A mild and regioselective C-H substitution reaction of thiazole by the catalysis of the palladium/copper system is found to proceed in the presence of tetrabutylammonium fluoride under mild conditions. Applying the new reaction, we synthesized various 2,5-diarylthiazoles that exhibit light-emitting and liquid crystalline characteristics in good yields. The color of light emission is tunable by the introduction of the substituent. Differently substituted 2,5-diarylthiazoles are found to exhibit a stable liquid crystal phase in a wide temperature range.

The coupling reaction of terminal alkynes with organic halides, Sonogashira-(Hagihara) coupling, takes place with only 2 equivalents of dilute aqueous ammonia as an additive. The reaction of phenylacetylene and 4-iodoanisole in the presence of 1 mol% of PdCl2(PPh3)2, 2 mol% of CuI, and 2 equiv of aqueous ammonia in THF proceeds at room temperature for 6 h to afford the coupling product in a quantitative yield.

Coupling reaction of terminal alkynes catalyzed by Pd/ Cu with several organic electrophiles is shown to proceed by use of tetrabutylammonium fluoride (TBAF) or tetrabutylammonium hydroxide (TBAOH) as an activator. Treatment of 1-octyne and 4-methoxy-iodobenzene in the presence of 2.0 equiv of TBAOH, 1 mol% of PdCl 2 (PPh 3 ) 3 , and 2 mol% of CuI affords 1-phenyl-1-octyne in 98% yield after stirring at room temperature for 1 h. The new coupling reaction is also applied to polymer synthesis via polycondensation of a diyne and a diiodoarene.

The synthesis of donor-acceptor-type 2,5-diarylthiazoles that bear electron-donating N,N-dialkylamine and electron-withdrawing cyano groups at the 2- and 5-position, respectively, were carried out with transition-metal-catalyzed C-H arylation reactions developed by us. The compounds were synthesized by the C-H arylation of unsubstituted thiazole at the 2-position with a palladium/copper catalyst in the presence of tetrabutylammonium fluoride (TBAF) as an activator. Further C-H arylation of the 2-arylated thiazole at the 5-position was carried out by the palladium-catalyzed reaction in the presence of silver(I) fluoride to afford the donor-acceptor-type 2,5-diarylthiazoles with N,N-dialkylamine groups of different chain lengths. The UV/Vis absorption, photoluminescence, and electrochemical behavior were similar regardless of chain length, whereas liquid-crystalline behavior and thermal characteristics were found to be dependent on the alkyl-chain length. The compounds with N,N-diethylamine or N-butyl-N-methyl groups showed a stable liquid-crystalline phase over a wide temperature range as well as higher stability to thermal decomposition.