L. I. Belen’kii

Quantum chemical studies of azoles 4. A novel elimination–addition mechanism of tetrazole and 1,2,4-triazole electrophilic substitution

Thermodynamic parameters of the addition–elimination and elimination–addition electrophilic substitution reactions of 1H-tetrazole and 1,2,4-1H-triazole obtained from DFT B3LYP/ 6-31G(d,p) quantum chemical calculations with proton as model electrophile are compared. According to calculations, the elimination–addition reactions can proceed without preliminary formation of N-protonated azolium salts.

Quantum chemical studies of azoles 5. Electrophilic substitution in tetrazole and 1, 2, 4-triazole: the influence of basis set on calculated thermodynamic parameters of the elimination—addition mechanism without preliminary formation of N-protonated azolium salts

Thermodynamic parameters of electrophilic substitution reactions of 1H-tetrazole and 1H-1, 2, 4-triazole proceeding by the addition–elimination and elimination–addition mechanisms were calculated by the DFT/B3LYP/6-31G(2df, p) method using proton as model electrophile and compared. The results obtained substantiate that the elimination–addition mechanism may not involve preliminary formation of N-protonated azolium salts, as was shown earlier in our DFT/B3LYP/6-31G(d, p) calculations.

Quantum chemical studies of azoles 3. Thermodynamic stabilities of neutral molecules and intermediates formed in electrophilic substitution of azoles containing three or four heteroatoms

Density functional theory quantum chemical calculations of thermodynamic stabilities in the gas phase and in water were carried out for 1,3,4-oxadiazole and 1,3,4-thiadiazole, 1,2,4-1H-triazole and 1,2,4-4H-triazole, 1,2,3,4-1H-tetrazole and 1,2,3,4-2H-tetrazole molecules, and for cationic and bipolar (carbenoid) intermediates formed by these molecules in electrophilic substitution reactions (with proton as model electrophile) and the results obtained are compared. Differences in the chemical behavior of pairs of isomeric 1H- and 4H-1,2,4-triazoles and 1H- and 2H-tetrazoles are analyzed.

The Literature of Heterocyclic Chemistry, Part X, 2005–2007

Publisher Summary This chapter is a sequel to nine already published surveys in “Advances in Heterocyclic Chemistry”. It includes a list of monographs and reviews published during the period 2005–2007, as well as some published earlier but omitted in Part IX. Like Parts III–IX, this survey is based partly on bibliographic papers published by the authors in Khimiya Geterotsiklicheskikh Soedinenii since 2006. The list has been divided according to contents of literature into classifications, such as general sources and topics, three-membered rings, four-membered rings, five-membered rings, six-membered rings, rings with more than six members, and hetrocycles containing unusual hetronatoms.

Photochromic dihetarylethenes. 14. Synthesis of symmetrical and unsymmetrical dihetarylcyclobutene-1,2-diones

A procedure was developed for the synthesis of symmetrical and unsymmetrical cyclobutene-1,2-dione derivatives bearing thiophene and thieno[3,2-b]thiophene substituents by the Friedel—Crafts reaction of the corresponding heterocyclic compounds with squaric acid dichloride in the presence of AlCl3. In addition to the target dihetarylcyclobutenediones, monoacylation products of methyl (2,5-dimethylthiophen-3-yl)acetate and methyl 5-methylthieno[3,2-b]thiophene-2-carboxylate with squaric acid dichloride were isolated and characterized.

The Literature of Heterocyclic Chemistry, Part VIII, 1999–2001

Publisher Summary This chapter presents monographs and reviews of heterocyclic chemistry published during the period 1999–2001. The sources in English, Russian, Japanese, Chinese, Czech, and other languages are surveyed and classified. This feature of the survey should not cause any problem because some of the sources are available in English translations and practically all others have informative English abstracts as well as quite understandable and useful schemes and lists of references. Carbohydrates are mentioned only in general cases e.g., anomeric effect when carbohydrates serve as starting compounds for the synthesis of other heterocycles or they are present as fragments of a complex system including another heterocyclic moiety such as nucleosides.

Quantum chemical studies of azoles 6. The effect of specific solvation on the calculated thermodynamic parameters of electrophilic substitution in tetrazole according to the elimination—addition scheme without preliminary formation of N -protonated azolium salts

Quantum chemical calculations (DFT/B3LYP/6-31G(d)) considering specific solvation effects were used to compare the thermodynamic parameters of electrophilic substitution reactions (with the hydroxonium ion as a model electrophile) in 1H-tetrazole according to the addition—elimination and elimination—addition schemes. The latter scheme can proceed without preliminary formation of N-protonated azolium salts, as demonstrated earlier by the DFT/ B3LYP/6-31G(d,p) and DFT/B3LYP/6-31G(2df,p) calculations considering the solvation effects in aqueous solution in terms of the polarizable continuum model (PCM) with a proton as a model electrophile.

Quantum chemical studies of azoles 2. Thermodynamic stability of neutral molecules and intermediates formed during electrophilic substitution of 1,2- and 1,3-azoles

The thermodynamic stability of 1,2- and 1,3-azole molecules, as well as of cationic and bipolar (carbenoid) intermediates in the gas phase and in aqueous solution formed by electrophilic substitution (proton as a model electrophile) was compared based on the analysis of quantum chemical calculations performed at the DFT/B3LYP/6-31G(D) level of theory with zero-point energy corrections. The differences in the chemical behavior of the isomeric 1,2- and 1,3-azole pairs, viz., pyrazole—imidazole, isoxazole—oxazole, and isothiazole—thiazole, were considered.

Quantum-Chemical Investigation of Azoles 1. Alternative Electrophilic Substitution Mechanisms in 1,2- and 1,3-Azoles*

Quantum-chemical calculations were performed for the molecular structures of 1,2-azoles (pyrazole, isoxazole, isothiazole), 1,3-azoles (imidazole, oxazole, thiazole), and the corresponding intermediates of electrophilic substitution reactions (with protons as the model electrophiles): azolium ions, bipolar ions (ylides/carbenes), cationic σ-complexes, as well as activation energy values were calculated for the decomposition of ylides. The calculations were performed for gas phase and aqueous solutions according to the B3LYP method in a 6-31G(d) basis set, with corrections for the zero-point vibration energy. The solvation effects were taken into account by using the overlapping spheres model (IEFPCM). The results of the calculations explained some features of electrophilic substitution in azoles according to two alternative mechanisms: the classical addition-elimination with cationic σ-complex intermediates, and the mechanism of elimination-addition that involves ylides (carbenes) as key intermediates.

Oxepanes and Oxepines

Based on the literature published during last 12 years, this chapter covers different aspects of the chemistry of oxepines and oxepanes from the simplest compounds formed in metabolism of aromatic hydrocarbons to complicated fused O-heterocyclic systems of marine origin. Some structural features, chemical properties, and reaction mechanisms of base systems are discussed concisely. Ring syntheses of oxepines and oxepanes are treated in detail. Especial attention is paid to modern cyclization procedures like ring-closing metathesis and transformations of different carbo- and heterocyclic compounds to oxepines and oxepanes, particularly to novel variants of Baeyer–Villiger reaction with cyclohexanones and manifold reactions leading from epoxides to seven-membered O-heterocycles. A separate section is devoted to brief summary of syntheses of particular classes of compounds under discussion. Finally, important compounds and applications of natural and synthetically prepared oxepines and oxepanes are demonstrated.