Catalytic Reduction of Alkyl and Aryl Bromides Using Propan-2-ol

Ruthenium pincer complex-catalyzed heterocycle compatible alkoxycarbonylation of alkyl iodides: substrate keeps the catalyst active

The electron pair of the heteroatom in heterocycles will coordinate with metal catalysts and decrease or even inhibit their catalytic activity consequently. In this work, a pincer ruthenium-catalyzed heterocycle compatible alkoxycarbonylation of alkyl iodides has been developed. Benefitting from the pincer ligand, a variety of heterocycles, such as thiophenes, morpholine, unprotected indoles, pyrrole, pyridine, pyrimidine, furan, thiazole, pyrazole, benzothiadiazole, and triazole, are compatible here.

Frustrated Lewis pair catalyzed hydrodehalogenation of benzyl-halides

10 mol% B(2,6-C₆F₂H₃)₃ in the presence of excess tetramethylpiperidine (TMP) and H₂ (or D₂) is shown to catalyze the hydrogenative dehalogenation of benzyl-halides to give corresponding toluene derivatives. These reactions proceed via an initial FLP activation of H₂ yielding the ammonium hydridoborate [TMPH][HB(2,6-C₆F₂H₃)₃]. This species acts in analogy to a FLP to cooperatively activate C-X bond (X = Cl, Br, I) of benzyl-halides delivering hydride and generating the corresponding ammonium halide salts.

Iridium-catalyzed highly stereoselective deoxygenation of tertiary cycloalkanols: stereoelectronic insights and synthetic applications

Excellent and unique diastereoselectivity is observed in the iridium-catalyzed deoxygenation of tertiary cyclohexanols and cyclopentanols. The substituent effect on the diastereoselectivity and detailed control models are analyzed case by case, using tertiary monocyclic and polycyclic cyclohexanols, bicyclic bridged cycloalkanols, and cyclopentanols as the model substrates. The selectivity is decided by the steric environment of the carbocation intermediates and is independent of the catalyst loading. Stereoelectronically, the iridium hydride approaches the carbocation in directions perpendicular to the carbocation plane. The sterically large iridium hydride delivers its hydride in the sterically least hindered direction to the carbocation. The deoxygenation has found important applications in the stereospecific arylations of sterically complex compounds. Our deoxygenation is stereochemically very different from the coupling reactions and can be used to specifically synthesize stereoisomers that are not available via cross-couplings.

Cross-Coupling Reactions of Nitroarenes

Cross-coupling reactions are powerful synthetic tools to construct diverse chemical bonds often found in, for example, advanced materials and pharmaceuticals. Since their discovery, haloarenes have habitually been used as electrophilic coupling partners both in academic and industrial contexts. However, concerning the efficiency and the often-negative environmental impact of haloarene-based cross-coupling processes, more readily available, inexpensive, and environmentally friendly electrophiles have been explored.Nitroarenes, for example, are obtained from the facile nitration of aromatic compounds and, thus, represent one of the most easy-to-access feedstock electrophiles. Furthermore, their electron-deficient arene core can be functionalized easily and site-selectively through a wide variety of reactions. Yet, despite these advantages and even though the direct transformation of the NO₂ group would be an attractive option in cross-coupling chemistry, it has so far remained difficult to convert nitroarenes via a cleavage of the Ar-NO₂ bond given the inherent reactivity (or the lack thereof) of the nitro group. Such denitrative conversion has been performed by a conventional sequence of reduction, diazotization, and Sandmeyer reactions, which severely lacks efficiency and generality.This Account summarizes our recent research progress on cross-coupling reactions that employ nitroarenes as electrophiles. First, we developed the Suzuki-Miyaura coupling of nitroarenes using a palladium/BrettPhos catalyst. This reaction proceeds via an (at the time) unprecedented oxidative addition of the Ar-NO₂ bond, which was supported by experimental results and theoretical calculations. A widely accepted catalytic cycle for Pd-catalyzed cross-couplings has since been extended to include nitroarenes as electrophiles, which significantly increases substrate generality. Second, this denitrative coupling protocol was applied to various bond-forming reactions, namely, Buchwald-Hartwig amination, etherification, and hydrogenation reactions. Such diversification has enhanced the utility of nitroarenes as cross-coupling partners. To develop each reaction, it was necessary to modify the reaction conditions as required to overcome the obstacles deriving from nitro functionality including transmetalation and side reactions, as well as oxidative addition. Third, we designed a new Pd/NHC catalyst that exhibits higher activity than Pd/BrettPhos. The improved performance of Pd/NHC system was supported by its strong electron-donicity and structural robustness, and it allows the reduction of the catalyst loading significantly, thus increasing the efficacy and practicality of this method.The field of nitroarene-based cross-coupling has just started to flourish. In addition to our original work, several research groups have already adopted Pd/BrettPhos or Pd/NHC catalysts to develop new denitrative functionalizations. The utility of nitroarenes in the context of organic synthesis should be now revisited.

Alkali Metal Adducts of an Iron(0) Complex and Their Synergistic FLP-Type Activation of Aliphatic C-X Bonds

We report the formation and full characterization of weak adducts between Li⁺ and Na⁺ cations and a neutral iron(0) complex, [Fe(CO)₃(PMe₃)₂] (1), supported by weakly coordinating [BAr^F₂₀] anions, [1·M][BAr^F₂₀] (M = Li, Na). The adducts are found to synergistically activate aliphatic C-X bonds (X = F, Cl, Br, I, OMs, OTf), leading to the formation of iron(II) organyl compounds of the type [FeR(CO)₃(PMe₃)₂][BAr^F₂₀], of which several were isolated and fully characterized. Stoichiometric reactions with the resulting iron(II) organyl compounds show that this system can be utilized for homocoupling and cross-coupling reactions and the formation of new C-E bonds (E = C, H, O, N, S). Further, we utilize [1·M][BAr^F₂₀] as a catalyst in a simple hydrodehalogenation reaction under mild conditions to showcase its potential use in catalytic reactions. Finally, the mechanism of activation is probed using DFT and kinetic experiments that reveal that the alkali metal and iron(0) center cooperate to cleave C-X via a mechanism closely related to intramolecular FLP activation.

Ruthenium-Catalyzed Dehydrogenation Through an Intermolecular Hydrogen Atom Transfer Mechanism

The direct dehydrogenation of alkanes is among the most efficient ways to access valuable alkene products. Although several catalysts have been designed to promote this transformation, they have unfortunately found limited applications in fine chemical synthesis. Here, we report a conceptually novel strategy for the catalytic, intermolecular dehydrogenation of alkanes using a ruthenium catalyst. The combination of a redox-active ligand and a sterically hindered aryl radical intermediate has unleashed this novel strategy. Importantly, mechanistic investigations have been performed to provide a conceptual framework for the further development of this new catalytic dehydrogenation system.

Facile and practical hydrodehalogenations of organic halides enabled by calcium hydride and palladium chloride

For the first time, calcium hydride and palladium chloride were used to reduce a wide range of organic halides including aromatic bromides, aromatic chlorides, aromatic triflates, aliphatic bromides, aliphatic chlorides and trihalomethyl compounds.

The SmI2/TEU--Mediated Cyclization of Unsaturated Halides

The combination of SmI₂ and the conjugate base of triethylurea (TEU^-) has been shown to favor the cyclization of unsaturated halides over direct reduction to a much greater extent than other SmI₂-based reductants. Aryl, heteroaryl, and alkyl halides (X = Br, Cl, F) readily undergo heterocyclization and carbocyclization in the presence of SmI₂/TEU^-.

Dehydroalkylative Activation of CNN- and PNN-Pincer Ruthenium Catalysts for Ester Hydrogenation

Ruthenium-pincer complexes bearing CNN- and PNN-pincer ligands with diethyl- or diisopropylamino side groups, which have previously been reported to be active precatalysts for ester hydrogenation, undergo dehydroalkylation on heating in the presence of tricyclohexylphosphine to release ethane or propane, giving five-coordinate ruthenium(0) complexes containing a nascent imine functional group. Ethane or propane is also released under the conditions of catalytic ester hydrogenation, and time-course studies show that this release is concomitant with the onset of catalysis. A new PNN-pincer ruthenium(0)-imine complex is a highly active catalyst for ester hydrogenation at room temperature, giving up to 15 500 turnovers with no added base. This complex was shown to react reversibly at room temperature with two equivalents of hydrogen to give a ruthenium(II)-dihydride complex, where the imine functionality has been hydrogenated to give a protic amine side group. These observations have potentially broad implications for the identities of catalytic intermediates in ester hydrogenation and related transformations.

Dehalogenative Deuteration of Unactivated Alkyl Halides Using D2O as the Deuterium Source

The general dehalogenation of alkyl halides with zinc using D₂O or H₂O as a deuterium or hydrogen donor has been developed. The method provides an efficient and economic protocol for deuterium-labeled derivatives with a wide substrate scope under mild reaction conditions. Mechanistic studies indicated that a radical process is involved for the formation of organozinc intermediates. The facile hydrolysis of the organozinc intermediates provides the driving force for this transformation.

Organopolymer with dual chromophores and fast charge-transfer properties for sustainable photocatalysis

Photocatalytic polymers offer an alternative to prevailing organometallics and nanomaterials, and they may benefit from polymer-mediated catalytic and material enhancements. MPC-1, a polymer photoredox catalyst reported herein, exhibits enhanced catalytic activity arising from charge transfer states (CTSs) between its two chromophores. Oligomeric and polymeric MPC-1 preparations both promote efficient hydrodehalogenation of α-halocarbonyl compounds while exhibiting different solubility properties. The polymer is readily recovered by filtration. MPC-1-coated vessels enable batch and flow photocatalysis, even with opaque reaction mixtures, via "backside irradiation." Ultrafast transient absorption spectroscopy indicates a fast charge-transfer process within 20 ps of photoexcitation. Time-resolved photoluminescence measurements reveal an approximate 10 ns lifetime for bright valence states. Ultrafast measurements suggest a long CTS lifetime. Empirical catalytic activities of small-molecule models of MPC-1 subunits support the CTS hypothesis. Density functional theory (DFT) and time-dependent DFT calculations are in good agreement with experimental spectra, spectral peak assignment, and proposed underlying energetics.

Phosphonic acid mediated practical dehalogenation and benzylation with benzyl halides

For the first time, by using H₃PO₃/I₂ system, various benzyl chlorides, bromides and iodides were dehalogenated successfully. In the presence of H₃PO₃, benzyl halides underwent electrophilic substitution reactions with electron-rich arenes, leading to a broad range of diarylmethanes in good yields. These transformations feature green, cheap reducing reagents and metal-free conditions. A possible mechanism was proposed.

A practical method for phosphonic acid mediated dehalogenation and benzylation with benzyl halides was developed.

Iron-catalyzed protodehalogenation of alkyl and aryl halides using hydrosilanes

A simple and efficient iron-catalyzed protodehalogenation of alkyl and aryl halides using phenylhydrosilane is disclosed.

Visible-light-mediated hydrodehalogenation and Br/D exchange of inactivated aryl and alkyl halides with a palladium complex

Photo-induced radical reductive dehalogenation of inactivated aryl/alkyl bromides and chlorides with a palladium complex is described. Reductive cyclization, dehalogenative deuteration, and radical addition process can be achieved smoothly.

Electron transfer-induced reduction of organic halides with amines

Reduction of a variety of organo halides was examined by using amines as a sacrificial hydrogen source. UV light-induced reduction of vinyl and aryl halides with triethylamine proceeded smoothly to give the corresponding reduced products. High temperature heating also caused the reduction and DABCO (1,4-diazabicyclo[2.2.2]octane) also served as a good reducing reagent.

Catalysis of Radical Cyclizations from Alkyl Iodides under H2: Evidence for Electron Transfer from [CpV(CO)3H]<sup/>

Radical cyclizations are most often achieved with Bu3SnH in the presence of a radical initiator, but environmental considerations demand that alternative reagents be developed-ones that can serve as a synthetic equivalent to the hydrogen atom. We have revisited [CpV(CO)3H]-, a known replacement for Bu3SnH, and found that it can be used catalytically under H2 in the presence of a base. We have carried out tin-free catalytic radical cyclizations of alkyl iodide substrates. The reactions are atom-efficient, and the conditions are mild, with broad tolerance for functional groups. We have, for example, achieved the first 5-exo radical cyclization involving attack onto a vinyl chloride. We suggest that the radicals are generated by an initial electron transfer.