Indian Journal of ChemistryVol. 4,1 B, August 2005, pp. 1708-1712

Ab initio investigation of electrophilic addition of chlorine to norbornadiene

Rza AbbasogluDepartment of Chemistry, Karadcniz Technical University, 61080 - Trabzon, Turkey

Email: rabbas@ktu.edu.tr

Received f May 2004; accepted (revised) 8 February 2005

Full geometry optimization of norbornadiene (Nil) has been done by ab initio HF/3-21 G*, HF/3-21 G**, HF/6-31 G*,HF/6-311 G* and HF/6-311 G* methods and the structure of the molecule is investigated. The double bonds of molecule areendo pyramidalized. The electron densities (q. IIOMO) on exo and endo faces of the double bonds arc not equal and it isbigger on the exo face. The NB-CI2 system is investigated by HF/3-2IG* method and the stable configurations aredetermined. The stable configurations of the NB-CI2 system correspond to NB ... CI2 (exo) and NB ... CI2 (endo) molecularcomplexes that are formed by the exo and endo orientation of Clj molccule to the double bond of N13, respectively. The exo-molecular complex has been found to be relatively more stable than the endo-complex. The cationic intermediates of thereaction have been studied by ab initio methods. The exo-chloronium cation is found to be more stable than endo-chloronium cation. The exo-facial selectivity should be observed in the addition reaction to N13 of chlorine. The resultsobtained indicate that a multicenter nonclassical chlorocarbonium cation formed by the rearrangement of exo-chloroniumcation is the most stable one among the cationic intermediates. It is likely that the ionic addition reaction proceeds via thenonclassical chlorocarbonium ion and results in the formation of the rearranged products. The mechanism of the additionreaction is also discussed.

Keywords: Electrophilic addition, norbornadiene, chlorocarbonium

IPC: Int.CI.7 C 07 C

The reactions between halogens and olefins have beenextensively studied both theoretically and experi-mentally over a long period of time. Factors such asreaction medium, solvent, temperature, structure of theolefin, polarity of the electrophile are the factors thataffect the kinetics and stereochemistry of thesereactions considerably. Recent studies have shown thatan important role is played by the halogen-olefinmolecular complexes, the existence of which has beenproved'". In order that the role of the complexes inthese reactions may be correctly understood, structuralinformation is required that is difficult to obtain byexperimental means because of the fast reactionsoccurring in the systems. Quantum chemical methodscan furnish data on the structures and stabilities of thecomplexes without recourse to experimentalmeasurements. Such calculations are also important tounderstand the stabilities and structures of the cationicintermediates that are formed through the reactions,Since these reactions proceed in the pscudostationaryconditions, it is difficult to obtain information about thereaction intermediates by experimental routes.Quantum-chemical calculations provide a reliablesource of information about the intermediates,

We have been interested for some time in theregiochemistry and stereochemistry of the additionsof halogens to unsaturated strained molecules7-J6. Incontinuation of our interest in the quantum-chemicalstudies related to the addition of halogens to rigid andunsaturated strained molecules, we wish to reportherein the results obtained from the investigation ofaddition of chlorine to norbornodiene (NB). Thiswork describes a theoretical investigation of theNB ... CIl molecular complexes and cationic inter-mediates. Stereoselectivity of this reaction depends onthe electron distribution of the double bonds to a largeextent. For a detailed analysis of the formationmechanism and the stereochemistry of the product, aquantum-chemical investigation of the structures andthe stabilities of the reaction intermediates areimportant.

Methods of calculationThe geometry and the electronic structure of NB

were investigated by ab initio SCF method in 3-21 G*,3-21 G**17, 6-31 G*J8, 6-311 G* and 6-311 G**J9 basis.The theoretical investigation of NB ... CIl molecularcomplexes was performed by ab initio HF/3-21G*

J

ABBASOGLU ELECTROPHILIC ADDITION OF CHLORINE TO NORBORNADIENE

method. The cationic intermediates predicted in thereaction were searched by ab initio SCF method in 3-2IG*, 3-2IG**, 6-3IG* and 6-31IG* basis. Fullgeometry optimization was carried out employing thePolak-Ribiere (conjugate gradient) algorithm/"(convergence of 0.00001 kcal mol") and RMSgradient at 0.0001 kcal/(A mol). All the calculationswere performed on an IBM PC PENTIUM IV usingl-IyperChem 6 program.

Results and Discussion

Full geometry optimization of NB was done byI-IF/3-2IG*, HF/3-2IG**, HF/6-3IG*, HF/6-31IG*and H F/6-3 II G* * methods and the structure of themolecule was investigated in detail. The pyrarnidali-zation parametersi':" were determined with the aim ofdetermining the structural deformation of the doublebond. The calculated double bond pyrarnidalizationangle (~)21 and the out-of-plane bending angle (X)22are given in Table I. Accordingly, the double bondsare endo pyramidalized and the two faces of thedouble bond are no longer equivalent. The electrondensity in exo direction is bigger than in endodirection. This geometrical feature causes a noticeablert-facial stereoselectivity in addition reactions of thedouble bond ". Thus, the addition reaction of chlorineto NB should show exo-selectivity. In general, thefacial selectivity of attack on a pyramidalized olefinparallels the pyramidalizatiorrf". When the degree ofpyramidalization of the double bond increases, thechemical reactivity also increases".

The analysis of frontier orbital (HOMO) of NBshowed that this orbital is principally localized in thedouble bonds. As seen in Figure 1, exo and endofaces of endo-pyramidalized double bonds of themolecule are not equal. The electron density on theexo-face of double bonds is higher than the electrondensity on the endo face. Therefore, the chlorinationreaction of N B should be stereoselective and theaddition must be realized from the exo direction.

1709

2D contour 3 D isosurface

Figure 1- Electron density distribution (HOMO) of thenorbornadiene molecue (I-I F/6-31 G*).

To find out the possible direction of approach andto determine the center of attack by Ch, the NB-CI2system was investigated in detail at the HF/3-2l G*level. As known, olefin-halogen molecular complex isformed in the first step of electrophilic addition ofhalogens to olefins. According to the thermodynamicstability of the moleculer complexes, it is possible todetermine from what direction the halogen attacks thedouble bond. The attack of bromine to the doublebond of NB is possible either from exo or endo side.Moreover, a chlorine molecule may approach thecarbon-carbon double bond in a manner that the enaxis of chlorine is either perpendicular or parallel tothe double bond plane. Taking these possibilities intoconsideration, full geometry optimization of variousconfigurations of NB-CI2 system was carried out andthe stable configurations corresponding to theminimum energy levels were searched. Twoconfigurations were found. One configurationcorresponds to exo NB ... C12 complex and the otherconfiguration corresponds to endo NB ... C12 complex,each having CYCI-CIperpendicular to the olefinic bond(Figure 2).

The stabilization energies, fiE = (ENI3 + Ec12) -ENB..CI2,of the exo and endo molecular complexes, thedistance between the carbon-carbon double bond andthe nearer chlorine atom (called as equilibriumdistance R) for each complex and the other properties

Table I - The calculated total energies (Hartree), energies of t'rontier molecular orbitals(e V), double bond lengths (A) and pyramidalization parameters (degrees) of N B

Method Etot tllOMO tLUMO rc=c <l> X

HF/3-21 G* -268.157 -8.723 4.287 1.320 2.111 2.273

HF/3-21 G** -268.201 -8.707 4.281 1.320 2.059 2.254

HF/6-3IG* -268.647 -8.603 4.249 1.319 2.586 2.753

HF/6-31IG* -268.695 -8.723 3.877 1.318 2.794 3.010

HF/6-311 G*.* -268.717 -8.734 3.852 1.318 2.801 3.030

1710 INDIAN J. CHEM., SEC B, AUGUST 2005

Exo

Figure 11-The optimized geometries of the NB ... C12 (exo) and NB ... C12 tendo) molecular complexes (HF/3-2 IG*).

endo

of the complexes were calculated and are given inTable II. The exo complex is 1.62 k.l/rnol more stablethan the endo complex. The results indicate that anexo selectivity must be considered in the electrophilicaddition of chlorine to norbornadiene. The n-facialselectivity observed in the [2.2.1] systems parallelsthe double bond pyrarnidalization. The electrondensity (q. HOMO) on the exo face of the endo-pyramidalized double bond of NB is higher than theelectron density on the endo face (Figure 1).

That is, HOMONB-LUMOchlorine interaction realizedfrom the exo face of the double bond in the formationof the exo molecular complex is more effective thanthe endo face and should be optimal. According to theFrontier Molecular Orbital theory, HOMOolI-LUMOhalogcn interaction is the decisive factor in theformation of olefin-halogen complex.". The formationof olefin-halogen molecular complex is realized withthe pyramidalization of the double bond6,'3. Thestability of complex increases by increasing thepyrarnidalization or the olefinic bond. HF/3-21 G*calculations showed that the value of the out-of-plane

bending angle (X) of the double bond in exo-NB ...Chcomplex is higher than that in el1do-NB ...Cl- complex(Table II). Hence, facial selectivity in the addition ofchlorine to NB is caused by the electronic effect in theimportant degree.

The chlorine molecule is partly polarized in boththe molecular complexes. The bond length betweenthe chlorine atoms in the molecular complexes isrelatively longer than that of the neutral chlorinemolecule (Table II). The results obtained indicatethat the molecular complexes play an important rolein the heterolytic splitting of chlorine moleculeleading to an ionic addition.

It is known that an olefin-halogen molecularcomplex is more stable in a solvent than in gas phaseand the stabilization energy of the complex becomeshigher as the solvent polarity increases". As a result,in the first step of addition of chlorine to NB, the exo-NB ... Cl2 molecular complex must essentially beformed either in a gas or a solvent medium. Thepolarization of chlorine and subsequently theheterolytic splitting of exo-NB ... Cl2 molecular

Table II - The properties ofNB ... Cl2 (exo) and NB ... C12 (endo) complexes (HF/3-21 G*)

Molecular Stabilization Equilibrium rCI-CI Transferred change fror Xcomplex energy distance, (A) NB to C12, (degree)

(kcal mol') R(A) E

exo 2.378 3.164 2.017 0.032 3.718endo 1.991 3.281 2.014 0.031 0.953

ABBASOGLU: ELECTROPHILIC ADDITION OF CHLORINE TO NORBQRNADIENE

f.

complex results in the formation of a cation. Thiscation and its isomers are the possible intermediatesof the addition reactions of chlorine to NB in gasphase and solvent medium (Scheme I). In order todetermine the structures and the relative stabilities ofthe predicted cationic intermediates, their fullgeometry optimization was performed using ab initioHF/3-21G*, I-IF/3-21G**, HF/6-31G* and I-IF/6-311 G* methods and the total energies (Etot) werecalculated (Table III).

The exo-chloronium cation I is relatively morestable than the endo-chloronium cation II. In otherwords, exo-bridged cation formed by the splitting ofthe exo-molecular complex is more stable than theendo-bridged cation. This reveals that an absoluteexo-selectivity must be considered in the mechanismof electrophilic addition of Cl2 to NB. As shown inTable III, the most stable cationic intermediate is therearranged nonclassical chlorocarbonium cation IVamong the possible cationic intermediates I-V. Theseresults are consistent with those calculated for the2-norbomyl, 2-exo-bromo-3-benzonorbomenyl and6-tricyclo[3.2.1.02•4]octyl cations using variousquantum chemical methods 10. 28-35. exo-Chloronium

./~~CI

!0JY~

CIH III

~CI J;CI6, 4 s /'@ 4, tI'@ ~ I""" .----sG---- 3 --- 6 '

IVa IVb

(CI

(l)v

~CI

IVeScheme I

Cl

5 Cl

1 2Scheme II

1711

Table 111- The calculated total energies of cations

Cations Etot(Hartree)HF/3-21 G* HF/3-21 G** HF/6-31 G* HF/6-311 G*

I

"IIIIVV

-725.191

-725.181

-725.209

-725.218

-725.216

-725.235

-725.224

-725.252

-725.262

-725.259

-728.857

-728.847

-728.880

-728.900

-728.887

-728.915

-728.906

-728.940

-72K96 I

-728.948

3

Cl

cation I is predicted to rearrange in order to form themore stable multicenter cation IV. It is reasonable toconsider that the ionic addition of chlorine to NBproceeds via the formation of the nonclassicalchlorocarbonium ion IV due to its high stability.

The rearranged cation IV possess' two effectivecationic centers as C-4 and C-6. Because of theinteraction between the C-4 cationic center and the n-electron clouds of the double bond, the electrophilicattack of the chloride ion to this center occurs fromthe opposite side of the double bond, and so, 2-exo-7-anti-dichloronorbom-5-ene 1 (Scheme II) is formedas a Wagner-Meerwein rearrangement product. In theionic addition of Ch to NB, the formation of 2-endo-A;rCI 7-anti-dichloronorborn-5-ene 2 is not possible due tothe electronic interaction. On the other hand, the C-6

Z ~\; cationic center of carbocation IV has a nearly planarstructure. For this reason, the attack of a bromide ionto this center is geometrically possible either from exoor endo, so exo, exo-dichloronortricyclane 3 and exo,endo-dichloronortricyclane 4 (Scheme II) are formed.The positive charge density for C-6 center ofchlorocarbonium ion IV is higher than that of C-4center according to each ab initio method. Hence, thenucleophilic attack of chloride ion is predicted tooccur easily and more effectively on C-6 as comparedto C-4. Since the ionic addition proceeds via cationIV, the formation of a trans product must not beexpected.

Consequently, the investigations of chlorineaddition to norbomadiene by ab initio methods give

CI4

IrV1712 INDIAN J. CHEM., SEC B, AUGUST 2005

some important results as mentioned below. Thedouble bonds of NB molecule are endo-pyramidali-zed. The electron density (q. 1I0MO) on exo and endofaces of the double bonds are not equal and it isbigger on the exo face. The exo-NB ... e12 molecularcomplex is more stable than the endo-NB ... e12complex and an essentially exo complex is formedthroughout the reaction. The molecular complex playsan important role in heterolytic splitting of chlorine.The bridged exo-chloroni um cation I is relativelymore stable than the endo-chloronium cation II. exo-Facial selectivity should be observed in the additionof chlorine to NB. Facial selectivity in the additionreaction by chlorine to NB molecule is caused byelectronic effect in the important degree. The moststable cation of cationic intermediates is therearranged nonclassical delocalized chlorocarboniumIV and the ionic addition occurs via this cation.

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