DFT study of the decomposition reactions of nitroethyl benzoates catalyzed by the 1 , 3-dimethylimidazolium cation

Article history: Received August 21, 2016 Received in revised form October 24, 2016 Accepted 8 November 2016 Available online 9 November 2016 DFT calculations indicate that the decomposition reaction of nitroethyl benzoates in the presence of 1,3-dimethylimidazolium cation takes place much faster than in the case of the non-catalyzed process. Additionally, our calculations suggest one-step polar mechanism of title reactions. © 2017 Growing Science Ltd. All rights reserved.


Introduction
The synthesis of nitroalkenes is very significant in organic chemistry.3][14] Furthermore, nitroalkenes are important precursors of many insecticides, 15 fungicides 16 and pharmaceuticals. 138][19][20] However, a great demand still exists for a method to prepare nitroolefins in a convenient and effective way.The present work is a continuation of our comprehensive study about the synthesis and fundamental properties of CNA. 21- 28Our previous works 27,28 present the decomposition of nitroalkyl carboxylates under thermal conditions.DFT calculations confirmed that these processes proceed via the one step mechanism, not "pericyclic", but quite a strong asynchronous one-step, two-stage mechanism.
This work is a continuation of our quantum chemical study of the decomposition reaction of nitroethyl benzoates in the presence of the 1,3-dimethylimidazolium (DMIM) cation.It should be mentioned, that our recent studies show that some nitroalkene cycloaddition reactions catalyzed by the imidazolium cation proceed via a stepwise, zwitterionic mechanism, 29,30 instead of the one step mechanism, which was noted in the case of non-catalytic conditions.This cation can be introduced for reaction as an ionic liquid.[33][34]

Computational details
All calculations were carried out using the Prometheus computer cluster in the CYFRONET regional computer centre in Cracow.The mechanism of the decomposition of nitroalkyl benzoates catalyzed by the 1,3-dimethylimidazolium cation have been examined as implemented in the GAUSSIAN 09 package. 35The geometric parameters for all the reactants, transition states and products of the reactions studied were fully optimized using the density functional theory (DFT) method.The calculations were performed using the B3LYP 36 with 6-31G(d) basic set.Additionally, calculations in more advanced 6-31+G(d) and 6-31G(d,p) basis sets, were carry out.
B3LYP is a combination of Becke's three parameter hybrid exchange functional 37 with the Lee, Yang and Parr correlated functional. 38Geometry optimization calculations have been carried out to obtain the global minima for the reactant and products, and to locate the saddle point for the transition state.Stationary points were characterised by frequency calculations.All reactants, and products had positive Hessian matrices.All transition states showed only one negative eigenvalue in their diagonalized Hessian matrices, and their associated eigenvectors were confirmed to correspond to the motion along the reaction coordinate under consideration.Transition states were located using the (QST2) algorithm.For the optimization process, the Berny analytical gradient was employed.Intrinsic reaction coordinate (IRC) calculations 39 have been made in all events to verify that the localized transition state structures connect with the corresponding minimum stationary points associated with reactants and products.The reaction environment polarity was simulated using PCM. 40It was assumed that the reaction environment has dielectric constant, ε= 13, for the reaction catalyzed by the 1,3dimethylimidazolium cation (the most typical 1,3-dimethylimidazolium ionic liquids have ε=~11,6-15,1 41 ).
Charge global electron density transfer (GEDT) 42 was calculated according to the formula: where qA is the net charge and the sum is taken over all the atoms of the substructure.
The values of enthalpies, entropies and free energies in all calculations were calculated with the standard statistical thermodynamics at 25ºC and 1 atm. 43

Energetical aspects of the decomposition reaction catalyzed by the DMIM cation
The reaction pathway of the decomposition reaction was studied using the B3LYP/6-31G(d) theoretical level.Recently published reports 26,[44][45][46][47][48] , indicate that a similar approach was used successfully for the exploration of a reaction involving several different nitrocompounds.These calculations proved that the first step of the decomposition of nitroethyl benzoate (1a) is the establishment of a pre-reaction complex ( [1][DMIM]) between the ester molecule and the 1,3dimethylimidazolium cation.Consequently, the creation of a pre-reaction complex entails the drop of reaction enthalpy by 3.54 kcal•mol -1 (Fig. 1 and Fig. 2).The DMIM cation is located near to the oxygen atom of the nitro group.We also analyzed many other orientations of the DMIM cation to the ester molecule.For further research we chose the most stable form.
Thereafter, the pre-reaction complex is recast to TS, which is associated with an increase in the enthalpy of activation over 33.31 kcal•mol -1 (Fig. 2).The decomposition reaction of nitroethyl benzoate 1a catalyzed by the DMIM cation proceeded faster than the same reaction without the catalyst.The enthalpy of the activation of the uncatalyzed process is equal to 38.73 kcal•mol -1 . 28Subsequently, the TS is converted to products 2 and 3 (Scheme 1).
Similar studies have been performed for the decomposition reactions of other nitroethyl benzoates which are substituted by NMe2 (1b) and NO2 (1c) functional groups.The decomposition process of substituted ester with the electrodonationg group (NMe2) will lower the activation barrier.In particular, for the decomposition of compound 1b, the activation barrier is 30.61kcal•mol -1 .The decomposition process of the 1c compound with an NO2 group has an activation barrier of 35.13 kcal•mol -1 .Therefore, as could be expected, the presence of an electroaccepting group will make the process more difficult (Table 1).For comparison, the uncatalyzed process give similar results (NMe2 -37.52 kcal•mol -1 , NO2 -39.65 kcal•mol -1 ). 28It turned out that, regardless of the nature of the substituent in the benzene ring, the decomposition process always followed the same mechanism (Scheme 1).
We have also performed similar a DFT study using more advanced B3LYP/6-31+G(d) and B3LYP/6-31G(d,p) theory levels.These calculations show that the mechanism of the decomposition reaction of nitroalkyl carboxylates also indicate one-step mechanism and makes that process proceed more mildly.The value of the enthalpy of activation for compound 1a are 30.07kcal•mol -1 and 30.47 kcal•mol -1 , respectively (Table 1).

Transition structure of the decomposition reaction
The TS, also in the decomposition reaction catalyzed by the 1,3-dimethylimidazolium cation, has a six-membered structure (Fig. 3).Simultaneously, new bonds are formed: between atoms O5-H6 and between atoms C1-C2 and O3-C4 double bonds.The C4-O5 bond is change to a single bond, and H6-C1 and C2-O3 bonds become broken (Table 2).Finally, we analysed the influence of the DMIM cation of structure TS.It was found that the H6-C1 bond in the reaction catalyzed by the DMIM cation is broken faster than in the uncatalyzed process.In turn, the C2-O3 bond of the TS of reaction with the ionic liquid cation is broken more slowly than in the case of the uncatalyzed reaction. 28The nature of the substituent in benzene ring also has an impact on the transition structure.In the case of reaction  2).
The GEDT value for the transition state for the decomposition reaction catalyzed by the 1,3dimethylimidazolium cation is 0.30.This value is bigger than the same GEDT value for the transition states of similar non-catalytic processes.

Conclusions
Our quantum chemical study proved that the decomposition reaction of nitroethyl benzoates catalyzed by the 1,3-dimethylimidazolium cation proceeded via a polar one-step mechanism.Compared to the uncatalyzed process, the decomposition reaction in the presence of the 1,3dimethylimidazolium cation proceed faster.All other research for the localization of different reaction channels lead via the ionic intermediate failed.