Selective Nucleophilic Attack of Trisulfides. An ab Initio Study

The nucleophilic substitution reactions of three trisulfides by thiolate were evaluated at MP2(full)/6-31+G* and MP4SDTQ(fc)/6-311+G**//MP2(full)/6-31+G*. The results show that the gas-phase mechanism at MP2/6-31+G* is addition−elimination. Kinetically, there is a 2−5 kcal mol-1 preference for attack at a terminal sulfur over a central sulfur in the same trisulfide. This preference for the terminal sulfur is due to more steric hindrance encountered when the nucleophile attacks the central sulfur versus attacking a terminal sulfur. Hydrogen bonding between the nucleophile and the trisulfide also assists in directing attack to the terminal position. Attack at the terminal sulfur is also thermodynamically favored, a result of the thiosulfenate being a better leaving group than the thiolate. These results provide theoretical support to the findings of Myers et al. that the trigger mechanism for activation of calichaemicin and related enediyne antitumor agents occurs by a multistep process, with a displacement at the terminal sulfur of the trisulfide followed by nucleophilic substitution of a disulfide.