Use of the Imine–Enamine Equilibrium in Cooperative Ligand Design

Abstract

The imine–phosphine ligands Ph₂PC₅H₇NAr, where Ar = 2,6-Prⁱ₂C₆H₃, 2,6-Me₂C₆H₃, were deprotonated using KH to generate the corresponding potassium salts, which were reacted with [(COD)­IrCl]₂ to generate the enamidophosphine derivatives (COD)­Ir­(Ph₂PC₅H₆NAr) (Ar = 2,6-Prⁱ₂C₆H₃, <b>4a</b>; Ar = 2,6-Me₂C₆H₃, <b>4b</b>). These complexes were exposed to alcohols, H₂, and CO to generate a series of products, some of which involve protonation of the enamido unit to generate the imine tautomer. The reaction of <b>4a</b> with isopropyl alcohol or H₂ generates the dinuclear hexahydride [(Ph₂PC₅H₇N-2,6-Prⁱ₂C₆H₃)­IrH₂]₂(μ-H)₂ (<b>5a</b>), while the reaction with primary alcohols generates the dicarbonyl enamidophosphine complex (CO)₂Ir­(Ph₂PC₅H₆NAr) (<b>6a</b>). The reaction of the hexahydride <b>5a</b> with CO generates <b>6a</b>, for which a mechanism is proposed on the basis of monitoring this reaction as a function of time by NMR spectroscopy. On the basis of these experiments, cooperative ligand effects can be replicated by imine–phosphine ligands by proton transfer to and from the ligand backbone