Isomerisation and controlled condensation in an aqueous medium of allyl alcohol catalysed by new water-soluble rhodium complexes with 1,3,5-triaza-7-phosphaadamantane (PTA)

Abstract

New aqua-soluble rhodium(I) [Rh(CO)(PTA)₄]Cl (1) (PTA = 1,3,5-triaza-7-phosphaadamantane) and rhodium(III) [RhCl₂(PTA)₄]Cl (2) complexes have been synthesized via the reaction of [{Rh(CO)₂(μ-Cl)}₂] or RhCl₃·3H₂O, respectively, with stoichiometric amounts of PTA in ethanol. Compound 1 is also obtained upon reduction of 2 in an H₂/CO atmosphere. They have been characterized by IR, ¹H and ³¹P{H} NMR spectroscopies, elemental and single crystal X-ray diffraction analyses. While compound 1 shows distorted square-pyramid geometry (τ₅ = 0.09) with a P₃C-type basal plane, compound 2 is octahedral with the chloro ligands in the cis position. The hydride rhodium(I) complex [RhH(PTA)₄] (3) is formed upon the addition of NaBH₄ to an aqueous solution of 1 or 2. Compounds 1–3 (in the case of 2 upon reduction by H₂) act as homogeneous catalysts, or catalyst precursors, in the isomerisation and condensation of allyl alcohol at room temperature and in an aqueous medium. The product selectivity is easily controlled by changing the concentration of the base in the reaction mixture, thus resulting in the exclusive formation of either 3-hydroxy-2-methylpentanal (HP) or 2-methyl-2-pentenal (MP) in quantitative yields.