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

Abstract

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 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 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 CC bond cleavage cannot rigorously be excluded. magnified image