Giancarlo Doddi

Thiopyrylium, Selenopyrylium, and Telluropyrylium Salts

Publisher Summary This chapter deals with thiopyrylium, selenopyrylium, and telluropyrylium salts. These compounds are considered the parent structures of an important class of six-membered heteroaromatic cations. It is observed that the reactivity of thiopyrylium salts resembles that of pyrylium salts with two notable differences— namely, the lesser tendency of 2 H adducts formed on nucleophilic attack to undergo ring opening and the ability of sulfur to accommodate more than eight electrons in its valence shell, leading, in some cases, to the formation of thiabenzene derivatives. The chapter also describes the structural and physical properties, the methods of synthesis, the reactions, and the practical uses of these pyrylium salts. There are large number of patents and applicative studies making use of a chalcogenopyrylium salt. Most applications of chalcogenopyrylium salts exploit their photophysical and photochemical properties, and principally regard photographic and reprographic technologies. Another important application of chalcogenopyrylium salts is as polymerization and crosslinking photoinitiators, especially in the preparation of photoresists, printing plates, and photosensitive compositions for laser imaging.

Synthesis of a ditopic cyclophane based on the cyclobutane ring by chalcone photocycloaddition

The intramolecular photocycloaddition of chalcones to give cyclobutanes has proven to be a fast and simple method to shrink a cyclophane ring to a tricyclic system, in order to prepare potential ditopic receptors. In particular, the chalcone 1, having dioxyethylene chains as spacers, is converted in high yield to the cyclobutane 2. NOESY spectroscopy indicates that the formation of 2 occurs by a head-to-head syn ring closure.

Template Effects and Kinetic Selection in the Self‐Assembly of Crown Ether Cyclobis(paraquat‐p‐phenylene) [2]Catenanes—Effect of the 1,4‐Dioxybenzene and 1,5‐Dioxynaphthalene Units

The template effects exerted by bis(p-phenylene)[34]crown-10 (3) and by 1,5-dinaphto[38]crown-10 (4) in the ring-closure reaction of the trication 2(3+) to yield the [2]catenanes 7(4+) and 8(4+) have been quantitatively evaluated in acetonitrile at 62 degrees C by UV/visible spectroscopy. The rate of ring closure of the trication 2(3+) dramatically increases in the presence of the templates 3 and 4, up to approximately 230 times at [3] approximately equals 0.1 molL(-1), and up to approximately 1,900 times at [4] approximately equals 0.01 molL(-1). The outcome of kinetic selection experiments, in which the two crown ethers compete for the incorporation into the catenane structure, has been discussed in the light of the obtained results. It has been shown that the product ratio of catenanes obeys the Curtin-Hammett principle only if the concentrations of the templates are equal and much greater than that of the substrate. Analysis of the rate profiles has shown that the 1,5-dioxynaphthalene unit, present in the template 4, has a greater affinity than the 1,4-dioxybenzene unit, present in the template 3, for the electron-deficient pyridinium rings present in both the transition-state and substrate structures. Ab initio calculations at the 3-21G and 6-31G(d) levels of theory indicate that the greater affinity of the 1,5-dioxynaphthalene unit cannot be explained on the basis of greater pi-pi stacking and [C-H...pi] interactions, but rather on the basis of the model of apolar complexation in which the solvent plays a major role.

Kinetic and thermodynamic study of the reaction of 2,4,6-triphenylthiopyrylium ion with butylamine and cyclohexylamine in dimethyl sulfoxide

A kinetic and thermodynamic study of the reaction of the 2,4,6-triphenylthiopyrylium ion (1) with butylamine and cyclohexylamine has been performed in Me2SO at 25°C. The reaction involves the initial formation of both the corresponding 2H- and 4H-thiopyrans, which equilibrate to form only the more stable 2H adduct. The kinetic data are consistent with a two-step process wherein the formation of the protonated 2H- and 4H-thiopyran intermediates is the rate-controlling step. A comparison with the addition reaction of 1 with CH3O- shows that the relative stabilities of the 2H and 4H isomers are strongly affected by the charge of the thiopyran derivative. Equilibrium data indicate that the acidity of the ammonium ion is strongly enhanced by the presence of the thiopyranyl moiety.

Remarkable catalysis by strontium ion in SN2 and E2 reactions occurring in proximity to a crown ether structure

Strontium ion catalyses the reactions of the crown ether derivatives 1 and 2 with methoxide ion in methanol at 40 °C to yield the product of substitution 3 and elimination 4, respectively.

Alkylation of 2,6-diphenylpyrylium and 2,6-di-t-butylpyrylium ions by photochemical reaction with tetraalkylstannanes

Abstract 2,6-diphenylpyrylium and 2,6-di- t -butylpyrylium perchlorates are easily converted into a number of the corresponding 4-alkyl derivatives by photochemical reaction with tetraalkylstannanes, in MeCN at room temperature. The mechanism of the process is discussed.

SYNTHESIS AND CONFORMATIONAL ASPECTS OF CORANDS INCORPORATING PYRYLIUM, THIOPYRYLIUM AND PYRIDINE SUBUNITS

Abstract The pyrylium corands 1a , 1b and the thiopyrylium corands 2a , 2b , were prepared in good yields by heterocyclization of the macrocyclic 1,5-pentanediones 4a , 4b , whereas the pyridine corands 3a , 3b were obtained by reacting 1a and 1b , respectively, with ammonium acetate. NMR data and molecular mechanics calculations suggest that the heteroaromatic nuclei tend to fill the cavity of the macrorings.

Effect of alkali cations on the methoxide ion addition to corands incorporating a thiopyrylium subunit

Abstract The incorporation of a thiopyrylium ring in a corand structure forms a system with a weak binding ability in methanol towards the alkali cations K + , Rb + and Cs + . The binding substantially increases in basic solution because of the reversible addition of methoxide ion to the heteroaromatic ring to form a neutral 4 H adduct. The effect of the length of the polyoxyethylene chain and of the presence of a bulky group ( tert -butyl) in the γ position of the heteroaromatic ring has been quantitatively investigated. Results obtained from molecular mechanics calculations were used to rationalise the depressing effect of the γ tert -butyl group on the ligand ability of thiopyrans.

The First Homolytic Substitution of Pyrylium Salts. C-4 Methylation of 2,6-Disubstituted Pyrylium Cations

Abstract 2,6-Diphenylpyrylium and 2,6-di-t-butylpyrylium cations react with methyl radical generated by ButOOH-Fe+2 in acidic aqueous acetonitrile, to yield substitution exclusively at C-4 position.

Template Effect in the Self-Assembly of a [2]Rotaxane Containing Diazapyrenium Units

The template effect of 1,5-Bis[2-(2-triisopropyloxyethoxy)ethoxy]naphthalene (5) on the ring closure reaction of 2-(4-Bromomethylbenzyl)-7-[4-(2,7-diazapyrenyl)-methylbenzyl]-2,7-diazapyrenium (3), to give the corresponding [2]rotaxane (7), has been quantitatively evaluated. The results are compared to those relative to the self-assembly of [2]rotaxane (8), and [2]pseudorotaxanes (9) and (10).