Palladium acetate assisted synthesis of five-membered N-polyheterocycles

Abstract

The metal-assisted synthesis of heterocyclic compounds is known to be one of the extremely developing as well as significant concepts of organic chemistry. Because of their expensive, complex working of the instrument and difficult procedures, the methodologies used earlier for the heterocycle synthesis were less amicable to the researchers. The Pd(OAc)₂-mediated cyclic reactions have been recognized to be very effective for both the stereoselective as well as regioselective formation of the 5-membered N-bearing heterocyclic compounds. The different uses of palladium acetate, as a catalyst in the formation of 5-membered N-containing polyheterocycles, are covered in this review article.     

Solvent Moisture-Controlled Self-Assembly of Fused Benzoimidazopyrrolopyrazines with Different Ring’s Interposition

This article shows that two extremely important families of fused heterocyclic assemblies, namely 6-methylbenzo[4,5]imidazo[1,2-a]pyrrolo[2,1-c]pyrazine and 5a-methyl-5a,6-dihydro-5H,12H-benzo[4,5]imidazo[1,2-a]pyrrolo[1,2-d]pyrazine, can be synthesized from only two available building blocks (N-allenylpyrrole-2-carbaldehyde and o-phenylenediamine) by controlling only one reaction parameter (water content of the medium). It should be emphasized that the latter class of compounds (with an a/d arrangement) is previously unknown. If the allene group is introduced not into the starting compound, but during the reaction (in superbase media), a heterocyclic ensemble, 5-methylbenzo[4,5]imidazo[1,2-a]pyrrolo[2,1-c]pyrazines, with a different position of the methyl group is formed.     

Enantioselective synthesis of N-heterocycles via intramolecular Pd(0)-catalysed allylic amination

An efficient and stereoselective synthesis of pyrrolidine-, piperidine-, and azepane-type N-heterocycles is described by the intramolecular Pd(0)-catalysed cyclisation of amino allylic carbonates. The use of chiral ligands gave the corresponding heterocyclic derivatives having er values that were from moderate to good.     

Intramolecular N-acyliminium ion versus Friedel-Crafts cyclization onto 3-indoles: synthesis of the novel rings pyrrolizino[2,1-b]indole and homologues

Acid treatment of indole-2-carboxylic acid β- and γ-oxoamides causes Friedel-Crafts intramolecular cyclization to β-carbolinones and dihydro-2H-azepino[3,4-b]indol-1-ones, in contrast to secondary δ-,?-, and ζ-oxoamides, which cyclize to the novel heterocyclic rings pyrrolizino[2,1-b]indole, indolizino[2,1-b]indole, and 9a,11-diaza-indeno[1,2-a]azulene, via an intermediate N-acyliminium ion. Tertiary amides lead only the Friedel-Crafts ring closure, thus allowing the synthesis of larger fused rings.     

Cyclic Amino(Ylide) Silylene: A Stable Heterocyclic Silylene with Strongly Electron‐Donating Character

An isolable heterocyclic silylene ( 4 ) with two different π‐donating substituents, namely a classical amino group and a more electropositive and stronger carbon‐based π‐donating phosphonium ylide, was synthesized and fully characterized. The combination of these two different π‐donating substituents confers high thermal stability and an unusual nucleophilic character on silylene 4 . Therefore, silylene 4 behaves as a strong donor ligand toward transition metals with a donating character comparable to N‐heterocyclic carbenes, in contrast to classical N‐heterocyclic silylenes, which generally present a weak donating character.     

Energetic Salts Based on Tetrazole N‐Oxide

Energetic materials (explosives, propellants, and pyrotechnics) are used extensively for both civilian and military applications and the development of such materials, particularly in the case of energetic salts, is subject to continuous research efforts all over the world. This Review concerns recent advances in the syntheses, properties, and potential applications of ionic salts based on tetrazole N‐oxide. Most of these salts exhibit excellent characteristics and can be classified as a new family of highly energetic materials with increased density and performance, alongside decreased mechanical sensitivity. Additionally, novel tetrazole N‐oxide salts are proposed based on a diverse array of functional groups and ions pairs, which may be promising candidates for new energetic materials.     

N-Acetyl-5-arylidenetetramic acids: synthesis,X-ray structure elucidation and application to the preparation of zinc(II) and copper(II) complexes

A new route to N-acetyl-3-acyl-5-arylidenetetramic acids was developed and applied to the synthesis of a small library of compounds with diversity at carbon 3 of the heterocyclic ring. The 3-acetyl derivative was used as ligand for complexation with zinc(II) and copper(II) acetates. Both the ligand and the zinc complex were studied by X-ray single crystal structure analysis, the zinc complex forms ribbons by H-bonding from the ethanol to a neighbouring molecule.     

Access to Pyrazolidin‐3,5‐diones through Anodic N–N Bond Formation

Pyrazolidin‐3,5‐diones are important motifs in heterocyclic chemistry and are of high interest for pharmaceutical applications. In classic organic synthesis, the hydrazinic moiety is installed through condensation using the corresponding hydrazine building blocks. However, most N,N′‐diaryl hydrazines are toxic and require upstream preparation owing to their low commercial availability. We present an alternative and sustainable synthetic approach to pyrazolidin‐3,5‐diones that employs readily accessible dianilides as precursors, which are anodically converted to furnish the N?N bond. The electroconversion is conducted in a simple undivided cell under constant‐current conditions.     

Polyisobutylene-supported N-heterocyclic carbene palladium catalysts

This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable when such solvent systems are perturbed to be biphasic. The results here show that such PIB-bound NHC ligands can be used to synthesize useful palladium catalysts. In this paper, both PIB-bound analogs of an N,N′-bis(2,6-diisopropylphenyl) heterocyclic carbene and simpler N,N′-dialkyl heterocyclic carbene ligand were prepared and were successfully used to form palladium cross-coupling catalysts. The reactivity, recycling and reusability of these catalysts has been examined.     

Stereoselective Direct N-Trifluoropropenylation of Heterocycles with a Hypervalent Iodonium Reagent

The availability and synthesis of fluorinated enamine derivatives such as N-(3,3,3-trifluoropropenyl)heterocycles are challenging, especially through direct functionalization of the heterocyclic scaffold. Herein, a stereoselective N-trifluoropropenylation method based on the use of a bench-stable trifluoropropenyl iodonium salt is described. This reagent enables the straightforward trifluoropropenylation of various N-heterocycles under mild reaction conditions, providing trifluoromethyl enamine type moieties with high stereoselectivity and efficiency.     

N-Alkoxymethylation of heterocyclic compounds with diethyl phosphite via cleavage of P-O bond

N-Alkoxymethylation of heterocyclic compounds with diethyl phosphite via cleavage of P-O bond was investigated and a series of N3-ethoxymethylated heterocyclic compounds were synthesized. A mechanism in which diethyl phosphite acts as an efficient surrogate of ethanol was proposed and supported by several evidences.     

Synthesis and Characterization of Novel Methyl (3)5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates

Series of methyl 3- and 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates were developed and regioselectively synthesized as novel heterocyclic amino acids in their N-Boc protected ester form for achiral and chiral building blocks. In the first stage of the synthesis, piperidine-4-carboxylic and (R)- and (S)-piperidine-3-carboxylic acids were converted to the corresponding β-keto esters, which were then treated with N,N-dimethylformamide dimethyl acetal. The subsequent reaction of β-enamine diketones with various N-mono-substituted hydrazines afforded the target 5-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates as major products, and tautomeric NH-pyrazoles prepared from hydrazine hydrate were further N-alkylated with alkyl halides to give 3-(N-Boc-piperidinyl)-1H-pyrazole-4-carboxylates. The structures of the novel heterocyclic compounds were confirmed by ¹H-, ¹³C-, and ¹⁵N-NMR spectroscopy and HRMS investigation.     

Syntheses and Photophysical Properties of N‐Pyridylimidazol‐2‐ylidene Tetracyanoruthenates(II) and Photochromic Studies of Their Dithienylethene‐Containing Derivatives

A series of tetracyanoruthenate(II) with chelating pyridyl N‐heterocyclic carbene ligands (NHC‐py) was synthesized and characterized. Their photophysical and electrochemical properties as well as the photochromic behavior of their dithienylethene‐containing complexes were studied. Photocyclization was found to take place upon irradiation into the metal‐to‐ligand charge transfer (MLCT) absorption bands of these complexes, and evidence is provided to support the triplet‐sensitizing reaction pathway.     

Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C‐Metalated N‐Methylquinoxalinium or N‐Methylpyrazinium Ligands

The cationic cluster complexes [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L¹Me)]⁺ ( 3 ⁺; HL¹=quinoxaline) and [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L²Me)]⁺ ( 5 ⁺; HL²=pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐Lⁿ)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N‐heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris‐sec‐butylborohydride (K‐selectride) to give neutral products that contain novel nonaromatic N‐heterocyclic ligands. The following are the products that have been isolated: [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹Me₂)] ( 6 ; from 3 ⁺ and methyl lithium), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹HMe)] ( 7 ; from 3 ⁺ and K‐selectride), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²Me₂)] ( 8 ; from 5 ⁺ and methyl lithium), and [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²HMe)] ( 11 ; from 5 ⁺ and K‐selectride). Whereas the reactions of 3 ⁺ lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N‐methyl group, the reactions of 5 ⁺ give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N‐methyl group. The LUMOs and the atomic charges of 3 ⁺ and 5 ⁺ confirm that the reactions of these clusters with anionic nucleophiles are orbital‐controlled rather than charge‐controlled processes. The N‐heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face‐capping modes. Those of compounds 6 – 8 have the atoms of a ligand C?N fragment σ‐bonded to two Ru atoms and π‐bonded to the other Ru atom, whereas the ligand of compound 11 has a C? N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable‐temperature ¹H NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above ?93 °C, the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through a conformational change of the ligand CH₂ group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180° rotation of the entire organic ligand over a face of the metal triangle.     

Synthesis of electroactive hydrazones derived from 3-(10-alkyl-10H-phenothiazin-3-yl)-2-propenals and their corresponding 3,3′-bispropenals

3-(10-Alkyl-10H-phenothiazin-3-yl)-2-propenals and their corresponding 3,3′-bispropenals that represent a previously unexplored class of functionalised phenothiazine derivatives were prepared upon reacting N-alkylated 3-bromo- and 3,7-dibromo-10H-phenothiazines with acrolein diethyl acetal under Pd catalysis. The obtained heterocyclic 2,3-unsaturated aldehydes were condensed with N-methyl-N-phenylhydrazine and N,N-diphenylhydrazine resulting in mono- and dihydrazones that act as effective hole transporting materials. Thermal, optical, electrochemical and photophysical properties of the synthesised new organic electroactive derivatives have been investigated.     

Energetic N‐Trinitroethyl‐Substituted Mono‐, Di‐, and Triaminotetrazoles

A series of dense energetic N‐trinitroethyl‐substituted mono‐, bis‐, and tri‐5‐aminotetrazoles were obtained by reacting primary amines with in situ generated cyanogen azide, followed by the trinitroethyl functionalization that involves a condensation of a hydroxymethyl intermediate (prepared by a reaction with formaldehyde) with trinitromethane. These compounds were fully characterized by using multinuclear NMR spectroscopy, IR, elemental analysis, differential scanning calorimetry (DSC), and, in one case with 9 , with single‐crystal XRD analysis. The heats of formation for all compounds were calculated with Gaussian 03 and then combined with experimental densities to determine the detonation pressures (P) and velocities (D_v) of the energetic materials. Interestingly, most of them exhibited high density, good thermal stability, acceptable oxygen balance, positive heat of formation, low impact sensitivity, and excellent detonation properties, which highlighted their practical application potentials as a fascinating class of highly energetic materials.     

Stereoselective Lewis Acid Mediated (3+2) Cycloadditions of N‐H‐ and N‐Sulfonylaziridines with Heterocumulenes

Alkyl and aryl isothiocyanates and carbodiimides are effective substrates in (3+2) cycloadditions with N‐sulfonyl‐2‐substituted aziridines and 2‐phenylaziridine for the synthesis of iminothiazolidines and iminoimidazolidines. Additionally, the stereoselective (3+2) cycloaddition of N‐H‐ and N‐sulfonylaziridines with isothiocyanates can be accomplished, allowing for the synthesis of highly enantioenriched iminothiazolidines. Evidence for an intimate ion‐pair mechanism is presented herein in the context of these chemo‐, regio‐, and diastereoselective transformations. The demonstrated ability to remove the sulfonyl group from the heterocyclic products displays the utility of these compounds for further derivatization and application.     

N‐Oxide 1,2,4,5‐Tetrazine‐Based High‐Performance Energetic Materials

One route to high density and high performance energetic materials based on 1,2,4,5‐tetrazine is the introduction of 2,4‐di‐N‐oxide functionalities. Based on several examples and through theoretical analysis, the strategy of regioselective introduction of these moieties into 1,2,4,5‐tetrazines has been developed. Using this methodology, various new tetrazine structures containing the N‐oxide functionality were synthesized and fully characterized using IR, NMR, and mass spectroscopy, elemental analysis, and single‐crystal X‐ray analysis. Hydrogen peroxide (50 %) was used very effectively in lieu of the usual 90 % peroxide in this system to generate N‐oxide tetrazine compounds successfully. Comparison of the experimental densities of N‐oxide 1,2,4,5‐tetrazine compounds with their 1,2,4,5‐tetrazine precursors shows that introducing the N‐oxide functionality is a highly effective and feasible method to enhance the density of these materials. The heats of formation for all compounds were calculated with Gaussian 03 (revision D.01) and these values were combined with measured densities to calculate detonation pressures (P) and velocities (ν_D) of these energetic materials (Explo 5.0 v. 6.01). The new oxygen‐containing tetrazines exhibit high density, good thermal stability, acceptable oxygen balance, positive heat of formation, and excellent detonation properties, which, in some cases, are superior to those of 1,3,5‐tritnitrotoluene (TNT), 1,3,5‐trinitrotriazacyclohexane (RDX), and octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX).     

Insensitive Nitrogen‐Rich Materials Incorporating the Nitroguanidyl Functionality

A new class of nitroguanidyl‐functionalized nitrogen‐rich materials derived from 1,3,5‐triazine and 1,2,4,5‐tetrazine was synthesized through reactions between N‐nitroso‐N′‐alkylguanidines and the hydrazine derivatives of 1,3,5‐triazine or 1,2,4,5‐tetrazine. These compounds were fully characterized using multinuclear NMR and IR spectroscopies, elemental analysis, and differential scanning calorimetry (DSC). The heats of formation for all compounds were calculated with Gaussian 03 and then combined with experimental densities to determine the detonation pressures (P) and velocities (D_v) of the energetic materials. Interestingly, some of the compounds exhibit an energetic performance (P and D_v) comparable to that of RDX, thus holding promise for application as energetic materials.     

1,2‐Thiazines: One‐Pot Syntheses Utilizing Mono and Diaza Analogs of Sulfones

A one‐pot Michael addition/cyclization/condensation reaction sequence for the regioselective synthesis of 1,2‐thiazines, starting from propargyl ketones and NH‐sulfoximines or NH‐sulfondiimines, has been developed. Under mild and operationally simple reaction conditions previously unprecedented 1,2‐thiazine 1‐imide and 1‐oxide derivatives are formed in good to excellent yields. The products represent heterocyclic building blocks, readily modifiable by a regioselective C?H bond functionalization, classical cross‐coupling reactions, and deprotection.