Article Cite This: J. Phys. Chem. A 2017, 121, 8247-8250
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Chiroptical Properties of Imines Derived from R‑(+)-Norbornenone: The Role of Electronegativity Differences Kenneth B. Wiberg*,∥ Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States S Supporting Information *
ABSTRACT: To allow a comparison with the specific rotations of R-(+)-5methylenenorbornene (1) and R-(+)-norbornenone (2) we performed calculations at the LC-wPBE/aug-cc-pVTZ level for the imines (5a and 5b) derived from norbornenone and also for their protonated derivative (6). In accord with our results for simpler systems, the specific rotations increase in the order of 1 < 5 < 2 ≈ 6. In addition, the specific rotation of the protonated ketone was calculated and found to be considerably larger than that for 2 or 6. These rotations were found to be linearly dependent on the Hirshfeld charges at the carbon of the exocyclic double bond. This leads to the conclusion that charge transfer from the endocyclic double bond to the π* MO of the exocyclic double bond is an important component of the process that leads to the optical activity of these compounds.
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INTRODUCTION In 1962 Mislow and Berger1 reported that S-(−)-norbornenone (1) had a remarkably large specific rotation, [α]D = −1233 deg dm−1 (g/cm3)−1 in isooctane solution. Subsequently, Stephens et al.2 calculated the rotation using B3LYP/ag-cc-pVTZ and found [α]D = −1146, in remarkably good agreement with the experimental value. However, when Lahiri et al.3 measured the rotation of R-(+)-norbornenone in the gas phase they found a considerably smaller value, [α]D = 755, the earlier larger value being a result of a large solvent effect. Norbornenone has been the subject of several theoretical studies,4−6 and there seems to be common agreement that the large rotation is associated with a magnetic transition dipole involving the lone pairs on oxygen. It is, of course, well-known that, for a transition to lead to optical activity, both electric and magnetic transition dipoles are needed.7 In the preceding paper, we found that the LC-wPBE density functional8 was quite effective in reproducing the observed rotations, and it will be used in the present study. To explore the role of lone pairs, in the preceding paper we examined R-(+)-5-methylenenorbornene (1)9 and found that it also had a large rotation, [α]D = 345, approximately half as large as that for 2. This suggested that the lone pairs for 2 may not be so significant in determining the specific rotation. What is the origin of the magnetic transition moment of 1?
examination of the rotamer of 3 that led to the largest specific rotation led to the conclusion that its magnetic transition moment arises from the ∼45° dihedral angle between the p-π orbitals of two CH2CH− groups in this rotamer. If it were 0° there could only be an electric transition dipole, and if it were 90° there could only be a magnetic transition dipole. At an intermediate angle, there could be both as required for an optical rotation. This also applies to 1. An examination of the corresponding imine, CH2CH− CH2−CHNH, found that the rotamer having the largest specific rotation gave a rotation between that of 1 and 2. Further, protonation at N, eliminating the lone pair, led to a marked increase in the calculated rotation. This suggested that the major part of the specific rotation was not a result of coupling with an n-π* transition but rather due to the electronegativity of the terminal atom in the CH2CH−CH2− CHX system. This could lower the energy of the −CHCX part permitting an increased charge transfer from the CC double bond to the π* molecular orbital (MO) of the CH CHX double bond.
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RESULTS AND DISCUSSION These were computational results for simple model systems. Would they carry over to the norbornenyl system? In accord with the suggestion of Moore, Srebro, and Autschbach4 we used a density functional theory (DFT) model that included long-range correlation, LC-wPBE.8 It was found to give rotations close to the gas-phase values for both 1 and 2, and
In a computational study10 of the chirality of twisted unconjugated dienes such as CH2CH−CH2−CHCH2 (3) and CH2CH−CH2−CHO (4) we found some of their rotamers to also have large calculated specific rotations. An © 2017 American Chemical Society
Received: August 18, 2017 Revised: September 17, 2017 Published: October 3, 2017 8247
DOI: 10.1021/acs.jpca.7b08275 J. Phys. Chem. A 2017, 121, 8247−8250
Article
The Journal of Physical Chemistry A therefore it would be reasonable to expect that it would also be successful with the imines. We examined norbornenonimine (5) by tight optimization11 at the B3LYP/aug-cc-pVTZ level, followed by calculation of the specific rotation using LC-wPBE/aug-cc-pVTZ. There are two possible conformations for 5, 5a, and 5b. The former had a slightly higher energy than the latter (ΔG = 0.18 kcal/mol).11 The calculated specific rotation for 5a was [α]D = 615, whereas that for 5b was [α]D = 440. Rotations at other wavelengths along with the corresponding B3LYP-derived rotations may be found in the Supporting Information. These rotations are between those for 1 and 2. Both imines give the same compound, 6, on protonation with [α]D = 767. The calculated rotation for 6 is approximately the same as that for 2 (770) showing that the order of increasing specific rotation is 1 < 5 < 2 ≈ 6, the order of increasing electronegativity of the exocyclic substituent.
Figure 1. Relationship between specific rotation and the Hirshfeld charge at the exocyclic carbon.
gives a lager contribution to the optical activity than that of the oxygen one pair. Also note that the optical rotation is a response property, and therefore chemical trends in the electronic ground state are not directly related to optical activity. Possible Experimental Tests of the above Conclusions. In the cases of 1 and 2, it is found that the specific rotation is approximately the same for both the LC-wPBE calculations and experiments. It would be desirable to have experimental confirmation of the computational results for 5 and 6. The imine, 5, would not be expected to be stable,13 but it is more likely that imines formed from primary amines could be formed and would be stable. To facilitate such a study, we calculated the geometries, vibrational frequencies, and the specific rotations of the imines formed using methylamine (8a and 8b) or t-butylamine (10a and 10b). The latter imine would be the more easily prepared, since methylamine is a gas, but the t-butyl group might lead to a steric interaction. There does appear to be such an interaction, since for the methyl imine pair (8) the difference in ΔG between the two rotamers was 1.08 kcal/mol, and for the t-butyl imine pair (10) the difference in ΔG = 1.85 kcal/mol.11 In both cases the “b” form had the lower energy. In addition we examined their protonated forms, 9a, 9b and 11a, 11b. For the 9 forms, the difference in ΔG increased to 0.7 kcal/mol, and for the 11 forms, the difference increased to 1.5 kcal/mol. All of these data may be found in the Supporting Information.
It seemed worthwhile to try to extend the electronegativity effect, and therefore protonated norbornenone was examined. As with the imine, there are two possible rotamers, 7a and 7b. The calculated rotation for 7a was [α]D = 1136, and that for 7b was 1099. These rotations are significantly larger than that for 2. Why should the electronegativity have such a large effect on the optical rotation? One effect of increasing electronegativity of the exocyclic group should be an increase in the charge at the Table 1. Calculated Specific Rotation and Charges on the Exocyclic Carbons compound
[α]D
charge, e
CH2 = norbornene (1) HN = norbornene (5a) HN = norbornene (5b) O = norbornene 2 H2N = norbornene 6 HO = norbornene (7a) HO = norbornene (7b)
404.9 614.5 439.5 770.3 767.0 1136.1 1098.7
0.0036 0.0935 0.0934 0.1701 0.1914 0.2472 0.2492
adjacent carbon. This was examined by calculating the Hirshfeld charges,12 and the data are shown in Table 1.
Obviously, there is a similar trend in the second and third columns. This is more conveniently examined in Figure 1. It can be seen that there is a fairly good linear relationship between these data. This result, along with our previous observation10 that changes in nonbonded distance between the nearest carbons of the two double bonds lead to a linear relationship between the distance and the calculated rotation, strongly suggests that the optical rotations involve charge transfer from the endocyclic double bond to the π* MO of the exocyclic double bonds. This conclusion is also in accord with Figure 5 of ref 4, which shows CC bond of norbornenone
The calculated rotations of all of these compounds are summarized in Table 2. In each case the b conformation has the lower energy, and the rotation for the protonated imines are larger than that for the imines themselves. However, the 8248
DOI: 10.1021/acs.jpca.7b08275 J. Phys. Chem. A 2017, 121, 8247−8250
Article
The Journal of Physical Chemistry A
Increasing electronegativity of X leads to increased specific rotations. The rotation is also dependent on the distance between the double bonds; decreasing distance leads to an increase in rotation. This strongly suggests that the specific rotations are related to charge transfer between the endocyclic double bond and the π* MO of the exocyclic double bond. Since norbornanonimine would not be expected to be stable, some substituted imines also were examined. The most favorable case for an experimental study appears to be the oxime derived from norbornenone, for which a quite large increase in specific rotation on protonation would be expected.
Table 2. Calculated LC-wPBE/aug-cc-pVTZ Specific Rotations ([α]D) of Norbornenone Derived Imines compound
imine
prot imine
5a 5b 8a 8b 10a 10b
615 440 718 441 657 345
767 767 794 637 650 510
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CALCULATIONS All of the calculations were performed using Gaussian.15
increase in rotation is smaller than for the simple imine (5), and in the case of 10a, there is essentially no increase. These results were interesting but not entirely satisfactory, because the “a” forms are predicted to have little change in specific rotation on protonation, and with the b form changes on protonation decrease with the larger alkyl group substituent. However, there was another possibility, the oxime derived from norbonornenone. It has been prepared in racemic form by the reaction of racemic norbornenone with hydroxylamine, and it is a crystalline solid.14 Geometry optimization and specific rotation calculations were performed for both the 12a and 12b conformers of the oximes as described above. Here again, the b form had the lower energy (ΔG = 1.2 kcal/mol). The protonated oximes (13a and 13b) also were studied. It is known that oximes undergo a Beckman rearrangement in acid media,14 but this requires strong acid and presumably involves diprotonation. This should not be a problem using a
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b08275. Tables of calculated energies, structures, and specific rotations for all of the imines in this report (PDF)
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oxime
protonated oxime
12a 12b
552 282
913 580
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kenneth B. Wiberg: 0000-0001-8588-9854 Present Address ∥
Present address: 865 Central Avenue, Apt. A404, Needham, MA 02492
Table 3. Computed Specific Rotations for the Oximes compound
ASSOCIATED CONTENT
S Supporting Information *
Notes
The author declares no competing financial interest.
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REFERENCES
(1) Mislow, K.; Berger, J. G. Dyssemetric Non-Conjugated Chromophores − Optical Rotatory Dispersion of Bicyclo[2.2.1]hept5-en-2-one and Bicyclo[2.2.2]octe-5-en-2-one. J. Am. Chem. Soc. 1962, 84, 1956−1961. (2) Stephens, F. J.; Devlin, F. J.; Cheeseman, J. R.; Frisch, M. J. Calculation of Optical Rotation using Density Functional Theory. J. Phys. Chem. A 2001, 105, 5356−5371. (3) Lahiri, P.; Wiberg, K. B.; Vaccaro, P. H.; Caricato, M.; Crawford, T. D. Large Solvation Effect on the Optical Rotatory Dispersion of Norbornenone. Angew. Chem., Int. Ed. 2014, 53, 1386−1389. (4) Moore, B., II; Srebro, M.; Autschbach, J. Analysis of Optical Activity in Terms of Bonds and Lone Pairs: The Exceptionally Large Optical Rotation of Norbornenone,. J. Chem. Theory Comput. 2012, 8, 4336−4346. (5) Caricato, M. Orbital Analysis of Molecular Optical Activity Based on Configuration Rotatory Strength. J. Chem. Theory Comput. 2015, 11, 1349−1353. (6) Mach, T. J.; Crawford, T. D. Basis Set Dependence of Coupled Cluster Optical Rotation Computation. J. Phys. Chem. A 2011, 115, 10045−10051. (7) Rosenfeld, L. Quantum Mechanical Theory of Natural Optical Activity of Liquids and Gases. Eur. Phys. J. A 1929, 52, 167−174. (8) Tawada, Y.; Tsuneda, T.; Yanagisawa, S.; et al. A Long-rangecorrected time-dependent Density Functional Theory. J. Chem. Phys. 2004, 120, 8425−8433. Iikura, H.; Tsuneda, T.; Yanai, T.; Hirao, K. A Long-range Correlation Scheme for Generalized-gradient-approximation Exchange Functionals. J. Chem. Phys. 2001, 115, 3540−3544. Chai, J.-D.; Head-Gordon, M. Systematic Optimization of Long-range Exchange Correlated Hybrid Functionals. J. Chem. Phys. 2008, 128, 084106.
stoichiometric amount of an acid in a dilute solution. The results of the calculations are shown in Table 3.
The oximes appear to be an ideal case for further study, because the predicted change in specific rotation on protonation is quite large. Unfortunately, the present author is no longer in a position to perform the needed experiments, and these results are presented so that some other investigator may choose to do so.
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SUMMARY The magnitude of the specific rotation derived from two double bonds, namely, CC and CX, joined by a methylene group is dependent on the substituent (X) attached to one of them. 8249
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The Journal of Physical Chemistry A (9) Lahiri, P.; Wiberg, K. B.; Vaccaro, P. H. Dispersive Optical Activity of R-Methylene-norbornene: Intrinsic Response and Solvation Effects, J. Phys. Chem. A, 2017, 121. 10.1021/acs.jpca.7b08193 (10) Wiberg, K. B. Chirality Induced by the Interaction of CC and CX Bonds Separated by a Methylene Group. J. Phys. Chem. A 2016, 120, 7771−7777. (11) The details of the energy calculations may be found in the Supporting Information. (12) Hirshfeld, F. L. Bonded-Atom Fragments for Describing Molecular Charge-Densities. Theor. Chim. Acta 1977, 44, 129−138. Wiberg, K. B.; Frisch, M. J. Effect of Conjugation on Electron Distributions. Separation of Sigma and Pi Terms. J. Chem. Theory Comput. 2016, 12, 1220−1227. (13) Marsh, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992; p 896. (14) VerHaeghe, D. G.; Weber, G. S.; Pappalardo, P. A. Beckman Fragmentation verses Beckman Rearrangement in Dehydronorcamphor Derivatives. Tetrahedron Lett. 1989, 30, 4041−4044. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, H.; Hratchian, H. P.; Bloino, J.; Janesko, B. G.; Izmaylov, A. F.; Marenich, A.; Lipparini, F.; Zheng, G.; Sonnenberg, J. L.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.; Morokuma, K.; V. Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. T.; Dapprich, S.; Parandekar, P. V.; Mayhill, N. T.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian Development, Version I.11; Gaussian, Inc: Wallingford, CT, 2012.
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DOI: 10.1021/acs.jpca.7b08275 J. Phys. Chem. A 2017, 121, 8247−8250
