Computational Study on Conformational Features of Protonated 5H,7H-Dibenzo- [b,g][1,5]dithiocin Oxides by ab initio MO Calculations

The ab initio MO calculations with HF/6-31G* basis set were performed in the neutral 5H, 7H-dibenzo[b,g][1,5]dithiocin 12-oxide (1), the mono-protonated (1-H + ) and the di-protonated (1-2H + ) sulfoxides.The calculations indicated that the protonation is very important process for the 1,5-oxygen shift and the oxygen atom approaches to another sulfur by protonation.The inside oxygen conformation (the TB i or the BB i ) is important for the intramolecular oxygen shift to form the oxygen-bridged intermediate in 1-2H + -BB b , and the outside oxygen conformation (the TB o or the BB o ) is the most probable conformation to form the sulfurane intermediate in 1-2H + -BB o and 2-2H + -BB o , respectively.2][3][4] Recently, theoretical investigation suggested a partial but significant S-S σ bond in the cationic species of ditiacyclooctane derivative. 5We reported an acid-induced 1,5-oxygen shift in 5H,7H-dibenzo[b,g][1,5]dithiocin 12-oxide (1) to produce the corresponding 6-oxide (2) (Scheme 2). 3 The transannular 1,5-oxygen shift in 1 proceeded smoothly in chloroform solution under acidic conditions but the reverse rearrangement from 2 to 1 was not observed.The oxygen isotope exchange experiments indicated that at least 70% of the 1,5-oxygen shift occurred intramolecularly through a direct path or coupling of intimate ion pair in the solvent cage (Scheme 3).Treatment of 1 with D 2 SO 4 f urnished the same dithiodication as that resulted from 2. Quenching of the dithiodication with aqueous sodium carbonate gave 2, quantitatively and the disproportionation was not observed (Scheme 4).