Palladium-Catalyzed C-H Activation Route to Dibenzothiophenes

Significance

Reported here is a palladium-catalyzed double C-H activation synthesis of dibenzo[b,d]thiophene-1-carbaldehyde derivatives 2 starting from 1 which involves the sulfoxide as a directing group. The combination of catalytic PdCl₂, AgOAc as an oxidant and iodoarene as an additive in acetic acid were found to give the best yields of products. EWG and EDG groups were well tolerated except in the ortho positions of both the benzylic and the sulfoxide moieties of 1. Several control experiments were carried out in order to understand the mechanism of this reaction. It is proposed that electrophilic attack of Pd(II) directed by the sulfoxide leads to 3 which upon addition of iodoarene gives the Pd(IV) complex 4. The latter undergoes the first C-H activation to produce 5 and hence 6 after reductive elimination. A Pummerer rearrangement (6 → 7), followed by Pd(II) coordination to the thiol and subsequent C-H activation (8) then affords product 2.