Silver oxide elimination reaction

Silver oxide acts by exchanging hydroxide ion for iodide ion in the quatei nary salt, thus providing the base necessary to cause elimination. The acluE elimination step is an 0,2 reaction (Section 11.8) in which hydroxide ioi removes a proton at the same time that the positively charged nitrogen atom leaves. 

Many of the reactions of amines are familiar from past chapters. Thus, amines react with alkyl halides in S 2 reactions and with acid chlorides in nucleophilic acyl substitution reactions. Amines also undergo E2 elimination to yield alkenes if they are first qualernized by treatment with iodomethane and then heated with silver oxide, a process called the Hofmann elimination. 

Alkyl halides can be hydrolyzed to alcohols. Hydroxide ion is usually required, except that especially active substrates such as allylic or benzylic types can be hydrolyzed by water. Ordinary halides can also be hydrolyzed by water, if the solvent is HMPA or A-methyl-2-pyrrolidinone." In contrast to most nucleophilic substitutions at saturated carbons, this reaction can be performed on tertiary substrates without significant interference from elimination side reactions. Tertiary alkyl a-halocarbonyl compounds can be converted to the corresponding alcohol with silver oxide in aqueous acetonitrile." The reaction is not frequently used for synthetic purposes, because alkyl halides are usually obtained from alcohols. 

The Hofmann elimination is useful synthetically for preparing alkenes since it gives the least substituted alkene. The reaction involves thermal elimination of a tertiary amine from a quaternary ammonium hydroxide these are often formed by alkylation of a primary amine with methyl iodide followed by reaction with silver oxide. The mechanism of the elimination is shown in Scheme 1.13 in this synthesis of 1-methyl-1-... 

The sequence of reactions leading to compound 83 by an elimination that liberates the aldehyde 82, which is immediately reduced to 83, is depicted. This result also explains the formation of 43 by Purdie-Irvine methylations of 4 (Ref. 29) and 33 (Ref. 24) in both, moist silver oxide is the base, and it initiates elimination followed... 

N-Acylated Cys derivatives 32 on treatment with silver(I) oxide or silver(I) carbonate/1061 mercury(II) oxide/1071 iron salts/1081 or DCC1 091 undergo a (3-elimination reaction to yield AAla derivatives 33. The various (3-leaving groups that have been reported include sulfoxide/1101 sulfone/1111 5,5-dimethylsulfonium [Me2+S]/1121 2,4-dinitrophenylsulfanyl11131 and 5-dimethylphosphinothionyl11141 (Scheme 12). 

Hofmann elimination (Section 24.7) a method for effecting the elimination reaction of an amine to yield an alkene. The amine is first treated with excess iodomethane, and the resultant quaternary ammonium salt is heated with silver oxide. 

Cesium fluoride reacts with epoxides to form cesium alcoholates that can be transformed into a plethora of ethers and esters in good yield, for example, reaction of 12 to give 13. Methyl-ation with iodomethane is common. For sensitive alcohols silver oxide is the base of choice." No elimination to the a,)S-unsaturated sulfoxides was reported, when alcohols 14 were treated w ith iodomethane in dimethylformamide to give ethers 15." ... 

Whereas nucleophilic substitution occurs on heating with water, aqueous potassium carbonate, silver oxide or sodium acetate, elimination reactions occur on heating an alkyl halide with ethanolic potassium hydroxide. Both unimolecular (El) and bimolecular (E2) pathways occur, the former with tertiary and the latter with primary and secondary halides. The reactions of alkyl halides with oxygen nucleophiles are summarized in Scheme 2.3. 

Apart from the reactions of diazonium salts, a number of other reactions are known in which the C-N bond is broken. The best known of these is the Hofmann elimination of quaternary ammonium hydroxides (Scheme 2.37). An amine is converted by methylation with methyl iodide to the quaternary ammonium salt ( exhaustive methylation ). The iodide, on treatment with moist silver oxide, forms the quaternary ammonium hydroxide which undergoes a bimolecular elimination to form an alkene. The bimolecular elimination of onium salts yields the least alkylated alkene. This substitution pattern is determined by the ease with which a hydrogen atom can be attacked by the base. 

Kawasaki et al. [91] looked at the oxidation kinetics of large photolytic silver clusters in redox buffer solutions. The process was analyzed in terms of a series of stepwise reactions involving electron transfer and silver ion elimination. It was shown that the single-step oxidation ... 

Amines are converted into alkenes by a two-step process called the Hofmann elimination. Reaction of the amine with excess CHjI in the first step yields an intermediate that undergoes E2 reaction when treated with basic silver oxide. Pentyl-amine, for example, yields 1-pentene. Propose a structure for the intermediate, and explain why it undergoes ready elimination. (See Section 24.7.)... 

The use of transition metal species can lower appreciably the decomposition temperature of ot-diazo-carbonyl compounds they can also alter the reactivity of the carbene intermediate (resulting from the initial nitrogen elimination see Section 3.9.2.1) by complex formation. Hence, the Wolff rearrangement may occur with difficulty or, usually, not at all. Thus, some copper species (excepting, for example, Cul), or Rh and Pd catalysts are inappropriate. Freshly prepared silver(I) oxide has been used most frequently, but silver salts (especially silver benzoate) are sometimes preferred.Silver-based catalysts are usually employed in combination with an alkaline reagent e.g. sodium carbonate or a tertiary amine). Even under silver catalysis competing reactions may be observed, and sometimes the products of Wolff rearrangement may not be obtained (see Section 3.9.2.3). 

An example of a reaction that shows this latter regiochemistry is the Hofmann exhaustive methylation, which is sometimes called, rather ambiguously, the Hofmann degradation (this is a term best avoided, because it is sometimes also used to describe the Hofmann rearrangement). In this reaction, an amine is methylated with methyl iodide until the quaternary ammonium iodide is formed. This is then treated with moist silver oxide to convert the iodide to the hydroxide. Write out the final elimination step that occurs on heating. 

Nitrosocarbonyl compounds, prepared by the oxidation of hydroxamic acids with silver oxide, add to 2,5-dimethylfuran to give adducts (27) in the expected way, but these immediately rearrange to 1,4,2-dioxazoles (28). A special feature is that the reaction must be easily reversible since heating the adducts causes them to eliminate dimethylfuran, leaving the nitrosocarbonyl intermediate to be trapped by some other means.68... 

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