On the Involvement of a Spiroepoxide Intermediate in N-Heterocyclic Carbene (NHC)-Catalyzed Benzoin Condensations - An Approach by Oxygenation of Deoxy-Breslow Intermediates

摘要

The involvement of a spiroepoxide, an isomer of the Breslow intermediate, in the catalytic cycle of the N-heterocyclic carbene (NHC)-cata-lyzed benzoin condensation was suggested by S. Gronert in 2007, based on calculations. To test this hypothesis, enediamines were generated in situ from (in part ~(13)C-labeled) N-heterocyclic carbenes, benzyl bromide and base, and subsequently treated with various single-oxygen atom oxidants. Clean oxidation occurred with a triazolylidene-derived ene-diamine 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 benzal-dehyde was observed, with subsequent oxidation of the carbene to a urea. Presumably, electrophilic oxygenation of the enediamine occurs at the exocy-clic 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 C=C bond cleavage cannot rigorously be excluded.