Dienes reactions with silylenes

The reactions of silylenes with 1,3-dienes giving the corresponding 3-silolenes are typical of the cycloaddition reactions of silylenes. The mechanism of these reactions has been investigated in detail, " " and it has been proposed that the reactions of silylenes with 1,3-dienes proceed via initial [1 + 2] addition followed by the isomerization of the resulting 2-vinylsiliranes to the corresponding 3-silolenes. However, the observation and isolation of the intermediary 2-vinylsiliranes has been limited to only a few examples because vinylsiliranes readily isomerize... 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

The reaction mechanism can be explained by the formation of a dimethylsilylene intermediate which is able to undergo an insertion reaction into a Si—H bond (for more about this type of reaction, see Chapter 7 Silylenes). The formation of the silylene can be explained by an a elimination from the n metal-complexed disilane. The presence of the silylene has been proved by a capture reaction with diphenylacetylene, when the expected 1,1,4,4-tetramethyl-1,4-disilacyclohexa-diene could be isolated. 

The best studied addition reactions of silylene have been those with conjugated dienes. Earlier studies using the nuclear recoil system have shown that 31SiH2 added to 1,3-butadiene to give silacyclopent-3-ene (44, 99, 100). 

Summary The formation of reactive intermediates via dehalogenation of chlorosilanes was investigated by using lithium powder and sonication. Whereas in the absence of a diene substrate mainly polysilanes are obtained, reactions with, e.g., dimethylbutadiene, yield the corresponding cycloaddition products, indicating silylenes and silaethenes as intermediates. 

The presence of very reactive intermediates is obvious from the formation of a variety of non identified by-products. However, the reactions with DMB as the diene substrate provide evidence for the formation of silylene and silaethene intermediates. Further investigations are necessary to elucidate the reaction pathways. 

Bis(diisopropylamino)silylene has been generated by photolysis of the precursor (76), and trapped chemically by reaction with triethylvinylsilane and 2,3-dimethylbuta-l,3-diene.Contrary to theoretical prediction, its dimer seems to have a disilene structure and not a bridged structure. 

Silylene and Disilene Reactions with some 1,3-Dienes and Diynes... 

Trichlorosilane also serves as a silylene precursor in the presence of phosphonium salt as a catalyst to undergo cycloaddition reactions with 1,3-dienes and alkynes (Scheme... 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

Silylene 59 also reacts with some dienes to give 1+4 cycloaddition products. With Iransjrans-1,4-diphenylbuladiene, stereospecific addition takes place to give the d.v-pro-duct, as expected for a singlet silylene (equation 112). A second example is the reaction... 

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