The curious case of mesityl azide and its reactivity with bpyNiEt2

Elucidation of the Reaction Mechanism of C2 + N1 Aziridination from Tetracarbene Iron Catalysts

A combined computational and experimental study was undertaken to elucidate the mechanism of catalytic C₂ + N₁ aziridination supported by tetracarbene iron complexes. Three specific aspects of the catalytic cycle were addressed. First, how do organic azides react with different iron catalysts and why are alkyl azides ineffective for some catalysts? Computation of the catalytic pathway using density functional theory (DFT) revealed that an alkyl azide needs to overcome a higher activation barrier than an aryl azide to form an iron imide, and the activation barrier with the first-generation catalyst is higher than the activation barrier with the second-generation variant. Second, does the aziridination from the imide complex proceed through an open-chain radical intermediate that can change stereochemistry or, instead, via an azametallacyclobutane intermediate that retains stereochemistry? DFT calculations show that the formation of aziridine proceeds via the open-chain radical intermediate, which qualitatively explains the formation of both aziridine diastereomers as seen in experiments. Third, how can the formation of the side product, a metallotetrazene, be prevented, which would improve the yield of aziridine at lower alkene loading? DFT and experimental results demonstrate that sterically bulky organic azides prohibit formation of the metallotetrazene and, thus, allow lower alkene loading for effective catalysis. These multiple insights of different aspects of the catalytic cycle are critical for developing improved catalysts for C₂ + N₁ aziridination.

Intramolecular C-H Functionalization Followed by a [2σ + 2π] Addition via an Intermediate Nickel-Nitridyl Complex

Irradiation of a disphenoidal Ni(II) azido complex, [Cz ^tBu(Pyr ^iPr)₂NiN₃] (1), revealed an unprecedented nickel complex, [Cz ^tBu(Pyr ^iPr)(NH₂-Pyr ^iPr)] (2), in >90% isolated yield. As evidenced by single-crystal X-ray diffraction, 2 is produced by double intramolecular C-H activation of a putative nickel-nitridyl intermediate, [Cz ^tBu(Pyr ^iPr)₂Ni-⃛N^•]. Calculations support the generation of an intermediate with significant nitridyl radical character after the loss of N₂, which, in turn, undergoes tandem C-H activations, leading to functionalized intermediates and products. This is an unprecedented example of transient Ni-⃛N^•-promoted intramolecular C-H functionalization, followed by a [2_σ + 2_π] addition, yielding bis-metallacyclic product 2. Complex 2 is also observed from the reaction of Ni(I) precursor Cz ^tBu(Pyr ^iPr)₂Ni (3) and Me₃SiN₃, suggesting a unique thermal route toward a masked nickel-nitridyl intermediate.