Dancing with Sulfur: Simple Preparation and Properties ofThiacalix[n]thiophene Derivatives

Abstract

In general, cyclization is the most crucial step in the synthesis of macrocyclic compounds formed out of repeated units of a π-system. However, a particular combination of aromatic wall unit and appropriate angular linkage can efficiently give cyclic compounds such as calix[n]arene and its analogues. This straightforward method allows material chemists to supply these compounds on multi-gram scale. Nevertheless, a more resourceful strategy is also required, because this simple approach is less feasible outside of specific combinations. Recently, we have newly developed a straightforward one-pot synthetic approach using a palladium coupling for a series of thiacalix[n]thiophene, thiacalix[n]dithieno[3,2-b:2’,3’-d]thiophene (thiacalix[n]DTT), and selenacalix[n]selenophenes, which are cyclic homologues of divalent chalcogen-bridged (S or Se) cyclic oligothiophene derivatives. A palladium-catalyzed reaction of (Bu₃Sn)₂S or (Bu₃Sn)₂Se with dibromothiophene, dibromoselenophene, or diboromo-DTT derivatives led to effective cyclization in good yield. In the presence of appropriate substituents, this method seems to kinetically favor the formation of macrocycles. The molecular and physical properties, including X-ray analysis, absorption spectra and redox properties, of the resultant macrocycles were also described. Unlike conventional calixarenes, they possess electron-donating ability, exhibiting multielectron redox processes due to electron delocalization. Thiacalix[n]DTT derivatives (n=4-6) acted as a cavitand for C₆₀ molecules; the cyclic 4-mer formed a 1:2 complex in the solid state, while the 5- and 6-mer formed 1:1 complexes in solution. Furthermore, thiacalix[4]thiophene and selenacalix[n]-selenophene having tert-butylphenyl groups exhibit gelling behavior in toluene, despite the absence of any hydrogen binding sites. Intermolecular chalcogen (S or Se) interactions facilitate the self-assembly that results in this gel formation.