Theoretical Study on the Transition State of N-nitropyrazoles Rearrangement Reaction

YANG Feng LI Yong-Xiang D ANG Xin GUO Heng-Jie CHAI Xiao-Xiao

(School of Chemical Engineering and Environment,North University of China, Taiyuan 030051, China)

1 INTRODUCTION

Nitropyrazoles have been studied as models of simple aromatic systems, some of which are of biological, pharmaceutical and energetic materials interest[1-6].Nitropyrazoles are usually prepared from the nitration of corresponding pyrazoles.Although nitrification is carried out with the same pyrazoles,the nitrification products obtained in different nitrifying agents are not the same, for instance the syntheses of 1-nitropyrazole and 4-nitropyrazole.But in many cases, it is impossible to introduce nitro group into the specified location of the pyrazole ring,while some certain nitro substituents at specific locations can be obtained by rearrangement of nitro groups, for instance the synthesis of 3-nitropyrazole.Therefore, in order to obtain nitro substituents at different positions of pyrazole, the rearrangement reactions of nitropyrazole have been studied extensively.1-Nitropyrazole can be rearranged to 3-nitropyrazole at 120～180 ℃ for 3～7 h in the solvent which has a high boiling point such as benzonitrile,anisole and so on[7-10].The thermolysis rearrangement of 1,3-dinitropyrazol in benzonitrile at 180 ℃gave 3,5-dinitropyazole in high yield.The rearrangement of nitropyrazole has been extensively studied and its possible rearrangement mechanism has been proposed.Although the molecular structure, vibrational properties and proton transfer reaction of nitropyrazoles were also studied by quantum chemistry in the study of nitropyrazole compounds[11-13],the quantum chemistry of nitropyrazole rearrange-ment reaction is rare, and the structure and charge distribution of the transition state and intermediates in the rearrangement process are not very perspicuous.Therefore, in order to be able to more intuitively understand the structure and charge distribution of the transition state and intermediates of the nitropyrazole rearrangement.The transition state of N-nitropyrazoles rearrangement reaction have been theoretically studied by Gaussian 09 package with gradient-corrected density functional theory (DFT)method at the B3LYP/6-311G (d, p) level of theory.

2 COMPUTIONAL DETAILS

The calculations described in this paper were carried out using Gaussian 09 package[14].All compounds were optimized using the gradient-corrected density functional theory (DFT) at the B3LYP/6-311G(d, p) level of theory[15,16]in the gas phase and solvent phase, respectively.The nature of all optimized structures was determined using harmonic frequency analysis as true minima with no imaginary frequency or transition state with only one imaginary frequency.The transition state geometry of the nitropyrazoles rearrangement reactions was determined using Berny method at the B3LYP/6-311G(d,p) level of theory.Meanwhile, the intrinsic reaction coordinate (IRC)[17,18]was calculated to confirm that the transition states are connected to the two corresponding stationary points of the reaction.To illustrate the population of charge of all atoms, natural bond orbital (NBO) analysis[19,20]is calculated at the B3LYP/6-311++G (d, p) level of theory.In order to accurately describe the effect of solvent on the structure and charge of the molecules at each stage of the rearrangement reaction, Truhlar’s implicit solvation model (SMD) was used[21].

3 RESULTS AND DISCUSSION

3.1 Geometry structure

We have optimized the structures at the B3LYP/6-311G(d,p) level of theory.No or only one imaginary frequency was found, confirming that these structures correspond to the true energy minimum or the first order saddle points.For ease illustration, we use(1,3)-rearrangement and (1,5)-rearrangement to express the rearrangement of N-nitropyrazole to 3-nitropyrazole and the rearrangement of 1,3-dinitropyrazole to 3,5-dinitropyrazole, respectively.And the molecular frameworks of pyrazoles with the numbering are presented in Fig.1.

Scheme 1. Syntheses of 3-nitropyrazole and 3,5-dinitropyrazole by the rearrangement of N-nitropyrazole

Fig.1. Molecular formula of nitropyrazoles with the numbering of atoms as used in the text:(a) N-nitropyrazole, (b) 3-nitropyrazole, (c) 1,3-dinitropyrazole, (d) 3,5-dinitropyrazole

3.1.1 Rearrangement of N-nitropyrazole to 3-nitropyrazole

As we all know, 3-nitropyrazole can not be synthesized by the nitrification of pyrazole directly.It must be prepared from the rearrangement of N-nitropyrazole in high boiling solvent.The optimized stationary structures of the rearrangement reaction are shown in Table S1 (shown in supporting information), and the possible pathways of the reaction are shown in Fig.2.As can be seen from the above figure, the reaction contains two transition states and one intermediate.Throughout the course of reaction, nitro group and hydrogen atom on adjacent carbon atoms are the main reactive groups.The nitro group is the major variant in the process from reactants to intermediates.By calculation we can see, the bond length between nitro nitrogen atom and pyrazole ring nitrogen atom is gradually drawn from 1.44 (RC) to 2.44 ? (INT) and the angle of nitro group (O–N–O) also has a significant change from the beginning of 129.25° (RC) to 134.53° (TS1)and finally changes to 126.89° (INT).Meanwhile,the migration of nitro groups is accompanied by the torsion itself.In transition state the plane of nitro group is almost perpendicular to the plane of pyrazole ring.Subsequently, the change of nitro group from intermediate to product stage is no longer obvious, and the main change is caused by the migration of hydrogen atoms and the change of angle between nitro plane and pyrazole ring plane.In transition state 2 (TS2), the hydrogen atom is no longer unique to carbon atom.At this time, hydrogen atom is almost on the bisector of the connection of carbon atoms and adjacent nitrogen atoms.During the migration of hydrogen atom, the distance between hydrogen and carbon atoms increases from 1.09 ? (INT) to 2.12 ? (PC), and the distance from nitrogen atom decreases from 2.10 ? (INT) to 1.01 ?(PC).With the migration of hydrogen atom, the angle between nitro plane and pyrazole ring plane is gradually reduced.In addition to the above functional groups and atomic changes, pyrazole ring also has an obvious change.During the rearrangement process, the pyrazole ring angles N(1)–N(2)–C(3)and C(4)–C(5)–N(1) showed an increase trend, while C(5)–N(1)–N(2), N(2)–C(3)–C(4) and C(3)–C(4)–C(5) showed a decrease trend.

Fig.2. Optimized structures of N-nitropyrazole (RC), transition state (TS1, 2), intermediates (INT), and 3-nitropyrazole(PC) of the (1, 3)-rearrangement in the gas phase (red = oxygen, blue = nitrogen, gray = carbon, white = hydrogen)

Table 1. Calculated Frontier Orbital Energy of (1,3)-Rearrangement at the B3LYP/6-311G(d,p) Level

3.1.2 Rearrangement of 1,3-dinitropyrazole to 3,5-dinitropyrazole

From Fig.3, it is clear that as in the case of (1,3)-rearrangement, the migration of nitro and hydrogen atom is included throughout the (1, 5)-rearrangement process.Moreover, the nitro group and hydrogen atom are not in the same side of the pyrazole ring plane throughout the process.The beginning of the rearrangement is the migration of nitro group, and the distance between nitro nitrogen atom and pyrazole ring nitrogen atom is continuously stretched from 1.47 ? (RC) to 2.45 ? (INT) during the migration, and the distance between nitro nitrogen atom and pyrazole ring carbon atom reduces from 2.52 ? (RC) to 1.53 ? (INT).The second half of the rearrangement occurs where the hydrogen atom migrates, and the distance between pyrazole ring carbon and hydrogen atom is from 1.09 ? (INT)to 2.12 ? (PC), while the distance between hydrogen and nitrogen atoms is gradually reduced from 2.15 ?(RC) to 1.01 ? (PC).During the whole process of rearrangement, there is also a significant change in the angle of nitro formation angle (O–N–O),especially in the process of nitro migration due to the change of interaction of nitro group and pyrazole ring.In the transition state 1 (TS1) period, the nitro formation angle is the largest 136.5°, owing to the weakest interaction between nitro and pyrazole ring.However, unlike the process of (1, 3)-rearrangement,the internal angle change of pyrazole ring is not particularly significant during (1, 5)-rearrangement process.It can be seen from Fig.3 that the internal angle of pyrazole ring changes in intermediate process (TS1, INT, TS2) of the rearrangement, but the difference in internal angle of reactants (RC) and product (PC) is small.And the optimized stationary structures of the rearrangement reaction are shown in Table S2 (in the supporting information).

Fig.3. Optimized structures of 1,3-dinitropyrazole (RC), transition states (TS1, 2), intermediates(INT) and 3,5-dinitropyrazole (PC) of the (1,5)-rearrangement in the gas phase (red = oxygen,blue = nitrogen, gray = carbon, white = hydrogen)

3.2 Electronic characteristics

In order to further understand the interaction between migrating and connecting atoms, the natural bond orbital (NBO) charge for optimized structures is detailedly calculated in this work.From previous part of discussion, we know that whether for the (1,3)- or (1, 5)-rearrangement, the main changes are nitro and hydrogen atom.Therefore, we mainly carry out electronic analysis of the atoms involved in the rearrangement process and the NPA charges for all atoms in the rearrangement process are shown in Tables S3 and S4 (listed in the supporting information).It can be seen from Figs.4 and 5 that the tendency of (1, 3)-rearrangement is similar to that of(1, 5)-rearrangement.Compared with the main reaction atoms on the pyrazole ring, the charge changes of the atoms on the nitro group are not obvious.The net charge of nitro nitrogen atom decreases with the progress of reaction, while the net charge of oxygen atom decreases first and then increases, and the maximum value of charge appears during the TS1 period.For the main reactive atoms on pyrazole ring,the positive charge of carbon and hydrogen atoms increases significantly as the reaction proceeds, while the negative charge of nitrogen atoms also increases,but the change process is not monotonically increased.The reason for the above phenomenon may be that the first stage of rearrangement process is that the nitro group gradually moves away from the pyrazole ring nitrogen atom and close to the carbon atom.In this process, the interaction between nitro nitrogen atom and pyrazole ring nitrogen atoms gradually weakens, and that of nitrogen atoms gradually increases.Furthermore, the interaction between nitro and pyrazole ring is the weakest at TS1 period, the angle of nitro is the largest, and the length of O–N bond is the smallest, so that the negative electron of oxygen atom is assigned to the nitro nitrogen atom, while the nitro and pyrazole ring systems of electronic effects are gradually weakened,which make the phenomenon of electron delocalization of pyrazole ring nitrogen atoms weaken and the electronegativity of nitrogen atoms increase.For the second stage, the electrons of nitro group and pyrazole ring are gradually enhanced due to the decrease of angle between nitro group and pyrazole ring plane.Because the hydrogen atom is gradually away from carbon atoms and close to nitrogen atoms, the nitrogen atom gets more electrons from hydrogen and carbon atoms, thus the negative charge of nitrogen atom increases, and for the same reason the positive electricity of carbon and hydrogen atoms increases.

Fig.4. NPA charges of migrating atoms and the connecting atoms of the (1, 3)-rearrangement

Fig.5. NPA charges of migrating atoms and the connecting atoms of the (1, 5)-rearrangement

Table 2. Calculated Frontier Orbital Energy of(1,5)-Rearrangement at the B3LYP/6-311G(d,p) Level

3.3 Molecular electrostatic potentials and Frontier molecular orbital analysis

The molecular electrostatic potential is the potential energy of a proton at a particular location near a molecule.In the figures, the blue and red parts represent positive and negative potential regions of the molecule, which correspond to the attraction and repulsion of protons, and the magnitude of this effect is represented by the depth of the color, so the greater the difference in color between the different regions of the molecule, the greater polarity of the molecule.From Figs.6 and 7 we can see that the polarity of each stage in the rearrangement process has a more obvious change, and the structural polarity of intermediate process is higher, and the polarity of molecular structure in the (1, 3)-rearrangement process is significantly higher than the molecular structure in the (1, 5)-rearrangement process.The reason for the above phenomenon can be attributed to the influence of molecular structure, as described in Figs.4 and 5 due to the transfer of nitro group and hydrogen atom during the reaction so that the charge distribution of the molecule changes, and for (1,5)-rearrangement, the electron-withdrawing effect of the nitro group on the 3-carbon atom makes the electron distribution of the system more uniform.

Fig.6. 3D molecular electrostatic potential maps of each stage in the gas phase during the (1, 3)-rearrangement reaction, where dark blue denotes positive charge and dark red denotes negative charge

Fig.7. 3D molecular electrostatic potential maps of each stage in the gas phase during the(1, 5)-rearrangement reaction, where dark blue denotes positive charge and dark red denotes negative charge

The frontier molecular orbital energies which involve the highest occupied molecular orbital(HOMO) energies and the lowest unoccupied molecular orbital (LUMO) energies are known to play a crucial role in governing the chemical reactions.In several studies it is revealed that the band gap between the frontier molecular orbital energies(ε(HOMO –LUMO)) is an important stability index of the molecules[25–31].A large band gap implies high stability and small band gap implies low stability; in turn, high stability indicates low chemical reactivity and low stability indicates high reactivity.The frontier molecular orbitals of the rearrangement computed from the B3LYP/6-311G(d,p) level of theory are shown in Figs.8 and 9 and the band gap values obtained from the B3LYP/6-311G(d,p) level of theory are listed in Tables 1 and 2.From the above tables we can see that the ε(HOMO – LUMO) values of the reaction transition state and the intermediate are smaller than that of the reactants and products,indicating that the reactants and products have a higher stability than the intermediate process of rearrangement reactions.As shown in the above table,the band gap values of 1-nitropyrazole, 3-dinitropyrazole, 1,3-dinitropyrazole, and 3,5-dinitropyrazole calculated from the B3LYP/6-311G(d,p) level are 0.191020, 0.205017, 0.187336 and 0.187435 a.u.respectively.From the above changes in the value of ε(HOMO – LUMO), the positions and number of nitro groups are the key to the impact of ε(HOMO –LUMO) values.Furthermore, the band gap is highly correlative with the Hess-Schaad resonance energy per π-electron, a measure of thermodynamic stability due to the cyclic conjugation[32].This correlation means that thermodynamically stable compounds are also kinetically stable.Thus the stability order of the above molecules is as follows: 3-nitropyrazole ＞N-nitropyrazole ＞ 3,5-dinitropyrazole ＞ 1,3-dinitropyrazole.

Fig.8. 3D frontier molecular orbital maps (HOMO and LUMO)of (1,3)-rearrangement computed at the B3LYP/6-311G(d,p) level

Fig.9. 3D frontier molecular orbital maps (HOMO and LUMO)of (1,5)-rearrangement computed at the B3LYP/6-311G(d,p) level

In order to explore the electronic structure and bonding characteristics of the compounds during the reactions, the orbital of each phase in the reaction process was systematically analyzed.The sum of squares of the atomic orbital coefficients is used to represent its contribution in the molecular orbital and normalized.The compounds are divided into the following sections: C, H, O, N(6), N(2) N(1), N(9).The calculation results are shown in Tables 3, 4 and Figures 10, 11.The compounds have the following bonding characteristics during the reaction: (1) In the frontier molecular orbitals, since the pyrazole ring has good conjugated delocalization, no matter which rearrangement reaction it is, the contribution of pyrazole ring to the entire orbital was greater than 70%.In the process of rearrangement, the com-position of carbon atom C is reduced while that of pyrazole N(1) and N(2) is significantly increased.(2)For the unoccupied molecular orbitals, the contribution of atoms to the molecular orbital is inconsistent with the occupied orbital.Nitro oxygen atoms O and nitrogen atom N(6) also occupy a higher composition.During rearrangement, the composition of O and N(6)atoms decreases, while the N(1), N(2) and C atoms increase.(3) Comparing various orbital compositions of HOMO and LUMO, it is easy to see that when the electrons are excited from HOMO to the LUMO orbital, the electrons of pyrazole ring carbon atoms and nitrogen atoms are mainly transferred to the oxygen and nitrogen atoms of the nitro group.

Table 3. Calculated Frontier Orbital Composition of(1,3)-Rearrangement at the B3LYP/6-311G(d,p) Level

Table 4. Calculated Frontier Orbital Composition of (1,5)-Rearrangement at the B3LYP/6-311G(d,p) Level

Fig.10. Calculated frontier orbital composition of the (1,3)-rearrangement

Fig.11. Calculated frontier orbital composition of the (1,5)-rearrangement

3.4 Solvent effects

It is well known that the rearrangement of nitropyrazole compounds often occurs in high-boiling solvents.In order to reflect the influence of solvent on the rearrangement reaction, the solvent effect is also explored by quantum chemistry.In the calculation results, we found that in addition to the nitro formation angle (O–N–O) with a significant change,the changes of other structural parameters are not obvious, so we only discuss the construction of the nitro formation angle.

The geometric parameters of the molecules at each stage of the rearrangement reaction in different solvents are shown in Tables S5 and S6 (supporting information).Obviously, it can be seen from the table that the solution has little effect on the geometrical parameters of each stage of the rearrangement reaction, but the dipole moments at each stage have a significant change.It is indicated that the electron density of the molecular system has changed from different solvents.From Tables 5 and 6, compared with the gas phase, the increase in the dipole moment in the acetonitrile and ethanol solutions is obvious,but the increasing trend has no correlation with the change trend of the solvent polarity, and in the ethanol solution, the maximum dipole moment value occurs.For nitro formation angle (O–N–O), the presence of acetonitrile and ethanol has reduced the angle of nitro formation, but it is contrary to the change trend of the dipole moment, and the angle of nitro group in ethanol solution is the smallest.Here we can find that the maximum angle of nitrification angle corresponds to the minimum dipole moment,and reverses the smallest nitro angle corresponding to the maximum dipole moment.Therefore, the change of nitro formation angle (O–N–O) may be one of the reasons leading to the change of dipole moment.By comparing the two tables, it can be seen that the change of dipole moment of dinitropyrazole is smaller than that of mononitropyrazole in the same solvent.

Table 5. Nitro Formation Angle (O–N–O) and Dipole Moments (D) of the (1,3)-Rearrangement Process at Different Stages in Various Solvents Using the SMD Implicit Solvation Model

Table 6. Nitro Formation Angle (O–N–O) and Dipole Moments (D) of the (1,5)-Rearrangement Process at Different Stages in Various Solvents Using the SMD Implicit Solvation Model

4 CONCLUSION

As suggested by Jassen, the rearrangement reaction of N-nitropyrazoles involves the transfer of nitro group to form 3-H intermediates process and the rapid transfer of 3-H intermediate hydrogen atom to form 3-nitropyrazole process.The migration of nitro and hydrogen atoms during the rearrangement process is not carried out on the same side of the pyrazole ring plane, which can be attributed to the change of interaction between the migration group and pyrazole ring during the rearrangement reaction.The system structure and charge distribution change significantly.In the whole process of rearrangement,part of the negative charge of the molecular system is transferred to the migrating nitro group.For HOMO,the pyrazole ring contributes the most to the orbital composition, while for LUMO, the nitro group has a significant contribution.In addition to the nitro formation angle (O–N–O), the structure of the rearrangement reaction in the solvent has no significant change compared with that in the air, and when the reaction occurs in the ethanol, the nitrification angle is the smallest and the molecular dipole moment is the largest.

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