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. 2022 Sep 6;27(18):5751.
doi: 10.3390/molecules27185751.

Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand

Sudheer S Kurup  1 , Natalie M Woodland  2 , Richard L Lord  2 , Stanislav Groysman  1
Affiliations

Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand

Sudheer S Kurup et al. Molecules. .

Abstract

Treatment of Mn(N(SiMe3)2)2(THF)2 with bulky chelating bis(alkoxide) ligand [1,1':4',1''-terphenyl]-2,2''-diylbis(diphenylmethanol) (H2[O-terphenyl-O]Ph) formed a seesaw manganese(II) complex Mn[O-terphenyl-O]Ph(THF)2, characterized by structural, spectroscopic, magnetic, and analytical methods. The reactivity of Mn[O-terphenyl-O]Ph(THF)2 with various nitrene precursors was investigated. No reaction was observed between Mn[O-terphenyl-O]Ph(THF)2 and aryl azides. In contrast, the treatment of Mn[O-terphenyl-O]Ph(THF)2 with iminoiodinane PhINTs (Ts = p-toluenesulfonyl) was consistent with the formation of a metal-nitrene complex. In the presence of styrene, the reaction led to the formation of aziridine. Combining varying ratios of styrene and PhINTs in different solvents with 10 mol% of Mn[O-terphenyl-O]Ph(THF)2 at room temperature produced 2-phenylaziridine in up to a 79% yield. Exploration of the reactivity of Mn[O-terphenyl-O]Ph(THF)2 with various olefins revealed (1) moderate aziridination yields for p-substituted styrenes, irrespective of the electronic nature of the substituent; (2) moderate yield for 1,1'-disubstituted α-methylstyrene; (3) no aziridination for aliphatic α-olefins; (4) complex product mixtures for the β-substituted styrenes. DFT calculations suggest that iminoiodinane is oxidatively added upon binding to Mn, and the resulting formal imido intermediate has a high-spin Mn(III) center antiferromagnetically coupled to an imidyl radical. This imidyl radical reacts with styrene to form a sextet intermediate that readily reductively eliminates the formation of a sextet Mn(II) aziridine complex.

Keywords: alkoxides; aziridination; iminoiodinane; manganese.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
Synthesis of Mn[O-terphenyl-O]Ph(THF)2 (2), along with the previously reported synthesis of iron analogue 1.
Figure 1
Figure 1
X-ray structure (50% probability) of Mn[O-terphenyl-O]Ph(THF)2 (2). Alternative conformations of one of the top THF ligands and one of the lateral phenyls, H atoms, and disordered solvent were omitted for clarity. Selected bond distances (Å) and angles (°): Mn-O1 1.888(5), Mn-O2 1.922(4), Mn-O3 2.205(4), Mn-O4 2.178(4), O1-Mn-O2 150.3(2), and O3-Mn-O4 91.23(2).
Figure 2
Figure 2
Aziridination of various olefins using 2. The yields of the aziridine products were determined by 1H NMR using internal standard (1,3,5-trimethylbenzene).
Figure 3
Figure 3
Optimized structure of the lowest energy quartet (left) and its spin density isosurface plotted using a cutoff value of 0.002 au (right). Blue and white surfaces correspond to excess alpha and beta spins, respectively.
Figure 4
Figure 4
Reaction energy profile calculated at the B3LYP-D3(BJ)/def2-TZVP//BP86-D3(BJ)/def2-SVP level of theory. * indicates a missing transition state. Up and down spins are indicated by red and blue electrons, respectively.
Scheme 2
Scheme 2
Possible mechanism for styrene aziridination catalyzed by 2.
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