Theoretical Investigation of Steric Effect Influence on Reactivity of

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Theoretical Investigation of Steric Effect Influence on Reactivity of Substituted Butadienes with Bromocyclobutenone ADILAKSHMI ARUMUGAM, Madhu Deepan Kumar, Madhavan Jaccob, and Venkatachalam Tamilmani J. Chem. Inf. Model., Just Accepted Manuscript • DOI: 10.1021/acs.jcim.9b00177 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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Journal of Chemical Information and Modeling

Theoretical Investigation of Steric Effect Influence on Reactivity of Substituted Butadienes with Bromocyclobutenone Adilakshmi Arumugama, Madhu Deepan Kumarb, Madhavan Jaccobb and Venkatachalam Tamilmani*c a Department

of Chemistry, University College of Engineering, Ariyalur (A Constituent College of Anna university-Chennai) Ariyalur – 621704, Tamil Nadu, India. b Computational Chemistry Laboratory, Loyola Institute of Frontier Energy (LIFE) and Department of Chemistry, Loyola College (Autonomous), Chennai – 600 034, Tamil Nadu, India. c Department of Chemistry, Bharathidasan Institute of Technology (A Constituent College of Anna university-Chennai), Tiruchirappalli – 620 024, Tamil Nadu, India. Email: [email protected] ABSTRACT The Diels-Alder reaction (DA) between various mono and disubstituted 1,3-Butadiene

(Dn-1 to Dn-10) and 2-Bromocyclobutenone (DPh) were carried out in gas phase using Density Functional Theory (DFT) at M06-2X/6-31+g** level. The reaction was found to proceed through concerted asynchronous transition state. Further, the asynchronous and early nature of transition state was clearly pinpointed with the Frontier Molecular Orbital (FMO) and bond order index (BOI) analyses. The intermolecular hydrogen bonding interaction along with steric encumbrance in the transition state were found to be the predominant factor in controlling the reactivity of the dienes. Among the investigated dienes, Dn-6 was found to be the most reactive diene which is attributed to low activation barrier due to presence of strong intermolecular H-bonding interactions. These factors were further supported by Quantum mechanical calculations using global descriptor indexes, NBO analysis and QTAIM analysis. These theoretical results were found to be in good agreement with the previous experimental findings. Key Words: DFT, Diels-Alder reaction (DA), 2-Bromocyclobutenone (DPh), 1,3-Butadiene (Dn1-Dn10)

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INTRODUCTION Diels-Alder (DA) reactions are the best known cycloaddition reactions used for organic synthesis and it was discovered by Otto Diels and Kurt Alder1. The remarkable stereoselectivity and regioselectivity of DA reactions made them as a wonderful synthetic tool2-4. A six membered ring resulting from DA reaction can be used as substructure in building new complex molecules and plays a vital role in designing new natural products5. The perspective of DA reactions was changed recently by the introduction of cyclobutenone as dienophile by Danishefsky6-8 since experimentally cyclobutenone was found to be more reactive and their reactions can be carried out in mild conditions than corresponding higher alkenones. The cyclo adduct formed during Diels Alder reaction involving cyclobutenone ring gives cyclopentanone, lactum lactones during ring expansion which cannot be able to prepared through direct Diels Alder reaction. These type of reactions attracted more attention towards its mechanistic elucidation. Further the experimental studies done by Danishefsky suggest that the dienophile reactivity gets enhanced by incorporation of a vinyl bromide or chloride at the =$

of cyclobutenone

9-10.

Several theoretical attempts were put forth by

many researchers to understand the reactivity aspects of DA reactions involving simple diene and dienophile using various levels of theory11-35. Notably, Richard Tia36 group investigated the regio and stereoselectivity of DA reactions of chloro, cyano, methyl and hydroxy substitution in 2, 3 and 4th position of cyclobutenones at MP2/6-31g* level of theory. Also, they have shown the versatile role of Lewis acid catalysts in cyclobutenone involving Intermolecular Diels Alder reactions (IMDA) for enhancing the selective formation of endo product. Further, through the quantum chemical calculations, Houk et al37 have studied the importance of Lewis acid coordination on the formation of endo product in cyclobutenone IMDA and also demonstrated how the reactivity of the intermolecular Diels–Alder reaction of tethered cycloalkenones and


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butadienes is being affected by bromo substitution at =$ carbon of cyclobutenone and terminal dienes. Among the cycloalkenones, cyclobutenone is found to have higher reactivity and bromine substitution on cyclobutenone produces a significant effect on the kinetics and lowers the activation energy of endo transition state. Further, the DA reaction involving cyclobutenone with cyclic dienes such as cyclopentadiene, cyclohexadiene and cycloheptadiene were explained and cyclopentadiene is found to be more reactive through their minimal out-of-plane distortion of their transition states than other cyclic dienes by DFT calculations38. The experimental result of Li and Danishefsky suggests that among various substituted butadienes Dn-6 and Dn-10 yields major product with 2-bromocyclobutenone (DPh) in gas phase conditions at ambient temperature., In order to explore the experimental observation, theoretical investigation for various substituted butadiene (Dn1 to Dn10) with 2-bromocyclobutenone dienophile (Dph) were attempted in gas phase (Scheme 1). The present work aims at thoroughly investigating, the mechanism of the reaction and the factors which drives the reaction. Specifically the role of substituents and bulky group, in enhancing the reaction rate have been investigated. This approach is commonly used to rationalize the reactivity differences among substituted dienes towards the Diels-Alder reaction with dienophile. hardness

39-45,

The electrophilicity,

activation free energy, deformation energy and bond order analyses were used to

pinpoint the difference in reactivity pattern between the substituted butadiene towards the DielsAlder reaction with dienophile.




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calculation to verify the nature of stationary points. In all the cases, the reactant and product had real frequencies and the TS had single imaginary frequency in the diagonlized hessian matrix. After locating TS, Intrinsic Reaction Coordinate (IRC) paths were traced to check whether the TS connected to appropriate minima on both sides by using the Second order Gonzalez Schlegel integration method 48-49. The DA reactions usually involve breaking of three G bonds and formation of one G and two H bonds. The bonding changes are therefore remarkable only in those bonds compared with the changes in the other parts of the reactants. For that reasons, the changes in these bonds alone were quantitatively calculated using the Wiberg bond indices50 which are derived from natural atomic orbital analysis. The second-order stabilization energies were also computed from Natural Bond Orbital (NBO) analysis and the bond order analyses were performed to monitor the progress of the reaction. The calculation of percentage of Bond Formation (BFi) and Cleavage (BCj) at the TS are described below51-53. BOiTS - BOiR X 100 BOiP - BOiR

BFi (or ) BC j

BFAve

BFC Ave =

1 n

n

BC Ave =

BFi i 1

and

1 n K BC n j =1 j

1 (BFAve + BC Ave ) 2

…. (2)

…. (3) …. (4)

Where, BFCAve is an indicative of the early/late nature of TSs for these reactions.

Bader's Quantum Theory of Atoms in Molecules (QTAIM) 54-55 analysis was also done to quantify the nature of possible hydrogen bonding interaction occurring within the TS using AIM2000 programme. The FOE analysis, activation and reaction energy calculations were also carried out.



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RESULTS AND DISCUSSION DFT calculations were performed to investigate the mechanistic aspects of DA reaction between substituted 1,3-butadienes and 2-bromocyclobuteneone as dienophile (DPh). In the present study, 1,3-butadiene is chosen as unsubstituted diene (Dn-1). Experimental results available only for reaction with 2-bromocyclobuteneone and Dn-2 and Dn-6 to Dn-10. In extension to this we have modelled the reaction of 2-bromocyclobuteneone with Dn-1, Dn-3, Dn-4 & Dn-5 for the better understading and to compare the results obtained. The nine different dienes (Dn-2 to Dn-10) were chosen based on mono and multiple substitutions of Me, OMe and OTBS groups (Scheme 1). It is well understood that endo adduct is more favored than that of exo-adduct by both experimentally and theoretically 10-36 and thus further discussions are mainly concerned on endo-adduct formation. DIELS-ALDER REACTION BETWEEN MONO SUBSTITUTED 1,3-BUTADIENES AND DIENOPHILE: Figure 1 displays the computed reaction profile for thermal intermolecular

N

reactions of mono substituted dienes (Dn-1 to Dn-6) and DPh. The activation and reaction energies for intermolecular DA reaction between 1,3-butadiene (Dn-1) and DPh is estimated as +17.55 and -47.27 kcal/mol respectively. Upon investigating its transition state geometries (Figure 2), the bond length for incipient C-C bonds and the breaking C=C bonds are found to be 2.414 & 2.147 Å and 1.369 & 1.381 Å respectively. This clearly indicates that the reaction is asynchronous and is found to be exothermic. Upon substitution of CH3 and OTBS (Dn-2 and Dn-4) groups on the C2 carbon of Dn-1, IMDA reaction is found to have higher activation barrier than un-substituted one and corresponding activation energies are 18.39 and 18.74 kcal/mol. Substitution of OMe group (Dn-3) on C2 carbon, the reaction is found to be kinetically


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more facile and favorable than Dn-1 by 3.0 kcal/mol. The asynchronous nature of the transition state is enhanced by substituting CH3, OMe and OTBS on C2 position of the Dn-1 (See Figure S1). This effect is more predominant in Dn4_TS where the incipient C-C and the breaking C=C bonds are found to be 2.714 & 1.996 Å and 1.369 & 1.354 Å respectively. Although there is not much difference in the kinetics aspects of the reaction in both Dn-2 and Dn-4, thermodynamically the product formation is favored for substitution of CH3 group on C2 carbon (Dn-3) over the substitution of OTBS group on C2 carbon (Dn-4). This stability differences among the products of Dn-2 and Dn-4 is mainly due to the encumbrance of steric interaction from bulky OTBS group. The stability of the products of Dn-2 and Dn-3 is almost similar (-46.45 and -46.04 kcal/mol respectively) which is presumably due to the electron donating nature of both the substituents and low kinetic barrier associated with Dn-3 is because of strong electron donating nature of OMe group than methyl group. Hence, activation barrier and reaction energies are perturbed depends on the steric and electron donating nature of the substituents on the C2 position of diene.




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interesting to note that the activation barrier of Dn-1 could substantially reduced by placing suitable electron donating substituents such as OMe on C1 and C2 position of 1,3-butadiene. This effect is much more pronounced on substituting OMe group on C1 position. The Intermolecular hydrogen bonding interaction between hydrogen atom of methyl group of Dn-5 and Dn-6 and oxygen atom of 2-bromocyclobuteneone plays a decisive role in reducing the activation barrier of the IMDA reaction. So that, dienes Dn-5 and Dn-6 with substitution of methyl and methoxy group at C1 position of 1,3-butadiene is found to be most effective diene for IMDA reaction with

2-bromocyclobuteneone through its electron donating and

mesomeric effects. The computed results are in good agreement with experimental observations made by Danishefsky10 where as Dn-6 found to provide more yield than other dienes.



C2

C1 2.414

1.369

C6 1.336

C1 1.342

C2 C5

C2

1.394

C3

C3

1.381

1.530

C6

1.334

C5

1.553

C3

C4 2.147

1.337

C1

1.508

C6

1.414

1.473

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C5

1.510

C4 1.537

C4 Dn-1_R

Dn-1_P

Dn-1_TS 2.448 2.477

C1 C2 C1

C1 C6

1.340

C3

C2

C1

C3 C2 C5

C4 C3

1.412

C4

2.114

1.537

1.334

C5

C4

1.383

Dn-5_P

Dn-5_TS

C7

C1

C5C6 C4 1.554 C3 1.538 C5 1.511 C4

C3 C2

1.396

C4

Dn-5_R

C7 O2

2.680

O2

2.624

C1

O1

1.340

2.503

C1

1.370

2.623

C5

1.340

C2

1.397

C3

1.343

1.332

C5

1.382

C4

1.559

C3

2.112

C5

1.509 C4 1.537

DN-6_TS Dn-6_TS

Dn-6_R

O1

1.504 C1 1.550 C6

C6

1.408

C6

C3 C4

2.499

O1

C2

C2 1.464

1.510

2.484C5 C6

C3

1.339

2.872

C6

C2

1.372

C6

C5

1.343

1.468

C1

C2

Dn-6_P 2.505

C1

2.738

1.357

1.337

1.343

1.507 C1

2.717

1.428

1.476

C4 C3

C1

C2

C2

1.400 1.396

1.342

Dn-9_R

2.511

2.542

C1

C2 1.467

Dn-9_P

DN-9_TS 2.810

1.337

C5

1.513 C4 1.536

2.029

Dn-9_TS

DN-9_R

1.552

C3

C5 C4

1.538 C6

1.336

C3

C5 C6

C2

C6

1.357

C2

C3 1.334 C6 C4 1.342 C5

1.501

1.395

C3

C5 C4

C2

C6

2.795

1.424

1.390

Dn-10_R

C1

2.093

Dn-10_TS

C1 1.552

1.333

C6

1.337

C3

1.535

1.510

C5

C4

Dn-10_P

Figure 2. Optimized geometries of reactant, transition state (TS) and product of DA cycloaddition reaction of Dn-1,Dn-5, Dn-6, Dn-9 and Dn-10 at M06-2X/6-31+g** level of theory.


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avoiding steric encumbrance in spite of forming stabilizing H-bonding interaction. This particular feature is verified theoretically by choosing diene moiety with Me and OMe on C1 position and bulky OTBS group on C3 position (Dn-9 and Dn-10). The computed activation barrier is found to be 14.57 and 13.59 kcal/mol respectively for Dn-9 and Dn-10. The entire reaction is exoergic for Dn-10 over all other dienes which is comparable with Dn-4 (-41.96 and 41.57 kcal/mol for

Dn-10 and Dn-4 respectively). This indicates that cycloadduct formation is

dictated by steric effects. The low activation barrier and higher exothermicity of IMDA between Dn-10 and DPh is solely combination effect of H-bonding interactions along with steric hindrance arises due to the bulky OTBS group. From the computed results, one could understand that position of the substituents on the diene unit is more vital in deciding the barrier heights and relative stabilities of cycloadduct formation. These entire aspects are enrooted from the possible steric encumbrance or additional stabilization evolving during the cycloadduct formation. Based on the computed energy profile, one could clearly understand the higher reactivity of the Dn-6 inferred from corresponding low activation which is also consistent with the experimental observations of Danishefsky who suggested that Dn-6 forming 97% product under thermal condition than other substituted butadienes.10 DEFORMATION ENERGY ANALYSIS: Deformation energies of diene and dienophile were computed to understand the role of strain and how for these two addends deform their geometries at the transition state. This could be obtained by performing single point calculations on the respective addends at the transition state. Deformation energy is obtained from the energy differences between the optimized ground state structure of reactant and their distortion structure.


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getting shortened from 2.112 to 2.093 Å for Dn-6 and Dn-10 respectively. This factor is predominant in deciding the kinetic and thermodynamic aspects of entire reaction. It is clearly envisaged from the high formation energy associated with cycloadduct formation in Dn-6 (-44.04 kcal/mol). Overall, Dn-6 and Dn-10 were found to have the lesser deformation energy among other dienes investigated (23.92 and 21.62 kcal/mol respectively). This clearly explains the higher reactivity associated with Dn-6 and Dn-10 over other dienes and the present computational studies are in good agreement with the experimental observations of Danishefsky10. Deformation of dienophile occurs largely at the transition state upon interacting with Dn-5 where methyl group substituted at C1 position of diene (15.66 kcal/mol) in addition to the weak intermolecular H-bonding stabilization between diene and dienophile. Similarly, dienophile found to possess significant deformation energy in Dn-4 (14.97 kcal/mol) where diene is substituted with bulky OTBS group in C3 position of diene. Rest of the other cases, dienophile found to have deformation energy in the range between 12.15 and 12.89 kcal/mol. The above fact is clearly depicted from the degree of asynchronous nature of transition state and is enhanced upon substituting bulky substituents on C1 or C3 positions of 1,3-butadiene. Even substitution at C3 position of 1,3-butadiene, the asynchronous nature of the transition state is larger in both Dn-4 and Dn-10. So the deformation of diene and dienophile occurs significantly to avoid the steric encumbrance evolving at the transition state, thereby asynchronous nature of transition state was also increased. Therefore, the role of steric effect is pivotal in enhancing the activation barrier of the reaction and deformation of both diene and dienophile.


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FRONTIER MOLECULAR ORBITAL (FMO) ANALYSIS: The Frontier Molecular Orbital’s (FMO) are often used to derive qualitative information about the electronic structural properties of molecules and is also used to estimate the chemical reactivity of the molecules and hence energy of FMOs are computed and analyzed. The computed HOMO, LUMO and band gap of dienes and dienophile along with quantum chemical descriptors are arranged in Table 2. The electron transfer in this [4+2] cycloaddition is from HOMO of electron rich diene towards LUMO of electron poor dienophile. The FOE gap value given in Table-2 shows that U"1 < U"2 which suggests that there is a normal electron demand in all the cases. Table 2. Computed HOMO and LUMO values of diene, band gap (eV), FOE gap (eV), chemical potential (µ) chemical hardness #X% and chemical softness (s) with dienophile at M06-2X/631+g** level of theory. Band Gap Dn1 -7.98 -0.19 7.79 Dn2 -7.71 0.14 7.85 Dn3 -7.54 -0.16 7.38 Dn4 -7.49 -0.08 7.41 Dn5 -7.58 0.02 7.60 Dn6 -7.18 0.16 7.35 Dn7 -7.8 -0.06 7.74 Dn8 -7.35 0.29 7.64 Dn9 -7.28 0.07 7.36 Dn10 -7.11 0.11 7.22 Dienophile -8.89 -1.01 7.88 a? 1 = EHOMO (of diene) - ELUMO (of dienophile) b? 2 = ELUMO (of diene) - EHOMO (of dienophile) Diene

HOMO

LUMO

FOE Gap ? 1a ? 2b 6.97 8.7 6.70 9.03 6.51 8.74 6.48 8.81 6.57 8.91 6.18 9.07 6.79 8.83 6.33 9.18 6.26 8.97 6.09 9.01 -

µ

A

S

-4.08 -3.78 -3.85 -3.79 -3.78 -3.51 -3.93 -3.53 -3.60 -3.50 -4.95

3.89 3.92 3.69 3.70 3.80 3.67 3.87 3.82 3.68 3.61 3.94

0.13 0.13 0.13 0.13 0.12 0.14 0.13 0.13 0.13 0.14 -

With respect to the unsubstituted diene (Dn-1), the band gap of mono and disubstituted dienes is substantially reduced from 7.79 to 7.22 eV except Dn-2 having band gap of 7.85 eV. The band gap of Dn-6 and Dn-10 is 7.35 and 7.22 eV respectively which clearly portrait the



higher reactivity attributed with low band gap. The energy of HOMO and LUMO level of Dn-1 is -7.98 and -0.19 eV. In general, both HOMO and LUMO levels are destabilized on substituting Me, OMe and OTBS groups on 1,3-butadiene moiety. The destabilization of HOMO and LUMO levels in both Dn-6 and Dn-10 occurs in greater extent when compared with other substituted dienes. So substitution of OMe group on 1,3-butadiene is due to its bulky nature and steric effect shows a prominent role in enhancing the reactivity of dienes thereby reducing the activation barrier of corresponding Diels-Alder reaction with dienophile. It is essential to correlate the quantum chemical descriptors with the observed reactivity of the dienes. So the present work is aiming to emphasis how for quantum chemical descriptors are helpful to correlate the chemical reactivity of the substituted dienes. In general, the reactivity of a molecule increases with decrease in the hardness values #Y in eV) and increases in the softness values (S, in eV). Softness is having inverse relationship with hardness of the molecule. These two global reactivity parameters are indicative index of the stability of chemical molecules. According to the calculated hardness and softness values listed in Table 2, the hardness value of a Dn-1 is 3.89 and for other substituted dienes (Dn-2 – Dn-10) is falls in the range of 3.92 to 3.61 eV. Low Y and high S values associated with methoxy substituted dienes Dn-6 and Dn-10 were found to possess higher reactivity giving high yield as observed in the experiments10. BOND ORDER ANALYSIS: The change in degree of bonding during the reaction is explained on the basis of calculating the percentage of Bond Forming (BFi), Cleaving (BCi) at the transition state of various bonds. Also, the relative maturity of transition states in the reaction path is identified through computing the average percentage of Bond Formation Cleavage (BFCave).


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In all the cases, BFCAve values fall in the range of 40 - 43% which shows the early nature of TS. This early nature of transition state is more predominant in Dn-5 and Dn-9. From the bond order values, it is revealed that diene first attacks the C5 carbon of dienophile as it has more electron deficient and as a result of it the C4-C5 bonds forms faster than C1-C6. It could be easily verified from the bond length values of newly forming bonds C4-C5 and C1-C6 from optimized geometries which is shown in Figure. 2 as well as from the computed bond order values presented in Table 3. Table 3. Computed bond formation (BFi) and bond cleavage (BCj) indices* for all chosen species in the Diels-Alder cycloaddition reaction with dienophile at M06-2X/6-31+g**/631+g**level of theory (Refer Figure 2 for atom numbering). Species

C1-C6

BFi C2-C3

C4-C5

C1-C2

BCj C3-C4

C5-C6

BFCAve

Dn1 22.27 75.66 37.72 34.77 43.36 42.30 42.68 Dn2 20.00 67.93 39.33 32.15 44.27 43.64 41.22 Dn3 17.90 74.82 37.30 32.90 44.20 43.16 41.71 Dn4 15.54 66.11 46.02 26.52 49.70 48.77 42.11 Dn5 11.90 62.66 46.75 22.60 51.44 49.90 40.88 Dn6 13.12 75.74 38.91 32.92 45.54 43.48 41.62 Dn7 19.85 71.09 39.83 32.38 44.57 43.77 41.92 Dn8 19.52 76.79 38.54 35.28 45.40 44.24 43.30 Dn9 10.60 63.90 45.20 23.54 50.36 48.68 40.38 Dn10 7.54 63.71 40.48 23.28 71.80 44.37 41.86 * By definition, bond indices BFi and BCj for various bonds listed above are 0 and 100, respectively, for reactants, transition states and products for species. The developing C2-C3 G bond is more pronounced than the C1-C6 and C4-C5 H bonds. Bond order analysis reveals that all TS are early TS and brings out bond breaking and making process in the reaction path. The IRC calculations provide that the geometry of species that lies at halfway between the TS and the reactant. These geometries are located in a smooth drop in energy, after the barrier height shows that C4-C5 bond is formed very fast, on the other hand C1C6 bond is much delayed. Moving the reacting system from the TS towards the product side



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confirms a gradual increase in formation of C1-C6 bond and this prove that the TSs obtained are indeed concerted asynchronous TS and not stepwise TS. NBO ANALYSIS: The nature of stabilizing interactions such as donor-acceptor interaction could be easily rationalized with the help of NBO analysis. Especially, second order perturbation interaction between donor and acceptor orbitals plays a significant role in identifying major interactions responsible for higher reactivity of the dienes. NBO analysis for all other dienes was provided in the electronic supporting information in Figure S2. Primarily G Z G interactions between diene and dienophile was found to be present in all the 10 dienes and ranging from 13.6 to 255.4 kJ/mol (See Table S1). The magnitude of these interactions is depends on the asynchronous nature of the transition state. Especially, the asynchronous nature of Dn4-DPh_TS is higher in Dn4 due to bulky nature of OTBS group, the corresponding stabilizing interactions were found as 17.5 and 255.4 kJ/mol corresponding incipient C-C bond distances (C1-C6 and C4-C5) is found to be 1.996 and 2.714 Å respectively. This clearly manifests the role of substituents is phenomenal in tuning the key stabilizing interactions between donor and acceptor orbitals. In addition to these interactions, only five dienes (Dn-5, Dn-6, Dn-7, Dn-9 and Dn-10) were found to have n Z H

H

interactions which is resulting from the intermolecular H-bonding

interactions. They are ranging from 1.84 to 3.26 kJ/mol. The n Z H

H

interactions and the

stabilization energies of dienes Dn-5, Dn-6, Dn-7, Dn-9 and Dn-10 were presented in Figure. 4. Dn6 was found to have both H-bonding interactions and G Z G interactions which could be major reason for their higher reactivity. Overall, good correlation between stabilization energies (in kcal/mol) and NBO stabilization energies # $H % was obtained for all the dienes. It is interesting to observe that NBO analysis also help to explain experimentally observed higher reactivity of Dn-6.


Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dn-5

Dn-7 nOD E * = 2.34

Dn-6 nOD E * = 3.26

nOD E* =2.51

Dn-9 nOD E * = 1.84

Dn-10 nOD E * = 2.51

Figure 4. Natural bond orbitals of strong hydrogen bonding interaction in certain transition states involved in the intermolecular DA reaction. Second-order stabilization energies are given in kJ/mol. QTAIM ANALYSIS: QTAIM analysis is an effective tool in rationalizing the existence of non-covalent interactions in ionic pairs54, host-guest complexes57and chemical systems 55-58. Especially, it is highly supportive in identifying the hydrogen bond interactions through the computation of (3,-1) Bond Critical Point (BCP) between H-bond donor and acceptor atoms. Electron density #\% is a direct measure of the strength of a particular interaction. Generally, larger electron density is associated with strong bonds. The magnitude of rho values #\% clearly pinpointed the nature of the interactions between any two atoms. For covalent bonding type of interactions, generally it is greater than 0.20 a.u and for non-covalent interactions, \ value is found to be less than 0.10 a.u. The computed rho values along with their Laplacian values of the (3,-1) critical points are provided in Table 4. The magnitude of \ value for incipient C-C bonds is found to be in the order



of 0.018 to 0.0808 a.u. The asynchronous nature of transition state in the present IMDA reaction between diene and dienophile is clearly observed from the rho values of (3,-1) BCP at their corresponding transition states. Dn4 was found to have higher asynchronous nature as observed in NBO and computed bond parameters and their corresponding rho values are 0.022 and 0.085 a.u. It is interesting to note that rho values derived from QTAIM analysis are in good accordance with computed activation energies and asynchronous nature of transition states. From the molecular graphs provided in Figure 5 and ESI Figure S3, only five dienes (Dn-5, Dn-6, Dn-7, Dn-9 and Dn-10) were found to have three BCPs at the transition state which are resulting from the intermolecular H-bonding interactions and rest of the dienes found to have only two BCPs. In the case of most reactive dienes Dn-6 and Dn-10, the orientation of DPh and Dn-6, Dn-10 renders them to interact with each other via many G$G interactions and through strong O-H interaction. The strong intermolecular interaction of Dn-6 was further supported by the Rho plot which is given in Figure 6. This strong intermolecular interaction of methyl hydrogen of diene and dienophilic oxygen in addition to ]$] interactions in Dn-6 and Dn-10 is responsible for their lower activation energy than other dienes. The activation energy calculation clearly predicts that Dn-6 and Dn-10 gives better yield with DPh than the other substituted butadienes.


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dn-1

Dn-5

Dn-6

Dn-7

Dn-9

Dn-10

Figure 5. Molecular graphs of particular transition states with strong hydrogen bonding interaction obtained from QTAIM analysis. All intermolecular BCPs between relevant atoms are circled in blue.



Page 22 of 30

Table. 4. Intermolecular bond critical points (BCPs) and the corresponding distances between atoms (Å), classifications of the intermolecular interactions, electron density #\ in a.u.) and its laplacian value ( 2\ in a.u). Compound Dn1

Dn2

Dn3

Dn4

Dn5

Dn6

Dn7

Dn8

Dn9

Dn10

length

\

(Å)

C•••C

C1···C6

2.414

0.039

-0.014

C4···C5

2.147

0.062

-0.009

C1···C6

2.468

0.035

-0.014

C4···C5

2.103

0.068

-0.007

C1···C6

2.843

0.042

-0.013

C4···C5

2.096

0.069

-0.006

C1···C6

2.714

0.085

-0.000

C4···C5

1.996

0.022

-0.012

C1···C6

2.484

0.034

-0.013

C4···C5

2.114

0.066

-0.007

lPO···H

2.477

0.009

-0.008

C1···C6

2.623

0.026

-0.013

C4···C5

2.112

0.067

-0.007

lPO···H

2.503

0.009

-0.008

C1···C6

2.459

0.036

-0.014

C4···C5

2.110

0.068

-0.007

lPO···H

2.502

0.009

-0.008

C1···C6

2.458

0.036

-0.014

C4···C5

2.107

0.067

-0.007

C1···C6

2.717

0.022

-0.012

C4···C5

2.029

0.0808

-0.002

lPO···H

2.505

0.009

-0.007

C1···C6

2.795

0.018

-0.011

C4···C5

2.093

0.071

-0.006

lPO···H

2.542

0.009

-0.008

Interactions


2\

C•••C

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a)

b)

C

H

O O C1

C1 C6

C5 C4

Br C

C2

C6 C

C3 C4

C5

Figure 6. Laplacian and Rho graph of Dn-6 transition states involved in the intermolecular DA reaction.

CONCLUSION: The DA reaction of substituted 1,3-butadiene (Dn-1 –Dn-10) and 2-bromocyclobutenone, (DPh) has been carried out using density functional theory. The calculations show that Dn-6 and Dn-10 are having lower activation energy barrier with 13.29 kcal/mol and 13.59 kcal/mol respectively. The reaction energy and deformation energy analysis further supports the same fact of high reacting tendency of Dn-6 and Dn-10 is observed. The FOE analysis shows that the reaction follows normal electron demand with U"1 < U"2 suggesting the electron transfer from HOMO of diene to LUMO of dienophile .The reactivity of diene and dienophiles were measured using global descriptors. The DA reaction proceeds through asynchronous transition state in which diene and dienophile approaches each other through fragile intermolecular interactions. This fragile intermolecular interaction significantly reduces activation energy barrier and signifying the better reacting mode of Dn-6 and Dn-10 among dienes (Dn-1-Dn-10) in the



reaction on gas phase. The QTAIM analysis further supports that the added O-H interaction with hydrogen bond stabilization found in Dn-6 and Dn-10 which is evident for the major product formation. The 0 $] interaction found using NBO also added on to the high reacting trend of Dn-6 and Dn-10 with DPh. The theoretical results obtained shows that Dn-6 and Dn-10 were found to be highly reactive which typically synchronizes with the experimental studies of Danishefsky where Dn-6 and Dn-10 shows high percentage yield among various dienes. ASSOCIATED CONTENT Supporting Information The Cartesian coordinates and energies of all optimized structures and transition structures. This material is available at free of cost via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

A. Adilakshmi- [email protected] Notes The author(s) declare that no competing financial assistance for this research, authorship, and/or publication of this article. REFERENCES: 1) Diels, O.; Alder, K., Syntheses of the Hydroaromatic Series, Justus Liebigs Ann. Chem. 1928, 460, 98–122. 2) Saver, J.; Sustmann, R. Mechanistic Aspects of Diels Alder Reactions: A Critical Survey Angew. Chem., Int. Ed. Engl. 1980, 19, 779-807. 3) Carruthers W Von., Cycloaddition Reactions in Organic Synthesis, Oxford, Pergamon. 1990, VIII, 373.


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Theoretical Investigation of Steric Effect Influence on Reactivity of

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