Insights into the Mechanism of the Ruthenium-Porphyrin-Catalysed Allylic Amination of Olefins by Aryl Azides

Direct synthesis of branched amines enabled by dual-catalyzed allylic C─H amination of alkenes with amines

Direct conversion of hydrocarbons into amines represents an important and atom-economic goal in chemistry for decades. However, intermolecular cross-coupling of terminal alkenes with amines to form branched amines remains extremely challenging. Here, a visible-light and Co-dual catalyzed direct allylic C─H amination of alkenes with free amines to afford branched amines has been developed. Notably, challenging aliphatic amines with strong coordinating effect can be directly used as C─N coupling partner to couple with allylic C─H bond to form advanced amines with molecular complexity. Moreover, the reaction proceeds with exclusive regio- and chemoselectivity at more steric hinder position to deliver primary, secondary, and tertiary aliphatic amines with diverse substitution patterns that are difficult to access otherwise.

Iron-Catalyzed Intermolecular C-H Amination Assisted by an Isolated Iron-Imido Radical Intermediate

Here we report the use of a base metal complex [(tBu pyrpyrr2 )Fe(OEt2 )] (1-OEt2 ) (tBu pyrpyrr2 2- =3,5-tBu2 -bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of Csp3 -H bonds of 9,10-dihydroanthracene (2 a) using 2,4,6-trimethyl phenyl azide (3 a) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1-OEt2 with control in selectivity for single C-H amination versus double C-H amination. Catalytic C-H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(tBu pyrpyrr2 )Fe{=N(2,6-Me2 -4-tBu-C6 H2 )] (1-imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 FeIV complex. Complex 1-imido reacts with 2 a to produce the ferrous aniline adduct [(tBu pyrpyrr2 )Fe{NH(2,6-Me2 -4-tBu-C6 H2 )(C14 H11 )}] (1-aniline) in 45 % yield. Lastly, it was found that complexes 1-imido and 1-aniline are both competent intermediates in catalytic intermolecular C-H amination.

Ruthenium(v) terminal arylimido corroles: isolation, spectroscopic characterization and reactivity

Terminal Ru(v)-imido species are thought to be as reactive to group transfer reactions as their Ru(v)-oxo homologues, but are less studied. With the electron-rich corrole ligand, relatively stable and isolable Ru(v)-arylimido complexes [Ru(tBu-Cor)(NAr)] (H3(tBu-Cor) = 5,15-diphenyl-10-(p-tert-butylphenyl)corrole, Ar = 2,4,6-Me3C6H2 (Mes), 2,6-(iPr)2C6H3 (Dipp), 2,4,6-(iPr)3C6H2 (Tipp), and 3,5-(CF3)2C6H3 (BTF)) can be prepared from [Ru(tBu-Cor)]2 under strongly reducing conditions. This type of Ru(v)-monoarylimido corrole complex with S = ½ was characterized by high-resolution ESI mass spectrometry, X-band EPR, resonance Raman spectroscopy, magnetic susceptibility, and elemental analysis, together with computational studies. Under heating/light irradiation (xenon lamp) conditions, the complexes [Ru(tBu-Cor)(NAr)] (Ar = Mes, BTF) could undergo aziridination of styrenes and amination of benzylic C(sp3)-H bonds with up to 90% product yields.

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.

The first crystallographically characterised ruthenium(vi) alkylimido porphyrin competent for aerobic epoxidation and hydrogen atom abstraction

The syntheses of [Ru^VI(Por)(NAd)(O)] and [Ru^VI(2,6-F₂-TPP)(NAd)₂] have been described. [Ru^VI(2,6-F₂-TPP)(NAd)(O)] capable of catalysing aerobic epoxidation of alkenes has been characterised by X-ray crystallography with Ru[double bond, length as m-dash]NAd and Ru[double bond, length as m-dash]O bond distances being 1.778(5) Å and 1.760(4) Å (∠O-Ru-NAd: 174.37(19)°), respectively. Its first reduction potential is 740 mV cathodically shifted from that of [Ru^VI(2,6-F₂-TPP)(O)₂].

Indoles from Alkynes and Aryl Azides: Scope and Theoretical Assessment of Ruthenium Porphyrin-Catalyzed Reactions

A symbiotic experimental/computational study analyzed the Ru(TPP)(NAr)₂ -catalyzed one-pot formation of indoles from alkynes and aryl azides. Thirty different C₃ -substituted indoles were synthesized and the best performance, in term of yields and regioselectivities, was observed when reacting ArC≡CH alkynes with 3,5-(EWG)₂ C₆ H₃ N₃ azides, whereas the reaction was less efficient when using electron-rich aryl azides. A DFT analysis describes the reaction mechanism in terms of the energy costs and orbital/electronic evolutions; the limited reactivity of electron-rich azides was also justified. In summary, PhC≡CH alkyne interacts with one NAr imido ligand of Ru(TPP)(NAr)₂ to give a residually dangling C(Ph) group, which, by coupling with a C(H) unit of the N-aryl substituent, forms a 5+6 bicyclic molecule. In the process, two subsequent spin changes allow inverting the conformation of the sp² C(Ph) atom and its consequent electrophilic-like attack to the aromatic ring. The bicycle isomerizes to indole via a two-step outer sphere H-migration. Eventually, a 'Ru(TPP)(NAr)' mono-imido active catalyst is reformed after each azide/alkyne reaction.

Polymeric Ruthenium Porphyrin-Functionalized Carbon Nanotubes and Graphene for Levulinic Ester Transformations into γ-Valerolactone and Pyrrolidone Derivatives

Polymeric ruthenium porphyrin-functionalized carbon nanotubes (Ru-PP/CNTs) were prepared by the metallation of polymeric porphyrin-functionalized carbon nanotubes (PP/CNTs) with Ru₃(CO)₁₂, whereas PP/CNTs were obtained by the condensation of terephthaldehyde and pyrrole in the presence of CNTs. The Ru-PP/CNTs have a thin layer of highly cross-linked polymeric ruthenium porphyrin coating over the CNT surface via strong π-π stacking interactions, thus showing a bilayered structure with an amorphous polymeric outer surface and an internal CNT core. Polymeric ruthenium porphyrin-functionalized reduced graphene oxide (Ru-PP/RGO) was prepared with a synthetic procedure similar to Ru-PP/CNTs, with RGO as the internal core. Both Ru-PP/CNTs and Ru-PP/RGO showed excellent catalytic performance toward hydrogenation of biomass-related ethyl levulinate (EL) to γ-valerolactone (GVL) with Ru-centered porphyrin units as the catalytic active species. Under optimized reaction conditions, a GVL yield higher than 99% with a complete conversion of EL was observed over both Ru-PP/CNTs and Ru-PP/RGO. In addition to GVL preparation, the versatile Ru-PP/CNTs can efficiently promote reductive amination of EL with various amines for the synthesis of pyrrolidone derivatives, with the corresponding yields ranging from 96.3 to 88.7%. Moreover, the composite materials of both Ru-PP/CNTs and Ru-PP/RGO behave as heterogeneous catalysts in the reaction system and can be easily reused.

Heterobimetallic Nitrido Complexes of Group 8 Metalloporphyrins

Heterobimetallic nitrido porphyrin complexes with the [(L)(por)M-N-M'(L_OEt)Cl₂] formula {por^2- = 5,10,15,20-tetraphenylporphyrin (TPP^2-) or 5,10,15,20-tetra(p-tolyl)porphyrin (TTP^2-) dianion; L_OEt^- = [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^-; M = Fe, Ru, or Os; M' = Ru or Os; L = H₂O or pyridine} have been synthesized, and their electrochemistry has been studied. Treatment of trans-[Fe(TPP)(py)₂] (py = pyridine) with Ru(VI) nitride [Ru(L_OEt)(N)Cl₂] (1) afforded Fe/Ru μ-nitrido complex [(py)(TPP)Fe(μ-N)Ru(L_OEt)Cl₂] (2). Similarly, Fe/Os analogue [(py)(TPP)Fe(μ-N)Os(L_OEt)Cl₂] (3) was obtained from trans-[Fe(TPP)(py)₂] and [Os(L_OEt)(N)Cl₂]. However, no reaction was found between trans-[Fe(TPP)(py)₂] and [Re(L_OEt)(N)Cl(PPh₃)]. Treatment of trans-[M(TPP)(CO)(EtOH)] with 1 afforded μ-nitrido complexes [(H₂O)(TPP)M(μ-N)Ru(L_OEt)Cl₂] [M = Ru (4a) or Os (5)]. TTP analogue [(H₂O)(TTP)Ru(μ-N)Ru(L_OEt)Cl₂] (4b) was prepared similarly from trans-[Ru(TTP)(CO)(EtOH)] and 1. Reaction of [(H₂O)(por)M(μ-N)M(L_OEt)Cl₂] with pyridine gave adducts [(py)(por)M(μ-N)Ru(L_OEt)Cl₂] [por = TTP, and M = Ru (6); por = TPP, and M = Os (7)]. The diamagnetism and short (por)M-N(nitride) distances in 2 [Fe-N, 1.683(3) Å] and 4b [Ru-N, 1.743(3) Å] are indicative of the M^IV═N═M'^IV bonding description. The cyclic voltammograms of the Fe/Ru (2) and Ru/Ru (4b) complexes in CH₂Cl₂ displayed oxidation couples at approximately +0.29 and +0.35 V versus Fc^+/0 (Fc = ferrocene) that are tentatively ascribed to the oxidation of the {L_OEtRu} and {Ru(TTP)} moieties, respectively, whereas the Fe/Os (3) and Os/Ru (5) complexes exhibited Os-centered oxidation at approximately -0.06 and +0.05 V versus Fc^+/0, respectively. The crystal structures of 2 and 4b have been determined.

Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications

Catalytic transformation of ubiquitous C-H bonds into valuable C-N bonds offers an efficient synthetic approach to construct N-functionalized molecules. Over the last few decades, transition metal catalysis has been repeatedly proven to be a powerful tool for the direct conversion of cheap hydrocarbons to synthetically versatile amino-containing compounds. This Review comprehensively highlights recent advances in intra- and intermolecular C-H amination reactions utilizing late transition metal-based catalysts. Initial discovery, mechanistic study, and additional applications were categorized on the basis of the mechanistic scaffolds and types of reactions. Reactivity and selectivity of novel systems are discussed in three sections, with each being defined by a proposed working mode.

Unexpected formation of a μ-carbido diruthenium(iv) complex during the metalation of phthalocyanine with Ru3(CO)12 and its catalytic activity in carbene transfer reactions

Catalytic activity of novel μ-carbido Ru(iv) bisphthalocyaninate was firstly demonstrated by the olefin cyclopropanation and carbene insertion into N–H bonds.

Synthesis and catalytic activity of μ-oxo ruthenium(IV) porphyrin species to promote amination reactions

This work describes the synthesis of ruthenium(IV) [Formula: see text]-oxo porphyrin complexes of general formula [RuIV(TPP)(X)]2O which have been applied as catalysts in nitrene transfer reactions using aryl azides (ArN[Formula: see text] as nitrene sources. Collected data indicated that the catalytic efficiency of [RuIV(TPP)(OCH[Formula: see text]]2O was comparable to that of RuII(TPP)CO because of their analogous reactivity towards aryl azides to give the same catalytically active bis-imido species RuVI(TPP)(ArN)2. The reaction of [RuIV(TPP)(OCH[Formula: see text]]2O with Ph3CN3 or (CH[Formula: see text]SiN3 afforded [RuIV(TPP)(N[Formula: see text]]2O which was fully characterised, its molecular structure was also determined by single crystal X-ray analysis.

Recent advances in C–H bond aminations catalyzed by ruthenium porphyrin complexes

This review deals with the use of ruthenium porphyrin complexes to catalyze hydrocarbon aminations. This class of versatile porphyrin catalysts are able to activate different nitrogen sources, such as iminoiodinanes and organic azides, and promote the synthesis of a variety of aza-derivatives. Many synthetic procedures have been discussed as well as catalytic mechanisms involved in order to give an overview on the use of ruthenium porphyrins to promote nitrene transfer reactions yielding aminated derivatives.

Resonance Raman spectroscopy as an in situ probe for monitoring catalytic events in a Ru-porphyrin mediated amination reaction

Resonance Raman microspectroscopy has been widely used to study the structure and dynamics of porphyrins and metal complexes containing the porphyrin ligand. Here, we have demonstrated that the same technique can be adapted to examine the mechanism of a homogeneously-catalysed reaction mediated by a transition-metal-porphyrin complex. Previously it has been challenging to study this type of reaction using in situ spectroscopic monitoring due to the low stability of the reaction intermediates and elevated-temperature conditions. We have made a straightforward modification to the sample stage on a microscope for time-lapsed Raman microspectroscopy from reaction mixtures in these media. The allylic amination of unsaturated hydrocarbons by aryl azides, which can be catalysed by a ruthenium-porphyrin complex, has been used as an illustrative example of the methodology. The mechanism of this particular reaction has been studied previously using density-functional theory and kinetic approaches. The Raman measurements support the mechanism proposed in the earlier publications by providing the first experimental verification of a precursor reaction complex between the aryl azide and the ruthenium metal ion, and evidence for the formation of a mono-imido intermediate complex under conditions of high concentration of the reactant olefin.

A mechanistic investigation of the ruthenium porphyrin catalysed aziridination of olefins by aryl azides

A combination of DFT and kinetic studies suggests a mechanism for olefin aziridination by organic azides catalysed by ruthenium porphyrins.

Density functional theory calculations on oxidative C-C bond cleavage and N-O bond formation of [Ru(II)(bpy)2(diamine)](2+) via reactive ruthenium imide intermediates

DFT calculations are performed on [Ru(II)(bpy)2(tmen)](2+) (M1, tmen = 2,3-dimethyl-2,3-butanediamine) and [Ru(II)(bpy)2(heda)](2+) (M2, head = 2,5-dimethyl-2,5-hexanediamine), and on the oxidation reactions of M1 to give the C-C bond cleavage product [Ru(II)(bpy)2(NH=CMe2)2](2+) (M3) and the N-O bond formation product [Ru(II)(bpy)2(ONCMe2CMe2NO)](2+) (M4). The calculated geometrical parameters and oxidation potentials are in good agreement with the experimental data. As revealed by the DFT calculations, [Ru(II)(bpy)2(tmen)](2+) (M1) can undergo oxidative deprotonation to generate Ru-bis(imide) [Ru(bpy)2(tmen-4 H)](+) (A) or Ru-imide/amide [Ru(bpy)2(tmen-3 H)](2+) (A') intermediates. Both A and A' are prone to C-C bond cleavage, with low reaction barriers (ΔG(≠)) of 6.8 and 2.9 kcal mol(-1) for their doublet spin states (2)A and (2)A', respectively. The calculated reaction barrier for the nucleophilic attack of water molecules on (2)A' is relatively high (14.2 kcal mol(-1)). These calculation results are in agreement with the formation of the Ru(II)-bis(imine) complex M3 from the electrochemical oxidation of M1 in aqueous solution. The oxidation of M1 with Ce(IV) in aqueous solution to afford the Ru(II)-dinitrosoalkane complex M4 is proposed to proceed by attack of the cerium oxidant on the ruthenium imide intermediate. The findings of ESI-MS experiments are consistent with the generation of a ruthenium imide intermediate in the course of the oxidation.

Organic azides: “energetic reagents” for the intermolecular amination of C–H bonds

This feature article highlights the potentiality of organic azides (RN₃) for the intermolecular amination of sp³ and sp² C–H bonds. A compendium of employed catalytic systems, together with a discussion of involved mechanisms, is provided.

Bis(sulfonylimide)ruthenium(VI) porphyrins: X-ray crystal structure and mechanism of C-H bond amination by density functional theory calculations

The X-ray crystal structure of [Ru(VI) (NMs)2 (tmp)] (Ms=SO2 - p-MeOC6 H4 ; tmp=5,10,15,20-tetramesitylporphyrinato(2-)), a metal sulfonylimide complex that can undergo alkene aziridination and C-H bond amination reactions, shows a Ru=N distance of 1.79(3) Å and Ru-N-S angle of 162.5(3)°. Density functional theory (DFT) calculations on the electronic structures of [Ru(VI) (NMs)2 (tmp)] and model complex [Ru(VI) (NMs)2 (por(0) )] (por(0) =unsubstituted porphyrinato(2-)) using the M06L functional gave results in agreement with experimental observations. For the amination of ethylbenzene by the singlet ground state of [Ru(VI) (NMs)2 (por(0))], DFT calculations using the M06L functional revealed an effectively concerted pathway involving rate-limiting hydrogen atom abstraction without a distinct radical rebound step. The substituent effect on the amination reactivity of ethylbenzene by [Ru(VI) (NX)2 (por(0) )] (X=SO2 -p-YC6 H4 with Y=MeO, Me, H, Cl, NO2 ) was examined. Electron-withdrawing Y groups lower the energy of the LUMOs of [Ru(VI) (NX)2 (por(0))], thus facilitating their interaction with the low-lying HOMO of the ethylbenzene C-H bond and hence increasing the reactivity of [Ru(VI) (NX)2 (por(0) )]. DFT calculations on the amination/aziridination reactions of [Ru(VI) (NSO2 C6 H5 )2 (por(0) )] with pent-4-enal, an aldehyde substrate bearing acyl, homoallylic, and allylic C-H bonds and a C=C bond, revealed a lower reaction barrier for the amination of the acyl C-H bond than for both the amination of the other C-H bonds and aziridination of the C=C bond in this substrate.