Azirine-triazole hybrids: selective synthesis of 5-(2H-azirin-2-yl)-, 5-(1H-pyrrol-2-yl)-1H-1,2,3-triazoles and 2-(5-(2H-azirin-2-yl)-1H-1,2,3-triazol-1-yl)pyridines.

Azirine-triazole hybrids, 1-R-5-(3-aryl-2H-azirin-2-yl)-1H-1,2,3-triazoles, were selectively synthesized by reaction of 1-(3-aryl-2H-azirin-2-yl)-2-(triphenylphosphoranylidene)ethanones with tosyl and (E)-2-benzoylvinyl azides in high yields at rt. The reaction with 2-azidopyridine makes it possible to obtain azirine-triazole-pyridine hybrids, albeit in moderate yields, at 170 °C. The mechanism and selectivity of the reaction of α-carbonylphosphoranes with azides are discussed on the basis of DFT calculations. According to the calculation, the reaction of α-carbonylphosphoranes with model mesyl azide, leading to 1,5-disubstituted triazoles proceeds via a non-concerted cycloaddition, while the reaction leading to 1,4-disubstituted triazoles proceeds via a concerted azide cycloaddition, but through the transition state which has much higher energy. In contrast to the reaction of α-(triphenylphosphoranylidene)ketones with TsN3, the reaction with TfN3 yields the α-diazo ketones. Ni-Catalyzed reaction of azirinyl-1,2,3-triazoles with acetylacetone provides pyrrole-triazole and pyrrole-triazole-pyridine hybrids in good yields under mild conditions.

Nucleophile-Induced Rearrangement of 2H-Azirine-2-carbonyl Azides to 2-(1H-Tetrazol-1-yl)acetic Acid Derivatives.

2H-Azirine-2-carbonyl azides undergo a rearrangement into derivatives of 2-(1H-tetrazol-1-yl)acetic acid when interacting with O- and S-nucleophiles at room temperature. The reaction is catalyzed by tertiary amines or hydrazoic acid. The reaction with primary alcohols and phenols gives alkyl/aryl 2-(1H-tetrazol-1-yl)acetates. Thiophenols react with 2H-azirine-2-carbonyl azides to afford S-aryl 2-(1H-tetrazol-1-yl)ethanethioates. The mechanism of the nucleophile-induced rearrangement of 2H-azirine-2-carbonyl azides is discussed on the basis of DFT calculations as well as kinetic and 15N labeling experiments.

Free-radical cyclization approach to polyheterocycles containing pyrrole and pyridine rings.

A wide range of derivatives with new pyrido[2,1-a]pyrrolo[3,4-c]isoquinoline skeleton was synthesized by free-radical intramolecular cyclization of o-bromophenyl-substituted pyrrolylpyridinium salts using the (TMS)3SiH/AIBN system. The cyclization provides generally good yields of pyrido[2,1-a]pyrrolo[3,4-c]isoquinoline hydrobromides having no additional radical-sensitive substituents. The free bases can be obtained from the synthesized hydrobromides in quantitative yield by basification at room temperature. The selectivity control of intramolecular arylation was achieved by replacing the halogen: the use of 1-(2-(ortho-bromophenyl)-4-(ortho-iodophenyl)pyrrol-3-yl)pyridinium bromide makes it possible to obtain a monocyclization product, and the bicyclization product from the dibromo derivative. The procedure is also applicable to obtain 3-arylpyrido[2,1-a]pyrrolo[3,2-c]isoquinoline derivatives including 2-unsubstituted skeletons that are inaccessible via Pd-catalyzed cyclization.

Metalated Ir(III) Complexes Based on the Luminescent Diimine Ligands: Synthesis and Photophysical Study.

A series of novel diimine (N∧N) ligands containing developed aromatic [2,1- a]pyrrolo[3,2- c]isoquinoline system have been prepared and used in the synthesis of Ir(III) luminescent complexes. In organic solvents, the ligands display fluorescence which depends strongly on the nature of solvents to give moderate to strong orange emission in aprotic solvents and shows a considerable blue shift and substantial increase in emission intensity in methanol. Insertion of electron-withdrawing and -donating substituents into peripheral phenyl fragment has nearly no effect onto emission parameters. The ligands were successfully used to prepare the metalated [Ir(N∧C)2(N∧N)]+ complexes (where N∧C = phenylpyridine (N∧C-1), p-tolylpyridine (N∧C-2), 2-(benzo[ b]thiophen-2-yl)pyridine (N∧C-3), 2-benzo[ b]thiophen-3-yl)pyridine (N∧C-4), and methyl 2-phenylquinoline-4-carboxylate (N∧C-5)) using standard synthetic procedures. The complexes obtained display moderate to strong phosphorescence in organic solvents; the emission characteristics is determined by the nature of emissive triplet state, which varies substantially with the variations in the structure and donor properties of the C- and N-coordinating functions in metalating ligands. TD-DFT calculations show that for complexes 1, 2, and 4 the emission originates from the mixed 3MLCT/3LLCT excited states with the major contribution from the aromatic moiety of the diimine ligand, whereas in 3 the emissive triplet manifold is mainly located at the N∧C ligand to give structured emission band typical for the ligand centered (LC) excited state. In the case of 5, the phosphorescence may be also assigned to the mixed 3MLCT/3LLCT excited state; however, the major contribution is attributed to the aromatic moiety of the metalating N∧C ligand.

Synthesis of Pyrrolotriazoloisoquinoline Frameworks by Intramolecular Cu-Mediated or Free Radical Arylation of Triazoles.

The cyclization of (2-bromophenyl)pyrrolyl-1,2,4-triazoles via copper-mediated intramolecular direct C-arylation of 1,2,4-triazoles was first accomplished under triazole-NHC control to give unknown fused heterocyclic skeletons, pyrrolo[3,2-c][1,2,4]triazolo[5,1-a] or [3,4-a]isoquinolines. The primary products underwent a triazole ring opening under the basic arylation conditions, providing N-(1H-pyrrolo[3,2-c]isoquinolin-5-yl)cyanamides. The formation of the cyanamides from isomeric pyrrolo[3,2-c][1,2,4]triazolo[3,4-a]isoquinolines involves, besides the triazole ring opening, the unusual migration of the cyano group. Cyanamides can be easily reduced to 1H-pyrrolo[3,2-c]isoquinolin-5-amines, the first NH2-substituted derivatives of 1H-pyrrolo[3,2-c]isoquinoline. An insight into the mechanism of the triazole ring cleavage was achieved by performing a DFT study at the B3LYP/6-31G+(d,p) level. Free radical cyclization of (2-bromophenyl)pyrrolyl-1,2,4-triazoles with TTMSS/AIBN under neutral conditions allows obtaining pyrrolo[3,2-c][1,2,4]triazolo[5,1-a] and [3,4-a]isoquinolines, as well two more new heterocyclic systems, pyrrolo[3,4-c][1,2,4]triazolo[5,1-a] and [3,4-a]isoquinolines, in good yields without triazole ring cleavage. The developed cyclizations provide a concise, atom-economical route to novel fluorescent fused polyheterocycles containing pyrrole and 1,2,4-triazole moieties.

Synthesis of functionalised azepanes and piperidines from bicyclic halogenated aminocyclopropane derivatives.

A series of 6,6-dihalo-2-azabicyclo[3.1.0]hexane and 7,7-dihalo-2-azabicyclo[4.1.0]heptane compounds were prepared by the reaction of dihalocarbene species with N-Boc-2,3-dihydro-1H-pyrroles or -1,2,3,4-tetrahydropyridines. Monochloro substrates were synthesised as well, using a chlorine-to-lithium exchange reaction. The behaviour of several aldehydes and ketones under reductive amination conditions with deprotected halogenated secondary cyclopropylamines was investigated, showing that this transformation typically triggers cyclopropane ring cleavage to give access to interesting nitrogen-containing ring-expanded products.

NHC as the Guiding Factor in a Copper-Catalyzed Intramolecular C Arylation of Pyrrolylimidazolium Salts: Synthesis of Luminescent Heterotetracyclic Frameworks.

3-(2/4-(2-Bromophenyl)-1H-pyrrol-3-yl)-1H-imidazol-3-ium bromides undergo a copper-catalyzed intramolecular direct C arylation under mild conditions to give new heterocyclic frameworks. The cyclizations involve the formation of betaines (imidazoliumylpyrrolides) under basic conditions and the tautomerizaton of the betaines to the corresponding NHCs, which are the reactive species responsible for the selectivity of the arylation via the formation of NHC-Cu complexes. The primary salt arylation products were dehydrohalogenated to obtain the first representatives of 7H-imidazo[2,1-a]pyrrolo[3,2-c]isoquinoline and 1H-imidazo[2,1-a]pyrrolo[3,4-c]isoquinoline heterocyclic skeletons, which were further transformed into thermodynamically more stable 1H- and 6H-tautomers, respectively, by removing of the benzyl-PG. The new heterotetracyclic systems are fluorescent in solutions with high quantum yields.

Synthesis, Transformations of Pyrrole- and 1,2,4-Triazole-Containing Ensembles, and Generation of Pyrrole-Substituted Triazole NHC.

Unprecedented pyrrole- and 1,2,4-triazole-containing ensembles, substituted 1-(1H-pyrrol-3-yl)-4H-1,2,4-triazol-1-ium bromides and 4-(1H-pyrrol-3-yl)-1H-1,2,4-triazol-4-ium bromides, were prepared from 2H-azirines and triazolium phenacyl bromides using a simple procedure. N-(1H-Pyrrol-3-yl)-N'-benzyltriazolium bromides undergo reductive debenzylation on Pd/C to give substituted 1-(1H-pyrrol-3-yl)-4H-1,2,4-triazoles and 4-(1H-pyrrol-3-yl)-1H-1,2,4-triazoles in high yields. Betaines (triazoliumylpyrrolides) and pyrrolyltriazole NHCs, which are possible products of dehydrobromination of pyrrolyltriazolium salts, have comparable thermodynamic stabilities in nonpolar solvents according to calculations at the DFT B3LYP/6-31G(d) level. The carbene forms can be easily trapped by the reaction of salts with base in the presence of sulfur. The corresponding 1- and 4-(1H-pyrrol-3-yl)-1H-1,2,4-triazole-5(4H)-thiones are formed in high yields. In the absence of sulfur as a trap, the opening of the triazole ring occurs with the formation of derivatives of N-cyanoformimidamide. According to the DFT calculations the latter is most probably formed via a pyrrolyltriazoliumide intermediate, which is the minor component of the equilibrium triazoliumylpyrrolide-pyrrolyltriazole NHC-pyrrolyltriazoliumide. Blocking of the pyrrolyltriazoliumide intermediate formation, by introduction of a substituent at the 3-position of the triazole ring, made it possible to generate the first pyrrole-substituted triazole NHC.

Fe(II)/Et3N-Relay-catalyzed domino reaction of isoxazoles with imidazolium salts in the synthesis of methyl 4-imidazolylpyrrole-2-carboxylates, its ylide and betaine derivatives.

A simple approach was developed for the synthesis of methyl 4-imidazolylpyrrole-2-carboxylates from easily available compounds, 5-methoxyisoxazoles and phenacylimidazolium salts under hybrid Fe(II)/Et3N relay catalysis. The products were easily transformed into the corresponding 3-(5-methoxycarbonyl-1H-imidazol-3-ium-3-yl)pyrrol-1-ides, which in turn can be hydrolyzed under basic conditions into the corresponding betaines. A carbene tautomeric form of the 4-methoxycarbonyl-substituted imidazolylpyrrolides was trapped by reaction with sulfur affording the corresponding imidazolethiones under very mild conditions.

Metal/organo relay catalysis in a one-pot synthesis of methyl 4-aminopyrrole-2-carboxylates from 5-methoxyisoxazoles and pyridinium ylides.

Methyl 4-aminopyrrole-2-carboxylates were synthesized in one-pot mode by the relay catalytic cascade reaction of 5-methoxyisoxazoles with pyridinium ylides by the use of a FeCl2/Et3N binary catalytic system leading to 1-(5-methoxycarbonyl-1H-pyrrol-3-yl)pyridinium salts followed by hydrazinolysis. The approach permits the introduction of a substituent at the pyrrole nitrogen via a nucleophilic reaction of the pyrrolylpyridinium ylide derived from the salt. Catalytic reduction of the ylides gives methyl 4-piperidinopyrrole-2-carboxylates.

A simple approach to pyrrolylimidazole derivatives by azirine ring expansion with imidazolium ylides.

A domino reaction of 2H-azirines with 1-alkyl-3-phenacyl-1H-imidazolium bromides in the presence of Et3N provides a facile route to 1-alkyl-3-(1H-pyrrol-3-yl)-1H-imidazol-3-ium bromides. 1-Benzyl derivatives can be reduced to 1-(1H-pyrrol-3-yl)-1H-imidazoles with HCO2NH4 on Pd/C. The action of KOH on pyrrolylimidazolium salts leads to a new type of stable ylide, 3-(1H-imidazol-3-ium-3-yl)-pyrrol-1-ides, which can, in principle, be in tautomeric equilibrium with the corresponding N-heterocyclic carbene. Although, according to the DFT B3LYP/6-31G(d) calculations in vacuo, electron-donating substituents in the 2-aryl-group cause the tautomeric equilibrium to shift to the carbene side, the investigated compounds exist in the ylide form in solution and in the solid state, which is in agreement with the relative stabilities of the species calculated with the PCM solvent model.