Synthesis of Unsaturated Compounds

Dehydrobromation of halogenated phosphonium salts is currently used in the synthesis of unsaturated compounds, (cf. Section II.C) provided that a labile carbanion is created in a suitable position relative to the halogen atom (reaction 261). Such bases as Et3N (for / - or (5-halogenated phosphonium salts)288 or bromide anion357 may induce this elimination. Fluorine atoms of the cationic part of 3-methyl-1,1-difluorocyclopent-2-ene-1-phosphonium hexafluorophosphate were substituted when the compound was treated with absolute methanol under mild conditions412. 

The synthesis of unsaturated compounds by C-C bond formation can also be carried out by coupling of alkenes with allenes, intermolecularly or intra-molecularly. Thus, 1,3-dienes were selectively obtained by coupling of allenes and vinyl ketones [28-30]. The reaction was catalyzed by the complex CpRuCl(COD) and with CeCl3 as a cocatalyst (Eq. 19). This cocatalyst is expected to decrease the chloride ion concentration to keep the active cationic ruthenium complex coordinatively unsaturated. 

The main limitation in the broad apphcation of alkene cross-metathesis reactions in the synthesis of unsaturated compounds is the formation of self-coupling byproducts. Moreover, these impurities, frequently difficult to separate, are usually formed alongside the desired compound in an almost statistical ratio. Another issue associated with this kind of reaction is the stereocontrol of fhe newly formed double bond. 

The addition of suhbne-substituted anions to carbonyl compounds yields -hydroxy sulfones. These are useful intermediates for the synthesis of unsaturated compounds via elimination processes. " -Hydroxy sulfones may also be prepared via the regioselective ring opening of epoxides by sodium p-toluenesulfinate. ... 

The y-trialkylsilyl-substituted allylboranes 1 and allenylboranes 2 and 3 have found useful applications in the synthesis of unsaturated compounds. The trimethyltin chloride-induced transformations of the organoborate complexes 4 also provide easy entries to a variety of unsaturated molecules for subsequent S3mthetic elaborations. 

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