Elimination reactions Subject

When optically pure 2 3 dimethyl 2 pentanol was subjected to dehydration a mixture of two alkenes was obtained Hydrogenation of this alkene mixture gave 2 3 dimethylpentane which was 50% optically pure What were the two alkenes formed in the elimination reaction and what were the relative amounts of each" ... 

Sulfonate esters are subject to the same limitations as alkyl halides Competition from elimination needs to be considered when planning a functional group transforma tion that requires an anionic nucleophile because tosylates undergo elimination reactions just as alkyl halides do... 

Because in this case an elimination reaction could occur to form an a,/ -unsaturated ester, use of two equivalents of the phosphorane should be avoided.1The (a-acylalk-ylidene)triphenylphosphorane can be subjected to a subsequent Wittig reaction with an aldehyde.[4] [5],[8]... 

A First, a reaction with phosphorus oxychloride and phosphorus pentachioride can be used to convert 2-pyridone into 2-chloropy= ridine. and then this compound is subjected to an addition-elimination reaction with methoxide ion (from sodium methoxide) (Scheme 2,26). Note chloride is a much better leaving group than methoxide. 

The cis/trans isomerization of platinum(II) complexes is a subject which will be discussed in some detail when the halide (Group VII) complexes are covered. Nevertheless the importance of reductive elimination reactions of platinum(II) alkyl and aryl complexes makes it imperative that this reaction be discussed here for alkyl and aryl platinum(II) compounds. 

The emphasis in the treatment that follows is on compounds that can be isolated, and the transient tantalum hydride species postulated in, for example, the work of Schrock et al. on alkylidenes 184, 194) are only briefly mentioned. The subject of metal hydride formation during hydrogen elimination reactions is worth a review in its own right, and indeed several exist. One of the most recent deals primarily with osmium hydrides and alkyls but is relevant to tantalum also 195). It can be consulted for other references, and the subject is not treated further here. 

When an alkoxide ion is used as the nucleophile, the reaction is called a Williamson ether synthesis. Because the basicity of an alkoxide ion is comparable to that of hydroxide ion, much of the discussion about the use of hydroxide as a nucleophile also applies here. Thus, alkoxide ions react by the SN2 mechanism and are subject to the usual Sn2 limitations. They give good yields with primary alkyl halides and sulfonate esters but are usually not used with secondary and tertiary substrates because elimination reactions predominate. 

This is the second chapter of a two-part review concerned with insertion reactions of transition metal-carbon a-bonded compounds. The first chapter, which appeared in Volume 11 of this series (137), provided a broad introduction to the subject of insertion reactions in general and a detailed treatment of the carbon monoxide insertion and decarbonylation. Presented herein are the insertion and elimination reactions of sulfur dioxide and of a few other unsaturated molecules. The reactions of sulfur dioxide are accorded a complete literature coverage, whereas those of the other inserting species are treated selectively. Metal-carbon a-bonded compounds of the main group elements are discussed only in the context of comparisons with their transition metal analogs. 

Disadvantage The method is limited to primary alkyl halides as secondary and tertiary alkyl halides are subject to unwanted elimination reactions. 

Kinetic studies of the nucleophilic reactions of azolides have demonstrated that the aminolyses and alcoholyses proceed via a bimolecular addition-elimination reaction mechanism, as does the neutral hydrolysis of azolides of aromatic carboxylic acids. Aliphatic carboxylic acid azolides which are subject to steric hindrance can be hydrolyzed in aqueous medium by an 5n1 process. There have been many studies of these reactions, and evidence supporting both 5n1 and 5n2 processes leaves the impression that there are features of individual olysis reactions which favour either an initial ionization or a bimolecular process involving a tetrahedral intermediate (80AHC(27)241, B-76MI40701). 

Besides substitution by amino, a chloro or bromo substituent may also be replaced by iodo, hydroxy, and alkoxy," though it is not possible to prepare 4-fluoroimidazoles by halogen exchange with metal fluorides on activated bromo- or chloroimidazoles." 4-Fluoroimidazoles are relatively resistant to nucleophiles, but the corresponding 2-fluoro compounds are subject to displacement (via an addition-elimination reaction) under mild conditions (see also Section III,B). Undoubtedly, the intermediate formation of a stable symmetrical 2-fluoroimidazolium cation (120) is responsible for this ease of displacement. (Eq. 28). Iodine at all ring carbons... 

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