Electrophilic reactions ketone trapping

The lithium salts of aldehyde t-butylhydiazones react with electrophiles (aldehydes, ketones, alkyl halides) to form C-trapped t-butylazo compounds isomerization and hydrolysis give a-hydroxy ketones or ketones in good yields, thereby providing a convenient path via a new acyl anion equivalent (Scheme 6). Reaction of th lithium salts with aldehydes and ketones, followed by elimination, provides a new route to azaalkenes, whereas homolytic decomposition of C-tnq>ped azo compounds of trityl and diphe-nyl-4-pyridylmethylhydrazones lead to the formation of alkanes, alkenes, alcohols or saturated esters. ... 

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

O-Substituted oxime derivatives are synthetically useful in a wide variety of transformations. Hoffman and Butani have observed that reaction of a series of aldehydes and ketones with the potassium salt of Af,0-bis(trimethylsilyl)hydroxylamine 4a or 4b (a rapid equilibrium between 4a and its Af,N-bis(silylated) isomer 4b probably exists in solution) gave high yields of the corresponding oximate anion 5, formed via the Peterson-type reaction, together with the silyl ether 6. Anion 5 could be protonated to the oxime 7 or trapped in situ with a variety of electrophiles to give 0-substituted oxime derivatives (Scheme 6). 

Satoh and coworkers further investigated this reaction and found that, in some cases, magnesium /3-oxido carbenoids gave better results. Trapping of the enolate intermediates with several electrophiles was successfully carried out and a new method for the synthesis of one-carbon expanded cyclic a,a-disubstituted ketones from lower cyclic ketones was realized. An example using 1,4-cyclohexanedione mono ethylene ketal (195) as a representative cyclic ketone is shown in Table 15. ... 

Erlenmeyer was first to consider ends as hypothetical primary intermediates in a paper published in 1880 on the dehydration of glycols.1 Ketones are inert towards electrophilic reagents, in contrast to their highly reactive end tautomers. However, the equilibrium concentrations of simple ends are generally quite low. That of 2-propenol, for example, amounts to only a few parts per billion in aqueous solutions of acetone. Nevertheless, many important reactions of ketones proceed via the more reactive ends, and enolization is then generally rate-determining. Such a mechanism was put forth in 1905 by Lapworth,2 who showed that the bromination rate of acetone in aqueous acid was independent of bromine concentration and concluded that the reaction is initiated by acid-catalyzed enolization, followed by fast trapping of the end by bromine (Scheme 1). This was the first time that a mechanistic hypothesis was put forth on the basis of an observed rate law. More recent work... 

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