Universität zu Köln

Berkessel, Albrecht ORCID: 0000-0003-0470-7428 and Elfert, Silvia (2014). On the Involvement of a Spiroepoxide Intermediate in N-Heterocyclic Carbene (NHC)-Catalyzed Benzoin Condensations - An Approach by Oxygenation of Deoxy-Breslow Intermediates. Adv. Synth. Catal., 356 (2-3). S. 571 - 579. WEINHEIM: WILEY-V C H VERLAG GMBH. ISSN 1615-4169

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Abstract

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.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Berkessel, Albrecht UNSPECIFIED orcid.org/0000-0003-0470-7428 UNSPECIFIED Elfert, Silvia UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-445963
DOI:	10.1002/adsc.201300801
Journal or Publication Title:	Adv. Synth. Catal.
Volume:	356
Number:	2-3
Page Range:	S. 571 - 579
Date:	2014
Publisher:	WILEY-V C H VERLAG GMBH
Place of Publication:	WEINHEIM
ISSN:	1615-4169
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Organic Chemistry
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language STABLE CARBENE; THIAZOLIUM SALTS; THIAMINE ACTION; UMPOLUNG; 1,3,4-TRIPHENYL-4,5-DIHYDRO-1H-1,2,4-TRIAZOL-5-YLIDENE; DIMETHYLDIOXIRANE; REACTIVITY; MECHANISM; ENAMINES; SPECTROSCOPY Multiple languages Chemistry, Applied; Chemistry, Organic Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/44596