•Indian Journal of ChemistryVol. 458, July 2006, pp. 1722-1728

Theoretical study of facial selectivity and regioselectivity in electrophilic additionreaction of chlorine to exo-tricyc1o[ 4.2.1.02,5]nona-3,7-diene

Rza Abbasoglu" & Sevil Savaskan YilrnazDepartment of Chemistry, Karadeniz Technical University, 6\080 Trabzon, Turkey

Email: rabbas@ktu.edu.tr

Received 24 May 2005; accepted (revised) 9 December 2005

For prediction and interpretation of facial selectivity and regioselectivity of the electrophilic addition reaction to exo-tricyclo[4.2.1.02•5]nona-3,7-diene(exoTND) of chlorine, various models(geometric distortions, electron density, orbitaleffects, electrostatic effects and intermediates stabilities) have been used. Full geometric optimization of exoTND moleculehas been done by ab initio and DFT methods and the structure of the molecule have also been investigated. Norbornenedouble bond(l) of molecule is endo pyramidalized and cyclobuten double bond(II) is also syn pyramidalized. The doublebond(l) is more pyramidalized than the double bond(II I) and it has higher reactivity. Exo face of the double bond(l) and antiface of the double bond(II) of the molecule are regions having much more electron density(qi.1I0MO) and bigger negativepotential. The exoTND-CI2 system has been investigated by 83L YP/6-311 +0* method and their stable configurations havebeen determined. The exoTND ... Cliexo) and exoTND ... CI2(anti) molecular complexes correspond to the most stableconfigurations of exoTND-CI2 system. The most stable cation of bridged cationic intermediates is the exo-bridgedchloronium cation. The results that are obtained by using the models for predicted and interpredicted facial selectivity andregioselectivity of the electrophilic addition reaction of chlorine to exoTND molecule agree with each other. Facialselectivity and regioselectivity of the addition reaction parallel the pyramidalization of the double bond.

Keywords: Ab initio and DFT calculations, facial selectivity and regioselectivity, exo-tricyclo[4.2.1.02•5]nona-3,7-diene,molecular electrostatic potential, pyramidalization

IPe: Int.Cl" C07D

Interest has been generated for quite some time in thestereo- and regiochemistry of addition reactions ofhalogens to strained 0lefinsl.6• In general, thestereochemical aspects" of addition of halogens tostrained olefins are the subject of a detailedinvestigation. Stereo- and regioselectivity of thesereactions depend on the geometry and the electronicstructure of the double bonds of strained olefin to alarge extent. Strained olefins of low symmetry have atendency to adopt a pyramidal geometry'. A largenumber of strained olefins that prefer non-planarstructures have been the subject of extensivetheoretical and experimental studies due to the factthat double bond pyramidalization plays an importantrole on the n-facial selectivity and regioselectivity inaddition reactions. As a consequence of the doublebond pyramidalization, the two faces of the doublebond are no longer equivalent. This extraordinarygeometrical feature causes the very noticable n-facialselectivity and regioselectivity in addition reactions tocarbon double bonds". The degree ofpyramidalizationis influenced by the electron density of the alkenyl n-

bond". In general, the facial selectivity of attack on apyramidalized olefin parallels the pyramidaliza-tion 10,11. When the pyramidalization degree of thedouble bond of olefins increases, their chemicalreactivities also increase'', Therefore, the investigationof the pyramidalization of the double bonds ofstrained unsaturated molecules is important. ThecaJculation of the pyramidalization of the doublebonds of strained unsaturated molecules helps todetermine the facial selectivity and regioselectiviteproperties of the addition reaction of halogens toolefins.

According to the frontier molecular orbital theory,the electrophile attacks the double bond which hashigher electron density(q;,lIoMo) in HOMO orbital ofunsaturated molecule'". Therefore, the analysis offrontier molecular orbitals of strained unsaturatedmolecules containing double bond with variousconfigurations is important.

One of the most accurate methods in thedetermination of the center and the approach of theelectrophilic attack of halogens on strained olefins is

ABBASOGLU et a/: ADDITION OF CHLORINE TO EXO-TRICYCLO[4.2.1.02•5jNONA-3,7-DIENE 1723

the molecular electrostatic potential (MESP)calculations. The MESP surfaces show considerabletopographical variation, with many minima, saddlepoints, and maxima. Every n-bond of an olefin has alocal minimum of electrostatic potential on eitherface. Because the regions with large negativepotentials should determine the initial approach of anelectrophile, the relative depths of the two minima canbe used to predict the preferred facial selectivity.Alternatively, integrated volumes of a certain negativepotential can be obtained for the two faces.Electrophilic attack is predicted to be more probableon the face with larger integrated volume. Theapproaches have been used effectively in a number ofsystems, qualitatively as well as rigorously'<l".

As known, olefin halogen molecular complex isformed in the first step of electrophilic addition toolefins of halogens 16.21. According to thethermodynamics of molecular complexes, it ispossible to determine the center and side of theelectrophilic attack against of halogen to olefin. Thestability of olefin-halogen molecular complexes arevery important in the determination of facialselectivity and regioselectivity of the addition reactionof halogens to olefins. Hence, to decide the possibleapproach and the determination of the center of attackof halogens to olefins, it is important to investigatethe olefin-halogen system and the determination ofthe stable configurations.

In the present work, various models (geometricdistortions, electron density, orbital effects,electrostatic effects and intermediates stabilities) havebeen used for prediction and interpretation of facialselectivity and regioselectivity of the electrophilicaddition reaction to exo-tricyclo[ 4.2.1.02.5]nona-3, 7-diene(exoTND) of chlorine. The ge~metry andelectronic structure of the exoTND molecule havebeen investigated in detail by ab initio methods. Thepyramidalization parameters22,23 of the double bondswith the different configurations of the molecule havebeen calculated. The analysis of frontier molecularorbitals has been done and the molecular electrostaticpotential (MESP) of the molecule has been calculated.ExoTND ...C12 molecular complexes have been studiedusing ab initio method and their stable configurationshave also been determined. The electronic and stericfactors that affect the structure and the stability ofmolecular complexes have been studied. The structureand the stability of cations formed by heterolyticsplitting of the molecular complexes have also beenexamined by ab initio methods.

Methodology

The exo-tricyclo[4.2.1.02,5]nona-3,7-diene(exoTND)molecule was investigated by ab initio SCF method in6-310*24 and 6_3110*25 basis. The calculations on themolecule have been also performed by using thedensity functional theory (DFT) method at theB3L YP/6-31 0*26,27 and B3PW9116-310*28 levels.The theoretical investigation of exoTND ...CIzmolecular complexes has been performed by using theB3L YP/6-311 +0*29 method. The predicted cationicintermediates formed by heterolytic splitting of themolecular complexes have also been' investigatedthrough the HF/6-310*, HF/6-3110* and 83LYP/6-3110* methods. The energy of electron correlationhas been calculated by using Moller-Plesset second-order perturbation theory". Full geometryoptimization was carried out employing the Polak-Ribiere (conjugate gradient) algorithm (convergenceof 0.00001 kcal mol') and an RMS gradient at 0.001kcal/(A mol). The calculations have been perfomedwith HyperChem 7.5 and Oaussian98 program withan IBM PC Pentium IV computer.

Results and Discussion

Full geometric optimization of exoTND moleculewas done by HF/6-3IG*, HF/6-3110*, B3LYP/6-310* and B3PW9116-31 G* methods and the structureof the molecule was also investigated in detail. In thelight of the results from each method, thepyramidalization parameters22.23 of molecule weredetermined with the aim of determining the structuraldeformation of double bonds. The values of thepyramidalization angle(~)22 and out-of-plane bendingangle(x)23 were calculated according to the resultsfrom each method of norbornene( I) (the double bondof norbornene fragment) and cyclobuten(II) (thedouble bond of cyclobuten fragment) double bonds(Scheme I) and are given in Table I. According toobtained results, norbornen double bond(l) is endopyramidalized and cyclobuten double bond(II) is alsosyn pyramidalized. Also, norbornene double bond(l)had greater structural deformation as compared tocyclobuten double bond(l I). That is, the doublebond(I) of exoTND molecule is more pyrarnidalizedthan the double bond(II). Therefore, norbornenedouble bond(l) in exo TND molecule has higherreactivity than that of cyclobuten double bond(II).Hence, the possibility of addition of halogens toexoTND molecule at the double bond(l) is higher.That is, the addition of halogens to exoTND molecule

Scheme I

1724 INDIAN J. CHEM., SEC B, JULY 2006

Table I - The calculated double bond lengths(A), pyramidalization parameters (degrees) and values ofelectron densities (q., HOMO) of exo-tricyclo[4.2.1.02,5]nona-3,7-diene

Method norbornene double bond(l) cyclobuten double bond(I!)rc=c <jJ X qi,HOMO rc=c <jJ X qi,1I0MO

HF/6-3IG* 1.323 4.825 5.132 0.456 1.324 0.R76 0.896 0.394

HF/6-31IG* 1.322 5.128 5.438 0.461 1.323 0.897 0.925 0.399

B3L YP/6-31 G* 1.343 5.600 5.994 0.395 1.346 1.774 1.837 0.370

B3PW91/6-3IG* 1.344 5.473 5.859 0.391 1.347 1.554 1.591 0.361

is realized on the norbornene double bond'preferentially. So, the regioselectivity in theelectrophilic addition reactions of halogens toexoTND molecule should be evident. Since thedouble bond(I) of exo-tricyclo[4.2.1.02,5]nona-3,7-diene molecule is endo pyramidalized, exo selectivityshould be observed in the addition reaction of halogento the double bond (I) of the molecule. Since thedouble bond(II) of exoTND molecule is synpyramidalized, anti selectivity should be observed inthe addition reactions of halogens to this double bond.

The analysis offrontier orbital(HOMO) of exoTNDmolecule showed that this orbital is principallylocalized in the double bonds. In the case of HOMO,the electron density(qi,lIoMo) in the double bondfl) ishigher than that of the double bond(I!) (Table I), asshown in Figure 1.

As seen in Figure 1, faces of endo and synpyramidalized double bonds of the molecule are notequal. The electron density in exo face of the doublebond (I) and anti face of the double bond/H) is more.Therefore, facial and regioselectivity in thechlorination reaction of the exoTND molecule shouldbe observed, the addition of chlorine should bereal ized from exo direction having higher electron

density at the double bond(l) which is highlypyramidalized. Also, the electrophilic addition to thedouble bond(II) of the molecule should occur fromanti direction which has more electron density.

In order to determine the possible centers and sitesof attack of chlorine to exoTND molecule, themolecular electrostatic potential (MESP) (inkcal mol") of the molecule was also calculated bymeans of HF/6-311 G*. The electrostatic potentialcontour maps (Figure 2) of the molecule reveal thatthe electrophilic attack of chlorine predominantlyoccurs on the exo face of the double bond(I). At thesame time, the probability of chlorine attack towardsthe anti direction of the double bond(1 I) is also high.

For determination of the exact centers of attack forCh to exoTND molecule, the exoTND-Ch system wasstudied in detail at the B3L YP/6-311 +G* level.Hence, the full geometry optimization of the variousconfigurations of exoTND-Ch system has beenperformed after taking into consideration the fact thatthe exoTND molecule involves two double bonds indifferent positions, and the stable configurations ofthe system corresponding to the minimum energylevel have been determined. It has been found that theexoTND-Ch system forms four stable configurations.

ABBASOGLU et at: ADDITION OF CHLORINE TO EXO-TRICYCLO[4.2. I .02,s]NONA-3,7-DIENE 1725

2D contours

Figure I-Electron density distribution(HOMO) of the exo-tricyclo[4.2. I .02,s]nona-3,7-diene molecule(HF/6-3 I IG*).

3D isosurface

2D contoursFigure 2-Electrostatic potential contour map of exo-tricyclo[ 4.2. I.02.5]nona-3,7-diene(HF/6-3 11G*).

3D isosurface

Two of them, exoTND",Cb(exo) (Figure 3) andexoTND".Cb(endo), are formed by the attack ofchlorine on the double bond(I) (Scheme I) throughexo and endo directions in axial position, The othertwo configurations.' are exoTND, ..Cl2(anti) andexoTND""CI2(syn) (Figure 3) formed by the attack ofchlorine molecule on the double bond(I!) through antiand syn directions in axial position. The stabilizationenergies of the molecular complexes, the equilibriumdistance Rx-ci (X is midpoint of the C=C bondexoTND) and the other calculated properties are givenin Table II. The exoT D".CI2(exo) molecularcomplex correspond to the most stable configurationof exoTND-CI2 system. The other (second) stableconfiguration correspond to .exoTND".Cb(anti)molecular complex. The exo molecular complex ismore stable (0.19 kcal mol") than anti complex. Thestability of the exo complex was found to be 0.57 kcalmol" more than that of the endo complex. The anticomplex is more stable than the syn complex by 0.48kcal mol". Hence, according to the stability of themolecular complexes too, facial selectivity andregioselectivity should be observed in theelectrophilic addition of chlorine to exoTND

molecule. The electrophilic attack of chlorine occurspreferentially from exo face of norbomene doublebond, after the anti face. As pointed out earlier, theelectron density(qi,lIoMo) in the double bond(I) ishigher than that of the double bond(II). Also, theelectron density is greater in the exo face of endopyramidalized double bond(I) and in the anti face ofthe syn pyramidalized double bond(II). According tothe frontier molecular orbital theory, HOMOolf.-LUMOhalogen interaction is the decisive factor in theformation of olefin-halogen complex". That is,HOMOTNO-LUMOchlorinc interaction realized from exoface of the double bond (I) in the formation of exomolecular complex is more effective than that of antiface of the double bond(lI) in the formation of antimolecular complex and should be optimal.

Also, HOMO-LUMO interaction should be muchmore effective in the formation of exo complex thanthat of endo complex and the higher of the stability ofthe complex can be explained by it. Therefore, theelectronic factor causes the exo complex to be morestable than anti and endo complexes. Hence, facialand regioselectivity in the addition reaction ofchlorine to exoTND molecule are a consequence of

1726 INDIAN J. CHEM., SEC B, JULY 2006

exo

anti

endo

Figure 3- The optimized geometries of exoTND

syn

Table 11- The properties of exoTND ...C12 molecular complexes(B3L YP/6-3ll +G*)

Molecular complex Stabilization Equilibrium rCl·CI Transferred changeenergy (kcallmol) distance, Rc (A) (A) from exoTND to

C12

exoTND ...Cl2(exo) 2.14 2.825 2.012 0.054

exoTND ... C12(endo) 1.57 2.982 2.001 0.044

exo'PND ...C12(anti) l.95 2.834 2.004 0.052

exoTND ...C12(syn) l.47 3.072 1.999 0.034

electronic effects to a significant extent. On the otherhand, the steric hindrance of methano bridge in theformation of exo complex is lower than that of ethanobridge in the anti complex. Since the steric hindrancethat is created by the methano bridge in the formationof syn complex is greater, the complex is moreunstable.

The bridged chloronium cations, which are formedby the hetereolytic splitting of the molecularcomplexes were investigated by ab initio methods. Inorder to determine the structure and the relativestability of these cations (Scheme II), the optimizedgeometries were determined by HF/6-31G*, HF/6-311 G* and B3L YP/6-311 G* methods followed by thecalculation of the total energies (Etal). Single pointenergy calculations at the MP2/6-311 G*//HF/6-311 G* level were used to evaluate the electronic

correlation effect on the energies and the order ofstability of cations. The results obtained are given inTable III.

As seen from Table III, exo-bridged cation isrelatively more stable than other types of bridgedcations. Anti-bridged cation is more stable than thesyn- and endo-bridged cations. Therefore, chlorineattacks from exo face of norbornene double bond ofexoTNO molecule preferentially, after the anti face ofcyclobuten double bond. Therefore, according to thestability of bridged cations, facial and regio selectivityshould be observed in the addition of chlorine toexoTNO molecule.

Consequently, the theoretical investigation of facialselectivity and regioselectivity in electrophilicaddition reaction of chlorine to exo-tricyclo-[4.2.1.02,5]nona-3,7 -diene by ab initio and OFT

ABBASOGLU et at: ADDITION OF CHLORINE TO EXO-TRICYCLO[4.2. L02•5jNONA-3,7-DIENE '1727

exo

anti

en do

synScheme II

TableIII-The calculated total energies of cations

Cations ElOt(Hartree)HF/6-3IG* HF/6-31IG* B3LYP/6-31IG* MP2/6-31IG*/ /

HF/6-31IG*

Exo -805.734 -805.821 -808.576 -807.399

Endo -805.724 -805.810 -808.565 -807.389

Anti -805.729 -805.815 -808.569 -807.394

Syn -805.706 -805.792 -808.546 -807.371

methods give some important result, as mentionedbelow. Norbornene double bond(J) of exo-tricyclo-[4.2.1.02•5]nona-3,7-diene is endo pyramidalized, andcyclobuten double bond(l!) is also syn pyramidalized.Norbornene double bond (I) is more pyramidalized ascompared to that of cyclobuten double bond(J I) and italso has higher reactivity. The electron density-(qi.HOMO) of the norbornen double bond(I) in theHOMO of the molecule is greater than that of thecyclobuten double bond(II). The electron density-(qi.1I0MO) in exo face of norbornene double bond(J)which is endo pyramidalized is greater than that ofendo face. Also the value of qi.IIOMO in anti face ofcyclobuten double bond(II) which is synpyramidalized, is greater than that of syn face. Exoface of norbornene double bond(l) and anti face ofcyclobuten double bond(II) of the molecule areregions having much more negative potential. Exo

and anti molecular complexes are more stable thanendo and syn complexes. Among the molecularcomplexes, the most stable is the exo complex. Themost stable of all bridged cationic intermediates is theexo-bridged chloronium cation. Anti-bridgedchloronium cation is relatively more stable ascompared to the syn- and endo-bridged chloroniumcations. The results that are obtained by using themodels for predicted and interpredicted facial andregioselectivity of electrophilic addition reaction ofchlorine to exoTND molecule are in agreement witheach other. Facial and regioselectivity should beobserved in the addition reaction. The electrophilicattack of chlorine should occur from exo face ofnorbornene double bond preferentially, after the antiface of cyclobuten double bond. Facial andregioselectivity of investigated addition reaction isparallel with the pyramidalization of the double bond.

1728 INDIAN J. Cl-IEM., SEC B, JULY 2006

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