Carbonyl compounds, reactions with silenes

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

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,... 

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. 

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 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. 

Stable silenes and germenes also react with various reagents as shown in Scheme 24." Addition reactions of metallenes with MeOH lead to the formation of R2M(OMe)-C(H)R2. The reactions of a,p-unsaturated carbonyl compounds afford the corresponding [4 - - 2] cycloadducts regiospecifically, reflecting the M +=c - polarity. 

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. 

Ene reactions of monoolefins with silenes have been known for a long time308. The addition of isobutene to 54 (equation 136) has been observed to proceed at —10 °C243. A similar reaction occurs between 54 and other olefins (Table 4)246 see also Reference 266. The rates of these reactions appear to be somewhat sensitive to steric hindrance but even the stable hindered silene 41 still adds propene and isobutene115. They are very much slower than the analogous ene additions of carbonyl compounds containing a hydrogens, such as acetone. 

The reaction shown in equation 170 is, however, the only known example where a silene reacts with its acylpolysilane precursor. Whereas sUenes of the type (Me3Si)2Si=C(OSiMe3)R do not react with their polysUane precursors, they readily react with less sterically congested acylsilanes. Thus, mesitylsilene 349 gives with benzoyl trimethylsilane a mixture of the diastereomeric sUoxetanes /Z-511 in 95% yield (equation 171). Typically, the adamantylsilene 150 gives with benzoyl trimethylsilane the nominal [4 + 2] cycloadduct 512 in 89% yield (equation 172). The silyl enol ether 513 is the result of an ene -type reaction which occnrs between 150 and acetyl triphenylsilane, a carbonyl compound having an activated hydrogen (eqnation 173). ... 

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