Hofmann elimination details

The purity of purchased aqueous [(C4H9)4N]OH varies in our experience in particular, concentrated aqueous solutions of [(C4H,)4N]OH (40%, Aldrich) gradually deposit solid at the bottom of the bottle, especially when stored below 30°C [as seems advisable to prevent possible Hofmann elimination reactions producing [(C4H,)3N + 1-butene + H20], In some instances, simple warming of the bottle briefly in a steam bath redissolves the solid however, it is better (as detailed below see also p. 1695 and footnote 11 elsewhere7) to prepare fresh aqueous... 

When both rings of the azoniaspiroalkane have five atoms or more, normal Hofmann elimination reactions occur, as in the early sequence shown in Equation (14) <06CB4347>. Where the choice is available, elimination can occur with opening of either ring. Jewers and McKeima <58JCS2209> have examined the degradation of 5-azoniaspiro[4,5]decanes (52) in some detail. For the parent system (52 R = R = H) a 1 1 mixture of alkenes (53) and (54) was obtained a substantial amount... 

Ionic liquid stability is known to be a function of temperature (for details see Section 3.1) but the presence of nucleophiles/bases and the water content also have to be considered. There is no doubt that, under the conditions of a catalytic reaction, temperature stability issues are more complicated than imder the conditions of a TGA experiment. The presence of the catalyst complex, the reactants and impurities in the system may well influence the thermal stability of the ionic liquid. Basic and nucleophilic counter-ions, reactants and metal complexes may not only lead to deprotonation of 1,3-dialkylimidazolium ions (to form carbene moieties that will undergo further consecutive reactions) but will also promote thermal dealkylation of the ionic liquid s cation. If basic reaction conditions are required for the catalysis only tetraalkylphosphonium ions can be recommended as the ionic liquid s cation at this point in time. Tetraalkylphosphonium cations have been recently shown to display reasonabe stability, even under strongly basic conditions [290]. In contrast, all nitrogen-based cations suffer to some extent from either carbene formation, Hofmann elimination or rapid dealkylation (with alkyl transfer onto the nucleophilic anion). 

One of the most common reasons for lowyields is an incomplete reaction. Rates of organic reactions can vary enormously, some are complete in a few seconds whereas rates of others are measured on a geological timescale. Consequently, to ensure that the problem of low yields is not simply due to low reactivity, reaction conditions should be such that some or all of the starting material does actually react. If none of the desired product is obtained, but similar reactions of related compounds are successful, the mechanistic implications should be considered. This situation has been referred to as Limitation of Reaction, and several examples have been given [32 ] the Hofmann rearrangement, for example, does not proceed for secondary amides (RCONHR ) because the intermediate anion 28 cannot form (Scheme 2.11). Sometimes, a substrate for a mechanistic investigation may be chosen deliberately to exclude particular reaction pathways for example, unimolecular substitution reactions of 1-adamantyl derivatives have been studied in detail in the knowledge that rear-side nucleophilic attack and elimination are not possible and hence not complications (see Section 2.7.1). 

The basic decomposition of tetraalkylammonium salts (the Hofmann degradation), has been reviewed extensively so.135) and will not be discussed here in detail. However, it should be noted that both displacement reactions and a-proton abstraction reactions may occur in addition to elimination reaction 30>. Ingold and Patel 75> report that the amount of substitution relative to elimination varies depending upon both the substituent on nitrogen and the base. 

In contrast to the relatively intractable laboratory oxidation products primary amines form on air and/or peroxide oxidation, the more stable oxides of tertiary amines have provided a platform for a significant amount of interesting chemistry. Among the reactions that have been explored in some detail are (1) the allylic N O rearrangements of suitably substituted A-oxides (Scheme 10.7), (2) the Cope (or Hofmann) type elimination reactions (vide infra and see Chapter 9) (Scheme 10.8), and (3) the Polonovsid reaction (Scheme 10.9), of particular interest because of its generality. 

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