Conformational isomers of 1,2,5,6-tetrathiocins and the photoisomerization of a 1,2,5,6-tetrathiocin into a 1,2,3,6-tetrathiocin: X-ray structures of (C6X4S2)2 (X = F, Cl) and C6F4SSSC6F4S

The 1,2,5,6-tetrathiocins (C₆X₄S₂)₂ (3a, X = F; 3b, X = Cl) are obtained in high yields by the oxidation of the dithiols 1,2-C₆X₄(SH)₂ (X = F, Cl) with I₂ or SO₂Cl₂, respectively. In the solid state 3a has the C_2h (chair) conformation and crystallizes in two different phases: alpha-(C₆F₄S₂)₂, monoclinic, P2₁/a, a = 9.351(2), b = 6.465(2), and c = 11.546(2) Å, ß = 95.60(1)°, V = 694.6(2) ų, Z = 2; and ß-(C₆F₄S₂)₂, monoclinic, P2₁/c, a = 4.825(2), b = 11.302(2), and c = 12.453(2) Å, ß = 91.45(3)°, V = 678.8(3) ų, Z = 2. By contrast, 3b displays a D₂ (twist-boat) structure and crystallizes in the C2/c space group with a = 15.243(3), b = 8.703(2), and c = 27.010(14) Å, ß = 92.81(4)°, V = 3578(1) ų, and Z = 8. The derivative 3a exists as an equilibrium mixture of two conformational isomers in toluene solution. The VT ¹⁹F NMR data afford the thermodynamic parameters H° = 9.9 ± 0.4 kJ mol^-1 and S° = 14 ± 1 J K^-1 mol^-1. Density functional theory calculations for 3a indicate that the D₂ conformation is lower in energy than the C_2h isomer by 4.6 kJ mol^-1. The photolysis of 3a in benzene promotes a transannular sulfur migration to give the 1,2,3,6-tetrathiocin, C₆F₄SSSC₆F₄S (6), which exists in a chair conformation with respect to antipodal sulfur atoms. Crystal structure of 6: orthorhombic, space group Pnma, a = 8.652(6), b = 19.084(4), and c = 8.301(6) Å, V = 1370.6(14) ų, and Z = 4. The isomers 3a and 6 were also characterized by EI mass spectrometry, ¹⁹F NMR, FTIR, and Raman spectroscopies.Key words: tetrathiocins, conformational isomers, photoisomerization.