ARTICLE pubs.acs.org/JPCA
Substituent Effects in Cationπ Interactions: A Unified View from Inductive, Resonance, and Through-Space Effects Fareed Bhasha Sayyed and Cherumuttathu H. Suresh* Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695019, India
bS Supporting Information ABSTRACT: The quantification of inductive (I), resonance (R), and throughspace (TS) effects of a variety of substituents (X) in cationπ interactions of the type C6H5X 3 3 3 Naþ is achieved by modeling C6H5(Φ1)nX 3 3 3 Na þ (1), C6H5(Φ2)nX 3 3 3 Na þ (2), C6H5(Φ2^)nX 3 3 3 Na þ (20 ), and C6H 6 3 3 3 HX 3 3 3 Na þ (3), where Φ1 = CH 2CH2 , Φ2 = CHCH, Φ2^ indicates that Φ2 is perpendicular to the plane of C6H5, and n = 15. The cationπ interaction energies of 1, 2, 20 , and 3, relative to X = H and fitted to polynomial equations in n have been used to extract the substituent effect E01, 2 20 E 0 , E0 , and E03 for n = 0, the C6H 5 X 3 3 3 Na þ systems. E01 is made up of 0 inductive (EI) and through-space (ETS) effects while the difference (E02 E02 ) 3 is purely resonance (ER) and E0 is attributed to the TS contribution (ETS) of the X. The total interaction energy of C6H5X 3 3 3 Naþ is nearly equal to the sum of EI, ER, and ETS, which brings out the unified view of cationπ interaction in terms of I, R, and TS effects. The electron-withdrawing substituents contribute largely by TS effect, whereas the electron-donating substituents contribute mainly by resonance effect to the total cationπ interaction energy.
’ INTRODUCTION Cationπ interactions are recognized as an important noncovalent binding force in structural biology.1 Further, the use of cationπ interactions is significantly growing in the design of molecular materials,2 supramolecular assembly,3 stereoselective synthesis,4 and synthetic receptors.5 The interaction of a cation to the π-system can be greatly affected by the nature of the attached substituent (X) due to inductive (I), resonance (R), and through-space (TS) effects. Therefore, the quantification of such effects in cationπ interactions is highly important to fine-tune the interactive behavior between the π-system and the cation. Hunter et al.6 used a chemical double mutant cycle to quantify the free energy change associated with the interaction between pyridinium cation and substituted aromatic ring and showed that the cationπ binding is sensitive to the electronic nature of the substituent (X). On the basis of the correlation between interaction energy of C6H5X 3 3 3 Naþ systems and the Hammett’s σm constants,7 Dougherty et al.8 reported that the inductive effects are much more relevant than the resonance effects. Recently, Wheeler and Houk9 argued that the substituent effects in C6H5X 3 3 3 Naþ are largely due to through-space interactions between the X and Naþ. Though the recent and prior reports8,9 diversified the concept of substituent effects in cationπ interactions, a challenge still remains to identify the most governing factor among I, R, and TS effects. Since the I, R, and TS effects of the X operate simultaneously in the C6H5X 3 3 3 Naþ, separating these effects into individual contributions is almost impossible. Energy decomposition r 2011 American Chemical Society
analysis (EDA) is often used to extract various energy terms such as electrostatic, induction, and dispersion effects that contribute to the binding of interacting molecules.1012 However, EDA alone is not sufficient to extract simultaneously the substituent contributions of I, R, and TS effects because for each of these effects, varying amounts of the various energy terms may contribute. Very often chemical models have been developed for the characterization and quantification of I, R, and TS effects. Among them, Hammett13 constants (σp and σm) developed from benzoic acid derivatives in the 1937 are the most widely used measure of substituent effects. Later, for a thorough understanding of substituent effects in a variety of chemical structures, inductive (through-bond), resonance, and through-space (sometimes called field effect, F) effects have been quantified in terms of the substituent constants σI, σR, and σF, respectively.7,14,15 Quantum chemically derived structural, electronic, and thermodynamic parameters have also been employed in the study of substituent effects.1417 However, to the best of our knowledge, I, R, and TS effects of X in cationπ interactions are not yet separated and quantified. In this paper, we show that these effects can be separated from one another and can be quantified accurately to arrive at a unified view to the cationπ interactions that agrees with the intuitive classical chemical concepts. Received: March 15, 2011 Revised: April 26, 2011 Published: May 13, 2011 5660
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Table 1. E0 and ΔE0 (in kcal/mol) of C6H5X 3 3 3 Naþ Systems Obtained at the B3LYP/6-311þG(d,p) Level X
E0
ΔE0
NH2
28.74
5.31
OH CH3
23.54 25.14
0.10 1.71
H
23.43
0.00
F
18.47
4.96
Cl
19.11
4.33
CF3
15.72
7.72
CN
12.99
10.44
NO2
10.93
12.50
’ COMPUTATIONAL DETAILS The structures of all the cationπ complexes involved in this study are optimized using the B3LYP method with triple-ζ quality 6-311þG(d,p) basis set. The interaction energies of cationπ complexes are calculated by subtracting the energy of the isolated monomers from its complex. Several substituents, viz., NH2, OH, CH3, H, F, Cl, CF3, CN, and NO2, which covers electron-donating, electron-withdrawing, and hyperconjugative effects are selected for the present study. All the interaction energies obtained at the B3LYP/6-311þG(d,p) level are counterpoise-corrected.18 Further, to examine the effect of different density functionals on the cationπ interaction energies, the C6H5CHCHX 3 3 3 Na þ complexes (X = NH2, OH, CH3, H, F, CN, and NO2) have been studied using B97-D,19 M06-2X, 20 and CAM-B3LYP12 methods with the 6-311þG(d,p) basis set. Moreover, the ab initio MP2/6-311þG(d,p) method is used to evaluate the cationπ interaction energies in C6H5 CHCHX 3 3 3 Na þ complexes. The B97-D functional developed by Grimme19 is based on the generalized gradient approximation (GGA), which includes damped atom-pair-wise dispersion corrections, and M06-2X is a hybrid meta exchange correlation functional developed by Truhlar. 20 All the computational calculations were carried out with Gaussian03/09 suite of programs.21 ’ RESULTS AND DISCUSSION Systems of the type C6H5X 3 3 3 Naþ are prototypical for the study of cationπ interactions.8,22 We use the notation E0 to indicate the cationπ interaction energy of C6H5X 3 3 3 Naþ systems, while the relative E0 with respect to X = H is denoted as ΔE0. For instance, in the case of benzene, aniline, and nitrobenzene, the E0 values are 23.43, 28.74, and 10.93 kcal/mol, respectively. It means that compared to X = H, NH2 group can increase the cationπ binding by 5.31 kcal/mol, while the NO2 group will decrease it by 12.50 kcal/mol. The increase or the decrease in the interaction energy (ΔE0) upon substitution can be attributed solely due to the substituent effect. For all the substituents, the E0 and ΔE0 values are presented in Table 1. In the case of electron-donating (NH2, OH, CH3) groups, ΔE0 is negative in the range from 5.31 to 0.10 kcal/mol, suggesting the enhancement in the cationπ interaction while the ΔE0 is positive for electron-withdrawing (F, Cl, CF3, CN, and NO2) groups in the range from 4.33 to 12.50 kcal/mol, indicating substantial reduction in the cationπ interaction. Thus, it is clear that the cationπ interaction is highly sensitive to the electronic nature of the X. Since the mechanism of electron donation/
Scheme 1. Cationπ Complexes Designed for the Quantification of Inductive (1), Resonance (2, 20 ), and Through-Space (3) Substituent Effects
withdrawal of X depends on the I, R, and TS effects, the quantification of these effects is essential for a comprehensive understanding of the nature of cationπ interactions in chemistry and biology. Herein we propose the use of four different chemical models that are appealing to the intuitive definitions of the I, R, and TS effects for the study of cationπ interactions. These four models, viz., 1, 2, 20 , and 3, are presented in Scheme 1. Model 1 is C6H5(Φ1)nX 3 3 3 Naþ (n = 15) wherein Φ1 is the CH2CH2 moiety. In 1, the CC single bond of Φ1 ensures that only the I and TS effects of the X are transmitted to the π-ring.7,23 Therefore, the cationπ interaction energy of 1 (En1) for a given X relative to X = H can be attributed to the sum of the I and TS effects of that X for the corresponding n. In the case of X = NH2, E11 = 0.29, E21 = 0.11, E31 = 0.06, E41 = 0.01, and E51 = 0.14 kcal/mol, while for X = NO2, E11 = 7.42, E21 = 4.81, E31 = 3.50, E41 = 2.05, and E51 = 1.61 kcal/mol (Supporting Information, Table S2), indicating that substituent effect progressively falls off with the increase in n. For instance, in the case of NO2, a 78.3% reduction of its contribution to the cationπ interaction can be seen from n = 1 to n = 5. For any X, a plot of En1 against n can be fitted almost perfectly by a second-order polynomial equation (Supporting Information, Figure S3). Since n = 0 for C6H5X 3 3 3 Naþ, the polynomial equation can be extrapolated to n = 0 to obtain the substituent effect, which is the sum of the I and the TS contributions (abbreviated as EIþTS). The EIþTS values for all the systems are presented in Table 2. Model 2 is C6H5(Φ2)nX 3 3 3 Naþ (n = 15), wherein the Φ2 is the CHCH group. The conjugated alkenyl chain ensures the transmission of the resonance effect of X to the π-system along with the inductive and TS effects. Model 20 [abbreviated as C6H5(Φ2^)nX 3 3 3 Naþ] is derived from 2 by restricting the orientation of the plane of the phenyl group in a position orthogonal to the plane of the substituted alkenyl unit, and this will prohibit the resonance between Φ2X and C6H5. 24 For a given n, the distance between the X and Naþ (throughspace distance) in 2 is ∼0.686 Å lower than that of 20 . Further, for a given n, the total CC bond distance of Φ2 in 2 (through-bond distance) is ∼0.035 Å higher than the CC bond distance of Φ2^ in 20 . Therefore, we may assume that the inductive and TS contributions of X in 2 and 20 are nearly identical.24 In other 5661
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Table 2. Relative Contributions of Inductive (EI), ThroughSpace (ETS), and Resonance Effects (ER) of C6H5X 3 3 3 Naþ Systems (all values in kcal/mol)
a
X
EIþTS
NH2
EI
ETS
ER
ETotala
0.49
0.98
0.49
3.96
4.45
OH
2.39
3.09
5.48
2.49
0.10
CH3
0.48
0.22
0.70
0.41
0.89
H
0.00
0.00
0.00
0.00
0.00
F Cl
6.46 6.34
0.77 1.55
7.23 4.79
1.32 0.95
5.14 5.39
CF3
7.49
1.47
6.02
0.74
8.23
CN
10.21
1.97
8.24
0.76
10.97
NO2
10.29
1.32
8.97
1.70
11.99
ETotal = EI þ ETS þ ER. 0
words, for a given n, the quantity (En2 En2 ) brings out the pure resonance 0effect of X in the cationπ interaction. By plotting (En2 En2 ) against n for n = 15 and extrapolating to n = 0 via a polynomial equation (Supporting Information, Figure S4), the resonance contribution of X in C6H5X 3 3 3 Naþ (designated as ER) can be obtained (Supporting Information, Table S5). The values of ER are presented in Table 2. The model 3 is used for the quantification of the TS effect, which is derived from 2 by chopping off the spacer unit between the phenyl ring and the X and subsequently adding “H” atom to the free valency created on the phenyl carbon and the X. The position of the newly added hydrogens are optimized by freezing all other atoms. The procedure used herein to develop 3 is similar to a model recently introduced by Wheeler and Houk9 for the determination of the TS effect of substituents in cationπ interactions. In their model, the X is connected directly on the aromatic ring, while in our case we use the spacer Φ2 to include distance dependency. Since the through-bond interaction between X and phenyl ring is absent in 3, the cationπ interaction energy for 3 (En3) will be affected only by the TS effect of the X.25 En3 can be fitted using a polynomial equation in n, which upon extrapolation to n = 0 (Supporting Information, Figure S5) will yield the TS effect of the X, termed as ETS (Table 2). By subtracting ETS from EIþTS, the through-bond inductive contribution, EI, can be obtained (Table 2). Figure 1 depicts the correlation plots of (EIþTS, σI), (ER, σR), and (ETS, σF). It is gratifying that all these plots are linear, which means that the computed energy terms serve as a good measure of the I, R, and TS substituent effects in C6H5X 3 3 3 Naþ. It may be noted that in the original definition of σI, the inductive and TS effects are combined and therefore only EIþTS can be correlated with σI and not the EI.7,25 According to IUPAC, “the field effect symbolized by F is an experimentally observable effect (on reaction rates, etc.) of intramolecular Coulombic interaction between the centre of interest and a remote unipole or dipole, by direct action through space rather than through bonds”. Hence, σF, the substituent field effect constant, is the right choice to understand the use of ETS in measuring the TS effect (Figure 1c). The data presented in Table 2 allow a comprehensive understanding of the substituent effects in cationπ interactions through the quantification of chemically intuitive concepts. The OH group shows the highest inductive effect, EI = 3.09 kcal/mol, and the rest can be grouped in the range from 0.98 to 1.97 kcal/mol. CH3 showed an enhancement in the cationπ interaction energy due to
Figure 1. Correlation between the inductive, resonance, and throughspace contributions of the total cationπ interaction energy of C6H5X 3 þ 3 3 Na with substituent constants: (a) EIþTS vs σI, (b) ER vs σR, and (c) ETS vs σF.
the TS effect, while all the other substituents decrease the interaction energy through the TS effect. The TS effect of NH2 is only 0.49 kcal/mol, whereas the OH group has a high value of 5.48 kcal/mol. All the electron-withdrawing groups showed high values of TS effect in the range from 4.79 to 8.97 kcal/mol. In fact, the TS contribution to the total effect is 78.0%, 78.9%, and 71.8% for CF3, CN, and NO2 substituted systems, respectively. From the ER values, it can be seen that the substituents bearing a lone pair of electrons, viz., NH2, OH, F, Cl, and hyperconjugative CH3 will enhance the cationπ interaction through the resonance effect. The electron-withdrawing groups CF3, CN, and NO2 can decrease the interaction energy through resonance by a small amount in the range from 0.74 to 1.70 kcal/mol. In the case of NH2, a remarkable resonance effect of 74.6% accounts to the total cationπ interaction, while the TS effect accounts only 9.2%. Interestingly, the electron-donating OH has negligible influence on the cationπ interaction when compared to benzene;1 the reason can be attributed to the near cancellation of the destabilizing TS effect (5.48 kcal/mol) and the stabilizing resonance and inductive effects (total 5.58 kcal/mol) (Table 2). In the case of electron-withdrawing CF3, CN, and NO2 substituted systems, the decrease in the E0 due to resonance is only 9.6%, 7.3%, and 13.6%, respectively. Thus, the models shown in Scheme 1 satisfy the criteria not only for the accurate quantification of inductive, resonance, and through5662
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space effects but also allow one to understand the most intriguing interplay of electronic features of X in cationπ interactions. We also define a quantity ETotal, the sum of EI, ER, and ETS (Table 2), to show the merit of the proposed approach of factoring the total cationπ interaction energy into I, R, and TS contributions. Surprisingly, the ETotal is nearly equal to E0 and the correlation plot of (ETotal, ΔE0) shown in Figure 2 indeed proves it (slope = 1.01). Hence, we can conclude that the substituent effects in cationπ interactions are primarily due to the interplay of I, R, and TS effects of the substituent. In order to assess the performance of various DFT methods and also the MP2 method in deriving the substituent effect, the cationπ interaction energies of 2 and 20 for n = 1 are calculated at B3LYP, CAM-B3LYP, M06-2X, B97-D, and MP2 levels (Table 3). In many computational studies, MP2 is used as an
affordable ab initio method to0 assess the quality of a DFT result. For the ordered pair (E12, E12 ), using the MP2 data as standard, the mean absolute deviation (MAD) in kcal/mol is found to be (0.38, 0.54) for B3LYP, (1.61, 1.88) for CAM-B3LYP, (2.84, 2.98) for M06-2X, and (9.18, 9.14) for B97-D. It means that B3LYP showed the best agreement to the MP2 data. Compared to MP2, DFT methods overestimate both E12 and 20 E1 and this overestimation is ∼2.6%, ∼8.0%, ∼12.9%, and ∼39.1%, respectively for B3LYP, CAM-B3LYP, M06-2X, and B97-D. The substantially high value of the cationπ interaction energy at the B97-D method is quite surprising, because the dispersion effect is expected to play only a minor (6%) role while the major contribution is due to the electrostatic interaction (∼67%).10 Further, in Table 4, the relative interaction energies ΔE12 and 20 ΔE1 of 2 and 20 systems with respect to X = H are shown. These values correspond to the total substituent effect of X. Surprisingly, all the DFT methods show nearly same values of substituent effect. The MP2 results also show good agreement with the DFT results, but the variation is slightly larger than that between any two DFT methods. For example, the contributions of (NH2, NO2) in kcal/mol toward E12 are (7.71, 10.48) at B3LYP, (7.22, 10.28) at CAM-B3LYP, (7.07, 10.39) at M06-2X, (8.20, 10.34) at B97-D, and (5.60, 7.88) at MP2. Moreover, the correlation matrix obtained (Supporting Information, Table S7) for all these methods suggests a correlation coefficient in the range from 0.995 to 1.000 for the data from any two methods. Thus, we can conclude that the individual substituent effect obtained using any of the DFT method or the MP2 method is accurate, as they do not vary significantly with respect
Figure 2. Correlation between the total substituent effect and the sum of the inductive, resonance, and through-space contributions.
0
Table 3. Cationπ Interaction Energies of C6H5(Φ2)1X 3 3 3 Naþ (E12) and C6H5(Φ2)1X 3 3 3 Naþ (E12 ) Systems (all values in kcal/mol) 0
E12 X
a
Aa
Bb
Cc
E12 Dd
Ee
Aa
Bb
Cc
Dd
Ee
NH2
33.00
33.90
35.06
42.05
30.56
29.19
30.61
31.58
37.78
27.66
OH CH3
27.40 27.61
28.40 28.89
29.57 30.16
36.42 36.32
26.01 26.65
25.00 26.67
26.31 28.06
27.44 29.20
33.64 35.17
24.09 25.70
H
25.29
26.68
27.99
33.85
24.96
24.88
26.30
27.47
33.32
24.23
F
23.10
24.27
25.56
32.00
22.64
21.90
23.24
24.43
30.54
21.52
CN
17.00
18.32
19.55
25.69
17.68
17.54
18.79
19.88
26.11
17.55
NO2
14.81
16.40
17.60
23.51
17.08
16.02
17.29
18.31
24.83
16.69
B3LYP results. b CAM-B3LYP results. c M06-2X results. d B97-D results. e MP2 results. 0
Table 4. Cationπ Interaction Energies of C6H5(Φ2)1X 3 3 3 Naþ (ΔE12) and C6H5(Φ2)1X 3 3 3 Naþ (ΔE12 ) Systems Relative to X = H System (all values in kcal/mol) 0
ΔE12 X
Bb
Cc
Dd
Ee
Aa
Bb
Cc
Dd
Ee 3.43
NH2
7.71
7.22
7.07
8.20
5.60
4.11
4.31
4.31
4.46
OH
2.11
1.72
1.58
2.57
1.05
0.03
0.12
0.01
0.32
0.14
CH3
2.32
2.21
2.17
2.47
1.69
1.73
1.79
1.76
1.85
1.47
H
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
F
2.19
2.41
2.43
1.85
2.32
3.04
2.98
3.06
2.78
2.71
8.29 10.48
8.36 10.28
8.44 10.39
8.16 10.34
7.28 7.88
7.59 9.16
7.34 8.86
7.51 9.01
7.21 8.49
6.68 7.54
CN NO2 a
Aa
ΔE12
B3LYP results. b CAM-B3LYP results. c M06-2X results. d B97-D results. e MP2 results. 5663
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The Journal of Physical Chemistry A to the applied methodology. Hence, we believe that the relationship given in Figure 2, ΔE0 ≈ EI þ ER þ ETS, is reliable.
’ CONCLUSIONS We have showed that the quantification of inductive (I), resonance (R), and through-space (TS) effects of a variety of substituents (X) in cationπ interactions of the type C6H5X 3 3 3 Naþ can be achieved by modeling 1, 2, 20 , and 3 systems and by applying an extrapolation procedure to the corresponding interaction energies. The extrapolation procedure avoids the direct interaction of the X with the aromatic π-system and facilitates the computation of the classical effects, such as inductive, resonance, and through-space effects. The electron-withdrawing substituents contribute largely through TS effect, whereas the electrondonating substituents give more resonance contribution to the total interaction energy. We have also confirmed that the computed substituent effect show only minor variation with respect to the choice of the method. The most unique result is that the total interaction energy of C6H5X 3 3 3 Naþ is a sum of EI, ER, and ETS, which brings out the unified view that the cationπ interactions are controlled by the combined effect of I, R, and TS of the substituents and not solely by any one of them. ’ ASSOCIATED CONTENT
bS
Supporting Information. Computational details and Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
ARTICLE
(15) Suresh, C. H.; Alexander, P.; Vijayalakshmi, K. P.; Sajith, P. K.; Gadre, S. R. Phys. Chem. Chem. Phys. 2008, 10, 6492–6499. (16) Exner, O.; B€ ohm, S. J. Phys. Org. Chem. 2007, 20, 454–462. (17) Suresh, C. H.; Gadre, S. R. J. Am. Chem. Soc. 1998, 120, 7049–7055. (18) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553–566. (19) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799. (20) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J., T; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision E.01; Gaussian Inc.: Wallingford CT, 2004. (22) Reddy, S. A.; Sastry, G. N. J. Phys. Chem. A 2005, 109, 8893–8903. (23) Nolan, E. M; Linck, R. G. J. Am. Chem. Soc. 2000, 122, 11497–11506. (24) Silva, P. J. J. Org. Chem. 2009, 74, 914–916. (25) Sayyed, F. B.; Suresh, C. H.; Gadre, S. R. J. Phys. Chem. A 2010, 114, 12330–12333.
Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This research work is supported by the Council of Scientific and Industrial Research (CSIR), Government of India. F.B.S. is thankful to CSIR for a senior research fellowship. ’ REFERENCES (1) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303–1324. (2) Mo, G. C. H.; Yip, C. M. Langmuir 2009, 25, 10719–10729. (3) Hamada, F.; Higuchi, Y.; Kondo, Y.; Kabuto, C.; Iki, N. Tetrahedron Lett. 2006, 47, 5591–5593. (4) Yamada, S. Org. Biomol. Chem. 2007, 5, 2903–2912. (5) Meadows, E. S.; De Wall, S. L.; Barbour, L. J.; Gokel, G. W. J. Am. Chem. Soc. 2001, 123, 3092–3107. (6) Hunter, C. A.; Low, C. M. R.; Rotger, C.; Vinter, J. G.; Zonta, C. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4873–4876. (7) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165–195. (8) Mecozzi, S.; West, A. P., Jr.; Dougherty, D. A. J. Am. Chem. Soc. 1996, 118, 2307–2308. (9) Wheeler, S. E.; Houk, K. N. J. Am. Chem. Soc. 2009, 131, 3126–3127. (10) Soteras, L.; Orozco, M.; Luque, F. J. Phys. Chem. Chem. Phys. 2008, 10, 2616–2624. (11) Frenking, G.; Frohlich, N. Chem. Rev. 2000, 100, 717–774. (12) Yanai, T.; Tew, D. P.; Handy, N. C. Chem. Phys. Lett. 2004, 393, 51–57. (13) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96–103. (14) Sayyed, F. B.; Suresh, C. H. Tetrahedron Lett. 2009, 50, 7351–7354. 5664
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