Catalyst-Free Decarboxylation of Carboxylic Acids and Deoxygenation of Alcohols by Electro-Induced Radical Formation

Electro-induced reduction of redox active esters and N-phthalimidoyl oxalates derived from naturally abundant carboxylic acids and alcohols provides a sustainable and inexpensive approach to radical formation via undivided electrochemical cells. The resulting radicals are trapped by an electron-poor olefin or hydrogen atom source to furnish the Giese reaction or reductive decarboxylation products, respectively. A broad range of carboxylic acid (1°, 2°, and 3°) and alcohol (2° and 3°) derivatives are applicable in this catalyst-free reaction, which tolerated a diverse range of functional groups. This method features simple operation, is a sustainable platform, and has broad application.     

Notiz über einen Zugang zu β, γ-ungesättigten Carbonsäurederivaten mit Hilfe der Amidacetal-Claisenumlagerung. Über synthetische methoden, 17. Mitteilung

Note on a preparation of β, γ-unsaturated carboxylic acid derivatives using the amide acetal Claisen rearrangement 3-(Trimethylsilyl)allyl alcohols smoothly undergo the amide acetal Claisen rearrangement furnishing allyl silanes. Subsequent protolysis with HF at ?20° provides a convenient, stereoselective method for the preparation of β, γ-unsaturated carboxylic acid derivatives. Three model examples illustrate the procedure.     

Enantio‐ and Site‐Selective α‐Fluorination of N‐Acyl 3,5‐Dimethylpyrazoles Catalyzed by Chiral π–CuII Complexes

Catalytic enantioselective α‐fluorination reactions of carbonyl compounds are among the most powerful and efficient synthetic methods for constructing optically active α‐fluorinated carbonyl compounds. Nevertheless, α‐fluorination of α‐nonbranched carboxylic acid derivatives is still a big challenge because of relatively high pK_a values of their α‐hydrogen atoms and difficulty of subsequent synthetic transformation without epimerization. Herein we show that chiral copper(II) complexes of 3‐(2‐naphthyl)‐l ‐alanine‐derived amides are highly effective catalysts for the enantio‐ and site‐selective α‐fluorination of N‐(α‐arylacetyl) and N‐(α‐alkylacetyl) 3,5‐dimethylpyrazoles. The substrate scope of the transformation is very broad (25 examples including a quaternary α‐fluorinated α‐amino acid derivative). α‐Fluorinated products were converted into the corresponding esters, secondary amides, tertiary amides, ketones, and alcohols with almost no epimerization in high yield.     

Pd/Light‐Accelerated Atom‐Transfer Carbonylation of Alkyl Iodides: Applications in Multicomponent Coupling Processes Leading to Functionalized Carboxylic Acid Derivatives

The atom‐transfer carbonylation reaction of various alkyl iodides thereby leading to carboxylic acid esters was effectively accelerated by the addition of transition‐metal catalysts under photoirradiation conditions. By using a combined Pd/hν reaction system, vicinal C‐functionalization of alkenes was attained in which α‐substituted iodoalkanes, alkenes, carbon monoxide, and alcohols were coupled to give functionalized esters. When alkenyl alcohols were used as acceptor alkenes, three‐component coupling reactions, which were accompanied by intramolecular esterification, proceeded to give lactones. Pd‐dimer complex [Pd₂(CNMe)₆][PF₆]₂, which is known to undergo homolysis under photoirradiation conditions, worked quite well as a catalyst in these three‐ or four‐component coupling reactions. In this metal/radical hybrid system, both Pd radicals and acyl radicals are key players and a stereochemical study confirmed the carbonylation step proceeded through a radical carbonylation mechanism.     

Enantio- and Site-Selective α-Fluorination of N-Acyl 3,5-Dimethylpyrazoles Catalyzed by Chiral π–CuII Complexes

Catalytic enantioselective α-fluorination reactions of carbonyl compounds are among the most powerful and efficient synthetic methods for constructing optically active α-fluorinated carbonyl compounds. Nevertheless, α-fluorination of α-nonbranched carboxylic acid derivatives is still a big challenge because of relatively high pK_a values of their α-hydrogen atoms and difficulty of subsequent synthetic transformation without epimerization. Herein we show that chiral copper(II) complexes of 3-(2-naphthyl)-l -alanine-derived amides are highly effective catalysts for the enantio- and site-selective α-fluorination of N-(α-arylacetyl) and N-(α-alkylacetyl) 3,5-dimethylpyrazoles. The substrate scope of the transformation is very broad (25 examples including a quaternary α-fluorinated α-amino acid derivative). α-Fluorinated products were converted into the corresponding esters, secondary amides, tertiary amides, ketones, and alcohols with almost no epimerization in high yield.     

Substrate-Mediator Duality of 1,4-Dicyanobenzene in Electrochemical C(sp2)−C(sp3) Bond Formation with Alkyl Bromides

Electrochemical approaches to form C(sp²)−C(sp³) bonds have focused on coupling C(sp³) electrophiles that form stabilized carbon-centered radicals upon reduction or oxidation. Whereas alkyl bromides are desirable C(sp³) coupling partners owing to their availability and cost-effectiveness, their tendency to undergo radical-radical homocoupling makes them challenging substrates for electroreductive cross-coupling. Herein, we disclose a metal-free regioselective cross-coupling of 1,4-dicyanobenzene, a useful precursor to aromatic nitriles, and alkyl bromides. Alkyl bromide reduction is mediated directly by 1,4-dicyanobenzene radical anions, leading to negligible homocoupling and high cross-selectivity to form 1,4-alkyl cyanobenzenes. The cross-coupling scheme is compatible with oxidatively sensitive and acidic functional groups such as amines and alcohols, which have proven difficult to incorporate in alternative electrochemical approaches using carboxylic acids as C(sp³) precursors.     

Synthesis of Aryl Ketones by Cross-Coupling Reaction of Arylboronic Acids with Carboxylic Anhydrides in Aqueous Phase

A highly efficient aqueous system of PdCl₂-catalyzed cross-coupling reaction of arylboronic acid with carboxylic anhydride (Suzuki-type reaction) without the use of phosphine ligands was developed. The reactions went smoothly in acetone/H₂O phase to give good yields of aryl ketones in short reaction times (1–3 h) at room temperature in air and were unaffected by the presence of electron-releasing and electron-withdrawing substituents in both the arylboronic acids and carboxylic anhydrides.     

Organic reactions in strong alkalis-III Fission of keto- and hydroxy-acids

Studies with suitable fatty acid derivatives have clarified many of the reactions of alcohols and ketones in concentrated alkalis.

Secondary alcohols are dehydrogenated to ketones which then yield acids by hydrolysis of the C—C bonds adjacent to the carbonyl group. Various side reactions are reported and discussed.

A mechanism is proposed for the fission of ricinoleic acid in concentrated alkalis.     

Studies on the polymerization of ethyl acrylate. II. Chain transfer studies

Transfer constants for different solvents representing hydrocarbons, halogenated compounds, alcohols, ketones, acids, and esters were determined in the thermal polymerization of ethyl acrylate at 80°C and they are compared with the available data on methyl acrylate and ethyl methacrylate. It was observed from the values of transfer constants that ethyl acrylate radicals are a little more effective than methyl acrylate or ethyl methacrylate in abstracting hydrogen atom from hydrocarbons and alcohols. In acetic and n-butyric acid media, it has been found, by the aid of endgroup analysis, that the derived solvent radicals from transfer reactions are not too efficient to start a new chain.     

Cu(II) bromide catalyzed oxidation of aldehydes and alcohols

A variety of aromatic, aliphatic and conjugated aldehydes and alcohols were transformed to the corresponding carboxylic acids and ketones with a quantitative conversion in high yields with 70% t‐BuOOH solution in water in the presence of catalytic (5 mol%) amounts of CuBr₂ under room temperature conditions. The conversion of 4‐methoxybenzaldehyde to 4‐methoxybenzoic acid is extremely facile in MeCN at ambient temperature in the presence of 5 mol% CuBr₂ and 2 equiv. 70% t‐BuOOH (water) as the oxidant. Oxidation with t‐BuOOH (water) alone in MeCN was found to be negligible. The scope of our catalytic system is applicable for a wide range of aromatic, conjugated and aliphatic substrates. These aldehydes were converted to the corresponding carboxylic acids in good isolated yields in reasonable times. It is pertinent to mention here that mild halogenic oxidants like hypochlorites, chlorites and NBS are not suitable for substrates with electron‐rich aromatic rings, olefinic bonds and secondary hydroxyl groups. Substitutions at different positions on the phenyl ring do not hinder the reaction, although the reaction time is affected. Oxidation of α,β unsaturated derivatives resulted in the formation of the expected acid in good yield. In addition, the transformation of secondary alcohols to ketones is extremely facile. No recemization was observed for menthone. This method possesses a wide range of capabilities since it can be used with other functional groups which may not tolerate oxidative conditions, involves fairly simple method for work‐up, exhibits chemoselectivity and proceeds under ambient conditions. The resulting products are obtained in good yields within reasonable time. Copyright © 2011 John Wiley & Sons, Ltd.     

Divergent Synthesis of Alcohols and Ketones via Cross-Coupling of Secondary Alcohols under Manganese Catalysis

A homogeneous manganese-catalyzed cross-coupling of two secondary alcohols for the divergent synthesis of γ-disubstituted alcohols and β-disubstituted ketones is reported. Employing the well-defined Mn-MACHO^Ph as the catalyst, this novel protocol has a broad substrate scope with good functional group tolerance and affords a diverse library of valuable disubstituted alcohols and ketones in moderate to good yields. The strong influence of the reaction temperature on the selective formation of alcohol products was theorized in preliminary DFT studies. Studies have shown that the Gibbs free energy of the formation of alcohols is thermodynamically more favourable than corresponding ketones at a lower temperature.     

PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

Regioselective Synthesis of Fused Furans by Decarboxylative Annulation of α,β‐Alkenyl Carboxylic Acid with Cyclic Ketone: Synthesis of Di‐Heteroaryl Derivatives

α,β‐Alkenyl carboxylic acids undergo Cu^II‐mediated decarboxylative annulation reactions with aliphatic cyclic ketones to provide synthetically valuable di‐heterocycles. The annulation process tolerates a variety of aliphatic ketones and heterocyclic alkenyl carboxylic acids, producing substituted fused furan derivatives with complete regioselectivity. The current protocol offers a synthetically applicable pathway to construct a variety of oligo‐heterocycles through Cu‐mediated single‐electron transfer and decarboxylation. Notably, synthesis of relatively inaccessible di‐heterocycles has been achieved successfully using this protocol.     

Unconventional titania photocatalysis: direct deployment of carboxylic acids in alkylations and annulations

Under dry, anaerobic conditions, TiO(2) photocatalysis of carboxylic acid precursors resulted in carbon-carbon bond-forming processes. High yields of dimers were obtained from TiO(2) treatment of carboxylic acids alone. On inclusion of electron-deficient alkenes, efficient alkylations were achieved with methoxymethyl and phenoxymethyl radicals. In reactions with maleic anhydride or maleimides, phenoxyacetic acid produced chromenedione derivatives in addition to adducts. These photocatalytic reactions are simple and cheap to perform, and the TiO(2) is easily removed by filtration. The anaerobic photocatalysis strategy offers a range of synthetic possibilities.     

Synthesis of substituted 6-cyclopropylpurine bases and nucleosides by cross-coupling reactions or cyclopropanations

Synthesis of novel purine bases and nucleosides bearing unsubstituted or substituted cyclopropyl rings in position 6 is reported. Unsubstituted 6-cyclopropylpurines were efficiently prepared by cross-coupling reactions of 6-chloropurines with cyclopropylzinc chloride. 6-Vinylpurines underwent Cu-mediated cyclopropanations with ethyl diazoacetate to give 6-[(ethoxycarbonyl)cyclopropyl]purines that were further transformed to carboxylic acids, amides and alcohols. 6-Cyclopropylpurine ribonucleoside exerted a significant cytostatic effect while all substituted derivatives were inactive.     

A convenient approach to β-heteroarylated (C-N bond) ketones from Cs2CO3 promoted reaction between propargyl alcohols and nitrogen-heterocycles

An efficient and direct approach to β-heteroarylated (C-N bond) ketones is demonstrated. Base promoted redox isomerization of propargyl alcohol to α,β-unsaturated ketone followed by conjugate addition to NH-heteroarenes affords a wide range of β-heteroarylated ketones in good to excellent yields. Aryl, heteroaryl, alkyl C(sp), and terminal alkynes containing unactivated propargyl alcohols effectively undergo redox-isomerization conjugate addition (RICA) with NH-heteroarenes. Reaction of 3-substituted pyrazoles or indazole with propargyl alcohols enables highly regioselective products. A diverse range of NH-bearing nucleophiles such as: pyrazoles, imidazole, triazoles, pyrrole, indoles and aniline participate in this reaction and deliver the corresponding β-heteroarylated ketones.     

Oxidative cyclizations in a nonpolar solvent using molecular oxygen and studies on the stereochemistry of oxypalladation

Oxidative cyclizations of a variety of heteroatom nucleophiles onto unactivated olefins are catalyzed by palladium(II) and pyridine in the presence of molecular oxygen as the sole stoichiometric oxidant in a nonpolar solvent (toluene). Reactivity studies of a number of N-ligated palladium complexes show that chelating ligands slow the reaction. Nearly identical conditions are applicable to five different types of nucleophiles: phenols, primary alcohols, carboxylic acids, a vinylogous acid, and amides. Electron-rich phenols are excellent substrates, and multiple olefin substitution patterns are tolerated. Primary alcohols undergo oxidative cyclization without significant oxidation to the aldehyde, a fact that illustrates the range of reactivity available from various Pd(II) salts under differing conditions. Alcohols can form both fused and spirocyclic ring systems, depending on the position of the olefin relative to the tethered alcohol; the same is true of the acid derivatives. The racemic conditions served as a platform for the development of an enantioselective reaction. Experiments with stereospecifically deuterated primary alcohol substrates rule out a "Wacker-type" mechanism involving anti oxypalladation and suggest that the reaction proceeds by syn oxypalladation for both mono- and bidentate ligands. In contrast, cyclizations of deuterium-labeled carboxylic acid substrates undergo anti oxypalladation.     

Pd/light-accelerated atom-transfer carbonylation of alkyl iodides: applications in multicomponent coupling processes leading to functionalized carboxylic acid derivatives

The atom-transfer carbonylation reaction of various alkyl iodides thereby leading to carboxylic acid esters was effectively accelerated by the addition of transition-metal catalysts under photoirradiation conditions. By using a combined Pd/hν reaction system, vicinal C-functionalization of alkenes was attained in which α-substituted iodoalkanes, alkenes, carbon monoxide, and alcohols were coupled to give functionalized esters. When alkenyl alcohols were used as acceptor alkenes, three-component coupling reactions, which were accompanied by intramolecular esterification, proceeded to give lactones. Pd-dimer complex [Pd(2)(CNMe)(6)][PF(6)](2), which is known to undergo homolysis under photoirradiation conditions, worked quite well as a catalyst in these three- or four-component coupling reactions. In this metal/radical hybrid system, both Pd radicals and acyl radicals are key players and a stereochemical study confirmed the carbonylation step proceeded through a radical carbonylation mechanism.     

Titanium-mediated cross-coupling reactions of imines with ketones or aldehydes: an efficient route for the synthesis of 1,2-amino alcohols

The cross-coupling reactions of imines with ketones using Ti(OⁱPr)₄/c-C₅H₉MgCl reagent lead to 1,2-amino alcohols after hydrolysis. The coupling reactions with aldehydes could also afford 1,2-amino alcohols, however, in some cases, aziridines were obtained as major products in a stereoselective manner.