Silylenes from rearrangements

In this model the reaction rate decreases with increasing temperature because dissociation of the intermediate complex to the reactants has a higher barrier (i.e. via TSi), than does the rearrangement of the complex (i.e. via TS2) to the product of the silylene reaction. Thus, as the temperature increases, product formation is disfavored relative to regeneration of the silylene from the intermediate complex. 

Two indirect routes to silenes, one derived from silylenes and the other from silylcarbenes, are of some generality and importance. Silylenes (e.g., Me3Si—Si—<]) (53) have been derived from the thermolysis of either methoxy or chloro polysilyl compounds. Thermolysis resulted in the elimination of trimethylmethoxy- or trimethylchlorosilane and yielded the silylene, which, based on products of trapping, clearly had rearranged in part to the isomeric silene [Eq. (5)]. Alternatively the silylene Me2Si has... 

The ability to insert in many element-element bonds is an important property of 1 the r -p1 rearrangement of the pentamethylcyclopentadienyl ligands during the reaction is a prerequisite to show a silylene-type reactivity. From a preparative point of view it is worth mentioning that element-silicon bonds which otherwise are difficult to form are easily accessible with the help of 1. In addition, the leaving group character of the pentamethylcyclopentadienyl substituents allows further chemical transformations (vide infra). 

Since, as could be shown by trapping experiments with 2,3-dimethylbutadiene, the primary products upon trimethylsilane extrusion are always the expected open-chain silylenes, the formation of the cyclic compounds must arise from a rearrangement reaction. Whether really a one-step process with a transition state of type 155 occurs as indicated in equation 43, or whether the reaction pathway from 154 to 156 involves intermediates,... 

Compounds with two or more silicon atoms directly attached to one another, subdivided into sections based first on the number of silicon atoms and then on the carbon functionality attached to the silicon atoms. Frequently, but not exclusively, the main photochemical behavior involves homolysis of a silicon-silicon bond yielding silyl radicals, but in some cases silylenes result directly from the photochemistry. The resulting compounds are frequently the products of a molecular rearrangement. 

Conlin and coworkers photolyzed vinyltris(trimethylsilyl)silane 188 in the presence of a variety of trapping reagents such as butadiene, substituted butadienes or silanes and observed products derived from intermediate silenes 189 (formed by rearrangement) or from silylenes 190 resulting from elimination of hexamethyldisilane93. In some cases complex mixtures of products which could have been derived from intermediate silyl radicals were also observed. The reaction products formed from the silene and the silylene in the presence of butadiene, 191 and 192 respectively, are shown in Scheme 32. 

The first 1,2,4-thiadisiletane results from the reaction of carbon disulphide and the hindered silylene [2,4,6- (Me3Si)2CH 3C6H2]MesSi (TbtMesSi ) formed from the Z-disilene precursor. The mechanism is thought to involve a skeletal rearrangement of the 3,3 -spirobi(l,2-thiasilirane) intermediate formed by silylene addition to each carbon-sulphur double bond (equation 31)64. 

In 1974, Caspar and coworkers suggested that 2 is formed in a gas-phase reaction of silicon atoms, resulting from the 31P (n,p) 31Si nuclear transformation, with 1,3-butadiene16. They proposed a mechanism in which the silylene 9, l-silacyclopent-3-ene-1,1-diyl, rearranges to silole 2. 

Product 19 in equation 39 is formed by trapping extruded Me2Sit, but 20, 21 and 22 are rearrangement products of silacyclopent-3-ene 23. Their formation is in accord with stepwise retroaddition with silylene extrusion from a vinylsilirane that is an intermediate in a reversible addition mechanism. As expected from such a mechanism, 20, 21 and 22 are coproducts with 23 in the addition of Me2Si to the piperylenes20. 

The silaylides formed from coordination of silylenes to chlorocarbons are believed to undergo competitive rearrangement (to the formal Cl—C insertion product) and fragmentation (to the product of formal HC1 abstraction by the silylene)5. In addition, these silaylides are believed to undergo dissociation into the radical pairs that would result from direct chlorine atom abstraction by the silylene111. 

A more complex reaction sequence may be responsible for the formation of the products of formal insertion of silylenes into the H—C bonds of carbaldimines110 116. As shown in equation 56, an initial Lewis base or TT-bond adduct can undergo rearrangement to both of the products observed from reactions of diarylsilylenes and pyridine-2-carbaldimines. A common intermediate is suggested by the thermal isomerization of the insertion product to the cycloadduct which can be the exclusive product from addition of a silylene to a 2,2/-bipyridyl (see Section ni.B)117. 

In the past decade there has been considerable mechanistic study of the addition of silylenes to 1,3-dienes. From the first report of the formation of l-silacyclopent-3-enes as major products thirty years ago, a stepwise mechanism including vinylsilirane intermediates was proposed, but concerted 1,4-addition was also considered1-5. Vinylsilirane intermediates were clearly identified in trapping experiments by 19752, but questions have remained about the concertedness of both the primary addition and rearrangement steps... 

Steady-state kinetic analysis of a competition experiment led to the conclusion that the siloxolane is formed by reaction of a vinylsilirane intermediate with acetone, and that the vinylsilirane arises from addition of the free silylene to butadiene. Since silylenes are known to react more rapidly with acetone than with butadiene, the kinetic analysis further suggested that the carbonyl sila-ylide dissociates more rapidly than it rearranges to the silyl enol ether shown in equation 64140. 

Analogous products were obtained from the reaction of silylene 85 with silyl lithium compounds, with alkali metal silylamides and alkali metal alkylamides, and sodium methoxide <2000CC1427, 2004JCD3288, 2005JCD2720, 2005CC5112>. In the case of the reaction with metallated silylamides a thermally initiated rearrangement (114 — 115) to give the new silylamide 115 took place (Scheme 11). 

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