Cobalt-catalyzed C–N,C–O,C–S bond formation: synthesis of heterocycles

In recent decades, a large number of reports related to the synthesis of N-, O- and S-containing heterocycles have appeared owing to a wide variety of their biological activity. Traditional approaches require expensive or highly specialized equipment or would be of limited use to the synthetic organic chemist due to their highly inconvenient approaches. New strategies have been developed for the preparation of heterocycles in the last decades. Metal and non-metal catalysts are used in organic reactions with high activity. These synthetic strategies are becoming important and highly rewarding protocols in organic synthesis. In this review article, the synthesis of heterocycles is presented with the application of cobalt metal as a catalyst. It describes the formation of different sized heterocyclic rings containing different heteroatoms.

     

Density functional study of the conformations and intramolecular proton transfer in thiohydroxamic acids

The conformational preferences of thiohydroxamic acids (N-hydroxythioamides) are investigated by the density functional B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) method in this work. Unlike hydroxamic acids, the thione and thiol forms are found to be equally stable in the gas phase, and the reaction pathways for the interconversion between the thione and thiol forms have been deduced to involve rotation about the C[double bond, length as m-dash]N bond of the thiol tautomer in the rate-determining step. The effect of aqueous solvation on the reactions has also been investigated. It is found that inclusion of a few explicit water molecules in an implicit solvent calculation is necessary in order to accurately account for hydrogen bonding effects. Thiohydroxamic acids, like their hydroxamic acid analogues, are found to be N-acids, both in the gas phase and in aqueous solution.     

Modern synthetic methods for copper-mediated C(aryl)[bond]O, C(aryl)[bond]N, and C(aryl)[bond]S bond formation

The copper-mediated C(aryl)[bond]N, C(aryl)[bond]O, and C(aryl)[bond]S bond formation is an important transformation and has been developed to include a wide range of substrates. This Review highlights the recent developments in the copper-mediated (both stoichiometric and catalytic) reactions of aryl boronic acids, aryl halides, iodonium salts, siloxanes, stannanes, plumbanes, bismuthates, and trifluoroborate salts as aryl donors. In particular, the recent introduction of boronic acids as reaction partners in both O- and N-arylation has been a significant discovery and will occupy centre-stage in this review. Clear improvements can be obtained by the correct choice of copper source, base, ligands, and other additives. Mechanistic investigations should provide insight into the catalytically active species, which would aid in the development of milder, more-efficient methods.     

Enhanced stereoselectivity in internucleotidic bond formation by the use of the chiral ribose moiety of thymidine

This paper deals with the synthesis of new cyclic thymidine 3'-phosphoramidite building blocks having a covalent linker between the trityl type 5'-hydroxyl protecting group and the phosphorus atom attached to the 3'-hydroxyl group of thymidine. The ring structures were designed to reduce the conformational freedom around the phosphorus center so that the stereoselectivity in the internucleotide linkage formation would be improved. The linkers were also designed to be removed readily by treatment with aqueous ammonia. These building blocks were synthesized in good yield by one-pot cyclization of the diol precursors with dichloro(N,N-diisopropylamino)phosphine, despite their large-membered ring. Various activators having 1H-tetrazole, imidazole, and triazole structures were investigated to find the best selectivity in the synthesis of thymidylyl(5'-3')thymidine phosphorothioate. It turned out that our cyclic phosphoramidites gave preferentially the R(p) diastereoisomer in high coupling yield applicable to the solid-phase synthesis of oligodeoxynucleotides. It should be noted that high stereoselectivity was achieved without any chiral sources other than the 2'-deoxyribose moiety itself. The mechanistic studies revealed the importance of the steric bulk and the acidity of the activators. It was also found that the steric bulk of the alcoholic nucleophile was an important factor that determined the stereoselectivity in our systems.     

Catalytic olefination reaction of carbonyl compounds. A study on stereoselectivity of alkene formation

The mechanism of formation of alkene stereoisomers in the catalytic olefination reaction of carbonyl compounds was studied. 4-Chlorobenzaldehyde hydrazone 1 stereoselectively reacts with a number of F-, Cl-, Br-, and I-containing polyhaloalkanes in the presence of catalytic amounts of CuCl to give -substituted styrenes 2 with the more thermodynamically stable alkene isomer being the major product. A model for the formation of the stereoisomers of alkenes 2 in the olefination reaction is proposed. Stereoselectivity of the reaction is determined by elimination of copper(ii) halides from the lowest-lying conformers of organocopper intermediates II. According to quantum-chemical calculations, the elimination should involve the staggered conformations with antiperiplanar arrangement of C—Hal and C—Cu bonds and proceed by the E2 anti-elimination mechanism. The results of quantum-chemical calculations are in good agreement with the experimental E/Z alkene isomer ratios.     

Amidation and esterification of carboxylic acids with amines and phenols by N,N′-diisopropylcarbodiimide: A new approach for amide and ester bond formation in water

The present study reports the successful synthesis of two important and abundant functional groups "ester and amide" by N,N′-diisopropylcarbodiimide (DIC) in water as a green solvent. A wide range of substrates could be employed with high functional group tolerance. The products were obtained in high yields after short reaction times. This method provides an efficient, economic, simple and very mild protocol for ester and amide bond formation in aqueous media. In addition, this work not only may lead to environmentally benign systems but also will provide a new aspect of organic chemistry in water.     

Quantum-chemical calculations of the methylation of hydroxamic and thiohydroxamic acids with diazomethane

We made calculations about the methylation of both hydroxamic and thiohydroxamic acids with CH₂N₂. The potential-energy surfaces of several proposed pathways leading to possible site-selective products (N-methylated and O-methylated hydroximates) are presented. Our results agree satisfactorily with an experimental observation by Liguori et al. who found site selectivity in the formation of dimethylated products. Simultaneous deprotonation and methylation occurs in both forms (E and Z) of hydroxamic acid and thiohydroxamic acid, and the net energy barrier via this pathway is the smallest. In most corresponding processes the energy barriers are smaller for thiohydroxamic acid, and the Z-form has an energy barrier smaller than that of the E-form in both hydroxamic and thiohydroxamic acids.     

Air stable,sterically hindered ferrocenyl dialkylphosphines for palladium-catalyzed C[bond]C,C[bond]N,and C[bond]O bond-forming cross-couplings

Pentaphenylferrocenyl di-tert-butylphosphine has been prepared in high yield from a two-step synthetic procedure, and the scope of various cross-coupling processes catalyzed by complexes bearing this ligand has been investigated. This ligand creates a remarkably general palladium catalyst for aryl halide amination and for Suzuki coupling. Turnovers of roughly 1000 were observed for aminations with unactivated aryl bromides or chlorides. In addition, complexes of this ligand catalyzed the formation of selected aryl ethers under mild conditions. The reactions encompassed electron-rich and electron-poor aryl bromides and chlorides. In the presence of catalysts containing this ligand, these aryl halides coupled with acyclic or cyclic secondary alkyl- and arylamines, with primary alkyl- and arylamines, and with aryl- and primary alkylboronic acids. These last couplings provide the first general procedure for reaction of terminal alkylboronic acids with aryl halides without toxic or expensive bases. The ligand not only generates highly active palladium catalysts, but it is air stable in solution and in the solid state. Palladium(0) complexes of this ligand are also air stable as a solid and react only slowly with oxygen in solution.     

A novel disproportionation reaction of aromatic aldehydes involving CC bond formation

The reaction of aromatic aldehydes with methyl thiocyanate in the presence of tributylphosphine afforded both S-methyl thiobenzoates and phenylacetonitriles via a novel disproportionation pathway involving carbon—carbon bond formation.     

A mild,rapid, and convenient esterification of α-keto acids

A new method for the esterification of a α-keto acids with alkyl chloroformates is described, which is compatible with many functional groups.     

A study on the esterification of terephthalic acid

The esterifications of terephthalic acid and monomethylterephthalate with methanol have been studied in order to compare the reaction rate constants of the first and second carboxyl groups of terephthalic acid. The ratio of the two constants is found to be 1 · 18 ± 0 · 07 at 110°. A value of the Hammett substituent constant for the group COOCH₃ at the para position is reported (σ_COOCH3 = 0 · 59).     

Quantum-chemical study of compounds containing intramolecular O-H…O=C bond

New results of the study of stable intermediates containing an intramolecular hydrogen bond O-H…O=C in the gas phase and solvents, carbon tetrachloride and dioxane, were analyzed. The structural and energy characteristics of the stable conformers of these compounds were determined by a MR2/6-311++G(d, p) method. The most stable is the hydrogen-bonded conformer of 1-hydroxy-1-chloroethyl acetate molecule. The approaches to accounting for the effects of the O-H…O=C intramolecular hydrogen bonding on the molecules reactivity were discussed.     

The use of iron carbonyls in the C—C bond formation reactions

Russian Chemical Bulletin - The review systematizes material by key types of reactions promoted by iron carbonyls. The reaction types are subdivided into several principal directions, focusing...     

A Molecular orbital study of the rotation about the CC bond in styrene

The geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C_βC₂C₁C₂ dihedral angle-Φ = 0°(cis), 15°, 30°, 60° and 90° — assuming that the vinyl and phenyl groups remain planar, but otherwise with full geometry optimization. Similar calculations have been carried out for 1,3-butadiene and 3-methylene-1,4-pentadiene (MPD) where rotation about 180° generates a different and not the same conformer. The torsional potential energy curve for styrene has a very flat minimum Φ = 0, i.e. the cis structure is the most stable, whereas butadiene and MPD have minima in the region Φ = 37° to 40°, indicative of more stable gauche structures. For styrene the barrier height Φ = 90° is 131.1 KJ mol^?1. These results provide strong support for the potential function obtained by Hollas and Ridley from single level vibronic fluorescence and other spectroscopic data. The distortion of the benzene ring brought about the vinyl group substitution is discussed, also the variation of the C/C and H/C bond lenghts with Φ and the change in charge on the vinyl group and the polarity of the various bonds in the conversion of the cis into the 90° gauche conformer. The stabilization energy for styrene relative to that for benzene has been evaluated according to various criteria, and, in addition, the energy associated with the distortion of the ring.     

Quantum-mechanical study on the mechanism of peptide bond formation in the ribosome

Ribosomes transform the genetic information encoded within genes into proteins. In recent years, there has been much progress in the study of this complex molecular machine, but the mechanism of peptide bond formation and the origin of the catalytic power of this ancient enzymatic system are still an unsolved puzzle. A quantum-mechanical study of different possible mechanisms of peptide synthesis in the ribosome has been carried out using the M06-2X density functional. The uncatalyzed processes in solution have been treated with the SMD solvation model. Concerted and two-step mechanisms have been explored. Three main points suggested in this work deserve to be deeply analyzed. First, no zwitterionic intermediates are found when the process takes place in the ribosome. Second, the proton shuttle mechanism is suggested to be efficient only through the participation of the A2451 2'-OH and two crystallographic water molecules. Finally, the mechanisms in solution and in the ribosome are very different, and this difference may help us to understand the origin of the efficient catalytic role played by the ribosome.     

Formation of carbon-carbon bonds with radicals derived from the esters of thiohydroxamic acids

Radicals formed from the esters of thiohydroxamic acids readily add to electron deficient olefins to give adducts of potential synthetic value in variable yield. In certain cases the added sulphur function is easily eliminated with reformation of olefin.