Metal-Free Dehydrogenative Double C-H Sulfuration To Access Thieno[2,3-b]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.