Thienannulation: Efficient Synthesis of π‐Extended Thienoacenes Applicable to Organic Semiconductors

This microreview focuses on several thiophene‐annulation reactions of less cyclic precursors on aromatic units (“thienannulation reactions”), which have been applied to the synthesis of a range of π‐extended thienoacene‐based organic semiconductors. Those thienannulation reactions can be applicable to the syntheses of two types of fused‐thiophene compounds: those with terminal fused thiophene units, represented by acene(di)thiophenes such as benzo[1,2‐ b :4,5‐ b ′]dithiophene (BDT), and those with internal thiophene units, such as [1]benzothieno[3,2‐ b ][1]benzothiophene (BTBT). In addition, this microreview discusses the merits, scope, and limitations of the thienannulation reactions and demonstrates their high reproducibility with different substrates. It also shows that the corresponding less cyclic precursor subunits for the thiophene‐fused substructures can be regarded as synthetic equivalents or “synthons” for certain types of thienoacene‐based organic semiconductors.