Journal
ADVANCED SYNTHESIS & CATALYSIS
Volume 356, Issue 2-3, Pages 571-578Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/adsc.201300801
Keywords
carbenes; NMR spectroscopy; oxidation; reaction mechanisms; umpolung
Categories
Funding
- Fonds der Chemischen Industrie
Ask authors/readers for more resources
The involvement of a spiroepoxide, an isomer of the Breslow intermediate, in the catalytic cycle of the N-heterocyclic carbene (NHC)-catalyzed benzoin condensation was suggested by S. Gronert in 2007, based on calculations. To test this hypothesis, enediamines were generated in situ from (in part C-13-labeled) N-heterocyclic carbenes, benzyl bromide and base, and subsequently treated with various single-oxygen atom oxidants. Clean oxidation occurred with a triazolylidene-derived enediamine and an N-sulfonyloxaziridine as oxidant, but NMR monitoring gave no hint to the accumulation of a spiroepoxide. Instead, oxidative cleavage of the enediamine to the free carbene and benzaldehyde was observed, with subsequent oxidation of the carbene to a urea. Presumably, electrophilic oxygenation of the enediamine occurs at the exocyclic methylene carbon atom and affords a zwitterion - identical to the primary carbene-aldehyde adduct of the benzoin cycle - which undergoes retro-cleavage to carbene and aldehyde. However, the occurrence of a spiroepoxide as transient intermediate of the oxidative CC bond cleavage cannot rigorously be excluded.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available