OH Bond Activation Research Articles

Electroreductive Deoxygenative C−H and C−C Bond Formation from Non‐Derivatized Alcohols Fueled by Anodic Borohydride Oxidation

AbstractAlcohols are one of the most common organic compound classes among natural and synthetic products. Thus, methods for direct removal of C−OH groups without the need for wasteful pre‐functionalization are of great synthetic interest to unlock the full synthetic potential of the compound class. Herein, electroreductive C−OH bond activation and subsequent deoxygenative C−H and C−C bond formation of benzylic and propargylic alcohols are demonstrated along with mechanistic insights. Experimental and theoretical studies indicate that the reductive C−OH bond cleavage furnishes an open shell intermediate that undergoes a radical‐polar crossover to the corresponding carbanion that subsequently undergoes protonation to furnish alkane products. Furthermore, we demonstrate that the carbanion can be trapped with CO2 to form arylacetic acids. The cathodic transformations are efficiently balanced by the anodic oxidation of sub‐stoichiometric borohydride additives, a strategy that serves as a highly attractive alternative to the use of sacrificial metal anodes.

Recent DFT Calculations on the Mechanism of Transition‐Metal‐Catalyzed C−O Activation of Alcohols

AbstractIn recent decades, density functional theory (DFT) calculations have been extensively employed to investigate the mechanism of transition metal‐catalyzed C−O activation reactions. These studies provide valuable insights into the structure‐reactivity/selectivity correlations via theoretical simulations and energy profile analysis. Alcohols, a category of molecules that are readily available from biomass and are low cost, represent one of the most extensively studied compounds in organic synthesis. In this review, we provide a brief overview of the DFT studies conducted since 2017 on the C−OH bond activation of alcohols catalyzed by transition metals. Specifically, the activation can be mainly divided into four categories: free radical cleavage, metal insertion, nucleophilic attack, and β‐OH elimination. We will also cover some promising strategies and give a perspective regarding future research directions in this field.

Calix[4]pyrrolato gallate: square planar-coordinated gallium(iii) and its metal-ligand cooperative reactivity with CO2 and alcohols.

Forcing a priori tetracoordinate atoms into planar configuration represents a promising concept for enhanced reactivity of p-block element-based systems. Herein, the synthesis, characterization, and reactivity of calix[4]pyrrolato gallates, constituting square planar-coordinated gallium(iii) atoms, are reported. Unusual structural constraint-induced Lewis acidity against neutral and anionic donors is disclosed by experiment and rationalized by computations. An energetically balanced dearomatization/rearomatization of a pyrrole unit enables fully reversible metal-ligand cooperative capture of CO2. While alcohols are found unreactive against the gallates, a rapid and selective OH-bond activation can be triggered upon protonation of the ligand. Secondary ligand-sphere modification adds a new avenue to structurally-constrained complexes that unites functional group tolerance with unconventional reactivity.

Cu-Catalyzed Direct Amination of Cyclic Amides via C-OH Bond Activation Using DMF.

Herein, we describe a Cu-catalyzed approach to directly accessing aromatic heterocyclic amines from cyclic amides. The most-reported methods for cyclic amide conversions to aromatic heterocyclic amines use an activating group, such as a halogen atom or a trifluoromethane sulfonyl group. However, subsequent elimination of activating groups during the amination process results in significant waste. This copper-catalyzed direct amination of cyclic amides in DMF forms aromatic heterocyclic amines with environmental friendliness and readily available reagents. A plausible radical mechanism has been proposed for the reaction. Meanwhile, the coordinating effect of the N1 atom is key to the success of this reaction, which provides assistance to the copper ions for the activation and amination of C-O bonds.

Reversible OH-bond activation and amphoterism by metal-ligand cooperativity of calix[4]pyrrolato aluminate.

Most p-block metal amides irreversibly react with metal alkoxides when subjected to alcohols, making reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(iii) center with metal–ligand cooperativity leverages unconventional reactivity toward protic substrates. Calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, which can be fully reversed under reduced pressure. The products exhibit a new form of metal–ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al–O bond by a dative Al–N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can invert thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.

Infrared analysis of methanol adsorption on mixed oxides derived from Mg/Al hydrotalcite catalysts for transesterification reactions

Mixed oxides derived from hydrotalcites by thermal decomposition are promising catalysts for transesterification reactions. To gain insight into its action mechanism, methanol OH bond activation was analyzed over MgAl mixed oxides using in-situ DRIFT spectroscopy to follow adsorbed species. After methanol adsorption, non-dissociative and dissociative species were identified. Dissociative adsorption resulted in two types of methoxy groups, type I or monodentate methoxy species and type II or bridging methoxy species. Catalytic tests of methanolysis of triacetin have shown a trend where, the higher the amount of adsorbed monodentate methoxy species the higher the catalytic activity of transesterification reaction.

Mechanistic insights on C[sbnd]O and C[sbnd]C bond activation and hydrogen insertion during acetic acid hydrogenation catalyzed by ruthenium clusters in aqueous medium

Catalytic pathways for acetic acid (CH3COOH) and hydrogen (H2) reactions on dispersed Ru clusters in the aqueous medium and the associated kinetic requirements for CO and CC bond cleavages and hydrogen insertions are established from rate and isotopic assessments. CH3COOH reacts with H2 in steps that either retain its carbon backbone and lead to ethanol, ethyl acetate, and ethane (47–95%, 1–23%, and 2–17% carbon selectivities, respectively) or break its CC bond and form methane (1–43% carbon selectivities) at moderate temperatures (413–523K) and H2 pressures (10–60bar, 298K). Initial CH3COOH activation is the kinetically-relevant step, during which CH3C(O)OH bond cleaves on a metal site pair at Ru cluster surfaces nearly saturated with adsorbed hydroxyl (OH∗) and acetate (CH3COO∗) intermediates, forming an adsorbed acetyl (CH3CO∗) and hydroxyl (OH∗) species. Acetic acid turnover rates increase proportionally with both H2 (10–60bar) and CH3COOH concentrations at low CH3COOH concentrations (<0.83M), but decrease from first to zero order as the CH3COOH concentration and the CH3COO∗ coverages increase and the vacant Ru sites concomitantly decrease. Beyond the initial CH3C(O)OH bond activation, sequential H-insertions on the surface acetyl species (CH3CO∗) lead to C2 products and their derivative (ethanol, ethane, and ethyl acetate) while the competitive CC bond cleavage of CH3CO∗ causes the eventual methane formation. The instantaneous carbon selectivities toward C2 species (ethanol, ethane, and ethyl acetate) increase linearly with the concentration of proton-type Hδ+ (derived from carboxylic acid dissociation) and chemisorbed H∗. The selectivities toward C2 products decrease with increasing temperature, because of higher observed barriers for CC bond cleavage than H-insertion. This study offers an interpretation of mechanism and energetics and provides kinetic evidence of carboxylic acid assisted proton-type hydrogen (Hδ+) shuffling during H-insertion steps in the aqueous phase, unlike those in the vapor phase, during the hydrogenation of acetic acid on Ru clusters.

A direct facile and effective synthesis of various 1,1-heterodiaryl alkenes through Pd catalyzed cross coupling reaction using N-tosylhydrazones via C–OH bond activation

At this event for the first time, the direct arylation with the generation of a facile and effective process for the synthesis of alkenes has been established through in situ C–OH activation afterward Pd catalyzed C–C bond formation of heteroarenols with N-tosylhydrazones. The noticeable features of these reactions are (1) No stoichiometric organometallic reagents desired, (2) No necessity of halides or pseudohalides (3) Environmentally benign, low toxic, and step economical reaction, (4) Easy to handle, mild conditions required, and give moderate to excellent yield.

Synthesis of 4-(Dimethylamino)quinazoline via Direct Amination of Quinazolin-4(3H)-one Using N,N-Dimethylformamide as a Nitrogen Source at Room Temperature

An efficient direct amination of quinazolin-4(3<i>H</i>)-ones using <i>N</i>,<i>N</i>-dimethylformamide as a nitrogen source is described that affords the corresponding 4-(dimethylamino)quinazolines in high yields. This transformation proceeds through efficient 4-toluenesulfonyl chloride mediated C–OH bond activation at room temperature.

ChemInform Abstract: A Tandem and Tunable Pd Catalyzed C—N Coupling of Heteroarenols with Ureas via C—OH Bond Activation.

At this circumstance for the first time, a facile and convenient method for heteroaryl ureas has been developed via a two-step process involving in situ C–OH activation followed by palladium catalyzed C–N coupling of heteroarenols with ureas, which show excellent functional group tolerance and give out rapid coupling in good to excellent yield.

ChemInform Abstract: A Convenient and Efficient C—OH Bond Activation, PdCl2(PPh3)2 Catalyzed, C—C Bond Formation of Tautomerizable Quinolinones with the Aid of BOP Reagent and Boronic Acids

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A tandem and tunable Pd catalyzed C–N coupling of heteroarenols with ureas via C–OH bond activation

At this circumstance for the first time, a facile and convenient method for heteroaryl ureas has been developed via a two-step process involving in situ C–OH activation followed by palladium catalyzed C–N coupling of heteroarenols with ureas, which show excellent functional group tolerance and give out rapid coupling in good to excellent yield.

Multimetallic Iridium‐Tin (Ir‐Sn3) Catalyst in N‐Acyliminium Ion Chemistry: Synthesis of 3‐Substituted Isoindolinones via Intra‐ and Intermolecular Amidoalkylation Reaction

AbstractThe multimetallic iridium‐tritin (Ir‐Sn3) complex [Cp*Ir(SnCl3)2{SnCl2(H2O)2}] (1) proved to be a highly effective catalyst towards COH bond activation of γ‐hydroxylactams, leading to a nucleophilic substitution reaction known as the α‐amidoalkylation reaction. Catalyst 1 can be easily synthesized from the reaction of (pentamethylcyclocyclopentadienyl)iridium dichloride dimer {[Cp*IrCl2]2} and tin(II) dichloride (SnCl2). In terms of catalyst loading, reaction conditions and yields of the product formed, 1 is found to be superior compared to classical Lewis acid catalysts. Different carbon (arenes, heteroarenes, allyltrimethylsilane, 1,3‐dicarbonyls) and heteroatom (alcohols, thiols, amides and sulfonamides) nucleophiles have been successfully employed in the intramolecular and intermolecular alkylations, as well as in heterocyclization reactions. In the majority of cases good to excellent yields of 3‐substituted isoindolinones and 5‐substituted pyrrolidin‐2‐ones have been obtained. Besides, the reactions are also atom economical and salt free. It is proposed that the multimetallic Ir‐Sn3 catalyst behaves as a mild and selective Lewis acid to activate the γ‐hydroxylactam towards the formation of the N‐acyliminium ion; the latter being trapped by potent nucleophiles leading to the desired products.magnified image

A Metal‐Free, Carbon‐Based Catalytic System for the Oxidation of Lignin Model Compounds and Lignin

AbstractNitrogen‐containing graphene material (LCN) has been identified as an effective catalyst for the oxidation of β‐O‐4 and α‐O‐4 types of lignin model compounds in the presence of tert‐butyl hydroperoxide, to provide aromatic aldehydes, acids and other organic chemicals in high yield. The transformations of five lignin model compounds over LCN were investigated systematically. Instrumentation analysis, kinetic study and radical trapping experiments highlight the mechanistic features of the reaction, including: 1) the reaction pathway starts by benzylic CH or COH bond activation, followed by CαCβ or CαO bond cleavage, and finally further oxidation of intermediate aromatics; and 2) the reaction follows a free‐radical mechanism with all the key steps involving radical species. In addition, the LCN proved to be a highly stable catalyst; no significant activity decrease was observed for four consecutive runs, and X‐ray photoelectron spectroscopy analysis indicates negligible decrease in the content of the active nitrogen species in the catalyst. Notably, this new catalytic system can be extended to the oxidative depolymerisation of real lignin, to produce a significant portion of liquefied, low‐molecular‐mass products.

A convenient and efficient C–OH bond activation, PdCl2(PPh3)2catalyzed, C–C bond formation of tautomerizable quinolinones with the aid of BOP reagent and boronic acids

C–C bond formation of tautomerizable quinolinones. C–OH bond activation using BOP reagent and boronic acids.

ChemInform Abstract: Rapid Access to 4‐Substituted‐pyrones and 2(5H)‐Furanones via a Palladium‐Catalyzed C—OH Bond Activation.

An efficient palladium-catalyzed cross-coupling reaction of 4-hydroxy-pyrone or 4-hydroxy-2(5H)-furanone with arylboronic acid is described, which affords the 4-substituted-pyrones and 2(5H)-furanones in good yields. This transformation proceeds through a C–OH bond activation under mild conditions.

Structural phase transitions induced by molecular motions within an (anilinium)(l-tartrate) ionic molecular crystal

Ionic molecular crystals of (anilinium+)(L-tartrate−) were prepared in C2H5OH–H2O via Bronsted acid–base reaction between aniline and L-tartaric acid. Strong intermolecular O–H⋯O− hydrogen-bonding interactions between the terminal –COOH and –COO− groups of the L-tartrate− monoanion (L-HTart−) are essential in forming the one-dimensional hydrogen-bonding (L-HTart−)∞ chains. Furthermore, N–H+⋯O− hydrogen-bonding interactions between L-HTart− and the ammonium moiety of anilinium (Ani+) form a π-stacking arrangement of the Ani+ cations that are orthogonal to the (L-HTart−)∞ chains. DSC analysis revealed a two-step behavior with reversible phase transitions at ∼220 and ∼270 K, in which three types of crystal phases (I, T < 220 K; II, 220 < T < 270 K; III, 270 K < T) were identified. Phase I possesses the same acentric space group (P1), while its unit cell volume is half of that of phases II and III, in which the π-stacking periodicity of the Ani+ cations of phases II and III are double that of phase I. Although the conformation of the hydroxy (–OH) groups of L-HTart− of phase II (T = 250 K) was identical to that of phase I (T = 100 K), the rotation of the C–OH bond (activation energy of ∼70 kJ mol−1) was observed for phase III. The thermal fluctuation of the polar –OH group in phase III was dominated by a single minimum-type potential energy curve, as evaluated from the calculations. The temperature- and frequency-dependent dielectric constants suggested a low-frequency thermal fluctuation at temperatures above ∼250 K.

Rapid access to 4-substituted-pyrones and 2(5 H)-furanones via a palladium-catalyzed C–OH bond activation

An efficient palladium-catalyzed cross-coupling reaction of 4-hydroxy-pyrone or 4-hydroxy-2(5 H)-furanone with arylboronic acid is described, which affords the 4-substituted-pyrones and 2(5 H)-furanones in good yields. This transformation proceeds through a C–OH bond activation under mild conditions.

Matrix Isolation Infrared Spectroscopic and Theoretical Study of the Reactions of Tantalum Oxide Molecules with Methanol

The reactions of tantalum monoxide (TaO) and dioxide (TaO(2)) molecules with methanol in solid neon were investigated by infrared absorption spectroscopy. The ground-state TaO molecule reacted with CH(3)OH in forming the CH(3)OTa(O)H molecule via the hydroxylic hydrogen atom transfer from methanol to the metal center spontaneously on annealing. The observation of the spontaneous reaction is consistent with theoretical predictions that the OH bond activation process is both thermodynamically exothermic and kinetically facile. In contrast, the TaO(2) molecule reacted with CH(3)OH to give primarily the TaO(2)(CH(3)OH) complex, which further rearranged to the CH(3)OTa(O)OH isomer via the hydroxylic hydrogen atom transfer from methanol to one of oxygen atom of metal dioxide upon visible light excitation.

ChemInform Abstract: Palladium‐Catalyzed Direct Arylation of 4‐Hydroxycoumarins with Arylboronic Acids via C—OH Bond Activation.

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