Chem. Sci., 2020, Vol. XX, pp XXXX-XXXX
Complexes of metals with N-heterocyclic carbene ligands (M/NHC) are typically considered the systems of choice in homogeneous catalysis due to their stable metal−ligand framework. However, it becomes obvious that even metal species with a strong M-NHC bond can undergo evolution in catalytic systems, and processes of M-NHC bond cleavage are common for different metals and NHC ligands. This review is focused on the main types of the M-NHC bond cleavage reactions and their impact on activity and stability of M/NHC catalytic systems. For the first time, we consider these processes in terms of NHC-connected and NHC-disconnected active species derived from M/NHC precatalysts and classify them as fundamentally different types of catalysts. Problems of rational catalyst design and sustainability issues are discussed in the context of the two different types of M/NHC catalysis mechanisms.
Catal. Sci. Technol., 2020, Vol. 10, pp 1228-1247
Many reactions catalyzed by Pd complexes with N-heterocyclic carbene (NHC) ligands are performed in the presence of amines which usually act as coupling reagents or mild bases. However, amines can react with Pd/NHC complexes in a number of ways: enhancing molecular catalysis, causing the catalyst deactivation or triggering the ligandless modes of catalysis by producing NHC-free active palladium species. This study gains insight into conditions required for the efficient use of amines as activators of molecular Pd/NHC catalysis and preventing the undesirable reductive cleavage of the Pd-NHC bond in catalytic systems. Reactions of Pd/NHC complexes with various amines within a temperature range of 25–140 °C and thermal stability of the resulting amino-complexes are examined. The results indicate the major influence of the amine structure and reaction temperature on the catalyst transformations. In particular, thermal decomposition of Pd/NHC complexes with aliphatic amine ligands predominantly leads to the reductive Pd-NHC bond cleavage, while deprotonation of the complexes with primary and secondary aliphatic amine ligands in the presence of strong bases at 25–60 °C promotes the activation of molecular Pd/NHC catalysis. Efficient Pd-PEPPSI complex – amine systems suitable for the strong-base-promoted C-S cross-coupling reactions between aryl halides and thiols are suggested on the basis of these findings.
Dalton Trans., 2019, Vol. 48, pp 17052-17062
Complexes of Ni, Pd, and Pt with N-heterocyclic carbenes (NHCs) catalyze numerous organic reactions via typically proposed M0/MII catalytic cycles comprising intermediates with the metal center in (0) and (II) oxidation states. In addition, MII/MIV catalytic cycles have also been proposed for a number of reactions. Catalytic intermediates in both cycles can suffer decomposition via R-NHC coupling, the side reductive elimination of NHC ligand and R groups (R = alkyl, aryl, etc.) to give [NHC-R]+ cations. In this study, relative stability of (NHC)MII(R)(X)L and (NHC)MIV(R)(X)3L intermediates (X = Cl, Br, I; L = NHC, pyridine) against the R-NHC coupling and other decomposition pathways via reductive elimination reactions is evaluated theoretically. The study reveals that R-NHC coupling represents the most favorable decomposition pathway for both types of intermediates (MII and MIV), while it is thermodynamically and kinetically more facile for the MIV complexes. Relative effects of the metal M (Ni, Pd, Pt), ligands L and X on the R-NHC coupling for the MIV complexes are significantly stronger than for the MII complexes. In particular, for (NHC)2MIV(Ph)(Br)3 complexes, Ph-NHC coupling is facilitated dramatically from Pt (ΔG = -36.9 kcal/mol, ΔG≠ = 37.5 kcal/mol) to Pd (ΔG = -61.5 kcal/mol, ΔG≠ = 18.3 kcal/mol) and Ni (ΔG = -80.2 kcal/mol, ΔG≠ = 4.7 kcal/mol). For the MII oxidation state of the metal, bis-NHC complexes (L = NHC) are kinetically and thermodynamically slightly more stable against R-NHC coupling than the mono-NHC complexes (L = pyridine). The inverse relation is observed for the MIV oxidation state of the metal, as (NHC)2MIV(R)(X)3 complexes are kinetically (4.3-15.9 kcal/mol) and thermodynamically (8.0-23.2 kcal/mol) significantly less stable than the (NHC)MIV(R)(X)3L (L =pyridine) complexes. For NiIV and PdIV complexes, additional decomposition pathways via the reductive elimination of NHC and X ligand to give [NHC-X]+ cation (X-NHC coupling) or reductive elimination of X-X molecule are found to be thermodynamically and kinetically probable. In overall, the obtained results demonstrate significant instability of regular Ni/NHC and Pd/NHC complexes (for example, not stabilized additionally by chelation) and high probability to initiate “NHC-free” catalysis in the reactions comprising MIV intermediates.
Chemistry — A European Journal, 2019, Vol. 25, pp 16564–16572
N‐Heterocyclic carbene ligands (NHC) are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M‐NHC binding in this metal‐ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M‐NHC bond (rather than its stabilization) could be more important for high‐performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal‐NHC binding and [PdX3]–[NHC‐R]+ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC‐R]+ azolium cation is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well‐defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC‐free anionic Pd in polar media via either Ph‐NHC coupling or reversible H‐NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.
ACS Catalysis, 2019, Vol. 9, pp 2984–2995
The mercury test is a rapid and widely used method for distinguishing truly homogeneous molecular catalysis from nanoparticle metal catalysis. In the current work, using various M0 and MII complexes of palladium and platinum that are often used in homogeneous catalysis as examples, we demonstrated that the mercury test is generally inadequate as a method for distinguishing between homogeneous and cluster/nanoparticle catalysis mechanisms for the following reasons: (i) the general and facile reactivity of both molecular M0 and MII complexes toward metallic mercury and (ii) the very high and often unpredictable dependence of the test results on the operational conditions and the inability to develop universal quantitatively defined operational parameters. Two main types or mercury-induced transformations, the cleavage of M0 complexes and the oxidative–reductive transmetalation of MII complexes, including a reaction of highly popular MII/NHC complexes, were elucidated using NMR, ESI-MS, and EDXRF techniques. A mechanistic picture of the reactions involving metal complexes was revealed with mercury, and representative metal species were isolated and characterized. Even in an attempt to not overstate the results, one must note that the use of the mercury tests often leads to inaccurate conclusions and complicates the mechanistic studies of these catalytic systems. As a general concept, distinguishing reaction mechanisms (homogeneous vs cluster/nanoparticle) by using catalyst poisoning requires careful rethinking in the case of dynamic catalytic systems.
Chemical Science, 2018, Vol. 9, (25), pp 5564-5577
Numerous reactions are catalyzed by complexes of metals (M) with N-heterocyclic carbene (NHC) ligands, typically in the presence of oxygen bases, which significantly shape the performance. It is generally accepted that bases are required for either substrate activation (exemplified by transmetallation in the Suzuki cross-coupling), or HX capture (e.g. in a variety of C–C and C-heteroatom couplings, the Heck reaction, C–H functionalization, heterocyclizations, etc.). This study gives insights into the behavior of M(II)/NHC (M = Pd, Pt, Ni) complexes in solution under the action of bases conventionally engaged in catalysis (KOH, NaOH, t-BuOK, Cs2CO3, K2CO3, etc.). A previously unaddressed transformation of M(II)/NHC complexes under conditions of typical base-mediated M/NHC catalyzed reactions is disclosed. Pd(II) and Pt(II) complexes widely used in catalysis react with the bases to give M(0) species and 2(5)-oxo-substituted azoles via an O–NHC coupling mechanism. Ni(NHC)2X2 complexes hydrolyze in the presence of aqueous potassium hydroxide, and undergo the same O–NHC coupling to give azolones and metallic nickel under the action of t-BuOK under anhydrous conditions. The study reveals a new role of NHC ligands as intramolecular reducing agents for the transformation of M(II) into “ligandless” M(0) species. This demonstrates that the disclosed base-mediated O–NHC coupling reaction is integrated into the catalytic M/NHC systems and can define the mechanism of catalysis (molecular M/NHC vs. “NHC-free” cocktail-type catalysis). A proposed mechanism of the revealed transformation includes NHC-OR reductive elimination, as implied by a series of mechanistic studies including 18O labeling experiments.
Organometallics, 2018, Vol. 37, (9), pp 1483–1492
The behavior of ubiquitously used nickel, palladium, and platinum complexes containing N-heterocyclic carbene ligands was studied in solution in the presence of aliphatic amines. Transformation of M(NHC)X2L complexes readily occurred according to the following reactions: (i) release of the NHC ligand in the form of azolium salt and formation of metal clusters or nanoparticles and (ii) isomerization of mono-NHC complexes M(NHC)X2L to bis-NHC derivatives M(NHC)2X2. Facile cleavage of the M–NHC bond was observed and provided the possibility for fast release of catalytically active NHC-free metal species. Bis-NHC metal complexes M(NHC)2X2were found to be significantly more stable and represented a molecular reservoir of catalytically active species. Slow decomposition of the bis-NHC complexes by removal of the NHC ligands (also in the form of azolium salts) occurred, generating metal clusters or nanoparticles. The observed combination of dual fast- and slow-release channels is an intrinsic latent opportunity of M/NHC complexes, which balances the activity and durability of a catalytic system. The fast release of catalytically active species from M(NHC)X2L complexes can rapidly initiate catalytic transformation, while the slow release of catalytically active species from M(NHC)2X2 complexes can compensate for degradation of catalytically active species and help to maintain a reliable amount of catalyst. The study clearly shows an outstanding potential of dynamic catalytic systems, where the key roles are played by the lability of the M–NHC framework rather than its stability.
ChemSusChem, 2018, Vol. 11, pp 3599–3608
Biomass processing wastes (humins) are anticipated to become a large‐tonnage solid waste in the near future, owing to the accelerated development of renewable technologies based on utilization of carbohydrates. In this work, the utility of humins as a feedstock for the production of activated carbon by various methods (pyrolysis, physical and chemical activation, or combined approaches) was evaluated. The obtained activated carbons were tested as potential electrode materials for supercapacitor applications and demonstrated combined micro‐ and mesoporous structures with a good capacitance of 370 F g−1 (at a current density of 0.5 A g−1) and good cycling stability with a capacitance retention of 92 % after 10 000 charge/discharge cycles (at 10 A g−1 in 6 m aqueous KOH electrolyte). The applicability of the developed activated carbon for practical usage as a supercapacitor electrode material was demonstrated by its successful utilization in symmetric two‐electrode cells and by powering electric devices. These findings provide a new approach to deal with the problem of sustainable wastes utilization and to advance challenging energy storage applications.
Mendeleev Communications, 2018, Vol. 28, (4), pp 431-433
The Pt/C catalysts with various Pt content (5-30 wt%) synthesized viaelectrochemical pulse alternating current technique have been evaluated for the base-free aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. The higher Pt content in the catalyst (30 wt%) provides the product yield up to 65% upon performing the process in concentrated (∼0.1 M) aqueous solutions of the substrate.
Organometallics, 2018, Vol. 37 (5), pp 787–796
Oxidative addition of organic halides (R–X) to (NHC)Pd0L complexes is involved in numerous metal-catalyzed reactions, and this step is expected to afford (NHC)PdII(R)(X)L intermediate complexes. However, these complexes may undergo further transformation via R–NHC coupling, which removes the NHC ligands from the metal and results in the generation of “bare” NHC-free metal species. The comparative theoretical study carried out in the present work revealed that the kinetic and thermodynamic stability of the (NHC)PdII(R)(X)L oxidative addition intermediates depends strongly on the nature of the organic group R. The predicted reactivity in the R–NHC coupling process decreases in the following order: R = Vinyl > Ethynyl > Ph > Me. Accordingly, for R = Me, a classical (NHC)PdII(R)(X)L intermediate can be expected as a product of the oxidative addition step, whereas for R = Ph, the outcome of the oxidative addition may already contain the NHC-free palladium complex. For R = Ethynyl, comparable amounts of both complexes should be formed, while for R = Vinyl, the NHC-free palladium complex can be the major product of the oxidative addition process. Unusual thermodynamic and kinetic instability of the (NHC)Pd(vinyl)(X)L complex and the tendency to vinyl–NHC coupling predicted by the computational modeling has been confirmed by experimental measurements with online mass spectrometric reaction monitoring. Thus, the outcome of the oxidative addition strongly depends on the type of organic group R and the R–NHC coupling process greatly influences the activity and stability of metal catalysts.
Catalysis Science & Technology, 2018, Vol. 8, (12), pp 3073-3080
Palladium complexes with fluorinated acetylacetonate chelating ligands were studied as catalysts for alkyne hydrothiolation. A ten-fold increase in the catalytic efficiency was achieved by using 0.1 mol% of Pd(hfpd)2complex (hfpd = hexafluoroacetylacetonate) with a variety of thiol–yne coupling partners. The principal possibility of a hundred-fold increase in the efficiency of Pd-catalyzed Markovnikov-type RSH addition with 0.01 mol% of the catalyst was successfully achieved with the hfpd ligand for the first time. The hexafluoroacetylacetonate chelating ligand not only enhanced the affinity of palladium centers to the triple bond of acetylene, but also stabilized the catalytic system against formation of insoluble polymeric [Pd(SPh)2]nspecies, thus ensuring that the reaction operates homogeneously. Utilizing other diketonate ligands resulted in cocktail-type catalysis with variable and poorly predictable contributions of homogeneous and heterogeneous pathways.
An efficient synthesis of 2-amino-1-R-[1,2,4]triazolo[1,5-a]-pyrimidinium or 3-amino-2-R-[1,2,4]triazolo[4,3-a]pyrimidi- nium chloride derivatives by heterocyclization of 3,5-diamino-1-R-1,2,4-triazoles (R = Alk or Ar) with pentane-2,4-diones was developed. The process is promoted by chlorotrimethyl- silane which plays the dual role of carbonyl-activating agent and water scavenger.
Tetrahedron Letters, 2017, Vol. 58, (36), pp 3517-3521
An efficient method for the oxidation of alcohols to aldehydes or ketones in a two-phase CH2Cl2/NaHCO3 (aq.) system, using iodine and catalytic amounts of 4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl and 2,4,6-trimethylpyridine, was developed. The performance of the method was demonstrated by the selective oxidation of 37 variously substituted alcohols in ≥90% yield, including the gram-scale synthesis of the important chemical 2,5-diformylfuran from biomass-derived 5-hydroxylmethylfurfural.
Organometallics, 2017, Vol. 36 (10), pp 1981–1992
Metal complexes bearing N-heterocyclic carbene (NHC) ligands are typically considered the system of choice for homogeneous catalysis with well-defined molecular active species due to their stable metal–ligand framework. A detailed study involving 19 different Pd-NHC complexes with imidazolium, benzimidazolium, and triazolium ligands has been carried out in the present work and revealed a new mode of operation of metal-NHC systems. The catalytic activity of the studied Pd-NHC systems is predominantly determined by the cleavage of the metal–NHC bond, while the catalyst performance is strongly affected by the stabilization of in situ formed metal clusters. In the present study, the formation of Pd nanoparticles was observed from a broad range of metal complexes with NHC ligands under standard Mizoroki–Heck reaction conditions. A mechanistic analysis revealed two different pathways to connect Pd-NHC complexes to “cocktail”-type catalysis: (i) reductive elimination from a Pd(II) intermediate and the release of NHC-containing byproducts and (ii) dissociation of NHC ligands from Pd intermediates. Metal-NHC systems are ubiquitously applied in modern organic synthesis and catalysis, while the new mode of operation revealed in the present study guides catalyst design and opens a variety of novel opportunities. As shown by experimental studies and theoretical calculations, metal clusters and nanoparticles can be readily formed from M-NHC complexes after formation of new M–C or M–H bonds followed by C–NHC or H–NHC coupling. Thus, a combination of a classical molecular mode of operation and a novel cocktail-type mode of operation, described in the present study, may be anticipated as an intrinsic feature of M-NHC catalytic systems.
Russian Chemical Reviews, 2017, Vol. 86, (5), pp 357-387
5-Hydroxymethylfurfural (HMF) is an important versatile reagent, a so-called platform chemical, that can be produced from plant biomass compounds: hexose carbohydrates and lignocellulose. In the near future, HMF and its derivatives could become an alternative feedstock for the chemical industry and replace, to a great extent, non-renewable sources of hydrocarbons (oil, natural gas and coal). This review analyzes recent advances in the synthesis of HMF from plant feedstocks and considers the prospects for the use of HMF in the production of monomers and polymers, porous carbon materials, engine fuels, solvents, pharmaceuticals, pesticides and chemicals. The most important HMF derivatives considered in the review include 2,5-furandicarboxylic acid, 2,5-diformylfuran, 2,5-bis(hydroxymethyl)furan, 2,5-bis(aminomethyl)furan, 2,5-dimethylfuran, 2,5-dimethyltetrahydrofuran, 2,5-bis(methoxymethyl)furan, and 5-ethoxymethylfurfural. In the nearest future, a significant extension of the HMF application is expected, and this platform chemical may be considered a major source of carbon and hydrogen for the chemistry of the 21st century.
Catalysis Today, 2017, Vol. 279, (1), pp 107-114
The influence of doping of Co/SiO2 catalysts with alumina on a performance of Fischer-Tropsch synthesis (FTS) was studied by extended FTS trials in a fixed-bed tubular pilot-scaled reactor. The addition of small amount of Al2O3 causes apparent promotional effect on the catalysts activity and C5+ hydrocarbons selectivity. The largest promotion effect was observed for the catalysts with 1 wt.% of alumina loading. The modification of the catalyst with alumina (1 wt.%) changes molecular weight distribution of the resultant C5+ paraffins with increasing the fraction of C8–C25 and decreasing the fraction of longer chain hydrocarbons. The addition of a proper amount of alumina into Co/SiO2 catalyst alters Co° particle size distribution making it narrower with the maximum at 8 nm and the same mean value for Co° particle size. A volcano-like dependence of CO chemisorption on alumina loadings with a maximum at 1 wt.% was observed. Relatively high CO chemisorption at the proper amount of alumina decreases the ratio of surface hydrogen to carbon monoxide and in such a way promotes formation of C5+ hydrocarbons.
Tetrahedron Letters, 2017, Vol. 58, (8), pp 748-754
A versatile approach for the synthesis of [1,2,4]triazolodipyrimidinones with various annulations of the triazole and pyrimidine rings was developed. The isomeric triazolodipyrimidinones were obtained by the stepwise condensation of partially hydrogenated [1,2,4]triazolo[1,5-a]pyrimidin-2-amines with β-ketoesters or diethyl ethoxymethylenemalonate, alkoxy base-mediated cyclization of the enamines, and subsequent cascade rearrangement of the 10-oxo-[1,2,4]triazolo[1,5-a:4,3-a′]dipyrimidines.
Chemistry — An Asian Journal, 2016, Vol. 11, (18), pp 2578-2585
A new method was developed for the selective gram‐scale synthesis of 2,5‐diformylfuran (DFF), which is an important chemical with a high application potential, via oxidation of biomass‐derived 5‐hydroxylmethylfurfural (HMF) catalyzed by 4‐acetylamino‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (4‐AcNH‐TEMPO) in a two‐phase system consisting of a methylene chloride and aqueous solution containing sodium hydrogen carbonate and potassium iodide. The key feature of this method is the generation of the I2 (co‐)oxidant by anodic oxidation of iodide anions during pulse electrolysis. In addition, the electrolyte can be successfully recycled five times while maintaining a 62–65 % yield of DFF. This novel method provides a sustainable pathway for waste‐free production of DFF without the use of metal catalysts and expensive oxidants. An advantage of electrooxidation is utilized in the preparation of demanding chemical.
Organometallics, 2015, Vol. 34 (24), pp 5759–5766
Metal complexes with N-heterocyclic carbene ligands (NHC) are ubiquitously used in catalysis, where the stability of the metal–ligand framework is a key issue. Our study shows that Ni-NHC complexes may undergo facile decomposition due to the presence of water in organic solvents (hydrolysis). The ability to hydrolyze Ni(NHC)2X2 complexes decreases in the order of NHC = 1,2,4-triazolium > benzimidazolium ≈ imidazolium. Depending on the ligand and substituents, the half reaction time of the complex decomposition may change from several minutes to hours. The nature of the halogen is also an important factor, and the ability for decomposition of the studied complexes decreases in the order of Cl > Br > I. NMR and MS monitoring revealed that Ni-NHC complexes in the presence of water undergo hydrolysis with Ni–Ccarbene bond cleavage, affording the corresponding N,N′-dialkylated azolium salts and nickel(II) hydroxide. These findings are of great importance for designing efficient and recyclable catalytic systems, because trace water is a common contaminant in routine synthetic applications.
The Journal of Organic Chemistry, 2015, Vol. 80 (21), pp 10694–10709
The acid-catalyzed condensation between 2-aminosubstituted [1,2,4]triazolo[1,5-a]pyrimidines and their analogues with various saturation of the pyrimidine ring and 1,3-diketones or 1,1,3,3-tetramethoxypropane was evaluated as a new approach for the synthesis of diversely substituted polycyclic derivatives of triazolopyrimidine. The reaction of 4,5,6,7-tetrahydro- or aromatic aminotriazolopyrimidines results in selective formation of the corresponding [1,2,4]triazolo[1,5-a:4,3-a′]dipyrimidin-5-ium salts, and the condensation of substrates containing the 4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine fragment is accompanied by a cascade rearrangement with unusual recyclization of the dihydropyrimidine ring to yield partially hydrogenated [1,2,4]triazolo[1,5-a:4,3-a′]dipyrimidin-5-ium or pyrimido[1′,2′:1,5][1,2,4]triazolo[3,4-b]quinazolin-5-ium salts. The proposed methodology exhibits a wide scope, providing rapid access to polycondensed derivatives of the [1,2,4]triazolo[1,5-a]pyrimidine scaffold. DFT calculations of the Gibbs free energies of possible isomers were performed to rationalize the experimentally observed reactivity and selectivity.
Tetrahedron, 2015, Vol. 71 (36), pp 6259-6271
2-Aminosubstituted 4,5,6,7-tetrahydro- and 4,7-dihydro[1,2,4]triazolo[1,5-a]pyrimidines heated with α-bromoketones in acetonitrile undergo selective quaternization at the N-3 atom of the triazolopyrimidine core with simultaneous oxidative aromatization of the dihydropyrimidine ring. The quaternized products can be cyclized into imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines by heating with alkali in ethanol solution. 6-Ethoxycarbonyl-4,7-dihydro[1,2,4]triazolo[1,5-a]pyrimidines, which are resistant to oxidative aromatization, in analogous conditions with α-bromoketones produce bromides of 5,8-dihydro-1H-imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines in one step. The unusual reaction of acid catalyzed hydrolytic cleavage of the imidazole ring of imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines in the presence of hydrobromic acid on heating is revealed. Tautomerism of partially hydrogenated imidazo[2′,1′:3,4][1,2,4]triazolo[1,5-a]pyrimidines was studied by experimental and computational methods. Some of these polycyclic compounds represent new partially hydrogenated mesoionic heterocycles.
The Journal of Organic Chemistry, 2015, Vol. 80 (1), pp 375–385
A combination of computational and experimental methods was used to examine the structure–reactivity relationships in the reactions of C-amino-1H-1,2,4-triazoles with electrophiles. The global nucleophilicity of 3-amino- and 3,5-diamino-1H-1,2,4-triazoles was predicted to be higher than that of 5-amino-1H-1,2,4-triazoles. Fukui functions and molecular electrostatic potential indicate that reactions involving an amino group should occur more easily for the 3-amino- than for the 5-amino-1H-1,2,4-triazoles. Increasing electrophile hardness should increase the probability of attack at the N-4 atom of the triazole ring, whereas increasing softness should enhance the probability of attack at the N-2 atom and 3-NH2 group. Calculated transition state energies of model SN2 reactions and experimental studies showed that quaternization of 1-substituted 3-amino- and 3,5-diamino-1H-1,2,4-triazoles by many alkyl halides proceeds with low selectivity and can involve the N-2 and N-4 atoms as well as the 3-NH2 group as reaction centers. A new method for the selective synthesis of 1,4-disubstituted 3-amino- and 3,5-diamino-1,2,4-triazoles based on quaternization of readily available 1-substituted 3-acetylamino-1,2,4-triazoles with subsequent removal of the acetyl protecting group by acid hydrolysis was developed.
Organic Letters, 2014, Vol. 16 (2), pp 406–409
The synthesis of 6-hetarylamino[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines is reported. The functionalized secondary amines were constructed via a K2CO3-mediated SNAr reaction of weakly basic hetarylamines with 3-(3,5-dimethylpyrazol-1-yl)[1,2,4]triazolo[4,3-b][1,2,4,5]tetrazines, which allowed displacement 3,5-dimethylpyrazolyl leaving group. Significantly, the reaction exhibited a broad substrate scope and proceeded in good yields.
Tetrahedron, 2014, Vol. 70 (3), pp 684-701
Partially hydrogenated 2-amino[1,2,4]triazolo[1,5-a]pyrimidines and 2-amino[1,2,4]triazolo[5,1-b]quinazolines react with the chlorides of chloroacetic, 3-chloropropanoic, and 4-chlorobutanoic acids at 0–5 °C to give amides through acylation of the 2-amino group. Heating the corresponding 3-chloropropanoyl derivatives at 80–90 °C in DMF leads to selective intramolecular alkylation at N-3 to form the chlorides of partially hydrogenated [1,2,4]triazolo[1,5-a:4,3-a′]dipyrimidin-5-ones and pyrimido[2′,1′:3,4][1,2,4]triazolo[5,1-b]quinazolin-12-ones. It may be more convenient to prepare such compounds through one-pot processes. Some reactions of the synthesized chlorides of polycondensed heterocycles have been studied. Conditions have been found to effect the selective synthesis of free bases, oxidative aromatization or hydrolysis of the dihydropyrimidine cycle, and the selective hydrolytic cleavage or elimination of the pyrimidone ring. Some of the resulting compounds represent new mesoionic heterocycles.