Ortho elimination

Note Although not being correct to describe a reaction, the term ortho effect is well established, and another occasionally used term, retro-1,4-addition, is redundant 1,4-elimination would describe the same. Unfortunately, this might be confused with l,4-H20-eliminations from aliphatic alcohols, for example. Therefore, the term ortho elimination is suggested and used here. 

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 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. 

Ort/io-substituted nitroarenes can be distinguished from meta- or para-substituted isomers due to a characteristic change in their mass spectra. In precence of a hydrogen-donating ortho subtituent, [M-OH] replaces the [M-O] the fragment ion, e.g., in case of the above nitrophenols, the ion at m/z 123 is replaced by one at m/z 122 (Fig. 6.49c). The mechanism of this OH loss is not quite clear, [209] because different proposals [210-212] are in good accordance with the experimental data. Here, the mechanism which resembles the previously discussed ortho eliminations is shown [212]... 

Ortho eliminations find widespread application in the structure elucidation of aromatic nitro compounds, e.g., nitroanilines, [200] dinitrophenols, [213] trini-troaromatic explosives, [214] and nitrophenyl-methanesulfonamides. [199] (Scheme 6.75 reproduced from Ref. [199] with permission. IM Publications 1997) ... 

Maltol 337 is one of the degradation products in monosaccharide solutions with amino acids forming Amadori compounds but not in the solution of monosaccharides alone. Heated solutions of monosaccharides yield 335, the logical precursor of 337, but not 337 itself. On the basis of the molecular mechanics calculation indicating that 335 adopts the conformation unfavorable for dehydration into 337, a possible route via the dehydrated product 336, an ortho-elimination product, has been postulated as a more favorable alternate reaction pathway [277]. 

Yaylayan, V.A. Mandeville, S. Structural features of ortho elimination reactions of amadori rearrangement products under electron impact condition. Spectroscopy 1991, 9, 73-78. 

Ortho Elimination from Even-Electron Ions... 

Ortho eliminations are widely considered in the structure elucidation of aromatic nitro compounds such as nitroanilines [175], dinitrophenols [189], or trini-troaromatic explosives [190]. 

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