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Fulvalene. Greater Antiaromaticity Than Aromaticity in Comparable Systems Ashley M

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Fulvalene. Greater Antiaromaticity Than Aromaticity in Comparable Systems Ashley M Dianion and Dication of Tetrabenzo[5.7]fulvalene. Greater Antiaromaticity than Aromaticity in Comparable Systems Ashley M. Piekarski,† Nancy S. Mills,* and Abraham Yousef‡ Department of Chemistry, Trinity UniVersity, San Antonio, Texas 78212-7200 Received June 11, 2008; E-mail: [email protected] Abstract: The dianion, 52-, and dication, 52+, of tetrabenzo[5.7]fulvalene represent an aromaticity/ antiaromaticity continuum in which the fluorenyl system changes from aromatic in 52- to antiaromatic in 52+. Conversely, the antiaromatic dibenzotropylium system of 52- becomes an aromatic system in 52+, allowing an examination of aromaticity/antiaromaticity within the same carbon framework. Dianion 52- was prepared and characterized by 1H NMR spectroscopy. The fluorenyl system of 52- showed the downfield shifts expected for an aromatic system, while the dibenzotropylium system showed the paratropic shifts expected for an antiaromatic system. The conclusions from 1H NMR spectroscopy were supported by 1 2- NICS(1)zz calculations for each system. Comparison of the H NMR spectrum and NICS(1)zz of 5 with those of 52+ supported the assignments of aromaticity/antiaromaticity for each system. Aromaticity/ antiaromaticity were further examined through comparison of the degree of bond length alternation, which showed that the bond length alternation was slightly greater for the antiaromatic ring systems than for the aromatic systems. However, when structures of 52- and 52+ with no bond length alternation were examined, there was a dramatic increase in the degree of antiaromaticity for the antiaromatic ring systems as evaluated through NICS. This result suggests that a decrease in bond length alternation results in an increase in antiaromaticity as well as an increase in aromaticity. The magnitude of the antiaromaticity of the fluorenyl system in 52+ was greater than the magnitude of the aromaticity in the fluorenyl system of 52-, with similar effects shown by the analogous tropylium systems. This is consistent with the behavior of the antiaromatic dication of tetrabenzo[5.5]fulvalene, compared to that of its aromatic dianion, and also with the behavior of the cyclopentadienyl cation/anion and tropylium cation/anion. Introduction species to probe the limits of those methods.12-24 Aromaticity 1-10 is often manifested within a very small range of values for The continuing intense interest in aromaticity demon- 1 11 conjugated cyclic hydrocarbons, i.e., 6.9-9.0 ppm for H strates the central nature of the concept in organic chemistry. ∼ The primary controversy in the area involves the questions of NMR shifts for neutral species. Inclusion of ionic species can 25 whether aromaticity can be quantified and, if so, which of the expand the range to 10.13 ppm for some dications and 5.73 methods commonly used to evaluate aromaticity is most ppm for the dilithiated cyclooctatetraene in tetrahydrofuran appropriate.7 We have entered this field through our work with antiaromatic dications, with the intent of using antiaromatic (12) Malandra, J. L.; Mills, N. S.; Kadlecek, D. E.; Lowery, J. A. J. Am. Chem. Soc. 1994, 116, 11622–11624. † Present address: Department of Chemistry and Biochemistry, University (13) Mills, N. S.; Malandra, J. L.; Burns, E. E.; Green, A.; Unruh, K. E.; of California-Santa Barbara, Santa Barbara, CA 93106. Kadlecek, D. E.; Lowery, J. A. J. Org. Chem. 1997, 62, 9318–9322. ‡ Present address: Department of Chemistry, Sweet Briar College, Sweet (14) Mills, N. S.; Burns, E. E.; Hodges, J.; Gibbs, J.; Esparza, E.; Malandra, Briar, VA 24595. J. L.; Koch, J. J. Org. Chem. 1998, 63, 3017–3022. (1) Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, (15) Mills, N. S.; Malinky, T.; Malandra, J. L.; Burns, E. E.; Crossno, P. R.; Schleyer, P. v. R. Org. Lett. 2006, 8, 863–866. J. Org. Chem. 1999, 64, 511–517. (2) Wannere, C. S.; Corminboeuf, C.; Allen, W. D.; Schaefer, H. F.; (16) Mills, N. S. J. Am. Chem. Soc. 1999, 121, 11690–11696. Schleyer, P. v. R. Org. Lett. 2005, 7, 1457–1460. (17) Mills, N. S.; Benish, M. M.; Ybarra, C. J. Org. Chem. 2002, 67, 2003– (3) Faglioni, F.; Ligabue, A.; Pelloni, S.; Soncini, A.; Viglione, R. G.; 2012. Ferraro, M. B.; Zanasi, R.; Lazzeretti, P. Org. Lett. 2005, 7, 3457– (18) Mills, N. S. J. Org. Chem. 2002, 67, 7029–7036. 3460. (19) Levy, A.; Rakowitz, A.; Mills, N. S. J. Org. Chem. 2003, 68, 3990– (4) Krygowski, T. M.; Cyranski, M. K. Phys. Chem. Chem. Phys. 2004, 3998. 6, 249–256. (20) Mills, N. S.; Levy, A.; Plummer, B. F. J. Org. Chem. 2004, 69, 6623– (5) Lazzeretti, P. Phys. Chem. Chem. Phys. 2004, 6, 217–223. 6633. (6) Aihara, J. Bull. Chem. Soc. Jpn. 2004, 77, 2179–2183. (21) Mills, N. S.; Tirla, C.; Benish, M. A.; Rakowitz, A. J.; Bebell, L. M.; (7) Cyranski, M. K.; Krygowski, T. M.; Katritzky, A. R.; Schleyer, P. v. Hurd, C. M. M.; Bria, A. L. M. J. Org. Chem. 2005, 70, 10709– R. J. Org. Chem. 2002, 67, 1333–1338. 10716. (8) Mitchell, R. H. Chem. ReV. 2001, 101, 1301–1316. (22) Mills, N. S.; Benish, M. A. J. Org. Chem. 2006, 71, 2207–2213. (9) Krygowski, T. M.; Cyranski, M. K. Chem. ReV. 2001, 101, 1385– (23) Mills, N. S.; Llagostero, K. B.; Tirla, C.; Gordon, S.; Carpenetti, D. 1419. J. Org. Chem. 2006, 71, 7940–7946. (10) Schleyer, P. v. R. Chem. ReV 2001, 101, 1115–1118. (24) Mills, N. S.; Llagostera, K. B. J. Org. Chem. 2007, 72, 9163–9169. (11) Balaban, A. T.; Oniciu, D.; Katritzky, A. R. Chem. ReV. 2004, 104, (25) Olah, G. A.; Staral, J. S.; Liang, G.; Paquette, L. A.; Melega, W. P.; 2777–2812. Carmody, M. J. J. Am. Chem. Soc. 1977, 99, 3349–3355. 10.1021/ja8042323 CCC: $40.75 © XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000 9 A Published on Web 10/14/2008 ARTICLES Piekarski et al. (THF).26 The inclusion of charged antiaromatic species sub- Scheme 1. Aromaticity/Antiaromaticity for 52+ and 52- stantially increases the range to 4.97 ppm for the dication of tetrabenzo[5.5]fulvalene12 and 2.58 ppm for the dianion of tetrabenzo[5.7]fulvalene, Vide infra. There has been limited use of antiaromatic species to explore aromaticity because the stability of these species has precluded experimental charac- terization. However, the antiaromatic dications we have exam- ined, shown below as 1 and 2, are much more stable than previously assumed. Part of their stability may be due to the fact that, as charged species, they are much less susceptible to dimerization. as a function of charge, with minimal changes to the geometry of the species. The extent of delocalization can be probed through the amount of bond length alternation in the optimized geometry.9,27-29 The evaluation of the aromaticity/antiaromaticity of 52-/52+ is challenging because examination of typical properties that reflect We have documented the antiaromaticity of 1 by using redox the aromaticity of an entire species would not give an accurate potentials to demonstrate the diminished stability of 1 compared 17,21 depiction of the aromaticity/antiaromaticity of individual ring to a non-antiaromatic reference system. Calculated values systems. Thus, evaluation of stability through experimental deter- of nucleus-independent chemical shifts (NICS) and magnetic mination of stability, calculation of aromatic stabilization energies,30 susceptibility exaltation, Λ, also support the categorization of or examination of magnetic susceptibility exaltation, Λ, all of which the fluorenyl system of 1 and the indenyl system of 2 as 16-20,22,23 1 examine the entire system, would be fruitless. The methods that antiaromatic. We have demonstrated paratropic H are suitable to probe local aromaticity include 1H NMR chemical NMR shifts for derivatives of 1 and 2 and have used these 8 31 32-35 1 shifts, NICS, calculated current density maps, and the experimental shifts to document the reliability of calculated H 9,27-29 17,19,21,23 harmonic oscillator measure of aromaticity (HOMA). These NMR shifts. The ability to evaluate the quality of measures all have some shortcomings. 1H NMR chemical shifts calculated properties through comparison with experimental ones 24 are the traditional magnetic measure of aromaticity, but because gives additional validity to calculated properties such as NICS. the chemical shift is affected by factors other than ring current, its We became involved in the preparation of dianions while direct relationship to aromaticity measurements has been recently exploring the aromaticity/antiaromaticity continuum of the questioned.2,3 Current density maps show the direction of the dications and dianions of tetrabenzo[5.5]fulvalene (3).22 The 2- circulation of electron density, indicating aromatic or antiaromatic properties of 3 , generally agreed to be aromatic, provided a ring currents, but normally do not provide quantitative information. context for the properties of 32+. By all measures examined, 2+ 2- We have found that the HOMA method was less sensitive than 3 was slightly more antiaromatic than 3 was aromatic, other measures,17 and therefore less indicative of the degree of which we had not anticipated. This result raised several aromaticity. questions: Was the enhanced antiaromaticity a result of the 2+ NICS values result from the calculated magnetic shielding dicationic nature of 3 ? Did the dication possess more effective tensor for a “dummy” atom located in the middle of the ring delocalization which then enhanced the antiaromaticity of the system under examination.31 Modifications in the calculation species, and is this more prevalent in dications rather than of NICS include positioning the dummy atom 1 Å above the dianions? Does more effective delocalization result in greater plane of the ring to avoid interaction with the electrons of antiaromaticity as well as greater aromaticity? the σ-system, NICS(1),36,37 and using only the component of the tensor perpendicular to the plane of the ring system, 1,38 NICS(1)zz.
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