Organic N-containing compounds, including amines, are essential components of many biologically and pharmaceutically important molecules. One strategy for introducing nitrogen into substrates with multiple reactive bonds is to insert a monovalent N fragment (nitrene or nitrenoid) into a C–H bond or add it directly to a C=C bond. However, it has been challenging to develop well-defined catalysts capable of promoting predictable and chemoselective aminations solely through reagent control. Herein, we report remarkable chemoselective aminations that employ a single metal (Ag) and a single ligand (phenanthroline) to promote either aziridination or C–H insertion by manipulating the coordination geometry of the active catalysts.

Asymmetric nitrene transfer reactions are a powerful tool for the preparation of enantioenriched amine building blocks. Herein, we report chemo- and enantioselective silver-catalyzed aminations that transform di- and trisubstituted homoallylic carbamates into [4.1.0]-carbamate-tethered aziridines in good yields and ee up to 92%. The effects of the substrate, silver counteranion, ligand, solvent and temperature on both chemoselectivity and ee were explored to complement the scope of traditional metal-catalyzed nitrene transfer reactions. Stereochemical models were proposed to rationalize the observed absolute stereochemistry of the aziridines, which undergo nucleophilic ring-opening to yield enantioenriched amines with no erosion in stereochemical integrity.

Catalyst-controlled, selective nitrene transfer is often challenging when both C−H and CC bonds are present in a substrate. Interestingly, a simple change in the Ag(I):L ratio (L = bidentate N,N-donor ligand) enables tunable, chemoselective nitrene transfer that favors either C C bond aziridination using an ∼1:1 Ag:L ratio (AgLOTf) or insertion into a C−H bond when the Ag:L ratio in the catalyst is 1:2 (AgL 2 OTf). In this paper, mechanistic studies, coupled with kinetic profiling of the entire reaction course, are employed to examine the reasons for this unusual behavior. Steady-state kinetics were found to be similar for both AgLOTf and AgL 2 OTf; both complexes yield electronically similar reactive intermediates that engage in nitrene transfer involving formation of a short-lived radical intermediate and barrierless radical recombination. Taken together, experimental and computational studies point to two effects that control tunable chemoselectivity: suppression of aziridination as the steric congestion around the silver center is increased in AgL 2 OTf and a decrease in the rate of C−H insertion with AgLOTf in comparison to AgL 2 OTf. The observation that the sterics of Ag catalysts can be varied, with minor effects on the electronic features of the putative nitrene, has important implications for the development of other silver catalysts that enable tunable, site-selective C−H bond aminations.

Allene aziridination generates useful bicyclic methylene aziridine scaffolds that can be flexibly transformed into a range of stereochemically complex and densely functionalized amine-containing stereotriads. The scope of this chemistry has been limited by the poor chemoselectivity that often results when typical dinuclear Rh(II) catalysts are employed with homoallenic carbamates. Herein, Ag(I) catalysts that significantly improve the scope and yield of bicyclic methylene aziridines that can be prepared via allene aziridination are described.

This article reviews methods for converting allenes to strained, three-membered methylene heterocycles, and also covers the reactivity of these products. Specifically, the synthesis and reactivity of methylene aziridines, allene oxides/spirodiepoxides, methylene silacyclopropanes, methylene phosphiranes, and methylene thiiranes are described, including applications to the synthesis of complex molecules. Due to the primary focus on heterocyclic motifs, the all-carbon analogue of these species (methylene cyclopropane) is only briefly discussed.