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

摘要

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 I-2 or SO2Cl2, respectively. In the solid state 3a has the C-2h (chair) conformation and crystallizes in two different phases: 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) Angstrom(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-boat) structure and crystallizes in the C2/c space group with a = 15.243(3), b = 8.703(2), and c = 27.010(14) Angstrom, beta = 92.81(4)degrees, 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 degrees = 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 migration to give the 1,2,3,6-tetrathiocin, C6F4SSSC6F4S (6), which exists in a chair conformation with respect to The lysis of 3a in benzene promotes a transannular sulfur antipodal sulfur atoms. Crystal structure 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 isomers 3a and 6 were also characterized by EI mass spectrometry, F-19 NMR, FTIR, and Raman spectroscopies. [References: 27]