EXAFS Investigations of Metal Organic Synthesis Tools

Abstract
Grignard Compounds RMgX (R=organic group ; X=Cl, 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 cornpounds. 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-(R₁R₂)₂-M (R=organic group ; M=metal, e.g. Li, Mg ; X=leaving group, e.g. Cl, Br, I, NO₂) 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-CH₂-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 α-carbon atom.