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 obta ined in high yields by the oxidation of the dithiols 1,2-C6X4(SH)(2) ( X = F, Cl) with I-2 or SO2Cl2, respectively. In the solid state 3a has the C-2h (chair) conformation and crystallizes in two different phase s: alpha-(C6F4S2)(2), monoclinic, P2(1)/a, a = 9.351(2), b = 6.465(2), and c = 11.546(2) Angstrom, beta = 95.60(1)degrees, V= 694.6(2) Angst rom(3), Z = 2; and beta-(C6F4S2)(2), monoclinic, P2(1)/c, a = 4.825(2) , b = 11.302(2), and c = 12.453(2) Angstrom, beta = 91.45(3)degrees, V = 678.8(3) Angstrom(3), Z = 2. By contrast, 3b displays a D-2 (twist-b oat) structure and crystallizes in the C2/c space group with a = 15.24 3(3), b = 8.703(2), and c = 27.010(14) Angstrom, beta = 92.81(4)degree s, V = 3578(1) Angstrom(3), and Z = 8. The derivative 3a exists as an equilibrium mixture of two conformational isomers in toluene solution. The VT F-19 NMR data afford the thermodynamic parameters Delta H degr ees = 9.9 +/- 0.4 kJ mol(-1) and Delta S degrees = 14 +/- 1 J K-1 mol( -1). Density functional theory calculations for 3a indicate that the D -2 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 mi gration to give the 1,2,3,6-tetrathiocin, C6F4SSSC6F4S (6), which exis ts in a chair conformation with respect to The lysis of 3a in benzene promotes a transannular sulfur antipodal sulfur atoms. Crystal structu re of 6: orthorhombic, space group Pnma, a = 8.652(6), b = 19.084(4), and c = 8.301(6) Angstrom, V= 1370.6(14) Angstrom(3), and Z = 4. The i somers 3a and 6 were also characterized by EI mass spectrometry, F-19 NMR, FTIR, and Raman spectroscopies.