Reduction Of Functional Groups Research Articles

Chemoselective Hydrosilylation of the α,β-Site Double Bond in α,β- and α,β,γ,δ-Unsaturated Ketones Catalyzed by Macrosteric Borane Promoted by Hexafluoro-2-propanol.

The B(C6F5)3-catalyzed chemoselective hydrosilylation of α,β- and α,β,γ,δ-unsaturated ketones into the corresponding non-symmetric ketones in mild reaction conditions is developed. Nearly 55 substrates including those bearing reducible functional groups such as alkynyl, alkenyl, cyano, and aromatic heterocycles are chemoselectively hydrosilylated in good to excellent yields. Isotope-labeling studies revealed that hexafluoro-2-propanol also served as a hydrogen source in the process.

Palladium supported on metal–organic framework as a catalyst for the hydrogenation of nitroarenes under mild conditions

Sustainable development demands an environmentally friendly and efficient method for the hydrogenation of organic molecules, including the hydrogenation of functionalized nitroarenes. In this study, a highly active and selective metal–organic framework‐supported palladium catalyst was prepared for the catalytic hydrogenation of nitroarenes. High selectivity (>99%) and excellent yield (98%) of aniline were realized after 2 hours in ethanol under hydrogen (1 atm) at room temperature. The reductions were successfully carried out in the presence of a wide range of other reducible functional groups. More importantly, the catalyst was very stable without the loss of its catalytic activity after five cycles.

Selective Acceptorless Dehydrogenation of Primary Amines to Imines by Core-Shell Cobalt Nanoparticles.

Core–shell nanocatalysts are attractive due to their versatility and stability. Here, we describe cobalt nanoparticles encapsulated within graphitic shells prepared via the pyrolysis of a cationic poly‐ionic liquid (PIL) with a cobalt(II) chloride anion. The resulting material has a core–shell structure that displays excellent activity and selectivity in the self‐dehydrogenation and hetero‐dehydrogenation of primary amines to their corresponding imines. Furthermore, the catalyst exhibits excellent activity in the synthesis of secondary imines from substrates with various reducible functional groups (C=C, C≡C and C≡N) and amino acid derivatives.

Selective Acceptorless Dehydrogenation of Primary Amines to Imines by Core–Shell Cobalt Nanoparticles

AbstractCore–shell nanocatalysts are attractive due to their versatility and stability. Here, we describe cobalt nanoparticles encapsulated within graphitic shells prepared via the pyrolysis of a cationic poly‐ionic liquid (PIL) with a cobalt(II) chloride anion. The resulting material has a core–shell structure that displays excellent activity and selectivity in the self‐dehydrogenation and hetero‐dehydrogenation of primary amines to their corresponding imines. Furthermore, the catalyst exhibits excellent activity in the synthesis of secondary imines from substrates with various reducible functional groups (C=C, C≡C and C≡N) and amino acid derivatives.

Monoterpenoid Oximes Hydrogenation Over Platinum Catalysts

Platinum nanoparticles supported on MgO, Al2O3, ZrO2, TiO2 were utilized for monoterpenoid oximes hydrogenation. Monocyclic menthone and carvone oximes synthesized from bio-derived monoterpenoids with a different structure were used to explore the effect of substrate structure on the reaction regularities. The oximes hydrogenation was carried out under hydrogen atmosphere at 100 °C using methanol as a solvent. Platinum catalysts were prepared by the impregnation methods. The catalysts were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, nitrogen physisorption. Hydrogenation of carvone oxime containing a conjugated oxime group and several reducible functional groups resulted in both hydrogenation and deoximation giving 5-isopropyl-2-methylcyclohexanamine and carvomenthone over Pt/Al2O3 and Pt/ZrO2 catalysts. Menthone oxime hydrogenation over Pt/Al2O3 catalysts with an average particle size of 0.8 nm provided the desired menthylamine formation with the selectivity of 90% at complete oxime conversion. Platinum catalysts based on MgO, ZrO2 and TiO2 enhanced menthone oxime deoximation to menthone.

Cis‐Selective Transfer Semihydrogenation of Alkynes by Merging Visible‐Light Catalysis with Cobalt Catalysis

AbstractHerein, the first example of visible‐light‐driven, cobalt‐catalyzed transfer semihydrogenation of alkynes to alkenes is reported. It is carried out by using Ir[dF(CF3)ppy]2(dtbbpy)]PF6 as photosensitizer, CoBr2/n‐Bu3P as proton‐reducing catalyst, and i‐Pr2NEt/AcOH as the hydrogen source. Under the established catalytic system, the semihydrogenation proceeds with Z as the major selectivity and with inhibition of over‐reduction. Under mild reaction conditions, both internal and terminal alkynes, as well as reducible functional groups such as halogen, cyano, and ester, are tolerated. Preliminary mechanistic studies revealed the dual role of the photosensitizer in initiating the reaction via a single‐electron transfer process and controlling the stereoselectivity via an energy transfer process.magnified image

Boric acid catalyzed chemoselective reduction of quinolines.

Boric acid promoted transfer hydrogenation of substituted quinolines to synthetically versatile 1,2,3,4-tetrahydroquinolines (1,2,3,4-THQs) was described under mild reaction conditions using a Hantzsch ester as a mild organic hydrogen source. This methodology is practical and efficient, where isolated yields are excellent and reducible functional groups are well tolerated in the N-heteroarene moiety. The reaction parameters and tentative mechanistic pathways are demonstrated by various control experiments and NMR studies. The present work can also be scaled up to obtain gram quantities and the utility of the developed process is illustrated by the transformation of 1,2,3,4-THQs into a series of biologically important molecules including the antiarrhythmic drug nicainoprol.

Photocatalytic hydrogenation of nitrobenzenes to anilines over noble metal-free TiO2 utilizing methylamine as a hydrogen donor

Since reduction and oxidation reactions simultaneously occur over a photocatalyst, a hole scavenger (a hydrogen source in the field of organic chemistry) is required to obtain a reduced product. The use of organic waste as the hole scavenger is environmentally favorable because the organic waste is degraded during the photocatalytic reduction. In this study, methylamine was used as the hole scavenger for photocatalytic reduction (hydrogenation) of nitrobenzenes over a titanium(IV) oxide photocatalyst. Corresponding anilines were obtained almost quantitatively at room temperature and under atmospheric pressure without the use of a precious metal and hydrogen gas. Nitrobenzenes having reducible functional groups such as chloro and acetyl groups were chemoselectively hydrogenated to corresponding anilines with the groups being preserved. Various parameters affecting the efficiency and the rate of the photocatalytic reaction are discussed on the basis of results of the physicochemical analyses.

Donor-Acceptor π-Conjugated Enamines: Functional Group-Compatible Synthesis from Amides and Their Photoabsorption and Photoluminescence Properties.

High functional group compatibility of iridium-catalyzed synthesis of enamines from amides and 1,1,3,3-tetramethyldisiloxane (TMDS) realized facile access of a series of donor (D)-π-acceptor (A)-conjugated enamines, in which enamine behaves as a donor functional group. The amide precursors containing reducible functional groups, such as halogen, carbonyl, and nitro groups, underwent reaction with TMDS to give the corresponding enamines in high yields. In most cases, chemoselective hydrosilane reduction of the amide group occurred while other reducible groups remained intact. Absorption and emission properties including solvatochromic behavior for the resulting D-π-A-conjugated enamines were determined using UV-visible and fluorescent spectra, which provided an understanding of the donor properties of the CH═CHNPh2 group and photofunctional properties of the D-π-A conjugated enamines as a fluorescent dye. Maximum absorption wavelength (λabs) of p-ZC6H4CH═CHNPh2 was predictable from λabs of p-ZC6H4NPh2, which was supported by density functional theory calculations. Some of the D-π-A-conjugated enamines showed fluorescence with moderate fluorescence quantum yields (Φfl). Of interest are unusually emissive π-conjugated enamines containing a nitro group, which generally behaves as strong quenchers of fluorescence. The additive effect of B(C6F5)3 resulted in significant red shifts of λabs and λfl. In some cases, high Φfl was observed in the solution state.

Selective Hydrogenation by Carbocatalyst: The Role of Radicals.

The selective hydrogenation of the nitro moiety is a difficult task in the presence of other reducible functional groups such as alkenes or alkynes. We show that the carbon-based (metal-free) catalyst can be used to selectively reduce substituted nitro groups using H2 as a reducing agent, providing a great potential to replace noble-metal catalysts and contributing to simple and greener strategies for organic synthesis.

Z-Selective Hydrofunctionalization of Dienes

Olefins play a fundamental role in synthetic organic chemistry because they are useful building blocks that create molecules. However, geometry control (E- vs Z-) in olefin synthesis is of utmost importance because the olefin geometry has a tremendous impact on its physical, chemical and biological properties. Additionally, Z-olefins are less stable compared to their E-olefin counterparts; due to this difference, general methods to make olefins results in more cases of E-olefins production with relatively fewer Z-olefins caused by its instability. It has been reported that Z-olefins can be synthesized from dienes through a rhodium-catalyzed formate mediated transformation, with tolerance to several reducible functional groups. With this successful method in hand, the focus is to make functionalized Z-alkenes while still maintaining tolerance to reducible functional groups under mild reaction conditions.
 Thus, this project presents the production of Z-olefins through rhodium-catalyzed hydrofunctionalization using the starting materials, dienes and aldehydes. This method requires an inert atmosphere and the reaction progress can be monitored by Nuclear Magnetic Resonance (NMR) using an internal standard to quantify the amount of product formed. In this process, it was observed that the starting material was consumed until more than 95% conversion. In addition, the possibility of using different dienes, such as diene esters and phenyl dienes, as well as different aldehydes could further broaden the scope of this method. The usefulness of this process can be applied to the production of complex molecules. For example, in the synthesis of a glucagon receptor antagonist, which is a drug that is used in the treatment of diabetes. Currently, there is a limited number of methods used to create Z-olefins; however, this proven procedure can be further applied in other hydrofunctionalization

Isolated Iron Single-Atomic Site-Catalyzed Chemoselective Transfer Hydrogenation of Nitroarenes to Arylamines.

Selective hydrogenation of nitroarenes to arylamines is a great challenge because of the complicated mechanism and competitive hydrogenation of reducible functional groups. Isolated single-atomic site catalysts, benefitting from their uniform and well-defined catalytic sites, are promising to achieve high activity and selectivity. Herein, we prepared an isolated iron single-atomic catalyst supported on ordered mesoporous nitrogen-doped carbon (Fe1/N-C). The as-prepared Fe1/N-C showed excellent activity and tolerance for functional groups in the transfer hydrogenation of nitroarenes over hydrazine hydrate. Density functional theory calculations revealed that the single atomically dispersed, partially positively charged Fe atoms and the lowered energy barrier collectively contribute to the superior hydrogenation performances for nitroarenes.

A versatile route to fabricate single atom catalysts with high chemoselectivity and regioselectivity in hydrogenation

Preparation of single atom catalysts (SACs) is of broad interest to materials scientists and chemists but remains a formidable challenge. Herein, we develop an efficient approach to synthesize SACs via a precursor-dilution strategy, in which metalloporphyrin (MTPP) with target metals are co-polymerized with diluents (tetraphenylporphyrin, TPP), followed by pyrolysis to N-doped porous carbon supported SACs (M1/N-C). Twenty-four different SACs, including noble metals and non-noble metals, are successfully prepared. In addition, the synthesis of a series of catalysts with different surface atom densities, bi-metallic sites, and metal aggregation states are achieved. This approach shows remarkable adjustability and generality, providing sufficient freedom to design catalysts at atomic-scale and explore the unique catalytic properties of SACs. As an example, we show that the prepared Pt1/N-C exhibits superior chemoselectivity and regioselectivity in hydrogenation. It only converts terminal alkynes to alkenes while keeping other reducible functional groups such as alkenyl, nitro group, and even internal alkyne intact.

Iridium-Catalyzed Alkene-Selective Transfer Hydrogenation with 1,4-Dioxane as Hydrogen Donor.

The iridium-catalyzed transfer hydrogenation of alkenes using 1,4-dioxane as a hydrogen donor is described. The use of 1,2-bis(dicyclohexylphosphino)ethane (DCyPE), featuring bulky and highly electron-donating properties, led to high catalytic activity. A polystyrene-cross-linking bisphosphine PS-DPPBz produced a reusable heterogeneous catalyst. These homogeneous and heterogeneous protocols achieved chemoselective transfer hydrogenation of alkenes over other potentially reducible functional groups such as carbonyl, nitro, cyano, and imino groups in the same molecule.

Visible light-induced diastereoselective semihydrogenation of alkynes to cis-alkenes over an organically modified titanium(IV) oxide photocatalyst having a metal co-catalyst

Hydrogen (H2)-free and poison (lead and quinoline)-free semihydrogenation of alkynes to cis-alkenes under gentle conditions is one of the challenges to be solved. In this study, a titanium(IV) oxide photocatalyst having two functions (visible light responsiveness and semihydrogenation activity) was prepared by modification with 2,3-dihydroxynaphthalene (DHN) and a copper (Cu) co-catalyst, respectively. The photocatalyst (DHN/TiO2-Cu) showed high performance for diastereoselective semihydrogenation of alkynes to cis-alkenes in water-acetonitrile solution under visible light irradiation without the use of H2 and poisons. Alkynes having reducible functional groups were converted to the corresponding alkenes with the functional groups being preserved. The addition of water to acetonitrile changed the amount of alkynes adsorbed on the photocatalyst, which was a decisive factor determining the rate of hydrogenation. A relatively large apparent activation energy, 27 kJ mol−1, was obtained by a kinetic study, indicating that the rate-determining step of this reaction was not an electron production process but a thermal catalytic semihydrogenation process over the Cu co-catalyst. Semihydrogenation and hydrogen evolution occurred competitively on Cu metals and the former became predominant at slightly elevated temperatures, which is discussed on the basis of the kinetic parameters of two reactions.

Low Temperature Chemoselective Hydrogenation of Aldehydes over a Magnetic Pd Catalyst

Chemoselective hydrogenation of aldehydes with heterogeneous catalysts under mild conditions is of great importance but remains a major challenge. Herein, an efficient strategy was developed for low temperature chemoselective hydrogenation of aldehydes with broad substrate scope over a magnetic material supported palladium catalyst (γ-Fe2O3@HAP-Pd). Aldehydes bearing various reducible functional groups readily underwent hydrogenation to give the corresponding primary alcohols with moderate to excellent yield at room temperature in aqueous solutions. The Hammett equation revealed that the hydrogenation of aromatic aldehydes proceeded via an anionic intermediate. Additionally, when the temperature increased to 70 °C, toluene was obtained by the deoxygenation of benzaldehyde in excellent yield. Furthermore, the γ-Fe2O3@HAP-Pd could be recycled up to six times without loss of activity and metal leaching.

Triphenylphosphine Oxide-Catalyzed Selective α,β-Reduction of Conjugated Polyunsaturated Ketones

The scope of the triphenylphosphine oxide-catalyzed reduction of conjugated polyunsaturated ketones using trichlorosilane as the reducing reagent has been examined. In all cases studied, the α,β-C=C double bond was selectively reduced to a C–C single bond while all other reducible functional groups remained unchanged. This reaction was applied to a large variety of conjugated dienones, a trienone, and a tetraenone. Additionally, a tandem one-pot Wittig/conjugate-reduction reaction sequence was developed to produce γ,δ-unsaturated ketones directly from simple building blocks. In these reactions the byproduct of the Wittig reaction served as the catalyst for the reduction reaction. This strategy was then used in the synthesis of naturally occurring moth pheromones to demonstrate its utility in the context of natural-product synthesis.

Selective reduction of nitro group using CuNi bimetallic nanoparticles

Bimetallic nanoparticles have shown a lot of promise as catalysts in the recent years. This piece of work highlights the use of nanocatalyst for the chemoselective reduction of nitro group in presence of other reducible functional groups. CuNi bimetallic nanoparticles of varying composition (1:1, 1:2) have been synthesized by polyol method using metal sulphates with hydrazine hydrate as reducing agent. The characterization of synthesised nanoparticles was done by XRD and FESEM with EDX, which confirm FCC structure and composition of synthesised bimetallic nanoparticles. Particle size varies with composition i.e. 23 nm for (1:1) and 20 nm for (1:2). The selective reduction of p-nitrophenol, p-nitrobenzaldehyde and 2,4 dinitrophenol was performed in water using both single Ni nanoparticles and bimetallic CuNi nanoparticles (1:1, 1:2) with hydrazine hydrate as reducing agent at room temperature. The various amino products obtained has various industrial applications. UV–visible spectroscopy was used to monitor the progress of the reaction. The bimetallic nanoparticles facilitated the rate of reaction much as compared to single Ni nanoparticles.

Metal-free chemoselective reduction of nitroaromatics to anilines via hydrogen transfer strategy

A novel protocol for chemoselective reduction of aromatic nitro compounds to aromatic amines has been established. The metal-free reduction goes through a hydrogen transfer process. Various easily reducible functional groups can be well tolerated under the optimized reaction conditions.

Trans-Hydrogenation, gem-Hydrogenation, and trans-Hydrometalation of Alkynes: An Interim Report on an Unorthodox Reactivity Paradigm.

cis-Delivery of H2 to the π-system of an unsaturated substrate is the canonical course of metal catalyzed hydrogenation reactions. The semireduction of internal alkynes with the aid of [Cp*Ru]-based catalysts violates this rule and affords E-alkenes by direct trans-hydrogenation. A pathway involving σ-complexes and metallacyclopropenes accounts for this unconventional outcome. Connected to this process is an even more striking reactivity mode, in which both H atoms of H2 are delivered to one and the same C atom. Such gem-hydrogenation of stable carbogenic compunds is a fundamentally new transformation that leads to the formation of discrete metal carbene complexes. Computational studies suggest that the trans- and the gem-pathway have similar barriers, but polar substituents in the vicinity of the reacting triple bond provide opportunities for imposing selectivity and control. Moreover, it is shown that catalytic trans-hydrogenation is by no means a singularity: rather, the underlying principle is also manifest in trans-hydroboration, trans-hydrosilylation, trans-hydrogermylation, and trans-hydrostannation, which are equally paradigm-changing processes. These reactions are robust and distinguished by excellent compatibility with many (reducible) functional groups and have already stood the test of natural product synthesis in a number of demanding cases.