Disilenes Tetraaryldisilenes

This method proved useful for the synthesis of a disilene having different substituents on each silicon atom, although it was successful only for a tetraaryldisilene.15 Reduction of the chlorine to hydrogen gave 1,2-dihy-drodisilane when R was i-Pr or f-Bu.26... 

CH=CH2)2, which was taken as a model for tetraaryldisilenes.47 While these values are all higher than those for tetraccordinate silicon, they are scattered and not very close to the experimental values for stable disilenes, as shown in Table II. 

However, the reverse trend is observed, albeit to a minor extent, in the case of tetraaryldisilenes, with the AH of the more hindered 2,6-diisopropylphenyl derivative (26.3 and 27.0 kcal mor1) being larger than that of the less hindered 2,6-diethylphenyl derivative (25.6 and 26.7 kcal mol-1). Although the reason for the difference between these two types of disilenes remains to be studied, it is probably due to the complexity of conformational equilibria in disilenes owing to the soft nature of the Si=Si bond. 

Thermal 1,2-diaryl rearrangement in disilenes was first demonstrated for tetraaryldisilene 13, which was found to give a mixture of (Z)- and (E)-S.64 More recently, kinetic data, as well as the scope and limitation of the 1,2-rearrangement, have been reported (Scheme 8).11,12 Similar rearrangement was also observed for 2,6-dimethyl-4-t-butylphenyl derivative 14 to give a mixture of (Z)- and ( )-6, but not for disilenes Mes(R) Si=SiMes(R) (3 R = /-Bu 7 R = N(SiMe3)2 8 R = Tip). These re-... 

Figure 56)155. Compounds investigated included three forms of tetramesityldisilene the solvent-free form 69156, the toluene adduct 69-C7Hs157 and the tetrahydrofuran solvate 69 THF158. Also studied were a second tetraaryldisilene (70)159 dialkyldiaryl-substituted disilene, 71160, and the only tetraalkyldisilene known to be stable as a solid, 72161. Three silyl-substituted disilenes, 73, 74 and 75, were also investigated162,163. To assist in the interpretation of the experimental results, ab initio molecular orbital calculations of the 29Si chemical shift tensors were carried out for model disilene molecules. 

Cathodic reduction potentials of disilenes were determined by cyclic voltammetry56. As shown in Table 18, tetraaryldisilenes are reduced at less negative potentials than dialkyl-diaryl derivatives. This is in sharp contrast to the fact that anodic oxidation potentials are similar for both types of these disilanes (see Table 13). 

The color of stable disilenes in the solid state ranges from pale yellow to blue. The longest absorption maxima of acyclic disilenes in solution are assignable to the it -mi transitions and are strongly dependent on the electronic and steric effects of substituents as shown in Tables I-IV. The Amax values are 393-493 nm for tetraalkyldisilenes, 394-452 nm for dialkyldiaryldisilenes, 400-460 nm for tetraaryldisilenes, 468 183 nm... 

Acyclic disilenes show the relatively low-field shifted resonances of unsaturated 29Si nuclei depending on the substituents. The 29Si resonances for A2Si = SiA2-type disilenes range from 53-66 ppm for tetraaryldisilenes, to 90-103 ppm for tetra-alkyldisilenes, and 131-156 ppm for tetrasilyldisilenes. 

The photolysis of trisilanes makes possible the formation of homo- and heteroleptic tetraaryldisilenes as well as 1,2-diary ldisilenes that may have alkyl, silyl or even amino groups as substituents. The heteroleptic disilenes formed in this way are usually obtained as mixtures of the E- and Z-isomers which can be isolated as pure compounds after fractional crystallization and/or can be converted thermally or photochemically to the other isomer. 

The longest wavelength absorptions of the tetraaryldisilenes occur in the region of 400-430 nm. The three tetrasilyldisilenes 16-18 also exhibit absorption bands between 412 and 425 nm that are attributed to tt — tt transitions. Disilene 18, the sterically most heavily overcrowded compound, exhibits an additional absorption band at 480 nm. The surprising color change from yellow in the solid state to deep red in solution suggests that 18 adopts a twisted form in solution in order to reduce the steric strain39. 

The 29 Si NMR chemical shifts, like the electron spectra, are characteristic for the disilenes and occur in the low field region between 50 and 155 ppm as a result of the coordination number 3. While the 29Si NMR signals of the tetraaryldisilenes appear at about 60 ppm, those of the 1,2-diaryldisilyldisilenes are found at ca 100 ppm and those of the tetrasilyldisilenes at a low field of approximately 150 ppm. 

Ene addition products have been isolated from reactions with various alkenes containing allylic hydrogen atoms compounds 49 and 50 are shown here as examples. Analogously, the reaction with a 1-alkyne furnishes the adduct 47 while styrene, in contrast, reacts to afford the [2 + 2] cycloaddition product 51. The latter mode of reaction, however, is no longer considered to be unusual since the tetraalkyldisilene 41 also forms [2 + 2] cycloadducts with various C=C double bond systems71-73. On the other hand, until very recently [2 + 2] cycloadditions of the tetraaryldisilene 9 were unknown. It has now been shown that 970, as well as 4171, can undergo cycloadditions with the C=C double bonds of styrene and 2-methylstyrene. [4 + 2] Cycloaddition reactions of disilenes with... 

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. 

Absolute rate constants and Arrhenius parameters have been determined for the thermal E,Z-isomerization of the stable disilene derivatives 92-96 in deuteriated aromatic solvents or THF-ds solution by XH or 29Si NMR spectroscopy133-136. With 1,2-dialkyl- and 1,2-diamino-l,2-dimesityldisilenes such as 92a-94, the (E)-isomers are considerably more stable than the (Z)-isomers, and so rate constants for E,Z-isomerization were determined after first generating mixtures enriched in the (Z)-isomer by UV-irradiation of the (El-isomer, and then monitoring the recovery of the solution to its equilibrium composition. On the other hand, little difference in thermodynamic stability is observed between the (Eland (Z)-isomers of tetraaryldisilenes such as 95a,b, and E,Z-isomerization kinetics were hence determined starting from solutions prepared from the individual, pure (or almost... 

The evidence that ,Z-isomerization of 92-95 proceeds by Si=Si bond rotation and not a mechanism involving silylene intermediates, produced by cleavage of the Si=Si bond followed by recombination, rests upon the fact that no trapping products consistent with the intermediacy of the corresponding diarylsilylenes could be detected upon heating the disilenes in the presence of known silylene traps such as methanol, triethylsilane or 2,3-dimethyl-l,3-butadiene. In fact, one tetraaryldisilene has been shown to isomerize by this mechanism, the 1,2-dimesityl-l,2-bis(2,4,6-tris[bis(trimethylsilyl)methyl]phenyl derivatives (E)- and (Z)-97a (equation 70)142,143. Arrhenius parameters for the thermal dissociation of (E)- and (Z)-97a to diarylsilylene 98 are listed in equation 70. 

Although the currently available results still do not provide a uniform scheme, they do clearly indicate that silylenes bearing bulky substituents such as 2, and also dimesitylsilylene [14], xmdergo [2+1]-cycloadditions to the double bonds of 1,3-dienes rather than [4+l]-cycloadditions. In contrast, the behavior of disilenes towards the C=C double bonds of alkenes and conjugated dienes is still not clear. While additions of the stable tetraaryldisilenes to such double bond systems are still unknown [1], the marginally stable disilene 3 is able, in individual cases, to take part in both [2+2]- and [4+2]-cycloaddition reactions. 

Since then, over 40 more acyclic and cyclic disilenes have been prepared and more than half of them have been structurally characterized. Most of these compounds are tetraaryldisilenes, although some examples with alkyl, silyl, or... 

Recently, we obtained the first and, as yet, only tetrasilabuta-1,3-diene 6 from the tetraaryldisilene as follows. The disilene was treated with excess lithium to give the putative disilenyllithium compound 4. In the second step of the reaction sequence, mesityl bromide was added in the expectation that the bulk of this aryl group and the poor solubility of mesityllithium would favor halogenation over the competing transarylation. In fact, the bromodisilene 5 does appear to be formed smoothly but, like 4, has not yet been unambiguously identified. Intermolecular cleavage of lithium bromide from the two intermediates 4 and 5 then furnished the tetrasilabutadiene 6 in up to 60% yield. 

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