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Volumn 73, Issue 9, 2008, Pages 3452-3459

Copper-catalyzed domino annulation approaches to the synthesis of benzoxazoles under microwave-accelerated and conventional thermal conditions

Author keywords

[No Author keywords available]

Indexed keywords

AMIDES; CATALYST ACTIVITY; COPPER; CYCLIZATION; REACTION KINETICS; REGIOSELECTIVITY; SYNTHESIS (CHEMICAL);

EID: 43449101554     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo702145d     Document Type: Article
Times cited : (257)

References (56)
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    • For selected examples of transition metal (Cu or Pd) catalyzed domino annulation processes leading to heterocycles, see the followingIndoles (Pd): (a) Willis, M. C.; Brace, G. N.; Holmes, I. P. Angew. Chem., Int. Ed. 2005, 44, 403-406.
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    • The only differences between the conditions reported in ref 25 and those reported herein are the catalyst loading (5% vs 10, and the choice of base (K2CO3 vs K3PO4, In addition, we initially encountered some problems with irreproducibility, which were found to be dependant upon maintaining efficient stirring during the course of the reaction, and upon the morphology of the K3PO4 base. Reactions were found to be reproducible only when conducted using K 3PO4 from a new bottle, which was free-flowing, fine granular material Fluka, catalog no. 04347, The importance of K 3PO4 morphology in copper-catalyzed N-arylation reactions has previously been observed by Buchwald and co-workers; see: Klapars, A, Huang, X, Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 7421-7428
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    • In an attempt to overcome this limitation for the poor control of regioselectivity and expand the scope of available substrates, we also evaluated the use of, o-bromo and iodoaryl triflates, which are readily synthesized by ortho-halogenation and triflation of phenols. Unfortunately only trace amounts (≤5, of the desired products were obtained, presumably because unlike Pd-catalyzed transformations, Cu does not undergo effective oxidative addition to the aryl triflate bond. The trace amounts of benzoxazole observed in these reactions may be due to a background reaction pathway not involving cross-coupling e.g, triflate hydrolysis followed by dehydrative annulation or nucleophilic aromatic substitution
    • In an attempt to overcome this limitation for the poor control of regioselectivity and expand the scope of available substrates, we also evaluated the use of, o-bromo and iodoaryl triflates, which are readily synthesized by ortho-halogenation and triflation of phenols. Unfortunately only trace amounts (≤5%) of the desired products were obtained, presumably because unlike Pd-catalyzed transformations, Cu does not undergo effective oxidative addition to the aryl triflate bond. The trace amounts of benzoxazole observed in these reactions may be due to a background reaction pathway not involving cross-coupling (e.g., triflate hydrolysis followed by dehydrative annulation or nucleophilic aromatic substitution).
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* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.