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

Abstract

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 π-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.

Cross-coupling of Aryl Sulfides Powered by <i><b>N</b></i>-Heterocyclic Carbene Ligands

“…7b Having developed interest in the uorescent properties of 2 arylbenzofurans, we then started to construct a library of more π extended 2 arylnaphthofurans. 9 PdCl 2 (dppf) usually worked nicely as a catalyst for the arylation. However, when it came to installing a bulky aryl group, the conventional protocol was ineffective.…”

Practical, Modular, and General Synthesis of Benzofurans through Extended Pummerer Annulation/Cross‐Coupling Strategy

“…Chemie strategy, we developed new Kumada-Tamao-Corriu conditions for bulky thioethers [7,8,19] [6] The reaction of 4 e with the 3-methoxy-4-siloxyphenylzinc reagent [21] provided TBS-protected E5 (TBS) in 57 % yield, and was smoothly deprotected with TBAF to give E5 in 96 % yield. Finally, we performed modular and tailor-made syntheses of multisubstituted difurans because substituents on the furan cores significantly alter their photophysical properties.…”

“…Finally, we performed modular and tailor-made syntheses of multisubstituted difurans because substituents on the furan cores significantly alter their photophysical properties. [3,6] Our iterative Pummerer annulation/cross-coupling strategy leads to the construction multisubstituted difurans at will ( Table 3). The details of the strategy are as follows.…”

“…We recently reported the synthesis of 3-trifluoromethylbenzofurans, [6] which have a convertible alkylthio group [7] at the 2-position, from simple phenols and perfluoroalkylsubstituted ketene dithioacetal monoxide (KDM) through an extended Pummerer annulation (Scheme 1). [8][9][10] Our diversity-oriented synthesis of substituted benzofurans was accomplished through an extended Pummerer annulation/ cross-coupling [11] sequence.…”

Pd‐NHC‐Catalyzed Alkynylation of General Aryl Sulfides with Alkynyl Grignard Reagents

“…Pyridyl sulfide 1 k was smoothly converted to afford 3 ka in 93 % yield. We have been interested in new extended Pummerer reactions of alkenyl sulfides, and previously developed concise approaches to 2‐methylsulfanylbenzo[ b ]furans, and 2‐methylsulfanylbenzo[ b ]thiophenes . Taking advantage of these synthetic routes, we could install a phenylethynyl group at the 2 positions of 1 l – s to give 3 la – sa .…”