Ortho Elimination from Molecular Ions

Commonly, ortho elimination refers to a hydrogen transfer via a six-membered transition state at ortto-disubstituted aromatic compounds. In practice, the reacting entities are almost in position to form this six-membered transition state. The general mechanism of the ortho elimination is as follows  

As indicated by metastable ion studies of isopropylbenzoic acids, [202] the formation of the ion at m/z 90 should be described as a two-step process, i.e., the product rather is [M-OH-CHO] than [M-HCOOH]. This would add yet another example to the list of violations of the even-electron rule (Chap. 6.1.3). 

As with the McLafferty rearrangement and the retro-Diels-Alder reaction before, the occurrence of the ortho elimination is not restricted to molecular ions. It may equally well proceed in even-electron species. 

Example Examination of the mass spectrum of isopropylbenzoic acid reveals that the molecular ion, m/z 164, as well as the [M-CHs] ion, m/z 149, eliminate H2O via ortho elimination yielding fragment ions at m/z 146 and 131, respectively (Fig. 6.47). [202] The [M-CH3-H20] ion, a homologue of the benzoyl ion, decomposes further by loss of CO, thereby creating a fragment that overall corresponds to [M-CHs-HCOOH]-, m/z 103. 

Example The molecular ion of l,2-bis(trimethylsiloxy)benzene, m/z 254, undergoes methyl loss by Si-C bond cleavage as typically observed for silanes (Fig. 6.48). Rearrangement of the [M-CHb] ion then yields [Si(Me)3], m/z 73 (base peak). This is not an ortho elimination with concomitant H transfer as defined in the strict sense, but the observed reaction is still specific for the orthoisomer. [190,203] In the spectra of the meta- and para-isomers the [Si(Me)3] ion is of lower abundance, the [M-CHb] ion representing the base peak in their spectra. Moreover, the m/z 73 ion is then generated directly from the molecular ion which is clearly different from the two-step pathway of the ortho-isomer. 

---

A partial insight in the complexity of such ortho effects (even in the case of simple compounds) permitted the thorough investigations of Middleton77. The data for 2H and 13C labelled ortho nitrostyrene show for example, that the loss of HCO from the molecular ion proceeds via the epoxide 70. The carbon atom of the neutral particle, however, originates from both side chain carbons, and the elimination probably corresponds to the well-known epoxide rearrangement/fragmentation78. ... 

The nitroarenes with an ortho substiment bearing a hydrogen show loss of H2O from the protonated molecule. This fragment frequently turns out as the predominant ion in the MS. It should be noted that this fragmentation is absent in the corresponding meta and para isomers . The H2O elimination resembles the characteristic loss of OH from the molecular ions in the electron impact mass spectra (cf Section IV.D.l) and is as such indicative of protonation of a nitro group followed by [1,4] elimination of H2O see Scheme 35. 

However, the intense fragment peak immediately below the molecular ion has an even mass and corresponds not to a simple cleavage product but to the multicenter elimination of water. Such an ortho effect is not uncommon in mass spectrometry and can be rationalized by a six-membered transition state. The subsequent loss of CO may occur by expulsion of the group from the ring or from the new ketene function (Equation 2.99). Meta and para isomers of this compound fragment in a manner parallel to that of benzoic acid loss of a hydroxy radical followed by loss of CO (Equation 2.100). 

An example of structural discrimination was found in the mass spectra of isomeric N-nitrophenyl benzenesulfonamides 11781. Common features of the mass spectra of ortho, meta and para isomers were the presence of [M — H]- ions, more intense than the molecular anion, and the presence of fragments at m/z 137 and m/z 141, originating from N—S bond rupture. However, the ratio of the intensities of the latter ions was markedly higher for the ortho and para compounds, which reflected the stabilization of the m/z 137 anion by a nitro group in a conjugatively effective relationship. The ortho isomer was further differentiated by the elimination of hydroxyl radical, to form [M — OH] - ions. 

---