Diastereoselectivity of cyclopropanation of substituted α-fluorostyrenes versus styrenes by different methods

The diastereoselectivity of cyclopropanations of styrene and alpha-fluorostyrene with diazoacetate depends on the catalyst used and the presence or absence of the fluorine substituent. The Cu(acac)(2) catalyzed reaction of styrene with diazoacetate led to 3:1 selectivity in favor of trans-2-phenylcyclopropane carboxylate, while a-fluor styrene gave a 1:1 mixture of cis/trans-isomers. A competition experiment proved that a-fluorostyrene reacted slower compared to styrene itself. With the bulkier tetraphenyl-iron(III)-porphyrin chloride as catalyst, 10:1 or 3:1 mixture, respectively, were obtained. An advantage of the latter protocol is the in situ formation of ethyl diazoacetate from ethyl glycinate hydrochloride in aqueous solution by diazotation avoiding the in-substance application of the potentially explosive ethyl diazoacetate. Accordingly, a series of diastereoisomeric ethyl 2-aryl-2-fluoro-cyclopropane carboxylates was synthesized from p- or m-substituted alpha-fluorostyrenes.