Conformational analysis oftrans-2,6-dithiabicyclo5.4.0undecane-4-spiro-1prime;-cyclobutane, -cyclopentane, and -cyclohexane by1H and13C nuclear molecular resonance spectroscopy

摘要

J. CHEM. SOC. PERKIN TRANS. 11 1988 Conformation a I AnaIysis of trans-2,6-Dithi a b icycIo5.4.01u n deca n e -4-spiro-1'-cyclobutane, -cyclopentane, and -cyclohexane by IH and I3CNuclear Molecular Resonance Spectroscopy Barbara Ryst Department of Organic Chemistry, Jagiellonian University, Karasia 3, Pl-30060 Krako w, Poland The high-field 'H and 13C d.n.m.r. spectra of the title compounds are presented. The coalescence effects in the spectra of spirocyclohexane derivatives are discussed and assigned to the conformational process, namely restricted pseudorotation of the dithiepane ring. We have recently published the results of 13C d.n.m.r. investigation of cis-2,6-dithiabicyclo5.4.0undecane-4-spiro-1'-cyclobutane, -cyclopentane, and -cyclohexane. ' In the temper- ature-dependent n.m.r.spectra two coalescence processes have been found. The first one interconverts the molecule into its mirror image and occurs by an inversion of the cis-fused bicyclic system. The second one is connected with the restricted pseudorotation of the dithiepane ring. In this paper we report the results of a high-field 'H and 13C d.n.m.r. investigation of the spiro-cyclobutane (la), -cyclo-pentane (lb), and -cyclohexane (lc) derivatives of trans-2,6- dithiabicyclo 5.4.0lundecane. The conformational mobility of this trans-bicyclic system is different from the previously investigated cis-isomer' as a trans fusion prevents the molecule from undergoing an inversion such as is seen in the trans-decalin molecule.2 However, inter- conversion and pseudorotation of the seven-membered ring is still possible.Results Variable-temperature 'H and '3C n.m.r. studies of compounds (la+) revealed spectral variations only for compound (lc). Table 1 summarises the 'H n.m.r. data obtained at high and low temperature for all three compounds while Table 2 summarises the corresponding '3C chemical shifts. The observed characteris- tic spectral changes are described next. The partial 400.13 MHz 'H n.m.r. spectrum of (lc) shown in Figure 1 reveals an AB pattern for the methylene protons at positions 3 and 5. This signal undergoes asymmetrical broadening at low temperatures. The high-field half of the AB signal broadens first and attains maximum broadening near 243 K, then at 203 K gives two t This research was carried out at Ruhr-Universitat Bochum.Table 1. 'H N.m.r. parameters for compounds (la-c) G/p.p.m.* Comp. T(K) A B CH,Sv,,/Hz J*,W (la) 283 233 3.15 3.13 2.95 2.97 68 60 15.8 15.8 (lb) 283 233 3.08 3.07 2.60 2.61 190 185 15.8 15.8 (lc) 283 2.84 2.68 67 15.8 203 2.96 2.19 310 15.0 2.85 2.56 115 14.3 doublets at 2.19 and 2.96 p.p.m. The low-field half of the high- temperature AB pattern reaches maximum broadening between 233 and 223 K and at 203 K two doublets at 6 2.56 and 2.85 are observed. These four signals in the lowest-temperature spectrum are of equal intensity. Similarly, in the 100.6 MHz 13C n.m.r. spectra of the compounds investigated the conformational-motion effects are observed only for (lc).Here coalescence processes involving signals of the seven- and the spiro-annulated six-membered ring and of C-8/C-ll leading to 1:l splittings in the lowest-temperature spectrum, were found. On the other hand, the signals due to the spiro atom C-4 and atoms C-4' and C-9/C-10 are unaffected. The free energy of activation (AGct = 11.3 amp; 0.2 kcal mol-') was calculated for four AX spin systems (C-7/C-1, C-3/C-5, C- 3'/C-5' and C-2'/C-6') using approximation equations for the reaction rates at the coalescence temperat~re.~ Discussion During the coalescence process, the I3C n.m.r. signals of C-2', C- 3', C-5', and C-6' split into two and the distance between C-2' and C-6' in the lowest-temperature spectrum was approxi-mately 4 p.p.m.This cannot be explained by a slowing down of the spirocyclohexane-ring inversion since that process inter- converts the molecule into itself. The observed process therefore seems to be an interconversion of two different conformations of the seven-membered ring. These conformations are equally populated as is shown by the integration of the 3-H/5-H region of 'H n.m.r. spectrum recorded at 203 K. The conformational behaviour of seven-membered saturated rings is well established. Experimental re~ults~,~ and molecular- mechanics calculations6 show that 1,4-dithiacycloheptane prefers a twist-chair (TC) rather than twist-boat (TB) conform- ation. These forms can undergo two different types of conformational pro~ess.~ One is TC #TB interconversion 1956 J.CHEM. SOC. PERKIN TRANS. II 1988 Table 2. 13C N.m.r. chemical shifts for compounds (la+) at high and low temperaturesldquo;rsquo;b T/K C-l/C-7 C-3/C-5 C-4 C-8/C-ll C-9/C-10 C-2rsquo; c-3lsquo; C-4 c-5lsquo; C-6rsquo; (la) 283 57.51 44.30 45.08 34.14 27.12 33.36 15.24 33.36 ---233 57.28 44.03 44.88 33.91 26.99 33.17 15.27 33.17 -(Ib) 283 57.55 44.44 50.16 33.98 27.15 39.11 25.81 25.8 1 39.1 1 -233 57.27 44.21 49.90 33.17 27.03 38.93 25.83 25.83 38.93 -(Ic) 283 57.16 42.98 39.67 33.99 27.25 36.41 22.8 5 27.15 22.85 36.41 56.55 47.28 33.55 37.77 22.93 22.93 37.77 203 39.17 27.02 or or 26.97 or or 56.34 37.77d 33.48 33.92 22.25 22.25 33.92 a For signal assignment see ref. (1). Signals show small up-field shifts with decreasing temperature.Signals are broadened due to coalescence effects. Signals of C-3(C-5) and C-2rsquo;(C-6rsquo;) in the lowest-temperature spectrum are overlapped. 263 K 253 KIrsquo;K 253 K 233A4 243 K 223 K 233 K 223 K 213 K 203 K I 315 2rsquo;16rsquo; I I I 3.0 2.5 50 40 30 6 1p.p.m Figure 1. Partial 400.13 MHz lsquo;H n.m.r. (left) and 100.6 MHz 13C n.m.r. spectra (right) of (Ic) in 2H,acetone-carbon disulphide (1 :1) at several temperatures (* denotes signal of water) and the second is a TC or TB pseudorotation. The energy barriers for these processes in 6,6-dimethyl- 1,4-dithiacyclo- heptane are less than 8 kcal mol-rsquo;.rsquo; An inspection of molecular models shows that the pseudo- rotation of the dithiepane ring in the molecules investigated is restricted due to the annelation.In Figure 2 conceivable TC conformations are depicted. Structure A is a flexible TC with C- 2lsquo; and C-6rsquo; being in isoclinal positions. Structure B is a rigid TC with C-2rsquo; and C-6rsquo; in very different stereochemical environments since both sulphur atoms are in gauche orientation with respect to C-2rsquo; but antiperiplanar to C-6rsquo;. The A IB interconversion is a pseudorotation with a barrier of ca. 11 kcal mol-rsquo;. This value is much higher than that for monocyclic dithiepanesrsquo; but is in the same range as for cis analogues of (lc).rsquo; In the transition state C, which has to be passed during A B pseudorotation, the C-2rsquo; methylene J. CHEM. SOC. PERKIN TRANS. II 1988 Experimental The synthesis of the compounds (la-c) has been reported earlier.rsquo; The temperature-dependent H and rsquo; 3C n.m.r.spectra were recorded at 400.13 and 100.6 MHz, respectively, on a lsquo;P A si +* 8 I Figure 2. Possible TC conformations of (lc) group is very close to the axial hydrogen atom at position 2. This steric compression may be the reason for such a high barrier for the process outlined, as well as the low barriers for (la) and (lb). In the last two cases this steric interference is somewhat relieved owing to the smaller bond angles C(2rsquo;)-C(4)-(4rsquo;) in (la) and C(2rsquo;)-C(4)-C(5rsquo;) in (lb). Bruker AM-400 spectrometer. The measurements were carried out with ca. 1 mol I-rsquo; solutions in 2H6acetone-carbon disulphide (1 : 1).All chemical shifts are referenced to internal SiMe,. Acknowledgements The author is indebted to Professor Helmut Duddeck, Bochum, for facilitating the measurements and Mrs. Doris Rosenbaum for recording the spectra. Financial support from the Polish Academy of Sciences (project CPBP 01.013) is acknowledged. References I B. Rys and H. Duddeck, Tetrahedron, 1985,41,889. 2 E. D. Eliel, N. L. Allinger, S. J. Angyal, and G. A. Morrison, lsquo;Conformational Analysis,rsquo; Wiley-Interscience, New York, 1965; J. Dale, lsquo;Stereochemie und Konformationsanalyse,rsquo; Verlag Chemie, Weinheim, 1978. 3 J. Sandstrom, Endeatlour, 1974, 33, 111; J. Sandstrom, lsquo;Dynamic N.M.R. Spectroscopy,rsquo; Academic Press, London, 1982. 4 M. J. Cook, G. Ghaem-Maghami, F. Kaberia, and K. Bergensen, Org. Magn. Reson., 1983,21, 339. 5 W. N. Setzer, G. S. Wilson, and R. S. Glass, Tetruhedron,1981,37,2735. 6 W. N. Setzer, B. R. Coleman, G. S. Wilson, and R. S. Glass, Tetruhedron, 1981,37,2743. 7 G. Favini, J. Mol. Struct. THEOCHEM., 1983,93, 139. 8 W. Tochterman, Top. Curr. Chem., 1970, 15,378. 9 S. Smoliriski and B. Rys, Monatsh. Chem., 1979,110,279. Received 20th February 1987;Paper 71325