Disilenes oxidation

The early stages in the oxidation of disilene have been treated theoretically for the parent molecule H2Si=SiH2.95 The first intermediate along the reaction coordinate is the open-chain trans diradical 64 (Scheme 16), which is in equilibrium with a gauche form, 65. From the latter, closure to the 1,2-dioxetane 66 would probably be rapid. The open-chain form can react with a second molecule of disilene to give the diradical 67, which could collapse into two molecules of the disilaoxirane 68. If similar steps are followed in the oxidation of 3, they must be quite rapid, since the relative configuration at the silicon atoms is maintained in both products, 59a and 61a.93... 

C. Disiladioxetanes Oxidation of Disilenes with Molecular Oxygen. 819... 

Disilenes have much lower oxidation potentials than olefins , and consequently they are much more reactive toward 02. Typically, disilenes 93 react in solution with triplet oxygen to give 1,2-disiladioxetanes 94 as the major product, accompanied at room temperature by a smaller amount of disilaoxirane 95 (equation 93) °. ... 

Silanone, disilene oxidation, 821 Silicon peroxides see Silyl peroxides Silicotungstate compound, olefin epoxidation,... 

Scheme 10.19 Formation of a 1,2-Si202 ring by oxidation of disilenes with oxygen.

FIGURE 32. Oxidation of disilenes with 302. Reprinted with permission from Reference 141. Copyright 1992 American Chemical Society... 

Disilenes undergo irreversible anodic oxidation at much less positive potentials, as shown in Table 1356. The oxidation potentials for these compounds are similar, indicating that the HOMO of each species lies at approximately the same energy level. 

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). 

Cyclic siloxanes are important precursors in the silicon industry, being formally dimers or trimers of silanone (R2Si=0), a known intermediate. Cyclic siloxanes have been synthesized by four routes, the conventional methods being the condensation of silanediol or the hydrolysis of species such as halosilanes or aminosilanes (Scheme l)16-20. Alternatively, oxidation of disilene by triplet oxygen (equation l)21-27 or oxidation of oxadisiliranes by singlet oxygen (equation 2)28-31 may be utilized. 

Stable disilenes are usually highly reactive toward oxidation and reduction because the 7i and n levels of disilene are much higher and lower than those of ethylene, as shown theoretically (Section III.A) and by the electrochemical oxidation and reduction potentials (Section III.E). Several stable disilenes are however stable in air for a long period. Disilene 27 undergoes very slow decomposition (half-life ca. 84 h) in wet THF solution at rt,60,61 and air oxidation of disilene 63 is completed in benzene at rt for a week.35 Disilene 27 survives for 4 months in the solid state. Since the oxidations and reductions of disilenes and their mechanisms have been extensively discussed in review OW, we mostly show herein the results of recent studies. 

The oxidation of disilene 2 affords unusual oxidation product 149 in addition to 150 via 148 76,105... 

The electrochemical oxidation of disilene 1 gives various monosilanes [Eq. 

As a consequence of the low electronegativity of silicon, many of the reactions described in the preceding sections can be formally classified as oxidations. Thus, this section is intended to provide a short survey of the extensive investigations on the air oxidation of disilenes as well as on the reactions of epoxides and P4 and AS4 molecules with the Si=Si double bond. 

At lower temperature in solution, the aerial oxidation of disilenes initially furnishes the 3,4-disiladioxetanes 119 together with small amounts of the disilaoxiranes 120, both of which react further in the solid state and in solution to give the cyclodisiloxanes 121 (Scheme 4). 

Oxidation of the disilene 13 follows a somewhat different course although a 3, 4-disiladioxetane is formed in a first step, the subsequent products are the cyclodisiloxane... 

The E- and Z-isomeric disilenes 35 are remarkably stable towards oxygen at 50 °C, 10 h are needed before the oxidation is completed. These reactions are also stereoselective and afford the E- and Z-cyclodisiloxanes46. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

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