Metal-Free Dehydrogenative Double C–H Sulfuration To Access Thieno[2,3‑<i>b</i>]indoles Using Elemental Sulfur

We report a base-promoted metal-free approach for the synthesis of thieno­[2,3-b]­indole derivatives. This method combined four C–H σ-bond cleavage reactions of two different kinds of C–H bonds and two C–S σ-bond formation processes. A series of thieno­[2,3-b]­indoles were obtained starting from 3-benzylindole derivatives with good yields and high regioselectivity, with the elemental sulfur serving as a cheap and readily available sulfur source. Good efficiency could be maintained even when the reaction was performed on a gram scale. A plausible mechanism was proposed on the basis of mechanistic studies.