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Tetrahedron 70 (2014) 7906e7911

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Tetrahedron

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Potential energy surface of the cationeneutral hydroaminationreaction: a computational study on the role of an ionemoleculecomplex in the reaction pathway

Sanyasi Sitha a,*, Linda L. Jewell b, Priya Bhasi a, Zanele P. Nhlabatsi a, Vijay M. Miriyala a

aDepartment of Chemistry, University of Johannesburg, PO Box 524, Auckland Park, Johannesburg 2006, South AfricabCollege of Science, Engineering and Technology, University of South Africa, PO Box 392, Pretoria 0003, South Africa

a r t i c l e i n f o

Article history:Received 9 June 2014Received in revised form 11 August 2014Accepted 27 August 2014Available online 2 September 2014

* Corresponding author. E-mail address: ssitha@uj.

http://dx.doi.org/10.1016/j.tet.2014.08.0610040-4020/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Detailed reaction profiles of the cationeneutral direct hydroamination reaction between ethylene andammonia are analyzed using B3LYP and Full-MP2 methodologies. By investigating the PES for the re-actions thoroughly we found that due to the presence of a stable ionemolecule complex, the transitionstate (TS) is now a true TS and thus the reaction proceeds through an energetic barrier. This observationcontradicts our earlier report (Tetrahedron 2010, 66, 3030) where have shown that cationeneutral directhydroamination reaction is barrierless. A detailed analysis of the reaction profile, shows that not only theenergetics of the stationary points but also the molecular dipole moments, the spin density distributionand the structural orientations of the reactant complex indicate a preference for the reaction ofCH2]CHþ

2 with NH3 over the reaction of NHþ3 with CH2]CH2. This observation is in strong agreement

with the experimental findings of Hamann et al. (Angew. Chem., Int. Ed. 2009, 48, 4643).� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The hydroamination of alkenes, which proceeds through theformal addition of an NeH bond across a carbonecarbon doublebond, is an elegant synthetic organic transformation, which offersan attractive route to numerous classes of organo-nitrogen mole-cules such as alkylated amines, enamines or imines.1e14 The syn-chronous direct addition of ammonia to an alkene to produce theamine is of seemingly fundamental simplicity and is highly desir-able from an industrial point of view, as several tons of amines areproduced worldwide every year.1,2,5,8 Hamann et al. have reportedan experimental investigation of the cationeneutral hydro-amination reaction of CH2]CH2 with NH3,15 and we have com-pared the hydroamination reaction of NH3 with CH2]CH2 for theneutraleneutral as well as the cationeneutral reaction cases usinga computational approach.16

In our earlier report we have shown that the cationeneutralhydroamination reaction is barrierless as the barrier was found tobe energetically more stable than the reactants.16 A question thatarises is that if the reaction has a structural potential barrier, why itis not a true barrier energetically? In the neutraleneutral hydro-amination reaction, the formation of a stable chemical complex is

ac.za (S. Sitha).

unlikely, due to the strong electronic repulsion arising from theinteraction between the p-cloud of the ethylene and the lone pairelectrons of the ammonia. Instead association of these two neutralreactants may lead to the formation of a very weak complex, wherethese twomolecules will be separated by a relatively large distance.This type of complex may have a slightly lower energy than thereactants, which will lead to a slightly larger activation barrier, butthe overall reaction trend will remain the same. On the other handwhen these two reactants react in an ionemolecule type of in-teraction (where one of the reactants is positively charged and theother is neutral), this will definitely lead to a favourable chemicalassociation complex. For example, if the ammonia is positivelycharged and reacts with the neutral ethylene, then the positivecentre of the ammonia (deficient in electron density) and the p-cloud of the ethylene (rich in electron density) will give rise toa strong electrostatic attraction and ultimately lead to chemicalbond formation. Hence, formation of a stable chemical complex inthe PES of the cationeneutral hydroamination reaction seems likelyand is expected to be more stable than the reactants. Now, if thiscomplex is less stable than the transition state (TS) of the cati-oneneutral hydroamination reaction, then the reaction can still betreated as barrierless.16 But, if the complex is more stable than theTS, then the reaction will no longer be barrierless, instead it willgive a completely different picture for the cationeneutral hydro-amination reaction. The effect of the energetics of such a complex

S. Sitha et al. / Tetrahedron 70 (2014) 7906e7911 7907

on the PES of the reaction is the main subject of this study and inthis work we have thoroughly investigated the PES of the cati-oneneutral hydroamination reaction using quantum chemicalmethods.

2. Computational methods

All of the calculations were carried out using MP2(Full)/6-31þþG(2df,2p) [henceforth referred as MP2] and B3LYP/6-31þþG(2df,2p) [henceforth referred as B3LYP] level of theory,implemented in the Gaussian 03 program package.17 For the closedshell species, the RMP2 and RB3LYPmethods, and for the open shellspecies, UMP2 and UB3LYP methods were employed during opti-mization. During the full-MP2 calculations, both the valence andcore electrons were taken into consideration. The true minima andthe transition states were confirmed from analysis of their fre-quencies by ensuring that all frequencies were positive for theminimum, with only one imaginary frequency for the transitionstate. We have also analysed the displacement vectors for theimaginary frequency to ascertain whether the TS is a true TSstructurally. All the thermodynamic quantities were calculatedfrom the zero point corrected energies of the reactants, reactantcomplex, TS and product. Results from these above two methodswere used to describe the reaction profile of this cationeneutralhydroamination reaction. As the discussions in our earlier report onthe same reaction are based on these twomethods, this will be ableto facilitate a direct comparison of the results.16 Besides these abovementioned two methods, we have also carried out calculations(geometry optimizations) using 6-31þþG(3df,2pd) and Dunning’scorrelation consistent basis sets like aug-cc-pVDZ and aug-cc-pVTZ(the prefix ‘aug’ represents that these basis sets are augmentedwith diffuse functions), for both MP2(Full) and B3LYP methods, toaccount for the effect of other higher level basis sets on the prop-erties of the stationary points and energetics of the reaction profile.Calculations were carried out using B3LYP-D3 to account for thedispersion correction to the DFT method and also for the complexwe have done counterpoise calculations to ascertain the Basis setsuperposition error.

3. Results and discussion

As proposed by Hamann et al., the cationeneutral hydro-amination reaction between the NH3 and CH2]CH2 proceedsthrough two reaction pathways:15 (1) the reaction of the cation ofethylene with neutral ammonia and (2) the reaction of the cation ofammonia with neutral ethylene. Hence, to thoroughly analyze thePES of these two reaction surfaces, every stationary point in thesesurfaces was analyzed using MP2 and B3LYP methods.

3.1. Reactants

The reactants are cationic and neutral ethylene and ammonia.Neutral ammonia is C3v symmetric and in the singlet 1A1 state. Itadopts a pyramidal shape with the angle of pyramidality being38.7� and 38.0�, respectively, for MP2 and B3LYP methodologies.The resultant dipole moment of the ammonia molecule is 1.70 Dand 1.60 D, respectively, for the MP2 and B3LYP methods and isunidirectional (negative z-axis, where the origin of the co-ordinates situated at the N-atom). On the other hand, neutralethylene adopts a fully planar configuration with D2h symmetryand is in the 1Ag state. Being symmetric, the dipole moment of theethylene is zero for both of the methods (the origin of the co-ordinates situated at the centre of CeC bond). The fully mini-mized geometry of the ammonia cation shows a planar structuralarrangement with D3h symmetry (the origin of the co-ordinatessituated at the N-atom), which is in good agreement with the

earlier computational and experimental works.18e22 The fullyminimized geometry of the ethylene cation shows a non-planar D2symmetric structural arrangement (the origin of the co-ordinatessituated at the centre of CeC bond) and agrees well with the ex-perimental results.23e26 Both the molecules in their cationic formhave a zero dipole moment in their ground state due to theirhighly symmetric structures. Note that there was very little spincontamination found during optimization of the cations ofethylene and ammonia (the hS2i values for CH2]CHþ

2 are 0.75396and 0.755939 and for NHþ

3 are 0.753639 and 0.760778 for theB3LYP and MP2 methods, respectively). Molecular structures ofthese reactants with the Cartesian axes are also providedin Supplementary data, along with their properties and energeticscalculated using various other methods (as discussed in Section 2).Also, a more detailed discussion on the reactants can be found inour earlier report.16

3.2. Reactant complex

In the cationeneutral hydroamination, either the cation ofethylene reacts with neutral ammonia, or the cation of ammoniareacts with neutral ethylene. This type of reaction creates an idealsituation for a favourable ionemolecule interaction. Hence, for-mation of an ionemolecule chemical complex in the cati-oneneutral hydroamination reaction is likely. So we carried outa search for a stable complex in the PES. Interestingly we were ableto locate a potential ionemolecule complex in the PES using MP2as well as B3LYP methods, and it was found to be more stable thanthe reactants. Note that there was very little spin contaminationfound during optimization of the complex (the hS2i values are0.753734 and 0.762303 for B3LYP and MP2 methods, respectively).A snapshot view of the optimized structure, key structural pa-rameters and some fundamental properties of the complex areshown in Table 1.

From analysis of the key structural parameters, it can be seenthat both the MP2 and B3LYP methods show almost the samegeometry for the reactant complex. R1,2 and R2,3 values show thatCeC and CeN, respectively, are characteristic of single bonds. Atthe same time, the angles of interaction between the NH3 andethylene subunits are 110.7� and 111.2�, respectively, in the MP2and B3LYP methods, and the pyramidality of the NH3 hydrogens inthe complex is retained, similar to that of the neutral NH3. The firstfive un-scaled harmonic frequencies of the complex in both of themethods are also shown in Table 1. Analysis of the harmonic fre-quencies shows that all of them are positive indicating that thecomplex is a true minimum. The dipole moment of the complex is3.8 D and 3.6 D in MP2 and B3LYP methods, respectively, andanalysis of the dipole component vectors shows that the dipolemoment is almost unidirectional with the maximum contributionto the total dipole moment coming from the x-direction (pleaserefer to the figure in Supplementary data for the Cartesian axesdirections).

Analysis of the charge distribution (hydrogens summed into theheavy atoms) shows that the unit positive charge is distributedthrough the backbone atoms of the molecule. The maximum pos-itive charge density is on the N-atom of the molecule (0.66 e and0.73 e in MP2 and B3LYP methods, respectively), with a smallercharge density on the C2 (which is directly connected to the N-atom), and finally a negligibly small positive charge density occurson the C1 atom. Analysis of the spin density values shows that thetotal spin density is unevenly distributed and the spin density ismostly concentrated at the C1 centre of the molecule. Also, themolecular structure of the complex with the Cartesian axes isshown in Supplementary data, along with its properties and en-ergetic details calculated using various other methods (as discussedin Section 2).

Table 1Key structural parameters (Rp,q are represented in�A, Ap,q,r and Dp,q,r,s are represented in degrees), dipole moments (in Debye), charges (in electrons), atomic spin densities andharmonic frequencies (in cm�1) of the reactant complex in the cationeneutral hydroamination reaction calculated using the MP2 and B3LYP methods

Methods Structural parameters Dipole moment Charges Spin densities Harmonic frequencies

MP2 R1,2¼1.479R2,3¼1.524A3,2,1¼110.7D8,9,10,3¼37.9

mx¼3.7my¼�0.7mz¼0.0mt¼3.8

C1¼0.04C2¼0.30N3¼0.66

C1¼1.19C2¼�0.14N3¼0.07

72.3, 240.0, 389.8, 586.7, 794.3

B3LYP R1,2¼1.481R2,3¼1.557A3,2,1¼111.2D8,9,10,3¼37.6

mx¼3.5my¼�0.7mz¼0.0mt¼3.6

C1¼0.05C2¼0.22N3¼0.73

C1¼1.01C2¼�0.05N3¼0.08

72.7, 221.9, 382.0, 625.9, 772.2

S. Sitha et al. / Tetrahedron 70 (2014) 7906e79117908

3.3. Transition state

Our search for a transition state in the PES of the cati-oneneutral hydroamination reaction resulted in similar geome-tries in both MP2 and B3LYP methods. Frequency analysis showsthat there is only one imaginary frequency (�1771.5 cm�1 and�1728.4 cm�1 in the MP2 and B3LYP methods) and so it can beconsidered as a transition state. Note that there was very littlespin contamination found during optimization of the transitionstate (the hS2i values are 0.756688 and 0.791504 for B3LYP andMP2 methods, respectively). Analysis of the displacement vectorsshows that the major displacement vector is in the direction to-wards the N-atom, with the H-atom oscillating between the eCH2and eNH2 units. The dipole moment values for this transitionstate are 1.83 D and 1.66 D from the MP2 and B3LYP methods.Analysis of the dipole vector components shows that the majordipole component is in the x-direction. Molecular structure ofthe transition state with the Cartesian axes is shownin Supplementary data, along with its properties and energeticdetails calculated using various other methods (as discussedin Section 2). A more detailed discussion on the structural pa-rameters and other properties can also be found in our earlierreport,16 where we have shown that though it has every charac-teristic to justify the status of a TS from a structural point of view,we concluded that this is not a true TS energetically as it wasfound to be more stable than the reactants.16 As discussed inSection 3.2, we are now able to locate a stable reactant complex inthe PES. Analysing the energies of the reactant complex and theTS, it was observed that the reactant complex is indeed morestable than the TS. In fact it creates a potential well in betweenthe reactants and the TS in the PES, indicating that the cati-oneneutral hydroamination reaction is definitely not barrierless.The detailed PES is discussed in Section 3.5.

3.4. Product

Cationic ethyl amine, [CH3eCH2eNH2]þ, was fully optimizedwith no symmetry restrictions and is in the 2A state. Both MP2 andB3LYP methods resulted in similar geometries for the productmolecule. The optimized geometry of the molecule shows that theNeC and CeC bond lengths have the characteristics of a single bondand the CeCeN bond angle is 113.1� and 115.3�, respectively, in the

MP2 and B3LYP methods. Also, the pyramidality of the eNH2 ni-trogen is completely lost in both of the methods. Note that therewas very little spin contamination found during optimization of thecomplex (the hS2i values are 0.754733 and 0.764403 for B3LYP andMP2 methods, respectively). The molecule has dipole momentvalues of 3.82 D and 3.15 D, respectively, in the MP2 and B3LYPmethods, and the major component is along the x-axis. Molecularstructure of the product with the Cartesian axes is shownin Supplementary data, along with the properties and energeticdetails calculated using various other methods (as discussedin Section 2). Also, a more detailed discussion of the structure andvarious other properties of this cationic ethyl amine can be found inour earlier report.16 As discussed by Hamann et al. once cationicethyl amine is formed; it then undergoes an electron re-combination process followed by structural relaxation to giveneutral ethyl amine as the final product.15

3.5. Potential energy surface

In calculating the PESs for the cationeneutral reactions we havefound an ionemolecule complex that is a local minimum betweenthe reactants and the TS. The ionemolecule complex is like a re-actant complex, indicating that the reactants first form a stablecomplex, and then from there pass through the TS and finally endin the product. It becomes clear that the barrier reported in ourearlier work is not only structurally a true barrier but also ener-getically, once the presence of the reactant complex comes into thepicture (Figs. 1 and 2). The detailed PESs of the reaction betweenthe cation of ethylene and neutral ammonia ½CH2]CHþ

2 þ NH3�and the reaction between the cation of ammonia and neutralethylene ½NHþ

3 þ CH2]CH2� are explored using MP2 and B3LYPmethodologies and discussed in Sections 3.5.1 and 3.5.2, re-spectively. We have also carried out calculations using the 6-31þþG(3df,2pd), aug-cc-PVDZ and aug-cc-pVTZ basis sets to ac-count for the effect of other higher level basis sets on the ener-getics of the PES. Energetics of all the stationary points for all theseabove mentioned methods are provided in Supplementary data.We have also carried out calculations using HF/6-31þþG(2df,2p)method and compared with the MP2(Full)/6-31þþG(2df,2p) re-sults to estimate the amount of correlation energy contribution.Results from these calculations are also provided in Supplementarydata.

Fig. 1. Potential energy surface diagram for the cationeneutral hydroamination reaction between CH2]CHþ2 and NH3. The ZPE corrected energy values are represented in kJ/mol for

MP2 and B3LYP methodologies (B3LYP values are given in brackets). The energy levels are not to scale.

Fig. 2. Potential energy surface diagram for the cationeneutral hydroamination reaction between NHþ3 and CH2]CH2. The ZPE corrected energy values are represented in kJ/mol for

MP2 and B3LYP methodologies (B3LYP values are given in brackets). The energy levels are not to scale.

S. Sitha et al. / Tetrahedron 70 (2014) 7906e7911 7909

3.5.1. Potential energy surface for reaction CH2]CHþ2 þ NH3. The

PES of the reaction of CH2]CHþ2 with NH3 is shown in Fig. 1. The

PES was mapped using MP2 and B3LYP methodologies. The B3LYPenergy values are shown in brackets. From Fig. 1, it can be seen thatthe reactant complex is highly stable compared to the reactants;�246.8 kJ/mol in MP2 and �207.6 kJ/mol in B3LYP methods. TheBSSE correction for the MP2 method is 14.1 kJ/mol and B3LYPmethod is 4.5 kJ/mol. The stability of the complex compared to thereactants is �240.2 kJ, �236.2 kJ and �245.1 kJ for MP2(Full)/6-31þþG(3df,2pd), MP2(Full)/aug-cc-pVDZ and MP2(Full)/aug-cc-pVTZ methods, respectively. Similarly, the stability of the complexcompared to the reactants is �204.9 kJ, �210.3 kJ and �203.8 kJ forB3LYP/6-31þþG(3df,2pd), B3LYP/aug-cc-pVDZ and B3LYP/aug-cc-pVTZ methods, respectively. Now, comparing the reactant com-plex with the transition state it can be seen that the transition stateis higher in energy than the reactant complex by þ159.1 kJ/mol inthe MP2 method and þ140.1 kJ/mol in the B3LYP method. For theMP2(Full)/6-31þþG(3df,2pd), MP2(Full)/aug-cc-pVDZ andMP2(Full)/aug-cc-pVTZ methods, the transition state is 162.3 kJ,158.3 kJ and 158.2 kJ higher in energy than the complex, re-spectively. Similarly, for the B3LYP/6-31þþG(3df,2pd), B3LYP/aug-

cc-pVDZ and B3LYP/aug-cc-pVTZ methods, the transition state is140.8 kJ, 138.3 kJ and 141.6 kJ higher in energy than the complex,respectively.

Again comparing the TS with the product, it can be seen that theTS has a higher energy state byþ125.6 kJ/mol andþ129.3 kJ/mol inthe MP2 and B3LYP methods, respectively. For the MP2(Full)/6-31þþG(3df,2pd), MP2(Full)/aug-cc-pVDZ and MP2(Full)/aug-cc-pVTZ methods, the transition state is 124.6 kJ, 123.3 kJ and122.4 kJ higher in energy than the product, respectively. Similarly,for the B3LYP/6-31þþG(3df,2pd), B3LYP/aug-cc-pVDZ and B3LYP/aug-cc-pVTZ methods, the transition state is 128.5 kJ, 126.9 kJand 130.3 kJ higher in energy than the product, respectively. Thisclearly indicates that the cationeneutral hydroamination reactionbetween CH2]CHþ

2 and NH3 is not barrierless.16 Now comparing allthe energetics, it can be seen that the reactant complex is the moststable stationary point in the PES. Comparing the energies betweenthe reactant and product shows that the product is energeticallymore stable than the reactant by �213.3 kJ/mol in the MP2 methodand �196.8 kJ/mol in the B3LYP method. For the MP2(Full)/6-31þþG(3df,2pd), MP2(Full)/aug-cc-pVDZ and MP2(Full)/aug-cc-pVTZ methods, the product is �202.5 kJ, �201.2 kJ and �209.3 kJ

S. Sitha et al. / Tetrahedron 70 (2014) 7906e79117910

lower in energy than the reactants, respectively. Similarly, for theB3LYP/6-31þþG(3df,2pd), B3LYP/aug-cc-pVDZ and B3LYP/aug-cc-pVTZ methods, the product is �192.7 kJ, �198.9 kJ and �192.4lower energy than the product, respectively. This indicates that thecationeneutral hydroamination reaction CH2]CHþ

2 and NH3 ishighly exothermic and thermodynamically feasible. Also, the re-sults related to the dispersion correction to DFT show that thereare substantial improvements in the energies (details providedin Supplementary data).

3.5.2. Potential energy surface for reaction NHþ3 þ CH2]CH2. The

PES of the reaction of NHþ3 with CH2]CH2 is shown in Fig. 2. The

PES was mapped using MP2 and B3LYP methodologies. The B3LYPenergy values are shown in brackets. Comparing the PES in Fig. 2with the previous PES (Fig. 1) it can be seen that they are similar,the only difference is that the energies of the reactants are slightlylower in Fig. 2 than in Fig.1 with respect to the reactant complex. Asfor the Fig. 1, analysis of Fig. 2 shows that the reactant complex ishighly stable compared to the reactants; �218.9 kJ/mol and�200.3 kJ/mol in the MP2 and B3LYP methods, respectively. TheBSSE correction for the MP2 method is 11.6 kJ/mol and B3LYPmethod is 3.5 kJ/mol.

Calculations using the MP2(Full)/6-31þþG(3df,2pd), MP2(Full)/aug-cc-pVDZ and MP2(Full)/aug-cc-pVTZ methods show that thedifference in energy between the reactants and the complex are�217.8 kJ, �209.4 kJ and �215.1 kJ, respectively. Similarly, calcula-tions using the B3LYP/6-31þþG(3df,2pd), B3LYP/aug-cc-pVDZ andB3LYP/aug-cc-pVTZ methods show that the difference in energybetween the reactants and the complex is �199.2 kJ, �203.3 kJ and�194.3 kJ, respectively. As the reactant complex, transition stateand the product are the same for both the surfaces (Figs. 1 and 2),the barrier heights for the forward and reverse reactions are thesame for both of the reaction paths. The same conclusion can bedrawn for Fig. 2 as for Fig. 1, namely that the cationeneutralhydroamination reaction NHþ

3 þ CH2]CH2 is not barrierless.Comparing the energies of the reactants and the product showsthat the product is energetically more stable by �185.4 kJ/mol inthe MP2 and �189.5 kJ/mol in the B3LYP methods. Calculationsusing the MP2(Full)/6-31þþG(3df,2pd), MP2(Full)/aug-cc-pVDZand MP2(Full)/aug-cc-pVTZ methods show that the difference inenergy between the reactants and the product is �180.1 kJ,�174.4 kJ and �179.4 kJ, respectively. Similarly, calculations usingthe B3LYP/6-31þþG(3df,2pd), B3LYP/aug-cc-pVDZ and B3LYP/aug-cc-pVTZ methods show that the difference in energy between thereactants and the products is �187.0 kJ, �191.9 kJ and �182.9 kJ,respectively. Similar to our earlier discussion, we can say that thecationeneutral hydroamination reaction between NHþ

3 and CH2]

CH2 is also highly exothermic and thermodynamically feasible.Also, the results related to the dispersion correction to DFT showthat there are substantial improvements in the energies (detailsprovided in Supplementary data).

3.6. Favourable reaction path

3.6.1. Dipole moments. Analysis of the dipole moment shows thatthe reactant complex has a large dipole moment compared to thereactants (the dipole moment values are 0.0 D for CH2]CH2 and0.0 D for NHþ

3 in both the methods; 1.7 D and 1.6 D in the MP2 andB3LYP methods, respectively, for NH3 and 0.0 D for CH2]CHþ

2 inboth the methods). Now, when the NH3 approaches the CH2]CHþ

2to form the complex, the interaction can be treated as a dipole-induced dipole type of interaction. This can lead to an enhancedresultant dipole moment in the reactant complex compared tothe reactants. On the other hand, when the NHþ

3 approaches theCH2]CH2 the interaction will be completely nondipolar in nature.A large dipole moment value of the reactant complex indicates

a preference for the CH2]CHþ2 þ NH3 reaction over the

NHþ3 þ CH2]CH2 reaction. For understanding of the dipole di-

rections of the reactants and complex, the molecular structureswith the Cartesian axes are shown in Supplementary data.

3.6.2. Spin densities. Now, if we consider the case of the NHþ3

reacting with CH2]CH2 leading to the formation of the complex,then it can be seen that there is a complete spin transfer from theNHþ

3 to the CH2]CH2 fragment of the reactant complex. On theother hand, if we consider the case of CH2]CHþ

2 reacting with NH3,then the spin density of the CH2]CHþ

2 is retained in the CH2]CH2fragment of the reactant complex. A retention of the spin density inthe CH2]CH2 fragment of the reactant complex indicates a prefer-ence for the CH2]CHþ

2 þ NH3 reaction over the NHþ3 þ CH2]CH2

reaction, as the reactions occur on a short time scale during theelectron exposure.15

3.6.3. Structural factors. Now analyzing the NHþ3 and NH3 it can be

seen that the former is fully planar, whereas the latter is pyramidalwith the angle of pyramidality around 38.0�. Analysis of the re-actant complex shows that the pyramidality remains almost thesame as that of NH3. It can also be seen that CH2]CH2 is fullyplanar, whereas CH2]CHþ

2 is slightly distorted from planarity anda similar distortion is maintained for the CH2]CH2 fragment in thereactant complex. Also, Hamann et al. reported that the actual re-action occurs mainly on a short time scale during the electron ex-posure.15 This observation indicates that during the course of thereaction in a short time scale the structure will probably exhibitfewer changes along the reaction co-ordinates. Our results alsoagree with this and indicates a preference for the CH2]CHþ

2 þ NH3reaction, which brings less structural changes to the reactantcomplex over the NHþ

3 þ CH2]CH2 reaction.

3.6.4. Energetics. Now comparing the energies of the reactantswith the product energy, it can be seen that the reactionCH2]CHþ

2 þ NH3 is more exothermic (�213.3 kJ/mol in the MP2and �196.8 kJ/mol in the B3LYP methods) than the reaction NHþ

3 þCH2]CH2 (�185.4 kJ/mol in the MP2 and �189.5 kJ/mol in theB3LYP methods). This gives a clear indication that the reactionbetween CH2]CHþ

2 and NH3 is thermodynamically favourable.Moreover, when the energy of the reactant complex is comparedwith the reactants, for the two methods it can be seen that thereaction of CH2]CHþ

2 with NH3 is more exothermic. Thus theanalysis of the energetics also indicates a preference for theCH2]CHþ

2 þ NH3 reaction over the NHþ3 þ CH2]CH2 reaction. As

both the reactions are highly exothermic, the energetic factor willhave very less contribution to the preference of one reaction overthe other.

All of the above discussion clearly shows that the reaction be-tween the CH2]CHþ

2 and NH3 is preferred over the reaction be-tween NHþ

3 with CH2]CH2. This is in agreement with the results ofHamann et al. who obtained more product in the case where theethylene is ionized than if the ammonia is ionized, indicating thatthe reaction between CH2]CHþ

2 and NH3 is more favourable thanthe reaction of NHþ

3 with CH2]CH2.15

4. Conclusions

In summary, we have carried out a computational study on thereaction paths of the cationeneutral direct hydroamination re-action, using the B3LYP and full-MP2 methodologies various basissets like 6-31þþG(2df,2p), 6-31þþG(3df,2pd), aug-cc-pVDZ andaug-cc-pVTZ. In our earlier report we showed that the TS is a trueTS structurally, but could not confirm that it was a true TS ener-getically.16 In this workwewere able to locate a reactant complex ina potential well in between the reactants and the TS. Thus the TS is

S. Sitha et al. / Tetrahedron 70 (2014) 7906e7911 7911

a true TS both structurally and energetically, and we conclude thatthe cationeneutral hydroamination reaction is definitely not bar-rierless. In fact in the cationeneutral hydroamination reactions,first the reactants form a highly stable reactant complex, whichthen passes through the TS to give the product ethyl amine. Ourcalculations show that molecular dipole, spin density distribution,structural factors and the energetics all favour the reaction betweenCH2]CHþ

2 and NH3 over the reaction between NHþ3 and CH2]CH2,

in agreement with the experimental results for this reaction.15 Thisstudy has an important application for bringing out future studiesto make the direct hydroamination into reality. As we know theionization of one of the reactants (which creates a positive chargeon it) makes this direct hydroamination reaction feasible withouta chemical catalyst. This study will help to explore the effect ofstrong electron withdrawing groups in the ethylene (which willcreate partial positive charges on the C]C backbone, whichmimicsthe cationic ethylene) on lowering the barrier height of the neu-traleneutral hydroamination reaction and making it feasiblewithout a catalyst.

Acknowledgements

Authors like to thank University of Johannesburg, South Africafor providing necessary facilities. This material is based on theworksupported by National Research Foundation, South Africa. Thestatements made and views expressed are, however, solely theresponsibility of the authors.

Supplementary data

Supplementary data associated with this article can be found inthe online version, at http://dx.doi.org/10.1016/j.tet.2014.08.061.

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