Quantum chemical simulation of the reaction of methane with gold(I) acetylacetonate and aquaacetylacetonate complexes

The interaction between methane and gold(I) acetylacetonate via electrophilic substitution (reaction (I)) and oxidative addition (reaction (II)) is simulated. In both cases, the formation of the products is thermodynamically favorable: the decrease in energy is 31 kcal/mol for reaction (I) and 26 kcal/mol for reaction (II). The product of reaction (II) is additionally stabilized by Au-H interaction. Both reactions have a low activation barrier and proceed via the formation of structurally different methane complexes reducing the energy of the system by 9.3 kcal/mol for reaction (I) and by 10.9 kcal/mol for reaction (II). The complex [Au(H₂O)(acac)] is also capable of forming methane complexes. These complexes result from a thermally neutral reaction and turn into products after overcoming a low energy barrier. The structure of the complex activating methane in the gold-rutin system is deduced from the data obtained.     

Synthesis of N-acyl-N,O-acetals from N-aryl amides and acetals in the presence of TMSOTf

Secondary amides undergo in situ silyl imidate formation mediated by TMSOTf and an amine base, followed by addition to acetal acceptors to provide N-acyl-N,O-acetals in good yields. An analogous, high-yielding reaction is observed with 2-mercaptothiazoline as the silyl imidate precursor. Competing reduction of the acetal to the corresponding methyl ether via transfer hydrogenation can be circumvented by the replacement of CY₂NMe with 2,6-lutidine under otherwise identical reaction conditions.     

Physicochemical Properties of Complexes of Zinc Iodide with Pyridine N-Oxides

Polycrystalline 1 : 2 complexes of ZnI₂ with N-oxides of pyridine, picoline, and 2,6-lutidine were studied by IR spectroscopy, dielectrometry, and conductometry. An equilibrium between three solid phases was observed. These phases are characterized by different enthalpies of formation of intermolecular bonds and different mechanism of electronic effect transmission via these bonds. Gas-like thermal molecular motion in one of the phases of the 2,6-lutidine N-oxide complex was observed. Reversible chemical reactions on the surface of the crystalline 3-picoline N-oxide complex are initiated on exposure to an alternating electric field. Two complexes, similarly to the ZnCl₂ complexes, are ionized in the cumulative mode on fast heating from 18-19 to 26-45°C. Partial activation energies of electrical conductivity of different ions were determined for a number of complexes and inorganic salts.     

Complexes of Copper (II) Aryl Carboxylates with 2, 6-Lutidine

Copper (II) aryl carboxylates are known to form co-ordination complexes with various oxygen and nitrogen donors such as pyridine-N-oxide¹⁾, quinoline and isoquinoline²⁾ diethylamine and dipropylamine¹⁾. There is no reference in literature regarding the preparation of complexes of copper (II) aryl carboxylates with 2,6-lutidine. The present communication describes the preparation of complexes of various copper (II) aryl carboxylates with 2,6-lutidine in acetone or ethylacetate medium.     

A Theoretical Investigation on N7(H)→N9(H)Tautomerization of Isolated and Monohydrated 2,6-Dithiopurine Combined with IPCM Solvent Model

The tautomerization reaction mechanism has been reported between N7(H) and N9(H) of isolated and monohydrated 2,6‐dithiopurine using B3LYP/6‐311+G(d,p). The isodensity polarized continuum model (IPCM) in the self‐consistent reaction field (SCRF) method is employed to account for the solvent effect of water on the tautomerization reaction activation energies. The results show that the two pathways P(1) (via the carbene intermediate I1) and P(2) (via the sp³‐hybrid intermediate I2) are found in intramolecular proton transfer, and each pathway is composed by two primary steps. The calculated activation energy barriers of the rate‐determining steps in isolated 2,6‐dithiopurine N7(H)→N9(H) tautomerism are 308.2 and 220.0 kJ·mol^?1 in the two pathways, respectively. Interestingly, in one‐water molecule catalyst, it dramatically lowers the N7(H)→N9(H) energy barriers by the concerted double proton transfer mechanism in P(1), favoring the formation of 2,6‐dithiopurine N9(H). However, the single proton transfer mechanism assisted with out‐of‐plane water in the first step of P(2) increases the activation energy barrier from 220.0 to 232.3 kJ·mol^?1, while the second step is the out‐of‐plane concerted double proton transfer mechanism, indicating that they will be less preferable for proton transfer. Additionally, the results also show that all the pathways are put into the aqueous solution, and the activation energy barriers have no significant changes. Therefore, the long‐range electrostatic effect of bulk solvent has no significant impact on proton transfer reactions and the interaction with explicit water molecules will significantly influence proton transfer reactions.     

Rhodium amino complexes [Rh(COD)(Amine)2]PF6 immobilized on poly(4-vinylpyridine) as catalysts in the water gas shift reaction

Catalysts for the water gas shift reaction prepared from Rh(COD)(amine)₂ PF₆ (COD=1,5-cyclooctadiene, amine=4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with 80% aqueoux 2-ethoxyethanol for 1×10⁻⁴ mol Rh/0.5 g of polymer, P(CO)=0.9 atm at 100 °C, are described. The role of the coordinated amine effect on the catalytic activity was investigated.     

Elemental F2 with Transannular Dienes: Regioselectivities and Mechanisms

Three reaction paths, namely, molecule‐induced homolytic, free radical, and electrophilic, were modeled computationally at the MP2 level of ab initio theory and studied experimentally for the reaction of F₂ with the terminal dienes of bicyclo[3.3.1]nonane series. The addition of fluorine is accompanied by transannular cyclization to the adamantane derivatives in which strong evidence for the electrophilic mechanism both in nucleophilic (acetonitrile) and non‐nucleophilic (CFCl₃, CHCl₃) solvents were found. The presence of KF in CFCl₃ and CHCl₃ facilitates the addition and substantially reduces the formation of tar products.     

Transformation,par 1′anhydride acètique,d'aldèhydes à caractére èlectrophile prononcè; catalyse par des pyridines. 1. Acyloïnes et hydrates di-o-acètylès d'aldèhydes

2-Benzothiazole-carbaldehyde is transformed into di-O-acetyl-enol-(benzothiazolecarboxyl-2)-oin in the presence of acetic anhydride and of pyridine as catalyst. Without pyridine or with 2,6-lutidine no reaction occurs. A mechanism of this reaction is proposed. No reaction was observed in the case of benzaldehyde. Choral reacts with acetic anhydride in the presence of pyridine as well as of 2,6-lutidine as catalyst to give 1,1-diacetoxy-2,2,2-trichloro-ethane. A mechanism is proposed, in which in an intermediate state the acetate ion (and not pyridine, for steric reasons) attacks the carbon of the carbonyl function of the conjugate acid with the acetylium cation to yield 1,1-diacetoxy-2,2,2-trichloro-ethane. These two reactions occur only with aldehydes whose carbonyl is very electrophilic, and seem to be a possible way to point out the presence of an acylium cation in pyridine medium.     

Electrophilic cyclization of 3-alkynyl-4-chalcogen-2-H-chromenes: synthesis of 3-halo-chalcogenophene[3,2-c]chromene derivatives

An efficient synthesis of 3-halo-chalcogenophene[3,2-c]chromene has been accomplished via electrophilic cyclization reaction of 3-alkynyl-4-chalcogen-2H-chromene using I₂, PhSeBr, and BuTeBr₃ as electrophilic sources. The cyclization reaction proceeded cleanly under mild reaction conditions, and 3-halochalcogen-chromenes were formed in good yields. In addition, the obtained 3-iodo-chalcogenophene-chromenes were readily transformed to more complex products using a metal-halogen exchange reaction with n-BuLi and trapping the lithium-intermediate formed with aldehyde, furnishing the desired secondary alcohol in good yield. Conversely, using the palladium catalyzed cross-coupling reactions with terminal alkynes and boronic acid, we were able to obtain the Sonogashira and Suzuki type products in good yields.     

High-Pressure Stopped-Flow Study of the Kinetics of Base Hydrolysis of the Dichromate Ion by Hydroxide Ion,Ammonia, Water,and 2,6-Lutidine in Aqueous Solution

Volumes of activation for the base hydrolysis of the dichromate anion have been measured at 298.2 K, using high-pressure stopped-flow spectrophotometry. The values of ΔV* (cm³ · mol^?1), ? 17.9 ± 0.6, ? 19.2 ± 0.9, ? 24.9 ± 0.9 and ? 26.0 ± 0.7 for OH^?, NH₃, H₂O and 2,6-lutidine, respectively, are consistent with an interchange mechanism with associative activation mode (I^a).     

Coupling Reactions of Alkynyl Indoles and CO2 by Bicyclic Guanidine: Origin of Catalytic Activity?

Density functional theory calculations were used to investigate the three possible modes of activation for the coupling of CO₂ with alkynyl indoles in the presence of a guanidine base. The first of these mechanisms, involving electrophilic activation, was originally proposed by Skrydstrup et al. (Angew. Chem. Int. Ed . 2015 , 54 , 6682). The second mechanism involves the nucleophilic activation of CO₂. Both of these electrophilic and nucleophilic activation processes involve the formation of a guanidine‐CO₂ zwitterion adduct. We have proposed a third mechanism involving the bifunctional activation of the bicyclic guanidine catalyst, allowing for the simultaneous activation of the indole and CO₂ by the catalyst. We demonstrated that a second molecule of catalyst is required to facilitate the final cyclization step. Based on the calculated turnover frequencies, our newly proposed bifunctional activation mechanism is the most plausible pathway for this reaction under these experimental conditions. Furthermore, we have shown that this bifunctional mode of activation is consistent with the experimental results. Thus, this guanidine‐catalyzed reaction favors a specific‐base catalyzed mechanism rather than the CO₂ activation mechanism. We therefore believe that this bifunctional mechanism for the activation of bicyclic guanidine is typical of most CO₂ coupling reactions.     

Hydroxycarbonylation of 1-hexene catalyzed by [Rh(COD)(amine)2](PF6) complexes immobilized on poly(4-vinylpyridine)

Summary Rhodium(I) complexes, [Rh(COD)(amine)₂](PF₆) (COD = 1,5-cyclooctadiene, amine = 4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with water catalyzed both the hydroxycarbonylation of 1-hexene to propionic acid and the water-gas shift reaction (WGSR). The role of the coordinated amine on the catalytic activity was examined.     

Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Alkynes through C?H Activation

The efficient Rh^I‐catalyzed cycloisomerization of benzylallene‐alkynes produced the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton through a C H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C H bond to Rh^I, an ene‐type cyclization to the vinylidenecarbene–Rh^I intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.     

Theoretical study of the mechanism of oxoiron(IV) formation from H2O2 and a nonheme iron(II) complex: O-O cleavage involving proton-coupled electron transfer

It has recently been shown that the nonheme oxoiron(IV) species supported by the 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane ligand (TMC) can be generated in near-quantitative yield by reacting [Fe(II)(TMC)(OTf)(2)] with a stoichiometric amount of H(2)O(2) in CH(3)CN in the presence of 2,6-lutidine (Li, F.; England, J.; Que, L., Jr. J. Am. Chem. Soc. 2010, 132, 2134-2135). This finding has major implications for O-O bond cleavage events in both Fenton chemistry and nonheme iron enzymes. To understand the mechanism of this process, especially the intimate details of the O-O bond cleavage step, a series of density functional theory (DFT) calculations and analyses have been carried out. Two distinct reaction paths (A and B) were identified. Path A consists of two principal steps: (1) coordination of H(2)O(2) to Fe(II) and (2) a combination of partial homolytic O-O bond cleavage and proton-coupled electron transfer (PCET). The latter combination renders the rate-limiting O-O cleavage effectively a heterolytic process. Path B proceeds via a simultaneous homolytic O-O bond cleavage of H(2)O(2) and Fe-O bond formation. This is followed by H abstraction from the resultant Fe(III)-OH species by an ?OH radical. Calculations suggest that path B is plausible in the absence of base. However, once 2,6-lutidine is added to the reacting system, the reaction barrier is lowered and more importantly the mechanistic path switches to path A, where 2,6-lutidine plays an essential role as an acid-base catalyst in a manner similar to how the distal histidine or glutamate residue assists in compound I formation in heme peroxidases. The reaction was found to proceed predominantly on the quintet spin state surface, and a transition to the triplet state, the experimentally known ground state for the TMC-oxoiron(IV) species, occurs in the last stage of the oxoiron(IV) formation process.     

Bifunctionalized Allenes,Part XI: Competitive Electrophilic Cyclization and Addition Reactions of 4‐Phosphorylated Allenecarboxylates

The reaction of the 4‐phosphorylated allenecarboxylates with different electrophilic reagents such as sulfuryl chloride, bromine, benzenesulfanyl, and benzeneselanyl chlorides takes place with a 5‐endo‐trig cyclization or 2,3‐addition reaction depending on the kind of the substituents in the phosphoryl group. Treatment of the 4‐(dimethoxyphosphopyl)‐allenoates with electrophiles gives a mixture of 2,5‐dihydro‐1,2‐oxaphospholes and furan‐2(5H)‐ones in the ratio of about 1.7:1 as a result of the neighboring group participation of phosphonate and carboxylate groups in the cyclization. On the other hand, (3E)‐4‐(diphenylphosphoryl)‐alk‐3‐enoates were prepared, in moderate yields, by chemo‐, regio, and stereoselective electrophilic addition to the C² C³‐double bond in the allenoate moiety. A possible mechanism involving cyclization and addition reactions of the 4‐phosphorylated allenecarboxylates was proposed.     

A ligand free copper(II) catalyst is as effective as a ligand assisted Pd(II) catalyst towards intramolecular C–S bond formation via C–H functionalization

Copper(I) catalysts are usually ineffective on the other hand Pd(II) catalysts are quite effective in promoting intramolecular sp² C–H functionalization (C–S bond formation). Herein, we have developed a ligand assisted Pd(II) catalyzed C–S bond formation via C–H activation from arylthioureas leading to the formation of 2-aminobenzothiazoles for substrates bearing electron donating (EDG) groups in the aryl ring. However without the assistance of ligand this Pd(II) catalyzed reaction is quite unproductive particularly for thioureas possessing strongly electron donating groups in the aryl rings. Interestingly, the ligand free Cu(II) catalyzed oxidative cyclization of arylthioureas are equally effective both for arylthioureas possessing electron donating as well as electron withdrawing groups in the aryl rings.     

Mechanism of the cyclization of dimethyl diethynyl silane with selenium tetrabromide: Computational and structural studies, and monitoring

The structure of 2,4-dibromo-2-dibromomethyl-3,3-dimethyl-1-selena-3-silacyclopentene-4, formed by regioselective electrophilic addition of SeBr₄ to dimethyl diethynyl silane, has been determined using X-ray analysis technique. Quantum chemistry methods were used to study elementary stages of the reaction. It was found that the first stage consisted of SeBr₄ conversion into bimolecular complex Br₂?SeBr₂, initiated by dimethyl diethynyl silane. Possible formation of five-membered and six-membered heterocycles involves different cyclization mechanisms. The formation of only five-membered heterocycle is explained by kinetically preferable ring closure through four-center transition state. The conclusions obtained by calculations were confirmed by monitoring of the reaction using ¹H NMR method.     

I2-Promoted Intramolecular Oxidative Cyclization of Butenyl Anilines: A Facile Route to Benzo[b]azepines

A metal-free approach for the synthesis of seven-membered N-heterocycles has been developed by the I₂-promoted intramolecular cross-coupling/annulation of butenyl anilines. This cyclization reaction involves C−H activation and C−C bond formation and exhibits good functional group tolerance. A series of benzo[b]azepine derivatives are obtained in moderate to good yields.     

Oxidation of Phenols and Hydroquinones by Mercury(II) Trifluoroacetate and Mercury(II) Oxide

Treatment of phenols with mercury(II) salts normally results in rapid electrophilic aromatic mercuration and formation of arylmercurial derivatives in good yield. The use of mercury(II) salts as oxidants for phenols and hydroquinones has not, however, been investigated in any detail. In 1935, Montignie reported that hydroquinone reduced mercury(II) iodide to mercury(O), but the organic reaction product was not identified.¹ A similar observation was made by Koton,² who found that reaction of hydroquinone, 4-hydroxy-aniline and pyrogallol with mercury(II) acetate did not result in mercuration but led to formation of mercury(0), while Ohno has described the “strong reducing action” of catechol and nitrohydroquinones to mercury(II) salts.³ Mercury(II) acetate has been shown to oxidise hydroquinone and 1,4-bis[β-(2,5-dihydroxyphenyl)-α-methylethylamino] anthraquinone to the corresponding quinones,^4,5 and treatment of 2,6-di-t-butyl-4-methylphenol—with mercury(II) oxide has been found to give the expected products of phenolic oxidative coupling via quinone methide formation.⁶     

Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Alkynes through CH Activation

The efficient Rh^I‐catalyzed cycloisomerization of benzylallene‐alkynes produced the tricyclo[9.4.0.0^3,8]pentadecapentaene skeleton through a C? H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C? H bond to Rh^I, an ene‐type cyclization to the vinylidenecarbene–Rh^I intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.