Substituted silenes

In agreement with other calculations the effect of methyl substitution is found to be small. However, some substituents such as hydroxy induce quite large geometrical effects. The effect of the substituents on the C=Si bond length is best understood in terms of the inherent polariiy of the C=Si bond shown in 43. Substituents that increase the 

TABLE 22. 3-21G optimized Si=C bond lengths (in A) in the substituted silenes H2C=SiHR (47) and RCH=SiH2 (48)fl 

The effects of the substituents on the thermodynamic stability of each of the isomeric silenes were evaluated by Apeloig and Kami by means of the isodesmic equations 16 and 17, in which the substituent R is transferred from a C=Si double bond to a C-Si (or C-C) single bond. The results are given in Table 24106. The substituent effects are relatively small, usually only a few kcal mol -19 especially when R is bonded to silicon. The largest effects are exerted by R = F (destabilizing by 7.8 kcal mol-1) and SiH3 (stabilizing by 6.0 kcal mol-1) when attached to carbon. 

More recently Morokuma calculated at 3-21G the barriers for dimerization of a variety of substituted silenes and the results are given in Table 25179. Although the calculations are very approximate, as the structures of the transition states were not optimized and various assumptions and simplifications were introduced, the trends in Table 25 are expected to hold also when more complete calculations become possible. 

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Generally, only simple silenes having small groups (H, Me, CH2=CH) are obtained as transient species from the thermolysis of silacyclobutanes. In part this is due to the high temperatures (usually above 450°C) required for the ring cleavage. Substitution on the carbon atom adjacent to silicon in the ring can lead to carbon-substituted silenes. 1,3-Disilacyclobutanes do not readily revert to silenes under thermal conditions, but examples... 

Another example of ring closure involving a 1,5-H shift appears to be that provided by Jung,119 who reported that the heteroatom-substituted silenes 144 rearranged to give 1,3-disilacyclobutanes 145 via a diradical intermediate (Eq. 51). When R = Cl the yield was 30%, and with R = MeO the yield of the disilacyclobutane was 44%. 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

When the chlorine atoms in silene 2 are substituted by other ir-donors the ability to partake in [2+2] cycloaddition reactions is conserved. This has been proved by studies in our group on the reactivity of amino substituted silenes of the type [11] ... 

A similar reaction of a silicon-amino substituted silene [11] supports this mechanism. The migration of a trimethylsilyl group from the Si-amino substituent to the nucleophilic carbon atom of the Si=C moiety leads to the diazasilacyclopentane 30. 

All results so far obtained prove the high synthetic potential of differently substituted neopentylsilenes. Especially silicon dichloro substituted silenes are useful for the preparation of a wide variety of new silacyclobutanes and -butenes. These SiC four membered ring compounds can be utilized as pre-... 

The products arising from the reaction of 431 with the alkyl-substituted silenes 149 and 150 suggest that the reaction occurs by a radical pathway, initiated by a homolytic Si—C bond cleavage of 431 and subsequent Si—Si bond formation giving the biradical 434. Intramolecular disproportionation of 434 gives 435, while 436 and 437 are the results of ring closure reactions without or with expulsion of tetramethylethene, respectively (equation 139)181. 

The [47r(ketone)-27T (silene)] reaction mode is dominant for the alkyl substituted silenes 149 and 150 The initially formed product is 479, which isomerizes in the dark in a non-reversible reaction to the siloxetane 480 (equation 159)235. 

Thus, the alkyl-substituted silenes of the family (Me3Si)2Si=C(OSiMe3)R give with triphenylimine the unstable 514 which is converted completely, faster upon photolysis or more slowly in the dark, into the silaazetidine 515 (equation 174). For the adamantylsilene 150 the complete conversion from the acylpolysilane to 515 (R = 1-Ad) requires 5 days and proceeds in an overall yield of 94%248. The mesitylsilene 349 forms no [4 + 2] cycloadduct, and the only product of the reaction with triphenylimine detected after 24 h is the silaazetidine 515 (R = Mes)248. The imine component also influences the product distribution of the reaction. For example, no [4+2] cycloadducts are formed in the reaction of silenes (Me3Si)2Si=C(OSiMe3)R with A-fluorenylidineaniline and only silaazetidines have been detected248. 

The UV absorptions of the tri- or tetrasubstituted silenes are red-shifted compared to the parent silene 2 (X = 258 nm) but occur at shorter wavelengths than in the highly substituted silenes of Brook (X = ca 340 nm). The introduction of trimethylsilyl groups on the carbon atoms results in slight red-shifts just as the conjugation of the silene system with carbonyl groups. 

TABLE 12. Absolute rate constants for reaction of 1-methyl- and 1-phenyl-substituted silenes (RR Si=CH2) with MeOH and acetone in hexane or isooctane solution at 23 - 25 °C... 

A few years later, Apeloig and Kami reported ab initio calculations of the structure, frontier molecular orbital energies and Mulliken charge distributions in a series of substituted silenes of structure RHSi=CH2 and H2Si=CHR, which indicated that all four of the substituents in the Brook silenes contribute to the reduction in Si=C bond polarity that was suggested to give rise to their unusual stability58. Their systematic study led to the conclusion that Si=C bond polarity is reduced by the presence of [Pg.995]

The absolute rate constants for reaction of the carbon-substituted silenes 2b, 6, 7, 23 and 85a-c are plotted against the resonance substituent parameter o 86 in Figure 10. The data appear to correlate reasonably well, if one assumes that the curvature in the plot results from the fact that MeOH for the more reactive derivatives in the series approaches... 

In contrast, the rate constants for methanol addition to the series of silicon-substituted silenes 2a-i (Table 13) do not vary in a straightforward way with either inductive io ) or resonance (o ) substituent parameters associated with the R substituent. However, a multi-parameter fit of the data to equation 64, in which Es is the steric substituent parameter of Unger and Hansch122 and p, pr and ps are the related standard reaction constants describing the individual effects of inductive, resonance and steric effects on the rate (and are the variables in the analysis), led to an excellent least-squares fit of the data (r2 = 0.965). This afforded the coefficients pr = —3.6 1.2, pi = 3.1 1.0 and ps = 0.21 0.08, where the quoted errors represent the 95% confidence limits of the analysis. Figure 11 shows a plot of the data against the function obtained from the least-squares fit (equation 65). 

FIGURE 12. Plots of the corrected rates of reaction of (a) silicon-substituted silenes and (b) carbon-substituted silenes vs. substituent parameter functions, accounting for diffusion effects on the overall rate constants... 

Fig. 11. Calculated pyramidalization angles at the substituted (Or) and unsubstituted (Oh) silicon atoms in mono-substituted silenes, H2Si=SiHR (6-3 IG //6-3 IG )...

Encounters between silyl radicals in solution or in the gas phase usually result in recombination and disproportionation (45, 46). Disproportionation results in the production of silanes and highly reactive silenes. The disproportionation reaction is thermodynamically favorable because of the formation of a silicon-carbon double bond, which, although subsequently chemically reactive, is worth —39 kcal/mol (44). For pentamethyldisilanyl radicals, disproportionation is kinetically competitive with radical dimerization (46). In an earlier study, Boudjouk and co-workers (47) demonstrated conclusively by isotopic substitution and trapping that the silyl radicals generated by photolysis undergo disproportionation, as well as, presumably, dimerization (Scheme I). In deuterated methanol, the silanes produced were predominantly undeuterated, whereas methoxymethyldiphenylsilane was extensively deuterated in the a position. The results of these experiments strongly implicated the substituted silene produced by disproportionation. 

TABLE 26. Calculated barriers (6-31G //3-21G) for the addition of HC1 and H20 to substituted silenes, and the calculated n and n energies (eV) and charge densities of the substituted silenes"... 

R = CH3) is small, and so also is the effect of silyl183. The calculated barriers are 42.2 kcal mol -1 in H2C=SiH2, 43.5 kcal mol"1 in H2C=SiH(CH3) and 42.8 kcal mol -1 in H2C=SiH(SiH3), all at the MP3/6-31G //6-31G level of theory183. The barriers may be smaller with substituents such as OH, which stabilize the silylene strongly, thus making equation 18 more exothermic. However, the isomerization barriers are still expected to be substantial so that at low temperatures substituted silenes are predicted to be kinetically stable towards 1,2-hydrogen shifts. Systematic studies on the effect of substituents on these isomerization barriers are needed for more quantitative estimations. 

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