Nitrogen Atom Transfer Catalysis by Metallonitrene C-H Insertion: Photocatalytic Amidation of Aldehydes

Abstract

C-H amination and amidation by catalytic nitrene transfer are well-established and typically proceed via electrophilic attack of nitrenoid intermediates. In contrast, the insertion of (formal) terminal nitride ligands into C-H bonds is much less developed and catalytic nitrogen atom transfer remains unknown. We here report the synthesis of a formal terminal nitride complex of palladium. Photocrystallographic, magnetic, and computational characterization support the assignment as an authentic metallonitrene (Pd-N) with a diradical nitrogen ligand that is singly bonded to Pd^II . Despite the subvalent nitrene character, selective C-H insertion with aldehydes follows nucleophilic selectivity. Transamidation of the benzamide product is enabled by reaction with N₃ SiMe₃ . Based on these results, a photocatalytic protocol for aldehyde C-H trimethylsilylamidation was developed that exhibits inverted, nucleophilic selectivity as compared to typical nitrene transfer catalysis. This first example of catalytic C-H nitrogen atom transfer offers facile access to primary amides after deprotection.

Keywords: Amides; C−H Activation; Nitrenes; Palladium; Photocatalysis.

Scheme 1

Stochiometric, intermolecular C−H nitrogen atom insertion reactions by (formal) terminal nitride complexes reported in the literature (top), and aldehyde amidation via catalytic C−H nitrogen atom transfer reported here (bottom).

Figure 1

a) Displacement ellipsoid plots of 1 and 2 N₂ from in crystallo N₂‐extrusion. 2 N₂ was refined from a sample with 40 % conversion of 1. Selected bond lengths [Å]: 1 Pd−N1, 2.050(2); 2 N₂ Pd−N1 A, 1.92(2). b) Solution UV/Vis spectra of 1 and the analogous Pt complex in THF. c) χ  _M T vs. T data (circles) from in situ photolysis (λ _exc=390 nm) of 1 obtained by SQUID magnetometry with simulated (red) and ab initio computed (blue) values.

Figure 2

Characteristic NLMOs derived by NBO analysis for the Pd−N σ‐bond (a), the N‐centered singly occupied p _y and p _z orbitals and s‐type lone pair (b), and the doubly occupied Pd d‐orbitals (c); isosurfaces at ±0.05 a₀ ^−3/2; doubly occupied orbitals obtained by spatial averaging over α and β spin orbitals; methyl groups and hydrogen atoms are omitted for clarity.

Scheme 2

a) Proposed catalytic cycle. b) Hammett plots for stochiometric (1→3, black; [ArCHO]₀:[PhCHO]₀:[1]=10 : 10 : 1, 5 min) and catalytic (red; [ArCHO]₀:[PhCHO]₀:[1]:[TMSN₃]₀=10 : 10 : 1 : 50, 5 h) amidation of para‐substituted benzaldehydes (60 °C, C₆D₆, 427 nm).

Figure 3

Computed pathway for transamidation with ΔG ^333K in kcal mol⁻¹ in parentheses and transition‐state imaginary frequencies in cm⁻¹; methyl groups and hydrogen atoms are omitted for clarity.

Scheme 3

Isotopic cross‐labelling experiment.