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Competitive Copper Catalysis in the Condensation of Primary Nitro Compounds with Terminal Alkynes: Synthesis of Isoxazoles
Author(s) -
Baglieri Ausilia,
Meschisi Luca,
De Sarlo Francesco,
Machetti Fabrizio
Publication year - 2016
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201600897
Subject(s) - chemistry , phenylacetylene , catalysis , nitro , copper , enyne , medicinal chemistry , selectivity , base (topology) , nitro compound , condensation reaction , coupling reaction , alkyne , organic chemistry , combinatorial chemistry , mathematical analysis , alkyl , mathematics
Isoxazoles, mainly 3,5‐disubstituted, are prepared by catalytic condensation of primary nitro compounds with terminal acetylenes by using a copper/base catalytic system. The additional catalytic effect of the copper(II) salts is evidenced by comparing the kinetic profiles. Selectivity dependence on reaction conditions is considered for phenylacetylene in the following competitive processes: oxidative coupling of terminal alkynes to conjugated diynes catalyzed by Cu II and base in the presence of air; production of furazans beside condensation with benzoylnitromethane to 3‐benzoylisoxazoles, as a result of the reaction of the dipolarophile with 3,4‐dibenzoylfuroxan; addition of electron‐poor alkynes (e.g., methyl propiolate) with themselves and with the nitro compound. Thus, oxidative coupling is negligible in reactions with “active” nitro compounds, whereas with nitroalkanes both products are observed: only trace amounts of isoxazoles are detected without copper. Similarly, in the presence of copper, 3‐benzoyl‐5‐phenylisoxazole is predominant over the furazan. Furthermore, condensations of electron‐poor alkynes give complex reaction mixtures in the presence of base alone, but cycloadducts are conveniently prepared with copper. The results indicate the practical and general utility of this catalytic method for synthetic practice.