Appel reaction mechanism

The previously unknown l,2,4-thiadiazole 4 oxides 64 were prepared by condensation of benzamidoximes or their derivatives with Appel salt (Scheme 4) <1996CC1273>. The reaction mechanism includes attack of the imino group in 65 on the S-l atom of the 1,2,3-clithiazole ring. The /V-oxidcs are shown to be the 4-isomers by analysis of the NMR and mass spectra of lsN-labeled and unlabeled products and X-ray structure determination of the derived carboxamide 66 <1999J(P1)2243>. 

N-monosubstituted 1,2-diaminobenzenes react with Appel salt 20 in dichloromethane to give the corresponding 2-cyanobenzimidazoles 87 either directly or through thermal or acid-catalyzed rearrangement of the intermediate imino-l,2,3-dithiazoles 88, which can be isolated (Scheme 9). The reaction mechanism includes reversible attack of the o-amino group on the dithiazole ring <1998T9639>. 

The next reaction in this sequence is an Appel reaction, which transforms an alcohol to an alkyl iodide. The simplified mechanism is depicted in the margin. The initial step of this reaction is the nucleophilic attack of triphenyl phosphine to iodine with formation of an iodine triphenyl phosphonium cation. The positive charge increases the oxophilicity of phosphorus, which is attacked in the next step by an alcohol. After elimination of hydrogen iodide, the resulting alkoxy triphenyl phosphonium cation is attacked by an iodide anion in an SN2-type reaction. Therefore, the reaction of chiral alcohols typically proceeds with inversion of configuration, although other behavior is also reported. The product of the Appel reaction is alkyl iodide 37 and triphenyl phosphine oxide is formed as byproduct. ... 

The first transformation is an Appel reaction the mechanism is shown below. The carbanion 55, which is generated in situ from PR3 and... 

S. Winstein, B. Appel, R. Baker, and A. Diaz, in Organic Reaction Mechanisms, The Chemical Society, London 1965, pp. 124-126. 

The reaction of the secondary alcohol with trioctylphosphine and carbon tetrabromide results in the formation of the 13. The conditions are similar to those established by Appel et al. for the conversion of alcohols to chlorides. The transformation proceeds through an Sn2 mechanism resulting in inversion of the configuration.18... 

Study of the mechanism of this complex reduction-liquefaction process led to the suggestion that part of the mechanism involves formate production from carbonate, dehydration of the vicinal hydroxyl groups in the cellulosic feed to carbonyl compounds via enols, reduction of the carbonyl group to an alcohol by formate and water, and regeneration of formate (Appell et al, 1975). The following reactions were suggested ... 

For almost every reaction presented thus far - acid and base catalyzed - a mechanism was shown. The understanding of a reaction in terms of its mechanism has always been a primary goal in Pines research. For several of his students like Eugene Aris-toff and Bozidar Stipanovic, his interest in mechanisms is one of their strongest memories of his research approach. Herbert Appel recalls that Pines taught him never to be satisfied with a mechanism until it explains all the products. 

Abstract A problem-solving approach to retrosynthesis is introduced. Basic principles for good disconnections are postulated. Examples of interconversion and disconnection of carbinols, alkenes, ketones and nitro compounds are discussed. Concepts of retro-Diels-Alder and re/ro-Wittig disconnections are presented and the mechanisms of reactions explained. Application of the Wittig reaction on the industrial scale is exemphfied by the synthesis of the analog of bombykol, the principal of pear odor and anti-appelizer chlorphentermine. 

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