Alkene Loss from Onium Ions

Aliphatic onium ions such as immonium, oxonium, and sulfonium ions have been introduced as even-electron ionic products of the a-cleavage occurring from molecular ions of amines, alcohols, and ethers or thiols and thioethers, respectively (Chap. 6.2.5). All these and analogous onium ions are capable of further fragmentation reactions, the majority of which are alkene losses [141] yielding fragments of high relevance for structure elucidation. 

The first of these reactions requires a y-hydrogen, and thus at least one Cs-alkyl substitutent. It may either be regarded as the even-electron analogy of the McLaf-ferty rearrangement (McL, Chap. 6.7) as it will be treated here, [85,86,142] or alternatively, as a retro-ene reaction. [143,144] 

The second alkene loss can occur from any onium ion bearing at least one C2-alkyl moiety, which obviously is the least demanding prerequisite for an alkene 

Whereas calculations seem to favor the concerted pathway, a large body of experimental data is in perfect agreement with the heterolytic stepwise mechanism (b). The first step of this, a 1,5-hydride shift, can be understood in terms of the onium-carbenium ion resonance  

Note Regardless of whatever the exact mechanism and regardless of whatever the correct name, the McLafferty rearrangement of onium ions is one of those processes allowing to reliably track ionic structures through a mass spectrum. 

The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

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Example The spectra of A7-ethyl-A -methyl-propylamine (Fig. 6.6), tripropylamine, and tributylamine (Fig. 6.43) also exemplify alkene loss from immonium ions via onium reaction. Oxonium ions are involved in the fragmentation of dieth-ylether and methylpropylether (Fig. 6.7) and many others have been published. [36,143,160-163] Below, the fragmentation of butyl-isopropylether is shown ... 

The loss of alkenes from aliphatic onium ions via onium reaction comprises scission of the C-X bond and concomitant transfer of a hydrogen from the leaving alkyl moiety to the heteroatom, and a merely phenomenological description of this reaction has already been included in the preceding schemes. 

The loss of alkenes from aliphatic onium ions via onium reaction comprises scission of the C-X bond and concomitant transfer of a hydrogen from the leaving alkyl moiety to the heteroatom, and a merely phenomenological description of this reaction has already been included in the preceding schemes. This seemingly simple reaction has to be regarded as a multistep process that involves ion-neutral complex (INC) intermediates. The properties of ion-neutral complexes are discussed in detail below (Chap. 6.12). Here, we restrict ourselves to the application of this concept to achieve a full and consistent explanation of the behavior of onium ions. 

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