Patai S., Rappoport Z. 1996 The chemistry of functional groups. The chemistry of amino, nitroso, nitro and related groups / 0249 294

NO

NO2

SCHEME 23

The transfer of two oxygens to the acetylenic carbons involving a series of rearrangements, followed by the loss of two CO molecules, leads to the phenothiazine radical cation, with the proposed fragmentation pathway being supported by collision activation MS89; see Scheme 24. Although the transfer of oxygen to sulphur apparently is of minor importance, the loss of a HSO2ž radical from a rearranged molecular ion seems to resemble the mechanism proposed for the equivalent fragmentation of allyl 2-nitrophenyl sulphide and 2-nitrophenyl styryl sulphides86,87,89.

NO2

N

H

SCHEME 24

2-nitrobenzenesulphenamides constitute an interesting class of compounds90. Thus, unexpected ortho interactions of the nitro groups apparently operate as the molecular

SCHEME 25

A competing oxygen migration in ortho nitroaryl thioamides on electron impact has been reported91. The loss of SO from the molecular ions is a highly significant process91. A mechanism involving an initial oxygen migration to the sulphur atom through a favourable six-membered TS followed by cyclization with concomitant expulsion of SO has been proposed91; see Scheme 26.

O

SCHEME 26

The migration of an oxygen atom from the nitrogen of the nitro group to the nitrogen moiety of the thioamide results in the formation of a stable o-nitrosothiobenzoyl cation91. It is assumed that the actual mechanism to be initiated by the migration of oxygen through a six-membered transition state is followed by an ˛-fission91; see Scheme 27.

NO

SCHEME 27

F. Polynitro Aromatics

1. Nitroadamantanes

The fragmentation pathways of adamantane, 1-nitroadamantane, 1,3-dinitroadamantane and 1,3,5,7-tetranitroadamantane have been studied by tandem high-resolution MS92. It was found that the fragmentation of the nitroadamantanes was initiated by consecutive losses of the nitro groups and followed by fragmentation of the hydrocarbon back-bone92.

2. Diand tri-nitroaromatics

The fragmentations of 2,4,6-trinitrotoluene have been studied using isotope labelling and MS/MS techniques93,94. The major pathways include the loss of OHž and H2O, followed by the subsequent loss of NO or NO2. The facile transfer of a methyl hydrogen to oxygen gives rise to the key fragment (m/z 210) initiating a number of important fragmentation sequences93. There is virtually no ring disintegration until the majority of the attached groups are lost94.

In addition, a series of dinitroaromatic compounds has been studied by collision activation MS95. Strong effects due to competitive processes were noted95.

G. Nitro-heteroaromatics

1. Nitroazoles

The substituted heteroaromatic compounds provide excellent opportunities for mechanistic studies of the interaction between the nitro group and other substituents. This is due

282 Helge Egsgaard and Lars Carlsen

to the unique synthetic possibilities as well as a more straightforward evaluation of the role of the aromatic back-bone. Thus, MS of nitroazoles, i.e. nitropyrazoles and nitroimidazoles, has been investigated in detail by the effect of substitution and isotope labelling by Luijten and Thuij96 99.

Various nitropyrazoles and nitroimidazoles have been studied96 99; see Table 1. The loss of OHž from methyl-substituted nitropyrazoles and nitroimidazoles appears as a useful probe in determining the actual location of the substituents. Thus, it could be established that in the case of adjacent methyl and nitro groups, the loss of OHž originates exclusively from these substituents96 98. In addition to the loss of OHž the presence of a mobile N-bonded hydrogen atom leads to the elimination of H2O, as is visualized by the mechanisms for loss of OHž and H2O from 3(5)-methyl-4-nitropyrazole and 4-methyl- 3(5)-nitropyrazole, respectively98; see Scheme 28.

The presence of a significant kinetic isotope effect for the losses of OHž from the nitroazoles (kH/kD D 4.6) supports the suggestion that the fragmentations are adequately described by a stepwise hydrogen transfer, followed by cleavage of the OH moiety or, alternatively, rearrangement to structures which can eliminate OHž and H2O98.

Loss of small O-containing molecules, such as CHOž and CH2O, can also take place. Three structural requirements for these reactions have been identified: (1) The methyl and nitro group must be in a mutual ortho position. Further, the reaction is strongly favoured if (2) the methyl group is bonded to a ring nitrogen and (3) a second heterocyclic moiety or a second heteroatom is close to the methyl group99. This is reflected in the relative intensities of the fragment ions. The reaction mechanisms for loss of the formyl radical and the elimination of formaldehyde are illustrated by the fragmentations of 1-methyl-5- nitroimidazole and 1-methyl-2-nitroimidazole, respectively99; see Scheme 29.

SCHEME 29

It should be noted that the operation of these effects is attributed to the interaction of substituents only. Thus, in no cases has involvement of ring atoms of the diazole in the losses of CHOž and CH2O been detected99. The use of the labelled compounds unambiguously demonstrated that the diazole ring of the molecular ion structures is retained in the fragment ion discussed above99.

The MS of some nitro derivatives of imidazole-4(5)-carboxaldehyde have been studied100. Based on the compounds 15 17, several new features caused by the interaction between the nitro group and adjacent substitutents were disclosed. It should be noted that for the compounds 16 and 17, there is a rapid equilibrium between the two possible tautomers resulting in equivalence of the 4 and 5 positions.

Surprisingly, the [M H2O]Cž and [M CH2O]Cž ions are observed, whereas there is virtually no loss of CHOž100 . The fragmentation pattern of 5(4)-nitro-imidazole-4(5)

carboaldehyde, 17, corroborates the presence of the formyl group. Thus, the primary losses of OHž and H2O are consistent with the H-shift from the formyl group to the nitro group followed by the loss of OHž or by elimination of H2O via mechanisms similar to that reported for methylnitroimidazoles; see Scheme 30. The ion corresponding to loss of H2O is the base peak in the MS of 17100.

SCHEME 30

The elimination of OHž and H2O has been explained by assuming an initial H-transfer from the formyl group to the nitro group, followed by a migration of a hydrogen atom from the methyl group to the methyl-carrying nitrogen atom in line with the fragmentation of ‘o-methylnitroimidazoles’100.

2. 3-Nitro-2H-chromenes and 3-nitrochromanes

The electron impact mass spectra of series of 2-aryl-3-nitro-2H-chromenes and 2-aryl- 3-nitrochromanes with varying functionalities have been reported101. The molecular ion is always of considerable intensity. Loss of NO2 from MCž is responsible for the base peak in the nitrochromenes101. In contrast, the [M HONO]Cž is apparently the more abundant ion in the nitrochromanes. This unusual loss of HONO is always observed in the metastable time-frame101.

The formation of [M HONO]Cž has been demonstrated by deuterium labelling to be associated with hydrogen transfer from the 2-position to the nitro group101. This leads to a stabilized radical cation which, following the expulsion of HONO, produces a very stable daughter ion; see Scheme 31.

SCHEME 31

All the compounds studied exhibited M ž accompanied by intense [M NO2] ž fragment ions in their electron capture negative ion chemical ionization spectra. Additional ions were of low intensity101.

Chemical ionization MS of 2-aryl-3-nitro-2H-chromenes and 4-hydroxy-3-nitroflavans have been studied using methane and ammonia as reagent gases102. The behaviour of 2-aryl-3-nitro-2H-chromenes was found to resemble that of aromatic nitro compounds. Thus, the methane spectra are characterized by the MHC ions, whereas the ammonia

spectra reveal dominant cluster ions, e.g. [M C NH4]C102 . Chemical ionization of the 4- hydroxy-3-nitroflavans leads, on the other hand, to a significantly increased fragmentation. Thus, the base peaks in the methane spectra are due to loss of H2OCNO2. In the ammonia spectra a rather complicated picture developed. Apparently, the dominant fragmentation is a retro Diels-Alder reaction of the ammonium adduct102; see Scheme 32.

SCHEME 32