Asymmetric Cyclopropanation of Vinyl Fluorides: Access to Enantiopure Monofluorinated Cyclopropane Carboxylates

The transition metal catalyzed cyclopropanation with alkyl diazoacetates of aliphatic or aromatic vinyl fluorides, prepared from the corresponding alkenes by bromofluorination and subsequent dehydrobromination, provides a smooth access to racemic 1 : 1 mixtures of cis/trans isomeric monofluorinated cyclopropane carboxylates. The application of enantiopure bis(oxazoline) ligands and copper(I) triflate makes the reaction trans-diastereoselective and enantioselective. For example, treatment of α-fluorostyrene (3a) with tert-butyl diazoacetate in the presence of 2 mol% of the catalyst prepared from (S)-tert-leucine-based 11b and CuOTf gave a 4 : 1 mixture of trans-2-fluoro-2-phenylcyclopropanecarboxylate (4e) with 93% ee and the corresponding cis-isomer 5e with 89% ee. The absolute configuration of the trans-isomer 4e has been determined to be (1S,2S) by X-ray structure analysis of a derivative.