Indian Journal of Chemistry Vol. 44A, February 2005, pp. 221-226

A quantum chemical investigation of electrophilic addition reaction of bromine to bicyclo[3.2.2]nona-6,8-diene

Rlza Abbasoglu*, S Sava~kan Yllmaz & Ya~ar G6k

Department of Chemistry, Karadeniz Technical University, 61080 Trabzon-Turkey Email: rabbas@ktu.edu.tr;sevily@ktu.edu.tr

Received 9 April 2004; revised 8 October 2004

Full geometric optimization of bicyclo[3.2.2lnona-6,8-diene (BND) has been done by semiempirical and ab initio methods and the structure of the molecule has also been investigated. The double bond (I) situated in the opposite direction of methylene group in END molecule is more exo pyramidalized than the other double bond (II). The electron density (qj,HOMO) of the double bond(l) in HOMO of the molecule is more than that of the (II) double bond. Exo and endo faces of exo pyramidalized double bonds of the molecule are not equal and electron density is higher in elldo faces. The molecular complexes of BND with bromine have been investigated by AM 1 method and their stable configurations determined. The reason for endo molecular complexes being more stable than exo is that the stability is caused by electronic and steric factors. Because of electronic factors, BND .. . Br2(endo I) complex is more stable than BND . .. Br2 (elldo 2). The endo-bridged bromonium cation(I) is relatively more stable than the elldo-bridged cation(IV). Endo- facial stereoselectiv ity and regioselectivity should be observed in the addition of bromine to BND molecule. Endo-facial stereoselectivity is caused by electronic and steric effects, regioselectivity by electronic effects. The rearranged bromocarbonium cation(V) is the most stable among the cationic intermediates and the ionic addition reaction occurs via this cation.

Electrophilic addition reactions of halogens to olefins have been theoretically and experimetally investigated to a large extent. However, the reaction mechanism, the nature of the structure and the stability of intermediates are still under discussion. In order to see a detailed analysis of the mechanism of such addition reactions, information on stability and structures of the intermediates is required. Since the intermediates have a low stability and a high reactivity, it is difficult to obtain this information experimentally. However, quantum-chemical calculations provide a reliable source for the structure and the stability of the intermediates without the aid of experimental data.

The addition reactions of halogens to ethylene and its derivatives and the reaction intermediates have been quantum chemically investigated l

-18

• In this connection, the theoretical investigations of the addition of bromine and chlorine to bicyclic olefins with rigid structure and stericaliy encumbered alkenes have recently been reported by US

I9-26. In continuation

of our interest in the quantum-chemical studies related to the addition of halogens to rigid and unsaturated bicyclic systems, we wish to report here the results obtained for the investigation of the addition of bromine to bicyc10[3.2.2Jnona-6,8-diene (BND).

Bromination of bicyclo[3 .2.2Jnona-6,8-diene (BND) molecule results in two ordered additional products : 4-exo-9-anti- and 4-endo-9-anti-dibrombicyc10[3 .3.1J non-2-ene, respectively27 It is interesting to investigate the reasons for the stereo- and regio­selectivity properties of this reaction. In order to carry out the detailed analysis of the formation mechanism and stereochemistry of the products in this reaction, a quantum chemical investigation of the structures and the stabilities of the reaction intermediates seem to be very important. In general, the sterochemical regularities of addition reactions of halogens to bicyclic systems are the subjects of detailed investigation. Stereo- and regioselectivity of these reactions depend on the geometry and the electron structure of the double bonds of bicyclic olefin to a large extent. The most important factors that affect the structure and the stability of olefin-halogen molecular complexes are the structure and the properties of olefins.

Calculations In this work, the geometry and the electronic

structure of the bicyclo[3.2.2Jnona-6,8-diene (BND) were calculated by the serniempirical methods

222 INDIAN] CHEM, SEC A, FEBRUARY 2005

MNDO/d2S, AM1 29

, PM3JO and ab initio SCF method in STO-3G*3 1 basis. BND ... Br2 molecular complexes have been studied through the semiempirical AMI method. The cationic intermediates possibly formed during this reaction have been investigated through the MNDO/d, AM I , PM3 and ab initio SCF/STO-3G* methods. All the calculations have been carried out using the HYPERCH EM 6.032 software on an IBM Pentium IV computer.

Results and Discussion Full geometric optimization of BND molecul e was

done by MNDO/d, AMI, PM3 semiempirical and HF/STO-3G* ab initio methods and the structure of the molecule was also investigated. In the light of the results of each method, the pyramidalization parameters3J,34 of molecule were determined with the aim of determining the structural deformation of double bonds.The values of the pyramidalization angle (<l» 3Jof twisting (torsion) angle (<l>D)J4and of out­of-plane bending angle (X)34 were calculated according to the results of each method of the double bonds which are opposite( I) and same(II) direction (Scheme I) with the methylene group in the middle of bridged methylene chain of BND molecule, given in Table I . According to the determined results, the structural deformation of the double bond(I), i.e. the values of pyramidalization parameters, are more than those of the double bond(II). Therefore, the double bond(l) of the BND molecule is more exo pyramidalized than

exo ':,)

endo

Scheme 1

exo LC ~n

)l

(II). Since the double bonds of the BND molecule are exo pyramidalized, elIdo selectivity should be observed in the addition reaction to the molecule. In general, the facial selectivity of attack on a pyramidalized olefin parallels the pyramidali za-. 1S 36 Wh I 'd I' . d f h tIOn' ·' . en t le pyraml a lzatlon egree ate

double bond of olefins increases, thei r chemical reactivites increase37

. Hence, the addition possibility to the BND molecule of bromine from the double bond (I) is hi gher.

The analysis of frontier orbital (HOMO) of BND molecule showed that this orbital is principally local ized in the double bonds. In the case of HOMO, the electron density (qj ,HOMO) in double bond(I) is higher than that of double bond (II) (Table I), as also shown in Fig. 1.

As seen in Fig. 1, exo and elldo faces of exo pyramidalized double bonds of the molecule are not equal. The electron density in endo face of each double bond is high. Therefore, the bromination reaction of the BND molecule should show regio- and st~reoselecti vity property, the addition of bromine should be realized from endo direction which has higher electron density of double bond (I).

To find out the possible way of approach and the determination of the center of attack fo r Br2 to BND molecule, the BND-Br2 sys tem was investigated in detail at the AMI level. It is known that a halogen can ~lttack a C-C double bond through both exo and elldo directions. On the other hand, the halogen can also attack the double bond in axial position (Coo axis of halogen is vertical to plane of double bond) and equatorial position (Coo axis of halogen is parallel to plane of double bond). Thus, the full geometrical opti mization of BND-Br2 system has been performed taking into consideration the fact that the BND molecule involves two double bonds in di fferent positions, and the stable configurations of the system corresponding to the minimum energy level have been determined. It has been found that the BND-Br2 system forms four stable configurations. Two of them,

Table I-The calculated double bond lengths (A), pyrnmidalization parnmeters(c1egrees) and val ues of electron densities (q j, 110 m ) of bicyclo[3 .2.2]nona-6,8-diene

Method Double bond(l) Double bond(II)

rc=c (jl c(lD X q j,HOMO rc=c (jl <PD X q j,HOMO

MNDO/d 1.349 0.0 0.0 0.238 0.458 1.35 1 0.0 0.0 0.168 0.325 AMI 1.339 0.436 0.0 0.531 0.409 1.341 0.308 0.0 0. 182 0.273 PM3 1.336 0.538 0.0 0.616 0.353 1.338 0.439 0.0 0.270 0.3 17 HFISTO-3G* 1.307 1.3 17 0.0 1.52 1 0.518 [,3 10 0.834 0.0 [,047 0.223

ABBASOGLU et al.: A QUANTUM CHEMICAL INVESTIGATION OF ELECTROPHILIC ADDITION REACTION 223

Table 2- The propenies of BND ... Br2 molecular complexes (AM 1) (the pyramidalization parameters are in degree unit)

Molecular Stabilization EquilibriulT!. rBr.Br Transferred complex energy distance. Rc(A) (ft.) charge from </l </lD X

(kJ/mol) BND to Br2. (e)

BND ... Br2(elldo 1) 3.096 3.102 2.187 0.022 1.572 0.0 1.887 BND ... Br2(exo 1) 1.506 3.2 16 2.185 0.011 0.533 0.0 0.563 BND ... Br2(elldo 2) 3.054 3.048 2.186 0.022 1.310 0.0 1.568 BND ... Bro(exo 2) 0.71 2 4.032 2.183 0.002 0.206 0.0 0.028

~ ..

II III

v

Scheme 2

BND .. . Br2(endol) (Fig. 2) and BND .. . Br2(exol), are formed by the attack of bromine on the double bond(l) (Scheme 1) opposite ~o trimethylene bridge through endo and exo directions in axial position. The other two configurations are BND ... Br2(endo2) (Fig. 2) and BND ... Br2(exo2) formed by the attack of bromine molecule on the double bond at the same direction as trimethylene bridge through endo and exo directions in axial position. The stabilization energy of BND ... Br2(endo 1) molecular complex has been calculated as 3.096 kllmo!. The equilibrium distance (Re) between bromine atom and C-C double bond plane, has been found to be 3.012 A. The stabilization energy and Rc of BND ... Br2(endo2) complex have been calculated as 3.054 kllmo! and 3.048 A, respectively. The properties of molecular complexes are compiled in Table 2.

Although stability energies of the two complexes are indistinguishable, a selectivity in fewer of BND .. . Br2(endo 1) has been observed. In comparison

with . the endo complexes, the exo complexes are less stable (Table 2) , Due to the steric hindrance of tri­methylene bridges of the BND molecule, the exo attack of bromine molecule on both double bonds is difficult. Thus, the exo complexes have become unstable, Since the steric hindrance forming of the bridge methylene chain in double bond (II) direction is higher, BND .. Br2(exo 2) complex is more unstable (Table 2) . On the other hand, as we pointed out, the electron densi ty (qi ,HOMO) in endo face of exo pyrarnidalized double bonds is higher (Fig. 1). That is , HOMOBND-LUMObrom interaction realized from endo face of the double bond in the formation of endo molecular complexes is more effective than that of exo face and should be optimal. According to the frontier molecular orbital theory, HOMOoleV LUMOhalogen interaction is the decisive factor in the formation of olefin-halogen complex38

, So, because of electronic and steric factors, endo molecular complexes are more stable than exo molecular

224 INDIAN J CHEM, SEC A, FEBRUARY 2005

2D contours 3D isosurface

Fig. I- Electron density distribulion(HOMO) of the bicyclo[3 .2.2]nona-6,8-diene molecule.

,endo 1 endo2

Fig. 2-The optimized geometries of BND ... Br2(elldo I) and BND ... Br2(ell do2) molec ular eompJexes(AM I).

co mplexes. Double bond(!) of the BND molecul e is much more exo pyramidaJized than doubl e bond (lI) and the electron density ( qj ,j'IOMO) in endo face of bond (J) is bigger than that of bond (II) (Table I and Fig.I). That is , HOMO[3ND-LUMObrmll interaction is more effective in th e forming of BND .. . Br2(endol) complex and it causes the increase in the stability of the complex.

The electronic factor causes BND ... Br2(endol) complex to be more stable than BND ... Br2(endo2) complex (Table 2). On the other hand, the formation of olefin-halogen molecular complex is realized with

the pyramidalizatioll of the double bond25. The

stability of co mplex increases on increaseing the pyramidalization of olefin double bond. The calculations using AMI method showed that the values of the pyramidali zation parameters (<p, <Po, X) of the double bond in BND .. . Br2(endo l ) complex are higher than that of BND .. . Br2(endo2) complex (Table 2). So, endo-facial stereoselectivi ty is caused by e lectronic and streric effect, regioselectivity is also caused by e lec tronic effec ts .

As seen from Table 2, the stabilities of BND ... Br:i(endo l ) and BND ... Br2Cendo 2) molecular complexes are close to each other, and so, both of the complexes may be formed in the first step of the addition of bromine to BND. However, BND ... Br2(endo 1) complex is observed to be favored. The bromine molecule has been partially polari zed in endo complexes, the bromine atom close to the double bond has a partial positive charge and the other one has a parti al negati ve charge . .It has been also found that the distance between bromine atoms in BND ... Br2(endo I) and BND ... Br2(endo2) is longer than in neutral bromine molecule. These results show that molecular complexes have an important role on heterolytic cleavage of bromine in ionic addition reactions.

The bridged bromonium cations, which might be formed by the heterolytic cleavage of bromine during the addition reaction have been investigated through the semiempirical and ab initio methods. The structures and relative stability of these bridged

ABBASOGLU et al.: A QUANTUM CHEMICAL INVESTIGATION OF ELECTROPHILIC ADDITION REACTION 225

Table 3--The calculated total energies and heats of formation of cations

Cation

I II III IV V VI

EIO,(Hartree)

HF/STO-3G*

-2888.757 -2888.748 -2888 .738 -2888.756 -2888.777 -2888.766

MNDO/d

980.225 1000.743 1005 .782 981.167 891.493 902 .624

bromonium cations and their isomers (Scheme 2) have been determined by canying out geometrical optimization using MNDO/d , AMI, PM3 and HF/STO-3G* methods. The calculated total energies

(EI01 ) and standard heat of formation ( I:1H~ ) values of

the cations are given in Table 3. According to the ab initio and semiempirical

methods, endo bridged bromonium cations(I and IV) are more stable than exa cations(1I and Ill) . These results confirm that Br2 prefers to attack the endo side, rather than exo side. Hence, endo-facial selectivity is preferred in the addition reaction. Nevertheless, it has been found that the stabili ty of bridged endo bromonium cations(J) and (IV) are close to each other and it is possible that both may occur. However, there has been a selectivity in fewer of bridged bromonium cation(I). The most stable cation among the investigated ions using these methods is the rearranged cation (V) which is formed by 1,2-migration of the trimethylene bridge of an endo bridged bromonium cations (I and IV). The conversion to the rearranged cation (V) of endo­bridged bromonium cation(IV) is realized by 1, 2-migration of the trimethylene bridge and by inversion of the methylene group in the middle of thi s bridge. Cation (VI) is computed to be less stable than the cation (V) by 13.982 kllmol at AMI and 27.196 kllmol at SCF/STO-3G*. Interconversion of cations (VI) and (V) is predi cted to be fac ile proceeding through a transition state with a planar trimethylene bridge and an activation barrier (AM I) of only -10.50 kllmo!.

According to the theoretical calculati ons, reananged cation(V) is the most stable cation among the other cations formed during the addi tion reaction and the reaction must be completed via this cation. For this reason, the rearranged products are predicted to be formed in the addition reaction . The cationic center of the ion (V) has a planar structure similar to

/',.H~ (kJ/mol)

AMI PM3

888.943 956.337 910.564 976.556 925 .79 1 994.552 890 .136 956.646 806.7 11 857 .032 820.693 871.767

the allyl cation. Bromide an;on (BO attacks the cationic center through enda and exa directions to form two reananged addition products. These results are in agreement with the experimental resul ts27.

Thus, theoretical study shows that the ionic addition of the bromine molecule to BND follows these steps: formation of the endo-molecular complexes and decomposition of these complexes to bridged endo­bromonium ions; reanangement of bromonium ions to bromocarbonium(V) cation; and nucleophilic attacks of bromide ion (BO to cationic center through exo and endo directions of this cation, respectively.

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