EXAFS Investigations of Metal Organic Synthesis Tools

Abstract

Grignard Compounds RMgX (R=organic group; X=Cl 1 , Br, I) are common tools in inorganic and organic synthesis. The structure of these substances in solution, however, is unknown. To answer this question we performed EXAFS measurements at the Br-K-edge in absorption mode and at the Mg-K-edge in fluorescence mode. From these complementary data a consistent model of the structure of the investigated Grignard compounds in solution is deduced. A similar class of reagents are Reformatsky compounds RZnBr (R=organic group), which possess a very high chemical selectivity in contrast to the Grignard compounds. In order to determine the degree of association in solution we performed EXAFS measurements at the Zn-K-edge as well as at the Br-K-edge. To learn more about the chemical behavior we varied the organic residue and the solvent. Carbenoids of the general formula XC-(R 1 R 2 ) 2 -M (R=organic group; M=metal, e.g. Li, Mg; X=leaving group, e.g. Cl , Br,I, NO 2 ) are modern tools in the synthesis of chiral organic compounds, but have the disadvantage to decompose at temperatures higher than -50°C. In detail we investigated Mg[Ph-CH 2 -C(H)Br] 2 disolved in THF. From the EXAFS spectroscopical determined data it was possible to derive a detailed structure model, especially to determine the geometry of the a-carbon atom.