Toward a Switchable Molecular Rotor. Unexpected Dynamic Behavior of Functionalized Overcrowded Alkenes
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J. Org. Chem. 1997, 62, 4943-4948 4943 Toward a Switchable Molecular Rotor. Unexpected Dynamic Behavior of Functionalized Overcrowded Alkenes Anne Marie Schoevaars,† Wim Kruizinga,† Robert W. J. Zijlstra,† Nora Veldman,‡ Anthony L. Spek,‡ and Ben L. Feringa*,† Department of Organic and Molecular Inorganic Chemistry, Groningen Center for Catalysis and Synthesis, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Department of Crystal & Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Received November 26, 1996X In an approach toward a photochemically bistable molecular rotor the synthesis of cis-1a and trans- 1b isomers of 2-(2,6-dimethylphenyl)-9-(2′,3′-dihydro-1′H-naphtho[2,1-b]thiopyran-1′-ylidene)-9H- thioxanthene (1), being sterically overcrowded alkenes functionalized with an o-xylyl group as a rotor, is described. The key steps in the synthesis are a Suzuki coupling to attach the xylyl moiety and a diazo-thioketone coupling with subsequent desulfurization to introduce the central olefinic bond in 1. The X-ray structure of cis-1a revealed an anti-folded helical conformation. Dynamic NMR studies on both isomers were performed, to elucidate the kinetics of their rotation processes and to investigate the possibility to control the biaryl rotation by photochemical cis-trans q isomerization. A rotation barrier was found by coalescence spectroscopy for cis-1a: ∆Gc ) 22 ( -1 q 1 kcal mol in DMSO-d6. 2D exchange spectroscopy (EXSY) showed barriers for cis-1a: ∆G303 -1 q -1 19.0 ( 0.2 kcal mol and trans-1b: ∆G303 ) 19.7 ( 0.2 kcal mol in DMSO-d6, respectively. Molecular mechanics and semiempirical calculations support the unexpected higher barrier for trans-1b. Since the rotation barriers are different for the cis and trans isomer, it can be concluded that control of a second mechanical effect, e.g. the rate of rotation of an attached biaryl rotor, is feasible in a photochemically switchable system. Introduction nent devices is an important challenge as well. An example of a gated molecular system based on chiroptical The development of functional devices at the molecular molecular switches in which fluorescence is reversibly and supramolecular level has attracted considerable modulated was demonstrated recently.10 Functionaliza- interest during the last decade.1,2 Elegant examples of tion of the optical switches with a rotor would open the functional molecular devices were reported recently, possibility to control a mechanical effect (e.g. the rate of among them a “molecular brake” by Kelly,3 a “molecular rotation around a single bond) by the photochemical cis turnstile” by Moore,4 a “light-switchable catalyst” by - trans isomerization, due to the difference in steric Rebek,5 “photoresponsive crown ethers” by Shinkai,6 hindrance in the cis and the trans isomer. This concept “photoswitchable fluorophores” by Lehn,7 and “chiroptical in fact represents the molecular analog of an accelerator molecular switches”.8 Extensive research in our group in an ordinary machine or a brake, if it is possible to block on sterically overcrowded alkenes provided the basis for the rotation completely in one of the isomers at a specific the application of these compounds as optical switches temperature. to control chirality of molecules and mesoscopic materi- als.8 These molecular switches are based on a photo- In this paper we describe the synthesis of a photo- chemical cis-trans isomerization of helically shaped chemically bistable molecular rotor and dynamic NMR alkenes which simultaneously inverts the helicity of the studies on both isomers, to elucidate the kinetics of their molecules. Optical data storage9 is one major objective rotation processes and to investigate the feasibility to for such photochromic bistable organic molecules, but control this mechanical effect by a chiroptical molecular photochemical control of other functions in multicompo- switch. Although an effective photochemical cis-trans isomerization could not be achieved in this model system † University of Groningen. since the UV/vis absorption spectra of the isomers are ‡ Utrecht University. nearly identical,11 unexpected dynamic behavior was X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Feynman, R. P. In Miniaturization; Gilbert, H. D., Ed.; Rein- observed. hold: New York, 1961; p 282. (2) (a) Drexler, K. E. Nanosystems: Molecular Machinery, Manu- facturing and Computation; Wiley: New York, 1992. (b) Lehn, J.-M. Results and Discussion Supramolecular Chemistry; VCH: Weinheim, 1995. (3) Kelly, T. R.; Bowyer, M. C.; Bhaskar, K. V.; Bebbington, D.; The principle of a photochemical switchable molecular Garcia, A.; Lang, F.; Kim, M. H.; Jette, M. P. J. Am. Chem. Soc. 1994, 116, 3657. rotor is shown in Scheme 1. Compound 1, a thioxan- (4) Bedard, T. C.; Moore, J. S. J. Am. Chem. Soc. 1995, 117, 10662. thene-based overcrowded ethylene which can adopt a cis- (5) Wu¨ rthner, F.; Rebek, J., Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 446. (6) Shinkai, S.; Ogawa, T.; Kusano, Y.; Manabe, O.; Kikukawa, K.; (9) For reviews see: Feringa, B. L.; Jager, W. F.; de Lange, B. Goto, T.; Matsuda, T. J. Am. Chem. Soc. 1982, 104, 1960. Tetrahedron 1993, 49, 8267. Feringa, B. L.; Huck, N. P. M.; Schoe- (7) Tsivgoulis, G. M.; Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1995, vaars, A. M. Adv. Mater. 1996, 8, 681. 34, 1119. (10) Huck, N. P. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun. (8) (a) Feringa, B. L.; Jager, W. F.; de Lange, B.; Meijer, E. W. J. 1995, 1095. Am. Chem. Soc. 1991, 113, 5468. (b) Jager, W. F.; de Jong, J. C.; de (11) Currently this concept is extended to an effectively switchable Lange, B.; Huck, N. P. M.; Meetsma, A.; Feringa, B. L. Angew. Chem., system by the attachment of electron-donating and -withdrawing Int. Ed. Engl. 1995, 34, 348. (c) Feringa, B. L.; Huck, N. P. M.; van groups in the molecule in order to increase the difference of the UV Doren, H. A. J. Am. Chem. Soc. 1995, 117, 9929. spectra of the cis and the trans isomer.8b S0022-3263(96)02210-4 CCC: $14.00 © 1997 American Chemical Society 4944 J. Org. Chem., Vol. 62, No. 15, 1997 Schoevaars et al. Scheme 1 The crystal structure of cis-1a is shown in Figure 1. The arrangement about the central double bond is nearly planar (dihedral angles C12-C11-C14-C26 ) 7.1(11)°; C10-C11-C14-C15 ) 4.7(11)°). The bond length obtained for the central double bond (1.345(12) Å, C11-C14)isa characteristic value for a simple nonconjugated double bond (1.337 Å).18 The anti-folded helical structure in which the top and bottom halves are tilted down and upward, respectively, relative to the plane of the central double bond, and by which severe nonbonding interac- tions are avoided, is clearly shown. Both heterocyclic or trans-geometry and is extended with a xylyl group as rings in cis-1a adopt twisted-boat conformations relieving a rotor, was designed as a model system. The presence the steric interactions present in this molecule. In the of the o-methyl groups of the rotor are expected to cause crystal structure the xylyl moiety is rotated strongly out restricted rotation around the biaryl bond due to steric of the plane of the adjacent aryl group resulting in a interactions with the upper part of the molecule. The torsion angle of this biaryl unit of -86.6(10)° (C18-C17- influence of these steric effects on the rate of rotation is C27-C32). presumed to be different in the cis and the trans isomers In the 1H-NMR spectra the isomers can be distin- of 1. Therefore the rate of rotation might be controlled guished on the basis of the resonance of the methyl by a photochemical cis-trans isomerization. Inspection groups of the xylyl moiety. Furthermore, both isomers of molecular models suggested severe steric hindrance cis-1a and trans-1b show two separate methyl signals for biaryl rotation in the cis isomer compared to the trans in their 1H-NMR spectra at 30 °C, indicating slow biaryl isomer, as a result of interactions with the upper part of rotation on the NMR time scale. The chemical shifts of the molecule due to the influence of the large naphtha- the methyl groups in DMSO-d6 are 0.64 and 1.62 ppm lene unit facing the rotor in the first case compared to for cis-1a and 1.91 and 2.18 ppm for trans-1b, respec- the small methylene group in the latter. Advantageous tively. The large chemical shift difference between the are also the singlets in the 1H-NMR spectra arising from methyl signals of cis-1a can be explained by the difference the methyl groups of the xylene rotor, allowing dynamic in electronic environment of both groups. From the X-ray NMR studies of the rotation process. structure of cis-1a (Figure 1) and molecular modeling A successful synthetic route to 1 is shown in Scheme studies, it becomes evident that one methyl group is 2. Key steps in the synthetic scheme are the Suzuki pointing toward the aromic upper part of the alkene, coupling12 to attach the xylyl rotor to the thioxanthone while the other methyl is not influenced by this naph- and the coupling of the upper and lower half of the helical thalene moiety. The rotation barriers for cis-1a and alkene using a diazo-thioketone method.13 Boronic acid trans-1b were examined first using temperature depend- derivative 6 was prepared from 2-bromo-1,3-dimethyl- ent 1H-NMR,19 focussing on the coalescence behavior of benzene 5 via lithiation, quenching with trimethyl borate the methyl signals.