Mechanistically Related Processes

A third method of aldehyde synthesis is one that we ll mention here just briefly and then return to in Section 21.6. Certain carboxylic acid derivatives can be partially reduced to yield aldehydes. The partial reduction of an ester by dhsobutylaluminum hydride (DIBAH), for instance, is an important laboratory-scale method of aldehyde synthesis, and mechanistically related processes also occur in biological pathways. The reaction is normally carried out at —78 °C (dry-ice temperature) in toluene solution. 

In a mechanistically related process, Fukamiya, Okano and Aratani have shown38 that the reaction of a sulphoxide with 3 mole equivalents of trifluoromethanesulphonyl chloride and iodine in cold, dry THF gives the corresponding sulphide in yields ranging from 52 to 96%. When compared to t-butyl bromide, this reagent is more expensive, and the products were isolated after chromatographic separation. The reaction pathway can most probably be described as depicted in equation (14) ... 

The introduction of at least one interconversion process (step) having a nature different from the activation-relaxation processes found along the reactant and product channels allows for a distinction between rate-related and mechanistic-related processes among quantum states. A simple reaction is defined by the existence of only one interconversion step. A real chemical reaction can, in principle, be decomposed in a series of simple reactions. 

The formation of azine derivatives by condensation of enamines and enamides with 1,3-dielectrophiles has been known for almost a century, and there are a number of reactions (e.g. the Hantzsch pyridine synthesis) which proceed by intermediate formation of such compounds. Examples are shown in equations (117)—(119). The transformations outlined in equations (120) and (121) are mechanistically related processes. 

Mechanistically related processes include the reaction of isocyanides with aldehydes for in situ generation of the activated species, which is then attacked by the azine (Scheme 41a). This strategy has been harnessed in a convenient s)mthesis of... 

Insertion of benzyl carbenoids into zirconacyclopentenes follows a different but mechanistically related process. 

As outlined below (Section 3.1.3), electronic energy transfer (EnT) between a donor and an acceptor (Figure 2) is a process that shares with electron transfer a number of formal and substantial similarities. In bimetallic dyads, energy transfer and PET are often strongly intertwined, either as competitive or as mechanistically related processes. Therefore, the discussion will not be strictly limited to ET processes, but will include, when required, EnT results as well. 

Condensations of pyrroles with aldehydes and ketones occur easily by acid catalysis but the resulting pyrrolylcarbinols cannot usually be isolated, for under the reaction conditions proton-catalysed loss of water produces 2-alkylidenepyrrolium cations which are themselves highly reactive electrophiles. Thus, in the case of pyrrole itself, reaction with aliphatic aldehydes in acid inevitably leads to resins, probably linear polymers. Reductive trapping of these cationic intermediates produces alkylated pyrroles all free positions react and as the example shows, acyl and alkoxycarbonyl substituents are unaffected," A mechanistically related process is the clean 4-chloromethylation of pyrroles carrying acyl groups at C-2." ... 

The standard aldol reaction involves the addition of an enolate to a ketone or an aldehyde. However, there are related processes and this chapter includes subsections on the isocyanide aldol, nitroaldol and Morita-Baylis-Hillmami reaction. In addition there are reactions involving additions of enolates to the C=N group and a large subsection is devoted to a discussion of the catalytic asynmietric Mamiich reaction. As well as these mechanistically related processes, the carbonyl-ene reaction is also discussed here. Whilst the mechanism of the carbonyl-ene reaction is different from the aldol reaction, the synthetic result is rather similar, and perhaps fits most comfortably into this chapter. 

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