Iluminada Gallardo

Anodic oxidation of some tertiary amines

In our investigations we focused our attention on aromatic amines leading to the formation of a benzidine structure where the loss of a proton occurs at the level of the phenyl group. This study was performed in acetonitrile with five amines using linear sweep voltammetry and ultramicroelectrode voltammetry

Nucleophilic Aromatic Substitution of Hydrogen: A Novel Electrochemical Approach to the Cyanation of Nitroarenes

The nucleophilic aromatic substitution of hydrogen through electrochemical oxidation of the intermediate sigma complexes (Meisenheimer complexes) in simple nitroaromatic compounds is reported for the first time. The studies have been carried out with hydride and cyanide anions as the nucleophiles using cyclic voltammetry (CV) and preparative electrolysis. The cyclic voltammetry experiments allow for the detection and characterization of the sigma complexes and led us to a proposal for the mechanism of the oxidation step. Furthermore, the power of the CV technique in the analysis of the reaction mixture throughout the whole chemical and electrochemical process is described.

Stable Spirocyclic Meisenheimer Complexes

Meisenheimer complexes are important intermediates in Nucleophilic Aromatic Substitution Reactions (SNAr). They are formed by the addition of electron rich species to polynitro aromatic compounds or aromatic compounds with strong electron withdrawing groups. It is possible to distinguish two types of Meisenheimer or σ-complexes, the σH-complex or σX-complex (also named ipso), depending on the aromatic ring position attacked by the nucleophile (a non-substituted or substituted one, respectively). Special examples of σX- or ipso-complexes are formed through intermediate spiro adducts, via intramolecular SNAr. Some of these spirocyclic Meisenheimer complexes, a type of σX-complex, are exceptionally stable in solution and/or as solids. They can be isolated and characterized using X-ray, and various spectroscopic techniques such as NMR, UV-Vis, IR, and fluorescence. A few of these stable spirocyclic Meisenheimer complexes are zwitterionic and exhibit interesting photophysical and redox properties. We will review recent advances, synthesis and potential applications of these stable spirocyclic Meisenheimer complexes.

Mechanistic studies on the reactivity of halodinitrobenzene radical-anion

Abstract The electrochemical behaviour of 1-F-2,4-dinitrobenzene, 1-Cl-2,4-dinitrobenzene and 1-Br-2,4-dinitrobenzene in DMF is described. The 1-F-2,4-dinitrobenzene radical anion dimerises before cleaving, whereas 1-Cl-2,4-dinitrobenzene and 1-Br-2,4-dinitrobenzene radical anions dimerise after cleavage. This change in mechanism allows the obtention of 2,2′,4,4′-tetranitrobiphenyl in a selective way, and with good efficiency. The electrochemical oxidation of σ-complexes is shown to be an alternative means of obtaining products that are difficult to obtain through traditional procedures.

Electrochemically promoted nucleophilic aromatic substitution in room temperature ionic liquids—an environmentally benign way to functionalize nitroaromatic compounds

The current manuscript shows the electrochemical studies performed to rationalize the mechanism and develop new green synthetic routes for the synthesis of substituted nitroaromatics based on the advantages of the electrochemical approach to the nucleophilic aromatic substitution reaction (such as (a) low cost and ready availability of reagents, (b) atom economy, (c) high yields, approaching 100%) and the use of Room Temperature Ionic Liquids (RTILs) as green alternative solvents to organic aprotic solvents. Four of the most popular RTILs (1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide ([BMIM]TFSI) and 1-butyl-3-methylimidazolium acetate ([BMIM]AcO) have been chosen since they have different properties in terms of solvation effects that can increase the regioselectivity of the reaction. The nucleophiles used to study the feasibility and viability of the reaction were the classical hydride, methoxide, ketones, cyanides and amines, whereas the nitroarenes selected were 4-nitrotoluene, 1,3-dinitrobenzene, 2,4-dinitroaniline, 1,3,5-trinitrobenzene, 1,3-dinitronaphthalene, 1-chloro-2,4,6-trinitrobenzene and 2,4,6-trinitroanisole. The electrocatalysis and regioselectivity effects of using RTILs are also investigated. The article concludes by analyzing the economic cost of performing this electrosynthesis in RTILs and organic solvent electrolyte systems, which contain 0.1 M of supporting electrolyte.

Electrochemical Synthesis of Nitroanilines

Alkylamines and amides are readily prepared by nucleophilic aromatic substitution of hydrogen in nitroarenes by electrochemical oxidation. Useful yields (15−85%) are achieved in a simple direct and regioselective amination process. The synthetic method has been examined in the absence and presence of external bases, used to promote the first step of the nucleophilic aromatic substitution reaction, i.e. the nucleophilic attack. In both cases, good results were obtained. The unreacted starting material can easily be recovered at the end of the electrochemical oxidation process. This new method represents an environmentally favourable route to amino- and amido-substituted nitroaromatic compounds.

Mechanistic study of the electrochemical oxidation of some aromatic amines in the presence of bases

Abstract Cyclic voltammetry was used to investigate the mechanism of anodic oxidation of four aromatic amines, together with the effect of the addition of bases (lutidine, sym-collidine, quinuclidine and hydroxide). Without a methoxy substituent in the para position of the phenyl group, the coupling of two radical cations occurs on the addition of weak bases, but a deprotonation is involved in the presence of hydroxide ion. This deprotonation occurs in all cases when a methoxy substituent is present on the phenyl group.

Electrochemical Synthesis of Nitroaromatic Ketones

Nitroaromatic ketones are readily prepared by nucleophilic aromatic substitution of hydrogen in nitroarenes by electrochemical oxidation. Carbanions of various ketones were added to selected nitroarenes in DMF/ketone mixtures leading to formation of the σH complexes. The reaction was promoted using potassium tert-butoxide as a base. Useful yields were achieved (80−100%) in the C-arylation of ketones. In most cases, the process proceeded with high selectivity. This new method represents an environmentally favourable route for obtaining nitroaromatic ketones.

Mechanistic studies on the electrochemical reductive coupling of some polyhalogenonitrobenzenes. A new example of a radical anion dimerization

Abstract Coupling processes with loss of one halogen atom per ring are observed when heavily substituted polyhalogenonitrobenzenes (polyfluoro- and polychloro-) are electrochemically reduced in DMF. Product analyses and cyclic voltametry mechanistic studies suggest that in the particular case of pentafluoronitrobenzene, direct dimerization of the radical anions occurs prior to CF bond fragmentation. On the contrary, reduction of the nitro group is the only observed process when less substituted polyhalogenonitrobenzenes are used in the same conditions. Nitro group reduction is also the main process for all the studied polyhalogenonitrobenzenes when the reactions are carried out in protic solvents.

On the electroreduction mechanism of halobenzenes: Part I. A quantum chemical approach

Abstract The half-wave potentials of various halobenzene derivatives in aprotic solvents correlate with the charge on the halogen atom being reduced and with the energy of the first unoccuppied σ molecular orbital, calculated using the MNDO method. These correlations explain the ortho effect observed in the dihalogen derivatives. Open-shell calculations of the radical anions demonstrate their existence as metastable intermediates. Chlorobenzene has been chosen as a model in order to study the breaking of the CX bond by the reaction coordinate method. According to these results, two possible reduction mechanisms are discussed.