Chromium Complex Catalyzed Synthesis of Spirocyclopropanes from Diaryl Diazo Compounds – Direct NMR-Spectroscopic Observation of a Carbene Complex Intermediate
The [2 + 1] cycloaddition reaction of electron-rich alkenes such as enolethers with 9-diazo-9H-fluorene (1) is efficiently catalyzed by pentacarbonyl(η2-cis-cyclooctene)chromium(0) (2). This cyclopropanation reaction shows a pronounced preference for electron-rich CC bonds, as demonstrated by the regioselective reactions of allyl vinyl ether and 2-vinyloxyethyl acrylate; the [2 + 1] cycloaddition proceeds