McLafferty rearrangement mechanism

McLafferty, Fred Warren, 732 McLafferty rearrangement. 416, 732 Mechanism (reaction), 139 acetal formation, 717-718 acetylide alkylation, 272 acid chloride formal ion with SOCl2, 795... 

Example The observed KER values and also peak shapes may change dramatically as Eor decreases. Here, the chain length of a leaving alkene increases with an increase of the reacting alkyl substituent. Nevertheless, the mechanism of the reaction - McLafferty rearrangement of immonium ions (Chap. 6.7) - remains unaffected. [58]... 

The loss of alkenes from molecular ions of carbonyl compounds has early been noted. [23,82] Soon, a mechanism involving y-H shift and P-cleavage has been proposed and studied in detail. [24-26,83,84] Strictly speaking, the term McLafferty rearrangement only describes an alkene loss from molecular ions of satu-... 

In addition to the already described benzylic and phenylic cleavages (Chap. 6.4), phenylalkanes may undergo alkene loss by a mechanism that is perfectly analogous to the true McLafferty rearrangement, provided the alkyl substituent fulfills all requirements. The y-hydrogen is transferred to the ortto-position where the aromatic ring serves as the accepting double bond ... 

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. 

Now let us consider esters in which the alcohol portion is the predominant portion of the molecule. Esters of fatty alcohols (except methyl esters) eliminate a molecule of acid in the same manner that alcohols eliminate water. A scheme similar to that described earlier for alcohols, involving a single hydrogen transfer to the alcohol oxygen of the ester, can be written. An alternative mechanism involves a hydride transfer to the carbonyl oxygen (McLafferty rearrangement). 

Mechanism of the McLafferty rearrangement. This rearrangement may be concerted, as shown here, or the y hydrogen may be transferred first, followed by fragmentation. 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

When the McLafferty rearrangement affects the w-octyl group, the ion with mfe 158 is formed. The mechanism proposed by the ION GENERATOR is ... 

Also, the McLafferty rearrangement (the sequence of operations shortlived 6-membered transition state with transfer of a hydrogen atom, followed by j8-cleavage) is the mechanism in 7 as well as in 8, a daughter ion of 7. 

Mechanistically, the McLafferty rearrangement can be either concerted with simultaneous hydrogen transfer and 0-cleavage, or stepwise with the initial hydrogen transfer being followed by )0-cleavage, j More and more experimental evidence supports the stepwise mechanism rather than the concerted one, although they cannot be differentiated from theoretical studies.Furthermore, the infrared multiple photon (IRMP) activated dissociation of the butyrophenone molecular cation shows the involvement of a distonic ion as the intermediate, which provides solid support for the stepwise mechanism. ... 

The loss of ethylene proceeds by a McLafferty rearrangement, i.e. the transfer of a y-hydrogen to a radical site via a six-membered transition state. The evidence favours the view that this occurs by a stepwise, and not by a concerted mechanism, two alternatives that have recently been considered. 

---