HETEROCYCLES

Communication | Special issue | Vol. 77, No. 1, 2009, pp. 151-155
Received, 3rd March, 2008, Accepted, 8th April, 2008, Published online, 11th April, 2008.
DOI: 10.3987/COM-08-S(F)5

■ Use of NaOH as a New Activator for the Palladium-Catalyzed Direct CH Arylation of Thiazole Derivatives

Takayuki Miyaoku and Atsunori Mori*

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan

Abstract

Sodium hydroxide is found to serve as a new activator for the palladium-catalyzed C-H arylation of thiazole derivatives with aryl bromides and aryl iodides. The reaction of benzothiazole proceeds smoothly with 4-bromoanisole to afford the corresponding coupling product in an excellent yield.

Thiazole, which is a five-membered heteroaromatic compound composed of nitrogen and sulfur atom, shows remarkable properties as functional organic materials as well as biologically important molecules.1 Synthesis of substituted thiazole derivatives is thereby a major concern in synthetic organic chemistry. The cross-coupling methodology with a transition metal catalyst is a practical tool for the introduction of a substituent into the thiazole ring.2 In particular, direct coupling of a thiazole derivative at the carbon–hydrogen bond by the catalysis of palladium is one of the practical way to introduce aryl and alkenyl groups via the carbon–carbon bond formation.3,4
We have recently shown that the reaction of thiazole with an aryl iodide with a palladium/copper catalyst system in the presence of tetrabutylammonium fluoride (TBAF) as an activator induces carbon–carbon bond formation at the 2-position of thiazole.5 The reaction was found to take place under mild conditions when several aryl iodies are employed as an organic electrophile, while use of aryl bromides was found to result in no reaction. Since the use of bromides as an electrophile for the coupling reaction considerably extends synthetic utility, our further concern has centered to investigate new catalytic reaction system to allow the reaction of aryl bromides for the introduction of the substituent into thiazole. We herein report that the use of sodium hydroxide as an activator undergoes the direct CH arylation of thiazole derivatives with aryl bromides when bulky trialkylphosphine is employed as a ligand for the palladium catalyst.6
We have examined the reaction of thiazole with 4-bromoanisole under the reaction conditions for aryl iodides using TBAF5a to result in no reaction. Switching the ligand of palladium to (t-Bu3)P was also ineffective. However, the reaction was found to occur when an inorganic base such as potassium carbonate and sodium hydroxide is employed instead of TBAF.
When the reaction of benzothiazole 1 (0.5 mmol) with 4-bromoanisole 2a (0.6 mmol) was carried out with PdCl2(NCPh)2 (10 mol%)-[t-Bu3PH]BF4 (20 mol%)/CuI (10 mol%) and sodium hydroxide (0.6 mmol) in 3 mL of DMSO, the CH arylation reaction to give the coupling product 3a was obtained in 66% yield after stirring at 60 ºC for 24 h (entry 1). Results on the reaction of 1 and 2a under several conditions are summarized in Table 1. Longer reaction period to 46 h improved the yield to 82% (entry 2). Use of aqueous solution of NaOH (1M) did not undergo the reaction at all (entry 3). By contrast, use of the aqueous solution of K2CO3 afforded 3a in moderate yields (entry 4), while the reaction with anhydrous K2CO3 was found to be ineffective (entry 5). The reaction with tricyclohexylphosphine as a ligand for the palladium catalyst resulted in lower yield (11%, entry 5) and attempted another phosphine ligands have been completely ineffective so far.

The reaction with other aryl bromides were examined as shown in Table 2. Bromides bearing an electron-donating substituents such as Me, NMe2 afforded 3 in good yields. On the other hand, the reaction of methyl 4-bromobenzoate resulted in a poor yield (18%).

The reaction of unsubstituted thiazole 4 was examined under the conditions with NaOH as an activator. Although the reaction of 4 with TBAF has been highly selective to take place at the 2-position to give 5a and only a trace amount of 2,5-diarylated 6a was furnished,5a the reaction with NaOH was found to be less selective to afford 37% of 5a and 22% of 6a. However, the improved selectivity was observed when the copper cocatalyst was switched to CuBr or CuCN. These results are summarized in Table 3.

The use of NaOH as an activator for the reaction of thiazole derivatives with aryl iodides was effective. Triphenylphosphine complex of palladium, PdCl2(PPh3)2 was available for the reaction. When the reaction of thiazole with 4-iodoanisole in the presence of PdCl2(PPh3)2 (3 mol%)/CuI (2 mol%) and NaOH (2 equiv) was carried out in DMSO at 60 ºC, 5a was obtained in 77% yield after stirring for 24 h. (eq 1-a) The reaction was also found to be applicable for the reaction of bifunctional aryl iodide 7 and thiazole to give 8, which is potentially available for the preparation of photoluminescent materials. Several spectroscopic and electrochemical properties of 7 and the further derivatives will be described in due course.

In summary, we have shown that sodium hydroxide is a new class of activator for the CH arylation reaction of thiazole derivatives. The availability of NaOH would extend synthetic usefulness of the CH arylation reaction of thiazoles, since NaOH shows advantage in the cost of synthesis toward TBAF. It is also worthy of note that aryl bromides are available for the reaction although further improvement of the catalytic reaction conditions is necessary.

ACKNOWLEDGEMENT
This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas, "Advanced Molecular Transformation of Carbon Resources" by Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

‡This paper is dedicated to Professor Emeritus Keiichiro Fukumoto on the occasion of his 75th birthday.

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