Tetramesityldisilene

Unusual Si/P compounds are also beginning to appear, for example, the tetrasilahexaphospha-adamantane derivative [(Pr Si)4(PH)6] (1), which is made by reacting Pc SiCl3 with Li[Al(PH2)4]- Again, reaction of white phosphorus, P4, with tetramesityldisilene, Mes2Si=SiMes2, in toluene... 

However, it was about 8 years after the first synthesis of tetramesityldisilene before stable coordination compounds became known. The main reason for this is the kinetic stabilization of the known disilenes by bulky substituents, which effectively prevents the coordination of the double bond to a metal fragment. Thus, a direct coordination of stable disilenes appeared to be reasonable only if metals with very low coordination numbers were used. 

The molar ratio of these compounds was approximately 3 2. They could not be transformed into each other, and separation of the isomers has not yet been successful. Evidently, the initial product mixture, containing phosphorus-rich polycyclic compounds, was degraded into the bicyclic 48 and 49. Similar product mixtures, for which structures have been established, were also observed by the analogous transformation of ( > 1,2-di-re rf-butyl- 1,2-dimesityldisilene and tetramesityldisilene with white phosphorus and yellow arsenic (As4), respectively.46... 

Successes in producing reactive intermediates like o-xylylene and carbene and in preparing bimetal lies in high yields using ultrasound led us to attempt to generate West s novel compound, tetramesityldisilene the first example of a stable species with a silicon-silicon double bond(32). We prepared this species in one step and trapped it with methanol(33). The disilene is reactive towards lithium, however, and we have found it very difficult to obtain consistent results. Most often, hexamesitylcyclotrisilane is isolated in very good yield(34). 

In some cases, the cathodic reduction of dichlorosilane gives the corresponding disilene. For example, the electrolysis of dimesityldichlorosilane in a divided cell equipped with a mercury pool cathode and silver anode under controlled potential conditions (-3.2V vs Ag/Ag+) affords tetramesityldisilene in 20% yield (Scheme 42) [90]. 

Cyclopropylaldehydes undergo addition reactions with tetramesityldisilene (Mes2Si=SiMes2) and with its germasilene analogue, apparently involving biradical intermediates. 

An organosilicon compound (tetramesityldisilene) containing a silicon to silicon double bond has been synthesized. It is a crystalline solid, mp 176C, and has reactive properties similar to olefins. Compounds of the type are silylenes. 

The average torsion angle in the general structure 311 (R SiSiR and R SiSiR ). cStructural data from tetramesityldisilene obtained from different crystals. 

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. 

Apeloig and Nakash have reported recently a Hammett-type study for the addition reactions of seven para- and meto-substituted phenols to tetramesityldisilene 39 (equation 10)54. They used a large excess of the phenol to enforce pseudo-first-order kinetics. The addition reactions are indeed firstorder in both the disilene and the phenol. 

The negative p value (—1.77) in the reaction of the tetramesityldisilene 39 with phenols with electron-donating substituents indicates that a positive charge is developing on the phenolic oxygen in the transition state, whereas the positive p value (1.72) observed... 

Among the limited studies of the photochemistry of disilenes, it has been reported that photolysis of tetramesityldisilene, 328, at 254 nm in pentane gave rise to the dihydrido compound 329 (equation 27), probably as a result of the radical character of the excited... 

Reacting tetramesityldisilene with PhHC=S gives the air-stable 1,2,3-disilathietane with a long Si—Si bond of 244.3 pm, longer than that in the oxetane. On photolysis in EtOH,... 

Addition of selenium and tellurium to tetramesityldisilene gives the selenirane and tellurirane (equation 42), which continue the trend with the chalcogen derivatives of a steady increase of the Si—Si bond length in going from the oxirane to the tellurirane94. 

Absorption bands of tetraaryldisilenes appear at a little longer wavelength than those of tetraalkyldisilenes. Leites et al. have found that the absorption and fluorescence spectra of tetramesityldisilene 1 depend on the forms of crystals, suggesting dependence of the conjugation between aryl n and Si = Si ti systems on the rotational conformations around Cm- Si bond.78 While both a 3-methylpentane solution of 1 and a thin film of 1 show the n-m absorption maxima at around 420 nm, crystals of unsolvated 1 exhibit the band at 465 nm. Unsolvated solids and a thin film of 1 show broad fluorescence bands at about 560 and 515 nm, respectively large Stokes shifts indicate a substantial geometry change upon excitation. 

In contrast to alkenes, disilene derivatives react very smoothly with various haloalkanes as shown in Eqs. (81) and (82). Tetramesityldisilene 1 reacts with tert-butyl chloride to give hydrogen chloride adduct 172 together with 2-methylpropene (173), while treatment of 1 with benzyl chloride affords l-benzyl-2-chlorodisilane 174.126 The reactions of tetrasilyldisilene 22 with haloalkanes proceed in a similar way to give l-halo-2-(haloalkyl)disilanes 175, whereas the reaction with carbon tetrahal-ides affords the corresponding 1,2-dihalodisilanes 176.127... 

Although the thermolysis of tetramesityldisilene 1 giving 191 through a possible intermediate 190 13 Constitutes a formal intramolecular ene reaction [Eq. (90)], no intermolecular ene reactions have been reported in review OW. 

Styrene and substituted styrenes react with tetramesityldisilene 1, tetra-tert-butyl-disilene 21, and tetrakis(tert-butyldimethylsilyl)disilene 22 to afford the corresponding disilacyclobutane derivatives.127,134 Similarly, [2 + 2] additions occur between the disilenes with a C = C double bond in an aromatic ring135 and acrylonitrile.136 Bains et al. have found that the reaction of disilene 1 with trans-styrene- provides a 7 3 diastereomeric mixture of [2 + 2] adducts, 201 and 202 [Eq. (95)] the ratio is changed, when czs-styrene-Ji is used.137 The formation of the two diastereomeric cyclic adducts is taken as the evidence for a stepwise mechanism via a diradical or dipolar intermediate for the addition, similar to the [2 + 2] cycloaddition of phenylacetylene to disilene ( )-3, which gives a 1 1 mixture of stereoiso-meric products.116,137... 

Stepwise radical mechanisms have been proposed for the apparent [2 + 2] cycloaddition of disilenes with ketones by Baines et a/.140,141 They have found that the reactions of tetramesityldisilene 1 with trara-2-phenylcyclopropane carbaldehyde (208a) and fra/M,fra/M-2-methoxy-3-phcnylcyclopropane carbaldehyde (208b), a mechanistic probe developed by Newcomb et al.,142 undergo characteristic cyclopropane ring-opening as shown in Eq. (99). 

The tetraaryldisilenes participate in a similar variety of cycloaddition reactions as the tetraalkyldisilene 41. The reactions of the classical tetramesityldisilene 9 have been most thoroughly investigated and some of them are summarized in Scheme 3. 

Most of the work done so far on the mechanism of alcohol additions to disilenes has been carried out with the kinetically stabilized derivatives ( )-92a151-153, ( )- and (Z)-92b152 and tetramesityldisilene (110)151,153 154. These compounds react with alcohols some 109 —1012 times more slowly than do the transient disilenes 103 and 104 and, as was shown specifically for 110, the rates vary more dramatically as a function of alcohol structure153. 

Apeloig and Nakash have reported absolute rate constants for the addition of seven met a- and para-substituted phenols (112a-g) to tetramesityldisilene (110) in benzene at 75 °C (equation 82)154, as well as deuterium kinetic isotope effects154 and Arrhenius... 

FIGURE 13. Hammett plot for the reactions of tetramesityldisilene with substituted phenols in benzene at 75 °C. Reproduced with permission from Reference 154. Copyright 1996 American Chemical... 

Disilenes react with ketones, aldehydes, esters and acid chlorides by formal [2 + 21-cycloaddition to yield the corresponding disiloxetanes (equation 73)8,16. The reaction is non-concerted and proceeds through the initial formation of a 1,4-biradical intermediate, as has been shown by the products of reaction of tetramesityldisilene (110) with the cyclopropyl aldehyde 117 (equation 90)163. The absolute rate constants listed in Table 19 indicate there to be a significant difference in reactivity between the monophenyl-substituted disilene 103 and the 1,2-diphenyl-substituted derivatives 104, consistent with a steric effect on the rate of formation of the biradical intermediate. As would be expected, no kinetic deuterium isotope effect is discernible from the relative rates of addition of acetone and acetone- to these compounds. 

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