J. Chem. Research (S), 1997, 38–39 J. Chem. Research (M), 1997, 0344–0354 Conformational Strains of (Z,Z)-, (E,Z)- and (E,E)-1,2,5,6-Tetrathiacycloocta-3,7-dienes based on ab initio Molecular Orbital Calculations Toshio Shimizu,a Kazuko Iwata,a Nobumasa Kamigata*a and Shigeru Ikutab aDepartment of Chemistry, Faculty of Science, Tokyo Metropolitan University, Minami-ohsawa, Hachioji, Tokyo 192-03, Japan bComputer Center, Tokyo Metropolitan University, Minami-ohsawa, Hachioji, Tokyo 192-03, Japan A comparison of the conformations and energies of (Z,Z)-, (E,Z)- and (E,E)-1,2,5,6-tetrathiacycloocta-3,7-dienes based on ab initio MO calculations is reported.Several conformational studies of cycloocta-1,5-diene have been reported based on forcefield calculations.2 We have also reported the conformational study of cycloocta-1,5-diene by ab initio molecular orbital calculations.3 Recently our interest has focused on the conformations of heterocyclic unsaturated compounds.More recently, we succeeded in isolating (Z,Z)-1,2,5,6-tetrathiacycloocta-3,7-diene (1,2,5,6-tetrathiocine) (1), and the conformation in the crystalline state was determined by X-ray crystallography.7 Here we report the conformations and strain energies of 1,2,5,6-tetrathiacycloocta- 3,7-diene through the Z,Z-1, E,Z-2 and E,E-3 isomers based on ab initio molecular orbital calculations. Six local minima were found for the Z,Z-1, E,Z-2 and E,E-3 isomers (Fig. 1). The twist conformer (1-T) of the Z,Z-isomer was calculated to be at a local minimum, in contrast to the corresponding twist conformer of cycloocta- 1,5-diene3 which by frequency analysis indicated the transition state.Furthermore, we attempted to optimize the chair conformer of the E,E-isomer 3 because the chair conformer of (E,E)-cycloocta-1,5-diene was found to lie at the local minimum.3 However, frequency analysis of the chair conformer of 3 showed one imaginary number assigned to be the transition state in the pathway from 3-TC to the other enantiomer of 3-TC.The trans-olefin moieties (S–C�C–S) for the E,Z- and E,E-isomers 2,3-T and 3-TC are twisted from the strain-free horizontal geometry: 144.9, 152.8 and 153.0° (average), respectively. These torsional strains are moderate compared with those of cycloocta-1,5-diene; 131–135°.3 The most interesting point of the conformations of the title compound is the twisted geometries around the sulfur–sulfur bonds. The torsional angles (C–S–S–C) of 3-T and 3-TC for the E,E-isomer are strongly reduced: 48.7 and 23.0°, respectively, corresponding to an increase in the sulfur–sulfur bond lengths.The twist conformer 1-T of the Z,Z-1 isomer was calculated to have the lowest potential energy among all the conformers at the local minima, as shown in Table 2 together with relative energies for cycloocta-1,5-diene. The calculated geometry of 1-T is in good agreement with the X-ray structure of 1 recently determined by us.7 The crystal structure of 1 was therefore the most stable form.The conformers of the E,E-isomer 3 were found to lie at higher energy levels, possibly because of the existence of ring strains. In order to estimate the strain energies of the olefin moieties, we calculated the torsional strain energies for the restricted geometries of but-2-ene as a model. The strain energies depend on the torsional strains around the olefin moieties for the six conformers 1-T, 1-C, 1-HC, 2, 3-TC and 3-TC and are 0.0, 0.0, 0.0, 5.0, 5.0 and 5.0 kcal molµ1, respectively.These strain energies for the E,Z-2 and E,E-3 isomers (5.0 kcal molµ1) are smaller than those for the corresponding geometries of cycloocta-1,5-diene.3 The torsional strains around the sulfur– sulfur bonds were also estimated using dimethyl disulfide as a model. The torsional strains of the disulfide units for the six conformers are 1.4, 1.2, 3.4, 4.3, 7.4 and 17.8 kcal molµ1, respectively.It was found that the torsional strains of the disulfide units for the conformers of the E,E-3 isomer are unexpectedly large compared with those of the olefin moieties in this ring system. The remaining relative energies after deduction of these torsional strain energies for the six conformers, relative to that of 1-T, are 0, 5.5, 9.8, 3.5, 3.0 and 1.8 kcal molµ1, respectively. As a result, the energy differ- 38 J. CHEM. RESEARCH (S), 1997 *To receive any correspondence.Fig. 1 Optimized geometries for (Z,Z)-1, (E,Z)-2 and (E,E)-3 at the local minimaences for 1-T, 2, 3-T and 3-TC are explained by the strain energies caused by the torsional strains of the olefin moieties and the disulfide units. The remaining energy for 1-HC may be explained by bond angle strains. The bond angles of S(4)–C(5)–C(6) and C(5)–C(6)–S(7) for 1-HC are strongly extended (143.6°), and the bond angle strain on the olefin sp2 carbons was estimated to be 13.7 kcal molµ1 based on 1,2-bis- (methylsulfanyl)ethene as a model.This value is 9.9 kcal molµ1 larger than the corresponding bond angle strains for 1-T. The remaining energy for 1-C is attributed to the repulsion of the sulfur atom lone pairs. Geometries were optimized using the Hartree–Fock (HF) method with double-zeta plus polarization basis set. Energies were obtained using the second-order Møller–Plesset perturbation (MP2) method with the same basis set. The HF-optimized geometries were applied to these energy calculations.This work was supported by Grant-in-Aid for Scientific Research on Priority Areas and General Scientific Research from the Ministry of Educations, Science and Culture, Japan. Technique used: ab initio molecular orbital calculations References: 12 Fig. 2: Correlation diagram between the energies and torsional angles for dimethyl disulfide 5 Table 1: Selected bond lengths, angles, interatomic distances and torsional angles for optimized geometries of 1, 2 and 3 Table 3: Estimated strain energies depending on torsional angles around the olefin moiety and disulfide units Received, 29th July 1996; Accepted, 29th October 1996 Paper E/6/05304A References cited in this synopsis 2 R.Pauncz and D. Ginsburg, Tetrahedron, 1960, 9, 40; N. L. Allinger and J. T. Sprague, J. Am. Chem. Soc., 1972, 94, 5734; Tetrahedron, 1975, 31, 21; O. Ermer, J. Am. Chem. Soc., 1976, 98, 3964; D. N. J. White and M. J. Bovill, J. Chem. Soc., Perkin Trans. 1, 1977, 1610; W. R. Roth, O. Adamczak, R. Breuckmann, H.-W. Lennartz and R. Boese, Chem. Ber., 1991, 124, 2499. 3 T. Shimizu, K. Iwata, N. Kamigata and S. Ikuta, J. Chem. Res. (S), 1994, 436. 7 T. Shimizu, K. Iwata and N. Kamigata, Angew. Chem., 1996, 108, 2505; Angew. Chem., Int. Ed. Engl., 1996, 35, 2357. J. CHEM. RESEARCH (S), 1996 39 Table 1 Relative energies for optimized geometries of (Z,Z)-1, (E,Z)-2 and (E,E)-3 and cycloocta- 1,5-dienes Z,Z E,E E,Z T C HC TB T TC C 25.3b 23.2 14.0 11.8 11.4 5.3 0 a Relative energies --------------------------------------------------------------------------------------------------- (kcal molµ1) 28.2 20.1 12.0 4.2b 2.8 2.0 0 a Relative energies -------------------------------------------------------------------------------------------------------- (kcal molµ1) aGeometry could not be optimized as a stationary point. bTransition stat