Oxidative Route to Abnormal NHC Compounds from Singly Bonded [M–M] (M = Ru, Rh, Pd) Precursors

A new base-free entry to metal–<i>a</i>NHC compounds from metal–metal bonded bimetallic precursors and imidazolium salts is reported. Regioselective metalation proceeds via C–I oxidative addition of an annulated imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine system to [Ru^I–Ru^I], [Rh^II–Rh^II], and [Pd^I–Pd^I] single bonds, affording C⁵-bound (abnormal) Ru^II–, Rh^III–, and Pd^II–NHC compounds, respectively, at room temperature and in high yields

Direct Synthesis of Benzimidazoles by Dehydrogenative Coupling of Aromatic Diamines and Alcohols Catalyzed by Cobalt

Herein, we present the base-metal-catalyzed dehydrogenative coupling of primary alcohols and aromatic diamines to selectively form functionalized 2-substituted benzimidazoles, liberating water and hydrogen gas as the sole byproducts. The reaction is catalyzed by pincer complexes of Earth-abundant cobalt under base-free conditions

Cyclometalations on the Imidazo[1,2‑<i>a</i>][1,8]naphthyridine Framework

Cyclometalation on the substituted imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine platform involves either the C₃-aryl or C₄′-aryl <i>ortho</i> carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄′-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅′-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd­(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work

Cyclometalations on the Imidazo[1,2‑<i>a</i>][1,8]naphthyridine Framework

Cyclometalation on the substituted imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine platform involves either the C₃-aryl or C₄′-aryl <i>ortho</i> carbon and the imidazo nitrogen N₃′. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄′-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅′-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd­(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅′) was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work

Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]₂ with excess ^<i>t</i>BuOK and subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh­(COD)­(κC₂-PIN)­Br] (<b>1</b>) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of <b>1</b> confirms ligand coordination to “Rh­(COD)­Br” through the carbene carbon featuring an unbound naphthyridine. Compound <b>1</b> is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^–1 was achieved for the acrylonitrile. A similar Rh­(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst–water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle

Bifunctional Water Activation for Catalytic Hydration of Organonitriles

Treatment of [Rh­(COD)­(μ-Cl)]₂ with excess ^<i>t</i>BuOK and subsequent addition of 2 equiv of PIN·HBr in THF afforded [Rh­(COD)­(κC₂-PIN)­Br] (<b>1</b>) (PIN = 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)­imidazol-2-ylidene, COD = 1,5-cyclooctadiene). The X-ray structure of <b>1</b> confirms ligand coordination to “Rh­(COD)­Br” through the carbene carbon featuring an unbound naphthyridine. Compound <b>1</b> is shown to be an excellent catalyst for the hydration of a wide variety of organonitriles at ambient temperature, providing the corresponding organoamides. In general, smaller substrates gave higher yields compared with sterically bulky nitriles. A turnover frequency of 20 000 h^–1 was achieved for the acrylonitrile. A similar Rh­(I) catalyst without the naphthyridine appendage turned out to be inactive. DFT studies are undertaken to gain insight on the hydration mechanism. A 1:1 catalyst–water adduct was identified, which indicates that the naphthyridine group steers the catalytically relevant water molecule to the active metal site via double hydrogen-bonding interactions, providing significant entropic advantage to the hydration process. The calculated transition state (TS) reveals multicomponent cooperativity involving proton movement from the water to the naphthyridine nitrogen and a complementary interaction between the hydroxide and the nitrile carbon. Bifunctional water activation and cooperative proton migration are recognized as the key steps in the catalytic cycle

A Highly Efficient Catalyst for Selective Oxidative Scission of Olefins to Aldehydes: Abnormal-NHC–Ru(II) Complex in Oxidation Chemistry

The utility and selectivity of the catalyst [Ru­(COD)­(<b>L¹</b>)­Br₂] (<b>1</b>) bearing a fused π-conjugated imidazo­[1,2-<i>a</i>]­[1,8]­naphthyridine-based abnormal N-heterocyclic carbene ligand <b>L¹</b> is demonstrated toward selective oxidation of CC bonds to aldehydes and CC bonds to α-diketones in an EtOAc/CH₃CN/H₂O solvent mixture at room temperature using a wide range of substrates, including highly functionalized sugar- and amino acid-derived compounds

Selective <i>N</i>‑Formylation of Amines with H₂ and CO₂ Catalyzed by Cobalt Pincer Complexes

<i>N</i>-formylation of amines utilizing CO₂ in the presence of reducing agents constitute an important methodology in organic synthesis. Presented herein is a selective <i>N</i>-formylation of amines with CO₂ and H₂ catalyzed by complexes of Earth-abundant cobalt. A wide range of amines were converted to their corresponding formamides under CO₂ and H₂ pressure, catalyzed by Co-PNP pincer complex, generating water as the sole byproduct

Selective <i>N</i>‑Formylation of Amines with H₂ and CO₂ Catalyzed by Cobalt Pincer Complexes

<i>N</i>-formylation of amines utilizing CO₂ in the presence of reducing agents constitute an important methodology in organic synthesis. Presented herein is a selective <i>N</i>-formylation of amines with CO₂ and H₂ catalyzed by complexes of Earth-abundant cobalt. A wide range of amines were converted to their corresponding formamides under CO₂ and H₂ pressure, catalyzed by Co-PNP pincer complex, generating water as the sole byproduct

Catalytic Conversion of Alcohols to Carboxylic Acid Salts and Hydrogen with Alkaline Water

A [RuH­(CO)­(py-NP)­(PPh₃)₂]Cl (<b>1</b>) catalyst is found to be effective for catalytic transformation of primary alcohols, including amino alcohols, to the corresponding carboxylic acid salts and two molecules of hydrogen with alkaline water. The reaction proceeds via acceptorless dehydrogenation of alcohol, followed by a fast hydroxide/water attack to the metal-bound aldehyde. A pyridyl-type nitrogen in the ligand architecture seems to accelerate the reaction