111 research outputs found
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: Acâ, HCO3â, Clâ, BF4â, PF6â, Iâ, CF3CO2â, (CF3SO2)2Nâ, CF3SO3â) 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 Xâand the ability of the counter-ion BMIm+to avoid the ion pairing with anion Xâwas 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 Acâor HCO3â), in ion pair formation (with anion Xâ) 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
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. Š 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Charge-tagged N-heterocyclic carbenes (NHCs): revealing the hidden side of NHC-catalysed reactions through electrospray ionization mass spectrometry
Nâheterocyclic carbenes (NHCs) are key intermediates in a variety of chemical reactions. Owing to their transient nature, the interception and characterization of these reactive species have always been challenging. Similarly, the study of reaction mechanisms in which carbenes act as catalysts is still an active research field. This Minireview describes the contribution of electrospray ionization mass spectrometry (ESIâMS) to the detection of chargeâtagged NHCs resulting from the insertion of an ionic group into the molecular scaffold. The use of different mass spectrometric techniques, combined with the chargeâtagging strategy, allowed clarification of the involvement of NHCs in archetypal reactions and the study of their intrinsic chemistry
Xanthine scaffold: available synthesis routes to deliver diversity by derivatization
The functionalization of the skeletal systems of heterocycles represents a significant goal
for the development of new compounds. The heterocyclic molecule xanthine (3,7-dihydro-1Hpurine-
2,6-dione) is a purine base with a bicyclic ring skeleton and four different nitrogen atoms,
three of them are -NH groups. The principal derivatives are the well known natural methylxanthines
(e.g., caffeine, theophylline and theobromine) that have prominent physiological effects at a very low
dose. The natural methylated xanthines, theophylline, theobromine and caffeine, are present in different
plants such as the tea, cocoa and coffee species. For this reason natural xanthines can be considered
as bio-based and renewable starting materials; their use in organic synthesis is strongly recommended
in order to carry out sustainable chemistry. Essentially, the xanthine scaffold led to the
preparation of numerous compounds very attractive in the pharmaceutical field, and these drugs are
commercialized for a wide range of biological activities. The scope of this mini-review is to consider
the use of natural xanthines as starting material in chemical transformations carried out in organic
solvents, without the intent to be exhaustive of all the synthetically chemical applications. More information
on the chemical and electrochemical reactivity of this structural core in an organic solvent
can be useful for the scientific community. The effectiveness of natural xanthines can be improved
by modifying the structures of these already biologically active compounds
New N,N-dimethylcarbamate inhibitors of acetylcholinesterase: design synthesis and biological evaluation
A series of N,N-dimethylcarbamates containing a N,N-dibenzylamino moiety was synthesized
and tested to evaluate their ability to inhibit Acetylcholinesterase (AChE). The most active
compounds 4 and 8, showed 85 and 69% of inhibition at 50 mM, respectively. Furthermore,
some basic SAR rules were outlined: an alkyl linker of six methylene units is the best spacer
between the carbamoyl and dibenzylamino moieties; electron-withdrawal substituents on
aromatics rings of the dibenzylamino group reduce the inhibitory power. Compound 4
produces a slow onset inhibition of AChE and this is not due to the carbamoylation of the
enzyme, as demonstrated by the time-dependent inhibition assay of AChE with compound 4
and by MALDI-TOF MS analysis of trypsinized AChE inhibited by compound 4. Instead,
compound 4 could act as a slow-binding inhibitor of AChE, probably because of its high
conformational freedom due to the linear alkyl chain
Organic electrochemistry: Synthesis and functionalization of β-lactams in the twenty-first century
Organic electrochemistry is a technique that allows for the heterogeneous redox reactions avoiding both the use of stoichiometric amounts of redox reagents and the resulting formation of stoichiometric by-pro- ducts. In fact, the redox reagent in these reactions is the electron, which is naturally eco-friendly and pro- duces no side compounds. It is therefore quite obvious that electrochemistry can be classified as a âgreenâ tech- nology. The use of this methodology in the synthesis of β-lactams is not a novelty, but the growing interest in this class of biologically active compounds, due to the dis- covery of new fields of application (after a moment of decrease in interest due to antibiotic resistance) has been a stimulus for the search for more efficient electro- chemical ways to synthesize and transform β-lactams. Thus, this review deals with the twenty-first-century applications of electroorganic technique to the chemistry of β-lactams, by analyzing first the syntheses classified by the type of reactions (cyclization, cycloaddition, etc.) and then by manipulating the β-lactam structure, using it as a synthon. Lastly, the importance of this technique is demonstrated by a study of a pilot plant scale reduction of a cephalosporanic acid derivative to a commercially important antibiotic
An insight into the reactivity of the electrogenerated radical cation of caffeine
Controlled potential electrolyses of caffeine (CAF) were carried out at a Pt electrode in undried acetonitrile (ACN) and ACN-H2O and the products of the anodic oxidation were analyzed by HPLC-PDA-ESI-MS/MS. A higher current efficiency occurred in ACN-H2O, but an analogous chromatographic outline was found in both media, evidencing a reactive pathway of the electrogenerated radical cation CAFâ˘+ with water, added or in trace, as nucleophile. No dimeric forms were evidenced, excluding any coupling reactions. Neither was 1,3,7-trimethyluric acid found, reported in the literature as the main oxidative route for CAF in water. Four main chromatographic peaks were evidenced, assigned to four proposed structures on the base of chromatographic and spectral data: a 4,5-diol derivative and an oxazolidin-2-one derivative were assigned as principal oxidation products, supporting a mechanism proposed in a previous work for the primary anodic oxidation of the methylxanthines olefinic C4 = C 5 bond. Two highly polar degradation products were also tentatively assigned, that seemed generating along two different pathways, one opening the imidazolic moiety and another one opening the purinic one
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