Ammonium compounds, eliminations Hofmann elimination

Sulfonium compounds (—C— SR2) undergo elimination similar to that of their ammonium counterparts (17-6 and 17-7) in scope and mechanism but this reaction is not of great synthetic importance. These syn-elimination reactions are related to the Cope elimination (17-8) and the Hofmann elimination (17-6). ... 

Hofmann elimination, quaternary ammonium compounds, 2 737 Hofmann-Gerhardt nomenclature,... 

New synthetic routes for the preparation of homologously pure dichloro DPX and tetrachloro DPX have been reported through the 1,6-Hofmann elimination of chlorinated y>-methylhenzyl trim ethyl ammonium hydroxide. In the case of dichloro DPX, yields of 30% were reported (21). In the presence of ketone compounds, yields were increased to 50% (20). 

A Pyrrolidine is a secondary amine and reacts with two moles of methyl iodide to form a quaternary ammonium salt, compound A. The analytical data for compound B shows that it has one double bond equivalent which is present as an alkene. Consequently, compound B has been formed from compound A by a Hofmann elimination ... 

In many cases it has been observed that reductive cleavage of quaternary ammonium compounds at lead is minor and that a Hofmann elimination is dominant this is brought about by an electrogenerated base. 

The stereochemistry of Hofmann elimination is commonly anti but less so than was formerly believed. Syn elimination is important for certain cyclic compounds, and can be made important even for open-chain compounds by the proper choice of base and solvent. Quaternary ammonium ions are more prone to syn elimination than alkyl halides and sulfonates. Electronically, anti formation of the double bond is favored in eliminations but when the alkene character of the transition state is slight—as here—other factors come into play conformational factors, it has been postulated. 

The parent thiophene 1,1-dioxide 1 has been generated by dehydrobromination of the bromide 2 (Scheme 2) [55-57]. The reagents used for this conversion include piperidine [55],R2CuLi,RLi,RMgBr,BuNH2 [56], and K/ultrasound [57]. The use of K/ultrasound system in toluene allowed the formation of 3-methyl-, 3-chloro-, and 3,4-dimethylthiophene 1,1-dioxides [57]. Hofmann elimination of the ammonium salt 4 led to the parent thiophene dioxide [58]. Treatment of compound 3 with bases also generates the parent thiophene dioxide by elimination of two molecules of HBr [57,59,60]. 

Hofmann degradation usually occurs with -elimination (El reaction). Wittig and Polster85 86 discovered a variant in which quaternary ammonium compounds are converted by strong bases such as phenyl- or butyl-lithium into olefins, by way of ylides, with intramolecular cw-elimination 87... 

For a quaternary ammonium ion to undergo an elimination reaction, the counterion must be hydroxide ion because a strong base is needed to start the reaction by removing a proton from a j8-carbon. Since halide ions are weak bases, quaternary ammonium halides cannot undergo a Hofmann elimination reaction. However, a quaternary ammonium halide can be converted into a quaternary ammonium hydroxide by treating it with silver oxide and water. The silver halide precipitates, and the halide ion is replaced by hydroxide ion. The compound can now undergo an elimination reaction. 

Once the fact that compound A is a tertiary amine is recognized, the reactions that lead to the formation of compound B should be seen as the formation of a quaternary ammonium salt with subsequent decomposition by heat (Hofmann elimination). The formation of quaternary ammonium salts can be generalized as follows ... 

However, it was suggested that if the residual anions could be completely removed from the organoclays, the primary degradation pathway would switch to an elimination-type mechanism [16]. The Hofmann elimination of ammonium compounds was most probably the source of additional amounts of vinyl-type unsaturation found in melt-processed OMMT-PE relative to both the polymer control and the Na+MMT-PE samples [28]. On the other hand, the presence of alkenes was also explained by three possible routes of decomposition (i) pyrolysis of alkanes derived from the major component of the organic part, i.e., hydrogenated tallow (HT) (ii) pyrolysis of the tallow (unsaturated fatty acids used for the preparation of the quaternary ammonium salt) and (iii) decarboxylation of RCOO and RCO radicals [17, 29]. 

The Hofmann fission of quaternary ammonium salts displays some unusual features in the steroid field. The elimination is normally of E2 character, and in acyclic structures gives olefinic products according to the familiar Hofmann rule [ 14]. The behaviour of 3-trimethylammonium steroidal salts is unexceptional [32, 3 ] the axial 3u compounds in the 5a-series giving A -olefins (5), or in the case of compounds already unsaturated at C-C 6) (6), the A -dienes (7). The 3j8-trimethylammonium-5a compounds are stable to bases, or... 

In E2 elimination with bases like KOH and CH30Na, most alkyl halides give Saytzeflf orientation. Certain other compounds (quaternary ammonium salts, Sec. 23.5, for example) give Hofmann orientation. Alkyl sulfonates fall in between. With each kind of compound, orientation is affected—sometimes drastically—by the choice of base and solvent, and by stereochemistry. (The percentage of 1-hexene from 2-hexyl chloride, for example, jumps from 33% in CH ONa/ CH3OH to 91% in /-BuOK//-BuOH, evidently for steric reasons.) In all this, we should remember that orientation is a matter of relative stabilities of competing transition states these stabilities are determined by electronic factors—alkene character and carbanion character—with superimposed conformational factors. 

Hofmann degradation of 5of-cholestan-4a- and -4/3-yI trimethylammonium salts closely resembled that of the C-6 quaternary ammonium salts the 4j -compound producing solely the 4-ene by Saytzeff elimination whilst the 4a-epimer gives largely demethylated 4a-dimethylamino-steroid, with some 3-ene. ... 

Usually ammonium, sulphonium and phosphonium compounds give the Hofmann products by 1, 2 elimination reactions. 

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