Studies on a General Method for the Demethylation of Quaternized Nitrogen Heterocycles

Probably the best general method so far developed for the dealkylation of quaternized nitrogen heterocycles is the reaction of the iodide salts in an aprotic solvent in the presence of triphenylphosphine.^1,2 It has since been established³ that the iodide counterion is the active dealkylating agent, with the triphenylphosphine acting as a “methyl sponge”, preventing the reverse reaction from occurring. It follows, therefore, that any heterocycle (1) ought to be able to fulfil the same role as the triphenylphosphine with respect to the dealkylation of the quaternary salt of a less nucleophilic heterocycle (2). Since the reactivity to methylation of a large number of heterocycles with a reactivity range of ca 10⁶ is known⁴, the choice of a convenient heterocycle (1) to go with any heterocycle (2) is, in principle, a wide one.     

Synthesis and mass spectrometry of 4-oxo-4,5,6,7-tetrahydroindole and 4-oxo-4,5,6,7-tetrahydrobenzofuran derivatives

We describe in this communication the synthesis and mass spectrometry of some l-aryl-2-phenyl-4-oxo-4,5,6,7-tetrahydroindole (IIIb, IVab-VIIIab, VIIIb, IXb) and 2-phenyl-4-oxo-4,5,6,7-tetrahydrobenzofuran derivatives (IIab). The fragmentations of these compounds are characterized by selectivity and the absence of any primary fragmentations involving substituents of the aryl rings. It is suggested that positive charges of the molecular ions of compounds II? IX are localized in the region of the carbonyl group and the conjugated double bond of the heterocycle.     

氮磷杂稠环化合物的质谱

有机磷杂环化合物的电子轰击正离子质谱已有报道。本文研究了18个具有植物生长调节活性的氮磷杂稠环有机磷化合物的电子轰击正离子质谱及其断裂机理。 实验采用VG-7070E双聚焦质谱仪和VG11-250数据处理系统,电子轰击源电子能量为70～100eV,离子源温度200℃,固体直接进样,温度100℃,RP=1000。采用B/E联动扫描研究亚稳断裂。试样的结构和纯度已用IR、~1H NMR、~(13)C NMR、~(31)P NMR、X-衍射及元素分析法测定。     

Palladium-catalyzed annulation of aryl heterocycles with strained alkenes

[reaction: see text] An annulation reaction proceeding by the intermolecular addition of an arylpalladium(II) halide across a strained alkene, followed by an intramolecular C-H functionalization of a pendant heterocycle is described. A variety of polycyclic heterocycles have been prepared from readily accessible haloaryl heterocycles by annulation with a range of strained alkene partners.     

Efficient One-Pot Synthesis of Naphthoquinone-Fused Phosphorous Heterocycles via Mannich-Type Reaction

A series of naphthoquinone-fused phosphorus heterocycles was synthesized via a three-component Mannich-type reaction. 2-Chloroethoxylphosphorodichloridite was used as a phosphorus substrate to explore the structure–activity relationships of the 2-position of the heterocycles. One spiral heterocycle compound was structurally characterized by a single-crystal X-ray diffraction analysis.     

New high temperature polymers. VI. Ordered heterocycle copolymers

A new class of linear, thermostable polymers is reported. The compositions are ordered heterocycle copolymers in which two different heterocycles alternate regularly along the polymer chain. Examples of combinations studied are: oxadiazole–benzimidazole, oxadiazole–pyromellitimide, and thiazole–pyromellitimide. The heterocycle copolymers, or alternatively, the corresponding precursor polymers, were prepared by condensing preformed di-or tetrafunctional blocks which contain one type of heterocycle with a second di- or tetrafunctional monomer under such conditions that no rearrangement of bonds occurred. The polymers are characterized in general by neither melting nor decomposing below 500°C. when heated in an inert atmosphere at a rate of about 10°C./min. Some of the copolymers are readily soluble in organic solvents; many, however, are soluble only in solvents such as concentrated sulfuric acid. In the case of the more intractable polymers, soluble precursor polymers can usually be prepared. In such precursor polymers only one of the heterocycles is preformed; the second heterocycle is formed by post-treatment after the polymer has been fabricated into an end product. All of the polymers yielded self-supporting films, some having very high strength; films of several of the polymers were hot-drawable. Drawn film of an ordered oxadiazoleimide copolymer was shown to be well oriented and moderately crystalline.     

4,4,5,5-Tetra(3,5-dibromophenyl)-2,2-diphenyl-1,3-dioxa-2-silole and related compounds as precursors to flame retardant oligomers

Certain strained five-membered heterocycles undergo thermally-induced, carbon-carbon bond homolysis to generate diradicals capable of initiating vinyl polymerization. These compounds usually contain two oxygen atoms and a larger heteroatom (silicon, phosphorus, sulfur) in addition to carbon. If the heterocycle is suitably substituted with halogen (particularly bromine) containing moities it may function as a vehicle to permit incorporation of flame-retarding units into a polymer or oligomer. When an appropriate heterocycle is used as an initiator for radical polymerization each polymer chain formed contains at least one flame-retarding unit. One such heterocycle is 4,4,5,5-tetra(3,5-dibromophenyl)-2,2-diphenyl-1,3-dioxa-2-silole. In instances in which the heterocycle also acts as a monomer, i.e., it is reactive toward chain-propagating radicals, additional flame-retarding activity may be incorporated into the polymer.     

The mass spectra of 2-aryl-4-arylidene-oxazol-5-ones,and the related thiazol-5-ones and imidazol-5-ones

Examination of the mass spectra of a series of substituted oxazol-5-ones and other related heterocycles showed no evidence of electron-impact induced isomerism. The electron-impact fragmentations of these compounds were characterized by a single dominant fragmentation process.     

Copper-diamine-catalyzed N-arylation of pyrroles, pyrazoles, indazoles, imidazoles, and triazoles

This paper details the copper-catalyzed N-arylation of pi-excessive nitrogen heterocycles. The coupling of either aryl iodides or aryl bromides with common nitrogen heterocycles (pyrroles, pyrazoles, indazoles, imidazoles, and triazoles) was successfully performed in good yield with catalysts derived from diamine ligands and CuI. General conditions were found that tolerate functional groups such as aldehydes, ketones, alcohols, primary amines, and nitriles on the aryl halide or heterocycle. Hindered aryl halides or heterocycles were also found to be suitable substrates using the conditions reported herein.     

From Thioureas to Thioquinolines through Isolated Benzothiazines by Gold Catalysis

New benzothiazine heterocycles have been formed from thiourea derivatives by using different gold catalysts. The catalyst and the conditions were optimised towards the selective synthesis of six-membered benzothiazine heterocycles, characterised by X-ray diffraction. Interestingly, these organic compounds evolved under gold catalysis in basic medium to achieve the formation of amino thioquinolines through an unprecedented aromatisation process of the heterocycle. The reaction was also carried out stoichiometrically by reaction with gold complexes to afford thioquinolines coordinated to the gold fragment. Benzothiazine, amino thioquinoline heterocycles and gold-derived species have a great potential for biological applications.     

Preparative Electrochemical Liberation of Arenes and Heterocycles from Their Respective Ironcyclopentadienyl Complexes

Electrolysis of arene and heterocycle cyclopentadienyliron hexafluorophosphates led to the liberation of arenes or heterocycles in 69–90% yield. The procedure described gives yields comparable with or superior to the yields obtained from pyrolytic sublimation used previously.     

Palladium catalyzed ring opening of furans as a route to α,β-unsaturated aldehydes

Furans may be ring opened via pallado-catalyzed reactions leading to α,β-unsaturated aldehydes and ketones tethered to indole and isoquinoline moieties. Besides their synthetic interest, these fragmentations bring interesting elements into the discussion around the reaction mechanisms involved in palladium C-H activations of electron-rich heterocycles.     

Coordination Assisted Distal C−H Alkylation of Fused Heterocycles

Distal C?H bond functionalization of heterocycles remained extremely challenging with covalently attached directing groups (DG). Lack of proper site for DG attachment and inherent catalyst poisoning by heterocycles demand alternate routes for site selective functionalization of their distal C?H bonds. Utilizing non‐productive coordinating property to hold the heterocycle into the cavity of a template system in a host–guest manner, we report distal C?H alkylation (C‐5 of quinoline and thiazole, C‐7 of benzothiazole and benzoxazole) of heterocycles. Upon complexation with heterocyclic substrate, nitrile DG in template directs the metal catalyst towards close vicinity of the specific distal C?H bond of the heterocycles. Our hypothesized pathway has been supported by various X‐ray crystallographically characterized intermediates.     

Photochemistry of quaternary salts of aromatic heterocycles and some of their mesoionic derivatives (Review)

The photochemical methods far the synthesis of quaternary salts of aromatic heterocycles, their photochemical reduction reactions, photoreactions with water, some photochemical transformations that proceed without a change in the charged heterocycle, and photoreactions involving valence isomerization and fragmentation of the mesoionic derivatives are examined.     

Coarctate cyclization reactions: a primer

The cleavage of five-membered heterocycles possessing an exocyclic carbene or nitrene to form conjugated ene-ene-yne systems has been documented for over 40 years; however, the reverse reaction, using a conjugated "ene-ene-yne" precursor to form a heterocycle is a relatively new approach. Over the past decade, the Haley and Herges groups have studied computationally and experimentally the cyclization of the "hetero-ene-ene-yne" motif via an unusual class of concerted reactions known as coarctate reactions. This feature article details our synthetic and mechanistic work involving triazene-arene-alkynes and structurally-related systems to generate heterocycles using coarctate chemistry.     

Intermolecular reactions of electron-rich heterocycles with copper and rhodium carbenoids

This tutorial review describes the reactions of the electron-rich heterocycles pyrrole, furan, indole and benzofuran with copper and rhodium carbenoids. Two main reaction pathways are possible, involving either a concerted non-synchronous cyclopropanation or zwitterionic intermediates. A diverse range of products are possible and the outcome is very dependent on the structure of the heterocycle and the carbenoid. To emphasize this point the carbenoids are considered in terms of three classes: acceptor, acceptor-acceptor and donor-acceptor carbenoids. Unusual catalytic asymmetric transformations can be achieved with this chemistry while the asymmetric induction is strongly influenced by how the carbenoid can approach the heterocycles. This tutorial review gives an overview of the general features that govern the chemistry of metal carbenoids with heterocycles and presents a mechanistic rationale for the range of products that can be formed.     

Strained heterocycles in radical chemistry

Over the last few decades the use of radicals in synthesis has witnessed an explosive growth through introduction of efficient chain and electron-transfer reactions. Strained heterocycles, in particular, have emerged as a highly versatile and readily available class of radical precursors. The generation of carbinyl radicals of heterocycles has resulted in many elegant applications of heteroatom-centered radicals, such as beta fragmentations, cyclizations, and intramolecular hydrogen atom abstractions. Direct electron transfer to strained heterocycles has been realized through the use of arene radical anions. The method combines the virtues of radical and organometallic chemistry to yield useful functionalized organolithium compounds. Epoxides have been opened with high regioselectivity by titanocene(III) reagents in either stoichiometric or catalytic quantities to yield beta-titanoxy radicals. This development has resulted in many new applications in natural product synthesis.     

Regioselective addition of sulfur dichloride and selenium tetrachloride to diallyldimethylsilane as the method of preparation of eight-membered heterocycles containing silicon and chalcogens

The electrophilic additiob of sulfur dichloride and selenium tetrachloride to diallyldimethylsilane proceeds strictly according Markownikoff rule and results in the formation of previously unknown saturated siliconcontaining heterocycles, 5,5-dimethyl-3,7-dichloro-1,5-thiasilacyclooctane and 5,5-dimethyl-1,1,3,7-tetrachloro-1,5-selena(IV)silacyclooctane. The structure of heterocycles obtained was confi rmed by ¹H, ¹³C NMR spectra, in the case of selenium-containing heterocycle, by ⁷⁷Se NMR spectrum, among them the 2D HMBC spectrum.     

C-C bond cleavage: Metal-free-catalyzed reaction of Betti bases with various heterocycles under microwave irradiation

The reaction of Betti bases with various heterocycles in the presence of p-toluenesulphonic acid (PTSA) under microwave irradiation gives bis(heterocycle)methanes through benzyl transfer. The reaction proceeds via the cleavage of C-N bond followed by C-C bond. The metal-free cleavage of C-C bond, which is in fact a C-dearylation, is rarely reported in the literature.     

A comparative study of the reactions of fluorinated oxi- and thiiranes with acyl chlorides

The CoCl₂-catalyzed reactions of fluorinated 1,2-oxi- and thiiranes with acyl chlorides were studied. It was found that a regioselective heterocycle opening reaction resulted in two isomers having normal and abnormal structure in a ratio predetermined by the substituents in both the starting heterocycles and acyl chlorides.