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Copper-catalyzed cross-coupling of amides and potassium alkenyltrifluoroborate salts: a general approach to the synthesis of enamides
Ist Teil von
Tetrahedron, 2010-07, Vol.66 (27), p.5283-5294
Ort / Verlag
Elsevier Ltd
Erscheinungsjahr
2010
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Potassium alkenyltrifluoroborate salts undergo coupling with amides to give enamides using a catalytic amount of Cu(OAc)
2 under mild oxidative conditions. The air and water stable alkenyltrifluoroborate salts offer a practical alternative to the use of alkenyl halides and alkenylboronic acids as cross-coupling partners. A range of amides participate in the cross-coupling, including heterocyclic amides, imides, carbamates, benzamides, and acetamides. Optimization studies established two sets of conditions, best suited to either high p
K
a or low p
K
a amide substrates. Lower p
K
a amide substrates worked best using a dichloromethane solvent system in the presence of 4
Å molecular sieves, 10
mol
% Cu(OAc)
2, and 20
mol
%
N-methylimidazole. Higher p
K
a amide substrates worked best using a ‘ligandless’ protocol using a 1:1 dichloromethane/DMSO solvent system in the presence of 4
Å molecular sieves and 10
mol
% Cu(OAc)
2. The cross-coupling reactions occur stereospecifically with retention of alkene configuration from the alkenyltrifluoroborate salt. The mild reaction conditions employed are tolerant of various functionalities, including nitro, acetals, alkyl and aryl halides, and α,β-unsaturated carbonyls. Finally, the importance of copper sources and the presence of minor impurities were investigated.
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