Chiral Iron(II) NPPN Complexes: Synthesis and Application in the Asymmetric Strecker Reaction of Azomethine Imines

A One-Step Preparation of Tetradentate Ligands with Nitrogen and Phosphorus Donors by Reductive Amination and Representative Iron Complexes

The synthesis and use of the first examples of unsymmetrical, mixed phosphine donor tripodal NPP₂' ligands N(CH₂CH₂PR₂)₂(CH₂CH₂PPh₂) are presented. The ligands are synthesized via a convenient, one pot reductive amination using 2-(diphenylphosphino)ethylamine and various substituted phosphonium dimers in order to introduce mixed phosphine donors substituted with P/P', those being Ph/Cy (2), Ph/ⁱPr (3), Ph/ⁱBu (4), Ph/o-Tol (5), and Ph/p-Tol (6). Additionally, we have developed the first known synthesis of a symmetrical tripodal NP₃ ligand N(CH₂CH₂PiBu₂)₃ using bench safe ammonium acetate as the lone nitrogen source (7). This new protocol eliminates the use of extremely dangerous nitrogen mustard reagents typically required to synthesize NP₃ ligands. Some of these tetradentate ligands and also P₂NN' ligands N(CH₂-o-C₅H₄N)(CH₂CH₂PR₂)₂ (P₂NN'-Cy, R = Cy; P₂NN'-Ph, R = Ph) prepared by reductive amination using 2-picolylamine are used in the synthesis and reactions of iron complexes. FeCl₂(P₂NN'-Cy) (8) undergoes single halide abstraction with NaBPh₄ to give the trigonal bipyramidal complex [FeCl(P₂NN'-Cy)][BPh₄] (9). Upon exposure to CO(g), complex 9 readily coordinates CO giving [FeCl(P₂NN'-Cy)(CO)][BPh₄] (10), and further treatment with an excess of NaBH₄ results in formation of the hydride complex [Fe(H)(P₂NN'-Cy)(CO)][BPh₄] (11). Our previously reported complex FeCl₂(P₂NN'-Ph) undergoes double halide abstraction with NaBPh₄ in the presence of the coordinating solvent to give [Fe(NCMe)₂(P₂NN'-Ph)][BPh₄]₂ (12). Ligand 3 can be coordinated to FeCl₂, and upon sequential halide abstraction, treatment with NaBH₄, and exposure to an atmosphere of dinitrogen, the dinitrogen hydride complex [Fe(H)(NPP₂'-ⁱPr)(N₂)][BPh₄] (13) is isolated. Our symmetrical NP₃ ligand 7 can also be coordinated to FeCl₂ and, upon exposure to an atmosphere of CO(g), selectively forms [FeCl(NP₃)(CO)][BPh₄] (14) after salt metathesis with NaBPh₄. Complex 14 can be treated with an excess of NaBH₄ to give the hydride complex [Fe(H)(NP₃)(CO)][BPh₄] (15), which can further be deprotonated/reduced to the Fe(0) complex Fe(NP₃)(CO) (16) upon treatment with an excess of KH.

Asymmetric FeII-Catalyzed Thia-Michael Addition Reaction to α,β-Unsaturated Oxazolidin-2-one Derivatives

A highly enantioselective Fe^II-catalyzed thia-Michael addition to α,β-unsaturated carbonyl derivatives was developed. The scope of the reaction was demonstrated with a selection of aromatic, heterocyclic and aliphatic thiols, and various Michael acceptors. The corresponding β-thioethers were obtained in good to excellent yields (up to 98%) and moderate to excellent enantioselectivities (up to 96:4 er). Unusual hepta-coordination of the metal and chelation to α,β-unsaturated oxazolidin-2-one derivatives allowed the construction of a coherent model rationalizing the enantioselective event. DFT calculations support the proposed model for observed stereoselectivities.

An efficient construction of N,N-bicyclic pyrazolidinones comprising enaminonitriles via asymmetric [3+2] cycloaddition

The first thiourea catalysed asymmetric [3+2] cycloaddition reaction between azomethine imines and malanonitriles was devoloped.