Silenes allylic

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

The alcohol 10 looks like it might be formed by the addition of a Grignard reagent to an aldehyde. In fact, Patrick Steel of the University of Durham prepared 10 (Tetrahedron Lett. 44 9135,2003) by Diels-Alder addition of the transient silene derived from 7 to the diene 8. More highly substituted dienes lead to more complex arrays of stereogenic centers. The intermediate silacyclohexenes, exemplified by 9, should also engage in the other reactions of allyl silanes. 

Silenes, bearing an allylic hydrogen, frequently give linear non-cyclic dimers, in which two silene molecules form a Si—Si bond39,86,88,89 107 112. This linear head-to-head dimer can be formed by intramolecular disproportionation of the initially formed biradical (path B in equation 101). Of course, an ene -reaction (path A in equation 101)... 

When simple alkenes were employed as reaction partners for silenes of the type (Me3Si)2Si=C(OSiMe3)R1, silacyclobutanes were obtained, provided that no allylic hydrogen is present in the alkene. In the reaction with alkenes with allylic hydrogens the ene reaction becomes predominant (see Table 11). Thus, while the reaction with styrene exclusively gives the four-membered ring compound 454, with 1-methylstyrene the ene products 455 were obtained (equation 144). Similarly, from the reaction of 150 with 1-octene only the ene product 456 was isolated (equation 145). 

In another study several simple silenes RR Si=CH2 (R, R = Me, Vinyl etc.) were formed by laser-powered pyrolysis and were found to form linear polymers, in contrast to the usual behavior of silenes which yield cyclodimers when formed by conventional thermolysis techniques16. Reactions of the silenes in the presence of several monomers such as vinyl acetate, allyl methyl ether and methyl acrylate were also studied. Laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane gave rise to silenes and other oxygen-sensitive deposits17,18. 

In this multi-authored monograph, several experts and leaders in the field bring the reader up to date in these various areas of research (synthesis and reactivity of zirconaaziridine derivatives, zirconocene-silene complexes, ste-reodefined dienyl zirconocenes complexes, octahedral allylic and heteroallylic zirconium complexes as catalysts for the polymerization of olefins and finally the use of zirconocene complexes for the preparation of cyclopropane derivatives). It is their expertise that will familiarize the reader with the essence of the topic. 

When examining the silylene transfer to gi OT-disubstituted alkenes to form silacyclopropenes, an unprecedented reaction of homoallylic ethers was discovered. Two di-/-butylsilylene units were found to be incorporated into the molecule followed by a complete rearrangement of the carbon backbone (Equation 6) <20050L5531 >. This reaction opens a new route to prepare organic synthetically useful allylic silenes to be used in annulation reactions. 

An intramolecular ene reaction between the silene and the H-C-C=0 moieties of 64 presumably occurs in the second step of the thermal rearrangement252 of the silicon analogue of a phenyl allyl ether discussed in Section III.A.2.C. 

In some cases the ene reaction of the silene with a diene competes with the Diels-Alder addition118,140,210,243"246,264. Thus, 54 undergoes a 4 + 2 cycloaddition with 1,3-butadiene but gives mixtures of products with its methyl derivatives (Table 4). In some cases the only observed products are those of an ene reaction, for instance in the reaction of 54 with Me2C=CH-CH=CMe2244 and in the addition of 2,3-dimethylbutadiene to the silene 75 with an activated allylic hydrogen (equation 134)140. 

When crotonate esters were employed the regioisomers 501 were mainly formed, accompanied by a significant amount of non-cyclic compounds 502 formally arising from insertion of the Si=C group in an allylic C—H bond (equation 167). The reaction of silenes 149 and 150 with crotonate esters are the only examples known so far where the product distribution is different when the reaction is conducted under photolytic conditions or in the absence of light. Thus, in the dark reaction of 150 with methyl crotonate the relative ratio 501 (R = 1-Ad, = Me) 502 (R = 1-Ad, R = Me) = 3.75 1 was... 

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