Publicaciones recientes

Aromatic guanidines as highly active binary catalytic systems for the fixation of CO2 into cyclic carbonates under mild conditions

Catal. Sci. Technol., 2019, 9, 3879-3886. DOI: 10.1039/C9CY00667B

We have synthesised a set of aromatic mono- and bis(guanidines) which are highly effective binary catalytic systems (guanidine/cocatalyst) for the formation of cyclic carbonates. The presence of multiple N–H bonds causes a modification in the traditional mechanism proposed for the synthesis of cyclic carbonates catalysed by guanidines through the formation of hydrogen bonds between the oxygen atom of the epoxide and the N–H groups of the guanidines. This change allows a considerable reduction of the reaction temperature and CO2 pressure employed in this process

Graphical abstract: Aromatic guanidines as highly active binary catalytic systems for the fixation of CO2 into cyclic carbonates under mild conditions

9-Borabicyclo[3.3.1]nonane: a metal-free catalyst for the hydroboration of carbodiimides

Chem. Commun., 2019, 55, 3073. DOI: 10.1039/c9cc00593e

The commercial 9-borabicyclo[3.3.1]nonane dimer is used as the first example of a metal-free catalyst for the monohydroboration of carbodiimides with pinacol borane. Stoichiometric reactions, kinetic studies, and DFT calculations have allowed us to propose a plausible mechanism involving a heterocyclic amidinate intermediate with a three center-two electron B–H–B bond.

Graphical abstract: 9-Borabicyclo[3.3.1]nonane: a metal-free catalyst for the hydroboration of carbodiimides

Unusual ligand rearrangement: from N-phosphinoguanidinato to phosphinimine-amidinato compounds

 Chem. Commun., 2019, 55, 2809. DOI: 10.1039/c9cc00432g 

Novel N-phosphinoguanidines (HNiPr)(Ph2PNiPr)C(NAr) (Ar = 2,6-iPr2C6H3, 2,4,6-Me3C6H2) react with AlMe3 to afford phosphinimine-amidinato derivatives, via an unprecedented rearrangement of an initial N-phosphinoguanidinato intermediate. A reasonable mechanism has been proposed for this transformation, supported by DFT calculations, involving carbodiimide de-insertion followed by a [3+2] cycloaddition.

Graphical abstract: Unusual ligand rearrangement: from N-phosphinoguanidinato to phosphinimine-amidinato compounds
 

Selective Three-Component Coupling for CO2 Chemical Fixation to Boron Guanidinato Compounds

Inorg. Chem., 2018, 57, 8404−8413 DOI: 10.1021/acs.inorgchem.8b01068

A selective three-component coupling was employed to fix carbon dioxide to boron guanidinato compounds. The one-pot reaction of carbon dioxide, carbodiimides, and borylamines (ArNH)BC8H14 afforded the corresponding 1,2-adducts {R(H)N}C{N(Ar)}(NR)-(CO2)BC8H14. Alternatively, the reaction with p-MeOC6H4NC or 2,6-Me2C6H3NC gave the corresponding isocyanide 1,1-adducts {i-PrHN}C{N(p-Me-C6H4)}(Ni-Pr)-{CNAr}BC8H14. The molecular structures of products (2,6-i-Pr2C6H3NH)BC8H14 7, {i-Pr(H)N}C{N(p-MeC6H4)}(Ni-Pr)(CO2)BC8H14 9, {Cy(H)N}C{N(p-MeC6H4)}(Cy)-(CO2)BC8H14 13, and {i-PrHN}C{N(p-MeC6H4)}(Ni-Pr)-{CNR″}BC8H14 (R″ = p-MeOC6H4, 2,6-Me2C6H3) 14 and 15 were established by X-ray diffraction. Density functional theory calculations at the M05-2X level of theory revealed that CO2 fixation and formation of the corresponding adduct is exothermic and proceeds via a nonchelate boron guanidinato intermediate.

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Carbodiimides as catalysts for the reduction of CO2 with boranes

Chem. Commun., 2018, 54, 4700 DOI: 10.1039/c8cc02139b

Carbodiimides catalyse the reduction of CO2 with H-BBN or BH3– SMe2 to give either mixtures of CH2(OBBN)2 and CH3OBBN or (MeOBO)3 and B(OMe)3 under mild conditions (25–60ºC, 1 atm CO2). Stoichiometric reactions and theoretical calculations were performed to unveil the mechanism of these catalytic processes.

 

Guanidine Substitutions in Naphthyl Systems to Allow a Controlled Excited-State Intermolecular Proton Transfer: Tuning Photophysical Properties in Aqueous Solution

J. Phys. Chem. C 2018, 122, 9363−9373 DOI: 10.1021/acs.jpcc.8b02576

The excited-state intermolecular proton transfer process (ESPT) in aqueous solution is achieved and controlled by the incorporation of guanidine groups in a fluorescent structure. The bisguanidine under investigation exhibits a dual fluorescence emission with a very high Stokes shifts in water, ≈86 (7890) and 210 (14 500) nm (cm−1), and an excited-stated deprotonation coupled to an intramolecular charge transfer (ICT) process contributes to this emission. The study demonstrates that the emission properties of the different protonation states are strongly dependent on the solvent environment, which also allows luminescence of the molecule to be tuned. The results of this work show the potential utility of guanidine substitution for the stabilization of ESPT−ICT processes in water and allow the subsequent logical design of new stimulus-responsive fluorophores.

Simple ZnEt2 as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies

Dalton Trans., 2017, 46, 12923 DOI: 10.1039/C7DT02700A

Expanding the possibilities of the use of simple and available ZnEt2 as a catalyst, the hydroalkynylation of carbodiimides with a variety of alkynes to obtain unsaturated substituted amidines is described in this work. Different stoichiometric studies allow proposing that amidinate complexes are intermediates in this catalytic process, produced by easy activation of the C–H bond of the alkyne and formation of alkynyl derivatives followed by a carbodiimide insertion step. Kinetics studies allowed the generation of a rate law for the hydroalkynylation of N,N′-diisopropylcarbodiimide with phenylacetylene which is second order in [carbodiimide], first order in [catalyst] and zero order in [alkyne], with a negligible PhC[triple bond, length as m-dash]CH/PhC[triple bond, length as m-dash]CD isotopic effect, consistent with a rate-determining state involving carbodiimide insertion. The hydroalkynylation reaction has been coupled with isocyanate (and isothiocyanate) insertion and intramolecular hydroamination to obtain imidazolidin-2-ones (or thione). The structures of different plausible intermediates have been determined by X-ray diffraction studies.

Graphical abstract: Simple ZnEt2 as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies
 

Insertion reactions of small unsaturated molecules in the N–B bonds of boron guanidinates

Dalton Trans., 2017,46, 10281-10299 DOI: 10.1039/C7DT02081C

We report here 1,1- and 1,2-insertion reactions of small unsaturated molecules in the N–B bonds of two boron guanidinates, (Me2N)C(NiPr)2BCy2 (1) and {iPr(H)N}C(NiPr){N(ptBu-C6H4)}BCy2 (2), and two bisboron guanidinates(2–), {iPr(BCy2)N}C(NiPr){N(ptBu-C6H4)}BCy2 (3) and {iPr(C8H14B)N}C(NiPr){N(p-Me-C6H4)}BC8H14 (4), the latter being prepared for the first time by double deprotonation of the corresponding guanidine with the 9-borabicyclo[3.3.1]nonane dimer, (H-BC8H14)2. Compounds 1–4 easily insert aromatic isonitriles, XylNC (Xyl = 2,6-Me2-C6H3) and (p-MeO-C6H4)NC, to give the expected diazaboroles 5–12, some of them being structurally characterised by X-ray diffraction. Interestingly, the BC8H14 derivatives 11 and 12 are in a fast temperature-dependent equilibrium with the de-insertion products, whose thermodynamic parameters are reported here. A correlation between these equilibria and the puckered heterocyclic structure found in the solid state for 11, and confirmed by DFT calculations, is also established. Reactions of the aforementioned guanidinates with CO are more sluggish or even precluded, and only one product, {iPr(H)N}C{N(ptBu-C6H4)}(NiPr)(CO)BCy2 (13), could be isolated in moderate yields. The 1,2-insertions of benzaldehyde in compounds 12 and 4 are reversible reactions in all cases, and only one of the insertion products, {iPr(H)N}C{N(ptBu-C6H4)}(NiPr)(PhHCO)BCy2 (16a), was isolated and diffractrometrically characterised. Likewise, CO2 reversibly inserts into a N–B bond of 2to give {iPr(H)N}C{N(ptBu-C6H4)}(NiPr)(CO2)BCy2 (19) with a conversion of ca. 9%. In all these equilibria, de-insertion is always favoured upon increasing the temperature.

Graphical abstract: Insertion reactions of small unsaturated molecules in the N–B bonds of boron guanidinates
 

Synthesis, Characterization, DNA Interactions and Antiproliferative Activity on Glioblastoma of Iminopyridine Platinum (II) Chelate Complexes

Journal of Inorganic Biochemistry 168 (2017) 46–54
 
 
 
A series of iminopyridine platinum chelate compounds has been prepared and characterized by NMR spectroscopy and single crystal X-ray diffraction. The complexes were evaluated in C6 tumoral cells as an in vitro model for glioblastoma multiforme. The DNA-binding properties of these complexes were studied by UV–Vis absorption and fluorescence spectroscopy and Density Functional Theory calculations were performed in an effort to rationalize the observed properties at the molecular level. The most promising drug candidate displayed a similar potency in inducing cell death to the clinically used reference compound and showed significant inhibition ofglioblastoma cell proliferation. Moreover, this compound had a safer profile than cisplatin on non-tumoral cells.
 
 
 
Structural and Mechanistic Insights into s-Block Bimetallic Catalysis: Sodium Magnesiate-Catalyzed Guanylation of Amines
 
Chem. Eur. J., 2016, 22, 17646-17656
 
 
To advance the catalytic applications of s-block mixed-metal complexes, sodium magnesiate [NaMg(CH2SiMe3)3] (1) is reported as an efficient precatalyst for the guanylation of a variety of anilines and secondary amines with carbodiimides. First examples of hydrophosphination of carbodiimides by using a Mg catalyst are also described. The catalytic ability of the mixed-metal system is much greater than that of its homometallic components [NaCH2SiMe3] and [Mg(CH2SiMe3)2]. Stoichiometric studies suggest that magnesiate amido and guanidinate complexes are intermediates in these catalytic routes. Reactivity and kinetic studies imply that these guanylation reactions occur via (tris)amide intermediates that react with carbodiiimides in insertion steps. The rate law for the guanylation of N,N’-diisopropylcarbodiimide with 4-tert-butylaniline catalyzed by 1 is first order with respect to [amine], [carbodiimide], and [catalyst], and the reaction shows a large kinetic isotopic effect, which is consistent with an amine-assisted rate-determining carbodiimide insertion transition state. Studies to assess the effect of sodium in these transformations denote a secondary role with little involvement in the catalytic cycle.
 
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Dialkylboron guanidinates: syntheses, structures and carbodiimide de-insertion reactions

Dalton Trans., 2016, 45, 15350 DOI: 10.1039/c6dt02913b

The synthesis of novel dialkylboron guanidinates is reported: the symmetrical compounds, (Me2N)-C(NR)2BR’2 [R = iPr, R’ = Nrb (1); R = Cy, R’ = Nrb (2); R = iPr, R’ = Cy (3); R = R’ = Cy (4); R = 2,6-iPr2-C6H3;R’ = Cy (5); Nrb = exo-2-norbornyl] and the asymmetrically coordinated {iPr(H)N}C(NiPr)(NAr)BCy2 [Ar = Ph (6), 4-Me-C6H4 (7), 4-tBu-C6H4 (8)] were prepared by the salt metathesis method from the appropriate lithium guanidinates and chloroboranes. Moreover, the bis(dicyclohexylboron)guanidinate(−2) {iPr(Cy2B)N}C(NiPr){N(4-tBu-C6H4)}BCy2 (9) was also prepared from the corresponding dilithium guanidinate Li2[{N(4-tBu-C6H4)}C(NiPr)2] and ClBCy2. The structures of compounds 1, 3, 6 and 9 were confirmed by X-ray diffraction and all displayed a chelate coordination of the guanidinate ligand to the BR’2 fragment, the latter displaying an additional BCy2 attached to the exocyclic N atom. Solutions of compounds 1–4 reached an equilibrium with the aminoboranes Me2NBR’2 [R’ = Nrb (10), Cy (11)] and the corresponding carbodiimides, which was slow at 25 °C. The thermodynamic parameters for these equilibria are also reported. The activation parameters for the equilibrium for compound 1 have been calculated after a kinetic study. Compounds 5–8, with one or two N-aryl fragments bound to a B centre, are more robust and need higher temperatures (80 °C) and prolonged times to give similar carbodiimide de-insertion reactions.

Graphical abstract: Dialkylboron guanidinates: syntheses, structures and carbodiimide de-insertion reactions Tris(pentafluorophenyl)borane as an efficient catalyst in the guanylation reaction of amines Dalton Trans., 2016, 45, 10717. DOI: 10.1039/C6DT01237J

Tris(pentafluorophenyl)borane, [B(C6F5)3], has been used as an efficient catalyst in the guanylation reaction of amines with carbodiimide under mild conditions. A combined approach involving NMR spectroscopy and DFT calculations was employed to gain a better insight into the mechanistic features of this process. The results allowed us to propose a new Lewis acid-assisted Brønsted acidic pathway for the guanylation reaction. The process starts with the interaction of tris(pentafluorphenyl)borane and the amine to form the corresponding adduct, [(C6F5)3B-NRH2] 1, followed by a straightforward proton transfer to one of the nitrogen atoms of the carbodiimide, iPrN=C=NiPr, to produce, in two consequent steps, a guanidine-borane adduct, [(C6F5)3B-NRC(NiPrH)2] 2. The rupture of this adduct liberates the guanidine product RNC(NiPrH)2 3 and interaction with additional amine restarts the catalytic cycle. DFT studies have been carried out in order to study the thermodynamic characteristics of the proposed pathway. Significant borane adducts with amines and guanidines have been isolated and characterized by multinuclear NMR in order to study the N–B interaction and to propose the existence of possible Frustrated Lewis Pairs. Additionally, the molecular structures of significant components of the catalytic cycle, namely 4-tert-butylaniline-[B(C6F5)3] adduct 1b and both free and [B(C6F5)3]-bonded 1-(phenyl)-2,3-diisopropylguanidine, 2a and 3a respectively, have been established by X-ray diffraction.

Graphical abstract: Tris(pentafluorophenyl)borane as an efficient catalyst in the guanylation reaction of amines

Half-Sandwich Guanidinate–Osmium(II) Complexes: Synthesis and Application in the Selective Dehydration of Aldoximes
 
European Journal of Inorganic Chemistry, 2016, 393

Abstract: The novel guanidinate-osmium(II) complexes [OsCl{k2-(N,N´)-C(NR)(NiPr)-NHiPr}(h6p-cymene)] [R = Ph (3a), 4-C6H4F (3b), 4-C6H4Cl (3c), 4-C6H4CF3 (3d), 3-C6H4CF3 (3e), 3,5-C6H3(CF3)2 (3f), 4-C6H4CN (3g), 4-C6H4Me (3h), 3-C6H4Me (3i), 2-C6H4Me (3j), 4-C6H4tBu (3k), 2,6-C6H3iPr2 (3l), 2,4,6-C6H2Me3 (3m)] have been synthesized in high yield (70-88%) by treatment of THF solutions of the dimeric precursor [{OsCl(m-Cl)(h6p-cymene)}2] (1) with four equivalents of the corresponding guanidine (iPrHN)2C=NR (2a-m) at room temperature. The easily separable guanidinium chloride salts [(iPrHN)2C(NHR)][Cl] (4a-m) were also formed in these reactions. The structures of 3a, 3d and 3h were unequivocally confirmed by X-ray diffraction methods. In addition, complexes 3a-m proved to be active in the catalytic dehydration of aldoximes. Best results were obtained with [OsCl{k2-(N,N´)-C(N-4-C6H4CF3)(NiPr)-NHiPr}(h6p-cymene)] (3d) (5 mol%) which, in acetonitrile at 80 ºC, was able to convert selectively a large variety of aromatic, heteroaromatic, a,b-unsaturated and aliphatic aldoximes into the corresponding nitriles, in high yields and short times.

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Phenyl-Guanidine Derivatives as Potential Therapeutic Agents for Glioblastoma Multiforme: Catalytic Syntheses, Cytotoxic Effects and DNA Affinity RSC Advances, 2016, 6, 8267. DOI: 10.1039/C5RA17920C

Glioblastoma is a highly malignant form of brain tumor. Current treatment with surgery, temozolamide (TMZ), and radiotherapy only leads to a modest median survival. There is clearly an unmet clinical need for new treatments that are able to arrest the rapid development of the disease through new drugs with antiproliferative activity on glioblastoma cells. In the work described here, several substituted phenyl-guanidine derivatives were developed for application in glioblastoma treatment. The compounds were synthesized by catalytic guanylation reactions and they were fully characterized and assessed for their affinity for DNA by UV titrations and fluorescent intercalator displacement assays. The cytotoxicity levels of the compounds were investigated in the C6 rat glioblastoma cell line by MTT, LDH, and BrdU proliferation assays. Some of the phenyl-guanidine derivatives displayed interesting antitumoral profiles, with a higher potency than the standard drug TMZ in reducing glioblastoma cell proliferation.

Graphical abstract: Phenyl-guanidine derivatives as potential therapeutic agents for glioblastoma multiforme: catalytic syntheses, cytotoxic effects and DNA affinity   Toward the Prediction of Activity in the Ethylene Polymerisation of ansa-Bis(indenyl) Zirconocenes: Effect of the Stereochemistry and Hydrogenation of the Indenyl Moiety ChemPlusChem 2015, 80, 963-972.

Abstract: A combined experimental and quantum chemical study has been performed on rac– and meso-[Zr{1-Me2Si(3-η5-C9H5Et)2}Cl2] (rac– and meso1) and their hydrogenated forms (rac– and meso2) to understand ligand effects and guide ligand design for more active ansa-bis(indenyl) zirconocenes for the polymerisation of ethylene. The rac-ansa-zirconocene rac-[Zr(1-Me2Si{3-Et-(η5-C9H9)}2)Cl2] (rac2) has been prepared and fully characterised by NMR spectroscopy and elemental analysis. The molecular structure of rac2 has also been determined by single-crystal XRD. The behaviour of the catalysts was analysed in the polymerisation of ethylene and higher activities were obtained for rac1 and its hydrogenated form rac2. The influence of the stereochemistry and hydrogenation of the indenyl ligand on the experimental activities has been evaluated by computational studies. The differences along the reaction pathway are dominated by changes in the relative stabilities of the catalytic intermediates. A hybrid density functional B3LYP study, in the presence of toluene as the solvent, indicates that the rac forms give rise to more active species than their meso counterparts. The hydrogenation of the rac forms is a very promising approach to increase activities in polymerisation, in contrast to the meso forms. Finally, the global mechanism rate constants for the polymerisation reaction for each metallocene were calculated by using the thermodynamic formulation of transition-state theory to complement the computational study.

Presentación2   Catalytically Generated Ferrocene-Containing Guanidines as Efficient Precursors for New Redox-Active Heterometallic Platinum(II) Complexes with Anticancer Activity

Organometallics, 2015, 34 (22), pp 5407–5417

Abstract: The potential of structurally new ferrocene-functionalized guanidines as redox-active precursors for the synthesis of heterometallic platinum(II)-guanidine complexes with anticancer activity was studied. To this end, an atom-economical catalytic approach was followed by using ZnEt2 to catalyze the addition of aminoferrocene and 4-ferrocenylaniline to N,N’-diisopropyl-carbodiimide. Furthermore, reaction of a platinum(II) source with the newly obtained guanidines Fc−N=C(NHiPr)2 (3) and Fc(1,4-C6H4)−N=C(NHiPr)2 (4) provided access to the heterometallic complexes [PtCl2{Fc−N=C(NHiPr)2}(DMSO)] (5), [PtCl2{Fc(1,4-C6H4)−N=C(NHiPr)2}(DMSO)] (6) and [PtCl2{Fc(1,4-C6H4)−N=C(NHiPr)2}2] (7). Electrochemical studies evidence the remarkable electronic effect played by the direct attachment of the guanidine group to the ferrocene moiety in 3, making its one-electron oxidation extremely easy. Guanidine-based Fe–Pt complexes 5 and 6 are active against all human cancer cell lines tested with GI50 values in the range 1.4–2.6 μM, and are more cytotoxic than cisplatin in the resistant T-47D and WiDr cell lines.

ferrocenoguanidina Reactivity of the dimer [{RuCl(µ-Cl)(η3:η3-C10H16)}2] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) towards guanidines: Access to novel guanidinate-ruthenium(IV) and ruthenium(II) complexes

Organometallics, 2015, 34 (12), pp 2796–2809

Abstract: The novel bis(allyl)-ruthenium(IV)-guanidinate complexes [RuCl{κ2-(N,N´)-C(NR)(NiPr)-NHiPr}(η3:η3-C10H16)] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl; R = Ph (3a), 4-C6H4F (3b), 4-C6H4Cl (3c), 4-C6H4Me (3d), 3-C6H4Me (3e) 4-C6H4tBu (3f)) have been synthesized by treatment of the dimeric precursor [{RuCl(µ-Cl)(η3:η3-C10H16)}2] (1) with 4 equivalents of the corresponding guanidine (iPrHN)2C=NR (2a-f). The easily separable guanidinium chloride salts [(iPrHN)2C(NHR)][Cl] (4a-f) are also formed in these reactions. Attempts to generate analogous Ru(IV)-guanidinate complexes from (iPrHN)2C=NR (R = 2-C6H4Me (2g), 2,4,6-C6H2Me3 (2h), 2,6-C6H3iPr2 (2i)) failed, due probably to the steric hindrance associated with the aryl group in these guanidines. On the other hand, the reaction of dimer [{RuCl(µ-Cl)(η3:η3-C10H16)}2] (1) with (iPrHN)2C=N-4-C6H4C≡N (2j) led to the selective formation of the mononuclear derivative [RuCl2(η3:η3-C10H16){N≡C-4-C6H4-N=C(NHiPr2)2}] (5), in which the guanidine coordinates to ruthenium through the pendant nitrile unit. This result contrasts with that obtained by employing the related Ru(II) dimer [{RuCl(μ-Cl)(η6p-cymene)}2] (6), whose reaction with 2j afforded the expected guanidinate complex [RuCl{κ2-(N,N´)-C(N-4-C6H4C≡N)(NiPr)-NHiPr}(η6p-cymene)] (7). Treatment of 7 with dimer 1 yielded a dinuclear Ru(II)/Ru(IV) derivative 8, via cleavage of the chloride bridges of 1 by the C≡N group of 7. Reductive elimination of the 2,7-dimethylocta-2,6-diene-1,8-diyl chain in complexes [RuCl{κ2-(N,N´)-C(NR)(NiPr)-NHiPr}(η3:η3-C10H16)] (3a-f) readily took place in the presence of an excess of 2,6-dimethylphenyl isocyanide, thus allowing the high-yield preparation of the octahedral ruthenium(II) compounds mer-[RuCl{κ2-(N,N´)-C(NR)(NiPr)-NHiPr}(CN-2,6-C6H3Me2)3] (9a-f). The structures of complexes [RuCl{κ2-(N,N´)-C(N-4-C6H4Me)(NiPr)-NHiPr}(η3:η3-C10H16)] (3d), [RuCl{κ2-(N,N´)-C(N-4-C6H4C≡N)(NiPr)-NHiPr}(η6p-cymene)] (7) and mer-[RuCl{κ2-(N,N´)-C(N-4-C6H4tBu)(NiPr)-NHiPr}(CN-2,6-C6H3Me2)3] (9f), as well as those of the guanidinium chloride salts 4a-c, were unequivocally confirmed by X-ray diffraction methods. In addition, the catalytic behaviour of the guanidinate complexes 3a-f and 9a-f in the redox isomerization of allylic alcohols was also explored.

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