J. Phys. Chem. 1995, 99, 5009-5015
5009
Thermal Rearrangement of 1,4=Dinitroimidazoleto 2,4-Dinitroimidazole: Characterization and Investigation of the Mechanism by Mass Spectrometry and Isotope Labeling S. Bulusu,* R. Damavarapu, and J. R. Autera Energetic Materials Division, US.Army Armaments Research, Development and Engineering Center, Picatinny Arsenal, New Jersey 07806-5000
R. Behrens, Jr., and L. M. Minier Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551-0969
J. Villanueva and K. Jayasuriya Geo Centers, Inc., at US.Army Armaments Research, Development and Engineering Center, Picatinny Arsenal, New Jersey 07806-5000
T. Axenrod Department of Chemistry, The City College of The City University of New York, New York 10031 Received: November 9, 1994@
The thermal rearrangement of 1,4-dinitroimidazole to 2,4-dinitroimidazole has been investigated by differential scanning calorimetry and mass spectrometry techniques. When mixtures of independently prepared deuteriumand 15N-labeled samples of the 1P-isomer were subjected to thermal rearrangement, the resulting 2,4-dinitroimidazole failed to show isotope-scrambled molecular ions in its mass spectrum, suggesting that the reaction was intramolecular in nature. This was interpreted to mean that the mechanism was of the (1,5)-sigmatropic type rearrangement. Extensive NMR measurements were used to obtain unequivocal evidence for the identity of the assumed structures of the isomeric dinitroimidazoles. Two byproducts (4-nitroimidazole and a trinitroimidazole), formed during the rearrangement reaction, have also been identified. Plausible mechanisms for their formation are discussed.
Introduction In an ongoing program to develop new and improved explosives and propellants in these laboratories, 2,4-dinitroimidazole (2,4-DNI, I), among others, has been found' to hold considerable promise. Firstly, 2,4-DNI is much less sensitive than RDX and HMX, which are high explosives extensively used at present, and secondly, it is about 15-20% more energetic than TATB, which is an important insensitive explosive with relatively low energy output. 2,4-DNI is also thermally more stable than HMX. Furthermore, since it can be made2s3a-cfrom the inexpensive starting material, imidazole, it is expected to be relatively low in cost. To fully explore its potential use in munitions, all its important properties such as thermal stability? decomposition kinetics and mechanism? crystal structure,4b detonation parameters,& and improved synthetic methods4dare currently being investigated. In the work presented here, the synthesis of 2,4-DNI from imidazole, in three steps which include the thermal rearrangement of the 1Q-dinitro isomer (1,4-DNI, II) to the 2,4-dinitro isomer, is studied with two objectives. The first objective was the investigation of the mechanism of thermal rearrangement, making use of mass spectrometry and isotope (15N and *H) labeling techniques. A second objective was to obtain conclusive evidence for the identity of the isomer structure obtained. This was accomplished by utilizing differential scanning calorimetry, mass spectrometry, and 13C NMR spectroscopy techniques.
* Author to whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, March 15, 1995.
0022-3654/95/2099-5009$09.00/0
The complete synthesis of 2,4-DNI starting from imidazole is outlined below. (In this paper the numbering shown for the imidazole ring in Scheme 1 is consistently followed. In the literature 2,4-DNI is sometimes referred to as 2,4(5)-DNI.) Its synthesis from imidazole by nitration to 1,4-DN13b3Cfollowed by thermal rearrangement represents an improvement over its first reported synthesis from 2-nitroimida~ole.~ The thermal rearrangement is carried out by heating the 1,4DNI at -115 "C in chlorobenzene (bp 132 "C) solution for several hours, causing the mp to change from 92 "C corresponding to 1,4-DNI to the mp of 2,4-DNI, 264-267 "C. However, there was a possibility of obtaining 4,5-dinitroimidazole derivative on thermal rearrangement. To our knowledge, the earlier published work did not present unequivocal evidence for the identity of the assumed isomer (2,4-dinitro). However, because of other indirect evidence, there is no apparent reason to doubt that 2,4-DNI was indeed the product obtained. One of the objectives of this work was to obtain direct evidence for the structure of the product by employing NMR spectroscopy. With respect to the migration of the NO:! group from the 1to the 2-position of imidazole in the last step, either an intramolecular 1,5-~igmatropicshift or a reaction path through an intermediate 1,3-shift to 4,4-dinitro-isomer seemed possible. It was of interest, therefore, to find whether or not the 4,4dinitro isomer was involved as an intermediate and to ascertain whether the rearrangement is intramolecular or intermolecular. The intra- versus intermolecular nature of the rearrangement was tested by designing an isotope-scramblingexperiment with 15N- and :!H-labeled analogues of 1,4-DNI. 0 1995 American Chemical Society
5010 J. Phys. Chem., Vol. 99, No. 14, I995
Bulusu et al.
1, 4-DNI SCAN RATE:
o'oO
'
50.00
60.00
3.00 deg/miri
liO.00
140.00
IjO. 00
200.00
230.00
TEMPERATURE
260.00
2d0.00
320.00
35[ DO
DSC Figure 1. Thermogram obtained by differential scanning calorimetry of 1.4-DNI showing the mp endotherm followed by two exothems. (C>
SCHEME 1
I
Imidazole
NO2
4-NI, (Ill)
Experimental Section Materials. 4-Nitroimidazole (4-NI) and imidazole44 were obtained from Aldrich Chemical Co., Inc., Milwaukee, WI. Nitric acid-15N(>99%) was prepared from sodium nitrate-I5N obtained from Cambridge Isotopes Laboratories, Inc., Andover, MA. Briefly, the procedure consisted of treatment with concentrated sulfuric acid on a vacuum line and distillation under high vacuum, according to the procedure previously described.6 4-Nitroimidazoled3. To imidazoled4 (1 g) contained in a reaction flask was added 70% nitric acid (4 mL) with stirring until a clear solution was obtained. To this solution, chilled to -5 "C, was added concentrated sulfuric acid (2 mL) dropwise under continuous stirring for 15 min. The solution was then refluxed gently for 2 h, cooled to room temperature, and poured over crushed ice (-40 g). The precipitate was collected by filtration, washed free of acid, and vacuum dried: yield 1.O g, mp 303 "C. The sample was recrystallized from ethyl alcohol. 1,4-Dinitroimidazoled2 and -I -15N02. To a solution of 4-nitroimidazole-d3 (0.55 g) in glacial acetic acid (1.25 mL) maintained at -17 "C was added dropwise (-30 min) under stirring 0.4 mL of 100%nitric acid. This was followed after a few minutes with dropwise addition of acetic anhydride, 1.0 mL. The temperature was held at 17 "C for another 2 h, following which the solution was stirred overnight at ambient
1,4-DNI,(11)
H
2.4-DNI, (I)
temperature. It was then poured over -20 g of crushed ice, and the precipitate obtained was filtered, washed free of acid with water, and vacuum dried: yield 0.4 g, mp 92-93 "C, molecular ion in mass spectrum at mlz 160. The l5N-labeled sample was prepared by an analogous method starting with 4-NI and substituting H15N03 for unlabeled nitric acid. It gave a molecular ion in the mass spectrum at mlz 159. 2,4-Dinitroimidazole-d~and -2-15N02. A thoroughly dried sample of 1,4-DNI (0.2 g), labeled either way as above, was heated with stirring in 10 mL of chlorobenzene at -115 "C overnight, using anhydrous conditions. Traces of moisture and higher temperatures promote formation of 4-nitroimidazole as a byproduct. About half of the solvent was then removed by evaporation and the solution allowed to cool to room temperature; 2,4-dinitroimidazole then crystallized out. It was collected by filtration and dried in vacuum at -60 "C: yield 0.175 g, mp 260 "C, molecular ions in mass spectra at mlz 160 and 159, respectively. Isotope-Scrambling Experiments. (1) 1,4-DNI samples (5 mg, each) labeled with deuterium and nitrogen-15, respectively, as described above, were dissolved together in acetone and recovered by evaporation of the solvent. This sample was then converted to 2,4-DNI by thermal rearrangement at -125 "C in the solid sample probe of the mass spectrometer, prior to
Thermal Rearrangement of 1,4-Dinitroimidazole
J. Phys. Chem., Vol. 99, No. 14, 1995 5011
0
Figure 2. Ion signals and weight loss observed in the heating of 1,4-DNI as measured with the STMBMS. The alumina reaction cell was fitted with a 25 p m diameter orifice and was heated at 4 "Clmin.
obtaining a mass spectrum of the product at -200 "C. (2) For thermal rearrangement in chlorobenzene, the two isotopic samples were directly dissolved in this solvent, in equal amounts, and treated as in the preceding procedure. Standard mass spectra of 1,4- and 2,4-&nitroimidazoles, their isotopic analogues, and the products of thermal rearrangements were obtained on a DuPont Model 21-492 mass spectrometer, served by a Teknivent Corp. Vector One data handling system. Differential Scanning Calorimetry. The thermogram of 1,4DNI shown in Figure 1 was obtained using a Perkin-Elmer DSC-4 equipped with a system 4 thermal analysis microprocessor controller and Perkin-Elmer thermal analysis Data Station. Approximately 2 mg of sample was placed in the aluminum sample holder and crimped with a cover in place. It was heated at 3 "C per min. Mass Spectrometry. For the data shown in Figure 2, a unique system known as simultaneous thermogravimetric molecular beam mass spectrometry apparatus (STMBMS) previously described in detail,7 was used. However, a brief description of this equipment is included in the Results and Discussion section. NMR Data. All the NMR spectral data were recorded on a Bruker N R 300 spectrometer. 'H and 13Cchemical shifts were measured from tetramethylsilane.
Results and Discussion
1. Thermal Rearrangement of 1,4- to 2,CDinitroimidazole. As mentioned in the Introduction, the thermal rearrangement of 1,4-DNI to 2,4-DNI is carried out by heating the sample at -115 "C in chlorobenzene2cfor several (-15-20) hours. A byproduct of this reaction is 4-nitroimidazole, the amount of which seems to depend on the exact conditions of the experiment such as moisture and higher temperature, both of which increase
its formation. However, the rearrangement could be carried out, without the presence of the solvent, by heating a sample either in a DSC sample pan or in a mass spectrometer solid sample probe. The thermogram obtained from the DSC experiment is shown in Figure 1. This shows, fist, the expected mp endotherm at 92 "C for the 1,4-DNI and, second, two exotherms starting at -115 and -215 "C, respectively. In a separate run which was quenched immediately after the first exotherm, the sample was found to have solidified, and its mp, obtained by the conventional capillary method, changed to -265 "C. This clearly suggests that the f i s t exotherm corresponded to the transformation of the 1,4- to the 2,4-isomer of the dinitroimidazole. This exotherm may also include denitration to 4-NI since the mass spectral data presented below clearly show its formation in greater amounts in the solid state than when the reaction was carried out in chlorobenzene. It was assumed that the second exotherm at -230 "C in the DSC (Figure 1) corresponded to the decomposition of the 2,4-DNL8 2. Confirmation of the Structural Identity of 2,4-DNI by NMR. Because of the possibility of obtaining the 4,5-dinitro isomer in the rearrangement reaction, NMR evidence was sought to exclude this possibility. Proton and 13C chemical shifts and the l3C-IH coupling constants (measured by gated decoupling experiments) of 4-nitroimidazole (4-NI), 1,4-DNI, and 2,4-DNI are summarized in Table 1. The structure of 4-NI was clearly established by the early work of Farger and P ~ m a nand , ~ ~it is assumed, therefore, to be correct. The labile protons of the imino groups in 4-NI and 2,4-DNI (11.2 and 11.7 ppm, respectively) are readily recognized by the broadening of their resonance lines. Next, the protons at the 5-position in all three compounds may be expected to be more deshielded compared to those at the 2-position by virtue of their proximity to the 4-nitro group in each one. It is reasonable, therefore, to assign
Bulusu et al.
5012 J. Phys. Chem., Vol. 99, No. 14, 1995
1
L
the resonances at 8.3,9.4, and 8.6 ppm, respectively, to protons at the 5-position in each of the three compounds. As to the 13C chemical shifts, the most "downfield' ones in all three compounds may be assigned to the nitro-substituted C-4 carbons. The C-2 carbon in each compound, being flanked by the imidazole-ring nitrogens, would be deshielded somewhat but may be expected to resonate at a slightly higher field than the C-4's. However, the C-2 in the presumed 2.4-DNI structure should be displaced downfield due to the nitro-substitution at this carbon. This is confirmed by the data shown in Table 1 (C-2, 144.2 ppm, and C-4, 144.9 ppm, respectively). The resonance of C-5 carbons in all cases should appear at higher fields relative to the C-2 and C-4 resonances, as, in fact, is the case (see Table 1). Additional confirmation of the C-2 resonance assignment comes from the NMR of the 15N-labeled 2,4-DNI (2-15N02). The data in Table 1 show that the C-2 resonance is split into a doublet (.hC(2)-lsN(2)= 30.6 Hz)in its l3C spectrum, presumably by the lSN. Thus, the fact that a doublet was observed for the 13Cresonance at the 2-position not only confirms the assignment of C-2 resonance but also establishes that the nitro group migrates from the 1-position to the 2-position and not to the 5-position. There exists strong NMR support, therefore, for the assumed identity of the 2,4-DNI structure. It is also interesting to see that the C-4 resonance in 2,4DNI (2-15N02) is a singlet. If the 1,4- 2,4-transformation proceeded through a 4.4-dinitro intermediate, statistically only 50% of the 15Nwould move to the 2-position. This would have produced a doublet for the C-4 resonance. In fact, both C-2 and C-4 resonances would have appeared as triplets because half the carbons would not have 15N's attached to them. Thus, this result effectively excludes the possibility of a 4,4-dinitro intermediate in the nitro group rearrangement. 3. Rearrangement Observed by Mass Spectrometry. Figure 2 presents typical mass spectrometric data showing the events taking place on heating 1Q-DNI at a linear rate. In this experiment, a small sample (-5-10 mg) contained in an alumina reaction cell is heated, with the temperature increasing linearly with time until the desired limit is reached. At the same time, the vapor evolving from the reaction cell which is positioned on top of the thermocouple probe of a thermogravimetric analyzer (TGA) is continuously analyzed in a molecular beam mass spectrometer located vertically above. The cap of the reaction cell has a variable size orifice for the escape of the vapor species, the larger size orifice (500 p m diameter) permitting vaporization of the sample itself and the smaller size orifice (25 pm diameter) forcing the decomposition of the sample due to increased containment time. In this procedure the signal at each m/z value is counted with a dwell time of 250 ms, sequentially, from m/z 1 to m/z 158 (MW of dinitroimidazoles). The repetition rate of the mass scans is l/min. Complete details of this apparatus and the procedures were previously de~cribed.~ The data presented in this discussion are collected from two slightly different experiments. In the first one, an 8.30 mg sample of 1,4-DNI was heated at 4 "C/min up to 350 "C using a 25 pm diameter orifice on the reaction cell. The results are shown in Figure 2, in which the billboard shows the weight loss (TGA) of the sample, recorded by the microbalance. The x-axis in Figure 2 shows temperature (as well as time since the two are linearly related), the y-axis m/z of the species measured, and the z-axis the evolution of the signal intensity with temperature. The TGA measurement shows two weight loss regions, the first between 90 and 150 "C and the second between 200 and 310 "C. In the first region there are ion signals at five
-
ii
c
Thermal Rearrangement of 1,4-Dinitroimidazole
v)
J. Phys. Chem., Vol. 99, No. 14, 1995 5013
TABLE 2: Percent Yield of the Major Components Formed from the Isothermal Decomposition of 1,4-DNI (10.68 mg) at 91 "C for 18 h
(*lo9 4.0
3-
compound 2,4-DNI
3.0
4-NI TNI
v
a)
c,
2
unreacted 1,4-DNI other products"
2.0
.-5
3
>
1.0
w
s
yield (%) 8.3 33.3
1.o 33.5 23.9
Products include NO, NOz, COZ, and other minor species.
c,
5
total mass (mg) 0.88 3.55 0.11 3.58 2.56
v)
0.0
0
50
100
150
Temperature
200
250
("C)
Figure 3. Gas evolution rates of the products formed from conversion of 1,4-DM held at 91 "C for 18 h. principal m/z values. These ion signals originate from 1,4-DNI (mlz = 46, l58), N2 or CO (mlz = 28), NO (mlz = 30), C02 or N2O (mlz = 44),and NO2 (mlz = 46). In the second region there are numerous mass peaks that arise from both the conversion products from 1,4-DNI and also their thermal decomposition products. In this region the main ion signals associated with the conversion products from 1,CDNI are 2,4DNI (mlz = 158, 30) and 4-NI (mlz = 113, 28). The large signals between m/z = 18 and 60 arise from the thermal decomposition of 2,4-DNI and 4-NI. The large size of the 4-NI signal (mlz = 113) compared to the 2,4-DNI signal indicates that more 4-NI is formed in this experiment than 2,4-DNI. This is the opposite of that observed in the transformation of 1,4DNI in a chlorobenzene solution. To see if this large ratio of 4-NI to 2,4-DNI may be due to the conversion taking place over a temperature range from 90 to 150 "C in a relatively short time (900 s), a second set of experiments was performed. In the second set of experiments, 10.68 mg of a 1,4-DNI sample was placed in an alumina reaction cell fitted with a 25 p m diameter orifice and held at 91 "C for 18 h. During this time the sample lost 2.2 mg (21%) from evaporation of 1,4DNI and 1.9 mg (19%) from volatile decomposition products. After the remaining sample cooled to room temperature, the identity and quantity of the resulting products of the isothermal reaction were obtained by substituting a 500 pm diameter orifice on the reaction cell and collecting the mass spectra of the products as they were allowed to evaporate. In the evaporation process, the sample was heated at 2 " C h i n , and data similar to that shown in Figure 2 were collected. In this experiment the mass spectra were collected above the molecular weight of 2,4DNI (mlz = 158) with the result that an ion signal at mlz = 203 was observed. In light of the high thermal stability (Figure 3) and previous studies on 2,4,5-trinitroimidazole (TNI),2 the signal at mlz = 203 is attributed to this product. In addition, the evaporation process was completed by 210 "C, and no thermal decomposition products from 4-NI, 2,4-DNI, or TNI were observed, as in the first experiment. The rates of gas evolution of the products from the reaction cell are shown in Figwe 3. Integrating the rates of gas evolution gives the amount of each product formed in the conversion process. The yield of the various products from the original 10.68 mg sample are shown in Table 2. In this experiment approximately 8% formed 2,4-DNI, 33% formed 4-NI, 1% formed TNI, 24% formed various other decomposition products (NO, NO2, C02/N20), and 33% was lost to evaporation (of the evaporation, 63% is lost in the isothermal part of the experiment and 37% during
TABLE 3: Molecular Ions in the Mass Spectra from Isotopic Analogues of 1,4- and 2,4-Dinitroimidazoles and the Thermal Rearrangement Products of a Mixture of 1,4-Dinitroimidazole-dzz and 1,4-Dinitroimidaz0le-~-~~NO~ Mf, m h ves no Pure Isotopic Analogue 1,4-dinitroimidazole-dz 160 X 1,4-dinitroimida~ole-I-'~NO~ 159 X 160 X 2,4-dinitroimidazole-dz 2,4-dinitroimidaz0le-2-'~NO2 159 X Products without Scrambling 2,4-dinitroimidazole-d2 160 X 159 X Products with Scrambling X 2,4-dinitroimidazole-d~-I~NO~ 161 2,4-dinitroimidaz01e-h2-'~NOz 158 X the analysis). Thus, it appears that even at relatively low temperatures (91 "C) approximately 6 times as much 4-NI than 2,4-DNI forms from neat 1,4-DNI, as opposed to conversion in chlorobenzene, where only minor amounts of 4-NI are formed. 4. Mechanism of Rearrangement from Isotope Labeling. The rearrangement of the nitro group from the 1-position in 1,CDNI to the 2-position appears to be similar to many sigmatropic rearrangements previously observed9 in which a B bond adjacent to a n system moves to a new position in the molecule. However, there is no net change in the n bonding in the current system as commonly observed in sigmatropic rearrangements. It was of interest, therefore, to determine if the rearrangement reaction fits the classical description of a sigmatropic reactiong and is, therefore, intramolecular as generally required for this type of pericyclic reaction.'O It was possible to do this by designing an isotope-scrambling experiment using deuterium- and 15N-labeled 1,4-DNI samples, as shown in Scheme 2. Deuterium- and 15N-labeled 1A-DNI samples were mixed in equal amounts in solution and reprecipitated. The resulting sample was then subjected to thermal rearrangement, as described in the Experimental Section. The molecular ions in the mass spectrum of the rearrangement product would in this case differentiate between the intra-(mlz = 159 and 160) and inter-(m/z = 158 and 161) molecular rearrangement species because in the latter case isotopescrambled (crossover) species would be observed. The molecular ions of various isotopic analogues of 1,4-DNI and 2,4-DNI observed are summarized in Table 3 along with the molecular ions observed in the product of the isotopescrambling experiment. It can be seen from Table 3 that molecular ions in the later are exclusively those corresponding to intramolecular rearrangement, namely, mlz = 159 and 160. It was remarkable that even minor amounts of intermolecular rearrangement species (m/z = 158 and 161) were not observed in the mass spectra of the 2,4-DNI formed in the thermal rearrangement. The conclusion is inescapable, therefore, that the nitro group migration in 1,4-DNI on heating takes place in a concerted intramolecular process rather than a stepwise
Bulusu et al.
5014 J. Phys. Chem., Vol. 99, No. 14, 1995
SCHEME 2 c
i, m/z
m/z 160
mlz 160
161
m/z 159
SCHEME 3 [ 1 SI sigmatropic
rearomatization
rearrangement
N 4
Step 1
ii
I
N 4
SCHEME 4 A
No2
H
SCHEME 5
SCHEME 6
RDX (M')
mechanism in which the N-NO2 bond undergoes a homolysis with the nitro group reattaching at the 2-position. In fact, the rearrangement from the N-nitro to a C-nitro has been previously observed in pyrazoles and shown to take place by an intramolecular, 2-step process.11s12Scheme 3 depicts this process for the dinitroimidazole. An interesting question now arises as to how 4-NI and TNI, which are apparently products of intermolec~larprocesses, are formed as byproducts.11*12For 4-NI to be formed a source of hydrogen could be the moisture present which was found to be extremely difficult to remove. A rationalization of the formation of 4-NI may be represented as shown in Scheme 4. The NO*+ released in this process could give rise to an isomer of TNI, as shown in Scheme 5. As the 2,4-DNI is formed by rearrangement of the 1,CDNI, an additional source of hydrogen becomes available because of the highly acidic (pKa = 2.85)13 nature of the imino-hydrogen. The anion left behind on its dissociation, together with the N02+ from Scheme 4, could also contribute to the formation of another TNI.
The isomer identity of the TNI formed cannot be distinguished in these small-scale experiments, and there may be more than one isomer present. Being products of intermolecular processes, 4NI and TNI should show isotope mixing. In fact, we did observe in the mass spectra obtained on the DuPont instrument (see the Experimental Section) ions mlz 113, 114, 115, and 116 corresponding to 4-NI with different numbers of deuterium atoms. Isotope mixing in TNI was not observed because the mass range scanned did not reach its mass, mlz 203, and in any case, it would be formed, presumably, in too minute an amount to be detectable. Rearrangement of a nitro group from nitrogen to a carbon has been observed in N-nitro aromatic amines9 as well as aliphatic nitraminesl4 under a variety of other conditions. Examples are rearrangement of N-nitro-N-alkylaniline to orthonitro-N-alkylaniline9 on treatment with acid and the electron impact induced migration of the nitro group in RDX (1,3,5trinitro-l,3,5-triazacyclohexane),14 respectively, shown in Scheme 6 for comparison.
J. Phys. Chem., Vol. 99, No. 14, I995 5015
Thermal Rearrangement of 1,4-Dinitroimidazole
In these reactions the nitro group moves to an adjacent carbon or in aromatic systems to a para position. The driving force in each of these reactions must be different, but it is not well understood. In each case the process is strictly intramolecular, as shown by isotopic ~ t u d i e s . ' ~ JThe ~ rearrangement of 1,4DNI to 2,4-DNI, which is shown to be intramolecular in this work, appears, however, to have more in common with sigmatropic rearrangements since it involves movement along a n system of electrons. Summary and Conclusions The work presented here collects together several points pertinent to the chemistry of dinitroimidazoles. First, it offers a confirmation of the rearrangement of 1,4-dinitroimidazoleto the 2,4-dinitro isomer, shows that it can be performed both in solution and in the solid state, and identifies two byproducts in this process, namely, 4-nitroimidazole and a trinitroimidazole isomer. Secondly, using 'H and 13CNMR chemical shifts and coupling constants, it presents unequivocal support for the correctness of the assumed isomeric structures. NMR data obtained on the 1,4-dinitroimida~ole-1-'~NU~ also helped to exclude the possibility, however slight, of the rearrangement occumng through a 4,4-dinitro intermediate. In mass spectrometry experiments using 2H and 15Nisotope labeled samples of 1,4-dinitroimidazole, failure to observe isotope scrambling during the rearrangement reaction showed that the rearrangement is strictly intramolecular. This suggests that the mechanism of the reaction is most likely to be a (1,5)-sigmatropic rearrangement analogous to the rearrangement of N-nitropyrazole. The rearrangement of 1,4-dinitroimidazole to 2,4-dinitroimidazole and the denitration reaction of 1,4-dinitroimidazole to 4-nitroimidazole are competing reactions, with the availability of Hf ions being a determining factor over which reaction is dominant. Formation of trinitroimidazole probably occurs in two steps: 1,4-dinitroimidazole rearrangement to 2,4-dinitroimidazole followed by nitration of 2,4-dinitroimidazole to trinitroimidazole.
Acknowledgment. The authors are thankful to D. Puckett for assistance with the mass spectrometry experiments and data collection. This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC0494AL8500. References and Notes (1) Jayasuriya, K.;Damavarapu, R.; Simpson, R. L.; Coon, C. L.; March 1993, Coburn, M. D. Technical Report, LLNL, UCRL-ID-1133G64, Lawrence Livermore National Laboratory, Livermore, CA, 1993 (patent pending). (2) Boyer, J. H. Nitrozoles: The C-nitro Derivatives of Five Membered N- and N,O Heterocycles; VCH Publishers, Inc.: Dee6ield Beach, FL, 1986; p 79. (3) (a) Farger, R.; Pyman, F. J. Chem. SOC. 1919, 217. (b) Sharnin, G. P.; Fassakhov, R. Kh.; Orlov, P. P. USSR Patent 458553,1975;Chem. Abstr. 1975, 82, 156316. (c) Sudarsanam, V.; Nagarajan, K.; George, T.; Shenoy, S. J.; Iyer, V. V.; Kaulgud, A. P. Indian J. Chem. 1982,218, 1022. (4)(a) Behrens, R.; Bulusu, S.;Minier, L. Sandia National Laboratory, Livermore, CA. (b) Gilardi, P. R. Naval Research Laboratory, Washington, DC. (c) Simpson, R. Lawrence Livermore National Laboratory, Livermore, CA. (d) Damavarapu, R. US.Army Armaments Research, Development and Engineering Center, Picatinny Arsenal, NJ. ( 5 ) Lancini, G. C.; Maggi, N.; Sensi, P. F a m c o (pavia), Ed.Sci. 1%3, 18, 390;Chem. Abstr. 1963, 59, 10032. (6) Bulusu, S.;Autera, 1.;Axenrod, T. J . Lubelled Compd. Radiopharm. 1980, XVll, 707. (7)Behrens, R., Jr. Rev. Sci. lnsfrum. 1986, 58, 451.;Int. J . Chem. Kinet. 1990, 22, 135; 1990, 22, 159. (8) Minier, L.; Behrens, R., Jr.; Bulusu, S. Proceedings of the International Symposium on Energetic Materials Technology, Orlando, FL, March 21-24, 1994;American Defense Preparedness Association, Arlington, VA. (9) March, J. Advanced Organic Chemistry, 4th ed.; John Wiley & Sons: New York, 1992;p 1037. (10) Jefferson, E.A.; Warkentin, J. J.Am. Chem. SOC.1992,114,6318. (11) Janssen, J. W. A. M.; Habraken, C. L.; Louw, R. J. Org. Chem. 1976, 41, 1758. (12) Suwinski, J.; Salwinska, E. Polish J . Chem. 1987, 61, 913. (13) Gallo, F. F.;Pasqualucci, C.R.; Radaelli, P.; Lancini, G. C. J . Org. Chem. 1964, 29, 862. (14) Bulusu, S.;Axenrod, T.; Milne, G. W. A. Org. Mass Spectrom. 1970, 3, 13. (15) Reference 9,p 508.
JP943020U
