L. I. Belen'kii

Substrate and positional selectivity in electrophilic substitution reactions in pyrrole, furan, thiophene, and selenophene derivatives and related benzoannelated systems

Data on the relative reactivities (substrate selectivity) of five-membered heterocycles in electrophilic substitution reactions and positional selectivity (α : β ratio) in these reactions were analyzed. Unlike the substrate selectivity (pyrrole ≫ furan > selenophene > thiophene) determined by the position of heteroatoms in the Periodic Table, the positional selectivity decreases in the order corresponding to the change in the relative stability of the onium states of the elements (O+ < Se+ ≤ S+ < N+) and reflects the predominant role of heteroatoms in the stabilization of σ complexes formed upon β-substitution. These differences in the positional selectivity of the parent heterocycles have a substantial effect on the orientation in electrophilic substitution reactions in their derivatives and the corresponding benzoannelated systems. This interpretation was confirmed by ab initio quantum chemical calculations (RHF/6–31G(d) and MP2/6– 31G(d)//RHF/6–31G(d)) and density functional theory calculations (B3LYP/6–31G(d)). Quantum chemical calculations were performed by the above-mentioned methods for model N-R-pyrroles (R = Me, Et, Pri, But, CH=CH2, C≡CH, Ph, PhSO2, and 4-O2NC6H4) and their α- and β-protonated σ complexes. The results of these calculations demonstrated that it is the steric factors and charges on the β-C, α-C, and N atoms and the substituents at the N atom (the kinetic control), as well as the nature of the electrophile, rather than the difference in the relative stabilities of the onium states of N+ (which depends on the nature of the substituent at the N atom and reflects the role of the heteroatom in stabilization of σ complexes formed via β-substitution; the thermodynamic control) that are responsible for the type of orientation (α or β) that prevails.

Photochromic Dihetarylethenes: XVII. * New Synthesis of Photochromic N-Alkyldithienylmaleimides **

A new convenient procedure for the synthesis of photochromic N-alkyldithienylmaleimides was developed on the basis of the reaction of 3,4-bis(2,5-dimethyl-3-thienyl)furan-2,5-dione with primary amines. Photochromic properties of the products were examined.

Substrate and Positional Selectivity in Electrophilic Substitution Reactions of Pyrrole, Furan, Thiophene, and Selenophene Derivatives

The relative reactivity (substrate selectivity) of five-membered heterocycles on electrophilic substitution (pyrrole >> furan > selenophene > thiophene) and their positional selectivity (furan > selenophene ≥ thiophene > pyrrole) are not consistent. The indicated differences in positional selectivity (α : β ratios) of the parent heterocycles show up essentially in orientation on electrophilic substitution reactions of their derivatives and the corresponding benzannelated systems. It was shown that the positional selectivity was reduced in a sequence corresponding to the change in relative stability of the onium states of the elements (O+ < Se+ ≤ S+ < N+) and reflects the predominant role of the heteroatoms in stabilizing the σ-complexes formed on β-substitution, in which the positive charge is distributed between the heteroatom and one α-carbon atom (in α -isom ers four atoms of the ring participate in delocalization of the charge). This interpretation was confirmed by quantum-chemical calculations carried out by the RHF/6-31G(d), MP2/6-31G(d)//RHF(6)-31G(d), and B3LYP/6-31G(d) ab initio methods.

The Literature of Heterocyclic Chemistry, Part IV

Publisher Summary This chapter discusses the survey done on the literature of heterocyclic chemistry. It includes monographs and is based mainly on short bibliographic papers published. Sources are not only in English, but also in Russian, German, Japanese, Chinese, French, Czech, Polish, and other languages are surveyed and classified. The chapter discusses about carbohydrates as it serves 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

A New Approach to the Synthesis of Dithienylethanediones and Dithienylacetylenes

Abstract1,2-Dithienylethanediones were synthesized by modified acylation of thiophene and its homologs in the presence of aluminum chloride and pyridine. The bishydrazones of the products were converted into dithienylacetylenes by oxidation with atmospheric oxygen in pyridine in the presence of CuCl.

The Literature of Heterocyclic Chemistry, Part V

Publisher Summary This chapter surveys the advances in heterocyclic chemistry. The structure, reactivity, and synthesis of specialized, natural, and synthetic biologically active heterocycles, such as nitrogen heterocycles and oxygen heterocycles, are described in the chapter. There are various kinds of heterocyclic rings: (1) three-membered rings, (2) four-membered rings, (3) five-membered rings, (4) six-membered rings, and (5) rings with more than six members. The three-membered rings consist of one heteroatom, two heteroatoms, and three heteroatoms; whereas, four-membered rings consist of one heteroatom and two heteroatoms. Five and six-membered rings comprise one heteroatom, two heteroatoms, three heteroatoms, and four heteroatoms. The heterocycle groups that contain unusual heteroatoms are— namely, group V element heterocycles, boron heterocycles, or selenium and tellurium heterocycles. The chemistry of individual classes of these heterocycles and their structure, stereochemistry, reactivity, and synthesis is also discussed in the chapter.

A new redox system: Trichloromethylarene — pyridine base. On the mechanism of the synthesis of N-(4-pyridyl)pyridinium dichloride

Abstract A redox reaction of trichloromethylarenes with pyridines results in respective N-(α-chloroarylmethyl)-substituted pyridinium chlorides which give on hydrolysis aromatic aldehydes and 4-chloropyridines or 1,4′-bipyridinium salts.

Novel synthesis of 2-arylbenzothiazoles

A new method for the synthesis of 2-arylbenzothiazoles by the reaction of dibenzyl disulfides witho-aminothiophenol is suggested. A possible mechanism of the new transformation is discussed. 2-Phenylbenzothiazole can also be prepared by reactions ofo-aminothiophenol with benzyl mercaptane or sodium benzyl thiosulfate.

Positional Selectivity in Electrophilic Substitution in π-Excessive Heteroaromatics

Publisher Summary Electrophilic substitution is regarded as an important type of reactions, for five-membered heterocycles, with one heteroatom. It enables compounds, with various substituents, to be obtained. This chapter discusses certain features of the substrate and positional selectivities in electrophilic substitution reactions of derivatives of pyrrole, furan, thiophene, and selenophene, along with the corresponding benzannulated systems. The effect of a heteroatom is displayed in an increased reactivity of α-positions that is usually interpreted to be the result of the higher stability of the corresponding σ-complex due to better charge delocalization, compared with its isomer formed on attack at a β-position. Many features of the reactions of pyrrole, furan, thiophene, and their derivatives, bearing electron-releasing substituents, with electrophiles, are governed by their ready formation and the high stability of corresponding 2H-hetarenium ions— namely, α-C-protonation products like σ-complex. The reversible formation of σ-complexes competes with acid-catalyzed oligomerization that is practically irreversible.

A convenient method for the preparation ofN-unsubstituted hydrazones of aromatic ketones and aldehydes

A general and useful method for the synthesis ofN-unsubstituted hydrazones of aromatic ketones and aldehydes in good yields was elaborated. The use of a large excess of hydrazine hydrate and catalytic amounts ofp-toluenesulfonic acid makes it possible to prepare the hydrazones without an admixture of the corresponding azine.