First direct evidence of N-heterocyclic carbene in BMIm acetate ionic liquid. An electrochemical and chemical study on the role of temperature

Cyclic voltammetry provides the first direct evidence of N-­heterocyclic carbene (NHC) presence in neat 1-‐butyl-­3-­methylimidazolium acetate ionic liquid (BMImAcO) at 120°C. The NHC existence, proved by its oxidation current in cyclic voltammetry, was confirmed by the formation of a PhCHO-­NHC adduct in pure ionic liquid. The role of the temperature was considered

Role of Anion and Cation in the 1-Methyl-3-butyl Imidazolium Ionic Liquids BMImX: The Knoevenagel Condensation

1-Methyl-3-butylimidazolium ionic liquids BMImX (X anions: Ac−, HCO3−, Cl−, BF4−, PF6−, I−, CF3CO2−, (CF3SO2)2N−, CF3SO3−) were used as catalysts in the Knoevenagel condensation reaction. The catalytic activity was compared with that of the inorganic salts NaAc, NaHCO3, NaCl. Whereas it is quite important to select suitable cation and anion depending on the purpose, the basicity of the anion X−and the ability of the counter-ion BMIm+to avoid the ion pairing with anion X−was investigated. 1-Methyl-3-butyl imidazolium acetate [BMImAc] and 1-methyl-3-butyl imidazolium hydrogen carbonate [BMImHCO3] show the most significant catalytic power and their catalytic effect was studied on a wide range of aromatic, heteroaromatic or aliphatic aldehydes or ketones with active methylene compounds. The corresponding substituted alkenes were obtained in excellent yields (up to 98%) in the absence of any solvent at room temperature within short times. The interesting feature of this study includes explanation on role of the cation BMIm+as proton donor (versus anions Ac−or HCO3−), in ion pair formation (with anion X−) and activator of the aldehyde structure

Electrochemically modified Corey-Fuchs reaction for the synthesis of arylalkynes. the case of 2-(2,2-dibromovinyl)naphthalene

The electrochemical reduction of 2-(2,2-dibromovinyl)naphthalene in a DMF solution (Pt cathode) yields selectively 2-ethynylnaphthalene or 2-(bromoethynyl)naphthalene in high yields, depending on the electrolysis conditions. In particular, by simply changing the working potential and the supporting electrolyte, the reaction can be directed towards the synthesis of the terminal alkyne (Et4NBF4) or the bromoalkyne (NaClO4). This study allowed to establish that 2-(bromoethynyl)naphthalene can be converted into 2-ethynylnaphthalene by cathodic reduction

Comparative study of the two large flares from SGR1900+14 with the BeppoSAX Gamma-Ray Burst Monitor

We report on spectral and temporal results of the 40-700 keV observations, obtained with the Gamma-Ray Burst Monitor (GRBM) on board BeppoSAX, of the two large flares from the Soft Gamma-ray Repeater SGR1900+14 occurred on August 27, 1998 and April 18, 2001. From their intensity, fluence and duration, the first one was classified as "giant" and the second as "intermediate". The spectral results have been obtained with an improved response function of the GRBM. We find that the two events have similar spectral properties, but different temporal properties. The major difference concerns the time profiles of the light curves, whereas the lack of evidence in the 2001 flare for the erratic time variability found at high frequencies (10-1000 Hz) in the 1998 flare could be ascribed to lower counting statistics. We discuss these results in the light of the magnetar model proposed for SGR sources.Comment: 15 pages, 20 figures, accepted for publication in A&

NHC in Imidazolium Acetate Ionic Liquids: Actual or Potential Presence?

Ionic liquids (ILs) are considered in the majority of cases green solvents, due to their virtually null vapor pressure and to the easiness in recycling them. In particular, imidazolium ILs are widely used in many fields of Chemistry, as solvents or precursors of N-heterocyclic carbenes (NHCs). The latter are easily obtained by deprotonation of the C2-H, usually using strong bases or cathodic reduction. Nevertheless, it is known that weaker bases (e.g., triethylamine) are able to promote C2-H/D exchange. From this perspective, the possibility of deprotonating C2-H group of an imidazolium cation by means of a basic counter-ion was seriously considered and led to the synthesis of imidazolium ILs spontaneously containing NHCs. The most famous of this class of ILs are N,N'-disubstituted imidazolium acetates. Due to the particular reactivity of this kind of ILs, they were appointed as "organocatalytic ionic liquids" or "proto-carbenes." Many papers report the use of these imidazolium acetates in organocatalytic reactions (i. e., catalyzed by NHC) or in stoichiometric NHC reactions (e.g., with elemental sulfur to yield the corresponding imidazole-2-thiones). Nevertheless, the actual presence of NHC in N,N'-disubstituted imidazolium acetate is still controversial. Moreover, theoretical studies seem to rule out the presence of NHC in such a polar environment as an IL. Aim of this Mini Review is to give the reader an up-to-date overview on the actual or potential presence of NHC in such an "organocatalytic ionic liquid," both from the experimental and theoretical point of view, without the intent to be exhaustive on N,N'-disubstituted imidazolium acetate applications

Electrogenerated N-Heterocyclic Carbene in Ionic Liquid: An Insight into the Mechanism of the Oxidative Esterification of Aromatic Aldehydes

An N-heterocyclic carbene (NHC), generated by cathodic reduction of BMImBF4, mediates the oxidative esterification of aromatic aldehydes with organic bromides in the corresponding ionic liquid as solvent. The product recovery by simple extractive work-up with diethyl ether allowed the ionic liquid to be recycled up to 9 times for subsequent electrolyses, with no significant loss in the product yield. The isolation of an intermediate, whose structure was confirmed by synthesis and transformation into the ester, provided the key for a mechanistic insight into the reaction. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The XMM-Newton view of GRS1915+105 during a "plateau"

Two XMM-Newton observations of the black-hole binary GRS1915+105 were triggered in 2004 (April 17 and 21), during a long "plateau" state of the source. We analyzed the data collected with EPIC-pn in Timing and Burst modes, respectively. No thermal disc emission is required by the data; the spectrum is well fitted by four components: a primary component (either a simple power law or thermal Comptonization) absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disc; emission and absorption lines; and a soft X-ray excess around 1 keV. The latter is not confirmed by RGS (which were used in the second observation only); if real, the excess could be due to reflection from the optically thin, photoionized plasma of a disc wind, in which case it may provide a way to disentangle intrinsic from interstellar absorption. Indeed, the former is best traced by the higher abundances of heavier elements, while an independent estimate of the latter may be given by the value we get for the disc wind component only, which roughly coincides with what is found for lower-Z species.Comment: 5 pages, 3 figures, 1 table. Submitte