McLafferty rearrangement with double hydrogen transfer

4 McLafferty Rearrangement with Double Hydrogen Transfer 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H)  

Based on thermochemical data, evidence has been presented for a two-step mechanism that finally yields a protonated carboxylic acid and the radical. However, while the final products are well described, the second step has not fully been elucidated [100,101] 

Studying the competition of McLafferty rearrangement either with charge retention or charge migration and double hydrogen transfer has revealed that ion-neutral complex intermediates (Chap. 6.12) can also play a role for the latter two processes. [102] 

Note The ion series m/z 51, 77, 105 is a reliable indicator for benzoyl substructures, e.g., from benzaldehyde, benzoic acid and its derivatives, acetophenone, benzophenone etc. Different from benzylic compounds, the peaks at m/z 39, 65, and 91 are almost absent. If a peak at m/z 105 and the complete series m/z 39, 51, 65, 77, 91 are present, this strongly points towards the composition [CgHg] , and thus to phenylalkanes. In case of doubt, HR-MS is the method of choice for their differentiation. 

---

Figure 9.35 presents the El mass spectrum of butyl propanoate and the McLafferty rearrangement with double transfer of hydrogen atoms from the molecular ion. 

With several differently D-labeled 4-nonanones it could be shown (as illustrated for 4-nonanone-l,l,l-d3) that the double McLafferty rearrangement ion exists in an enolic form, which is created by hydrogen migration at the carbon-carbon double bond (c) and not by the isomerization of the oxonium ion (b) or by ketonization (e) of the intermediate single McLafferty enol. It is remarkable that no equivalent proton-transfer reaction could be observed for the dienol pent-l,3-dien-4-ol ion 218). 

---