Transition Metal Aziridination Catalysts

Abstract

Aziridines are three‐membered rings containing one nitrogen and two carbon atoms. This functional group is present in many bioactive molecules in addition to being a useful synthetic intermediate that is analogous to epoxides. Since they are a strained ring system, aziridines are reactive toward nucleophilic ring opening, ring expansion, and ring rearrangement reactions. Due to their importance in synthetic chemistry, numerous approaches to their synthesis have been developed. In addition to traditional organic reaction approaches to aziridination, transition metal catalyzed versions of this reaction have been developed. The primary rationales for catalyzed aziridination includes broadening the substrate scope, increasing yields (particularly under mild conditions), and improving enantiomeric purity. These catalytic reactions can mostly be designated as one of three classes. In this article, each of these important transition metal catalyzed aziridination reaction paths will be reviewed in addition to the catalysts that perform them.

Reactivity of Pyridine Dipyrrolide Iron(II) Complexes with Organic Azides: C–H Amination and Iron Tetrazene Formation

“…The five-coordinate geometry around the iron center can be described as heavily distorted square-pyramidal or trigonal bipyramidal with N(2)−Fe(1)−(N4), N(2)− Fe(1)−O(1), and O(1)−Fe(1)−N(4) angles of 118.46(7)°, 149.61(6)°, and 91.04(6)°, respectively. Due to the small bite angle of the pincer ligand, the two pyrrole nitrogen atoms N(1) and N(3) are not perfectly trans to each other with an N(1)−Fe(1)−N(3) angle of 147.17 (7)°. The long Fe−N and Fe−O bond lengths (Table 2) are consistent with a high-spin iron center.…”

“…Reactions between organic azides and various transition metal complexes have been intensively studied due to their relevance in nitrogen-group transfer reactions . As a consequence of this research, nitrene transfer to olefins and insertion into C–H bonds has become a viable strategy for the synthesis of important nitrogen-containing organic compounds such as aziridines and functionalized amines, respectively. − Inspired initially by the related oxygen atom-transfer reactivity of P450 and nonheme iron enzymes, nitrene transfer via iron catalysis has received particular attention. − Based on the seminal work on biomimetic iron porphyrin complexes that promote C–H amination and the presence of iron-oxo intermediates in biological systems, − a substantial number of isoelectronic iron-imido species with numerous supporting ligand systems has been prepared and studied over the last two decades. − This class of compounds displays a remarkable range of different electronic structures with iron centers in oxidation states from +II to +V and various spin states combined with dianionic imido (NR 2– ) or monoanionic iminyl radical (NR •1– ) ligands.…”

Nitrene-Transfer Chemistry to C–H and C═C Bonds Mediated by Triangular Coinage Metal Platforms Supported by Triply Bridging Pnictogen Elements Sb(III) and Bi(III)

“…The direct C–H/CC functionalization obviates the need for constructing energetic C–X precursors (X = leaving or directing group) but raises challenges for achieving acceptable levels of reactivity and selectivity . Taking C–N bond construction from C–H/CC feedstock as an example, both metal-free and metal-dependent catalytic systems , have been advanced to overcome these challenges. Notwithstanding the expense and toxicity involved, platinum-group metal catalysts have been frequently invoked, but nonprecious, first-row transition metal reagents have found increasing use.…”

“…The development of C–N bond construction methodologies encompasses various pathways directed toward inserting nitrogen functionalities into carbonaceous feedstock for the synthesis of a diverse body of commodity and high-value chemicals (pharmaceuticals, agrochemicals, polymers, semiconductors, catalysts, solvents, and household chemicals) . Among different approaches, the insertion of nitrenes (NR) or nitrenoids (NR­(X)) into C–H bonds or their addition to CC bonds affords valuable products of amination and aziridination, respectively, as well as derivatives (such as amidines or five-membered N -heterocycles) in the presence of additional substrates (usually unsaturated entities) . This direct functionalization of C–H/CC feedstock belongs to the general category of atom/group-transfer chemistry, pertaining to a wide variety of common atoms (e.g., H/D, N, O, S, and halogen) or groups (e.g., BR 2 , CR 2 /CR 3 , NR/NR 2 , and N 3 ). − Biological atom/group-transfer analogs have frequently provided inspiration and opportunities for further development via biomimetic approaches and enzyme engineering .…”

Direct Electrocatalytic N–H Aziridination of Aromatic Alkenes Using Ammonia

“…Regarding existing aziridination methods (Scheme a), a large body of work has used hypervalent iodine compounds such as N -tosyliminophenyliodinane (PhINTs) − and derivatives , but also phthalimides, , sulfonamides, , chloramine-T, N -sulfonyl imines, azides like 2,2,2-trichloroethoxysulfonyl azide (TcesN 3 ), , and others, − with frequent use of homogeneous transition metal catalysts. Although these methods can produce aziridines even from unactivated alkenes in some cases, they all form N -protected aziridines.…”