McLafferty rearrangement with double

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) ... 

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. 

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]... 

McLafferty rearrangements miz 164 (loss of ethylene), 150 (loss of propylene), and a double-McLafferty loss to 122. Compound 490 exhibits two major fragments at mIz 100 and 98 in accordance with the fragmentation of piperazines. An assumed structure was confirmed by synthesis 464). 

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). 

Like the McLafferty rearrangements, retro Diels-Alder reactions occur in many types of compounds with various double-bond systans. The aromatic ring of tetralin, for example, promotes the expulsion of ethylene from its molecular ion (Equation 2.73). 

The empirical formula of the compound is C8H14O3 with two rings or double bonds. The M - C2H4 ion (m/z 130) along with the M - OC2H5, ion (m/z 113) is characteristic of an ethyl ester function. This is corroborated by the ion at m/z 88 (C4H8O2), which corresponds to the McLafferty rearrangement product. 

Many other examples of McLafferty-like rearrangements will be encountered with various classes of compounds. In general, this rearrangement requires only a double bond and a hydrogen atom in the proper relative positions, as shown by Equation 2.65, where the letters represent various types of atoms. The hydrogen migration may occur before cleavage of the C-D bond, but it cannot, of course, occur afterward. 

Fragmentation Loss of alkyl by fragmentation of the C-O bond with concomitant double H rearrangement to form the protonated sulfonic acid ion (m/z 97 for methanesulfonates), which then loses water. Loss of the alkoxyl residue (fragmentation of the S-O bond). Formation of an alkene ion from the sulfonate alkyl by a McLafferty-type rearrangement. In aryl esters, the phenoxy ion and the phenol radical cations dominate the spectrum. 

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