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. 2018 May 28;57(22):6496-6500.
doi: 10.1002/anie.201802087. Epub 2018 May 8.

The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-Coupling with Simple Ferric Salts and MeMgBr

Salvador B Muñoz 3rd  1 , Stephanie L Daifuku  1 , Jeffrey D Sears  1 , Tessa M Baker  1 , Stephanie H Carpenter  1 , William W Brennessel  1 , Michael L Neidig  1
Affiliations

The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-Coupling with Simple Ferric Salts and MeMgBr

Salvador B Muñoz 3rd et al. Angew Chem Int Ed Engl. .

Abstract

The use of N-methylpyrrolidone (NMP) as a co-solvent in ferric salt catalyzed cross-coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross-coupled product in reactions utilizing alkyl Grignard reagents. Despite the critical importance of NMP, the molecular level effect of NMP on in situ formed and reactive iron species that enables effective catalysis remains undefined. Herein, we report the isolation and characterization of a novel trimethyliron(II) ferrate species, [Mg(NMP)6 ][FeMe3 ]2 (1), which forms as the major iron species in situ in reactions of Fe(acac)3 and MeMgBr under catalytically relevant conditions where NMP is employed as a co-solvent. Importantly, combined GC analysis and 57 Fe Mössbauer spectroscopic studies identified 1 as a highly reactive iron species for the selective formation generating cross-coupled product. These studies demonstrate that NMP does not directly interact with iron as a ligand in catalysis but, alternatively, interacts with the magnesium cations to preferentially stabilize the formation of 1 over [Fe8 Me12 ]- cluster generation, which occurs in the absence of NMP.

Keywords: MCD spectroscopy; Mössbauer spectroscopy; N-methylpyrrolidone; cross-coupling; iron.

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Conflict of interest statement

Conflict of interest

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
80 K 57Fe Mössbauer spectra of a frozen solution of 57Fe-(acac)3 (9 equiv of NMP relative to MeMgBr) with (A) 20 equiv and (B) 5 equiv of MeMgBr (black dots), total fit (black line) and individual fit components are shown. (C) 5 K, 7 T NIR MCD spectrum of 20 equiv of MeMgBr reacted with Fe(acac)3 and 180 equiv of NMP at RT in 1:1 THF/2-MeTHF.
Figure 2
Figure 2
(A) Structure of [Mg(NMP)6][FeMe3]2 (1), drawn at 50% probability level. Hydrogen atoms omitted for clarity. (B) 80 K 57Fe Mössbauer spectrum of a frozen solution of 1 (black dots), total fit (green line). (C) 5 K, 7 T NIR MCD of 1 dissolved in 1:1 THF/2-MeTHF. (D) Saturation magnetization data (dots) and best fit (lines) collected at 10486 cm−1.
Figure 3
Figure 3
Calculated molecular orbital energy diagram for 1.
Figure 4
Figure 4
80 K 57Fe Mössbauer spectra of a frozen solution of 57Fe-(acac)3 (9 equiv of NMP relative to MeMgBr) with (A) 5 equiv of MeMgBr and subsequent addition of (B) 0.25 equiv, (C) 0.75 equiv of β-bromostyrene and (D) 0.75 equiv of β-bromostyrene followed by 1 equiv of MeMgBr (black dots), total fit (black line) and individual fit components are shown.
Scheme 1
Scheme 1
Iron-catalyzed alkyl-alkenyl cross-coupling reactions with ferric salts reported by Kochi and Cahiez and the molecular structure of [Fe8Me12]−.
See this image and copyright information in PMC

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