Antiaromaticity Proved by the Anisotropic Effect in 1H NMR Spectra

Article pubs.acs.org/JPCA

Antiaromaticity Proved by the Anisotropic Effect in 1H NMR Spectra Erich Kleinpeter* and Andreas Koch Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Potsdam (Golm), Germany S Supporting Information *

ABSTRACT: The spatial magnetic properties (through-space NMR shieldings, or TSNMRSs) of the antiaromatic 9-oxaanthracene anion 12− and of the corresponding 9-dimeric dianion 112− have been calculated by the gauge-invariant atomic orbitals (GIAO) perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSSs) of various size and direction. The TSNMRS values, thus obtained, can be employed to indicate antiaromaticity by paratropic ring currents of the anionic compounds of 112− and 12− studied and other neutral and ionic antiaromatic molecules from previous studies because anisotropic effects of functional groups in 1H NMR spectra have quantitatively proven to be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). antiaromaticity could be qualified above five-membered ring moieties on the surface of the fullerenes23 C50, C60, C606−, C70, and C706−, and the absence of homoantiaromaticity in 3Hcyclonoa[def ]biphenylene 8 was proved by the same approach. However, we never tried to compare the antiaromaticity of different structures employing our approach,3 and because Black et al.28 found different answers concerning the antiaromaticity of 9-oxaanthracene anion 12− and of the corresponding 10-dimeric dianion 112− (Scheme 1) when employing various methods [NICS(1)zz,29 magnetic susceptibility exaltation Λ and average 1H chemical shifts],28 we attempted our method,3 and this more so, as employing proton chemical shifts to quantify (anti)aromaticity proves to be a dubious parameter.30 This is the topic of this paper. Of significant note, though, there have been some recent developments of the NICS index31 showing that, not the average NICS, but only the NICS(1)zz component could rigorously be used to quantify aromaticity,32 and average NICS have proven to not be generally suitable for the quantitative evaluation of aromaticity.33

1. INTRODUCTION The shielding constant at or above the center of aromatic ring systems (a nucleus independent chemical shift, or NICS)1 can be used to characterize the aromaticity of organic compounds. NICS values on a grid around molecules can be calculated to locate the diatropic and paratropic regions of the molecules involved.2,3 These through-space NMR shieldings (TSNMRSs) have been visualized3 as iso-chemical-shielding surfaces (ICSSs) and employed to quantify the anisotropic effects of functional groups (to determine the stereochemisty of nuclei proximal to the functional group).3−16 Even if there are persistent and strong reservations17 to qualifying molecular response properties as the experimentally proven anisotropic effects arising from functional or aromatic groups on the 1H chemical shifts of proximate protons by unobservable quantities such as spacial NICS,18 our results2−16 can serve as definitive proof that TSNMRSs (spatial NICS) successfully assign not only the configuration and diastereoisomerism of structures,2−16 but also the conformational state if the underlying dynamic process is fast on the NMR time scale.19 Thus TSNMRSs visualize and quantify the anisotropic effects of functional groups in NMR spectra, which can be measured experimentally, and can thus be evaluated as the molecular response property of spatial NICS.19 We have also employed TSNMRSs to indicate planar,20,21 spherical22,23 and chelatoaromaticity.24 Antiaromaticity with opposite ICSS (deshielding above/below the plane, shielding in-plane), when employing our approach,3 was indentified in cyclobutadiene derivatives3,22 2 and 4 and pentalene 3,3 respectively, and the partial antiaromaticity of the involved seven-membered ring moieties in fulvenes25 5 and fulvalenes 6 and 7,26 respectively, could be assigned as well. Homoantiaromaticity was proved to exist in the transition states of the ring interconversions of cycloheptatriene 9 and the homotropylium cation 10. 27 Further, by the same method,3 exohedral © 2012 American Chemical Society

2. COMPUTATIONAL DETAILS The quantum chemical calculations were performed using the Gaussian 09 program package.34 The structures of 2−7, 112− and 12− were fully optimized at the MP2/6-311G** level of theory.35 NICS values1 were computed on the basis of MP2/ 6-311G** geometries of 2−7, 112−, and 12− using the gaugeinvariant atomic orbitals (GIAO) method36 at the B3LYP/ 6-31G* theory level.37 To calculate the NICS, ghost atoms were placed on a lattice of −10 Å to +10 Å with a step size of 0.5 Å in the three directions of the Cartesian coordinate system. Received: January 26, 2012 Revised: April 18, 2012 Published: April 18, 2012 5674

dx.doi.org/10.1021/jp300860q | J. Phys. Chem. A 2012, 116, 5674−5680

The Journal of Physical Chemistry A

Article

Scheme 1

The zero points of the coordinate system were positioned at the centers of the structures of 2−7, 112−, and 12−. The resulting 68 921 NICS values, thus obtained, were analyzed and adjusted by the SYBYL 7.3 molecular modeling software;38 different ICSSs of −0.5 ppm (orange) and −0.1 ppm (red) deshielding, and 5 ppm (blue), 2 ppm (cyan), 1 ppm (greenblue), 0.5 ppm (green), and 0.1 ppm (yellow) shielding were used to visualize the TSNMRSs of 2−7, 112−, and 12− in the figures. NMR chemical shifts were calculated by using the GIAO method36 at the same level of theory (reference compound TMS was calculated at the same level); the PCM solvent model39 was employed to consider the corresponding NMR solvents. All calculations were carried out on SGI workstations and LINUX clusters.

Figure 1. Correlation of experimental40 and computed shifts of the carbon atoms of 12−.

13

C chemical

ammonia40 and, therefore, 1H NMR chemical shifts are hardly comparable). 3B. Comparison of Antiaromatic Molecules Employing TSNMRS of 1−10. The TSNMRS visualizations by employing ICSSs of different size and direction of the antiaromatic molecules studied so far3,22,23,25−27 are given in Figure 2. Employing our approach3 concerning antiaromaticity [ICCS(−0.1 ppm), red, from the central point of the coordinate system (in Å) above/below the plane of the antiaromatic ring system], the following consolidated findings can be concluded: (i) The paratropic ring currents in cyclobutadienes 2 and 4, respectively, and in pentalene 3 are significantly different [ICSS (−0.1 ppm)], indicating antiaromaticity: 2 (6.4 Å), 4 (8.0 Å), and 3 (6.0 Å). This results is in complete agreement with the parameter ΣNICS(1)zz/ring area as an excellent measure of (anti)aromaticity.41 (ii) When comparing the paratropic ring currents of [3−7]fulvalene 6 (ICSS (−0.1 ppm) = 6.7 Å),26 with the corresponding heptafulvene 5 (ICSS (−0.1 ppm) = 5.0 Å),25 the former (6a; Scheme 2) is by far more generated and even more than in the 4π antiaromaticity prototypes (cyclobutadienes 2 and 4, respectively).3,22 The cycloheptatrienyl moiety obviously exhibits the tendency to generate paratropic ring currents and therefore antiaromaticity (6a and 5a, respectively) and not diamagnetic ones via useful

3. RESULTS AND DISCUSSION 3A. Structures and 13C NMR Chemical Shifts. The corresponding 13C chemical shifts of the compounds 1−10, already studied previously,3,22,23,25−27 were employed as a quality check for the correctness of the structures obtained theoretically; especially in the case of the homoaromatic compounds, 13C chemical shifts proved to be extremely dependent on even small structural differences;27 1H chemical shifts can be employed as well but prove extremely dependent on the surrounding environment (in the case of 112− and 12− on both charges of the molecules, cations and the extreme solvents that were applied).28 The 13C NMR spectrum of 112− is not published yet, but the corresponding values for 12− are,40 and the correlation δ(exp) versus δ(calc) is quite satisfying (cf. Figure 1). Considering also the good agreement of calculated and experimental 1H chemical shifts of 112−,28 it was concluded that in the case of the anions 112− and 12−, correct geometries have been obtained as well. For the following (3C) calculation of the anisotropic effects on the protons in 1H NMR spectra of the antiaromatic dianion of bixanthylidene 112− with respect to xanthenide anion 12− from TSNMRS, only gas phase structures of 112− and 12− were studied because of the strong media influences on the two proton NMR spectra (112− was studied in deuterated tetrahydrofuran (THF-d8),28 while 12− was studied in liquid 5675



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Figure 2. Structure and TSNMRS (visualized as ICSSs: blue represents 5 ppm shielding, cyan is 2 ppm shielding, green-blue is 1 ppm shielding, green is 0.5 ppm shielding, red represents −0.1 ppm deshielding, and orange is −0.5 ppm deshielding); (1st row) benzene 1,3 cyclobutadiene 2,3 pentalene 3,3,22 and tetratrimethylsilylbutadiene 4;22 (2nd row) heptafulvene 5,25 [3−7]fulvalen 6 and [7,9]fulvalene 7;26 (3rd row): the transition states of the ring interconversions of cyclobutadiene 9 and of the homotropylium cation 10.27

Scheme 2

conjugation (5b and 6b, respectively; cf. Scheme 2). This tendency is strongly strengthened by generation of partial

aromaticity (in the three-membered ring) in [3−7]fulvalene 6 (6a) at the same time (Scheme 2). Shift of 5676



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Figure 3. Structure and TSNMRS (visualized as ICSSs: blue represents 5 ppm shielding, cyan is 2 ppm shielding, green-blue is 1 ppm shielding, green is 0.5 ppm shielding, and red is −0.1 ppm deshielding) of 3H-cyclonona[def ]biphenylene 8 (different views; above)27 and of fullerene C60 [below, closed (left) and open surface (right)].23

π-electron density into the three-membered ring moiety is out of question (6b). Another interesting conclusion comes from Figure 2: These seven-membered ring moieties in 5 and 6 are not completely planar, but are rather bent and still paratropic ring currents, and therefore partial antiaromaticity can develop. The partial antiaromaticity of the cycloheptatrienyl moiety in 5 and 6 goes quickly in [7−9]-fulvalene 7;26 both the seven- and the nine-membered rings are hardly twisted (cf. Figure 2), there is no residual π-shift into the 7-membered ring moiety (7b, cf. Scheme 2), and the cycloheptatrienyl moiety itself is so heavily twisted that paratropic ring currents cannot really develop and the spatial magnetic properties of 7 are determined by the anisotropic effects of the CC double bonds only. (iii) This tendency of the almost planar cycloheptatrienyl moiety is corroborated by the paratropic ring currents of the planar transition states of the ring interconversion process of cycloheptatriene 9 (ICSS (−0.1 ppm) = 5.5 Å) and the homoaromatic analogue, the homotropylium cation 10 (ICSS (−0.1 ppm) = 6.1 Å).27 Near planarity of the seven-membered ring moiety as one requirement for the generation of paratropic ring currents was also found in the structure of the [7− 9]fulvalene in ref 26 (ICSS (−0.1 ppm) = 4.0 Å). The structure in Figure 2, however, is 3.77 kcal mol−1 more stable (when employing the present computation conditions), the seven-membered ring moiety is rather

bent, and the paratropic ring current drops down adequately. (iv) Finally, any expected homoantiaromaticity in 3Hcyclonona[def ]biphenylene 842 was proved to be absent27 (cf. Figure 3; the spatial magnetic properties of 8 are dominated by the ring currents of the two separated benzene moieties with usual shielding above/below the units and deshielding in-plane; a completed paratropic ring current in the nine-membered ring unit in 8 containing the 4n (eight π electrons) is not generated),27 and the main result of the fullerene study23 was that the exohedral antiaromaticity above five-membered ring moieties of the fullerene surfaces23 of C50, C60, C606−, C70, and C706− was relatively stronger than the exohedral aromaticity above the corresponding six-membered ring moieties of these fullerenes (cf. Figure 3). Thus, the indication of antiaromaticity by ICSS of TSNMRS in certain conjugated molecules proved to be possible and applicable for these purposes, as is aromaticity employing the inversed TSNMRS values (shielding above/below the ring system) of aromatic compounds.20 With this optimistic outcome of our previous studies concerning antiaromaticity, we turned toward the anions 112− and 12− employing the same successful approach3 concerning this matter. 3C. Antiaromaticity of the Anions 112− and 12− Employing TSNMRS. Structures of the two anions are given in Figure 4 together with the TSNMR as obtained with our approach.3 The structure of 112− is in complete agreement with ref 28: the two oxaanthracene moieties are twisted by 90° to 5677



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Figure 4. Structure and TSNMRSs (visualized as ICSSs: blue represents 5 ppm shielding, cyan is 2 ppm shielding, green-blue is 1 ppm shielding, green is 0.5 ppm shielding, red is −0.1 ppm deschielding, and orange is −0.5 ppm deshielding) of 9-oxaanthracene anion 12− (above, from various directions and sides) and dioxaanthracene dianion 112− (below), respectively.

Figure 5. Energy minima structures of 9-[o,o′,p-trismethylphenyl]fluorene43 13 (left) and the dixanthylidene dianion 112− (right).

112−, the ortho protons H-1,8 are positioned above the antiaromatic moiety of the second, by the 90° twisted antiaromatic moiety of 112−, about the same position of the ortho methyl protons in the 9-aryl fluorenes 13 studied by us (cf. Figure 5).43 When in 13 these methyl protons are strongly high-field shifted due to the ring current effect of the fluorene moiety, in the present case of 112− it should be strongly low field shifted due to the paratropic ring current effect of the antiaromatic structure fragments in 112−; the following 1H chemical shift differences between 112− and 12− have been computed (cf. Table 1). The difference between two values each proves to be the anisotropic effect of the paratropic ring current of the second antiaromatic oxaanthracene moiety on the protons at the first oxaanthracene part of 112−. The same effect was now computed from the TSNMRS values of one-half of the molecule (employing the TSNMRS of one-half of 112− for it)

each other, and the two via C-9 connected moieties are structurally in complete agreement with those in 12−. As noted,28 both compounds are antiaromatic and, employing our basis approach,3 are similar in antiaromaticity [112− ICSS (−0.1 ppm) = 6.9 Å), 12− ICSS (−0.1 ppm) = 7.5 Å). Concerning aforementioned antiaromatic molecules (2−7, 9, and 10) this property is, employing our approach,3 in 112− and 12− rather large and in line with the parameter ΣNICS(1)zz/ring area as the measure of (anti)aromaticity.41 However, this is a computational result, found out already by Black et al.28 and needs further experimental proof. We identified the experimental anisotropic effects of functional groups in the 1H NMR spectra to quantitatively be the molecular response property of theoretical spatial NICS,19 and the differences between Δδcalc and Δδexp were also quantified as arising from steric compression.43 In the dianion 5678



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Table 1. Theoretical Chemical Shifts of the Protons in 112− and 12− (Quantification of the Anisotropic Effect of the Paratropic Ring Current)

Notes

The authors declare no competing financial interest.

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δ(1H)/ppm

a

proton

112−

12−

Δδ(1H)/ppm

anisotropic effect Δσ/ppma

H-1,8 H-2,7 H-3,6 H-4,5

5.51 5.25 4.23 4.55

4.45 5.42 4.44 4.80

+1.06 −0.16 −0.21 −0.25

−0.71 −0.12 −0.02 −0.005

(1) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; von Ragué Schleyer, P. Chem. Rev. 2005, 105, 3842−3888. (2) von Ragué Schleyer, P.; Manoharan, M.; Wang, Z. X.; Kiran, B.; Jiao, Y.; Puchta, R.; v. E. Hommes, N. J. R. Org. Lett. 2001, 3, 2465− 2468. (3) Klod, S.; Kleinpeter, E. J. Chem. Soc., Perkin Trans. 2 2001, 1893− 1898. (4) Tóth, G.; Kovács, J.; Lévai, A.; Koch, A.; Kleinpeter, E. Magn. Reson. Chem. 2001, 39, 251−258. (5) Kleinpeter, E.; Holzberger, A. Tetrahedron 2001, 57, 6941−6946. (6) Germer, A.; Klod, S.; Peter, M. G.; Kleinpeter, E. J. Mol. Model. 2002, 8, 231−236. (7) Klod, S.; Koch, A.; Kleinpeter, E. J. Chem. Soc., Perkin Trans. 2 2002, 1506−1509. (8) Kovács, J.; Tóth, G.; Simon, A.; Lévai, A.; Koch, A.; Kleinpeter, E. Magn. Reson. Chem. 2003, 41, 193−201. (9) Kleinpeter, E.; Klod, S.; Rudorf, W.-D. J. Org. Chem. 2004, 69, 4317−4329. (10) Kleinpeter, E.; Klod, S. J. Am. Chem. Soc. 2004, 126, 2231−2236. (11) Szatmári, I.; Martinek, T. A.; Lázár, L.; Koch, A.; Kleinpeter, E.; Neuvonen, K.; Fülöp, F. J. Org. Chem. 2004, 69, 3645−3653. (12) Ryppa, C.; Senge, M. O.; Hatscher, S. S.; Kleinpeter, E.; Wacker, Ph.; Schilde, U.; Wiehe, A. Chem.Eur. J. 2005, 11, 2427−2442. (13) Kleinpeter, E.; Schulenburg, A.; Zug, I.; Hartmann, H. J. Org. Chem. 2005, 70, 6592−6602. (14) Kleinpeter, E.; Schulenburg, A. J. Org. Chem. 2006, 71, 3869− 3875. (15) Heydenreich, M.; Koch, A.; Klod, S.; Szatmári, I.; Fülöp, F.; Kleinpeter, E. Tetrahedron 2006, 62, 11081−11089. (16) Kleinpeter, E.; Koch, A.; Sahoo, H. S.; Chand, D. K. Tetrahedron 2008, 64, 5044−5050. (17) (a) Lazzeretti, P. Phys. Chem. Chem. Phys. 2004, 6, 217−223. (b) Pelloni, St.; Lazzeretti, P.; Zanasi, R. J. Phys. Chem. A 2007, 111, 8163−8169. (c) Stanger, A. Chem. Commun. 2009, 1939−1947. (18) (a) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; von Ragué Schleyer, P. Org. Lett. 2006, 8, 863−866. (b) Corminboeuf, C.; Heine, T.; Seifert, G.; von Ragué Schleyer, P.; Weber, J. Phys. Chem. Chem. Phys. 2004, 6, 273−276. (19) Kleinpeter, E.; Lämmermann, A.; Kühn, H. Org. Biomol. Chem. 2011, 9, 1098−1111. (20) Kleinpeter, E.; Klod, S.; Koch, A. J. Mol. Struct. (THEOCHEM) 2007, 811, 45−60 and references therein.. (21) Kleinpeter, E.; Klod, S.; Koch, A. J. Mol. Struct. (THEOCHEM) 2008, 857, 89−94. (22) (a) Kleinpeter, E.; Koch, A.; Shainyan, B. A. J. Mol. Struct. (THEOCHEM) 2008, 863, 117−122. (b) Kleinpeter, E.; Koch, A. J. Mol. Struct. (THEOCHEM) 2008, 851, 313−318. (23) Kleinpeter, E.; Klod, S.; Koch, A. J. Org. Chem. 2008, 73, 1498− 1507. (24) Kleinpeter, E.; Koch, A. Phys. Chem. Chem. Phys. 2011, 13, 20593−20601. (25) Kleinpeter, E.; Fettke, A. Tetrahedron Lett. 2008, 49, 2776− 2781. (26) Kleinpeter, E.; Holzberger, A.; Wacker, Ph. J. Org. Chem. 2008, 73, 56−65. (27) Kleinpeter, E.; Koch, A. Tetrahedron 2009, 65, 5350−5360. (28) Black, M.; Woodford., C.; Mills, N. S. J. Org. Chem. 2011, 76, 2286−2290. (29) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; von Ragué Schleyer, P. Org. Lett. 2006, 8, 863−866. (30) (a) Wannere, C. S.; von Ragué Schleyer, P. Org. Lett. 2003, 5, 605−608. (b) Viglione, R. G.; Zanasi, R.; Lazzeretti, P. Org. Lett. 2004, 6, 2265−2267. (c) Wannere, C. S.; Corimboeuf, C.; Allen, W. A.; Schaefer, H. F., III; von Ragué Schleyer, P. Org. Lett. 2005, 7, 1457− 1460. (d) Faglioni, F.; Ligabue, A.; Pelloni, S.; Soncini, A.; Viglione, R.

Minus for deshielding, plus for shielding.

on the corresponding protons in the second half of the molecule and additionally included in Table 1. When comparing the two columns, there is a margin of error of 0.2 to 0.3 ppm; however, within this uncertainty, deshielding of H-1,8 due to the anisotropic effect of the paratropic ring current of the fixed second xylidene moiety of 0.7 ppm is corroborated by the chemical shift difference of these protons in 112− and 12−, respectively, and this both in size and correct low-field direction. Thus, these two results are congruent: The deshielding anisotropic effect of the paratropic ring current in antiaromatic 112− was proved by chemical shift differences between 112− and 12− and, hereby, both the generation and similarity in antiaromaticity of 112− and 12− is proved as well.

4. SUMMARY AND CONCLUSIONS Previous results of TSNMRS values of antiaromatic molecules, studied so far and applied for the identification of antiaromaticity only,20,22,24−27 are compared with respect to molecular reasons for the antiaromaticity obtained [employing ICSS (0.1 ppm) deshielding in Å above the center of the corresponding molecule]20 and were found to be dependent on 4π/8π antiaromaticity, the relative conjugation of π electrons and antiaromaticity/homoantiaromaticity, respectively. With this encouraging result in hand, we studied, employing the same method,20 the relative antiaromaticity of the oxaanthracene anion 12− and dioxaanthracene dianion 112−, which proved to be unclear,28 employing other precise indicators of (anti)aromaticity.42 The antiaromaticity of one-half of the dianion 112− compared with the one of the anion 12− was found to be consistent with the magnetic susceptibility data of 12− compared with a single ring moiety of 112−.28 Hereby TSNMRS values could be successfully employed for visualization and qualification of antiaromaticity if well-established theoretical parameters of (anti)aromaticity fail. That the results are confident was proved by the anisotropic effect of the paratropic ring current of 112−, which is quantitatively the molecular response property of theoretical spatial NICS.19,43

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

REFERENCES

S Supporting Information *

Absolute energies, Cartesian coordinates of 2−7, 112−, and 12− computed at the MP2/6-311G** level of theory, 1H and 13C chemical shifts of 112− and 12−computed on the basis of MP2/ 6-311G** geometries of 2−7, 112−, and 12− using the GIAO method at the B3LYP/6-31G* theory level. This information is available free of charge via the Internet at http://pubs.acs.org Corresponding Author

*E-mail: [email protected]. 5679



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G.; Ferraro, M. B.; Zanasi, R.; Lazzeretti, P. Org. Lett. 2004, 7, 3457− 3460. (31) Corminboeuf, C.; Heine, T.; Seifert, G.; von Ragué Schleyer, P.; Weber, J. Phys. Chem. Chem. Phys. 2004, 6, 273−276. (32) Stanger, A. Chem.Eur. J. 2006, 12, 2745−2751. (33) (a) Lazzeretti, P. Phys. Chem. Chem. Phys. 2004, 6, 217−223. (b) Pelloni, St.; Lazzeretti, P.; Zanasi, R. J. Phys. Chem. A 2007, 111, 8163−8169. (c) Stanger, A. Chem. Commun. 2009, 1939−1947. (34) 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.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (35) Møller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618−622. (36) Ditchfield, R. Mol. Phys. 1974, 27, 789−807. (37) The lattice points (“ghost atoms”) should be sensor points only without energy contribution in the present calculations. Only if DFT or HF calculations are applied is this true; in the case of electron correlation calculations, the “ghost atoms” get their own electron density and show some influence on the energy of the studied molecule. In these cases, the TSNMRS surfaces are heavily distorted. (38) SYBYL 7.3; Tripos Inc.: St. Louis, MO, 2007. (39) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999−3094. (40) Anastassiou, A. G.; Kasmai, H. S. Angew. Chem., Int. Ed. Engl. 1980, 19, 43−44. (41) Mills, N. S.; Llagostera, K. B. J. Org. Chem. 2007, 72, 9163− 9169. (42) Wilcox, C. F., Jr.; Blain, A. D.; Clardy, J.; van Duyne, G.; Gleiter, R.; Eckert-Maksić, M. J. Am. Chem. Soc. 1986, 108, 7693−7702. (43) Kleinpeter, E.; Koch, A. Tetrahedron 2011, 67, 5740−5743.

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Antiaromaticity Proved by the Anisotropic Effect in 1H NMR Spectra

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