Thiago C. Correra

Selectivity and Mechanisms Driven by Reaction Dynamics: The Case of the Gas-Phase OH– + CH3ONO2 Reaction

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH(-) + CH(3)ONO(2) reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S(N)2@C) or the nitrogen center (S(N)2@N) as well as a proton abstraction followed by dissociation (E(CO)2) pathway. Direct dynamics simulations yield an S(N)2:E(CO)2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO(2) group that scatters the incoming OH(-). In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion-molecule gas-phase reactions.

Gas-Phase Nucleophilic and Elimination Reactions in Simple Alkyl Nitrates

There has been increasing interest in the gas-phase reactivity of alkyl nitrates because of their well-known applications as explosives and because of their role in atmospheric and in marine processes. This manuscript describes an experimental study by FT-ICR techniques of the gas-phase reactions of OH(-) and F(-) with methyl and ethyl nitrate. For methyl nitrate, the main reaction channel is found to be an elimination process promoted by abstraction of an α proton from the methyl group. Nucleophilic displacement of nitrate anion through an S(N)2 process at the carbon center is also found to be an important reaction channel with methyl nitrate. In ethyl nitrate, formation of NO(3)(-) is greatly enhanced and this is attributed to the ease of an E2-type elimination process promoted by proton abstraction at the β position of the ethyl group. Theoretical calculations at the MP2/6-311+G(3df,2p)//MP2/6-31+G(d) level of theory are consistent with the relative importance of the reaction channels and suggest that these reactions proceed through a double well potential. The calculations also predict that nucleophilic attack by OH(-) at the nitrogen center (Sn2@N) is energetically the preferred pathway but experiments with (18)OH(-) showed no evidence for this channel. Single-point calculations reveal a strong preference for approach to the carbon center and may explain the lack of reactivity at the nitrogen center. Calculations were also carried out for NH(2)(-) and SH(-) to establish the reactivity pattern to provide a better understanding of environmentally relevant nitrate esters.

Ytterbium(III)-catalyzed three-component reactions: synthesis of 4-organoselenium-quinolines

4-Organoselenium-quinolines were synthesized via model multi-component Povarov (MCR) reactions between p-anisidine, ethyl glyoxylate and ethynyl(phenyl)selane, catalyzed by the Lewis acid Yb(OTf)3, with yields ranging from moderate to good. This method is advantageous in that there is no requirement for an oxidant and that the reaction has a wide scope. These 4-organoselenium-quinolines allowed access to 4-alkynyl-quinolines, which were synthesized via copper free Sonogashira cross-coupling reactions catalyzed by a new palladium catalyst that employs an oxazoline derivative as a ligand. A plausible mechanism was proposed based on HRMS studies.

Gas-Phase Nucleophilic Reactivity of Alkoxysilanes

A reatividade em fase gasosa das alcoxissilanas do tipo Me 4-n Si(OEt) n (n = 1-3) foi investigada por ressonância ciclotronica de ions por transformada de Fourier (FT-ICR) com o intuito de se caracterizar os mecanismos de ataque nucleofilico nesses importantes precursores de novos materiais. Nucleofilos como F - , MeO - e EtO - reagem rapidamente e de forma preferencial atacando o atomo de Si, formando a especie pentacoordenada de Si. Esta em seguida sofre processos de eliminacao iniciados, ou por um ion metidio nascente Me - , ou um ion etoxido EtO - , que por sua vez, podem abstrair um proton gerando carbânions ou siloxidos. Carbânions do tipo X(Me)3-nSi(OEt)nCH2 - (X = F, MeO, EtO e n = 1-3) e siloxidos do tipo X(Me)4-nSi(OEt)n-1O - (X = F, MeO, EtO e n = 2-3) podem ser facilmente dissociados por excitacao multifotonica no infravermelho induzida por um laser de CO 2 gerando uma grande variedade de siloxidos simples e silicatos analogos a especies intermediarias encontradas nas primeiras etapas de processos sol-gel. Para as alcoxissilanas que contem mais grupos etoxido, uma reacao competitiva analoga a uma eliminacao E2 e observada, onde o nucleofilo abstrai um hidrogenio β de um grupo etoxila, eliminando etileno. A cinetica, distribuicao relativa de produtos e termoquimica dessas reacoes tambem sao apresentadas para alguns casos especificos. The gas-phase reactivity of the Me 4-n Si(OEt) n (n = 1-3) alkoxysilanes was investigated by Fourier transform ion cyclotron resonance (FT-ICR) technique in order to characterize the fundamental mechanisms associated with nucleophilic attack on these important precursors of new materials. Typical nucleophiles such as F - , MeO - and EtO - react readily and preferentially by attack at the silicon center with formation of a pentacoordinated siliconate that undergoes elimination processes initiated by either a nascent methide ion, Me, or an ethoxide ion, EtO, that can abstract a proton to yield either a carbanion or a siloxide-type anion. Carbanions of the type X(Me) 3-n Si(OEt) n CH 2 - (X = F, MeO, EtO and n = 1-3) and siloxide ion of the type X(Me)4-nSi(OEt)n-1O - (X = F, MeO, EtO and n = 2-3) can be easily dissociated by infrared multiphoton excitation with a CO2 laser to give rise to a large variety of simple siloxide and silicate-type anions that are reminiscent of intermediate species in the early stages of sol-gel processes. For the higher-ethoxy containing substrates, a competing reaction is observed that is analogous to an E2 elimination reaction in which the nucleophile abstracts a β-proton from the ethoxy group leading to elimination of ethylene. The kinetics, relative product distribution and thermochemistry of these reactions are also reported for some specific cases.

Revisiting the Mechanism of Hydrolysis of Betanin

Betanin (betanidin 5‐O‐β‐D‐glucoside) is a water‐soluble plant pigment used as a color additive in food, drugs and cosmetic products. Despite its sensitivity to light and heat, betanin maintains appreciable tinctorial strength in low acidic and neutral conditions, where the color of other plant pigments, such as anthocyanins, quickly fades. However, betanin is an iminium natural product that experiences acid‐ and base‐catalyzed hydrolysis to form the fairly stable betalamic acid and cyclo‐DOPA‐5‐O‐β‐D‐glucoside. Here, we show that the decomposition of betanin in aqueous phosphate solution pH 2–11 is subject to general base catalysis by hydrogen phosphate ion and intramolecular general acid and base catalysis, providing new insights on the mechanism of betanin hydrolysis. UV/Vis absorption spectrophotometry, 1H NMR spectroscopy and mass spectrometry were used to investigate product formation. Furthermore, theoretical calculations support the hypothesis that the nitrogen atom of the tetrahydropyridine ring of betanin is doubly protonated, as observed for structurally simpler amino dicarboxylic acids. Our results contribute to the study of betanin and other pigments belonging to the class of betalains and to deepen the knowledge on the chemical properties of imino acids as well as on iminium‐catalyzed modifications of carbonyl compounds in water.

Sequential Methyl–Fluorine Exchange Reactions of Siloxide Ions in the Gas Phase

Exchange Me for a fluorine: Trimethylsiloxide ions in the presence of NF(3) in the gas phase undergo an unusual and sequential metathesis-type reaction wherein methyl groups are exchanged for fluorine. Theoretical calculations suggest that the reaction proceeds by a three-step internal-nucleophilic-displacement mechanism which features a pentacoordinated siliconate species as a transition state rather than as an intermediate.

(R)-2-Phen-oxy-1-(4-phenyl-2-sulfan-ylidene-1,3-oxazolidin-3-yl)ethanone.

The central 1,3-oxazolidine-2-thione ring in the title compound, C17H15NO3S, is approximately planar with maximum deviations of 0.036 (4) and −0.041 (5) Å for the O and methylene-C atoms, respectively. The dihedral angles formed between this plane and the two benzene rings, which lie to the same side of the central plane, are 86.5 (2) [ring-bound benzene] and 50.6 (3)°. The ethan-1-one residue is also twisted out of the central plane, forming a O—C—N—C torsion angle of 151.5 (5)°. The dihedral angle formed by the benzene rings is 62.8 (2)° so that overall, the molecule has a twisted U-shape. In the crystal, molecules are linked into supramolecular arrays two molecules thick in the bc plane through C—H⋯O, C—H⋯S and C—H⋯π interactions.

Investigation of the Photocatalytic Activity of Titanium Dioxide Films Under Visible Light Measured by Electrospray Mass Spectrometry

We report, for the first time, the photocatalytic activity of pure TiO2 P-25 in the form of thin homogeneous mesoporous films under visible light activation. The study was accomplished by analyzing the photodegradation rate of two water contaminant molecules, 2-naphthol and methyl orange, via an electrospray mass spectrometry technique with the use of ammonium hexafluorophosphate as an internal standard. The employed methodology can also be used to run different semiquantitative assays and study other film mediated photocatalytic processes. It was observed that TiO2 films can easily promote photooxidation of 2-naphthol under the incidence of visible light and in the presence of oxygen gas by showing almost complete photodegradation after about 6 hours of reaction. Photooxidation of methyl orange was also observed, yet the decay rate was slower. This process was also confirmed through identification of by-products of the reaction via mass spectrometry and through the decay of the absorbance peak at 468 nm via UV-vis spectrophotometry. This photoactivity of pure TiO2 P-25 films under visible light is attributed to its narrower band gap that occurs because of the thermal treatment that titania undergoes in the process of synthesis of the films. The use of these films as photocatalysts is an advantage since they work as a heterogenous catalyst in the absence of UV radiation.

Evaluation of Ca2+ Binding Sites in Tacrolimus by Infrared Multiple Photon Dissociation Spectroscopy

Tacrolimus (TAC) is an efficient immunosuppressant used in organ transplantation procedures. There is an intrinsic correlation between TAC and Ca2+ because of the dependence of its action mechanism on calcium and calcineurin, and the role of ion coordination on TAC identification and quantitation. To depict the Ca2+ binding sites in TAC, this work carried out gas-phase vibrational infrared multiple photon dissociation spectroscopy of [Ca(TAC)]2+ and of three other TAC mimetic molecules (probes 1-3). Density functional theory (DFT) and Monte Carlo (MC) simulations were also used to support the experimental data assignment, and natural bond orbital (NBO) analysis was carried out to depict the coordination sphere. PM3 and B3LYP/6-31G(d) levels of theory displayed similar trends during the MC simulations, suggesting that PM3 is a viable alternative to more expensive DFT calculations, at least during the conformational analysis step. Infrared spectroscopy of the [Ca(probe X)1]2+ and [Ca(probe X)3]2+ ( X = 1-3) complexes allowed for a useful guide for building guess geometries and for the band assignment of the [Ca(TAC)]2+ complex. Nevertheless, the MC approach was particularly useful for exploring the potential energy surface. The lowest energy conformation for [Ca(TAC)]2+ was found by MC simulations and is 32.92 kJ mol-1 lower in energy than the one found by comparing the results obtained for Ca2+ coordination in probes, despite the calculated spectra being virtually identical. Both approaches are good ways to depict the coordination sites, and these results suggest that using small molecules as models is a reliable approach to depict the geometry or coordination sites of extensive ions, yielding a robust correlation between experimental and theoretical spectra. Furthermore, MC survey produced a lower energy conformation with a good match to the experimental results. Both methods depict the Ca2+ coordination sphere as a hexacoordinated environment where the main coordination centers are carbonyl groups.

Dynamic/Thermochemical Balance Drives Unusual Alkyl/F Exchange Reactions in Siloxides and Analogs

A recent report has shown that siloxides can undergo an unusual Me/F exchange reaction promoted by NF3 in the gas phase ( Angew. Chem. Int. Ed. 2012, 51, 8632-8635). A more extensive study of this kind of exchange has been carried out using mass spectrometry techniques (FT-ICR), DFT calculations, natural bond orbital (NBO) analysis, and Born-Oppenheimer molecular dynamics simulations (BOMD), using NF3, SO2F2, and CF4 as fluorine donors and evaluating the effect of replacing the Si center by Ge and C. This comprehensive approach shows that NF3 is crucial for the exchange reaction, as SO2F2 forms SO3F(-) via a pentacoordinated channel whereas no reaction is observed for CF4. The uniqueness of NF3 is caused by favorable thermochemical consideration and by dynamic effects that preclude the formation of the ubiquitous Si-F pentacoordinated species. Me3GeO(-) was shown to be as reactive as siloxides toward NF3, whereas C analogs showed no reactions under our experimental conditions. The exchange reaction was also shown to take place for triethylsiloxides. These exchange reactions are examples of reaction systems that avoid the lower energy pathway and are driven by dynamic effects that cannot be explained by the potential energy surface.