Synthesis of Phenanthro[9,10‐c]thiophenes by 2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ)‐Promoted Cyclo‐Oxidation

Abstract

A new method to prepare phenanthro[9,10‐c]thiophenes has been developed. In the presence of triflic acid, 3,4‐diaryl thiophenes undergo 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ)‐promoted cyclo‐oxidation. NMR and computational studies indicate that protonation of the thiophene plays a key role in this reaction. The reaction can be used to prepare phenanthro[9,10‐c]thiophene, as well as derivatives with alkyl, bromo, and methoxy substituents. However, the yields and selectivity of the reaction depend on both the nature and location of the substituents. Bis(3‐methoxyphenyl)thiophene reacts under these conditions to give the desired product in 57 % yield, while bis(4‐methoxyphenyl)thiophene gives no product. Bis(3‐bromophenyl)thiophene did not react, but cyclo‐oxidation of bis(4‐bromophenyl)thiophene provides the desired product in 34 % yield.