Aldehydes, vide carbonyl compounds

The principal synthetic application of cyclopropylidenetriphenylphosphoranes 4 is the conversion with carbonyl compounds (aldehydes and ketones) into alkylidenecyclopropanes 5. As cyclopropyl halides do not react with phosphanes to form cyclopropylphosphonium halides vide supra), the generation of these precursors 3 is generally performed by condensation of 1,3-dibromopropanes 1 with triphenylphosphane and subsequent 1,3-dehydrobromination of the resulting salt 2 with one equivalent of base. A second equivalent of base generates the... 

As in the alcohol activation step, like the TM-catalyzed direct hydrogen transfer route (Schemes 5, 14), the MPV-O process cannot provide carbonyl compounds directly according to the reaction mechanism (Scheme 39). This may make the alcohol activation reaction the rate-limiting step of the whole process. Most probably for this reason, the early TM-free iV-alkylation reactions [4—9] were performed under harsh conditions to facilitate the initial formation of aldehydes by high temperature dehydrogenation of the alcohols. Otherwise, alternative ways for alcohol activation should be adopted (vide infra). 

Dithioketals and dithioacetals can be induced to undergo reductive desulfurization on heating alcoholic solutions of those dithio-derivatives in the presence of moist Raney nickel (see, e.g., page 746). These sulfur compounds are prepared by treating the aldehyde or ketone with 1,2-ethanedithiol (CAUTION STENCH) in the presence of an add catalyst (an addition to the carbon of the carbonyl with loss of water, a process that will be discussed at length, vide infra) (Scheme 9.13). 

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