C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)–Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)–Pd(ii) pathway or Pd(ii) catalysis.

C4-arylation via Pd(i)–Pd(ii) catalysis and domino C4-arylation/3,2-carbonyl migration of indoles via Pd(ii) catalysis tuning by acids have been developed.     

Rh(i)-catalyzed stereoselective desymmetrization of prochiral cyclohexadienones via highly exo-selective Huisgen-type [3 + 2] cycloaddition

A Rh(i)-catalyzed highly stereoselective desymmetrization of 2-alkynylbenzaldehyde-tethered cyclohexadienones triggered by intramolecular Huisgen-type [3 + 2] cycloaddition has been developed. This method enables convergent construction of complex epoxy-bridged polycyclic ring systems with five contiguous stereocenters with excellent exo-selectivity and broad substrate scope. The highly atom-economical process involves 6-endo-dig cyclization of carbonyl oxygen onto an activated alkyne resulting in a highly reactive metal–benzopyrylium intermediate, which readily undergoes intramolecular [3 + 2] annulation/hydration. Asymmetric induction is also achieved for the first time in Rh(i)-catalyzed 1,3-dipolar cycloaddition using an easily accessible chiral diene as the ligand.

A Rh(i)-catalyzed highly stereoselective desymmetrization of 2-alkynylbenzaldehyde-tethered cyclohexadienones triggered by intramolecular Huisgen-type [3 + 2] cycloaddition has been developed.     

The role of hypervalent iodine(iii) reagents in promoting alkoxylation of unactivated C(sp3)–H bonds catalyzed by palladium(ii) complexes

Although Pd(OAc)₂-catalysed alkoxylation of the C(sp³)–H bonds mediated by hypervalent iodine(iii) reagents (ArIX₂) has been developed by several prominent researchers, there is no clear mechanism yet for such crucial transformations. In this study, we shed light on this important issue with the aid of the density functional theory (DFT) calculations for alkoxylation of butyramide derivatives. We found that the previously proposed mechanism in the literature is not consistent with the experimental observations and thus cannot be operating. The calculations allowed us to discover an unprecedented mechanism composed of four main steps as follows: (i) activation of the C(sp³)–H bond, (ii) oxidative addition, (iii) reductive elimination and (iv) regeneration of the active catalyst. After completion of step (i) via the CMD mechanism, the oxidative addition commences with an X ligand transfer from the iodine(iii) reagent (ArIX₂) to Pd(ii) to form a square pyramidal complex in which an iodonium occupies the apical position. Interestingly, a simple isomerization of the resultant five-coordinate complex triggers the Pd(ii) oxidation. Accordingly, the movement of the ligand trans to the Pd–C(sp³) bond to the apical position promotes the electron transfer from Pd(ii) to iodine(iii), resulting in the reduction of iodine(iii) concomitant with the ejection of the second X ligand as a free anion. The ensuing Pd(iv) complex then undergoes the C–O reductive elimination by nucleophilic attack of the solvent (alcohol) on the sp³ carbon via an outer-sphere S_N2 mechanism assisted by the X⁻ anion. Noteworthy, starting from the five coordinate complex, the oxidative addition and reductive elimination processes occur with a very low activation barrier (ΔG^‡ 0–6 kcal mol⁻¹). The strong coordination of the alkoxylated product to the Pd(ii) centre causes the regeneration of the active catalyst, i.e. step (iv), to be considerably endergonic, leading to subsequent catalytic cycles to proceed with a much higher activation barrier than the first cycle. We also found that although, in most cases, the alkoxylation reactions proceed via a Pd(ii)–Pd(iv)–Pd(ii) catalytic cycle, the other alternative in which the oxidation state of the Pd(ii) centre remains unchanged during the catalysis could be operative, depending on the nature of the organic substrate.

This work uses DFT calculations to explore Pd(ii)-catalysed iodine(iii)-mediated alkoxylation of unactivated C(sp³)–H bonds and reveals how important the isomerization is in triggering the oxidative addition of ArIX₂ to Pd(ii).     

Mangana(iii/iv)electro-catalyzed C(sp3)–H azidation

Manganaelectro-catalyzed azidation of otherwise inert C(sp³)–H bonds was accomplished using most user-friendly sodium azide as the nitrogen-source. The operationally simple, resource-economic C–H azidation strategy was characterized by mild reaction conditions, no directing group, traceless electrons as the sole redox-reagent, Earth-abundant manganese as the catalyst, high functional-group compatibility and high chemoselectivity, setting the stage for late-stage azidation of bioactive compounds. Detailed mechanistic studies by experiment, spectrophotometry and cyclic voltammetry provided strong support for metal-catalyzed aliphatic radical formation, along with subsequent azidyl radical transfer within a manganese(iii/iv) manifold.

The merger of manganese-catalyzed C–H functionalization with electrosynthesis enabled C(sp³)–H azidation devoid of chemical oxidants or photochemical irradiation. Detailed mechanistic studies are supportive of a manganese(iii/iv) electrocatalysis.     

Visible light driven generation and alkyne insertion reactions of stable bis-cyclometalated Pt(iv) hydrides

Hydride complexes resulting from the oxidative addition of C–H bonds are intermediates in hydrocarbon activation and functionalization reactions. The discovery of metal systems that enable their direct formation through photoexcitation with visible light could lead to advantageous synthetic methodologies. In this study, easily accessible dimers [Pt₂(μ-Cl)₂(C^N)₂] (C^N = cyclometalated 2-arylpyridine) are demonstrated as a very convenient source of Pt(C^N) subunits, which promote photooxidative C–H addition reactions with different 2-arylpyridines (N′^C′H) upon irradiation with blue light. The resulting [PtH(Cl)(C^N)(C′^N′)] complexes are the first isolable Pt(iv) hydrides arising from a cyclometalation reaction. A transcyclometalation process involving three photochemical steps is elucidated, which occurs when the C^N ligand is a monocyclometalated 2,6-diarylpyridine, and a detailed analysis of the photoreactivity associated with the Pt(C^N) moiety is provided. Alkyne insertions into the Pt–H bond of a photogenerated Pt(iv) hydride are also reported as a demonstration of the ability of this class of compounds to undergo subsequent organometallic reactions.

The photochemical generation of isolable bis-cyclometalated Pt(iv) hydrides via photooxidative C–H addition reactions is demonstrated from easily accessible Pt(ii) precursors using visible light.     

Asymmetric synthesis of dihydro-1,3-dioxepines by Rh(ii)/Sm(iii) relay catalytic three-component tandem [4 + 3]-cycloaddition

Heterocycles have been widely used in organic synthesis, agrochemical, pharmaceutical and materials science industries. Catalytic three-component ylide formation/cycloaddition enables the assembly of complex heterocycles from simple starting materials in a highly efficient manner. However, asymmetric versions remain a yet-unsolved task. Here, we present a new bimetallic catalytic system for tackling this challenge. A combined system of Rh(ii) salt and chiral N,N′-dioxide–Sm(iii) complex was established for promoting the unprecedented tandem carbonyl ylide formation/asymmetric [4 + 3]-cycloaddition of aldehydes and α-diazoacetates with β,γ-unsaturated α-ketoesters smoothly, affording various chiral 4,5-dihydro-1,3-dioxepines in up to 97% yield, with 99% ee. The utility of the current method was demonstrated by conversion of products to optically active multi-substituted tetrahydrofuran derivatives. A possible reaction mechanism was provided to elucidate the origin of chiral induction based on experimental studies and X-ray structures of catalysts and products.

Catalytic asymmetric tandem carbonyl ylide formation/[4 + 3]-cycloaddition of β,γ-unsaturated α-ketoesters, aldehydes and α-diazoacetates was achieved by using a bimetallic rhodium(ii)/chiral N,N′-dioxide–Sm(iii) complex catalyst.     

Ruthenium-catalyzed cascade C–H activation/annulation of N-alkoxybenzamides: reaction development and mechanistic insight

A highly selective ruthenium-catalyzed C–H activation/annulation of alkyne-tethered N-alkoxybenzamides has been developed. In this reaction, diverse products from inverse annulation can be obtained in moderate to good yields with high functional group compatibility. Insightful experimental and theoretical studies indicate that the reaction to the inverse annulation follows the Ru(ii)–Ru(iv)–Ru(ii) pathway involving N–O bond cleavage prior to alkyne insertion. This is highly different compared to the conventional mechanism of transition metal-catalyzed C–H activation/annulation with alkynes, involving alkyne insertion prior to N–O bond cleavage. Via this pathway, the in situ generated acetic acid from the N–H/C–H activation step facilitates the N–O bond cleavage to give the Ru-nitrene species. Besides the conventional mechanism forming the products via standard annulation, an alternative and novel Ru(ii)–Ru(iv)–Ru(ii) mechanism featuring N–O cleavage preceding alkyne insertion has been proposed, affording a new understanding of transition metal-catalyzed C–H activation/annulation.

A highly selective ruthenium-catalyzed C–H activation/annulation through a pathway involving N–O bond cleavage prior to alkyne insertion is developed.     

Electrochemical studies of tris(cyclopentadienyl)thorium and uranium complexes in the +2, +3, and +4 oxidation states

Electrochemical measurements on tris(cyclopentadienyl)thorium and uranium compounds in the +2, +3, and +4 oxidation states are reported with C₅H₃(SiMe₃)₂, C₅H₄SiMe₃, and C₅Me₄H ligands. The reduction potentials for both U and Th complexes trend with the electron donating abilities of the cyclopentadienyl ligand. Thorium complexes have more negative An(iii)/An(ii) reduction potentials than the uranium analogs. Electrochemical measurements of isolated Th(ii) complexes indicated that the Th(iii)/Th(ii) couple was surprisingly similar to the Th(iv)/Th(iii) couple in Cp′′-ligated complexes. This suggested that Th(ii) complexes could be prepared from Th(iv) precursors and this was demonstrated synthetically by isolation of directly from UV-visible spectroelectrochemical measurements and reactions of with elemental barium indicated that the thorium system undergoes sequential one electron transformations.

Electrochemical determination of the reduction potentials for a variety of tris(cyclopentadienyl)uranium and thorium complexes, including data on U(ii) and Th(ii) complexes.     

Heterobimetallic Eu(iii)/Pt(ii) single-chain nanoparticles: a path to enlighten catalytic reactions

We introduce the formation and characterization of heterometallic single-chain nanoparticles entailing both catalytic and luminescent properties. A terpolymer containing two divergent ligand moieties, phosphines and phosphine oxides, is synthesized and intramolecularly folded into nanoparticles via a selective metal complexation of Pt(ii) and Eu(iii). The formation of heterometallic Eu(iii)/Pt(ii) nanoparticles is evidenced by size exclusion chromatography, multinuclear NMR (¹H, ³¹P{¹H}, ¹⁹F, ¹⁹⁵Pt) as well as diffusion-ordered NMR and IR spectroscopy. Critically, we demonstrate the activity of the SCNPs as a homogeneous and luminescent catalytic system in the amination reaction of allyl alcohol.

A bifunctional terpolymer containing two orthogonal ligand moieties was synthesized, giving way to the facile formation of heterometallic Eu(iii)/Pt(ii) single-chain nanoparticles, which display both catalytic and luminescent properties.     

Phase transfer of metal cations by induced dynamic carrier agents: biphasic extraction based on dynamic covalent chemistry

In contrast to the classical method where a single molecule is designed to extract metal cations under specific conditions, dynamic covalent chemistry provides an approach based on the implementation of an adaptive dynamic covalent library for inducing the generation of the extractant species. This approach has been applied to the liquid–liquid extraction of copper(ii) nitrate based on a dynamic library of acylhydrazones constituents that self-build and distribute through the interface of a biphasic system. The addition of copper(ii) cations to this library triggers a modification of its composition and the up-regulation of the ligand molecules driven by coordination to the metal cations. Among these, one species has proven to be sufficiently lipophilic to play the role of carrier agent and its formation by component exchange enables the partial extraction of the copper(ii). The study of different pathways to generate the dynamic covalent library demonstrates the complete reversibility and the adaptability of the system. The detailed analytical investigation of the system provides a means to assess the mechanism of the dynamic extraction process.

Phase transfer of Cu(ii) cations is achieved by component exchange in a dynamic covalent library of acylhydrazone ligands. B1/B2 component exchange leads to the generation of a lipophilic carrier agent that extracts Cu(ii) into chloroform.     

Decarbonylative ether dissection by iridium pincer complexes

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported, mediated by iridium(i) complexes supported by aminophenylphosphinite (NCOP) pincer ligands. The decarbonylation, which involves the cleavage of one C–C bond, one C–O bond, and two C–H bonds, along with formation of two new C–H bonds, was serendipitously discovered upon dehydrochlorination of an iridium(iii) complex containing an aza-18-crown-6 ether macrocycle. Intramolecular cleavage of macrocyclic and acyclic ethers was also found in analogous complexes featuring aza-15-crown-5 ether or bis(2-methoxyethyl)amino groups. Intermolecular decarbonylation of cyclic and linear ethers was observed when diethylaminophenylphosphinite iridium(i) dinitrogen or norbornene complexes were employed. Mechanistic studies reveal the nature of key intermediates along a pathway involving initial iridium(i)-mediated double C–H bond activation.

A unique chain-rupturing transformation that converts an ether functionality into two hydrocarbyl units and carbon monoxide is reported.     

Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

In this work, we describe the synthesis, structural and photophysical characterization of four novel Pd(ii) and Pt(ii) complexes bearing tetradentate luminophoric ligands with high photoluminescence quantum yields (Φ_L) and long excited state lifetimes (τ) at room temperature, where the results were interpreted by means of DFT calculations. Incorporation of fluorine atoms into the tetradentate ligand favors aggregation and thereby, a shortened average distance between the metal centers, which provides accessibility to metal–metal-to-ligand charge-transfer (³MMLCT) excimers acting as red-shifted energy traps if compared with the monomeric entities. This supramolecular approach provides an elegant way to enable room-temperature phosphorescence from Pd(ii) complexes, which are otherwise quenched by a thermal population of dissociative states due to a lower ligand field splitting. Encapsulation of these complexes in 100 nm-sized aminated polystyrene nanoparticles enables concentration-controlled aggregation-enhanced dual emission. This phenomenon facilitates the tunability of the absorption and emission colors while providing a rigidified environment supporting an enhanced Φ_L up to about 80% and extended τ exceeding 100 μs. Additionally, these nanoarrays constitute rare examples for self-referenced oxygen reporters, since the phosphorescence of the aggregates is insensitive to external influences, whereas the monomeric species drop in luminescence lifetime and intensity with increasing triplet molecular dioxygen concentrations (diffusion-controlled quenching).

Pt(ii) and Pd(ii) complexes with unprecedented photophysical properties were developed. Encapsulation in nanoparticles boosted their performance while rendering them as self-referenced oxygen sensors.     

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate

We report the use of electron rich iron complexes supported by a dianionic diborate pentadentate ligand system, B₂Pz₄Py, for the coordination and activation of ammonia (NH₃) and hydrazine (NH₂NH₂). For ammonia, coordination to neutral (B₂Pz₄Py)Fe(ii) or cationic [(B₂Pz₄Py)Fe(iii)]⁺ platforms leads to well characterized ammine complexes from which hydrogen atoms or protons can be removed to generate, fleetingly, a proposed (B₂Pz₄Py)Fe(iii)–NH₂ complex (3_Ar-NH₂). DFT computations suggest a high degree of spin density on the amido ligand, giving it significant aminyl radical character. It rapidly traps the H atom abstracting agent 2,4,6-tri-tert-butylphenoxy radical (ArO˙) to form a C–N bond in a fully characterized product (2_Ar), or scavenges hydrogen atoms to return to the ammonia complex (B₂Pz₄Py)Fe(ii)–NH₃ (1_Ar-NH₃). Interestingly, when (B₂Pz₄Py)Fe(ii) is reacted with NH₂NH₂, a hydrazine bridged dimer, (B₂Pz₄Py)Fe(ii)–NH₂NH₂–Fe(ii)(B₂Pz₄Py) ((1_Ar)₂-NH₂NH₂), is observed at −78 °C and converts to a fully characterized bridging diazene complex, 4_Ar, along with ammonia adduct 1_Ar-NH₃ as it is allowed to warm to room temperature. Experimental and computational evidence is presented to suggest that (B₂Pz₄Py)Fe(ii) induces reductive cleavage of the N–N bond in hydrazine to produce the Fe(iii)–NH₂ complex 3_Ar-NH₂, which abstracts H˙ atoms from (1_Ar)₂-NH₂NH₂ to generate the observed products. All of these transformations are relevant to proposed steps in the ammonia oxidation reaction, an important process for the use of nitrogen-based fuels enabled by abundant first row transition metals.

Synopsis: a highly reactive Fe(iii)–NH₂ complex is generated via activation of ammonia or hydrazine in reactions of relevance to fundamental steps in ammonia oxidation processes mediated by an abundant, first row transition metal.     

Cross dehydrogenative C–O coupling catalysed by a catenane-coordinated copper(i)

Catalytic activity of copper(i) complexes supported by phenanthroline-containing catenane ligands towards a new C(sp³)–O dehydrogenative cross-coupling of phenols and bromodicarbonyls is reported. As the phenanthrolines are interlocked by the strong and flexible mechanical bond in the catenane, the active catalyst with an open copper coordination site can be revealed only transiently and the stable, coordinatively saturated Cu(i) pre-catalyst is quickly regenerated after substrate transformation. Compared with a control Cu(i) complex supported by non-interlocked phenanthrolines, the catenane-supported Cu(i) is highly efficient with a broad substrate scope, and can be applied in gram-scale transformations without a significant loss of the catalytic activity. This work demonstrates the advantages of the catenane ligands that provide a dynamic and responsive copper coordination sphere, highlighting the potential of the mechanical bond as a design element in transition metal catalyst development.

The use of a catenane-supported copper(i) complex for the cross dehydrogenative C–O coupling of phenols and bromodicarbonyls is described.     

Ortho-aryl substituted DPEphos ligands: rhodium complexes featuring C–H anagostic interactions and B–H agostic bonds

The synthesis of new Schrock–Osborn Rh(i) pre-catalysts with ortho-substituted DPEphos ligands, [Rh(DPEphos-R)(NBD)][BAr^F₄] [R = Me, OMe, ⁱPr; Ar^F = 3,5-(CF₃)₂C₆H₃], is described. Along with the previously reported R = H variant, variable temperature ¹H NMR spectroscopic and single-crystal X-ray diffraction studies show that these all have axial (C–H)⋯Rh anagostic interactions relative to the d⁸ pseudo square planar metal centres, that also result in corresponding downfield chemical shifts. Analysis by NBO, QTAIM and NCI methods shows these to be only very weak C–H⋯Rh bonding interactions, the magnitudes of which do not correlate with the observed chemical shifts. Instead, as informed by Scherer''s approach, it is the topological positioning of the C–H bond with regard to the metal centre that is important. For [Rh(DPEphos–ⁱPr)(NBD)][BAr^F₄] addition of H₂ results in a Rh(iii) ⁱPr–C–H activated product, [Rh(κ³,σ-P,O,P-DPEphos-ⁱPr′)(H)][BAr^F₄]. This undergoes H/D exchange with D₂ at the ⁱPr groups, reacts with CO or NBD to return Rh(i) products, and reaction with H₃B·NMe₃/tert-butylethene results in a dehydrogenative borylation to form a complex that shows both a non-classical B–H⋯Rh 3c-2e agostic bond and a C–H⋯Rh anagostic interaction at the same metal centre.

Rh(i) complexes of ortho-substituted DPEphos-R (R = H, Me, OMe, ⁱPr) ligands show anagostic interactions; for R =ⁱPr C–H activation/dehydrogenative borylation forms a product exhibiting both B–H/Rh 3c-2e agostic and C–H/Rh anagostic motifs.     

A chiral cobalt(ii) complex catalyzed enantioselective aza-Piancatelli rearrangement/Diels–Alder cascade reaction

A chiral N,N′-dioxide/cobalt(ii) complex catalyzed highly diastereoselective and enantioselective tandem aza-Piancatelli rearrangement/intramolecular Diels–Alder reaction has been disclosed. Various valuable hexahydro-2a,5-epoxycyclopenta[cd]isoindoles bearing six contiguous stereocenters have been obtained in good yields with excellent diastereo- and enantio-selectivities from a wide range of both readily available 2-furylcarbinols and N-(furan-2-ylmethyl)anilines.

An asymmetric aza-Piancatelli rearrangement/Diels–Alder cascade reaction between 2-furylcarbinols and N-(furan-2-ylmethyl)anilines was realized by using a chiral N,N′-dioxide/cobalt(ii) complex catalyst.     

Single metal four-electron reduction by U(ii) and masked “U(ii)” compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a “clear-cut” single metal four-electron transfer in f-element chemistry.

Both a masked and the actual complex [U(ii){N(SiMe₃)₂}₃]⁺ effect the reduction of azobenzene to yield a U(vi) bis-imido species providing the first example of a “clear-cut” metal centred four-electron reduction in f-element chemistry.     

Rhodium-catalyzed cascade reactions of triazoles with organoselenium compounds – a combined experimental and mechanistic study

Herein, we report on our studies on the reaction of organoselenium compounds with triazoles under thermal conditions using simple Rh(ii) catalysts. These reactions do not provide the product of classic rearrangement reactions. Instead two different cascade reactions were uncovered. While allyl selenides react in a cascade of sigmatropic rearrangement and selenium-mediated radical cyclization reaction to give dihydropyrroles, cinnamyl selenides undergo a double rearrangement reaction cascade involving a final aza-Cope reaction to give the product of 1,3-difunctionalization. Theoretical and experimental studies were conducted to provide an understanding of the reaction mechanism of these cascade reactions. The former provide an important insight into fundamental question on the nature of the ylide intermediate in rearrangement reactions and reveal that organoselenium compounds take up multiple roles in rearrangement reactions and mediate a free ylide reaction mechanism.

Herein, we report on our studies on the reaction of organoselenium compounds with triazoles under thermal conditions using simple Rh(ii) catalysts.     

Iron porphyrin catalysed light driven C–H bond amination and alkene aziridination with organic azides

Visible light driven nitrene transfer and insertion reactions of organic azides are an attractive strategy for the design of C–N bond formation reactions under mild reaction conditions, the challenge being lack of selectivity as a free nitrene reactive intermediate is usually involved. Herein is described an iron(iii) porphyrin catalysed sp³ C–H amination and alkene aziridination with selectivity by using organic azides as the nitrogen source under blue LED light (469 nm) irradiation. The photochemical reactions display chemo- and regio-selectivity and are effective for the late-stage functionalization of natural and bioactive compounds with complexity. Mechanistic studies revealed that iron porphyrin plays a dual role as a photosensitizer and as a catalyst giving rise to a reactive iron–nitrene intermediate for subsequent C–N bond formation.

An iron(iii) porphyrin catalysed sp³ C–H amination and alkene aziridination with broad substrate scope under mild conditions is conducted, with selectivity through the use of organic azides as the nitrogen source under blue LED light irradiation.     

Rh(iii)-Catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates

Herein, we report a mild and highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C–H annulation of 2-alkenylanilides with allenyl acetates, which has been elusive so far. The reaction proceeds via vinylic C–H activation, regioselective 2,3-migratory insertion, β-oxy elimination followed by nucleophilic cyclization to get direct access to 1,2-dihydroquinoline derivatives. The strategy was also successfully extended to C–H activation of 2-alkenylphenols for constructing chromene derivatives. In the overall [5 + 1] annulation, the allene serves as a one carbon unit. The acetate group on the allene is found to be crucial both for controlling the regio- and chemoselectivity of the reaction and also for facilitating β-oxy elimination. The methodology was scalable and also further extended towards late stage functionalization of various natural products.

A highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C–H annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates was described for accessing dihyroquinoline and chromene derivatives.