Journal
ADVANCED SYNTHESIS & CATALYSIS
Volume 355, Issue 6, Pages 1098-1106Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adsc.201300034
Keywords
carbenes; cooperative catalysis; organocatalysis; oxidation; ruthenium
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Funding
- Alexander von Humboldt Foundation
- SFB858 [Z1]
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The paper describes a cooperative NHC (N-heterocyclic carbene) and ruthenium-based redox catalysis for the mild aerobic oxidative esterification of various aromatic and heteroaromatic aldehydes. The ruthenium(II) complex Ru(bpz)3(PF6)2 (bpz=2,2-bipyrazine) as catalyst is shown to be compatible with free NHCs. The NHC is used in these cascade reactions for the umpolung of the aldehyde to form the corresponding Breslow intermediate which in turn gets oxidized to an acylazolium ion by the ruthenium redox catalyst. Air is used as a terminal oxidant for oxidation (regeneration) of the ruthenium catalyst. In addition, we will show that in the absence of the ruthenium redox catalyst and alcohol, NHC-catalyzed aerobic oxidation of aldehydes delivers the corresponding acids in good to excellent yields. Mechanistic studies and DFT calculations supporting our suggested mechanisms are provided.
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