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. 2022 Feb 16;28(9):e202104078.
doi: 10.1002/chem.202104078. Epub 2021 Dec 29.

Characterization of Cyclic N-Acyliminium Ions by Infrared Ion Spectroscopy

Jona Merx  1 , Kas J Houthuijs  2 , Hidde Elferink  1 , Eva Witlox  1 , Jasmin Mecinović  3 , Jos Oomens  2 , Jonathan Martens  2 , Thomas J Boltje  1 , Floris P J T Rutjes  1
Affiliations

Characterization of Cyclic N-Acyliminium Ions by Infrared Ion Spectroscopy

Jona Merx et al. Chemistry. .

Abstract

N-Acyliminium ions are highly reactive intermediates that are important for creating CC-bonds adjacent to nitrogen atoms. Here we report the characterization of cyclic N-acyliminium ions in the gas phase, generated by collision induced dissociation tandem mass spectrometry followed by infrared ion spectroscopy using the FELIX infrared free electron laser. Comparison of DFT calculated spectra with the experimentally observed IR spectra provided valuable insights in the conformations of the N-acyliminium ions.

Keywords: DFT calculations; N-acyliminium ion; heterocycles; ion spectroscopy; stereoselectivity.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
A) Cyclic N‐acyliminium ion conformations 1 and 2 and their preferential facial selectivity upon nucleophilic attack; Nu=nucleophile. B) NAI precursor molecules 5–10 used in this study.
Figure 1
Figure 1
A) Isolation of the NAI (m/z 142) derived from precursor 5, corresponding to cation 5a. B) Comparison of the experimental spectrum corresponding to m/z 142 (black line) with the DFT calculated spectrum (B3LYP) of cation 5a (color filled). C) Comparison of the experimental spectrum corresponding to m/z 142 (black line) with the DFT calculated (B3LYP) spectrum of cation 5b (color filled). Energies are relative to the lowest‐energy structure, carbamate omitted for clarity in the 3D schematic structures.
Figure 2
Figure 2
A) Isolation of the ionized of precursor 6 (m/z 200) corresponding to cation 6a. B) Comparison of the experimental spectrum corresponding to m/z 200 (black line) with the DFT calculated (B3LYP) spectrum of cation 6a (color filled). C) Comparison of the experimental spectrum corresponding to m/z 200 (black line) with the DFT calculated (B3LYP) spectrum of cation 6b (color filled). Energies are relative to the lowest‐energy structure, carbamate omitted for clarity in the 3D schematic structure in B.
Figure 3
Figure 3
A) CID of precursor 7 results in fragmentation to m/z 200, corresponding to the mass of cation 7a. Comparison of the IR spectrum of m/z 200 (black line) with the DFT calculated cations: B) 7aeq (B3LYP). C) 7aeq (M06‐2x). D) 7beq (B3LYP). E) 7aax (B3LYP). F) 7cax (M06‐2x). G) 7bax (B3LYP). Energies are relative to the lowest‐energy structure, carbamate omitted for clarity in the 3D schematic structures.
Figure 4
Figure 4
A) CID of precursor 8 results in fragmentation to m/z 220, corresponding to the mass of cation 8a. Comparison of the spectrum m/z 220 (black line) with the DFT calculated (color filled) cations. B) 8aeq (B3LYP). C) 8beq (B3LYP). D) 8aax (B3LYP). E) 8bax (B3LYP). Energies are relative to the lowest‐energy structure, carbamate omitted for clarity in the 3D schematic structures.
Figure 5
Figure 5
A) CID of precursor 9 results in fragmentation to m/z 273, corresponding to the mass of cations 9a–c. Comparison of the spectrum m/z 273 (black line) with the DFT calculated spectrum of cations 9a and 9b (color filled) of: B) 9aeq (B3LYP) C) 9beq (MP2). D) 9bax (B3LYP). E) 9bax (MP2). Energies are relative to the lowest‐energy structure, carbamate omitted for clarity in the 3D schematic structures.
Scheme 2
Scheme 2
NAI structures characterized by IRIS and the expected facial selectivity of the nucleophile approach (blue). Observed selectivity of nucleophilic addition in the solution phase (Table).
Figure 6
Figure 6
CID of precursor 10 results in fragmentation to m/z 198, corresponding to the mass of cations 11–15. Comparison of the spectrum m/z 273 (black line) with the DFT calculated cations 11–15 (color filled) of: B) 15 (B3LYP). C) 13 (B3LYP). D) 14 (B3LYP). E) 11 (B3LYP). F) 12 (B3LYP). Energies are relative to the lowest‐energy structure, carbamate omitted for clarity in the 3D schematic structures.
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