed as abstraction reactions. Thus equation (12) involves F-abstraction to give SiF2 " equations (13) and (14) involve Cl-abstraction to give SiCl2 and equation (15) involves 0-abstraction to give SiO." ... 

The decomposition method is probably the most significant one for the formation of silylenes. From the fundamental point of view, this method involves a variety of decomposition modes with many different precursor molecules. Even from the practical point of view this method is important because it generally involves relatively less dramatic reaction conditions and can produce silylenes in relatively larger quantities. In the following sections, the classification of decomposition modes is generally according to the type of reactant molecules and the type of reactions involved rather than the kind of silylenes being formed. 

Strictly speaking, the formation of silylenes as described in Sections IV.l.C and III.6 concerning reactions of elemental silicon with substrates can also be classified as reduction methods. Besides these, two other types of reductions to produce silylenes are observed. 

---