Catalytic Intramolecular Hydroamination with a Bifunctional Iridium Pyrazolato Complex: Substrate Scope and Mechanistic Elucidation

Catalytic intramolecular cyclization of nonactivated aminoalkene with functional group compatibility provides an atom-economical and concise route to valuable nitrogen-containing heterocycles yet remains a challenge. In this paper, we report the detailed substrate scope and mechanism of catalytic intramolecular hydroamination with a half-sandwich-type iridium pyrazolato complex we have recently developed. This metal–ligand bifunctional catalyst promoted the hydroamination of various primary and secondary aminoalkenes at mild temperatures (50–110 °C) without side reactions such as oxidative amination. Cyclization of secondary aminoalkenes containing ester, cyano, bromo, and hydroxy groups occurred with maintenance of these functional groups, while the reactions of aminoalkenes bearing allylic substituents proceeded with a perfect diastereoselectivity. Catalyst optimization revealed that the proton-responsive functional group at the position β to the metal is crucial to efficient catalytic turnover. Kinetic analysis indicated a highly ordered transition state associated with N–H bond cleavage in the rate-determining step. On the basis of these data along with the stoichiometric reactions and DFT calculations, we propose an unprecedented metal–ligand cooperating mechanism, in which cyclization occurs through <i>syn</i> addition of the amino group to the coordinated olefin bond with the aid of the Brønsted basic pyrazolato ligand

N–N Bond Cleavage of Hydrazines with a Multiproton-Responsive Pincer-Type Iron Complex

N–N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal–ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ¹<i>N</i>-phenylhydrazine complex

A Bifunctional Iridium Catalyst Modified for Persistent Hydrogen Generation from Formic Acid: Understanding Deactivation via Cyclometalation of a 1,2-Diphenylethylenediamine Motif

Thermal degradation of a bifunctional Ir complex with a 1,2-diphenylethylenediamine (DPEN) framework was investigated, which is relevant to catalyst deactivation in the acceptorless dehydrogenation of formic acid. The well-defined hydridoiridium complex <b>1b</b>, derived from <i>N</i>-triflyl-1,2-diphenylethylenediamine (TfDPEN), proved to be solely transformed at the reflux temperature of 1,2-dimethoxyethane (DME) into two iridacycles (<b>2</b> and <b>3</b>) via C–H bond cleavage at the ortho carbon atoms of the phenyl substituents on the diamine backbone. These products were successfully isolated and characterized by NMR, elemental analysis, and X-ray crystallography. The iridacycle formation was significantly enhanced in the presence of water, possibly due to facile deprotonative orthometalation via a hydroxidoiridium intermediate. To prevent the deactivation process caused by the cyclometalation of the DPEN moiety, a hydridoiridium complex (<b>5b</b>) without phenyl substituents was synthesized from <i>N</i>-triflylethylenediamine (TfEN). The modified complex <b>5b</b> showed a pronounced ability to catalyze hydrogen evolution from formic acid in a 1/1 mixed solvent of water and DME even in the absence of base additives. The initial rate was maintained for a longer time relative to <b>1b</b>, and thus formic acid was mostly converted within 80 min under the conditions of a HCOOH/<b>5b</b> ratio of 15900 at 60 °C

Catalytic Intramolecular Hydroamination with a Bifunctional Iridium Pyrazolato Complex: Substrate Scope and Mechanistic Elucidation

Catalytic intramolecular cyclization of nonactivated aminoalkene with functional group compatibility provides an atom-economical and concise route to valuable nitrogen-containing heterocycles yet remains a challenge. In this paper, we report the detailed substrate scope and mechanism of catalytic intramolecular hydroamination with a half-sandwich-type iridium pyrazolato complex we have recently developed. This metal–ligand bifunctional catalyst promoted the hydroamination of various primary and secondary aminoalkenes at mild temperatures (50–110 °C) without side reactions such as oxidative amination. Cyclization of secondary aminoalkenes containing ester, cyano, bromo, and hydroxy groups occurred with maintenance of these functional groups, while the reactions of aminoalkenes bearing allylic substituents proceeded with a perfect diastereoselectivity. Catalyst optimization revealed that the proton-responsive functional group at the position β to the metal is crucial to efficient catalytic turnover. Kinetic analysis indicated a highly ordered transition state associated with N–H bond cleavage in the rate-determining step. On the basis of these data along with the stoichiometric reactions and DFT calculations, we propose an unprecedented metal–ligand cooperating mechanism, in which cyclization occurs through <i>syn</i> addition of the amino group to the coordinated olefin bond with the aid of the Brønsted basic pyrazolato ligand

N–N Bond Cleavage of Hydrazines with a Multiproton-Responsive Pincer-Type Iron Complex

N–N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal–ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ¹<i>N</i>-phenylhydrazine complex

N–N Bond Cleavage of Hydrazines with a Multiproton-Responsive Pincer-Type Iron Complex

N–N bond cleavage of hydrazines on transition metals is of considerable importance in understanding the mechanism of biological nitrogen fixation under ambient conditions. We found that a metal–ligand-bifunctional complex of iron with a pincer-type ligand bearing two proton-responsive pyrazole arms catalyzes the disproportionation of hydrazine into ammonia and dinitrogen. The NH groups in the pyrazole ligands and hydrazines are crucial for the reaction, which most likely occurs through multiple and bidirectional proton-coupled electron transfer between the iron complex and hydrazine. The multiproton-responsive pincer-type ligand also stabilizes the intermediate diazene complex through a hydrogen-bonding network, as revealed by structural characterization of a κ¹<i>N</i>-phenylhydrazine complex

A Bifunctional Iridium Catalyst Modified for Persistent Hydrogen Generation from Formic Acid: Understanding Deactivation via Cyclometalation of a 1,2-Diphenylethylenediamine Motif

Thermal degradation of a bifunctional Ir complex with a 1,2-diphenylethylenediamine (DPEN) framework was investigated, which is relevant to catalyst deactivation in the acceptorless dehydrogenation of formic acid. The well-defined hydridoiridium complex <b>1b</b>, derived from <i>N</i>-triflyl-1,2-diphenylethylenediamine (TfDPEN), proved to be solely transformed at the reflux temperature of 1,2-dimethoxyethane (DME) into two iridacycles (<b>2</b> and <b>3</b>) via C–H bond cleavage at the ortho carbon atoms of the phenyl substituents on the diamine backbone. These products were successfully isolated and characterized by NMR, elemental analysis, and X-ray crystallography. The iridacycle formation was significantly enhanced in the presence of water, possibly due to facile deprotonative orthometalation via a hydroxidoiridium intermediate. To prevent the deactivation process caused by the cyclometalation of the DPEN moiety, a hydridoiridium complex (<b>5b</b>) without phenyl substituents was synthesized from <i>N</i>-triflylethylenediamine (TfEN). The modified complex <b>5b</b> showed a pronounced ability to catalyze hydrogen evolution from formic acid in a 1/1 mixed solvent of water and DME even in the absence of base additives. The initial rate was maintained for a longer time relative to <b>1b</b>, and thus formic acid was mostly converted within 80 min under the conditions of a HCOOH/<b>5b</b> ratio of 15900 at 60 °C

Hydrodefluorination of Fluoroarenes Using Hydrogen Transfer Catalysts with a Bifunctional Iridium/NH Moiety

The hydrodefluorination of fluoroarenes with transfer hydrogenation catalysts using 2-propanol or potassium formate is described. With the aid of metal/NH cooperation, the C–N chelating Ir complexes derived from benzylic amines can efficiently promote the reduction involving the C–F bond cleavage under ambient conditions even in the absence of hydrosilanes or H₂ gas, leading to the partially fluorinated products in good yields and with high selectivity

Intramolecular 1,3-Dipolar Cycloaddition of Nitrile <i>N</i>-Oxide Accompanied by Dearomatization

Intramolecular 1,3-dipolar cycloaddition of 2-phenoxybenzonitrile <i>N</i>-oxides to benzene rings, accompanied by dearomatization, formed the corresponding isoxazolines in high yields. The X-ray single-crystal structure analysis revealed that the reaction formed the <i>cis</i>-adduct as a single isomer. The substituents on the benzene rings markedly affected the reaction rate, yield, and structure of the final product

Nucleophilic Aromatic Substitution in Hydrodefluorination Exemplified by Hydridoiridium(III) Complexes with Fluorinated Phenylsulfonyl-1,2-diphenylethylenediamine Ligands

In connection with the mechanism of the catalytic reduction of fluoroarenes, the intramolecular defluorinative transformation of a family of iridium hydrides utilized as a hydrogen transfer catalyst is studied. Hydridoiridium­(III) complexes bearing fluorinated phenylsulfonyl-1,2-diphenylethylenediamine ligands are spontaneously converted into iridacycles via selective C–F bond cleavage at the <i>ortho</i> position. NMR spectroscopic studies and synthesis of intermediate model compounds verify the stepwise pathway involving intramolecular substitution of the <i>ortho</i>-fluorine atom by the hydrido ligand, i.e., hydrodefluorination (HDF), and the following fluoride-assisted cyclometalation at the transiently formed C–H bond. A hydridoiridium complex with a 2,3,4,5,6-pentafluorophenylsulfonyl (Fs) substituent is more susceptible to HDF than its analog with a 2,3,4,5-tetrafluorophenylsulfonyl (Fs^H) group. The Fs^H-derivative clearly shows that C–F bond cleavage occurs in preference to C–H activation. These experimental results firmly support the nucleophilic aromatic substitution (S_NAr) mechanism in HDF by hydridoiridium species