Synthesis of Citronellal by RhI-Catalysed Asymmetric Isomerization ofN,N-Diethyl-Substituted Geranyl- and Nerylamines or Geraniol and Nerol in the Presence of Chiral Diphosphino Ligands, under Homogeneous and Supported Conditions

Abstract

For the asymmetric isomerization of geranyl- or neryldiethylamine ((E)- or (Z)-1, resp.) and allyl alcohols geraniol or nerol ((E)- or (Z)-2, resp.) to citronellal (4) in the presence of a [RhI(ligand)cycloocta-1,5-diene)]+ catalyst, the atropic ligands 5 – 11 are compared under homogeneous and polymer-supported conditions with the non-C2-symmetrical diphosphino ferrocene ligands 12 – 16. The tBu-josiphos ligand 13 or daniphos ligand 19, available in both antipodal series, already catalyse the reaction of (E)-1 at 20° (97% e.e.) and favourably compare with the binap ligand 5 (see Table 1). Silica-gel- or polymer-supported diphosphino ligands usually afford similar selectivity as compared to the corresponding ligands applied under homogeneous conditions, but are generally less reactive. In this context, a polymer-supported ligand of interest is the polymer-anchored binap (R)-6, in terms of reactivity, selectivity, and recoverability, with a turnover of more than 14400.