Carboranylpyrans and carboranylpyrylium salts

Relatively difficult-to-oxidize 4-carboranylpyrans were obtained by the action of (phenyl-o-carboranyl)lithium on 2,6-disubstituted pyrylium salts. The conversion of the 4-carboranyl-pyrans to the corresponding 4-carboranylpyrylium salts is achieved by the action of a one-electron oxidizing agent-tris(p-bromophenyl)aminium hexachloroantimonate. The 4-(o-car-boranyl)pyrylium salts readily undergo one-electron electrochemical reduction. It is assumed that the first step in dehydrogenation of the pyrans is one-electron transfer.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1172–1175, September 1977.     

Cyclic transmembrane charge transport mediated by pyrylium and thiopyrylium ions

Transient spectroscopy revealed that 2,4,6-trimethylpyrylium, 2,4,6-triphenylpyrylium, and 2,4,6-triphenylthiopyrylium ions oxidatively quench excited triplet [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) to form the corresponding neutral radicals and the zinc porphyrin pi-cation. The measured quenching rate constants were proportional to the pyrylium one-electron reduction potentials, that is, the reaction driving force. In the presence of anionic dihexadecyl phosphate vesicles, only the fraction of pyrylium not bound to the vesicle was capable of reacting with the photoexcited zinc porphyrin. Nonetheless, the pyrylium radicals mediated highly efficient transmembrane reduction of tris(2,2'-bipyridine)cobalt(III) contained within the inner aqueous core of the vesicles with apparent quantum yields that approached unity. Permeability coefficients (P) determined for the pyrylium radicals, pyrylium cations, and the proton were 10(-4)-2 x 10(-5) cm/s, 10(-10) cm/s, and < 5 x 10(-7) cm/s, respectively, so that only the neutral radicals are membrane-permeable on the time scale of the transmembrane redox reactions. However, each electron carrier was demonstrated to transport up to 200 electrons, at which point the internal pool of electron acceptors was exhausted. Since the cations are membrane-impermeable, a reaction cycle is proposed that includes hydrolysis of the pyrylium cations formed within the aqueous core to the corresponding 1,5-diketones which, as neutral molecules, can diffuse across the bilayer. According to this mechanism, while undergoing redox cycling the pyrylium ions function as cyclical antiporters of OH(-) and the electron, thereby maintaining electroneutrality in the reaction compartments.     

2-Ferrocenyl-substituted pyrylium ions from coupling of ethynylferrocene with cyclomanganated chalcones

2-Ferrocenyl-substituted pyrylium salts are produced when orthomanganated chalcones are reacted with ethynylferrocene in CCl₄. When the reaction is carried out in benzene, intermediate ferrocenyl-substituted (η⁵-pyranyl)Mn(CO)₃ species can be isolated which give the pyrylium cations on oxidation. The electrochemistry of the 2-ferrocenyl-pyrylium cations shows both oxidation (of the ferrocenyl) and reduction (of the pyrylium) processes, and the UV-visible spectra show a broad band at ca 680 nm which can be assigned to an intramolecular charge transfer transition.     

Pyrylium Mediated Transformations of Primary Amino Groups into Other Functional Groups. New Synthetic Methods (41)

The first pyrylium salt was isolated some 80 years ago, yet up to the 1950s only moderate interest was taken in the preparation, properties and uses of such salts. However, the past thirty years has seen a phenomenal growth in the literature pertinent to this area of chemistry: the importance of pyrylium salts as intermediates has been realized. They are readily prepared by a variety of generally applicable routes, and they are highly reactive towards nucleophiles. Together, this enables the convenient synthesis of a great variety of acyclic and heterocyclic compounds. We have used highly substituted pyrylium salts for the two-step conversion of the amino group in alkylamines RNH₂ into numerous other functionalities. In the first step, the pyrylium salts are converted with the amines into N-substituted pyridinium salts, which, in the second step, react with Nu^? to give the desired products RNu. In some cases the R moiety is also changed, e.g. by elimination. Studies of the reactions of these pyridinium salts have allowed interesting insights into the mechanisms of nucleophilic substitution, in addition to rendering aliphatic amines important synthetic intermediates. Thus, the method complements the diazotization procedure for the transformation of arylamines.     

A versatile synthesis of benzothieno-annelated 1,2-dihydropyridine and 1,2,3,4-tetrahydropyridine derivatives: the effect of the structure of benzothieno-annelated pyridinium salts on their reduction by sodium borohydride

Abstract  

Benzothieno[2,3-c]pyridinium and benzothieno[2,3-c]quinolinium salts were synthesized either by quaternization of benzothieno[2,3-c]pyridines, or recyclization of benzothieno[2,3-c]pyrylium salts with primary amines. One-pot synthesis of benzothieno[3,2-g]indolizinium salts from 1-(3-chloropropyl)-benzothieno[2,3-c]pyrylium included consequent recyclization of the pyrylium core by ammonia into a pyridine intermediate and its further intramolecular quaternization reaction. Depending on the structure of benzothieno-annelated pyridinium salts, their reaction with sodium borohydride in methanol results in reduction of the pyridine core into tetrahydropyridine or dihydropyridine derivatives. Whereas reduction of benzothieno[2,3-c]pyridinium and benzothieno[3,2-g]indolizinium salts readily yields benzothieno-annelated tetrahydropyridines as a complex mixture of stereoisomers, reduction of benzothieno[2,3-c]quinolinium salts results in dihydropyridine derivatives. The structure of the latter, in particular, was confirmed by single-crystal X-ray diffraction analysis.     

Polymers by the reaction of bis(pyrylium salts) with diamines: A novel approach to ionene polymers

The following reactions of pyrylium salts with amines are described: (1)bis(pyrylium salts) with amines; (2) diamines with pyrylium salts; and (3) bis(pyrylium salts) with diamines. Both (1) and (2) give bis(pyridinium salts) in high yields, and (3) gives the corresponding polymers which are isolated and characterized. This procedure was applied to cationic bis(pyrylium salts) to give cationic dimers and polymers, and further to zwitterionic bis(pyrylium salts) to yield the corresponding zwitterionic dimers and polymers.     

Electrochemical reduction of pyrylium salts in an aprotic medium

The electrochemical reduction of pyrylium salts in anhydrous acetonitrile is not complicated by adsorption and may serve as a convenient method for the evaluation of the effect of structural factors on the ability of pyrylium cations to undergo conversion to the corresponding pyranyl radicals and the ability of the latter to undergo dimerization. The stepwise two-electron reduction of conjugated dipyrylium salts leads successively to cation radicals and dipyranylidene structures.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 318–321, March, 1977.     

The preparation of 4-substituted pyrylium salts and their use in dienal synthesis

The unsubstituted pyrylium nucleus is shown to undergo reaction with cuprate or secondary Grignard organometallic reagents to give intermediate 4-substituted pyrans which are converted into the corresponding 4-substituted pyrylium salts (7 examples) in fair to good overall yield. The synthetic utility of the 4-substituted pyrylium heterocycles is demonstrated by their reaction with organolithium reagents to give 3,5-disubstituted dienals in a highly stereospecific manner (7 examples) via electrocyclic ring opening of the intermediate 2-substituted pyrans.     

Étude voltamétrique des sels de styryl-2 et styryl-4 pyrylium. 1re partie

Voltammetric study of styryl-2 and styryl-4 pyrylium salts. Part 1. The redox properties of diphenyl-2,6 styryl-4 and diphenyl-4,6 stryl-2 pyrylium salts have been studied by means of cyclic voltammetry. These salts disclose irreversible reduction and oxidation steps that are characterised by one reduction peak and two oxidation peaks, respectively. The second oxidation peak vanishes when the switching potential is −0.75 V. The influence of water on the electrochemical behaviour of these salts as well as their degradation over several months has been studied.     

Over one century after discovery: pyrylium salt chemistry emerging as a powerful approach for the construction of complex macrocycles and metallo-supramolecules

Over one century after its discovery, pyrylium salt chemistry has been extensively applied in preparing light emitters, photocatalysts, and sensitizers. In most of these studies, pyrylium salts acted as versatile precursors for the preparation of small molecules (such as furan, pyridines, phosphines, pyridinium salts, thiopyryliums and betaine dyes) and poly(pyridinium salt)s. In recent decades, pyrylium salt chemistry has emerged as a powerful approach for constructing complex macrocycles and metallo-supramolecules. In this perspective, we attempt to summarize the representative efforts of synthesizing and self-assembling large, complex architectures using pyrylium salt chemistry. We believe that this perspective not only highlights the recent achievements in pyrylium salt chemistry, but also inspires us to revisit this chemistry to design and construct macrocycles and metallo-supramolecules with increasing complexity and desired function.

This perspective summarizes the representative efforts of synthesizing and self-assembling large, complex architectures using pyrylium salt chemistry.     

Free Radicals: XXVII. Synthesis of 2,6-Diphenyl-4-[p-(2,6-diphenyl-4-pyridyl)phenyl]pyranyl

Starting with 2,6-diphenyl-4-(p-tolyl)pyridine a synthesis of 2,6-diphenyl-4-[p-(2,6-diphenyl-4-pyridyl)phenyl]pyrylium perchlorate was performed. By reduction of the latter the corresponding radical was obtained. The radical was shown to be unstable with respect to oxygen.     

Pyrylium,pyridine, and N-phenylpyridinium derivatives of cyclopentadienyltricarbonylmanganese

2,4,6-Trisubstituted pyrylium salts containing a cymantrenyl substituent in the 6 position were synthesized by condensation of acetylcymantrene with chalcones. As compared with the corresponding pyrylium derivatives of ferrocene, the pyrylium salts obtained exchange a heteroatom more readily and are more stable. The pyrylium derivatives of cymantrene were converted to the corresponding pyridines by the action of ammonium acetate in glacial acetic acid or an aqueous solution of ammonia and to the corresponding N-phenylpyridinium salts by the action of aniline in acetic acid or alcohol.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 21–24, January, 1979.     

Synthesis of N-methylpentafluorophenylpyridinium salts

Di- and triaryl-N-methylpyridinium perchlorates bearing pentafluorophenyl groups have been obtained by reaction of the appropriate pyrylium salts with methylamine. In the case of 4-phenyl-2,6-bis(pentafluorophenyl)pyrylium perchlorate, a stable 2H-pyran intermediate was isolated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1100–1103, August, 1991.     

Reaction of methyl-substituted pyrylium salts with tertiary amines

2,4,6-Trimethylpyrylium perchlorate reacts with heterocyclic compounds containing apyridine nitrogen atom and having basicities higher than 9 pK_a units (in acetonitrile) through a step involving the formation of a methylenepyran. 4-Methyl-2,6-diphenyl- and 2-methyl-4,6-diphenylpyrylium perchlorates react with benzimidazole to give 1,2-ethanediylidenebispyrans. Methyl-substituted pyrylium salts react with 2,6-diphenylpyrylium and flavylium perchlorates in the presence of benzimidazole to give methylidynecyanines and with acetic anhydride to give trimethylidynecyanines.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1484–1489, November, 1976.     

Synthesis and spectral properties of furyl-substituted pyridines and pyrylium and pyridinium salts

2,6-Diphenyl-4-(5-R-2-furyl) pyrylium and 2,4-diphenyl-6-(5-bromo-2-furyl) pyrylium perchlorates and the corresponding pyridines and 1-methylpyridinium perchlorates were synthesized. On the basis of the UV and PMR spectra it was concluded that there is significant electronic interaction of the furylium substituent with the pyrylium and pyridinium cations.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1320–1324, October, 1988.     

Donor-functionalized polydentate pyrylium salts and phosphinines: synthesis, structural characterization, and photophysical properties

A series of donor-functionalized pyrylium salts have been prepared by classical condensation reactions which were further converted into the corresponding thienyl- and pyridyl-substituted polydentate lambda(3)-phosphinines by reaction with P(SiMe(3))(3). Further chemical modification of these phosphorus heterocycles with Hg(OAc)(2) in the presence of methanol resulted in the formation of lambda(5)-phosphinines. The photophysical properties of a selected series of thienyl- and pyridyl-functionalized pyrylium salts, lambda(3)- and lambda(5)-phosphinines, were investigated and the results compared and supported by theoretical calculations on the DFT level. Significant fluorescence was observed for the pyrylium salts and lambda(5)-phosphinines. In contrast, the heteroaromatic substituted lambda(3)-phosphinines show very little emission which is consistent with the low oscillator strength predicted by DFT calculations for this pi-->pi* transition. Furthermore, all three classes of compounds show readily observable phosphorescence in solution, which was determined by time-gated detection at low temperature.     

Functional molecules from single wall carbon nanotubes. Photoinduced solubility of short single wall carbon nanotube residues by covalent anchoring of 2,4,6-triarylpyrylium units

Raw, micrometric HiPCO single wall carbon nanotube (SWNT) material was submitted to harsh acid oxidative treatment with a 3:1 H2SO4/HNO3 mixture to give short residues of SWNT (s-SWNT, <200 nm length measured by TEM). s-SWNT was functionalized through the tip carboxylic groups by peptide bonds using 3-mercatopropanamine linkers that subsequently were reacted with 2,6-diphenyl-4-(4-vinylbiphenyl)pyrylium using azobis(isobutyronitrile) as a radical initiator. After purification by dialysis, the resulting s-SWNT having covalently linked through an ethylthiopropylamide tether the strong electron-transfer pyrylium photosensitizer (Py-sSWNT) was characterized by solution 1H NMR spectroscopy (observation of specific signals due to the heterocyclic protons). Emission spectroscopy shows that the fluorescence of 2,6-diphenyl-4-(4-dodecylthiobiphenyl)pyrylium (Py-SC12) tetrafluoroborate (a model compound to the tethered pyrylium moiety in Py-sSWNT) (lambdaem 533 nm) is quenched by s-SWNT and vice versa that the emission of s-SWNT (lambdaem 330 nm) is quenched by Py-SC12. Depending on the excitation wavelength, Py-sSWNT exhibits dual emission corresponding to each of the two moieties, but with much less intensity than each of the model components independently. Laser flash photolysis of model Py-SC12 allows detection of the triplet (lambdaT-T 750 nm, tau 11.7 micros) and the much longer-lived pyrylium centered radical (lambdamax 525 nm, tau 147 mus). The latter species arises from photoinduced electron transfer from the sulfur atom, as the donor, to the pyrylium heterocycle in its electronic excited-state, as the electron acceptor. Laser flash photolysis (355 nm) of Py-sSWNT also allows detection of the pyrylium centered radical together with a broad absorption spanning from 200 to 500 nm and peaking at 280 nm. The latter band is absent in the laser flash photolysis of the model s-SWNT and was attributed to the electron hole localized on the nanotube moiety of Py-SWNT. The most remarkable effect of the steady-state irradiation is a 1 order of magnitude increase in the solubility of Py-sSWNT. According to TEM images this photoinduced solubility can be attributed to the debundling of the nanotubes due to photoinduced charge separation through the nanotube walls. In addition to exemplify how molecular compounds with photoresponsive properties can be derived from SWNT materials, the observation of photoinduced solubility can serve to develop SWNT layers suitable for photolithography patterning.     

Reaction of indolylpyrylium salts with some hydrazine derivatives

The reaction of 4-(3-indolyl)pyrylium salts with phenylhydrazine in alcohol gives 5-(3-indolyl)pyrazoles. The pyrylium salts react with acid hydrazides in refluxing dimethylformamide to give monohydrazones of the pseudobases of these salts (unsaturated 1,5-diketones).Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 184–186, February, 1972.     

Ring transformations of heterocyclic compounds. XVI. Spiro[cyclohexadiene-dihydroacridines]. A novel class of spirodihydroacridines by ring transformation of pyrylium salts with 9-methylacridine and its quaternary salts

2,4,6-Triarylpyrylium salts 1 react with the in situ generated anhydrobase of 9,10-dimethylacridinium methosulfate ( 2a ) in the presence of anhydrous sodium acetate in ethanol by a 2,5-[C₄+C₂] pyrylium ring transformation to give the hitherto unknown 6-aroyl-3,5-diaryl-10′-methylspiro[cyclohexa-2,4-diene-1,9′-9′,10′-dihydro-acridines] 3 . When the pyrylium perchlorate 1a is treated under the same conditions with the N-ethyl, N-allyl or N-benzyl substituted acridinium salts 2b-d a dealkylation of these salts occurs and the N-unsubstituted spiro[cyclohexadiene-dihydroacridine] 4a is formed. The same compounds 4 can also be obtained by transformation of the pyrylium salts 1 with 9-methylacridine ( 7 ) and triefhylamine/acetic acid in ethanol. Structure elucidation is performed by an X-ray crystal structure determination of the spiro[cyclohexadiene-dihydroacridine] 3a . Spectroscopic data of the transformation products and their mode of formation are discussed.     

取代氧翁盐化合物的分子内电荷转移和光谱行为的研究

合成了一组带不同取代基的2,4,6-三芳基氧翁盐化合物, 对它们在不同溶剂中的光物理行为进行了研究表明: 取代基的性质(推或拉电子)和位置(X轴或Y轴)对化合物的发光有很大影响。本工作还利用荧光去偏振方法研究了氧翁盐化合物分子内的能量转移以及通过加入三氟醋酸使叔胺季铵盐化方法, 为这类化合物提供了在非水溶剂中作为荧光指示剂应用的可能性。