Theoretical study on the Diels–Alder reaction of bromo-substituted 2H-pyran-2-ones and some substituent vinyls J. Serb. Chem. Soc. 80 (9) 1139–1148 (2015) UDC 546.14’11+547.81+519.677+ JSCS–4786 541.124:66.095.252.091.7:543.637 Original scientific paper 1139 Theoretical study on the Diels–Alder reaction of bromo-substituted 2H-pyran-2-ones and some substituent vinyls MINA HAGHDADI*, HAMED AMANI and NASIM NAB Department of Chemistry, Islamic Azad University, P. O. Box 755, Babol branch, Babol, Iran (Received 5 December 2014, revised 8 February, accepted 8 February 2015) Abstract: A DFT study of the reactivity, regio- and stereoselectivity of Diels– –Alder reactions between 3-bromo, 5-bromo, and 3,5-dibromo-2H-pyran-2- -ones and some weakly activated and unactivated alkenes was performed using the density functional theory (DFT). Four possible reaction channels, which are related to the formation of meta- and para- and endo- and exo-cycloadducts, were explored and characterized. The energy and natural bond orbital analysis showed that the meta-regioselectivity on the exo pathway was preferred and followed an asynchronous concerted mechanism with a polar nature in all Diels–Alder cycloadditions. Moreover, the activation free energies of the Diels– Alder cycloadditions of 3,5-dibromo-2H-pyran-2-one were lower than those for 3-bromo-2H-pyran-2-one and 5-bromo-2H-pyran-2-one, which is in line with experimental observations. DFT-based reactivity indices clearly predicted the regiochemistry of the isolated cycloadducts. Keywords: bromo-2H-pyran-2-ones; DFT study; reaction mechanism; reac- tivity indices; regio- and stereoselectivity. INTRODUCTION During investigations on the role of substituents on the cycloaddition reac- tion of 2H-pyran-2-ones, it was found that 3-bromo and 5-bromo-2H-pyran-2- -ones are the most interesting and unique, and have useful features.1–5 These two 2H-pyran-2-ones are ambident dienes3 and react with electron-rich, electron-poor and electron-neutral dienophiles with good regio- and stereoselectivity.4,6,7 The cycloadditions of 2H-pyran-2-one itself are not selective.5 Moreover, in contrast to the bromo-pyrones, 4-chloro-2H-pyran-2-one, in line with 2H-pyran-2-one itself, is neither ambident diene nor undergoes regioselective cycloadditions.8 It undergoes cycloadditions only with electron-deficient dienophiles that were stereoselective, but not regioselective.3a During the course of a study of 2H- * Corresponding author. E-mail: mhaghdadi2@gmail.com doi: 10.2298/JSC141205014H _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1140 HAGHDADI, AMANI and NAB -pyran-2-ones, Cho and co-workers investigated 3,5-dibromo-2H-pyran-2-one in Diels–Alder (DA) reactions with a series of electronically and sterically distinct dienophiles.3b Their results showed that it is a highly potent ambident diene, being more reactive and stereoselective than monobromo-2H-pyran-2-ones, and thus capable of generating a variety of bicycloadducts in much higher chemical yields and endo/exo rations than monobromo-2H-pyran-2-ones.3 Afarinkia and co-workers studied the Diels–Alder reactions of 3- and 5-halo- -substituted 2H-pyran-2-ones with poor electron-rich, electron-rich and deficient dienophiles.7–9 Their experimental results showed that these cycloadditions proceed with excellent regioselectivity and very good stereoselectivity. In con- trast, the 4-halo-substituted-2H-pyran-2-ones reactions proceed with only moder- ate regio- and stereoselectivity.9 Furthermore, their results showed that the nature of halogen substituent had only a small, sometimes negligible, influence on the cycloaddition of 2H-pyran-2-ones, and also, in both the 3- and 5-substituted series, the distribution of the products did not appear to be significantly dif- ferent.8 Therefore, changing the halogen substituent did not significantly change the electronic demand of the 3- and 5-halo-substituted 2H-pyran-2-one, although it may influence their reactivity. Furthermore, they performed a range of calcul- ations on substituted 2H-pyran-2-one cyclo-additions, at the B3LYP/6-31G level of theory, to demonstrate the advantage of 3- and 5-halo-substituted 2H-pyran-2- -ones over 4-halo-substituted 2H-pyran-2-ones.8,9 Although there are many reports about the alternative synthetic routs,3–9 there are no theoretical investigations about the detailed molecular mechanism and electronic parameters. As a part of a program directed toward the inves- tigation of related DA cycloadditions, herein the results of a theoretical study on the mechanism of cycloaddition reactions between 3-bromo, 5-bromo and 3,5- dibromo-2H-pyran-2-ones 1a–c, with a range of vinyl derivatives: vinyl acetate (2a), vinyl benzoate (2b), 2-ethenyl-1H-isoindole-1,3(2H)-dione (N-vinylphthal- imide) (2c) and 2-propenenitrile (2d), to give the bridged bicyclic lactones 3–13 are presented (Scheme 1). The purpose of the present study was to provide a better understanding the mechanistic features of these processes, especially by localization and characterization of all stationary points involved in these formally [2+4]cycloadditions. A density functional theory (DFT) analysis was performed to explain both the exo/endo stereocontrol and regioselectivity of these processes in order to find a possible mechanism that may explain the different reactivity observed in each case. Although DA cycloadditions of 5-bromo and 3,5-dibromo-2H-pyran-2-ones with poor electron-rich dienophiles 2a–c were not prepared as part of expe- rimental studies, the calculations based on them provided a better understanding of the trends, differences, and similarities between halogen substituted 2H-pyran- -2-ones. _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ THEORETICAL STUDY ON THE DIELS-ALDER REACTION 1141 Scheme 1. The calculated possible reaction channels for the DA reaction of bromo-substituted 2H-pyran-2-ones 1a–c with the vinyl derivatives 2a–d at the B3LYP/cc-pVDZ level. COMPUTATIONAL DETAILS The density functional theory calculations were realized using the Gaussian 09 pack- age.10 The relative energies and free energies were computed at 298 K for the various sta- tionary points at the B3LYP/cc-pVDZ level. The electronic structures of the stationary points were analyzed by the natural bond orbital (NBO) method.11 The global reactivity indexes were estimated according to the equations recommended by Parr.12 The global electrophilicity index, ω, is given by the following expression:13 2 2 μ ω η = (1) in terms of the electronic chemical potential, μ, and the chemical hardness, η. Both quantities may be approached in terms of the one-electron energies of the frontier molecular orbitals HOMO and LUMO, ɛH and ɛL,14 as: H L( ) / 2μ ε ε= + (2) L Hη ε ε= − (3) Recently, Domingo introduced an empirical (relative) nucleophilicity index, N,15 based on the HOMO energies obtained within the Kohn Sham scheme,13 and defined as: HOMO HOMO(Nu) (TCE)ε ε− (4) Nucleophilicity is referred to tetracyanoethylene (TCE), because it presents the lowest HOMO energy in a large series of molecules already investigated within the context of polar cycloadditions. This choice allows the convenient handling of a nucleophilicity scale of posi- tive values. Recently, Domingo proposed two new electrophilic, kP + , and nucleophilic, kP − , Parr functions based on the atomic spin density distribution at the radical anion and cation of a neutral molecule.16 The electrophilic, kP + , and nucleophilic, kP − , Parr functions, were obtained through the analysis of the Mulliken atomic spin density of the radical anion and cation by single-point energy calculations over the optimized neutral geometries using the _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1142 HAGHDADI, AMANI and NAB unrestricted UB3LYP formalism for radical species. The local electrophilicity indices, ωk,17 the local nucleophilicity indices, Nk,198 were calculated using the following expressions: k k Pω ω += (5) k kN NP −= (6) where kP + and kP − are the electrophilic and nucleophilic Parr functions, 16 respectively. RESULTS AND DISCUSSIONS In the present study, the regio- and stereoselectivity of the cycloaddition reaction between bromo-substituted 2H-pyran-2-ones 1a–c and vinyl substituents 2a–d were studied, and then an analysis based on the reactivity indices of stationary points was performed. Study of the DA reactions of bromo-substituted 2H-pyran-2-ones 1a–c with some vinyl derivatives (2a–d) Due to the asymmetry of bromo-substituted 2H-pyran-2-ones 1a–c, four regio-isomeric channels are feasible for each of the DA reactions, meta and para, which are related to the endo and exo approach modes of the diene systems 1a–c relative to the R group of the vinyl compounds 2a–d (Scheme 1). Analysis of the stationary points associated with these DA reactions indi- cated that they could occur via a one-step mechanism and consequently, four ste- reoisomeric TSs, named TS1, TS2, TS3, and TS4, and the corresponding pro- ducts 3–13 were located and characterized. The activation and relative energies associated with these stationary points are given in Table I. Analysis of the geo- metries at the TS structures shows that the TSs of meta pathways correspond to asynchronous bond formation processes. The extent of bond formation along a reaction pathway is provided by the concept of bond order (BO).19 These values are within the range of 0.180 to 0.636. These results show that for all DA reac- tions, TS1 and TS2 (meta pathways) are more asynchronous than TS3 and TS4 (para pathways), and that the TSca, TScb and TScc (for the N-phthalimide sub- stituent) are the most asynchronous ones. The asynchronicity shown by the geo- metrical data is accounted for by the BO values. Furthermore, the asynchronicity in bond formation at the TSs measured by ∆r = (r2–r1) ranges from 0.72 to 1.10 at TS1 and TS2, indicating that the TSs of meta process correspond to highly asynchronous bond-formation processes. Nat- ural population analysis (NPA)11 allowed the evaluation of the charge transfer (CT) along these DA reactions, at the TSs. Charge transfer (CT) plays a relevant role in most of organic reactions. In fact, in Diels–Alder reactions, the CT value is one of the most relevant characteristics of their transition states (TSs) and, in most cases, it is responsible of the height of their energy barrier. The calculated CT values for these DA reactions are given in Fig. 1. In general, the CT values in the TSs associated with the para pathways were lower than 0.090 e, in clear agree- _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ THEORETICAL STUDY ON THE DIELS-ALDER REACTION 1143 TABLE I. Activation energies, ΔE#, activation free energies, ∆G#, and reaction energies, ∆Er, (all in kJ mol-1), with the formation of DA cycloadducts between bromo-substituted 2H- -pyran-2-ones 1a–c and vinyl derivatives 2a–d in the meta pathways Entry Species TS ∆E# ∆G# ∆Er 1 1a+2a→3a-exo TS1aa 108.48 166.33 –48.53 2 1a+2a→4a-endo TS2aa 112.94 169.31 –46.22 3 1a+2b→3b-exo TS1ab 112.12 168.26 –49.94 4 1a+2b→4b-endo TS2ab 116.66 170.56 –45.98 5 1a+2c→3c-exo TS1ac 106.75 163.59 –28.80 6 1a+2c→4c-endo TS2ac 112.64 168.23 –27.73 7 1a+2d→3d-exo TS1ad 117.59 171.64 –29.45 8 1a+2d→4d-endo TS2ad 107.65 161.71 –33.67 9 1b+2a→5a-exo TS1ba 92.82 150.75 –69.94 10 1b+2a→6a-endo TS2ba 102.04 157.18 –65.45 11 1b+2b→5b-exo TS1bb 98.36 154.17 –68.60 12 1b+2b→6b-endo TS2bb 100.47 155.97 –66.38 13 1b+2c→5c-exo TS1bc 64.58 120.92 –75.89 14 1b+2c→6c-endo TS2bc 76.74 131.41 –71.40 15 1b+2d→5d-exo TS1bd 108.51 162.17 –51.16 16 1b+2d→6d-endo TS2bd 100.17 153.82 –54.83 17 1c+2a→7a-exo TS1ca 94.23 151.39 –62.01 18 1c+2a→8a-endo TS2ca 100.63 156.30 –59.89 19 1c+2b→7b-exo TS1cb 98.40 153.68 –63.50 20 1c+2b→8b-endo TS2cb 100.22 155.34 –59.66 21 1c+2c→7c-exo TS1cc 91.26 148.47 –43.06 22 1c+2c→8c-endo TS2cc 101.83 157.25 –41.22 23 1c+2d→7d-exo TS1cd 106.23 160.32 –41.75 24 1c+2d→8d-endo TS2cd 98.22 152.36 –45.00 ment with the non-polar character of these pathways. On the other hand, the CT values at the TSs of the DA reactions of 1a–c and 2a–c in the most favorable regioisomeric pathways (meta–exo), were between 0.205 and 0.157 e, which indicate the polar nature of the meta channels in these DA reactions. Only the most unfavorable DA reactions of 1a–c and 2d presented low CT values (lower than 0.050 e). These results with the proposal that for the DA reactions of 1a–c with 2a–d, an increase in the polar character as the reaction proceeds is accompanied by an acceleration of the reaction.7–9 The energy barrier (∆E#) and activation Gibbs free energy values (∆G#), related to the occurrence of transition states for the DA reactions of 1a–c with 2a–d are lower for the meta approaches than those for the para ones (Table I). The mea- sured stereoselectivity indicated that the meta–exo cyclization modes are more favorable than the meta–endo ones, leading to the formation of meta–exo adducts for the DA reactions of 1a–c with 2a–c, while the lowest barrier energies for the DA reactions of 1a–c with 2d occur on the meta–endo pathway, which yields the meta–endo cycloadducts 4d, 6d and 8d. Therefore, the presence of a cyano group _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1144 HAGHDADI, AMANI and NAB TS2ab TS1ab TS2aa TS1aa TS2ad TS1ad TS2ac TS1ac TS2bb TS1bbTS2ba TS1ba TS2bd TS1bd TS2bc TS1bc TS2cb TS1cbTS2ca TS1ca TS2cd TS1cdTS2cc TS1cc Fig. 1. Optimized geometries (B3LYP/cc-pVDZ) of the transition structures involved in the meta pathways of the DA reactions between the bromo-substituted 2H-pyran-2-ones 1a–c and the vinyl derivatives 2a–d. The bond distances are given in Å, the Wiberg bond indices are given in parenthesis and the natural charges (CT) of the TSs are also given. _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ THEORETICAL STUDY ON THE DIELS-ALDER REACTION 1145 on the dienophile, neither changes the stereoselectivity (exo to endo), nor inc- rease the energy barriers relative to the other dienophiles. On the other hand, the results of energy values in Table I showed that the DA reactions of 1a–c with 2a and 2b are less stereoselective than the cycloaddi- tions to dienophiles 2c and 2d. These differences in stereoselectivity could be explained as follows. The lack of stereoselectivity in the cycloadditions of vinyl acetate 2a pre- sumably arises from a lack of strong secondary orbital interactions, suggesting that the cycloaddition to the weakly activated dienophile may be much more sus- ceptible to steric interaction.7 This was confirmed from the results of the cyclo- additions of 2-ethenyl-1H-isoindole-1,3(2H)-dione (2c), where the reactions are highly exo selective. Here, the steric congestion arises directly from an unfavor- able steric interaction between the second nitrogen substituent and the bromine atom in the TS, leading to the endo cycloadduct. Therefore, TS1ac, TS1bc and TS1cc leading to the exo cycloadduct are favored. This does not arise in the TS of endo cycloadduct of vinyl benzoate 2b since the benzoate group can swing away from the bromine in the transition state. Moreover, the endo predomination in the cycloaddition of 2-propenenitrile (2d) is attributed to secondary orbital interactions and therefore it was not expected that cycloaddition to the bromo- 2H-pyran-2-ones 1a–c would give an endo to exo ratio of nearly one. As can be seen in Table I and Scheme 1, it is possible to correlate the cal- culated energy of the transition state to the final yield of the cycloadducts 3–13. The calculated values of all transition states confirmed that the ones likely to be the most abundant are the 3, 5 and 7 isomers in all cases, which occurred in the DA reactions of 1a–c with 2c. The cycloadditions of 1a–c with 2d in all of reac- tions had the highest relative energy and it was expected to be the most disfav- ored cycloadduct. The results of calculated free activation energies (∆G#) for DA reactions of 3,5-dibromo-2H-pyran-2-one (1c) with 2a, 2b and 2d demonstrate the lowest activation free energy, while an increasing barrier energy has been seen for 1c and 2c.With considering FMO approach (Table II), broadly speaking, 3-bromo, 5-bromo and 3,5-dibromo-2H-pyran-2-one should undergo normal and inverse electron demand cycloadditions with dienophiles bearing weakly electron-donat- ing (1a–c) and electron-withdrawing (2d) substituents, respectively. DFT-based reactivity indices The molecular DFT-based parameters, electronic chemical potential (μ), chemical hardness (η), global electrophilicity (ω) and global nucleophilicity (N) of the reactants 2a–d and 1a–c are displayed in Table II. As can be seen in Table II, the bromo-2H-pyran-2-one derivatives 1a–c are more electrophilic than the dienophiles 2a–d and 3,5-dibromo-2H-pyran-2-one _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1146 HAGHDADI, AMANI and NAB (1c) with the highest electrophilicity (ω = 2.43 eV) being classified as a strong electrophile on the electrophilicity scale.20 On the other hand, 1a has a high nuc- leophilicity index, N = 2.52 eV, and thus is classified as a strong nucleophile on the nucleophilicity scale.21 This ambiphilic behavior is the consequence of the presence of the enone and oxygen atom inside 1a–c. The electronic chemical pot- ential (μ) of the bromo-2H-pyran-2-one derivatives 1a–c are lower than those of the dienophiles, 2a (–0.134), 2b (–0.156) and 2c (–0.130), indicating that charge transfer along the corresponding reactions will occur from the dienophiles 2a–c to the electron deficient dienes 1a–c. While as expected, a CN group (2d) dec- reases the chemical potential and increases the electrophilicity toward the dieno- philes 2a–d, and hence, these results are in agreement with the increase in the activation energy. TABLE II. HOMO and LUMO energies, electronic chemical potential, µ, chemical hardness, η, (all in a.u.), global electrophilicity, ω, and nucleophilicity, N (both in eV), for the reactants obtained at the B3LYP/cc-pVDZ level of theory Species EHOMO ELUMO µ η ω N 1a –0.24366 –0.07747 –0.160 0.166 2.09 2.52 1b –0.24586 –0.07990 –0.162 0.166 2.15 2.40 1c –0.24777 –0.08881 –0.168 0.158 2.43 2.41 2a –0.25309 –0.01470 –0.134 0.238 1.02 2.27 2b –0.25205 –0.06044 –0.156 0.192 1.71 2.29 2c –0.24195 –0.01862 –0.130 0.223 1.03 2.57 2d –0.26633 –0.08384 –0.178 0.182 2.28 1.90 The polar character of a cycloaddition process can be predicted using the electrophilicity difference of the reaction pair, ∆ω.22 In this sense, the electro- philicity differences between the diene 1c and the dienophiles 2a and 2c are about 1.40, indicating a large polar character for these cycloadditions, while the small ∆ω between 1a and 2b (0.38 eV) and between 1b and 2d (0.21 eV) show a low polar character for these cycloaddition reactions. The Parr indices, local electrophilicity indices and local nucleophilicity indices for the atoms C6 and C3 of the pyrones 1a–c, and C7 and C8 of the dienophiles 2a–d are given in Table III (see Scheme 1 for atom numbering). The Parr functions (the electrophilic, kP + , and nucleophilic, kP − ) were computed based on Mulliken atomic spin density analysis. According the Domingo model,15,17 along a polar cycloaddition involving asymmetric reagents, the most favorable reactive channel is that involving the initial two-center interaction between the most electrophilic center (ωk) at the electrophile and the most nucleophilic center (Nk) at the nucleophile. According to this model, in the cycloaddition reactions of 1a–c with dienophiles 2a–d, the most favorable two-center interaction occurs between C6 of the dienes and C8 of _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ THEORETICAL STUDY ON THE DIELS-ALDER REACTION 1147 dienophiles 2a–d, leading to the formation of the 3–13 regioisomers, which is in agreement with the experimental results.7–9 TABLE III. The Parr functions ( k k,P P − + / au), local electrophilicity indices (ωk / eV) and local nucleophilicity (Nk / eV) indices for the C6 and C3 atoms of the pyrones 1a–c and for atoms C7 and C8 of the dienophiles at the reactive sites for the reactants obtained at the B3LYP/cc- -pVDZ level of theory Species k kP − kP + Nk ωk 1a C6 0.188 0.381 0.474 0.795 C3 0.253 0.204 0.639 0.426 1b C6 0.394 0.205 0.945 0.492 C3 0.232 0.264 0.558 0.634 1c C6 0.240 0.384 0.580 0.932 C3 0.188 0.222 0.453 0.540 2a C7 0.181 0.171 0.410 0.174 C8 0.291 0.548 0.660 0.559 2b C7 0.018 0.074 0.040 0.127 C8 0.092 0.357 0.212 0.610 2c C7 0.009 0.037 0.024 0.038 C8 0.019 0.476 0.050 0.490 2d C7 0.219 0.260 0.146 0.594 C8 0.606 0.426 1.152 0.972 CONCLUSIONS DFT computations using the B3LYP functional in conjunction with the cc-pVDZ basis set were used to analyze the outcome of the DA reactions of the bromo-2H-pyran-2-ones 1a–c with some weakly activated and unactivated vinyls. The following conclusions could be inferred from the results of the energies: I. The activation energies associated with the DA reaction of cyclic dienes 1a–c with dienophile 2c is more favorable than those for the reactions with 2a, b and 2d. The low reactivities of the dienophiles in these DA reactions correspond with their nucleophilic character. II. While the DA reactions with 2a–c are exo selective, the reaction with 2d is endo selective. III. 3,5-Dibromo-2H-pyran-2-one is more active than 3- and 5-bromo-2H- -pyran-2-ones, having a lower energy barrier. IV. These DA reactions proceed via a polar, regioselective and highly asyn- chronous process. _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ 1148 HAGHDADI, AMANI and NAB И З В О Д ТЕОРИЈСКА СТУДИЈА ДИЛС–АЛДЕРОВЕ РЕАКЦИЈЕ БРОМО-СУПСТИТУИСАНИХ 2H-ПИРАН-2-ОНА И НЕКИХ СУПСТИТУИСАНИХ ВИНИЛА MINA HAGHDADI, HAMED AMANI и NASIM NAB Department of Chemistry, Islamic Azad University, P. O. Box 755, Babol branch, Babol, Iran Извршено је DFT испитивање реактивности, регио- и стереоселективности Дилс– –Алдерове реакције између 3-бромо, 5-бромо и 3,5-дибромо-2H-пиран-2-она и неких слабо активираних и неактивираних алкена. Истражена су четири могућа реакциона пута, који обухватају формирање мета-, пара-, ендо- и егзо-циклоадукта. Анализа засно- вана на енергији и природним орбиталама показује да је преферирана мета-региоселек- тивност и егзо-реакциони механизам. (Примљено 5. децембра 2014, ревидирано 8. фебруара, прихваћено 8. фебруара 2015) REFERENCES 1. K. Afarinkia, T. D. Nelson, M. V. Viader, G. H. Posner, Tetrahedron 48 (1992) 9111 2. B. T. Woodward, G. H. Posner, Adv. Cycloaddit. 5 (1999) 47 3. a) C.-G. Cho, Y.-W. Kim, Y.-K. Lim, J.-S. Park, H. Lee, S. Koo J. Org. Chem. 67 (2002) 290; b) C.-G. Cho, J.-S. Park, I.-H, Jung, H. Lee, Tetrahedron Lett. 42 (2001) 1065 4. G. H. Posner, T. D. Nelson, C. M. Kinter, K. Afarinkia, Tetrahedron Lett. 32 (1992) 5295 5. K. Afarinkia, G. H. Posner, Tetrahedron Lett. 33 (1992) 7839 6. G. H. Posner, K. Afarinkia, H. Dai, Org. Synth. 73 (1995) 231 7. K. Afarinkia, N. T. Daly, S. Gomez-Farnos, S. Joshi, Tetrahedron Lett. 83 (1997) 2369 8. K. Afarinkia, M. J. Bearpark, A. Ndibwami, J. Org. Chem. 70 (2005) 1122 9. K. Afarinkia, M. J. Bearpark, A. Ndibwami, J. Org. Chem. 68 (2003) 7158 10. Gaussian 09, Revision A, Gaussian, Inc., Wallingford, CT, 2009 11. A. E. Reed, R. B. Weinstock, F. Weinhold, J. Chem. Phys. 83 (1985) 735 12. R. G. Parr, R. G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512 13. R. G. Parr, L. Von Szentpaly, S. Liu, J. Am. Chem. Soc. 121 (1999) 1922 14. R. G. Parr, W. Yang, Density functional theory of atoms and molecules, Oxford University Press, New York, 1989, p 16 15. L. R. Domingo, P. Pérez, J. Org. Chem. 73 (2008) 4615 16. L. R. Domingo, P. Pérez, J. A. Saez, RSC Adv. 3 (2013) 1486 17. L. R. Domingo, M. J. Aurell, P. Pérez, J. Phys. Chem., A 106 (2002) 6871 18. P. Pérez, L. R. Domingo, M. Duque-Noreña, E. Chamorro, J. Mol. Struct.: THEOCHEM 895 (2009) 86 19. K. B. Wiberg, Tetrahedron 24 (1968) 1083 20. L. R. Domingo, M. J. Aurell, P. Perez, R. Contreras, Tetrahedron 58 (2002) 4417 21. I. Kim, K. A. Hoff, E. T. Hessen, T. Haug-Warberg, H. F. Svendsen, Chem. Eng. Sci. 64 (2009) 2027 22. H. Chemouri, S. M. Mekelleche, Int. J. Quantum Chem. 112 (2012) 2294. _________________________________________________________________________________________________________________________ (CC) 2015 SCS. All rights reserved. Available on line at www.shd.org.rs/JSCS/ << /ASCII85EncodePages false /AllowTransparency false /AutoPositionEPSFiles true /AutoRotatePages /None /Binding /Left /CalGrayProfile (Dot Gain 20%) /CalRGBProfile (sRGB IEC61966-2.1) /CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2) /sRGBProfile (sRGB IEC61966-2.1) /CannotEmbedFontPolicy /Error /CompatibilityLevel 1.4 /CompressObjects /Tags /CompressPages true /ConvertImagesToIndexed true /PassThroughJPEGImages true /CreateJobTicket false /DefaultRenderingIntent /Default /DetectBlends true /DetectCurves 0.0000 /ColorConversionStrategy /CMYK /DoThumbnails false /EmbedAllFonts true /EmbedOpenType false /ParseICCProfilesInComments true /EmbedJobOptions true /DSCReportingLevel 0 /EmitDSCWarnings false /EndPage -1 /ImageMemory 1048576 /LockDistillerParams false /MaxSubsetPct 100 /Optimize true /OPM 1 /ParseDSCComments true /ParseDSCCommentsForDocInfo true /PreserveCopyPage true /PreserveDICMYKValues true /PreserveEPSInfo true /PreserveFlatness true /PreserveHalftoneInfo false /PreserveOPIComments true /PreserveOverprintSettings true /StartPage 1 /SubsetFonts true /TransferFunctionInfo /Apply /UCRandBGInfo /Preserve /UsePrologue false /ColorSettingsFile () /AlwaysEmbed [ true ] /NeverEmbed [ true ] /AntiAliasColorImages false /CropColorImages true /ColorImageMinResolution 300 /ColorImageMinResolutionPolicy /OK /DownsampleColorImages true /ColorImageDownsampleType /Bicubic /ColorImageResolution 300 /ColorImageDepth -1 /ColorImageMinDownsampleDepth 1 /ColorImageDownsampleThreshold 1.50000 /EncodeColorImages true /ColorImageFilter /DCTEncode /AutoFilterColorImages true /ColorImageAutoFilterStrategy /JPEG /ColorACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description << /ARA /BGR /CHS /CHT /CZE /DAN /DEU /ESP /ETI /FRA /GRE /HEB /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. Stvoreni PDF dokumenti mogu se otvoriti Acrobat i Adobe Reader 5.0 i kasnijim verzijama.) /HUN /ITA /JPN /KOR /LTH /LVI /NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken die zijn geoptimaliseerd voor prepress-afdrukken van hoge kwaliteit. De gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe Reader 5.0 en hoger.) /NOR /POL /PTB /RUM /RUS /SKY /SLV /SUO /SVE /TUR /UKR /ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.) >> /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ << /AsReaderSpreads false /CropImagesToFrames true /ErrorControl /WarnAndContinue /FlattenerIgnoreSpreadOverrides false /IncludeGuidesGrids false /IncludeNonPrinting false /IncludeSlug false /Namespace [ (Adobe) (InDesign) (4.0) ] /OmitPlacedBitmaps false /OmitPlacedEPS false /OmitPlacedPDF false /SimulateOverprint /Legacy >> << /AddBleedMarks false /AddColorBars false /AddCropMarks false /AddPageInfo false /AddRegMarks false /ConvertColors /ConvertToCMYK /DestinationProfileName () /DestinationProfileSelector /DocumentCMYK /Downsample16BitImages true /FlattenerPreset << /PresetSelector /MediumResolution >> /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ] >> setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice