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.
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Meyer et al. (2000) studied this question.