Highly Selective Synthetic Reactions by the Combined Use of Organometallic Reagents and Radical Species

Abstract

Abstract During these thirty years, we have pursued new methodologies in organic synthesis and developed many synthetically useful reactions. Among them, four topics are described in this article. (1) Regio- and stereoselective silylmetalation of acetylenes has been examined. Whereas platinum- or copper-catalyzed silylmagnesation of terminal acetylenes afforded (E)-1-silyl-1-alkene exclusively, palladium-catalyzed silylalumination with PhMe2Si-AlEt2 provided 2-silyl-1-alkenes with high regioselectivity. (2) Triethylborane induced radical addition of Ph3SnH to acetylenes gave 1-triphenylstannyl-1-alkenes in the presence of small amount of oxygen. The reaction has two distinguishing characteristics. One feature is that Et3B can initiate the radical reaction at low temperature, such as −78 °C. (3) The other distinctive feature of Et3B-induced radical reaction is that many solvents could be used for the reaction. Thus, water was chosen as a solvent, and Et3B-induced atom-transfer radical cyclization of iodo acetals and iodoacetates in water was examined. (4) Three types of organometallic ate complexes, R3MnMgBr, R3MgLi, and R3Co(L2)MgBr, were prepared and used for organic synthesis. Treatment of gem-dibromocyclopropane with trialkylmanganate provided alkylated cyclopropane after aqueous workup. Aryl and alkenyl halides could be converted into the corresponding magnesium reagents by the action of trialkylmagnesate via halogen–magnesium exchange. Finally, synthetic reactions catalyzed by cobalt complexes are described. Without suffering from β-elimination, cobalt complexes allow cross-coupling reactions of alkyl halides with Grignard reagents.