Former publications

2000

Applications of microwave irradiation in fullerene chemistry

Fernando Langa, Pilar de la Cruz, Antonio de la Hoz, Eva Espildora.
A review with 37 refs.  The authors present their results on the application of microwave irradn. as an efficient methodol. in [60]- and [70]fullerene chem.  In most cases, the effect of microwaves on the yield of a reaction is particularly important and shorter times and/or lower temps. can be used.  In [70]fullerene chem., modifications of the selectivity, in relation to classical heating, have also been obsd. – Proc. – Electrochem. Soc. (2000) vol. 2000-11 (Fullerenes 2000–Volume 9: Functionalized Fullerenes) pp. 168-178

Synthesis and Properties of Isoxazolo[60]fullerene-Donor Dyads

Fernando Langa, Pilar de la Cruz, Eva Espildora, Araceli Gonzalez-Cortes, Antonio de la Hoz, Vicente Lopez-Arza.
A series of isoxazolo[60]fullerenes I [R = 1-phenyl-4-pyrazolyl, 1-phenyl-5-pyrazolyl, Me, 4-pyridyl, 2-thienyl, 4-(dimethylamino)phenyl, ferrocenyl, 2-furyl] has been prepd. in one pot from aldoximes RCH:NOH and C60 under microwave irradn.  Several donors and acceptors were used as substituents.  The absorption and emission spectra of I in polar solvents suggest a weak charge-transfer interaction between the oxygen atom of the isoxazoline moiety and the C60 cage, as well as a stronger interaction between the donor and the fullerene cage for I [R = 4-(Me2N)C6H4, ferrocenyl].  The redn. potentials of I were investigated and they show the same or better acceptor character than C60 in all cases.  Theor. calcns. of the HOMO and LUMO energies, frontier orbitals, and conformations of I were performed which support the results obtained.  Solvent effects in the 1H NMR spectra have been detd. and provide useful information concerning the polarization of dyads. – J. Org. Chem. (2000) vol. 65 (25) pp. 8675-8684

Pd(II) Complexes with Polydentate Nitrogen Ligands. Molecular Recognition and Dynamic Behavior Involving Pd-N Bond Rupture. X-ray Molecular Structures of [{Pd(C6HF4)2}(bpzpm)] and [{Pd(η3-C4H7)}2(bpzpm)] (CF3SO3)2 [bpzpm = 4,6-Bis(pyrazol-1-yl)pyrimidine]

Felipe Gómez-de la Torre, Antonio de la Hoz, Felix Jalon, Blanca Manzano, Ana Rodriguez, Jose Elguero, Martin Martinez-Ripoll.
The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm) and 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Me-bpzpm) were synthesized and their reactions with some Pd derivs. explored.  Mononuclear or dinuclear neutral or cationic complexes were obtained by reaction of the ligands with 1 or 2 equiv of Pd(C6XF4)2(cod) (cod = 1,5-cyclooctadiene; X = F, H) or the Pd fragment [Pd(η3-2-Me-C3H4)(S)2]+ (S = acetone).  The reaction of the dinuclear derivs. with 1 equiv of the resp. free ligand immediately led to the regeneration of the mononuclear complexes.  Except in the case of the synthesis of [{Pd(C6HF4)2}{Pd(C6F5)2}(bpzpm)], where two similar metallic groups are present, all attempts to obtain dinuclear asym. complexes with two different Pd fragments failed.  Instead, the dinuclear sym. complexes were formed.  This result could be considered as an example of mol. recognition with the ligand acting as a ditopic receptor.  This behavior is comparable to chem. symbiosis but in this case applied to the ligand rather than to the metal center as occurs normally.  The polyfluorophenyl rings are situated on av. in a perpendicular orientation with respect to the coordination plane.  Their restricted rotation results in several atropisomers for the complexes with m-C6HF4.  Different cross-reaction expts. were carried out, and these showed the mobility of the metallic fragments, with the more difficult process being that involving the more strongly bonded polyfluorophenyl Pd groups.  By 1H NMR variable temp. studies, the interconversion of the two isomers of [{Pd(η3-C4H7)}2(bpzpm)]Tf2 (Tf = CF3SO3) was analyzed.  In the case of [{Pd(η3-C4H7)}(bpzpm)]Tf the existence of two processes, an intramol. apparent allyl rotation and an intermol. exchange of the allylpalladium fragments, was demonstrated.  Different ΔGc.thermod. values at the coalescence temps. also were detd.  An x-ray single-crystal anal. was carried out on [{Pd(η3-C4H7)}2(bpzpm)]Tf2, which crystallizes in the monoclinic system, space group I2/m, with a = 9.368(2), b = 16.191(3), c = 20.228(6) Å, β = 101.26(3), and Z = 4.  [{Pd(C6HF4)2}(bpzpm)] crystd. in the triclinic system, space group P.hivin.1, with a = 8.845(6), b = 12.6609(9), c = 12.826(3) Å, α = 88.45(2), β = 74.36(3), γ = 89.32(2)°, and Z = 2. – Inorg. Chem. (2000) vol. 39 (6) pp. 1152-1162

Microwave irradiation in solvent-free conditions: an eco-friendly methodology to prepare indazoles, pyrazolopyridines and bipyrazoles by cycloaddition reactions

Angel Diaz-Ortiz, Antonio de la Hoz, Fernando Langa.
A review with >42 refs. on the behavior of vinylpyrazoles, pyrazolyl imines and pyrazolyl hydrazones as dienes or dipoles.  Cycloaddn. of the aforementioned compds. with electron-poor reagents under microwave irradn. in solvent-free conditions permits the prepn. of indazoles, pyrazolopyridines and bipyrazoles in moderate to good yields.  This represents an eco-friendly, green approach to these valuable pyrazole derivs. that avoids the harsh or highly contaminating conditions involving classical heating and offers a redn. or even elimination of solvent use and recovery, simplification of the work-up procedures, facility of scale-up and savings in energy consumption, in addn. to higher yields. – Green Chem. (2000) vol. 2 (4) pp. 165-172

Synthesis of pyrazolo[3,4-b]pyridines by cycloaddition reactions under microwave irradiation

Angel Diaz-Ortiz, Jose Carrillo, Fernando Cossio, Maria Gomez-Escalonilla, Antonio de la Hoz, Andres Moreno, Pilar Prieto.
Microwave irradn. induces the cycloaddn. of pyrazolylimines with arom. and aliph. nitro alkenes to afford pyrazolo[3,4-b]pyridines within 5-20 min.  Some of these reactions do not occur under classical heating.  Tricyclic heterocycles can be synthesized from cycloalkenes.  The reactivity and regiochem. can be understood in terms of the energy and at. coeffs. of the frontier orbital, except in the case of 1-ethyl-6-(4-methoxyphenyl)pyrazolo[3,4-b]pyridine-4-nitrile.  Calcns. at the HF/3-21G level predict an asynchronous transition structure for this cycloaddn. with regiocontrol detd. by Coulombic interaction. – Tetrahedron (2000) vol. 56 (11) pp. 1569-1577

Modification of Regioselectivity in Cycloadditions to C70 under Microwave Irradiation

Fernando Langa, Pilar de la Cruz, Antonio de la Hoz, Eva Espildora, Fernando Cossio, Begona Lecea.
The regioselectivity of the cycloaddn. of N-methylazomethine ylide to C70 can be modified by using microwave irradn. as the source of energy.  Under microwave irradn. and by choosing the appropriate solvent and irradn. power, the 5-6 isomer is the major product, a situation that is in contrast to conventional heating where the 1-2 isomer predominates.  Moreover, isomer 7-21, which represents 13% of monoadducts under classical heating, is not formed under microwave irradn. and with ODCB as solvent.  Theor. calcns. predict an asynchronous mechanism and suggest that the modification of the regiochem. outcome is related to the relative energies and hardnesses of the transition structures involved. – J. Org. Chem. (2000) vol. 65 (8) pp. 2499-2507

Determination of syn/anti isomerism in DCNQI derivatives by 2D exchange spectroscopy: theoretical underpinning
Fernando Cossio, Antonio de la Hoz, Pilar de la Cruz, Fernando Langa, Nazario Martin, Pilar Prieto, Luis Sanchez.
The dynamic syn/anti isomerism resulting from the inversion of the cyano group at the C=N double bond in a series of substituted DCNQIs has been investigated in soln. by two-dimensional exchange spectroscopy (2D EXSY).  The isomers formed were characterized by 1H NMR at 223 K to slow down the inversion process of the cyanoimine group as well as by H,H COSY spectra.  Whereas most compds. show only one isomer, for some compds. two isomers with two inversion processes and for others three isomers with four inversion processes were obsd.  The energy barrier for the syn/anti isomerization of the NCN groups has been estd. from the rate consts. detd. from the 2D EXSY spectra, and very close values (13.16-13.93 kcal/mol for a single inversion) were found for all compds.  Isomerizations involving two NCN groups exhibited higher activation free energy values (13.41-14.40 kcal/mol).  Theor. calcns. of these free energy barriers are in excellent agreement with the exptl. values, esp. when solvent effects are taken into account.  Theor. calcns. at B3LYP/6-31G* level predict planar geometries for the DCNQI derivs. studied, in particular when solvent effects are considered.  This is in perfect agreement with the exptl. data.  Two-electron stabilizing interactions, as well as solvent effects, are related to the relative energies of the different stereoisomers.  The relative equil. populations of the different isomers have been calcd. using semiempirical energies and Boltzmann’s distribution.  Although the most stable isomer as detd. by AM1 method is in qual. agreement with that deduced from NMR expts. in all DCNQI derivs. studied, better quant. correlations are obtained at the B3LYP(L1A1)/6-31G* calcn. level.  Comparison of both semiempirical and ab initio calcns. reveal that in these DCNQI systems AM1 is the method of choice for those larger systems whose size prevents energy calcns. at higher levels. – Eur. J. Org. Chem. (2000) (13) pp. 2407-2415

2001

New complexes with pyrazole-containing ligands and different metallic centres. Comparative study of their fluxional behaviour involving M-N bond rupture

Jose Elguero, Ana Guerrero, Felipe Gómez de la Torre, Antonio de la Hoz, Felix Jalon, Blanca Manzano, Ana Rodriguez.
New complexes of palladium(II), silver(I), copper(I) and ruthenium(II) contg. the ligands 2-(pyrazol-1-yl)pyridine (pzpy), 2-(pyrazol-1-yl)pyrimidine (pzpm), 2-(3,5-dimethylpyrazol-1-yl)pyrimidine (pz*pm) and 4,6-bis(pyrazol-1-yl)-pyrimidine (bpzpm) were prepd. and characterized.  Only the isomer with the least steric hindrance was found for the derivs. PdClR(N-N’), R = Me or COMe, N-N’ = pzpy or pzpm, while the two possible isomers were found for similar derivs. with pz*pm.  In the reaction of [RuCl2(cod)]n with pzpy, pzpm and bpzpm, the isomer trans-RuCl2(cod)(N-N’) was always formed.  Three different isomers were consecutively formed after the reaction of RuCl2(PPh3)3 with bpzpm, with the thermodynamically stable isomer being cis,trans-RuCl2(PPh3)2(bpzpm).  A fluxional process involving interchange of the H4/H6 pyrimidine protons, which implies M-N(pm) bond rupture, was found in the silver, copper and some palladium derivs.  Such an interchange was not obsd. in the ruthenium complexes.  The trans influence of the group situated trans to the pyrimidine ring is extremely important for Pd-N bond rupture.  The isomer interconversion for the derivs. PdClR(pz*pm), R = Me or COMe, was studied by variable temp. 1H NMR studies.  The process is of higher energy than the H4/H6 interchange and is facilitated by the presence of the acyl group. – New J. Chem. (2001) vol. 25 (8) pp. 1050-1060

C60-Based Triads with Improved Electron-Acceptor Properties: Pyrazolylpyrazolino[60]fullerenes

Fernando Langa, Antonio de la Hoz, Eva Espildora, Pilar de la Cruz, Jose Bourdelande, Luis Sanchez, Nazario Martin.
A series of triad pyrazolylpyrazolino[60]fullerenes has been prepd. in one pot from suitably functionalized hydrazones by 1,3-dipolar cycloaddn. reactions under microwave irradn.  The electrochem. properties of the compds. obtained were investigated by cyclic voltammetry, and they show better electron acceptor character than the parent C60 in all cases.  Fluorescence expts. and time-resolved transition spectroscopy indicate the existence of photoinduced charge-transfer processes with the C60 triplet acting as the acceptor. – J. Org. Chem. (2001) vol. 66 (15) pp. 5033-5041

Strained π-systems as hydrogen bond acceptors: the case of benzyne

Pilar Prieto, Antonio de la Hoz, Ibon Alkorta, Isabel Rozas, Jose Elguero.
The behavior of strained π-systems with regard to their interaction with H bond (HB) donors was studied.  The interaction of a model system, strained acetylene, and ortho-benzyne with HF in their singlet and triplet electronic configuration was explored at the B3LYP/6-311++G** and MP2/6-311++G** levels.  The energetic results indicate that there are two preferred approaches, the first with the HF pointing towards one of the radical C atoms and the second that corresponds to the HB perpendicular to the π-bond.  The atoms in mols. (AIM) anal. of the electron d. shows a conflict catastrophic situation for several of the approaches. – Chem. Phys. Lett. (2001) vol. 350 (3,4) pp. 325-330

Microwave-assisted cyclocondensation under solvent-free conditions: quinoxaline-2,3-dione

Ester Vazquez, Antonio de la Hoz, Nils Elander, Andres Moreno, Sharon Stone-Elander.
Use of monomodal microwave techniques together with solvent-free conditions in the condensation of 1,2-phenylenediamine with di-Et oxalate gives the quinoxaline-2,3-dione in good to high yields after 2-3 min.  The method is simple, rapid and avoids prolonged heating with corrosive mineral acids. – Heterocycles (2001) vol. 55 (1) pp. 109-113

Synthesis, x-ray Structure, and Properties of 2-(1′-Pyridin-2′-one)benzimidazole

Antonio de la Hoz, Ines Almena, Concepcion Foces-Foces, Manuel Yanez, Otilia Mo, Manuel Alcami, Nadine Jagerovic, Jose Elguero.
2-(1′-Pyridin-2′-one)benzimidazole was synthesized to det. its structure in the solid state, its NMR properties, and its behavior in absorption-emission electronic spectroscopy.  To rationalize these properties, ab initio calcns. were carried out on its ground and excited states.  A three-center N-H···O:C H bond connects the mols. in the crystal forming chains parallel to the c axis.  The intramol. H bond promotes coplanarity of both ring systems.  In the ground state, only the N-H tautomer exists as a local min. of the potential energy surface, while in the excited-state, both tautomers are stable.  The main consequence is that the photochem. behavior of this compd. is governed by the existence of a proton transfer in its excited state. – J. Phys. Chem. B (2001) vol. 105 (51) pp. 12759-12770

Diels-Alder cycloaddition of 4,6-dimethyl-1,2,3-triazine with enamines, or their precursors, under microwave irradiation

Angel Diaz-Ortiz, Antonio de la Hoz, Pilar Prieto, Jose Carrillo, Andres Moreno, Hans Neunhoeffer.
4,6-Dimethyl-1,2,3-triazine undergoes Diels-Alder cycloaddn. with enamines, derived from cyclic ketones, under microwave irradn. within 20 min.  This methodol. represents a dramatic improvement on the classical method and allows fused pyridine systems to be obtained, in some cases, in good yields.  E.g., 4,6-dimethyl-1,2,3-triazine was reacted with enamine I for 20 min. at 150° under microwave irradn. to give 2,3-cyclopentenopyridine II in 40% yield. – Synlett (2001) (2) pp. 236-237

Tandem diels-alder aromatization reactions of furans under unconventional reaction conditions – experimental and theoretical studies

Jose Fraile, Jose Garcia, Maria Gomez, Antonio de la Hoz, Jose Mayoral, Andres Moreno, Pilar Prieto, Luis Salvatella, Ester Vazquez.
Lewis acids such as zinc chloride, diethylaluminum chloride, and titanium tetrachloride supported on silica are good catalysts for Diels-Alder reactions between furan and acrylonitrile and Me acrylate to give oxabicycloheptenes at room temp.  When 2,5-dimethylfuran is used as the diene, yields of the oxabicycloheptene Diels-Alder adducts with Me acrylate are lower, due in part to the appearance of the aromatization product Me 2,5-dimethylbenzoate.  Reaction of 2,5-dimethylfuran with Me acrylate and acrylonitrile in the presence of catalyst and microwave activation gives the aromatization products Me 2,5-dimethylbenzoate and 2,5-dimethylbenzonitrile in up to 80% yield; the use of microwave heating and silica-supported catalysts thus constitutes a good synthetic route to polysubstituted arom. compds.  Computational studies on the Diels-Alder and aromatization mechanisms with calcd. transition state structures and the role of the catalyst on the product distribution show that “hard” Lewis acids, such as aluminum derivs., make ring-opening of the adduct to afford arom. products easier.  The theor. results are in excellent agreement with the relative reactivity obsd. for the different dienes and dienophiles. – Eur. J. Org. Chem. (2001) (15) pp. 2891-2899

Synergy between heterogeneous catalysis and microwave irradiation in an efficient one-pot synthesis of benzene derivatives via ring-opening of Diels-Alder cycloadducts of substituted furans

Antonio de la Hoz, Angel Diaz-Ortiz, Jose Fraile, M Victoria Gómez, Jose Mayoral, Andres Moreno, Alfonso Saiz, Ester Vazquez.
The use of silica-supported Lewis acids as catalysts under microwave irradn. promotes regiospecific opening of the 7-oxa bridge of Diels-Alder cycloadducts, producing the corresponding arenes in a single step.  This rapid and efficient procedure permits the synthesis of 1,2-, 1,2,4- and 1,2,3,4-substituted benzenes by reaction of 2,5-dimethylfuran, 2-methoxyfuran, and 2-ethylfuran with dienophiles such as acrylonitrile, Me acrylate, N-methylmaleimide and fumaronitrile. – Synlett (2001) (6) pp. 753-756

2002

Relation between charge transfer and solvent polarity in fullerene derivatives: NMR studies

Fernando Langa, Pilar de la Cruz, Juan Delgado, Eva Espildora, Maria Gomez-Escalonilla, Antonio de la Hoz.
In this paper, we show that solvent-dependent NMR studies can be used as an indication of charge transfer processes in the ground state in isoxazolino- and pyrazolinofullerenes.  NMR studies have several advantages over other techniques, not only can the occurrence of the CT process be detected but the power of NMR spectroscopy in structural detn. can be used to indicate the group responsible for the CT process in [60]fullerene-based materials. – J. Mater. Chem. (2002) vol. 12 (7) pp. 2130-2136

Synthesis of 1,3,5-triazines in solvent-free conditions catalyzed by silica-supported lewis acids

Angel Diaz-Ortiz, Antonio de la Hoz, Andres Moreno, Ana Sanchez-Migallon, Gema Valiente.
The prepn. of sym. substituted 1,3,5-triazines was performed by cyclotrimerization of nitriles in solvent-free conditions using silica-supported Lewis acids as catalysts.  Although microwave irradn. gives the best overall results in a short time, the best yields were obtained by conventional heating over periods of 24 h.  The use of these recyclable and environmentally benign catalysts produces yields similar to those obtained with the more complex and contaminant lanthanide catalysts.  This fact, together with the use of solvent-free conditions, makes this procedure an environmentally benign synthetic method. – Green Chem. (2002) vol. 4 (4) pp. 339-343

Synthesis and properties of pyrazolino[60]fullerene-donor systems

Eva Espildora, Juan Delgado, Pilar de la Cruz, Antonio de la Hoz, Vicente Lopez-Arza, Fernando Langa.
A series of pyrazolino[60]fullerenes has been prepd. in one pot by 1,3-dipolar cycloaddn. to C60 of the corresponding nitrile imine, which was generated in situ from the corresponding hydrazone.  A range of donors and acceptors were introduced as substituents.  Electrochem. and photophys. studies have revealed weak ground-state interactions between the org. addends and the fullerene sphere.  Steady-state fluorescence has shown that, in both toluene and benzonitrile solns., an efficient electron transfer process takes place when a strong donor is attached to the pyrazolino[60]fullerene system. – Tetrahedron (2002) vol. 58 (29) pp. 5821-5826

Solvent-free synthesis and structural characterization of azolyl-substituted pyrimidines

Antonio de la Hoz, Hector Blasco, Angel Diaz-Ortiz, Jose Elguero, Concepcion Foces-Foces, Andres Moreno, Ana Sanchez-Migallon, Gema Valiente.
Base-catalyzed trimerization of N-cyanomethylazoles under pressure and in solvent-free conditions afforded 4-amino-2,6-bis(azol-1-ylmethyl)-5-(azol-1-yl)pyrimidines [azol-1-yl = 1-pyrazolyl, 1-imidazolyl, 1,2,4-triazol-1-yl] in 33-67% yields.  The structures of these compds. was detd. by a combination of NMR techniques and X-ray crystallog.  The 4-amino groups show a restricted rotation around the C-N bond; the free energy of activation for this process was detd. by variable temp. expts.  In the crystal structure of the pyrazol-1-yl deriv., the amino group shows a distorted planar geometry in both independent mols. and acts as a double hydrogen bond donor towards two of the three pyrazole rings, forming ribbons of R22(10) rings. – New J. Chem. (2002) vol. 26 (7) pp. 926-932

The importance of the linking bridge in donor-C60 electroactive dyads

Fernando Langa, Pilar de la Cruz, Juan Delgado, Maria Gomez-Escalonilla, Araceli Gonzalez-Cortes, Antonio de la Hoz, Vicente Lopez-Arza.
The synthesis, spectroscopic characterization, electrochem. and photophys. measurements of three 2-methoxyphenylfullerene derivs. are described and their properties compared.  Cyclic and Osteryoung square-wave voltammetry studies indicate that the pyrazolino[60]fullerene shows improved electron affinity in comparison to the parent C60.  Preliminary photophys. studies suggest the occurrence of an electron transfer process. – New J. Chem. (2002) vol. 26 (1) pp. 76-80

Theoretical study on the reaction between 4,6-dimethyl-1,2,3-triazine and enamines

Pilar Prieto, Fernando Cossio, Jose Carrillo, Antonio de la Hoz, Angel Diaz-Ortiz, Andres Moreno.
The reaction between 4,6-dimethyl-1,2,3-triazine and cyclopentanone pyrrolidine enamine has been studied using ab initio SCF-MO computational methods.  Solvent effects have also been taken into account.  The reaction is predicted to be a concerted Diels-Alder cycloaddn.  Self-consistent reaction field methods based on multipole expansions of the free energy of solvation tend to overestimate the stability of zwitterionic intermediates at the Hartree-Fock level.  This overestimation results in a stepwise mechanism, although higher levels of theory predict a concerted mechanism. – J. Chem. Soc., Perkin Trans. 2 (2002) (7) pp. 1257-1263

Microwaves in cycloadditions

Antonio de la Hoz, Angel Diaz-Ortiz, Fernando Langa.
A review on the techniques and applications of microwave irradn. in cycloaddn. reactions.  Modifications of chemo-, regio- and stereoselectivity which result from use of microwaves are discussed.  – Microwaves Org. Synth. (2002) pp. 295-343

Microwave-enhanced reactivity of non-activated dienophiles towards pyrazine o-quinodimethanes

Angel Diaz-Ortiz, Antonio de la Hoz, Andres Moreno, Pilar Prieto, Rafael Leon, Maria Herrero.
Microwave irradn. in solvent-free conditions induces cycloaddn. reactions of pyrazine o-quinodimethane intermediates with electron-rich dienophiles within 10-15 min to afford quinoxaline derivs. in acceptable yields. – Synlett (2002) (12) pp. 2037-2038

2003

Enhancing stereochemical diversity by means of microwave irradiation in the absence of solvent: synthesis of highly substituted nitroproline esters via 1,3-dipolar reactions

Angel Diaz-Ortiz, Antonio de la Hoz, M Herrero, Pilar Prieto, Ana Sanchez-Migallon, Fernando Cossio, Ana Arrieta, Silvia Vivanco, Concepcion Foces-Foces.
Microwave irradn. induces the 1,3-dipolar cycloaddn. of imines derived from α-aminoesters with β-nitrostyrenes in the absence of solvent within 10-15 min.  The reaction proceeds to give yields in the range 81-86% and three isomeric pyrrolidines are obtained in the cycloaddn.  Consequently, the use of three imines and two β-nitrostyrenes gives rise to a library of 18 nitroproline esters.  The use of classical heating with longer reaction times (24 h) gives lower yields of products (below 50%) and only two stereoisomers can be detected in each reaction. – Mol. Diversity (2003) vol. 7 (2-4) pp. 175-180

Five Different Fluxional Processes in Polyfluorophenyl Palladium(II) Complexes with 2,4,6-Tris(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine. The Driving Effect of the Solvent

M Carrion, Ana Guerrero, Felix Jalon, Blanca Manzano, Antonio de la Hoz, Rosa Claramunt, Viktor Milata, Jose Elguero.
The polydentate N-donor ligand 2,4,6-tris(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (Me2-TpzT) was used to synthesize the new palladium derivs. Pd(R)2(Me2-TpzT), R = C6F5, 1; R = m-C6ClF4, 2.  In the case of complex 2, four different atropisomers were detected at low temp.  The new complexes exhibit a rich dynamic behavior, including three metallotropic processes (metal-hurdling, 1,4-metallotropic shifts, and an intermol. process) and two processes involving restricted rotation of arom. rings (the polyfluorophenyl groups and the uncoordinated pyrazole group adjacent to the metal fragment).  The fluxional behavior was studied by 1H and 19F NMR spectroscopy using variable temp. NMR studies and 1H,1H and 19F,19F EXSY expts.  The study of solns. of 2 in 1,1′,2,2′-tetrachloroethane-d2 gave the following order for the free energy of activation: pyrazole rotation < 1,4-metallotropic shift < intermol. exchange < polyfluorophenyl rotation.  The process of metal hurdling was not found in this solvent.  However, in acetone-d6 such a process was detected and was found to be of lower energy than the 1,4-metallotropic shift.  In dil. acetone-d6 or 1,1′,2,2′-tetrachloroethane-d2 solns., the intermol. process was not obsd.  Conclusions concerning the different mechanisms were deduced from the data obtained. – Inorg. Chem. (2003) vol. 42 (3) pp. 885-895

Synthesis, structural determination and dynamic behavior of 2-chloro-4,6-bis(pyrazolylamino)-1,3,5-triazines

Angel Diaz-Ortiz, Jose Elguero, Concepcion Foces-Foces, Antonio de la Hoz, Andres Moreno, Sergio Moreno, Ana Sanchez-Migallon, Gema Valiente.
A series of 2-chloro-4,6-bis(pyrazolylamino)-1,3,5-triazines with applications in crystal engineering have been prepd.  At low temp., the presence of two or three isomers has been detected and these assigned to 4,6-diamino-1,3,5-triazine structures on the basis of comparison with model compds. 2D-Exchange spectroscopy studies in various solvents and at different temps. have been used to det. the equil. consts. and the activation free energies of the restricted rotation about the amino-triazine bond.  A plot of the activation free energy vs. temp. showed a good linear correlation and confirmed that the same process is present in all of the compds. under investigation.  Comparison with model compds. also confirmed both the occurrence of the restricted rotation and the 4,6-diamino-1,3,5-triazine tautomerism for triazines in soln.  The structure of compd. I has been detd. in the solid state by X-ray crystallog. and consists of a 4,6-diamino-1,3,5-triazine structure stabilized by intra and intermol. hydrogen bonds. – Org. Biomol. Chem. (2003) vol. 1 (24) pp. 4451-4457

The structure of N1-hydroxylophine N3-Oxide (=1-hydroxy-2,4,5-triphenyl-1H-imidazole 3-oxide) in the solid state

Ana Sanchez-Migallon, la de, Concepcion Lopez, Rosa Claramunt, Lourdes Infantes, Sam Motherwell, Kenneth Shankland, Harriott Nowell, Ibon Alkorta, Jose Elguero.
The crystal structure of 1-hydroxy-2,4,5-triphenyl-1H-imidazole 3-oxide (1) has been detd. from X-ray powder-diffraction data.  The two independent mols. in the asym. unit form chains via O-H···O hydrogen bonds related by a twofold screw axis.  One of the O···O distances is extremely short (2,32(1) and 2.43(1) Å).  Solid-state NMR spectroscopy (CPMAS) combined with calcn. of abs. shieldings (GIAO/B3LYP/6-31G*) allowed us to det. that the compd. behaves as if the O-H···O hydrogen bond has the proton in the middle (single-well potential), resulting in the near identity of both 15N-NMR signals. – Helv. Chim. Acta (2003) vol. 86 (4) pp. 1026-1039

Application of microwave irradiation, solid supports and catalysts in environmentally benign heterocyclic chemistry

Jose Carrillo, Angel Diaz-Ortiz, Antonio de la Hoz, Andres Moreno, Maria Gomez, Pilar Prieto, Ana Sanchez-Migallon, Ester Vazquez.
A review on applications of supported reagents and microwave irradn. in green chem. protocols for heterocyclic chem.  Included are the synthesis of coumarin derivs., arom. compds. from furan derivs., α- and β-substituted alanine derivs., 1,3,5-triazines, and quinoxaline-2,3-diones.  In all cases shorter reaction times were required than in the corresponding reactions using classical thermal heating and the contaminating inorg. acids commonly used as catalysts in these reactions were avoided. – Targets Heterocycl. Syst. (2003) vol. 7 pp. 64-85

First Diels-Alder reactions of 3-styrylchromones under microwave irradiation

Diana Pinto, Artur Silva, Lucia Almeida, Jose Carrillo, Angel Diaz-Ortiz, Antonio de la Hoz, Jose Cavaleiro.
Microwave irradn. in solvent-free conditions induces Diels-Alder cycloaddn. reactions of 3-styrylchromones with N-methyl- and N-phenylmaleimide giving 4-aryl-1,3-dioxopyrrolo[3,4-c]-3a,4,11a,11b-tetrahydroxanthones in good yields.  This is the first example of [4+2] cycloaddn. reactions of 3-styrylchromones. – Synlett (2003) (10) pp. 1415-1418

2004

Green synthesis and self-association of 2,4-diamino-1,3,5-triazine derivatives

Angel Diaz-Ortiz, Jose Elguero, Concepcion Foces-Foces, Antonio de la Hoz, Andres Moreno, Carmen del, Ana Sanchez-Migallon, Gema Valiente.
2,4-Diamino-1,3,5-triazines I [R = Ph, 4-MeOC6H4, 4-O2NC6H4, 4-ClC6H4, 2-(1-pyrazolyl)C6H4, 3-(1-pyrazolyl)C6H4, 1-Ph-4-pyrazolyl, 4-pyridinyl, 1-piperidinyl, 4-morpholinyl] and bis(diaminotriazinyl)benzene II are prepd. by cyclocondensation of dicyandiamide H2NC(:NH)NHCN with nitriles RCN (or, in the case of II, with 1,2-benzenedicarbonitrile) by an environmentally friendly procedure using microwave irradn.; the procedure decreases the vol. of solvent used, is simple to perform, and requires less time than previous methods.  The structures of I (R = Ph, 1-Ph-3-pyrazolyl, 1-piperidinyl) are detd. by NMR; in addn., the structures of I are detd. by X-ray crystallog.  The barriers to rotation about the amino-triazine C-N bonds of I [R = Ph, 3-(1-pyrazolyl)C6H4, 4-pyridinyl, 4-morpholinyl] are detd. by variable-temp. NMR expts.  Intermol. hydrogen-bonding interactions differ among the aminotriazines, resulting in variations in crystal structure motif from pseudo-honeycomb networks to corrugated rosette layers. – New J. Chem. (2004) vol. 28 (8) pp. 952-958

An efficient one-pot synthesis of phenol derivatives by ring opening and rearrangement of Diels-Alder cycloadducts of substituted furans using heterogeneous catalysis and microwave irradiation

Andres Moreno, Maria Gomez, Ester Vazquez, Antonio de la Hoz, Angel Diaz-Ortiz, Pilar Prieto, Jose Mayoral, Elisabet Pires.
The use of silica-supported Lewis acids as catalysts under microwave irradn. promotes regiospecific opening of the 7-oxa bridge of Diels-Alder cycloadducts of furan derivs. and produces polysubstituted phenols in a single step.  This rapid and efficient procedure permits the synthesis of tri-, tetra- and pentasubstituted benzene derivs. by reaction of 2,5-dimethylfuran, 2-ethylfuran and 2-methoxyfuran with dienophiles such as di-Me acetylenedicarboxylate and Me propiolate. – Synlett (2004) (7) pp. 1259-1263

2005

Use of different microporous and mesoporous materials as catalyst in the Diels-Alder and retro-Diels-Alder reaction between cyclopentadiene and p-benzoquinone

M Gomez, Angel Cantin, Avelino Corma, Antonio de la Hoz.
Diels-Alder cycloaddn. between cyclopentadiene and p-benzoquinone has been studied using amorphous silica and different ITQ-2 and MCM-41 pure silica and metal contg. materials as catalysts.  The reaction can afford different products depending on the mol. reacting site, and the possibility of consecutive addns.  Structured solid catalysts increase the selectivity to the endo-endo isomer.  Silanol groups have not enough Broensted acidity to interact with the carbonyl groups present in the dienophile, to reduce LUMO’s energy and provide a better overlap between HOMO and LUMO, according to the frontier MO theory.  The introduction of transition metal atoms in the framework increases the reaction rate for the Diels-Alder reaction while preserving the selectivity to the endo-endo isomer.  The presence of more acidic OH groups enhance the retro-Diels-Alder reaction increasing the selectivity to the endo-exo isomer. – J. Mol. Catal. A: Chem. (2005) vol. 240 (1-2) pp. 16-21

Microwave-assisted synthesis and dynamic behaviour of N2,N4,N6-tris(1H-pyrazolyl)-1,3,5-triazine-2,4,6-triamines

Angel Diaz-Ortiz, Jose Elguero, Antonio de la Hoz, Agustin Jimenez, Andres Moreno, Sergio Moreno, Ana Sanchez-Migallon.
A series of N2,N4,N6-tris(1H-pyrazolyl)-1,3,5-triazine-2,4,6-triamines, e.g. I, has been synthesized under microwave irradn. in solvent-free conditions.  By reaction of pyrazolylamines with cyanuric chloride and 2-chloro-4,6-diamino-1,3,5-triazines under microwave irradn., 1,3,5-triazine-2,4,6-triamines with sym. and asym. substitution, resp., can be obtained.  In the latter case, the procedure can be easily adapted by addn. of a small amt. of DMSO for the prepn. of polymer-supported triazines, with application in supramol. combinatorial synthesis.  At low temp., the presence of two or four conformers has been detected for sym. and asym. substituted derivs. resp.  1D- and 2D-exchange spectroscopy studies in various solvents and at different temps. have been used to det. the equil. consts. and the activation free energies of the restricted rotation about the amino-triazine bond.  A plot of the activation free energy vs. temp. shows a good linear correlation and confirms that the same process is present in all of the compds. under investigation. – QSAR Comb. Sci. (2005) vol. 24 (5) pp. 649-659

Activation of organic reactions by microwaves

Antonio de la Hoz, Angel Diaz-Ortiz, Andres Moreno.
A review.  Microwave irradn. was successfully applied in org. chem.  Spectacular accelerations, higher yields under milder reaction conditions and higher product purities have all been reported.  Indeed, a no. of authors have described success in reactions that do not occur by conventional heating and even modifications of selectivity (chemo-, regio- and stereoselectivity).  An overview of the application of microwave irradn. in org. synthesis and the combination with other synthetic methodologies is included.  The effect of microwave irradn. in org. synthesis is a combination of thermal effects, arising from the heating rate, superheating or hot spots, the selective absorption of radiation by polar substances and the existence of nonthermal effects of the highly polarizing radiation – the specific microwave effect.  Such phenomena are not usually accessible by classical heating. – Adv. Org. Synth. (2005) vol. 1 pp. 119-171

Microwaves in organic synthesis. Thermal and non-thermal microwave effects

Antonio de la Hoz, Angel Diaz-Ortiz, Andres Moreno.
The review discusses the current state of thermal and non-thermal microwave effects along with a view on how these phenomena can be effectively used in org. synthesis.  Thermal effects arise from the different characteristics of microwave dielec. heating and conventional heating and include overheating of polar liqs., which affect the product distribution, “hot spots” and the selective absorption of radiation by polar substances.  Such phenomena are not usually accessible by classical heating and the existence of non-thermal effects of highly polarizing radiation-the “specific microwave effect”-is still a controversial topic. – Chem. Soc. Rev. (2005) vol. 34 (2) pp. 164-178

The Unusual Transformation of an Aromatic 1H-Imidazole into a Non-Aromatic 2H-Imidazole

Antonio de la Hoz, Ana Sanchez-Migallon, Maria Mateo, Pilar Prieto, Lourdes Infantes, Jose Elguero.
2H-Imidazole deriv. I has been synthesized and characterized by the X-ray diffraction (XRD) method.  The compd. crystallizes in the monoclinic space group Cc with cell parameters a = 19.398(1), b = 8.890(1), c = 10.247(1), β = 110.76(1), Z = 4.  The mols. are inter-linked through C-H···O and C-H···π interactions forming infinite chains. – Struct. Chem. (2005) vol. 16 (5) pp. 485-490

Reactivity of 3-styrylchromones as dienes in Diels-Alder reactions under microwave irradiation: A new synthesis of xanthones

Diana Pinto, Artur Silva, Cristela Brito, Angela Sandulache, Jose Carrillo, Pilar Prieto, Angel Diaz-Ortiz, Antonio de la Hoz, Jose Cavaleiro.
Microwave irradn. under solvent-free conditions induced 3-styrylchromones to undergo Diels-Alder cycloaddn. reactions with N-methyl- and N-phenylmaleimide as well as with other dienophiles.  In some of these reactions the cycloadducts underwent in situ oxidn. to give xanthones, whereas in other cases it was necessary to add DDQ as an oxidant.  The reactions of 3-styrylchromones with N-methyl- and N-phenylmaleimide gave the cycloadducts 4-aryl-1,3-di-oxo-3a,4,11a,11b-tetrahydropyrrolo[3,4-c]xanthones, which were oxidized to 4-aryl-1,3-dioxopyrrolo[3,4-c]xanthones with DDQ.  The reactions of 3-styrylchromones with 2-(2-nitrovinyl)thiophene gave 2-aryl-3-(2-thienyl)xanthones directly through a Diels-Alder reaction followed by HNO2 elimination and oxidn.  The cycloaddn. reactions of 3-styrylchromones described herein are stereoselective.  The reactions of (Z)- and (E)-3-styrylchromones with N-methylmaleimide gave, resp., the endo and exo cycloadducts, while reactions of (Z)-3-styrylchromones with the less reactive dienophile N-phenylmaleimide gave a mixt. of compds.  This mixt. contained the expected endo cycloadduct and another compd. that was identified as the exo adduct of the reaction between the (E)-3-styrylchromone, obtained in situ from the isomerization of its (Z) isomer, and N-phenylmaleimide.  The regio- and stereoselectivity of these cycloaddn. reactions have been studied by ab initio calcns. and can be understood in terms of a thermodynamically controlled reaction.  Theor. calcns. are in complete agreement with the exptl. results. – Eur. J. Org. Chem. (2005) (14) pp. 2973-2986

 

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