Silenes 1,2-eliminations

The temperature at which a cycloaddition reaction of a neopentylsilene takes place (detected by the elimination of LiCl) has turned out to be dependent on the reaction partners added as substrate. This implies that an interaction between the substrate and A or B or the substrate and C occurs somewhere along the reaction pathway depicted above. For the system Cl3SiCH=CH2/LiBut/R2C=NR it was observed that the imine initiates and supports the salt elimination from the species A/B. Based on the knowledge that silenes are stabilized by external donors [1] we conclude that with carbon unsaturated compounds x-donor interactions instead of cr-donor complexes may be possible as well for the lithiated species (D) as for the silene itself (E). 

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 other important route to silenes via eliminations has been studied by Jones el al. through addition of /-butyllithium to vinylchloro- or fluorosi-lanes followed by 1,2-elimination of the LiX [Eq. (11)]. While silenes... 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

Jones and coworkers developed a new method of generating silenes based on the addition-elimination reaction. Addition of t-BuLi to an appropriately substituted chloro(vinyl)silane produces a neopentyl-substituted silene67,68. Among many reactions, it has been shown that the transient silene adds to anthracene to afford stereoisomers, 40a and 40b, as isolable compounds (Scheme 11). Fractional crystallization of the adduct 40 from hexane gave pure 40a, leaving a 69/31 mixture of 40a and 40b. 

The Wiberg -type silenes like 92, available through salt elimination reactions from 93, react with nonenolisable aldehydes, ketones and the corresponding imino derivatives to give in a first step donor adducts 9459, which are then transformed to the [2 + 2] and [2 + 4] cycloadducts 95 and 96, respectively (equation 21)60-62. These cycloadducts may liberate the silene 92 upon heating and it can be trapped by suitable reagents. 

The attempted synthesis of 104 from its LiF adduct by salt elimination leads exclusively to the silene 104a65,66. 104a can be reacted with benzophenone to give the [2 + 4] and [2 + 2] cycloadducts 105 and 10667. The [2 + 4] cycloadduct of silene 104 cannot be obtained directly. The adducts 105 and 106, however, rearrange to the thermodynamically more stable 107, probably via the donor adducts 108, 109 and the free silenes 104 and 104a (equation 24). 

More reaction pathways are opened up when one of the substituents at silicon is a vinyl group as in 116 (equation 28)70. The intermediate silaallylic radical 117 (alternatively, the authors suggest an ionic mechanism) can be trapped directly by methanol to give the silyl ether 118. Alternatively, 117 closes to the silene 119, which was identified by its reaction product with methanol, 120. A third reaction channel is the elimination of a silene, which again was identified by its trapping product 121 with methanol. 

The most straightforward synthesis of unsaturated Si=C compounds is the formation of the double bond by 1,2-elimination of a salt. This method has been widely used by N. Wiberg s and N. Auner s groups in recent years to produce a variety of different silenes. 

The formation of the a-metalated species in the case of the Auner/Jones Type silenes (neopentylsilenes) 132 (which is deduced by formation of dimers 133 or by trapping) is achieved through addition of f-butyllithium to a vinylchlorosilane in an inert solvent79,80. The initially formed lithiated intermediate eliminates LiCl at ca 0°C (equation 31), the exact temperature depends on the substituents R. The multitude of transient silenes that are available through this method will be discussed in Section I.B.4.b on the reactivity of neopentylsilenes. The basic reactions of Cl3Si—CH=CH2 with t-BuLi have... 

A relatively new synthetic approach to silenes was established independently in the laboratories of Oehme101-110, Apeloig39,111 and Ishikawa112,113. The key-step is a base-initiated 1,2-elimination of silanolate from at-hydroxydi si lanes 157 and formation of silenes 158 analogous to the original Peterson olefination reaction (equation 39). 

Three different routes to the key compounds for the sila-Peterson elimination, the a-alkoxydisilanes 157, are described in the literature, namely A, reaction of silyllithium reagents with ketones or aldehydes B, addition of carbon nucleophiles to acylsilanes C, deprotonation of the polysilylcarbinols. In addition, method D, which already starts with the reaction of 2-siloxysilenes with organometallic reagents, leads to the same products. The silenes of the Apeloig-Ishikawa-Oehme type synthesized so far are summarized in Table 4. 

Alternatively, the reaction may proceed via 1,6-ring closure giving the isolated 2,3-disilanaphthalene 372109. It should be mentioned, however, that under the reaction conditions it is possible that silyl lithium compound 375 formed from the precursor of the silene 376 (obtained from metalation of 377) by a 1,3-trimethylsilyl shift adds to the intermediate silene 371 and that subsequent ring closure with elimination of lithium silanolate gives rise to the observed products 372 and 373 (equation 105). 

While the germanium-containing tungsten complex 166 rearranged to 167 on photolysis, the isomeric species 168 directly eliminated the silene Me2Si=CH2 yielding 169 (Scheme 26)80. 

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. 

Reactions of the silene hydride complex Cp Ru(P(Pr-/)3)(I I)(r 2-C.I I2=SiPh2) with hydrosilanes proceed via an initial migration of the hydride to the silene ligand, affording as the final products either the disilyl hydride or the mono silyl dihydrido ruthenium(IV) complexes as described in Scheme 21. Similar reductive elimination and... 

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