Silylenes with dienes

Cycloaddition Reactions of Dimethylaminomethyl-silylene with Dienes and Heterodienes... 

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... 

Silylene and disilene adducts are obtained with dienes (equation 18). With thiophene, desulfurization takes place and the products are more complicated, as shown in equation 1949. 

Summary Copyrolytic gas-phase reactions of l,l,2,2-tetra(alkoxy)- and tetrakis-(dimethylamino)dimethyldisilanes with dienes, heterodienes, and heterotrienes in a flow-system furnish a number of unsaturated silicon heterocycles via silylene-intermediates with the exception of 1,4-diaza- and 1,4-oxazadienes, yielding 40-65 % of 1,3,2-diaza- and l,3,2-oxazasilacyclopent-4-enes, respectively, mixtures of diastereoisomeric 1-oxa- or 1-aza-2-silacyclopent-4-enes and -3-enes are formed. Conjugated oxatrienes were found to undergo additionally [6+l]-cycloadditions giving l-oxa-2-silacyclohepta-4,6-dienes. 

In the course of our investigations we have tried to synthesize cyclization products of silylenes and DMB directly from chlorosilanes without using UV light. After that, we extended our studies to the formation of silaethenes and their trapping products with dienes by dehalogenation of chloromethyl-chlorosilanes. 

Summary Monoaminosilylenes and bis(diethylamino)silylene are formed by thermolysis of amino-substituted disilanes and characterized by trapping with dienes. In the case of MeSiNMe2, cycloaddition reactions were extended to 1,4-diheterodienes, unsaturated ketones and imines allowing an easy synthetic access to functionally substituted unsaturated silicon heterocycles. The syntheses of an isolable, unsymmetric diaminosilylene and of related germylenes and stannylenes are described. The results are presented and discussed in relation to theoretical work on stabilization of donor-substituted silylenes and recent work on isolable diaminosilylenes done by or in cooperation with other groups. 

However, Bu3SnSiMe2H (154) reacts with terminal aikynes to give the 3,4-disubs tituted silacyclopenta-2,4-diene 156 at room temperature. The dimetliyl-silylene 155 is an intermediate[80]. (Me3Sn)2 undergoes facile addition to aikynes to give the 1,2-distannylalkene 157[8I-83],... 

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. 

Silacyclopropenes are commonly formed from the addition of a silylene to an alkyne, or in some cases as the result of photolysis of an alkynyldisilane (see Section III.C). Substituted silacyclopropenes have been shown to undergo both 1,2- or 1,3-shifts when photolyzed, yielding silyl-substituted allenes or alkynes, respectively2. More complex behavior was observed with methylenesilacyclopropenes such as 2323 which ring-opened to a diene, as shown in Scheme 4. 

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. 

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