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The Search for Homoaromatic Semibullvalenes. 5.1 1,5-Dimethyl-2,4,6,8-semibullvalenetetracarboxylic Dianhydride Richard Vaughan Williams,* Vijay R. Gadgil, and Kamlesh Chauhan Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343
Lloyd M. Jackman and Errol Fernandes Department of Chemistry, Pennsylvania State University, 152 Davey Laboratory, University Park, Pennsylvania 16802 Received November 3, 1997
The synthesis and study of a novel bisannelated semibullvalene that is calculated to be a neutral homoaromatic ground state molecule is presented. Through variable-temperature X-ray, 13C solution-phase NMR, and 13C solid-state CP-MAS NMR studies and by a modified Saunders’ isotopic perturbation method, 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride is shown to be a highly fluxional molecule that is not homoaromatic in the solution or solid states. The barrier to the Cope rearrangement of this dianhydride, in solution, is estimated (from 125 MHz 13C NMR data) to be less than 14 kJ mol-1. Introduction The concept of homoaromaticity was advanced more than four decades ago,2 and since that time the area has remained one of intense interest and extensive investigation.3 Despite this extreme activity, there is to date no well-accepted example of an electrically neutral ground state homoaromatic molecule.3,4 The phenomenon is well founded, as many homoaromatic cationic species are known.3 From the results of calculations on the trimerization of acetylene to give benzene, Houk et al. postulated that it was very unlikely that any closed-shell neutral homoaromatic system could ever exist.5 However, recent calculations, at a much higher level of theory, suggest that there is weak (homo)aromatic stabilization of the planar D3h transition state for the trimerization of acetylene to benzene.6 Subsequently, Liebman et al. presented thermochemical evidence for neutral homoaromaticity in triquinacene 1.7 Using similar techniques, Scott et al. have suggested that permethyl [5]pericyclyne 2 is a neutral homoaromatic,8 and Rogers et al. assert that 1,3,5-cycloheptatriene is homoaromatic.9 Roth et al. also conclude from heats of hydrogenation data and (1) (a) Part 1: Williams, R. V.; Kurtz, H. A. J. Org. Chem. 1988, 53, 3626. (b) Part 2: Williams, R. V.; Kurtz, H. A.; Farley, B. Tetrahedron 1988, 44, 7455. (c) Part 3: Williams, R. V.; Kurtz, H. A. J. Chem. Soc., Perkin Trans. 2 1994, 147. (d) Part 4: Willams, R. V.; Gadgil, V. R.; Chauhan, K.; van der Helm, D.; Hossain, M. B.; Jackman, L. M.; Fernandes E. J. Am. Chem. Soc. 1996, 118, 4208. (2) Applequist, D. E.; Roberts, J. D. J. Am. Chem. Soc. 1956, 78, 4012. Doering, W. v. E.; Laber, G.; Vonderwahl, R.; Chamberlain, N. F.; Williams, R. B. J. Am. Chem. Soc. 1956, 78, 5448. Winstein, S. J. Am. Chem. Soc. 1959, 81, 6524. (3) Williams, R. V.; Kurtz, H. A. Adv. Phys. Org. Chem. 1994, 29, 273. (4) Williams, R. V. Adv. Theor. Interesting Mol., in press. (5) Houk, K. N.; Gandour, R. W.; Strozier, R. W.; Rondan, N. G.; Paquette, L. A. J. Am. Chem. Soc. 1979, 101, 6797. (6) Wagenseller, P. E.; Birney, D. M.; Roy, D. J. Org. Chem. 1995, 60, 2853. (7) Liebman, J. F.; Paquette, L. A.; Peterson, J. R.; Rogers, D. W. J. Am. Chem. Soc. 1986, 108, 8267. (8) Scott, L. T.; Cooney, M. J.; Rogers, D. W.; Dejroongruang, K. J. Am. Chem. Soc. 1988, 110, 7244. (9) Rogers, D. W.; Podosenin, A.; Liebman, J. F. J. Org. Chem. 1993, 58, 2589.
comparison of experimental and calculated, using force field methods (MM2ERW), heats of formation that a series of cycloheptatrienes and norcaradienes are homoaromatic.10 Calculations by us11 and others12 do not support the claims that either triquinacene or the [5]pericyclynes should be considered as neutral homoaromatics. It is believed that the nonconjugated models and inappropriate force fields used to account for strain energy differences in these thermochemical studies are inadequate for the reliable assignment of homoaromaticity.3,12a-d The case of cycloheptatriene is controversial. Early work was equivocal, whereas our more recent calculations do not support homoaromaticity for this system1b and those of Schleyer13 and Cremer14 et al. suggest some homoaromatic stabilization as do the thermochemical studies of Roth et al.10 The semibullvalenes have long been recognized as the system most closely approaching neutral homoaromaticity.3,4,15 It has even been suggested that the homoconjugative interactions in semibullvalenes may be viewed as the open-shell through-space (homoconjugative) in(10) Roth, W. R.; Kla¨rner, F.-G.; Siepert, G.; Lennartz, H.-W. Chem. Ber. 1992, 125, 217. (11) First presented at the 72nd Canadian Chemical Conference, Victoria, British Columbia, June 1989 (Williams, R. V.; Kurtz H. A.), and the 7th International Symposium on Novel Aromatic Compounds, Victoria, B.C., Canada, July 1992 (Williams, R. V.; Chauhan, K.; Kurtz, H. A.); see also ref 3. (12) (a) Miller, M. A.; Schulman, J. M.; Disch, R. L. J. Am. Chem. Soc. 1988, 110, 7681. (b) Schulman, J. M.; Miller, M. A.; Disch, R. L. THEOCHEM 1988, 169, 563. (c) Dewar, M. J. S.; Holder, A. J. J. Am. Chem. Soc. 1989, 111, 5384. (d) Storer, J. W.; Houk, K. N. J. Am. Chem. Soc. 1992, 114, 1165. (e) Jiao, H.; Schleyer, P. v. R. 8th International Symposium on Novel Aromatic Compounds, Braunschweig, Germany, Jul 30-Aug 4 1995; Abstract 73. (f) Jiao, H.; Hommes, N. J. R. v. E.; Schleyer, P. v. R.; de Meijere, A. J. Org. Chem. 1996, 61, 2826. (13) Schleyer, P. v. R. Personal communication at the 8th International Symposium on Novel Aromatic Compounds, Braunschweig, Germany, Jul 30-Aug 4, 1995. (14) Cremer, D.; Childs, R. F.; Elia, G. In The Chemistry of the Cyclopropyl Group; Rappoport, Z., Ed.; J. Wiley & Sons: Chichester, 1995; Vol. 2, p 411. (15) Paquette, L. A.; Liao, C. C.; Burson, R. L.; Wingard, R. E., Jr.; Shih, C. N.; Fayos, J.; Clardy, J. J. Am. Chem. Soc. 1977, 99, 6935. (16) See, for example, Quast, H.; Geissler, E.; Mayer, A.; Jackman, L. M.; Colson, K. L. Tetrahedron 1986, 42, 1805 and references therein.
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The Search for Homoaromatic Semibullvalenes
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state.1a,c These predictions are confirmed, for 8 and 9, by recent high-order ab initio and density functional theory results.12e,19 This paper contains a full account of our studies on the bisannelated semibullvalenes 1,5dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride 10 and (1,5)-ethyl,methyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride 11.25
teraction of two allyl radicals (e.g., 3d),16 which thus avoids the destabilization predicted for closed-shell systems.5 The degenerate Cope rearrangement of semibullvalene (3a a 3b) proceeds through the homoaromatic transition state (3c) with a very low activation barrier (∆Gq298 ) 25.94 kJ mol-1,17a ∆Gq173 ) 24.27 kJ mol-1,17a ∆Gq123 ) 23.01 kJ mol-1 17b).18,19 There have been many experimental and theoretical investigations aimed at lowering this barrier even to the point of eliminating it and achieving a ground state homoaromatic semibullvalene analogue of (3c).3,4 One approach to this goal is by small-ring annelation of the semibullvalene nucleus.1,3,4,19,20 Paquette and Chamot suggested that in the bisannelated semibullvalene 4 the “breathing motion” of the Cope process would be inhibited, and this would perhaps lead to a homoaromatic molecule.21 Early extended Hu¨ckel investigations by Hoffmann and Stohrer suggested that the Cope rearrangement of the bisethenoannelated semibullvalene 5 would be strongly inhibited and that the perturbed transition state would be the strained cyclodecapentaene 6.22 Very recent high-order ab initio and density functional theory calculations12e,19 completely validate our earlier semiempirical calculations on several annelated semibullvalenes.1a,c These calculations clearly demonstrate that small ring annelation at the 1,5, 2,8, or 2,8:4,6 position(s) results in homoaromatic semibullvalenes. Supporting the contention that suitable small-ring annelation of the semibullvalene nucleus will result in homoaromaticity is the fact that the Cope rearrangement of 7 is still fast on the NMR time scale at -160 °C.23 7 is not an ideal candidate for neutral homoaromaticity as it is well-known that substitution at the 3,7-positions of the semibullvalene4,17a,24a,b and barbaralane4,16,24c nuclei increases the activation barrier for the Cope rearrangement. Thus, the NMR results for 7 bode well for the prospect of neutral homoaromatic systems based on appropriately designed bisannelated semibullvalenes. Using a set of semiempirical discriminators for the detection of energy-lowering homoconjugative interactions,1b we predicted that the annelated semibullvalenes 8-10 will have a homoaromatic ground (17) (a) Moskau, D.; Aydin, R.; Leber, W.; Gu¨nther, H.; Quast, H.; Martin, H.-D.; Hassenru¨ck, K.; Miller, L. S.; Grohmann, K. Chem. Ber. 1989, 122, 925. (b) Cheng, A. K.; Anet, F. A. L.; Mioduski, J.; Meinwald, J. J. Am. Chem. Soc. 1974, 96, 2887. (18) Jiao, H.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1760. (19) Jiao, H.; Nagelkerke, R.; Kurtz, H. A.; Williams, R. V.; Borden, W. T.; Schleyer, P. v. R. J. Am. Chem. Soc. 1997, 119, 5921. (20) Miller, L. S.; Grohmann, K.; Dannenburg, J. J. J. Am. Chem. Soc. 1983, 105, 6862. (21) Chamot, E.; Paquette, L. A. J. Org. Chem. 1978, 43, 4527. (22) Hoffmann, R.; Stohrer, W.-D. J. Am. Chem. Soc. 1971, 93, 6941. (23) Kohnz, H.; Du¨ll, B.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1989, 28, 1343. (24) (a) Quast, H.; Go¨rlach, Y.; Christ, J.; Peters, E.-M.; Peters, K.; von Schnering, H. G.; Jackman, L. M., Ibar, G.; Freyer, A. J. Tetrahedron Lett. 1983, 24, 5595. (b) Schnieders, C.; Mu¨llen, K.; Braig, C.; Schuster, H.; Sauer, J. Tetrahedron Lett. 1984, 25, 749. (c) Gu¨nther, H.; Runsink, J.; Schmickler, H.; Schmitt, P. J. Org. Chem. 1985, 50, 289.
Results and Discussion The known tetraester 12 (R ) Me) (Scheme 1A) appeared to be the ideal starting material for the synthesis of a variety of bisannelated semibullvalenes. In particular, the dianhydride 10 was considered to be a readily accessible synthetic target. Initially, only a single minimum corresponding with 10c was located by semiempirical calculations on 10.1d During a recent reinvestigation of this system, a higher energy minimum corresponding with the localized forms (10a,b) was discovered. Using our discriminators,1a-c the ground state species 26
(25) Some of the work presented herein was communicated in ref 1d. (26) Miller, L. S.; Grohmann, K.; Dannenberg, J. J.; Todaro, L. J. Am. Chem. Soc. 1981, 103, 6249.
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Williams et al. Scheme 1a
a Key: (A, R ) Me) (a) NaHCO , H O (82%), (b) Br , CHCl (88%), (c) Et N, CHCl (78%), (d) i-Bu Al, toluene, ambient, 4 h, R′ ) H, 3 2 2 3 3 3 3 R′′ ) OH (76%), (e) MsCl, Et3N, CH2Cl2, 5 °C f ambient, 40 h, R′ ) H, R′′ ) OMs (83%), (f) NaI, acetone, reflux, 3.5 h (63%); (B, R ) Me) (a) NaHCO3, H2O (82%), (b) Br2, CHCl3 (88%), (c) Et3N, CHCl3 (78%), (d) NaBH4, THF, H2O,
