Determining the diastereoselectivity of new synthetic molecules can be a challenge when NMR methods fail and the compounds are difficult to crystallize. Encapsulating organic crystals (EnOCs) can be used to overcome this challenge. Here we show that the diastereomeric configuration of racemic mixtures of propargyl cyclopropanes can be determined by co-crystallization with 1,3,5,7-tetrakis(2-bromo-4-methoxyphenyl)adamantane (TBro) and X-ray crystallography. Three crystal structures are reported that unambiguously identify the products of Fe-catalyzed cyclopropanations as cis- or trans-isomers. These findings expand the scope of co-crystallization with tetraaryladamantanes as a method to determine the stereochemical configuration of organic molecules that are difficult to crystallize by themselves.
The nucleophilic Fe-complex Bu4N[Fe(CO)3(NO)]
(TBA[Fe]) catalyzes the cyclopropanation of enynes to substituted
propargyl cyclopropanes using diazoesters as carbene surrogates. The
catalyst can be activated either thermally in the presence of catalytic
amounts of 4-nitroanisole or under photochemical conditions. Cyclopropanation
occurs selectively at the enyne moiety; alternative olefinic moieties
remain intact.
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