Thiiranes ring synthesis from epoxides

Whereas epoxides are readily cleaved by ammonia and amines (6), ring opening of thiiranes is rather sluggish. The situation is ameliorated by adding silver ion (7) to coordinate with the sulfur atom which then renders the ring C-S bond more permanently polarized (and hence less polarizable), i.e., the ring carbon becomes harder and more responsive to hard bases. Precedence for this activation is found in a one-step synthesis of sulfenamides from disulfides (8). 

---