Photodecarboxylative Amination of Redox-Active Esters with Diazirines

Diazirines have been recently demonstrated to serve as electrophilic amination reagents that afford diaziridines, versatile heterocycles that are readily transformed into amines, hydrazines, and nitrogen-containing heterocycles. Here, we report the photodecarboxylative amination of redox-active esters with diazirines using inexpensive photoactivators under mild conditions with an enhanced scope for primary substrates. The stability of diazirines to blue light is demonstrated, paving the way for further research into other photochemical amination methods with these unique heterocycles.

Decarboxylative Amination: Diazirines as Single and Double Electrophilic Nitrogen Transfer Reagents

The ubiquity of nitrogen-containing small molecules in medicine necessitates the continued search for improved methods for C-N bond formation. Electrophilic amination often requires a disparate toolkit of reagents whose selection depends on the specific structure and functionality of the substrate to be aminated. Further, many of these reagents are challenging to handle, engage in undesired side reactions, and function only within a narrow scope. Here we report the use of diazirines as practical reagents for the decarboxylative amination of simple and complex redox-active esters. The diaziridines thus produced are readily diversifiable to amines, hydrazines, and nitrogen-containing heterocycles in one step. The reaction has also been applied in fluorous phase synthesis with a perfluorinated diazirine.

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.