Synthesis of a Library of Fluorescent 2-Aryl-3-trifluoromethylnaphthofurans from Naphthols by Using a Sequential Pummerer-Annulation/Cross-Coupling Strategy and their Photophysical Properties

A library of 2-aryl-3-trifluoromethylnaphthofurans was synthesized with high efficiency from simple naphthols. In this synthesis, the Pummerer-type annulation of naphthols with 3-(2,2,2-trifluoroethylidene)-2,4-dithiapentane 2-oxide was followed by a cross-coupling of the resulting 2-methylthio-3-trifluoromethylnaphthofurans with a variety of arylzinc reagents. A palladium complex, Pd-PEPPSI-IPr, was the most efficient catalyst for the arylation step, which represents the first cross-coupling of aryl sulfides by using an N-heterocyclic-carbene-ligated palladium complex. This library consists of new p-expanded molecules, all of which are fluorescent in the solid state as well as in solution. Their photophysical properties, such as absorption and emission, fluorescence quantum yields, and fluorescence lifetimes, were thoroughly investigated. This library was also useful to identify acidochromic molecules.