Low and high temperature bromination of 2,3-dicarbomethoxy and 2,3-dicyano benzobarrelene: unexpected substituent effect on bromination

Abstract

The bromination of 2,3-dicarbomethoxy benzobarrelene yielded the dibromide regio- and stereospecifically arising from the aryl shift where the bromine exclusively attacks the double bond from the exo face of the double bond. High temperature bromination of this compound yielded aryl-shift products. From the low temperature bromination of 2,3-dicyano benzobarrelene, non-rearranged products were obtained along with the rearranged products. Surprisingly, we observed that the yield of non-rearranged products decreased on increasing the temperature. A possible role of the substituent was discussed using DFT calculations on model compounds.

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Introduction

Electrophilic bromination of simple olefins has been extensively studied within the last two decades. In general, the reaction is stereoselective and leads to trans-1,2-dibromides via a three-membered bromonium intermediate.¹ The bromination of unsaturated bicyclic systems with molecular bromine leads to rearrangements of the molecular skeleton.² For example, bromination of benzobarrelene (1) at 10 °C gives only rearranged products.^2b No trace of non-rearranged products was observed from this reaction. While studying the bromination reaction, it was noticed that the reaction temperature has a dramatic influence on the product distribution. The bromination reaction of benzobarrelene (1) at 77 °C results in the formation of non-rearranged products in 15% yield, whereas increasing temperature to 150 °C allowed formation of non-rearranged products up to 50% yield via radical intermediates.^2a An electronegative bromine atom substituted on the olefin also directs the reaction. For example, bromination of tribromobenzobarrelene (3) gives only non-rearranged products, even at room temperature,³ whereas bromination of 2,3-dibromobenzoberelene (2) requires higher temperatures to obtain non-rearranged products.^2d,e Reaction of 2 with bromine at 77 °C mainly gives non-rearranged products up to 78% yield.^2d,e Similar to related compounds,^2,4 these results show that attachment of an additional bromine atom to the double bond leads to an increase in the amount of the non-rearranged products from 0% (in the case of 1) up to 100% (in the case of 3). The temperature employed to prevent skeletal rearrangement during bromination depends on the substituent condensed to benzobarrelene. In this paper, we report on the bromination reaction of benzobarrelene derivatives (4 and 5) which have substituents which are highly electron-withdrawing via inductive and mesomeric effects (Fig. 1).

Fig. 1 Structures of benzobarrelene derivatives.

Results and discussion

Synthesis of starting materials 4 and 5

Firstly, the starting materials 4 and 5 were prepared according to the published method. The Diels–Alder cycloaddition reaction of naphthalene (6) with dimethyl acetylene dicarboxylate (7) at 160 °C gave 2,3-dicarbomethoxy benzobarrelene (4).⁵ The second target compound, 5,⁶ was synthesized by successive replacement of bromine atoms in 2,3-dibromobenzobarrelene (2) with nitrile groups by reaction of 2 with cuprous cyanide in dimethylformamide (Scheme 1).

Scheme 1 Synthesis of starting materials.

Bromination of 4 and 5 at 25 °C

The electrophilic addition of bromine to 2,3-dicarbomethoxy benzobarrelene (4) was performed in chloroform solution at 25 °C. Contrary to the parent molecule 1 and dibromide derivative 2, which produce complex rearranged product mixtures, the bromination of 2,3-dicarbomethoxy benzobarrelene (4) yielded the dibromide regio- and stereospecifically, arising from the aryl shift where the bromine exclusively attacks the double bond from the exo face of the double bond, in quantitative yield (Scheme 2).

Scheme 2 Bromination of 4 at 25 °C.

The bromination reaction of 2,3-dicyanobenzobarrelene (5) at the same temperature resulted in the formation of the rearranged product 9 and non-rearranged products, trans-dibromide 10, exo-cis-dibromide 11 and endo-cis-dibromide 12, in 70, 17, 9 and 2% yields, respectively (Scheme 3).

Scheme 3 Bromination of 5 at 25 °C.

To rationalize the formation of the isomeric bromides 8–12 formed at 25 °C, we propose the following mechanism by comparing the bromination of 5 with brominations of the parent molecule benzobarrelene (1)^2b and 2,3-dibromobenzobarrelene (2)^2d,e (Scheme 4). Electrophilic bromine can attack the double bond in 1/2/4/5 from both the exo and endo faces of the double bond to form the cyclic bromonium ions 13–16 and 17–20, respectively. The ions 13–20 can be trapped by the bromide anion to give the trans addition products 10/34–36. The bromide anion can attack the carbon atom C1 in 21–24 to form the rearranged dibromides 8/9/25–30 and the carbon atom C2 from the exo side to form exo-cis products 11/31–33. The endo-bromonium ions 17–20 follow a similar course to generate the non-classical carbocations 37–40. The bromide anion can attack the carbon atom C1 in 37–40 to form the rearranged dibromides 41–48, the carbon atom C4 to form the cyclopropanoid dibromides 49–56 and the carbon atom C2 from endo side to form endo-cis products (12/57–59).

Scheme 4 Mechanism for low temperature bromination of benzobarrelene derivatives.

In the bromination of benzobarrelene (1), it was observed that the ratio of products obtained via the exo-bromonium ion 13 to products obtained via the endo-bromonium ion 17 is approximately 4 : 1. This is evidently due to the higher rate of the formation of ion 13, which is stabilized by the π participation of the aromatic ring (ion 21). Such participation is significantly more effective than the participation of the double bond, as in ion 37. In the case of 2,3-dibromobenzobarrelene (2), we found that the ratio of the products formed from the exo ion 14 and the products formed from the endo ion 18 amounts to ∼6 : 1.^2d,e In the present work, we observed that during the bromination of 4, only the exo bromonium ion 15 is formed and the reaction results in the formation of only dibromide 8 in quantitative yield. These differences result from the fact that the intermediate 19 (and ion 39) is less favourable because of the strongly electron-withdrawing ester group at the double bond in molecule 4. Similarly, the bromination reaction of nitrile derivative 5 mainly gives product 9via the exo-bromonium ion 16. No trace of alkyl migration products 45–48 and cyclopropanoid compounds such as 53–56 is observed in these reactions. Unlike benzobarrelene derivatives 1, 2 and 4, the reaction of 5 with bromine at 25 °C results in the formation of non-rearranged products 10–12 in a total yield of 28%. The formation of cis products 11 and 12 is also straightforward. It is likely that these compounds were formed by the opening of the intermediates such as 24 and 40 (Scheme 4).

The structures and relative stabilities of possible cation intermediates of the addition reactions of bromine with 2,3-dicarbomethoxy benzobarrelene (4) at low temperatures have been determined by carrying out geometrical optimizations using DFT/B3LYP (density functional theory with B3LYP—the hybrid Becke’s three parameter functional and Lee–Yang–Parr exchange–correlation potential)⁸ method with the 6-311+G(d,p)⁹ basis set and the total energies (E_tot) have been calculated. Solvent effects were calculated on the basis of the same theory as the optimizations were performed by single-point calculations on the optimized structures using the CPCM (conducting polarized continuum model)¹⁰ method (with UAKS cavities¹¹) in chloroform (ε = 4.90). All calculations were performed with the GAUSSIAN 03 software on an IBM PC Pentium IV computer.

According to the results of the calculations, the most stable cation of cationic intermediates derived from 15 is the non-classical, delocalized bromocarbonium ion 23 (Scheme 4 and Fig. 2). The non-classical bromocarbonium cation has a stability of 7.691 kcal mol⁻¹ (B3LYP/6-311+G(d,p)) and 7.868 kcal mol⁻¹ (CPCM-B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)) and is more stable than the exo-bridged-bromonium cation 15. The skeletal isomerization of the exo-bromonium cation 15 into the non-classical bromocarbonium cation 23 is thermodynamically feasible. In the non-classical bromocarbonium ion 23, the positive charge is partially transferred to the benzene ring and the ion becomes more stable due to the overlapping of the π orbitals of the aromatic ring. The lengths of the C1–C2, C1–C3 and C2–C3 bonds of the non-classical bromocarbonium ion 23 (Fig. 2) optimized by the B3LYP/6-311+G(d,p) method are 1.441, 1.664 and 1.642 Å, respectively. For the non-classical cation 23, the C1–C3 bond tends to break, while the C2–C3 bond tends to form and the C1–C2 bond has a double bond character. In the non-classical ion, both the C1 cation center and carbomethoxy group are located in the same plane (Fig. 2). Interaction between the C1 cation center and the electron cloud of the double bond positively affects the stability of cation 23. The ionic addition of bromine to 2,3-dicarbomethoxy benzobarrelene (4) is predicted to proceed via the non-classical bromocarbonium ion 23. The C1 atom of the non-classical delocalized cation 23 has the maximum positive charge (+0.144 e); in other words, because of the aryl shift, the cationic centre is shifted to the C1 atom. Because of the interaction between the C1 cationic centre and the π-electron clouds of the benzene ring, the nucleophilic attack of the initially formed bromide ion (Br⁻) at the cationic center occurs from the opposite side of the benzene ring, and only one rearrangement product, exo,anti-4,8-dibromo-2,3-dicarbomethoxy-6,7-benzobicyclo[3.2.1]octa-2,6-diene (8), is formed. In the ionic addition of Br₂ to 2,3-dicarbomethoxy benzobarrelene (4), the formation of endo,anti-4,8-dibromo-2,3-dicarbomethoxy-6,7-benzobicyclo[3.2.1]octa-2,6-diene (29) is not possible due to the electronic interaction (Fig. 2, cation 23).

Fig. 2 The optimized geometries of cations (B3LYP/6-311+G(d,p)) (bond lengths are in Å).

The structures and stabilities of cationic intermediates formed in the addition reaction of bromine to 2,3-dicyanobenzobarrelene (5) at low temperature have been investigated by the B3LYP/6-311+G(d,p) method. By using the optimized geometries of cations at the B3LYP/6-311+G(d,p) level, their single-point energies have been computed by the CPCM-B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) method. The results show that the difference between the total energies of the non-classical (24) and exo-bromonium (16) cations is small. The non-classical cation 24 is more stable than the exo-bromonium cation 16. The lengths of the C1–C2, C1–C3 and C2–C3 bonds (Fig. 2) are 1.450, 1.618 and 1.665 Å, respectively, optimized by the B3LYP/6-311+G(d,p) method in non-classical bromocarbonium cation 24. For non-classical cation 24, the C1–C3 bond distance is extended and the C2–C3 distance is shortened. In non-classical cation 24, the double bond electrons and cyano group electrons are conjugated (Fig. 2). The non-classical cation 24 possesses two effective cationic centers, which are C1(+0.110 e) and C2 (+0.090 e). As a result of the attack of the bromide (Br⁻) ion at the C1 center of the non-classical cation 24, the weak C1–C3 bond is broken and the C2–C3 bond is formed; thus, exo,anti-4,8-dibromo-2,3-dicyano-6,7-benzobicyclo[3.2.1]octa-2,6-diene (9) is formed as a Wagner–Meerwein rearrangement product. Furthermore, between the C2 cationic center and the π-electron clouds of the benzene ring, nucleophilic attack of the bromide ion at this center occurs on the opposite side of the benzene ring and, thus, exo-cis product 11 (Scheme 4) is formed. According to the theoretical results, there is a small difference between the stabilities of the exo-bromonium (16) and endo-bromonium (20) cations. The non-rearranged trans-adduct 10 is formed via the exo-bromonium (16) and endo-bromonium (20) ions. The difference between the total energies of the endo-bromonium (20) and classical bromocarbonium (40) cations is small. The bromide anion can attack the carbon atom C2 in 40 on the endo side to form endo-cis product 12 (Scheme 4).

High temperature bromination of 4 and 5

In the following experiments, we aimed to minimize the formation of rearranged products and obtain non-rearranged products in high yield. The reaction of 4 with bromine at 77 °C was investigated. Unlike bromine analogue 2, from the bromination reaction of 4, rearranged product 8 was obtained in high yield. This is an unexpected situation because our previous studies and the literature have demonstrated that it is possible to prevent skeletal rearrangement by using high temperature bromination methods. When we tried the reaction at a higher temperature, 150 °C, we unexpectedly observed the formation of vinyl migration products 45, 46, 60 and 61. No trace of non-rearranged products such as 33/36/59 and aryl-shift products such as 8/29 were detected from this reaction (Scheme 5).

Scheme 5 Bromination of 4 at 77 and 150 °C.

Seeing that bromination of 5 at 25 °C gave non-rearranged products 10–12 in moderate yield, we expected that these would be the major products of the high temperature bromination. Contrary to our expectations, we found that the rate of non-rearranged products 10–12 was decreased on increasing the temperature. From the reaction at 25 °C, dibromides 10–12 are formed in 28% yield, whereas in bromination reactions at 77 and 150 °C, this was determined to be 23% and trace yields, respectively. At a higher temperature of 150 °C, mainly alkyl-shift products 47, 48, 62 and 63 were observed (Scheme 6).

Scheme 6 Bromination of 5 at 77 and 150 °C.

To rationalize the formation of the isomeric dibromides formed at a high temperature, we propose the following mechanism by comparing the bromination of 4 and 5 with the bromination of the parent molecule benzobarrelene (1)^2b and 2,3-dibromobenzobarrelene (2)^2d,e (Scheme 7).

Scheme 7 Reaction mechanism for radical brominations.

The mechanism of bromine addition at higher temperatures is different from the mechanism at low temperatures. We have already demonstrated that at high temperatures, bromine radicals are also involved as well as ionic reactions.^2a,b,4 As is known, the radicals have a very low tendency for rearrangement; however, as we previously demonstrated, rearranged products are also formed via radical intermediates in benzobarrelene systems.^2a,b,d,e,7 Yet, bromination of 2,3-dibromobenzobarrelene (2) at 77 °C gives non-rearranged products up to 88% yield.^2d,e The following question is raised as a result of the present study: why do cyanide and ester derivatives of benzobarrelene behave differently to dibromide derivative 2? Bromine, cyano and ester groups act as inductively-electron-withdrawing groups. In addition, the bromine atom also acts as a mesomeric electron-donating group. But this cannot account for the different behaviour of the dibromides 4 and 5. Bordwell and Lynch¹² have calculated the relative radical stabilizing effects of the ester and cyano groups and have shown that the resonance stabilization energies are of a size comparable with the methyl group. Balci’s studies^6,13 also comply with Bordwell and Lynch’s results. We assumed that the differences arise from the stabilization of radicals 74, 75, 78 and 79, which are responsible for the formation of radicals 82, 83, 86 and 87. Since radicals 74, 75, 78 and 79 are stabilized by the resonance effect (Fig. 3), the lifetimes of radicals 66, 67, 70 and 71 are very short and they rapidly change to more stable radicals (Fig. 3). Radicals 72/73 and 76/77 are not as stable as radicals 74, 75, 78 and 79; therefore, conversion of radicals 65 and 69 to 81 and 85 is not so fast and radicals 65 and 69 have adequate lifetime to react with a bromine atom to give normal addition products 32, 35 and 58.

Fig. 3 Stabilization of radicals by the resonance effect.

To find support for this postulate, we carried out some DFT calculations on both possible radical intermediates. The structure and the stability of the predicted radical intermediates (65, 69, 73, 77, 81 and 85, Scheme 7) of bromination reaction of 2,3-dibromo benzobarrelene (2) at high temperatures have been determined by carrying out geometrical optimizations using UB3LYP/6-311+G(d,p) method, which was followed by the calculation of the total energies. According to the results, there is a very small difference between the stabilities of the radicals 65 and 69. The results obtained by the UB3LYP/6-311+G(d,p) method indicate that radicals 65 and 69 are more stable than radicals 73 and 77, respectively. The differences between the total energies of radicals 73 and 65 and of radicals 77 and 69 are 4.558 and 4.913 kcal mol⁻¹, respectively. Therefore, radicals 65 and 69 cannot be converted into the more stable radicals 81 and 85via radicals 73 and 77, respectively. For this reason, radicals 65 and 69 are the essential intermediates in the radicalic addition of bromine to 2,3-dibenzobarrelene (2) and the reaction products are predicted to be formed via these radicals.

The radical intermediates (66, 70, 74, 78, 82 and 86, Scheme 7) formed in the addition reaction of bromine to 2,3-dicarbomethoxy benzobarrelene (4) at high temperatures have been investigated by the UB3LYP/6-311+G(d,p) method with full geometry optimization and their total energies have been calculated. The results show that the stabilities of radicals 66 and 70 are close to each other. Radicals 74 and 78 have stabilities of 4.747 and 4.641 kcal mol⁻¹ and are more stable than radicals 66 and 70, respectively. Overlapping of p orbitals of the radical centers of 74 and 78 radicals with carbomethoxy group orbitals leads to uncoupled electron delocalization and this increases the stability of the radical (Fig. 4, radical 74). The radicals 82 and 86 have stabilities of 9.973 and 9.543 kcal mol⁻¹ and are more stable than the radicals 66 and 70, respectively. Therefore, radicals 66 and 70 can be converted to radicals 82 and 86via radicals 74 and 78, respectively. The most stable radicals of the radical intermediates are radicals 82 and 86. For this reason, radicals 82 and 86 are the essential intermediates in the radicalic addition of bromine to 2,3-dicarbomethoxy benzobarrelene (4) and the reaction products are predicted to form via these radicals.

Fig. 4 The optimized geometries of the radicals (UB3LYP/6-311+G(d,p)) (bond lengths are in Å).

The geometrical optimization of radical intermediates (67, 71, 75, 79, 83 and 87, Scheme 7) of bromination reaction of 2,3-dicyano benzobarrelene (5) at high temperatures have been determined by the UB3LYP/6-311+G(d,p) method and their total energies have also been calculated. The results show that radical 67 and radical 71 have almost equal stability. Radicals 75 and 79 have stabilities of 4.986 and 4.922 kcal mol⁻¹ and are more stable than radicals 67 and 71, respectively. Overlapping of p orbitals of radical centers of 75 and 79 radicals with the cyano group π orbitals leads to uncoupled electron delocalization and this increases the stability of the radical (Fig. 4, radical 75). Moreover, radicals 83 and 87 have stabilities of 10.864 and 10.090 kcal mol⁻¹ and are more stable than the radicals 67 and 71, respectively. Therefore, it is thermodynamically possible that radicals 67 and 71 are converted to radicals 83 and 87via radicals 75 and 79, respectively.

NMR spectral studies and configurational assignments. The structures of these compounds have been elucidated on the basis of ¹H and ¹³C NMR data and extensive double resonance experiments and by comparison with some spectral data of related systems reported in the literature.^2b,d,e,7,14 Below is a summary of the proton coupling constants exhibited by these closely-related [3.2.1]octadienes (structures A and B in Fig. 5).

Fig. 5 Representation of numbering of carbons.

The coupling patterns that are important for stereochemical characterization of this duo are J_8,10syn, J_8,10anti, J_8,9exo and J_8,9endo in A, and J_9,10syn, J_9,10anti, J_8exo,9 and J_8endo,9 in B. As a consequence of the rigid geometries and reliability of the Karplus rule¹⁵ in [3.2.1]octane systems,^2b,d,e,7,14 the dihedral relationship of the H8 proton to H10_anti in A and the H9 proton to H10_syn in B (∼40°), and to H10_syn in A and to H10_anti in B (∼80°) is sufficiently distinctive to be revealed by the magnitude of the spin–spin interaction. Thus, the high value of J_H10anti in A and J_H10syn in B (J = 4.4–4.7) is uniquely accommodated by the syn-orientation of the bromine atom in A (anti-orientation of bromine in B) bonded to the bridge atom. H10_syn in A and H10_anti in B give a singlet with line broadening (J ≤ 1 Hz). The configuration of bromine at the C9 atom in A and at the C8 atom in B were determined from the coupling constants J_8,9 (in A and in B). Inspection of Dreiding’s models indicates that the dihedral angle between protons H8 (H9 in B) and H9_exo (H8_exo in B) is approximately 40°, whereas the dihedral angle between H8 (H9 in B) and H9_endo (H8_endo in B) is 60°. Large coupling constants of J = 5.1–5.4 Hz are observed in the case of the endo-orientation of bromine (exo-proton) and J = 1.0–2.2 Hz in the case of the exo-orientation of bromine (endo-proton). Aryl-shift products type B and alkyl-shift products type A can also be easily distinguished in the typical pattern of aromatic resonances.^3b The dibromide 61 formed initially could not be isolated in a pure state because of its instability on the column. The spectral data for the isomer 61 were extracted from the NMR spectra of the mixture. The coupling constant (J = 5.4 Hz) between the protons H8 and H9 shows that the bromine atom attached to carbon C9 is endo and that there is no measurable coupling constant between the protons H10 and H5/H8 shows that the bromine atom at carbon C10 is anti.

Conclusion

The results of the present work demonstrate that bromination of 2,3-dicarbometohoxy benzaberrelene (4) at room temperature resulted in the formation of aryl-shift product 8 in quantitative yield. Increasing the reaction temperature to 150 °C generated alkyl-shift products 45, 46, 60 and 61, instead of the expected non-rearrangement products. Reaction of 2,3-dicyanobenzobarrelene (5) at room temperature mainly yielded aryl-shift product 9, along with non-rearranged products 10–12. For the reaction of this compound at higher temperatures, we mainly expected the formation of non-rearrangement products 10–12. However, we observed that the rate of non-rearrangement products was decreased, while the ratio of aryl-shift products was increased with increasing temperature. In the light of our present and previous results, we conclude that the high-temperature bromination method is a useful synthetic method for generating non-rearranged bromine addition products in norbornene 92 (Fig. 6), benzonorbornadiene 93 and related systems, which have a great tendency to undergo Wagner–Meerwein rearrangement because the bromination reaction is radicalic at high temperatures and the radicals have a very low tendency towards rearrangement. However, in benzobarrelene (1) and norbornadiene (94) systems, rearrangements occur via vinyl–vinyl bridging intermediates under radicalic conditions and benzo-vinyl bridging is not favorable. So, bromination of norbornene and benzonorbornadiene derivatives in radicalic conditions results in the conservation of skeletal rearrangement, while bromination of benzobarrelene and norbornadiene systems in the same conditions also gives rearranged products via vinyl–vinyl bridging intermediates. Substituents connected to molecules also direct the reaction. The experimental results were supported by theoretical calculations.

Fig. 6 Norbornene derivatives.

Experimental

General. Melting points are uncorrected. Infrared spectra were obtained from solution in 0.1 mm cells or KBr pellets on a regular instrument. The ¹H and ¹³C NMR spectra were recorded on 400 (100)-MHz spectrometers. Apparent splitting is given in all cases. Column chromatography was performed on silica gel (60-mesh, Merck). All substances reported in this paper are in their racemic form.

Caution: it has been reported¹⁶ that of three laboratory workers who have used dibromides and bromohydrin derived from norbornadiene, two later developed similar pulmonary disorders that contributed to their subsequent deaths. The third exhibited minor skin sensitivity reactions. In the case of dibromide derived from benzonorbornadiene there are no toxicological reports in the literature. However, we recommend that the compounds must be handled only with extreme caution.

Bromination of dimethyl 1,4-dihydro-1,4-ethenonaphthalene-2,3-dicarboxylate (4) at 25 °C. 400 mg (1.48 mmol) 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarboxylate (4) was dissolved in 10 mL of dry chlorofom in a 50 mL flask. A solution of bromine (249 mg, 1.55 mmol) in 2 mL chlorofom was added dropwise over 1 min at 25 °C. The bromine colour disappeared immediately. The solvent was evaporated and crude product recrystallised from dichloromethane–n-hexane (1 : 2) to give dibromide 8 as the sole product (632 mg, 100%). Dimethyl-(5R(S),8S(R),9R(S),10S(R))-8,10-dibromo-8,9-dihydro-5H-5,9-methanobenzo[a][7]annulene-6,7-dicarboxylate (8). mp: 145–146 °C, yellow crystals [Found: C, 44.21; H, 3.39; requires C₁₆H₁₄Br₂O₄: C, 44.68; H, 3.28%]; ¹H-NMR (400 MHz, CDCl₃): 7.37–7.13 (m, 4H, H_aryl), 4.86 (bd, J_8,9 = 1.5 Hz, 1H, H8), 4.80 (t, J_5,10 = J_9,10 = 4.1 Hz, 1H, H10), 4.19 (d, J_5,10 = 4.1 Hz, 1H, H5), 3.88 (bd, J_9,10 = 4.1 Hz, 1H, H9), 3.83 (s, 3H, OCH₃), 3.79 (s, 3H, OCH₃) ppm. ¹³C-NMR (100 MHz, CDCl₃): 166.64, 165.65, 146.01, 140.05, 136.99, 134.53, 128.84, 128.75, 124.68, 122.56, 53.05, 52.74, 50.29, 47.66, 45.79, 42.21 ppm. IR (KBr, cm⁻¹): 2947, 2840, 1724, 1460, 1435, 1343, 1272, 1253, 1236, 1152, 1080, 1043, 937, 881, 726.

Bromination of 1,4-dihydro-1,4-ethenonaphthalene-2,3-dicarbonitrile (5) at 25 °C. 250 mg (1.22 mmol) 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarbonitrile (5) was dissolved in 10 ml of dry chloroform in a 50 mL flask. A solution of bromine (205 mg, 1.28 mmol) in 2 ml chloroform was added dropwise over 1 min at 25 °C. The bromine colour disappeared immediately. The solvent was evaporated and crude product was chromatographed on silica gel (30 g) eluting with ethylacetate–n-hexane (1 : 9).

Four compounds were isolated:

The first fraction. (1S(R),4R(S),9R(S),10R(S))-9,10-dibromo-1,4-dihydro-1,4-ethanonaphthalene-2,3-dicarbonitrile (10). (75 mg, 17%) mp: 147–148 °C, white crystals from dichloromethane–n-hexane (1 : 1). [Found : C, 46.34; H, 2.31; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.41–7.38 (m, 4H, H_aryl), 4.61 (d, J_1,10 = 2.9 Hz, 1H, H1), 4.43 (d, J_4,9 = 2.9 Hz, 1H, H4), 4.36 (t, J_4,9 = J_9,10 = 2.9 Hz, 1H, H9), 4.03 (t, J_1,10 = J_9,10 = 2.9 Hz, 1H, H10) ppm. ¹³C-NMR (100 MHz, CDCl₃): 134.65, 133.80, 132.75, 130.84, 129.16, 129.02, 127.55, 125.16, 113.55, 112.91, 52.55, 51.55, 50.67, 50.33. IR (KBr, cm⁻¹): 3003, 2964, 2919, 2851, 2224, 1474, 1463, 1259, 1222, 1161, 1022, 856, 772, 735, 596 ppm. MS (EI, 70 eV) m/z: 366/364/362(M⁺, 2), 285/283(M⁺, −Br, 8), 203(M⁺, −2Br, 16), 178(100), 151(7), 102(3), 88(5).

The second fraction. (1S(R),4R(S),9S(R),10R(S))-9,10-dibromo-1,4-dihydro-1,4-ethanonaphthalene-2,3-dicarbonitrile (12). (9 mg, 2%) mp: >300 °C, white crystals from dichloromethane– n-hexane (2 : 3). [Found : C, 46.37; H, 2.19; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.43–7.36 (AA′BB′ system, 4H, H_aryl), 4.57 (AA′ part of AA′BB′ system, 2H, H1 and H4), 4.50 (BB′ part of AA′BB′ system, 2H, H9 and H10) ppm. ¹³C-NMR (100 MHz, CDCl₃): 133.56, 132.51, 128.65, 127.38, 112.74, 52.14, 47.17 ppm. IR (KBr, cm⁻¹): 2923, 2846, 2218, 1457, 1373, 1206, 1152, 1110, 1068, 913, 748.

The third fraction. (5R(S),8S(R),9R(S),10S(R))-8,10-dibromo-8,9-dihydro-5H-5,9-methanobenzo[a][7]annulene-6,7-dicarbonitrile (9). (310 mg, 70%) mp: 203–204 °C, white crystals from dichloromethane–n-hexane (1 : 2). [Found : C, 46.55; H, 2.38; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.43–7.31 (m, 4H, H_aryl), 4.81 (t, J_5,10 = J_9,10 = 4.2 Hz, 1H, H10), 4.61 (d, J_8,9 = 1.6 Hz, 1H, H8), 4.00 (d, J_5,10 = 4.2 Hz, 1H, H5), 3.90 (bd, J_9,10 = 4.2 Hz, 1H, H9) ppm. ¹³C-NMR (100 MHz, CDCl₃): 144.44, 139.19, 130.59, 130.15, 129.62, 125.04, 124.38, 123.09, 114.56, 113.99, 50.29, 48.70, 43.53, 39.15 ppm. IR (KBr, cm⁻¹): 2963, 2857, 1465, 1262, 2224, 1586, 1410, 1261, 1236, 1197, 1088, 1024, 845, 800, 732, 585. MS (EI, 70 eV) m/z: 366/364/362(M⁺, 13), 285/283(M⁺, −Br, 77), 204(M⁺, −2Br, 100), 177(34), 151(15), 128(9), 102(18), 75(16), 62(8).

The fourth fraction. (1S(R),4R(S),9R(S),10S(R))-9,10-dibromo-1,4-dihydro-1,4-ethanonaphthalene-2,3-dicarbonitrile (11). (40 mg, 9%) mp: 260–261 °C, pale yellow crystals from dichloromethane–n-hexane (1 : 2). [Found: C, 46.38; H, 2.34; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.37–7.32 (AA′BB′ system, 4H, 4H, H_aryl), 4.76 (AA′ part of AA′XX′ system, 2H, H1 and H4), 4.27 (XX′ part of AA′XX′ system, 2H, H9 and H10) ppm. ¹³C-NMR (100 MHz, CDCl₃): 135.94, 131.50, 129.34, 125.22, 113.77, 53.17, 47.67 ppm. IR (KBr, cm⁻¹): 2919, 2851, 2218, 1474, 1460, 1267, 1236, 764, 737, 702. MS (EI, 70 eV) m/z: 366/364/362(M⁺, 1), 285/283(M⁺, −Br, 7), 204(M⁺, −2Br, 26), 178(100), 151(14), 102(12), 75(10).

Bromination of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarboxylate (4) at 77 °C. 700 mg (2.60 mmol) of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarboxylate (4) was dissolved in 10 mL of carbon tetrachloride in a 50 mL flask which was equipped with a reflux condenser. The solution was heated until carbon tetrachloride started to reflux while stirring magnetically. To the refluxing solution, a hot solution of bromine (497 mg, 3.11 mmol) in 2 mL of carbon tetrachloride was added dropwise over 2 min. The resulting reaction mixture was heated for 5 min at reflux temperature. After being cooled to room temperature, the solvent was evaporated and the crude product recrystallised from dichloromethane–n-hexane (1 : 2) to give dibromide 8 as the sole product (908 mg, 82%).

Bromination of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarboxylate (4) at 150 °C. 600 mg (2.22 mmol) of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarboxylate (4) was dissolved in 10 mL of decalin in a 50 mL two-necked flask equipped with reflux condenser and an inlet glass tube touching the bottom of the reaction flask. The inlet glass tube was connected to a 2 mL round-bottom flask that contained 426 mg (2.66 mmol) of bromine. Bromine vapors, obtained by heating of the flask to 100 °C, were transferred directly to a decalin solution having a temperature of 150 °C, in 5 min while stirring magnetically. The color of the bromine disappeared immediately. The solvent was removed under reduced pressure. The residue was chromatographed on silica gel (50 g) eluting with ethylacetate–n-hexane (1 : 9). Three compounds were isolated:

The first fraction. Dimethyl (5S(R),8S(R),9R(S),10S(R))-9,10-dibromo-8,9-dihydro-5H-5,8-methanobenzo[a][7]annulene-6,7-dicarboxylate (60), (313 mg, 33%) mp: 168–169 °C, white crystals from dichloromethane–n-hexane (1 : 2). [Found: C, 45.01; H, 3.51; requires C₁₆H₁₄Br₂O₄: C, 44.68; H, 3.28%]; ¹H-NMR (400 MHz, CDCl₃): 7.37–7.05 (m, 4H, H_aryl), 5.54 (d, J_8,9 = 2.1 Hz, 1H, H9) 5.13 (s, 1H, H10), 4.04 (m, 1H, H5), 3.91 (m, 1H, H8), 3.82 (s, 3H, OCH₃), 3.75 (s, 3H, OCH₃) ppm. ¹³C-NMR (100 MHz, CDCl₃): 164.43, 163.54, 149.10, 137.84, 135.74, 133.40, 133.30, 129.46, 128.67, 127.28, 61.34, 56.90, 55.40, 52.78, 52.67, 46.73 ppm. IR (KBr, cm⁻¹): 2952, 2851, 1722, 1636, 1480, 1435, 1348, 1262, 1220, 1196, 1092, 1078, 950, 892, 833,796, 758. MS (EI, 70 eV) m/z: 432/430/428(M⁺, 1), 351/349(M⁺,-Br, 100), 318(22), 289(24), 269(M⁺,-2Br, 36), 210(60), 179(18), 152(35), 128(19), 76(10).

The second fraction. Dimethyl (5S(R),8S(R),9S(R),10R(S))-9,10-dibromo-8,9-dihydro-5H-5,8-methanobenzo[a][7]annulene-6,7-dicarboxylate (45). (275 mg, 29%) mp: 94–95 °C, white crystals from dichloromethane–n-hexane (1 : 1). [Found: C, 45.00; H, 3.43; requires C₁₆H₁₄Br₂O₄: C, 44.68; H, 3.28%]; ¹H-NMR (400 MHz, CDCl₃): 7.57–7.09 (m, 4H, H_aryl), 5.74 (d, J_8,9 = 5.2 Hz, 1H, H9), 4.82 (t, J_5,10 = J_8,10 = 4.7 Hz, 1H, H10), 3.86 (ddd, J_8,9 = 5.2 Hz, J_8,10 = 4.7 Hz, J_5,8 = 1.0 Hz, 1H, H8), 3.82 (dd, J_5,10 = 4.7 Hz, J_5,8 = 1.0 Hz, 1H, H5), 3.80 (s, 3H, OCH₃), 3.77 (s, 3H, OCH₃) ppm. ¹³C-NMR (100 MHz, CDCl₃): 164.07, 163.29, 147.04, 139.27, 135.87, 133.17, 132.19, 129.26, 128.63, 128.15, 53.30, 52.83, 52.65, 52.62, 52.61, 45.85 ppm. IR (KBr, cm⁻¹): 2997, 2953, 2846, 1720, 1644, 1480, 1435, 1337, 1270, 1205, 1154, 1120, 1085, 1025, 952, 782. MS (EI, 70 eV) m/z: 432/430/428(M⁺, 5), 398(18), 351/349(M⁺, −Br, 36), 317(47), 269(M⁺, −2Br, 35), 210(100), 183(32), 152(58), 128(21), 76(12).

The third fraction. Dimethyl (5S(R),8S(R),9R(S),10R(S))-9,10-dibromo-8,9-dihydro-5H-5,8-methanobenzo[a][7]annulene-6,7-dicarboxylate (46). (228 mg, 24%) mp: 123–124 °C, yellow crystals from dichloromethane–n-hexane (1 : 1). [Found: C, 44.85; H, 3.38; requires C₁₆H₁₄Br₂O₄: C, 44.68; H, 3.28%]; ¹H-NMR (400 MHz, CDCl₃): 7.57–7.07 (m, 4H, H_aryl), 5.47 (s, 1H, H9), 4.91 (t, J_5,10 = J_8,10 = 4.5 Hz, 1H, H10), 3.92 (bd, J_5,10 = 4.5 Hz, 1H, H5), 3.82 (s, 3H, OCH₃), 3.76 (dt, J_8,10 = 4.5 Hz, J_5,8 = J_8,9 = 1.0 Hz, 1H, H8), 3.74 (s, 3H, OCH₃) ppm. ¹³C-NMR (100 MHz, CDCl₃): 163.75, 163.05, 148.17, 138.83, 135.01, 133.58, 133.22, 129.33, 128.57, 128.35, 52.95, 52.79, 52.57, 52.41, 46.39, 41.39 ppm. IR (KBr, cm⁻¹): 2952, 2846, 1722, 1636, 1452, 1435, 1320, 1270, 1217, 1152, 1082, 1015, 948, 878, 839, 760. MS (EI, 70 eV) m/z: 432/430/428(M⁺, 8), 398(17), 351/349(M⁺, −Br, 86), 317(35), 289(33), 269(M⁺, −2Br, 36), 210(100), 183(29), 152(60), 128(32), 76(12).

Bromination of 1,4-dihydro-1,4-ethenonaphthalene-2,3-dicarbonitrile (5) at 77 °C. 700 mg (3.43 mmol) of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarbonitrile (5) was dissolved in 10 mL of carbon tetrachloride in a 50 mL flask which was equipped with a reflux condenser. The solution was heated until carbon tetrachloride started to reflux while stirring magnetically. To the refluxing solution, a hot solution of bromine (659 mg, 4.12 mmol) in 2 mL of carbon tetrachloride was added dropwise over 2 min. The resulting reaction mixture was heated for 1 h at reflux temperature. After being cooled to room temperature the solvent was evaporated and crude product was chromatographed on silica gel (30 g) eluting with ethylacetate–n-hexane (1 : 9). Seven compounds were isolated:

The first fraction. (1S(R),4R(S),9R(S),10R(S))-9,10-dibromo-1,4-dihydro-1,4-ethanonaphthalene-2,3-dicarbonitrile (10) (161 mg, 13%).

The second fraction. ((1S(R),4R(S),9S(R),10R(S))-9,10-dibromo-1,4-dihydro-1,4-ethanonaphthalene-2,3-dicarbonitrile (12) (49 mg, 4%).

The third fraction. (5S(R),8S(R),9R(S),10R(S))-9,10-dibromo-8-9-dihydro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (48). (115 mg, 9%) mp: 192–193 °C, white crystals from dichloromethane–n-hexane (1 : 2). [Found: C, 46.26; H, 2.29; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.63–7.12 (m, 4H, H_aryl), 5.25 (m, 1H, H9), 4.87 (t, J_5,10 = J_8,10 = 4.4 Hz, 1H, H10), 3.90 (d, J_5,10 = 4.4 Hz, 1H, H5), 3.77 (dt, J_8,10 = 4.4 Hz, J_5,8 = J_8,9 = 1.0 Hz, 1H, H8) ppm. ¹³C-NMR (100 MHz, CDCl₃): 137.70, 133.63, 132.54, 131.75, 130.86, 129.67, 128.47, 126.35, 111.27, 111.23, 53.55, 52.32, 44.40, 38.48 ppm. IR (KBr, cm⁻¹): 2964, 2919, 2851, 2224, 1600, 1476, 1454, 1259, 1203, 1172, 1138, 1085, 1038, 881, 772, 744. MS (EI, 70 eV) m/z: 366/364/362(M⁺, 1), 285/283(M⁺, −Br, 74), 204(M⁺, −2Br, 100), 178(64), 149(25), 102(39), 175(32).

The fourth fraction. (5S(R),8S(R),9R(S),10S(R))-9,10-dibromo-8-9-dihydro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (62). (248 mg, 20%) mp: 153–154 °C white crystals from dichloromethane–n-hexane (1 : 2). [Found: C, 46.09; H, 2.32; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.42–7.08 (m, 4H, H_aryl), 5.34 (d, J_8,9 = 2.2 Hz, 1H, H9), 5.19 (s, 1H, H10), 4.02 (m, 1H, H5), 3.90 (m, 1H, H8) ppm. ¹³C-NMR (100 MHz, CDCl₃): 138.91, 134.61, 133.68, 132.27, 131.04, 129.87, 127.27, 123.67, 112.15, 111.82, 61.44, 58.10, 53.08, 43.55 ppm. IR (KBr, cm⁻¹): 3070, 2997, 2919, 2851, 2224, 1600, 1477, 1454, 1315, 1264, 1236, 1150, 892, 772, 741, 716, 654. MS (EI, 70 eV) m/z: 366/364/362(M⁺, 1), 285/283(M⁺, −Br, 74), 204(M⁺, −2Br, 100), 177(34), 151(13), 140(4), 128(20), 102(22), 89(23), 75(21), 74(11).

The fifth fraction. (5S(R),8S(R),9S(R),10R(S))-9,10-dibromo-8-9-dihydro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (47). (260 mg, 21%) mp: 149–150 °C, white crystals from dichloromethane–n-hexane (1 : 2). [Found: C, 46.60; H, 2.59; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.63–7.09 (m, 4H, H_aryl), 5.69 (d, J_8,9 = 5.1 Hz, 1H, H9), 4.76 (t, J_5,10 = J_8,10 = 4.6 Hz, 1H, H10), 3.82 (bd, J_5,10 = 4.6 Hz, 1H, H5), 3.72 (ddd, J_8,9 = 5.1 Hz, J_8,10 = 4.6 Hz, J_5,8 = 0.9 Hz, 1H, H8) ppm. ¹³C-NMR (100 MHz, CDCl₃): 136.42, 132.62, 132.57, 131.97, 130.67, 129.61, 127.79, 127.34, 112.29, 111.24, 53.48, 52.88, 50.50, 43.04 ppm. IR (KBr,cm⁻¹): 3070, 2958, 2919, 2851, 2224, 1594, 1480, 1454, 1328, 1292, 1266, 1245, 1197, 1166, 920, 895, 875, 827, 765. MS (EI, 70 eV) m/z: 366/364/362(M⁺, 1), 285/283(M⁺, −Br, 70), 204(M⁺, −2Br, 100), 177 (39), 151 (13), 140 (3), 128 (17), 102 (18), 89 (13), 75 (20), 74 (9).

The sixth fraction. (5S(R),8S(R),9S(R),10S(R))-9,10-dibromo-8,9-dyhidro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (63). (161 mg, 13%) mp: 175–176 °C, yellow crystals from dichloromethane–n-hexane (1 : 2). [Found: C, 46.48; H, 2.39; requires C₁₄H₈Br₂N₂: C, 46.19; H, 2.22%]; ¹H-NMR (400 MHz, CDCl₃): 7.61–7.09 (m, 4H, H_aryl), 5.57 (d, J_8,9 = 5.2 Hz, 1H, H9), 4.55 (s, 1H, H₁₀) 3.99 (bd, J_8,9 = 5.2 Hz, 1H, H8), 3.98 (m, 1H, H5) ppm. ¹³C-NMR (100 MHz, CDCl₃): 135.72, 134.05, 133.10, 131.90, 130.92, 129.93, 126.75, 126.42, 113.10, 111.89, 59.90, 57.92, 54.26, 42.94 ppm. IR (KBr, cm⁻¹): 2924, 2857, 2224, 1706, 1597, 1476, 1451, 1379, 1270, 909, 749.

The seventh fraction. (1S(R),4R(S),9R(S),10S(R))-9,10-dibromo-1,4-dihydro-1,4-ethanonaphthalene-2,3-dicarbonitrile (11) (74 mg, 6%).

Bromination of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarbonitrile (5) at 150 °C. 720 mg (3.53 mmol) of dimethyl 1,4-dihydro-1,4-ethanonaphtalene-2,3-dicarbonitrile (5) was dissolved in 10 mL of decalin in 50 mL two-necked flask equipped with reflux condenser and an inlet glass tube touching the bottom of the reaction flask. The inlet glass tube was connected to 2 mL of round-bottom flask that contains 678 mg (4.24 mmol) of bromine. Bromine vapours, obtained by heating of the flask to 100 °C, were transferred directly to the decalin solution having a temperature of 150 °C, in 5 min while stirring magnetically. The color of the bromine disappeared immediately. The solvent was removed under reduced pressure. The crude product was chromatographed on silica gel (30 g) eluting with ethylacetate–n-hexane (1 : 9). Three compounds were isolated:

The first fraction. (5S(R),8S(R),9R(S),10S(R))-9,10-dibromo-8-9-dihydro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (62) (384 mg, 30%).

The second fraction. (5S(R),8S(R),9S(R),10R(S))-9,10-dibromo-8-9-dihydro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (47) (269 mg, 21%).

The third fraction. (5S(R),8S(R),9S(R),10S(R))-9,10-dibromo-8,9-dyhidro-5H-5,8-methanobenzo[a][7]annulen-6,7-dicarbonitrile (63) (358 mg, 28%).

Acknowledgements

The authors are indebted to the Scientific and Technical Research Council of Turkey (TUBITAK project NO: TBAG-106T658) and Ataturk University for financial support.

References

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR and mass spectra of compounds. See DOI: 10.1039/b9nj00372j

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