Indian Journal of Chemistry Vol. 43A, December 2004, pp. 2497-2502

A quantum chemical investigation of electrophilic addition reaction of bromine to exo-tricyclo[3.2.1.02.4]oct-6-ene

Rtza Abbasoglu* & Sevil Sava~kan Ytlmaz

Karadeniz Technical University, Chemistry Department 61080 Trabzon, Turkey

Email: rabbas@ktu .edu.tr;sevily@ktu.edu.tr

Received 19 January 2004; revised 16 September 2004

Full geometric optimization of exo-tricyclo[3.2.1.02.4]oct-6-ene(exoTCO) has been done by the semiempirical methods and the structure of the molecule investigated. The double bond of molecule is endo-pyramidalized and the two faces of double bond are no longer equivalent. Exo face of the double bond of the molecule has regions having high electron density(qi.HoMo) and bigger negative potential. The exoTCO ... Br2 system has been investigated by AM I method and exoTCO ... Br2(exo) molecular complex has been found to be relatively more stable than the exo ... Br2(endo) complex. The cationic intermediates of the reaction have been studied by semiempirical methods. Exo-bromonium cation is found to be more stable than endo bromonium cation. Exo-facial selectivity has been observed in the addition reaction to exoTCO of bromine which is caused by electronic and steric effects. Exo-classical bromocarbonium cation(lIl) is more stable than rearrangament cation(V) which is formed with Wagner-Meerwein rearrangament. Bromocarbonium cation(III) is the most stable ion among the cationic intermediates and the ionic addition occurs via the formation of this cation. The mechani sm of the addition reaction has also been di scussed.

IPC Code: Int.Cl.7 C07B 37/02

Halogenation of organic compounds is an important step in the preparation of various synthetic intermediates or products. Addition of bromine to the carbon-carbon double bond with molecular bromine is formally one of the simplest reactions typical of unsaturated compounds. The nature of the intermediates of the addition depends on temperature, steric factors, torsional effects, 11:- and <J-participation in the transition state and the formation of non­clasical ions or a fast equilibrium of classical ions 1.2.

The bromination of unsaturated bicyclic systems with molecular bromine leads to reaITangements of the molecular skeleton3

.8

. In order to analyze the reaction mechanism and stereochemistry details, some data about the structures and stabilities of the intermediates such as olefin-halogen molecular complexes and cations formed during the reactions are needed. Since the intermediates possess low stabilities and high reactivities, it is difficult to obtain such data experimentally. Nevertheless, quantum-chemical calculations provide a reliable source of information about the structure and stability of intermediates without the aid of experimental measurements.

We have been interested for some time in the regiochemistry and stereochemistry of the addition reactions of halogens to unsaturated strained molecules5.6.9. 15. We report herein the results obtained

for the investigation of the addition bromine to exo­tricyclo[3.2.1.02.4] oct-6-ene. Reaction of exo­tricyclo[3.2.1.02.4]oct-6-ene(exoTCO) with bromine gave two non-rearranged products; 6-exo-7 -exo­dibromo-exo-tricyclo[3.2 .1.02.4]octane(2), 6-exo-7-endo-dibromo-exo-tricyclo[3.2.1.02.4]octane( I) and three reaITanged products; 5-exo-bromo-3-exo­bromomethyltricyclo [2.2.1.02,6]heptane(3), 6-exo­bromotri cyclo [3,2 ,1 ,02.7] oct -3-ene( 4), 4-exo-6-exo­dibromotricyclo [3,2, I ,02.7] octane(5)16. In order to carry out the detailed analysis of the formation mechanism and stereochemistry of the product of this reaction, a quantum chemical investigation of the structures and stabilities of the reaction intermediates seem to be very important. On the other hand, the formation of 5-exo-bromo-3-exo-bromomethyl­tricyclo[2.2.1.02,6]heptane(3) product from the cation through the Wagner-Meerwein reaITangament or exo classical is still a subject of discussion 16. In general, the stereochemical regularities of addition reactions of halogens to unsaturated strained systems are the subjects of detailed investigation. Stereoselectivity of these reactions depends the electron structure of the double bonds of strained olefins 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.

2498 INDI AN J CHEM, SEC A, DECEMBER 2004

Methodology The geometry and the electronic structure of the

exo-tricyclo[3.2. l.02,4]oct-6-ene(exoTCO) were calculated by the semiempirical methods MNDO/d (ref. 17), AM I (ref. 18) and PM3 (ref. 19). ExoTCO .. . Br2 molecular complexes have been studied through the semi empirical AMI method and their stab le configurations have been determined. The structure and the stabi lity of cations formed by hetereolytic splitting of the molecular complexes and their isomers have also been investigated through the MNDO/d , AM1 and PM3 methods. Full geometry optimization was carried out employing the Polak-Ribiere (conjugate gradient) algorithm (convergence of 0 .00001 kcallmol) and RMS gradient at 0 .0001 kcall(A mol) . All the calcu lations have been carried out using the HYPERCHEM 6.0 software20 on an IBM Pentium IV computer.

Results and Discussion Full geometric optimization of exoTCO molecule

was done by MNDO/d, AM I and PM3 semiempirical methods and the structure of the molecule was also investigated in detail. In the light of the results of each

h d I 'd l' . 2122 f met 0, t le pyraml a 1zatIOn parameters ' 0

molecule were determined with the aim of determining the structural deformation of double bond. The calculated values of the pyramidalization

angle21 (<»), twisting(torsion) angle21 (<»D) and out-of­

plane bending angle2\x) are given in Table l.

According to the determined results, the double bond of exoTCO molecule is endo pyramidalized and the two faces of double bond are no longer equ ivalent. The electron density in exo direction of endo pyramidalized double bond of the molecule must be larger than in endo direction . This extraordinary geometrical feature causes a very noticeable IT-fac ial stereoselectivity in addition reactions to doubl e bond23 . Then, the addition of bromine to exoTCO molecu le in which the double bond is elldo pyramidalized, should show the exo-selectivity. In general, the facial selec ti vity of attack on a pyracnidalized olefin pan-allels the pyraml­dalization24.25

. When the pyramidalization degree of the double bond of olefins increases, their chemi cal reacti vi tes also i ncrease23 .

The analys is of frontier orbital (HOMO) of exoTCO molecule showed that this orbital IS

principally localized in the double bond (Fig. I). As seen in Fig 1 exo and endo faces of endo

pyramidalized double bond of the molecu le are not equal. The electron density in exo face of double bond is high. Therefore, the bromination reaction of the exoTCO molecule should show the stereoselec tivity property and the addition of bromine should be realized from exo direction which is hav ing higher electron densi ty.

One of the most accurate method" in determining the direction of the electrophilic attack of halogen to the double bonds of strained olefins is the molecular

Table ) -The calculated heat of formation energies (kcal/mol), energies of frontier molecular orbita ls (e V), double bond lengths (1\) and pyramidali zation parameters (degrees) of exo-tricyclo[3 .2.1 .02.4]oct-6-ene

Method /)'H o EHOMO ELUMO rc;c <1> <1>D X f

MNOO/d 65.905 -9.638 0.999 1.360 0.21 3 0.0 0.343 AMI 70.890 -9.6 13 1.21 6 1.355 0.792 0.0 0.902 PM3 61.122 -9.794 1.027 1.348 0.923 0.0 1.059

.--~.~}

-"-.

2D Contour 3D isosurface

Fig. I- Electron densi ty di stribution (HOMO) of the exo-tricycloI3,2. 1.02,4Joct-6-ene molecule

ABBASOGLU et at.: SEMIEMPIRICAL CALCULATIONS FOR ADDITION OF BROMINE TO exo-TCO 2499

electrostatic potential (MESP) calculations. The MESP surfaces show considerable topographical variation, with many minima, saddle points, and maxima. Every rr-bond of olefin has a local minimum of electrostatic potential on either face. Because the regions with large negative potentials shou ld direct the initial approach of an electrophile, the relative depths of the two minima can be used to predict the preferred facial selectivity . Alternatively, integrated volumes of a certain negative potential can be obtained for the two faces. Electrophilic attack is predicted to be larger on the face with larger integrated volume. The approaches have been used effectively in a number of systems, qualitatively as well as rigorously26-28 . For understanding from which direction the double bond of exoTCO molecule will be attacked by bromine, the molecular electrostatic potential (MESP) of the molecule was calculated by AM I method (Fig. 2). The electrostatic potential contour maps of the molecule indicate that the electrophi lic attack of bromine predominantly occur on exo face of double bond.

As known, olefin-halogen molecular complex is formed in the first step of electrophilic addition to olefins of halogens29-34

. According to the thermodynamic stability of the molecular complexes, it is possible to determine from what direction the halogen attacks the double bond. Therefore, the stability and the electron structure of exoTCO ... Briexo) and exoTCO .. . Briendo) molecular complexes formed with the addition from exo and endo directions to double bond of exoTCO molecule of bromine are investigated using AM I method. The electrophi lic attack of bromine to double bond of exoTCO molecule is possible either from exo or endo side. Moreover, a bromine molecule may

, \.

\ \

'-..." .... -._-

2D contour

approach the carbon-carbon double bond in either axial (the C1) axis of bromine molecule is perpendicular to the double bond plane) or equatorial (CX) axis of bromine molecule is parallel to the double bond plane) position. By considering these, the full geometric optimization of the various configurations of exoTCO ... Br2 system has been performed and the stable configuration corresponding to the minimu m energy levels have been determined. In thi s connection, two configurations corresponded to the local minima of the exoTCO-Br2 system have been found . These configurations correspond to exoTCO ... Br2(exo) and exoTCO .. . Br2(elldo) molecular complexes which are formed by the exo and endo orientation of Br2 molecule to the double bond of exoTCO in axial position, respectively (Fig. 3) .

The stabilization energies (!1E=(E"xoT("0+EBr2)­

EexoTCO Br2} of the molecular complexes, the heat of

formation (!1H ~ ), the equi librium distance RX.Br (X

is midpoint of the C=C bond of exoTCO) and the other calculated properties are given in Table 2 . The exo complex is 0.759 kJ/mol relatively more stable than the other and has 0 .8 kJ/mol lower heat o f formation than that of the endo complex (Table 2). The results obtained indicate that an exo selectivity must be considered in the electrophilic addition of bromine molecule to exo-tricyclo[3.2 . 1.02.4 Joct-6-ene . The rr-facial selectivity observed in the [2 .2. 1] systems parallels the double bond pyramidalization and also results in the minimization of steric interactions by approaching from the same face as the methano bridge rather than the ethano bridge. On the other hand , as we pointed out, the electron density (qi .HOMO) in exo face of endo pyramidalized double bond of TCO molecule is higher (Fig. I). That IS,

3D isosurface

Fig. 2- Electrostatic potential contour map of exo-tricyc\o[3.2. 1.02•4

] oct-6-enG (AM I)

2500 INDIAN J CHEM, SEC A, DECEMBER 2004

! exo endo

Fig 3-The optimized geometries of the exoTCO . . .. Br2(exo) and exoTCO . .. Br2(endo) molecular complexes (AM I).

Table 2- The properties of TCO .. . Briexo) and TCO ... Br2 (endo) molecular complexes (AM 1) (the pyramidalization parameters are in degree unit)

Molecular complex

Stabilization energy (kllmol)

Heat formation (kllmol)

Equilibrium distance R(A)

Charge transferred from TeO to Br2, e

<1>0 x

Exo

Endo

2.89

2.09

271.57

272.37

3.045

3. 125

HOMOTCO-LUMObrom interaction realized from exo face of the double bond in the formation of exo molecular complex is more effective than that of endo face and is optimal. According to the frontier molecular orbital theory , HOMOolf.LUMOhalogen interaction is the decisive factor in the formation of olefin-halogen complex35

. So, because of electronic and steric factors, exo molecular complex is more stable than endo molecular complex (Table 2).

On the other hand, the formation of olefin-halogen molecular complex is realized with the pyramidalization of the double bond 13.34. The stability of complex increases by increasing the pyramidalization of o lefin double bond. The calculations done using AM 1 method showed that the

values of the pyramidalization parameters (<», <»D, X) of the double bond in TCO ... Br2(exo) complex are higher than those of TCO ... Br2(endo) complex (Table 2). Thus, exo-facial stereoselectivity of electrophilic addition reaction of bromine to exoTCO is caused by electronic and steric effects. It is obvious that exo­selectivity must be taken into consideration in this addition.

Bromine molecule is partly polarized In

exoTCO ... Brz(exo) molecular complex and the bromine atom nearer to the double bond of exoTCO possesses a partial positive charge while the other bromine has a partial negative charge. The bond length between the bromine atoms in the molecular complex is relatively longer than that of the neutral bromine molecLtle. Also, the length of the double

0.022

0.0 16

1.358 2.187 2. 170 0.0 2.429

1.355 2.186 0.457 0.0 0.419

bond of the molecular complex is 0.003,A. longer than that of the exoTCO molecule. The results obtained reveal that exoTCO ... Br2(exo) molecular complex plays an important part in the heterolytic splitting of bromine molecule leading to an ionic addition. The investigation of the molcular complexes have been shown to be very important on the overall reaction coordinate of brominations due to autocatalytic action of bromine.

It is known that an olefin-halogen molecular complex is more stable in a solvent than in a gas phase medium and the stabilisation energy of the complex becomes higher as the solvent polarity increases36

. As a result, in the first step of the addition of bromine to exoTCO, exoTCO . . . Br2 molecular complex mills t be formed either in a gas or a solvent medium. Subsequently, the splitting of exoTCO .. . Br2 molecular complex is predicted to occur. The cations and their isomers shown in Scheme 1 are formed as the possibk cationic intermediates of the reaction. The structures and relative stability of these cations have been determined by carrying out geometrical optimization using MNDO/d , AMI and PM3 methods

and then the standard heat of formation (11H~) have

been also calculated (Table 3) .

According to the semi empirical methods, exo bridged bromonium cation(l) is more stable than endo cation(II). This confirms that Br2 prefers to attack the exo side, rather than endo side, hence exo-facial selectivity in the addition reaction. The results

ABBASOGLU et al. : SEMIEMPIRICAL CALCULATIONS FOR ADDITION OF BROMINE TO exo-TCO 250 1

I II ill

IV v

Scheme 1

Table 3- The calcu lated heats of formation of cations

Cation

II

III

IV

V

MNDO/d

276.44

277.19

274.76

275.11

275.85

Mi~ (kcal/mol)

AMI PM3

263.61 265.80

265.97 266.27

260.40 260.34

261.66 261.55

261.65 260.93

obtained from the three semi empirical methods indicate that the exo bridged bromonium cation (I) is higher in energy than the classical bromocarbonium cation(III). The formation of rearranged cation(V) can be expected by a Wagner-Meerwen rearrangement from cation(III). But, according to the three semiempirical methods, the classical bromocarbonium cation(lII) is more stable than cation(V). In other words, the conversion of the cation(III) to ion(V) is not easy. As shown in Table 3, the most stable cation of cationic intermediates is the classical bromocarbonium(III) and the ionic addition reaction occurs via this cation. According to the theoretical results obtained in the study, a plausible mechanism of the ionic addition of Br2 to exoTCO molecule can be considered as shown in Scheme 2.

It is known that the trans-adducts are formed via bridged-halogenium ions in the addition of halogens to olefins37

. As seen in Scheme 1, the non-rearranged trans-adduct 1 is also formed via the bridged exo­bromonium ion(I), and the bromine atoms of the adduct 1 are in exo and endo configurations, respectively. On the other hand, trans product can also be formed by the endo attack of bromide ion to

exoTCO ... Br2

Scheme 2

cation(III). Furthermore, the non-rearranged cis­adduct 2 (an exo, exo-dibromide) is formed by the exo attack of bromide ion to cation(III). C2C3 and C2C4 cyclopropyl bonds (Scheme 2) become weak and the

2502 INDIAN J CHEM, SEC A, DECEMBER 2004

interaction results in electron cloud of cyclopropane ring of cationic centre in classical bromocarbonium cation(lII). In other words, carbocation(IIJ) can occur with participation of the C2C3 cyclopropyl bond to give rna, when captured by bromide ion giving rearrangement product (3) (Scheme 2). With participation of the internal C2C4, cyclopropyl bond, carbocation(llI) can rearrange to give IIIb that may undergo a proton loss to give product (4). Bromide ion attack to cation(IIIb) results in the rearrangement product (5) (Scheme 2).

To conclude the double bond of exoTCO molecule is endo pyramidalized. The electron densities (qi.HOMO) in exo and endo faces of the double bond are not equal and it is more in endo face. Exo face of the double bond of the molecule has larger negative potential.The exo molecular complex is more stable than the endo complex. The bridged exo-bromonium cationO) is relatively more stable than the endo­bromonium cation(II). Exo-facial selectivity should be observed in the addition reaction to exoTCO molecule of bromine and it is caused by electronic and steric factors. Exo-classical bromocarbonium cation(III) IS

more stable than rearrangement cation(V)

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