Propenal, reaction with silenes

Ions containing a Si—C double bond can be generated in the gas phase via dissociative ionization of silacyclobutane derivatives. The silene radical cation [Me2Si=CH2]+ has been obtained by El ionization of 1,1-dimethyl-l-silacyclobutane145. The reactions with ethylene-dr and propene are suggested to occur by way of a [2 + 2] cycloaddition, leading to a cyclic four-membered intermediate. 

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. 

These studies proved that the reactions of 22 with butadienes and propenes take place both regioselectively and stereoselectively and are accelerated by electron-donating groups on propenes and butadienes (e.g., 2-methylpropene in relation to propene) and retarded by increasing bulkiness of substituents in 1,4- or 1,3-positions. As in the case of alkenes and silenes, the reactions of 22 occur in a concerted way and are HOMO (dienes or enes)-LUMO (dienophiles or enophiles) controlled.31 However, some small differences are observed between germene 22 and the analogous... 

The reactions of bare Fe" " ions and related species in the gas phase continue to attract much interest. The remote functionalisation of 1,6-hexanediol by Fe occurs by C-H activation at C(3) and C(4).26 Functionalisation of 3-methyl-2-pentanone at C(4) is diastereoselective, probably because of the conformation of a chair-like intermediate. Reactions of Fe with anisoles and phenols have also been studied.28 Interaction of Fe with silanes gives both silene and silylene species, though the two are not interconvertible. The reactions of Fe(alkene)+ complexes with pentane were found to differ dramatically from those of bare Fe" , and C-H and C-C activation were also observed in reactions of Fe(C2H4) with oxygen. 0,31 interaction of Fe(benzyne)+ with alkyl halides led to C-X or C-C addition followed by p-elimination and loss of HX.32 The gas phase reaction of Fe(NH2)Me" with C2H4 is best explained by insertion into the Fe-C bond followed by P-elimination and loss of propene. The reaction of FeMe with 1-octyne also leads to C-C bond forming processes. 

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