Silenes with carbonyls

In view of the evident reactivity of the Brook-type silenes toward carbonyl compounds and the fact that these silenes were prepared by the photolysis of acylsilanes, it is natural to ask why the silenes apparently did not react with their acylsilane precursors. This question has been answered recently. On the one hand, as shown in Scheme 19, the silene Ph2Si=C(OSiMe3)Ad apparently did add in a [2 + 2] manner to its acylsi-... 

Some remarkable chemistry is observed when silenes react with heteroatom systems, in particular carbonyl compounds (]>C=0) and imines Q>C=N—R). The reaction with ketones was first described by Sommer (203), who postulated formation of an intermediate siloxetane which could not be observed and hence was considered to be unstable even at room temperature, decomposing spontaneously to a silanone (normally isolated as its trimer and other oligomers) and the observed alkene [Eq. (14)]. Many efforts have been made to demonstrate the existence of the siloxetane, but it is only very recently that claims have been advanced for the isolation of this species. In one case (86) an alternative formulation for the product obtained has been advanced (204). In a second case (121) involving reaction of a highly hindered silene with cyclopentadienones,... 

The stereochemistry of the ene reaction with carbonyl compounds has been investigated by Ishikawa and coworkers with me.vo-(550) and rac-Ph(Et)MeSi—SiMe(Et)Ph as silene precursors251. With acetone and acetaldehyde the reactions were found to be highly diastereoselective when run in ether or toluene. In electron-donating solvents like THF or acetonitrile the diastereoselectivity decreased, and this was ascibed to a stepwise reaction. 

All known silenes react violently with molecular oxygen with the exception of the stable 1-silaallenes which are air-stable. Along with carbonyl compounds the products of the oxidation of silenes are cyclic siloxanes from the corresponding silanone, depending on the reaction conditions3,8,28. For example, when the stable adamantylsilene 150 is... 

The UV absorptions of the tri- or tetrasubstituted silenes are red-shifted compared to the parent silene 2 (X = 258 nm) but occur at shorter wavelengths than in the highly substituted silenes of Brook (X = ca 340 nm). The introduction of trimethylsilyl groups on the carbon atoms results in slight red-shifts just as the conjugation of the silene system with carbonyl groups. 

The reactions of silenes with aldehydes and ketones is another area whose synthetic aspects have been particularly well-studied4,6 7 10 12. The favoured reaction pathways for reaction are generally ene-addition (in the case of enolizable ketones and aldehydes) to yield silyl enol ethers and [2 + 2]-cycloaddition to yield 1,2-siloxetanes (equation 44), but other products can also arise in special cases. For example, the reaction of aryldisilane-derived (l-sila)hexatrienes (e.g. 21a-c) with acetone yields mixtures of 1,2-siloxetanes (51a-c) and ene-adducts (52a-c) in which the carbonyl compound rather than the silene has played the role of the enophile (equation 45)47,50 52 98 99. Also, [4 + 2]-cycloadducts are frequently obtained from reaction of silenes with a,/i-unsaturated- or aryl ketones, where the silene acts as a dienophile in a formal Diels-Alder reaction6 29,100-102. 

The reactions of 1,1-dimethyl-2,2-bis(trimethylsilyl)silene (54) with carbonyl compounds such as acetone and ethyl acetate, and particularly with benzophenone imine, are very much faster than its ene reactions with hydrocarbons such as isobutene244,246,266, pointing to the involvement of the nucleophilic lone pairs in the former case. 

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