DFT study for the Diels-Alder reactions between butadiene ethylene for evaluate the effects of electro-attractors substituents

Vol. 13, 2021, pp. 01~08 ISSN: 2605-6895

DFT study for the Diels-Alder reactions between butadiene ethylene for evaluate the effects of electro-attractors

substituents

Imad HAMMOUDAN ⁽¹⁾, Samir CHTITA ^(2,*), Driss RIFFI-TEMSAMANI ⁽¹⁾

1 Laboratory physical chemistry, Faculty of sciences of Tetouan, University Abdelmalek Essaadi, Tetouan, Morocco.

2 Laboratory physical chemistry of materials, Faculty of sciences Ben M’Sik, Hassan II University of Casablanca, PO Box 7955 Sidi Othmane, Casablanca, Morocco.

* Corresponding author: Tel: 00212 660005554; E-mail: samirchtita@gmail.com

Article Info ABSTRACT

Article history:

Received 21 April 2021 Revised 28 June 2021 Accepted 30 June 2021

Investigation of the effect of three electron-withdrawing atoms of the periodic table on the activation energy of the Diels-Alder reaction by substituting an ethylene hydrogen CH2=CH2 by F, Cl, Br, following the decrease in electronegativity. An exception was found for the boron atom Br, which is that the activation energy decreases remarkably from 27 to 20.86. This drop-in activation energy can be explained by the secondary orbital interaction.

Keyword:

DFT calculations, Diels-Alder reaction, ethylene, activation energy.

Corresponding Author:

Adress: PO Box 7955 Sidi Othmane, Casablanca, Morocco.

Email: samirchtita@gmail.com Phone: 00212 660005554

1. INTRODUCTION

Diels-Alder reaction since its discovery by Otto Diels and Kurt Alder (Nobel Prize in

cycloaddition reaction [4π + 2π] In which a conjugated diene reagent with a molecule containing a double or triple bond, called dienophile, results in a six-atom ring containing an unsaturation, called Diels-Alder Adduct Scheme1:

Scheme1

The reaction is a very effective way to prepare carbocyclic and heterocyclic compounds with high regio and stereo-selectivity control. This has paved the way for the synthesis of many natural products such as terpenes, and alkaloids [2].

R. B. Woodward even used the Diels-Alder reaction to predict the synthesis of many naturally occurring complexes such as cortisone, cholesterol, reserpine [3-4-5-6].

This cycloaddition is controlled by the Woodward-Hoffmann [7] rule and can only be possible when the molecular orbitals of the reactants have the same symmetry as those of the products [8]. It is also influenced by the nature of diene and dienophile. The more the diene is rich in electrons and the dienophile is poor in electrons, the more the reaction is favored [9-10-11].

In this part of our work, we analyze some factors that control the reactivity of Diels Alder's reaction by studying its activation energy Ea. We will study the effect of the electro- attractor substituents carried by the dienophile.

2. METHODS OF COMPUTATION

The optimization of the reagents balances geometries and transition states was performed by the DFT B3LYP method using 6-31G (d) as a basis. The corresponding energies were calculated at the same level B3LYP/6-31G (d). The location of the transition states was confirmed by the presence of one and only one imaginary frequency in the hessian matrix.

All calculations were performed with the Gaussian 09 programs [12].

3. RESULTS AND ANALYSIS

3.1. Case of electro-attractive substituents carried by the dienophile

To investigate the influence of the electron attractive substituents carried by the dienophile on the activation energy of the Diels-Alder reaction, we successively replace hydrogen of ethylene H2C=CH2 (dienophile) by Fluorine, Chlorine and Bromine according to the following reaction:

+

Scheme 2

With X= F, Cl et Br.

We compare the activation energies of their reactions with that of the unsubstituted dienophile ethylene H2C=CH2

3.2. Calculation of activation energies.

The geometries of the transition state corresponding to reaction (scheme 2) are shown in figure 1 and table 1 following:

TS-F

TS-Cl

TS-Br

Figure 1: The structures of transition states DFT/B3LYP(6-31G(d))[13]

Table 1: The bond length of the dienophile, diene, and structure of the transition state (Å).

Dienophile diene TS

F Cl Br TS-F TS-Cl TS-Br

C1C2 1.234 1.237 1.326 1.471 2.1889 2.15562 2.15856

C2C3 1.339 1.3793 1.38395 1.38281

C3C4 2.3245 2.38737 2.38474

C4C11 1.3807 1.37848 1.37849

C11C12 1.4069 1.40761 1.40780

C12C1 1.3874 1.38882 1.38839

From the structures shown in figure 1, we notice that the C2C3 distances in the transition states are longer than the C1C2 links of the corresponding dienophiles. This shows the beginning of the disappearance of the π C1C2 links from dienophiles and the appearance of σ C2C3 links in transition structures. We also observe the elongation of the C4C11, C1C12 bonds of the transition state structures concerning their diene C1C2 and C3C4 counterparts, while the C11C12 bond of the transition states narrows for C2C3 of the diene, which shows the beginning of the formation of the next six-atom ring with unsaturation:

The HOMO/LUMO energy gaps of the reagents are given in the following table 2. These calculations are made to highlight the donor (nucleophilic) or acceptor (electrophilic) character of the two reactants.

Table 2: Difference between the two possible combinations of HOMO/LUMO for 1, 3- Butadiene diene and CH2CHX dienophile (ev values).

X

F 5.58 6.26

Cl 6.15 6.31

Br 6.11 6.10

H 6.6 6.43

According to the results of table 2, all the X (F, Cl, Br) electro-attractors substituents favors the reaction of Diels-Alder, in addition, we notice that the gaps are smaller than the gaps for TS-F and TS-F in Schme3 which correspond to the substituents X=F, and Cl, which means that the corresponding dienes behave like nucleophiles while the dienophiles behave like electrophiles. But the opposite is observed for the substituent Br, its gap is smaller than its gap showing that the diene behaves as an electrophilic while the dienophile behaves as a nucleophile.

The behavior of these substituents will also be observed for the activation energy values given in table 3. While it was expected that activation energies would decrease in the direction of decreasing electronegativity of the X substituents going from Fluorine, Chlorine and Bromine, on the other hand, Brome leaves this rule, its activation energy is lower than that of Chlorine and Fluorine.

Table 3: energies of activation of the Diels-Alder reactions of 1,3-Butadiene diene with CH2CHX dienophiles (X= F, Cl, Br).

TS Ea (Kcal/mol) Ref

TS-F 22.17 This work

TS-Cl 22.27 This work

To explain these results, we used the molecular orbital analysis of transition state complexes. The figures below show the interactions between the molecular orbitals of dienes and dienophile in the transition state:

TS-F

TS-Cl

TS-Br

From these illustrations, we notice that in the TS-Br transition state, there is the overlap of the orbitals of Brome with those of one, 3-butadiene diene, while in the TS-Cl transition state is less visible and is absent. This shows the existence of the most important secondary orbital interactions in the TS-Br state, further stabilizing the transition state and decreasing the corresponding activation energy. This explains the values of the activation energies in table 3.

4. CONCLUSION

The substitution of hydrogen of ethelyne by the bromine Br decreases the activation energy for the Diels-Alder reaction with the diene, the localization of the structure of the transition state by a method already studied, which gives good results, allows us to visualize secondary interactions which explain this decrease.

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