“Orthogonal” Lewis Acids: Catalyzed Ring Opening and Rearrangement of Acylaziridines

Branched-Selective Cross-Electrophile Coupling of 2-Alkyl Aziridines and (Hetero)aryl Iodides Using Ti/Ni Catalysis

The arylation of 2-alkyl aziridines by nucleophilic ring-opening or transition-metal-catalyzed cross-coupling enables facile access to biologically relevant β-phenethylamine derivatives. However, both approaches largely favor C-C bond formation at the less-substituted carbon of the aziridine, thus enabling access to only linear products. Consequently, despite the attractive bond disconnection that it poses, the synthesis of branched arylated products from 2-alkyl aziridines has remained inaccessible. Herein, we address this long-standing challenge and report the first branched-selective cross-coupling of 2-alkyl aziridines with aryl iodides. This unique selectivity is enabled by a Ti/Ni dual-catalytic system. We demonstrate the robustness of the method by a twofold approach: an additive screening campaign to probe functional group tolerance and a feature-driven substrate scope to study the effect of the local steric and electronic profile of each coupling partner on reactivity. Furthermore, the diversity of this feature-driven substrate scope enabled the generation of predictive reactivity models that guided mechanistic understanding. Mechanistic studies demonstrated that the branched selectivity arises from a TiIII-induced radical ring-opening of the aziridine.

Recent updates and future perspectives in aziridine synthesis and reactivity

In this review, selected recent advances in the preparation and reactivity of aziridines using modern synthetic approaches are highlighted, while comparing these new strategies with more classical approaches. This critical analysis is designed to help identify current gaps in the field and is showcasing new and exciting opportunities to move the chemistry of aziridines forward in the future.

Biocatalytic One-Carbon Ring Expansion of Aziridines to Azetidines via a Highly Enantioselective [1,2]-Stevens Rearrangement

We report enantioselective one-carbon ring expansion of aziridines to make azetidines as a new-to-nature activity of engineered "carbene transferase" enzymes. A laboratory-evolved variant of cytochrome P450_BM3, P411-AzetS, not only exerts unparalleled stereocontrol (99:1 er) over a [1,2]-Stevens rearrangement but also overrides the inherent reactivity of aziridinium ylides, cheletropic extrusion of olefins, to perform a [1,2]-Stevens rearrangement. By controlling the fate of the highly reactive aziridinium ylide intermediates, these evolvable biocatalysts promote a transformation which cannot currently be performed using other catalyst classes.

Synthesis of chiral pyridine-oxazolines via catalytic asymmetric Heine reaction of meso-N-(2-picolinoyl)-aziridines

An asymmetric Heine reaction of (meso)-N-(2-picolinoyl)-aziridines catalyzed by a chiral ytterbium(III)–N,N’-dioxide complex was established. A novel library of pyridine-oxazolines was obtained in decent yields and enantioselectivities, which show potential as...

Titanium and Cobalt Bimetallic Radical Redox Relay for the Isomerization of N-Bz Aziridines to Allylic Amides

Herein a bimetallic radical redox-relay strategy is employed to generate alkyl radicals under mild conditions with titanium(III) catalysis and terminated via hydrogen atom transfer with cobalt(II) catalysis to enact base-free isomerizations of N-Bz aziridines to N-Bz allylic amides. This reaction provides an alternative strategy for the synthesis of allylic amides from alkenes via a three-step sequence to accomplish a formal transpositional allylic amination.

RuV -Acylimido Intermediate in [RuIV (Por)Cl2 ]-Catalyzed C-N Bond Formation: Spectroscopic Characterization, Reactivity, and Catalytic Reactions

Metal-catalyzed C-N bond formation reactions via acylnitrene transfer have recently attracted much attention, but direct detection of the proposed acylnitrenoid/acylimido M(NCOR) (R=aryl or alkyl) species in these reactions poses a formidable challenge. Herein, we report on Ru(NCOR) intermediates in C-N bond formation catalyzed by [Ru^IV (Por)Cl₂ ]/N₃ COR, a catalytic method applicable to aziridine/oxazoline formation from alkenes, amination of substituted indoles, α-amino ketone formation from silyl enol ethers, amination of C(sp³ )-H bonds, and functionalization of natural products and carbohydrate derivatives (up to 99 % yield). Experimental studies, including HR-ESI-MS and EPR measurements, coupled with DFT calculations, lend evidence for the formulation of the Ru(NCOR) acylnitrenoids as a Ru^V -imido species.

Synthesis of Aminomethylene-gem-bisphosphonates Containing an Aziridine Motif: Studies of the Reaction Scope and Insight into the Mechanism

A broad range of N-carbamoylaziridines were obtained and then treated by the diethyl phosphonate anion to afford α-methylene-gem-bisphosphonate aziridines. Study of the reaction's scope and additional experiments indicates that the transformation proceeds via a new mechanism involving the chelation of lithium ion. This last step is crucial for the reaction to occur and disfavors the aziridine ring-opening. A phosphonate-phosphate rearrangement from a α-hydroxybisphosphonate aziridine intermediate is also proposed for the first time. This reaction provides a simple and convenient method for the synthesis of a highly functionalized phosphonylated aziridine motif.

Synthesis of a meso-Oxa-Diaminopimelic Acid Containing Peptidoglycan Pentapeptide and Coupling to the GlcNAc-anhydro-MurNAc Disaccharide

The syntheses of peptidoglycan (PG)-derived peptides containing meso-diaminopimelic acid (meso-Dap) are typically quite lengthy due to the need to prepare orthogonally protected meso-Dap. In this work, the preparation of the PG pentapeptide containing the isosteric analog meso-oxa-Dap is described. The synthesis relies on the ring opening of a peptide embedded aziridine via the attack of a serine residue. The pentapeptide was attached to a GlcNAc-anhydro-MurNAc disaccharide, to produce a putative substrate for the AmpG pore protein.

Copper-based catalysts derived from salen-type ligands: synthesis of 5-substituted-1H-tetrazoles via [3+2] cycloaddition and propargylamines via A3-coupling reactions

Base-metal copper(ii) complexes derived from unsymmetrical salen-type ligands were designed and synthesized using ligands L¹H to L⁴H. These complexes were employed as catalysts for [3+2] cycloaddition and A³-coupling reactions.

Peptides Containing meso-Oxa-Diaminopimelic Acid as Substrates for the Cell-Shape-Determining Proteases Csd6 and Pgp2

The enzymes Csd6 and Pgp2 are peptidoglycan (PG) proteases found in the pathogenic bacteria Helicobacter pylori and Campylobacter jejuni, respectively. These enzymes are involved in the trimming of non-crosslinked PG sidechains and catalyze the cleavage of the bond between meso-diaminopimelic acid (meso-Dap) and d-alanine, thus converting a PG tetrapeptide into a PG tripeptide. They are known to be cell-shape-determining enzymes, because deletion of the corresponding genes results in mutant strains that have lost the normal helical phenotype and instead possess a straight-rod morphology. In this work, we report two approaches directed towards the synthesis of the tripeptide substrate Ac-iso-d-Glu-meso-oxa-Dap-d-Ala, which serves as a mimic of the terminus of an non-crosslinked PG tetrapeptide substrate. The isosteric analogue meso-oxa-Dap was utilized in place of meso-Dap to simplify the synthetic procedure. The more efficient synthesis involved ring opening of a peptide-embedded aziridine by a serine-based nucleophile. A branched tetrapeptide was also prepared as a mimic of the terminus of a crosslinked PG tetrapeptide. We used MS analysis to demonstrate that the tripeptide serves as a substrate for both Csd6 and Pgp2 and that the branched tetrapeptide serves as a substrate for Pgp2, albeit at a significantly slower rate.

Triflic Acid Promoted Decarboxylation of Adamantane-oxazolidine-2-one: Access to Chiral Amines and Heterocycles

We have developed a one-step procedure to a variety of chiral lipophilic and conformationally rigid amines and heterocycles by decarboxylation of adamantane-oxazolidine-2-one. Triflic acid or aluminum triflate promote the addition of diverse nucleophiles to the oxazolidine-2-one moiety accompanied by the release of carbon dioxide. The resulting amine or heterocycle is then protonated/metalated by the catalyst (promotor). Additionally, the starting racemic material, adamantane-oxazolidine-2-one, was resolved into single enantiomers using a chiral auxiliary to access enantio-enriched products and to study the racemization pathway of chiral 1,2-disubstituted adamantane derivatives.

Metal triflate promoted synthesis of naphthalenes

A synthetic route to derive the skeleton of naphthalenes starting with isovanillin is described with modest total yields via the key transformation of metal triflate-mediated intramolecular benzannulation of o-formyl or o-benzoyl allylbenzenes in MeNO₂ at rt.

Visible-Light Promoted Distereodivergent Intramolecular Oxyamidation of Alkenes

The visible-light-promoted diastereodivergent intramolecular oxyamination of alkenes is described to construct oxazolindinones, pyrrolidinones and imidazolidones via mild generation of primary amidyl radicals from functionalized hydroxylamines. A unique phenomenon of highly diastereoselective ring-opening of aziridines controlled by electron sacrifices was observed. Highly diastereoselective amino alcohols derivatives were obtained efficiently through this protocol in gram scales. The mechanistic studies suggested the isolatable anti-aziridine intermediates were generated quickly from primary amidyl radicals and the diastereoselectivities were controlled by pK_a values of the electron sacrifices.

Organo-Catalytic Synthesis of 1,3-Thiazole Derivatives

An efficient synthesis of 1,3-thiazolidine derivatives has been achieved in which 2-pyridinecarboxaldehyde oxime was employed as a base catalyst to effect cyclisation between aziridines and CS2 in DMF at 40 °C. A similar reaction between aziridines and thiocyanates yielded 1,3-thiazoline-2-imine derivatives

Palladium-Catalyzed, Enantioselective Heine Reaction

Aziridines are important synthetic intermediates for the generation of nitrogen-containing molecules. N-Acylaziridines undergo rearrangement by ring expansion to produce oxazolines, a process known as the Heine reaction. The first catalytic, enantioselective Heine reaction is reported for meso-N-acylaziridines where a palladium(II)-diphosphine complex is employed. The highly enantioenriched oxazoline products are valuable organic synthons and potential ligands for transition-metal catalysis.

Visible light C-H amidation of heteroarenes with benzoyl azides

Benzoyl azides were used for the direct and atom economic C-H amidation of electron rich heteroarenes in the presence of phosphoric acid, a photocatalyst and visible light. Hetero-aromatic amides are obtained in good yields at very mild reaction conditions with dinitrogen as the only by-product. The reaction allows the use of aryl-, heteroaryl- or alkenyl acyl azides and has a wide scope for heteroarenes, including pyrroles, indole, furan, benzofuran and thiophene giving good regio-selectivities and yields.

Vicinal diamination of alkenes under Rh-catalysis

The synthesis of 1,2-diamines has been achieved through a single-step, tandem sequence involving Rh-catalyzed aziridination followed by NaI-promoted rearrangement to an isomeric cyclic sulfamide. Facile ring opening of these products in hot water and pyridine affords differentially protected vicinal diamines. Demonstration of the utility of this method for the syntheses of (±)-enduracididine and (±)-allo-enduracididine is highlighted.

Theoretical investigations on the mechanistic pathway of the thermal rearrangement of substituted N-acyl-2,2-dimethylaziridines

The mechanism of the thermal rearrangement of substituted N-acyl-2,2-dimethylaziridines 1 has been studied using quantum chemistry methods. Geometries of reactants, transition states and products have been optimized at the B3LYP/6-311++G(2d,2p) level. Relative energies for various stationary points have been determined and reaction identified by IRC calculations. The results show that thermal rearrangements occur in three ways. Firstly, the transition state TS 1 in which a hydrogen atom of methyl groups migrates from primary carbon to oxygen of amid group to give the N-methallylamide 2. The second is via the transition state TS 2 in which the attack of oxygen to the tertiary carbon yields the oxazoline 3. The third is via the transition state TS 3 in which a hydrogen migrate from the secondary carbon to oxygen to give the vinylamide 4. In order to get insights into the factors determining the exact nature of its interactions with electrophiles, the application of reactivity parameters derived from density functional theory in a local sense, in particular the softness and Fukui function, to interpret and predict the mechanisms of the thermal decomposition of the N-acyl-2,2-dimethylaziridines 1, has been discussed.

Catalyst-controlled C-O versus C-N allylic functionalization of terminal olefins

The divergent synthesis of syn-1,2-aminoalcohol or syn-1,2-diamine precursors from a common terminal olefin has been accomplished using a combination of palladium(II) catalysis with Lewis acid cocatalysis. Palladium(II)/bis-sulfoxide catalysis with a silver triflate cocatalyst leads for the first time to anti-2-aminooxazolines (C-O) in good to excellent yields. Simple removal of the bis-sulfoxide ligand from this reaction results in a complete switch in reactivity to afford anti-imidazolidinone products (C-N) in good yields and excellent diastereoselectivities. Mechanistic studies suggest the divergent C-O versus C-N reactivity from a common ambident nucleophile arises due to a switch in mechanism from allylic C-H cleavage/functionalization to olefin isomerization/oxidative amination.

Synthesis of a potent antimalarial amphilectene

7-Isocyano-11(20),14-epiamphilectadiene, the most potent of antimalarial amphilectenes, is synthesized in seven steps from readily available materials. The synthesis is enabled by a new dendrimeric triene (Danishefsky [3]-dendralene) and a new method for stereo- and chemoselective isocyanation. This chemistry provides a useful entry into an underexplored yet promising family of antimalarial terpenoids.