Alkyl Grignard cross-coupling of aryl phosphates catalyzed by new, highly active ionic iron(ii) complexes containing a phosphine ligand and an imidazolium cation

Abstract

A novel family of ionic iron(II) complexes of the general formula [HL][Fe(PR′₃)X₃] (HL = 1,3-bis(2,6-diisopropylphenyl)imidazolium cation, HIPr, R′ = Ph, X = Cl, 2; HL = HIPr, R′ = Cy, X = Cl, 3; HL = HIPr, R′ = Ph, X = Br, 4; HL = HIPr, R′ = Cy, X = Br, 5; HL = 1,3-bis(2,4,6-trimethylphenyl)imidazolium cation, HIMes, R′ = Cy, X = Br, 6) was easily prepared via a stepwise approach in 88%–92% yields. In addition, an ionic iron(II) complex, [HIPr][Fe(C₄H₈O)Cl₃] (1), has been isolated from the reaction of FeCl₂(THF)_1.5 with one equiv. of [HIPr]Cl in 90% yield and it can further react with one equiv. of PPh₃ or PCy₃, affording the corresponding target iron(II) complex 2 or 3, respectively. All these complexes were characterized by elemental analysis, electrospray ionization mass spectrometry (ESI-MS), ¹H NMR spectroscopy and X-ray crystallography. These air-insensitive complexes 2–6 showed high catalytic activities in the cross-coupling of aryl phosphates with primary and secondary alkyl Grignard reagents with a broad substrate scope, wherein [HIPr][Fe(PCy₃)Br₃] (5) was the most effective. Complex 5 also catalyzes the reductive cross-coupling of aryl phosphates with unactivated alkyl bromides in the presence of magnesium turnings and LiCl, as well as the corresponding one-pot acylation/cross-coupling sequence under mild conditions.