Efficient and Selective Catalysts for Asymmetric Synthesis
[RhOH(S)-BINAP]2 (1) and Rh(I) Pre-catalysts
The Hayashi group at Kyoto University has spearheaded the development of rhodium-based catalysts applied in the asymmetric, conjugate addition of arylboronic acids to C=C bonds.1 They utilized Rh-BINAP catalysts to effect several advantages over other enantioselective 1,4-addition reactions: 1) high selectivities (> 95% ee) have been attained, 2) the reaction is performed in an aqueous environment, 3) the reaction temperature is > rt (60 - 90 °C) and thus is advantageous for process design, 4) a multitude of aryl and alkenyl groups can be incorporated, and 5) a variety of electron-deficient olefins can be effectively coupled with boronic acids in an asymmetric fashion.
These Rh(I) catalysts have also been applied in the enantioselective synthesis of 2-aryl-4-piperidones (Scheme 3), which are known to be building blocks for biologically active molecules such as the tachykinin antagonists.3,4 It should be noted that the asymmetric, catalytic 1,4-addition of organoboron or organozinc reagents involves a broad scope of electronically and sterically diverse reagents. In all cases the authors have convincingly produced highly enantiopure 2-aryl-4-piperidones in good to excellent yields.
Miura and co-workers have explored the use of [Rh(OH)(cod)]2 in the regio- and stereoselective homocoupling of γ-arylated tert-propargyl alcohols (Scheme 4).5 This efficient and unprecedented alkyne coupling catalyzed by a Rh(I)-dppp complex (dppb = diphenylphosphinobutane) liberates ketone byproduct through β-carbon elimination to afford 2-hydroxymethyl-(E)-enynes. These organic compounds are useful precursors to dihydrofuran derivatives that produce intense fluorescent emissions in the solid state.