Pentamethylcyclopentadienyl-iridium(iii) complexes with pyridylamino ligands: synthesis and applications as asymmetric catalysts for Diels–Alder reactions

Abstract

Reaction of the dimer [(Cp*IrCl)₂(µ-Cl)₂] with chiral pyridylamino ligands (pyam, L₁–L₅) in the presence of NaSbF₆ gave complexes [Cp*IrCl(pyam)][SbF₆] 1–5 as diastereomeric mixtures, which have been fully characterised, including the X-ray molecular structure determination of the complexes (S_Ir,R_N,R_C)–[Cp*IrClL₁][SbF₆] 1a and (R_Ir,S_N,S_C)–[Cp*IrClL₅][SbF₆] 5a. Treatment of these cations with AgSbF₆ affords the corresponding aqua species [Cp*Ir(pyam)(H₂O)][SbF₆]₂6–10 which have been also fully characterised. The molecular structure of the complex (S_Ir,R_N,R_C)–[Cp*IrL₁(H₂O)][SbF₆]₂6 has been determined by X-ray diffractometric methods. The aqua complexes [Cp*Ir(pyam)(H₂O)][SbF₆]₂ (6, pyam = L₂ (7), L₃ (8)) evolve to the cyclometallated species [Cp*Ir{κ³(N,N′,C)-(R)-(C₆H₄)CH(CH₃)NHCH₂C₅NH₄}][SbF₆] (11), [Cp*Ir{κ³(N,N′,C)-(R)-(C₁₀H₆)CH(CH₃)NHCH₂C₅NH₄)}][SbF₆] (12), and [Cp*Ir{κ³(N,N′,C)-(R)-(C₁₀H₆)CH(CH₃)NHCH₂C₉NH₆)}][SbF₆] (13) respectively, via intramolecular activation of an ortho C–H aryl bond. Complexes 6–10 are enantioselective catalysts for the Diels–Alder reaction between methacrolein and cyclopentadiene. Reaction occurs rapidly at room temperature with good exo : endo selectivity (from 81 : 19 to 98 : 2) and moderate enantioselectivity (up to 72%). The involved intermediate Lewis acid–dienophile compounds [Cp*Ir(pyam)(methacrolein)][SbF₆]₂ (pyam = L₄ (14), L₅ (15)) have been isolated and characterised.