Crystal-Free Crystallography Method Clarifies Structure Of Iodine Reagent

When chemists in Japan last year reported a crystal-free X-ray diffraction method for determining structures of molecules by inserting them in metal-organic framework (MOF) hosts, researchers rejoiced. The work didn’t live up to all its promises, however—the team realized that the method could not unambiguously determine the stereochemistry of a marine sponge metabolite, as they previously claimed. An independent group at MIT has now shown that the method still works for determining atom connectivity (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201310897). Ekaterina V. Vinogradova, Peter Müller, and Stephen L. Buchwald determined the structure of another group’s iodine reagent, used for transferring trifluoromethyl-sulfur groups to carbon atoms. The structure shows that the SCF₃ group is bound to the oxygen atom of the reagent, not the iodine atom as previously reported. The revised structure suggests that the reagent may be working through an unexpected mechanism. The hardest part of the crystal-free method was getting the reagent inside the MOF scaffold, says Müller, who directs MIT’s X-ray facility. Vinogradova spent considerable time preparing the MOF and soaking it in the pure compound, which is a liquid. The method isn’t for every molecule, Müller says, and to do it right, “you need to be patient.”