Tetramesityldisilene reactions

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... 

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

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... 

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. 

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. 

Polyalkenes are invariably made by polymerization of alkenes. Why is an analogous route, the polymerization of disilenes, R2Si=SiR2, not used to make polysilanes The reason is that the barrier to polymerization of disilenes is simply too low, so that in most cases they polymerize or oligomerize as soon as they are generated. Stable disilenes can be made and isolated, but only with very bulky substituents at the silicon which make the polymer less stable than the disilene. An example is tetramesityldisilene (7), a highly reactive compound that undergoes many novel chemical reactions but does not polymerize. 

Tetramesityldisilene quantitatively desulfurizes cyclohexene sulfide at room temperature <1993CC1348>. Products with Si-S bonds were obtained from a reaction between cyclohexene sulfide and decamethylsilicocene (Scheme 43) <1989AGE1518>. 

Since the first isolation of tetramesityldisilene by West et al. in 1981, a number of stable disilenes have been synthesized, and their structures and properties have been extensively investigated up to date [1,2]. However, known types of stable disilenes had been limited to simple acyclic derivatives with one Si=Si double bond until 1996, when the first stable cyclotetrasilene was synthesized [3]. Recently, the number of known skeletal types of disilenes has increased remarkably, and studies of their structures and reactions have made astonishing progress. The chemistry of disilenes is now approaching a stage to be compared with the extensive and profound chemistry of olefins. Here, we discuss the synthesis, structure, and reactions of novel cyclic and bicyclic disilenes and a trisilaallene, the first compound with a formally sp-hybridized silicon atom. 

The original report76 that the irradiation into the UV but not the visible absorption band of tetramesityldisilene in cyclohexane leads to 1,1,2,2-tetramesityldisilane, presumably by hydrogen abstraction, was later retracted and the observations were attributed to the initial use of crude reaction mixtures74. Such a side-reaction was not observed during the cis-trans isomerization studies of the two above disilenes. 

Reactions of disilenes with elemental sulfur92,93 probably belong to the miscellaneous rather than radical attack category. The products from the reactions of elemental sulfur with tetramesityldisilene and ( )-l,2-di-fert-butyl-l,2-dimesityldisilene have been characterized as disilathiiranes (equation 32), and X-ray structure analysis of the former (38) has been performed93. A single isomer of the latter is formed in the reaction, presumably trans. 

A process that is more clearly of radical nature is the reaction of elemental chlorine with tetramesityldisilene. In solvents containing abstractable hydrogens the main product is the HC1 adduct, 1,1,2,2-tetramesityl-l-chlorodisilane76,82. Its formation can be rationalized best by the free radical chain process shown in equation 33. With cyclohexane as solvent, the expected solvent-derived products, chlorocyclohexane and cyclohexene, were indeed formed. The radical 39 postulated here as the first product of the reaction of tetramesityldisilene with chlorine has now been prepared independently and observed in an ESR cavity78. The addition of Cl2 to (E)-l,2-di-terf-butyl-l,2-dimesityldisilene in cyclohexane proceeds similarly and yields a mixture of the diastereomeric HC1 adducts. 

The reaction of tetramesityldisilene with tri-n-butyltin hydride82 yields no tetramesityl-disilane but gives the air-stable 1,2-adduct 40, presumably by a radical chain mechanism (equation 35). 

The reaction mixture is cooled for 5 min in a solid C02/acetone bath, and then the tube is inverted to filter the cooled mixture. The round bottom flask is then immersed in the cooling bath to pull the solvent through the frit. The tetramesityldisilene that remains on the side of the photolysis tube is washed into the frit by condensing solvent onto the side of the tube. This is best done by soaking a pair of cotton gloves in liquid nitrogen and applying it to the side of the tube. This process is repeated until all the tetramesityldisilene is washed into the filter frit. 

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