Dimerization of disilene

Similar reaction series aboimd. Thus, in a series of Woodward-Hoff-mann forbidden 2 + 2 dimerizations, the promotion gap is proportional to the sum of the AEst (jhi ) quantities of the two reactants. Consequently, the barrier decreases from 42.2 kcal/mol for the dimerization of ethylene, where SAEst nn ) is large ( 200 kcal/mol) down to <10 kcal/mol for the dimerization of disilene for which SAEsr(itit ) is small ( 80 kcal/mol). A similar trend was noted for Woodward-Hoffmann allowed reactions (4 + 2 or 2 + 2 + 2), where the barrier jumps from 22 kcal/mol for the Diels-Alder reaction where SAEsT(7tit ) is small ( 173 kcal/mol) to 62 kcal/mol for the trimerization of acetylene where SAEsrlitit ) is very large ( 297 kcal/mol). 

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

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. 

The addition of t-Bu2>Si to 1,4-diaza-l,3-butadienes competes with dimerization of the silylene only when the concentration of t-BinSi is low170. Subtle steric effects must also be responsible for the addition of /-BinSi to the W-cyclohexyl mono-imine of benzil, while only the silylene dimer undergoes addition under similar conditions in the presence of the IV-methyl mono-imine171. It may be that t-Bi Si and its dimer t-Bu2Si=SiBu-t2, both formed simultaneously upon photolysis of cyclo (t-Bu2Si)3, are in equilibrium, and the steric effect is upon the (2+4) cycloaddition of the disilene. 

In 1981, West et al. synthesized the first stable disilene 1 via the dimerization of the corresponding silylene generated by the photolysis of a trisilane and characterized the structure by conventional spectroscopies [Eq. (2)].5 Availability of 1 and other stable disilenes has stimulated theoretical and experimental studies of various aspects of disilenes such as their bonding and structure, spectroscopic properties, reactivities, applications to the synthesis of novel types of organosilicon compounds, etc. 

Jutzi et al. synthesized an (/ )-A BSi = SiAB-type disilene by the reaction of stable silyliumylidene cation Me5C5Si + 83 with lithium bis(trimethylsilyl)amide 84.19 Formation of disilene 44 is explained by the dimerization of the initially formed (Me5C5)[(Me3Si)2N]Si ... 

The above theoretical analysis for a variety of dimer structures of silylenes requires inevitably a definition of disilenes different from that of alkenes, molecules with carbon carbon double bonds. Geometry around a typical C=C double bond is planar and the double bond length (134 pm) is shorter than the corresponding single bond (154 pm). BDE of ethylene to two methylenes is ca. 170 kcal mol-1 which is 1.9 times larger than for the C C single bond (90 kcal mol-1 for H3C-CH3) the BDE of ethylene really almost doubles the BDE for ethane ... 

Cycloaddition reactions of transient or isolable disilenes with heterocumulenes such as CX2 (X = S, Se) produce heterocyclic carbenes, for example, carbene 59, which has a disilane backbone. These carbenes are only transient species and were not isolated but were either trapped with C6o. or dimerization of the carbenes occurred to give the tetrathiafulvalene or tetraselenafulvalene analogues 28 <2002CEJ2730, 2005AGE7567>. 

An alternative method for the preparation of disilenes involves the photolysis of cyclo-trisilanes18 in which a disilene and a silylene are formed simultaneously through cleavage of two Si—Si bonds. The latter species undergoes dimerization to furnish the corresponding disilene (equation 2). 

The focus of the section on silene reaction kinetics is mainly on studies of bimolecular reactions of transient silene derivatives, because little absolute kinetic data exist for the reactions of stable derivatives and there have been few quantitative studies of the kinetics of unimolecular isomerizations such as ,Z-isomerization and pericyclic rearrangements, although a number of examples of such reactions are of course well known. In contrast, most of the studies of disilene reaction kinetics that have been reported have employed kinetically stable derivatives, and E,Z-isomerization has thus been fairly well characterized. The paucity of absolute rate data for unimolecular isomerizations of transient silenes and disilenes is most likely due to the fact that it is comparatively difficult to obtain reliable data of this type for transient species whose bimolecular reactions (including dimerization) are so characteristically rapid, unless the unimolecular process is itself relatively facile. Such instances are rare, at least for transient silenes and disilenes at ambient temperatures. 

The reaction is initiated by extrusion of a silylene 2 from 3a or 3b, thus paralleling the well known equilibrium between hexamethylsilacyclopropane and dimethylsilylene [7]. In the absence of a silylene trapping reagent [8] dimerization of 2 to disilene 5 takes place. Addition of a third silylene to the Si=Si double bond eventually yields cyclotrisilane 1 [9]. The reversibility of the cyclotrisilane formation from 3a and 3b provides evidence, that the reverse reaction of 1 with olefins includes free silylenes 2 as reactive species as well. 

Major reaction patterns of silylenes are insertion into the O-H (equation 82), Si-H (equation 59) and Si Si bonds (equation 60), addition to multiple bonds (equation 83), rearrangement (equation 84), and dimerization. Dimerization of the bis(mesityl)silylene (equation 85) opened a pathway to the first isolable disilene. ... 

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