Molecular electrochemistry of multi-redox functionalized 5-membered heterocycles

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Reactions of diazo derivatives of 5-membered heterocycles with hydrazines

Interaction of hydrazine or dimethylhydrazine with 5-diazoimidazole-4-carboxamide produced, presumably via tetrazenes, high yields of 5-azidoimidazole-4-carboxamide. In contrast, when semicarbazide reacted with either the diazoimidazole or the analogous diazo-1,2,3-triazole, the yield of the azidoheterocycle was low, and the yield of biurea, and presumably the aminoheterocycle, was high. 3-Azidopyrazole-4-carboxamide was obtained from a reaction of hydrazine with the diazopyrazole. Both azido and amino derivatives were formed from reactions of diazo-imidazole esters with hydrazine or thiosemicarbazide.     

Three-component strategy toward 5-membered heterocycles from isocyanide dibromides

A three-component strategy starting from isocyanides allows a straightforward synthesis of five-membered ring heterocycles. New cascades were developed involving the addition of a nitrogenated nucleophile-an azide or a tetrazole-on isocyanide dibromides, an electrocyclization, and a Suzuki coupling, which afford new accesses to tetrazole and triazole scaffolds.     

Synthesis of 5-and 6-membered carbon-boron heterocycles by pyrolysis of pyridine-arylboranes

Pyrolysis of pyridine(2-benzylphenyl)borane and pyridine(2-benzhydrylphenyl) borane yielded the 9,10-dihydro-9-boraanthracene system. The ring-closed products were isolated as the ethanolamine esters. An analogou? conversion of pyridine-2-biphenylylborane gave pyridine-9-borafluorene.     

Formation of new organoselenium heterocycles and ring reduction of 10-membered heterocycles into seven-membered heterocycles

Reacting 2,4-bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide [{PhP(Se)(μ-Se)}₂], Woollins’ reagent, with an equivalent of aromatic diol in refluxing toluene afforded 10-membered phosphorus-selenium heterocycles 1 and 2 with an O-P(Se)-Se-Se-P(Se)-O linkage. Two equivalents of aromatic diol and Woollins’ reagent in refluxing toluene gave seven-membered phosphorus-selenium heterocycles 3 and 4 with an O-P(Se)-O linkage together with 10-membered phosphorus-selenium heterocycles 1 and 2. It was also found that the diphosphorus species O-P(Se)-Se-Se-P(Se)-O rings 1 and 2 could be readily ring contracted into the monophosphorus rings 3 and 4 in almost quantitative yields via further reaction with another equivalent of corresponding aromatic diol. One representative X-ray structure is reported.     

Palladium-catalyzed arylation of electron-rich heterocycles with aryl chlorides

[reaction: see text] Palladium-catalyzed C-H activation: cheap aryl chlorides can now be used for the arylation of a wide variety of electron-rich heterocycles. The key to the success of this reaction is the use of a bulky, electron-rich phosphine ligand. No copper additives are needed.     

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4–8 transition metal catalysts

In this Perspective, we discuss recent syntheses of 5- and 6-membered aromatic heterocycles via multicomponent reactions (MCRs) that are catalyzed by group 4–8 transition metals. These MCRs can be categorized based on the substrate components used to generate the cyclized product, as well as on common mechanistic features between the catalyst systems. These particular groupings are intended to highlight mechanistic and strategic similarities between otherwise disparate transition metals and to encourage future work exploring related systems with otherwise-overlooked elements. Importantly, in many cases these early- to mid-transition metal catalysts have been shown to be as effective for heterocycle syntheses as the later (and more commonly implemented) group 9–11 metals.

In this Perspective, we discuss recent syntheses of 5- and 6-membered aromatic heterocycles via multicomponent reactions (MCRs) catalyzed by group 4–8 transition metals, with a focus on common mechanisms and synthetic strategies across the series.     

Vinylation of 5-membered heterocycles by ethylene in the presence of palladium(II)

The reaction between ethylene and 5-membered heterocyclic compounds (thiophene, furan, furfural) in solution in the presence of a palladium salt at 25–100°C and atmospheric pressure is found to give good yields of 2-vinyl derivatives of the heterocycles indicated.

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5- (, , ) 25–100° 2- .

     

Low energy excited singlet states of some monophenyl substituted 5-membered heterocycles and their photoisomerization

The two low-lying singlet excited states of 14 mono-phenylated 5-membered heterocycles have been studied with respect to their energy level ordering, the oscillator strengths of the pertinent transitions, the state wave functions and the one-particle density matrices. Two types of lowest energy singlet states could be revealed by means of the configuration analysis (CA) and the similarity analysis (SA). One state (labelled “state A”) is characterized by a delocalization over the whole π system, whereas the other (labelled with “state B”) is locally excited at the phenyl moiety. An attempt is made to link this classification with the different photoisomerization pathways of the title compounds.     

Synthesis, reactivity and stereochemistry of new phosphorus heterocycles with 5- or 6-membered rings

Syntheses of novel phosphorus heterocycles containing α-amino or α-hydroxyphosphonic or phosphinic acids motifs are developed. 2,3-dihydro-1,3-oxaphospholes (1) and 1,4,2-oxazaphosphinanes (2) exhibit a reactive part, respectively the enolether moiety and the P-H bond, which allows various structural modifications: (i) for 1a, by introduction of amino substituents, (ii) for 2a, by hydroxy- or aminoalkylation, by Michael addition or by P-arylation. These reactions present generally a good or even an excellent kinetic diastereoselectivity which can often be predicted by molecular models of the transition states.     

Synthesis of heterocycles from products of the anion arylation of unsaturated compounds. 3. 2-arylimino-5-arylmethyl-4-thiazolidones

The reaction of esters of 3-aryl-2-bromopropionic acids with N-arylthioureas gives 2-arylimino-5-arylmethyl-4-thiazolidones, which exist in solution as (E) and (Z) isomers of the imino form.For Communication 2, see [1].I. Franko Lvov State University, Lvov 290602, Ukraine. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 555–559, April, 1998.     

A palladium-catalyzed alkylation/direct arylation synthesis of nitrogen-containing heterocycles

A norbornene-mediated palladium-catalyzed sequence is described in which an alkyl-aryl bond and an aryl-heteroaryl bond are formed in one reaction vessel. The aryl-heteroaryl bond-forming step occurs via a direct arylation reaction. A number of six-, seven-, and eight-membered ring-annulated indoles, pyrroles, pyrazoles, and azaindoles were synthesized from the corresponding bromoalkyl azole and an aryl iodide.     

Oxidative coupling of 5-membered heterocycles catalyzed by a Pd(II)-heteropolyacid system in dimethylformamide

Oxidative coupling of thiophene and 2-methylfuran to the corresponding dihetaryls in DMFA medium occurs in the presence of the homogeneous catalytic system Pd(II)-molybdovanodophosphoric heteropolyacid (HPA). The data obtained show that HPA may act as a reversible oxidant in polar aprotic media.

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, Pd(II)- 2- . , .

     

Stereoselective template-directed C-glycosidation. Synthesis of 5-membered oxygen heterocycles via cation-mediated intramolecular cyclization reactions

Construction of 5-membered oxygen heterocycles by intramolecular template-directed C-glycosidation is described. Alkylation of thioglycoside 5 with enone 6 followed by enol etherification gave cyclization substrates 8. Compound 8a underwent silver(I) trifluoromethanesulfonate-mediated ring-closure to give a 1.7:1 mixture of 9 and 9β; isomer 8s gave an 8.5:1 ratio. Some derivatization reactions of the bicyclic products are described.     

Vilsmeier formylation of 2-methyl-3 -carbethoxy-5-methoxy- and 2-methyl-5-methoxyindoles

The corresponding 6- and 3-formyl derivatives were obtained by formylation of 2-methyl-3-carbethoxy-5-methoxy- and 2-methyl-5-methoxyindole derivatives.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 339–341, March, 1971.     

Nickel-catalyzed sp2 C-H bonds arylation of N-aromatic heterocycles with Grignard reagents at room temperature

A novel protocol for nickel-catalyzed direct sp(2) C-H bond arylation of purines has been developed. This new reaction proceeded efficiently at room temperature using Grignard reagent as the coupling partner within 5 hours in good to high yields. This approach provides a new access to a variety of C8-arylpurines which are potentially of great importance in medicinal chemistry.