Palladium-Catalyzed Cross-Coupling Reaction of Heteroaryltriolborates with Aryl Halides for Synthesis of Biaryls

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Abstract
Cyclic triolborates possessing a heteroaromatic ring on the boron atom [ArB(OCH2)3CHCH3]M (M=K, Na, Li) were prepared in high yields from heteroarylboronic acids, 1,1,1-tris(hydroxymethyl)ethane (triol) and KOH or NaH. The corresponding lithium salts were synthesized from aryllithiums, B(OMe)3 and triol. They were air-stable white solids that were convenient for handling in air. High performance of these triolborates for metal-catalyzed bond-forming reactions was demonstrated in palladium-catalyzed cross-coupling reactions with haloarenes. Although the use of heteroarylboronic acids often results in very low yields due to competitive hydrolytic B-C bond cleavage with water under typical conditions using aqueous bases, triolborates possessing a 2-pyridyl, 3-pyridyl or 2-thiophenyl ring afforded biaryls in high yields at 50-120℃ in anhydrous DMF. There was a strong accelerating effect of CuI for reactions of 2- and 3-pyridylborate derivatives, whereas 2-thiophenyl derivatives reacted smoothly resulting in high yields in the absence of CuI
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