Polar Aprotic Media Research Articles

Effect of various aqueous extracting agents on fluorescence properties of waste tea residue derived carbon dots (WTR-CDs): Comparative spectroscopic analysis.

Fluorescent carbon dots (CDs) are one of the important carbonaceous nanomaterials in the area of nanoscience and nanotechnology because of their interesting physical as well as chemical properties. Herein we studied the effect of various aqueous extracting agents on fluorescence properties of waste tea residue-based carbon dots (WTR-CDs). WTR-CDs are firstly synthesized by utilizing kitchen waste-based carbonaceous biomass. To check the role of various aqueous media during the course of WTR-CDs synthesis from carbonized carbon powder, extraction of WTR-CDs was carried out in various kinds of aqueous media viz., only aqueous (100% water, WT), aqueous-alcoholic (10% ethanol, ET), aqueous-acidic (10% acetic acid, AA), and aqueous-basic (10% ammonia, AM). The consequences of extracting agents on the photophysical properties of final WTR-CDs-WT, WTR-CDs-ET, WTR-CDs-AA and WTR-CDs-AM were also discussed in detail. We have observed interesting blue shift fluorescence spectra in acidic medium for WTR-CDs-AA and polar protic solvents compared to polar aprotic medium. The solvatochromic behaviour of WTR-CDs-WT in model polar and non-polar solvent was also studied. The effect of cationic, anionic and non-anionic surfactants on the fluorescence of WTR-CDs-WT was also evaluated. The proposed findings may help researchers in the near future to obtain fast, easy and direct synthesize CDs from a variety of biomass-based precursors under different aqueous conditions.

Specific locations of blue and green-emitting units in dual emissive carbon dots and their reversible emitting properties due to switchable inter-chromophoric interactions

Carbon dots (CDs) are the unique class of luminescent nanomaterials consist of various chromophoric units heterogeneously distributed throughout the nanoparticle, resulting intriguing multistate emissive properties. Herein, we have critically investigated the specific locations of the blue and green-emitting centers inside dual emissive CDs by steady-state and time-resolved polarized emission study. It is further clarified by a temperature-dependent fluorescence study for both the emitting domains. Results suggest that the blue chromophoric units are located at the interior part of CDs, while green units are mostly at the exterior region. Furthermore, we have investigated the solvent-dependent inter-chromophoric interactions between the two emissive domains by the Time-Resolved Area Normalized Emission Spectroscopy (TRANES). Results suggest that at polar aprotic solvent acetone, time-dependent positive evolution of green-emitting states and negative evolution of blue emissive domains have been observed. This reversible emitting properties evolve due to the excited state energy migration from blue emissive domains to green emissive domains at polar aprotic medium, while in the case of polar protic solvent water, this phenomenon is missing. This switchable inter-chromophoric interaction are correlated further with the inter-particle interactions of CDs.

Self-Assembly of Hydrogen-Bonded Cage Tetramers of Phosphonic Acid

The self-association of phosphonic acids with general formula RP(O)(OH)2 in solution state remains largely unexplored. The general understanding is that such molecules form multiple intermolecular hydrogen bonds, but the stoichiometry of self-associates and the bonding motifs are unclear. In this work, we report the results of the study of self-association of tert-butylphosphonic acid using low temperature liquid-state 1H and 31P NMR spectroscopy (100 K; CDF3/CDF2Cl) and density functional theory (DFT) calculations. For the first time, we demonstrate conclusively that polar aprotic medium tert-butylphosphonic acid forms highly symmetric cage-like tetramers held by eight OHO hydrogen bonds, which makes the complex quite stable. In these associates. each phosphonic acid molecule is bonded to three other molecules by forming two hydrogen bonds as proton donor and two hydrogen bonds as proton acceptor. Though the structure of such cage-like tetramers is close to tetrahedral, the formal symmetry of the self-associate is C2.

Electro-Carboxylation of Furfural By CO2 in γ-Valerolactone

The continuous rise of global warming and depletion of fossil fuel resources has led to the development of new ways of valorization of non petro-sourced chemicals. In this objective, it is important to develop electrochemical valorization processes of CO2, a greenhouse gas, for the synthesis of valuable molecules. When the CO2 electroreduction is performed in aqueous media, the competitive HER will decrease the energetic efficiency and the rapid deactivation of reaction intermediates will lead to side products such as CO, formic acid, oxalic acid and small chain hydrocarbons [1]. To circumvent these issues and create value added chemicals, reductive electro-carboxylation reactions can be performed in polar aprotic solvents (DMF, DMSO, acetonitrile or NMP) in the presence of tetraalkylammonim bromide or chloride as electrolytes [2].In this work, we focused on non-aqueous bio-sourced solvent, the γ-valerolactone (GVL), to perform a carboxylation reaction of bio-sourced molecules like furfural by electroreduction of CO2. The reductive electro-carboxylation of furfural was studied at a Pt/C electrode was studied in a bio-sourced GVL with tributylmethylammonium chloride (TBMACl) as electrolyte. The solvent behavior in the CO2 reduction potential range was studied by in situ FTIRS measurements which indicated that GVL reacted at the electrode in the absence of furfural and/or of CO2, while the presence of furfural and/or of CO2 made the solvent much less reactive. The electrochemical study showed that the CO2 electroreduction started at from the high cathodic potential of −0.8 V vs SCE (saturated calomel electrode) and the coupling of electrochemistry with in situ FTIRS demonstrated that it reacted with furfural.The reductive electro-carboxylation was then performed in an electrolysis cell. The cell voltage was maintained at 2.0 V (corresponding to a cathode potential of -1.9 V vs. SCE) for 24 h. In absence of furfural, gas chromatography coupled with mass spectroscopy (GC–MS) analyses revealed the formation of oxalic acid (coupling reaction of CO2) as main reaction product. In the presence of furfural in the reaction medium, a new carboxylated specie, 2-furyl(hydroxy)acetic acid, was identified as the main reaction product. 1H NMR and LC-MS analyses further confirmed the formation of this compound, demonstrating the potentialities of bio-sourced solvents for reduction electro-carboxylation of a bio-sourced molecule by reduction of CO2[3].[1] Y. Hori, Electrochemical CO2 reduction on metal electrodes, in: C. Vayenas, R. White, M. Gamboa-Aldeco (Eds.), Modern Aspects of Electrochemistry, 42, Springer, NewYork 2008, pp. 89–189[2] R. Matthesses, J. Fransaer, K. Binnemans, D.E. De Vos, Electrocarboxylation: towards sustainable and efficient synthesis of valuable carboxylic acids, Beilstein J. Org.Chem. 10 (2004) 2484–2500.[3] Florent Boissou, S. Baranton, M. Tarighi, K. De Oliveira Vigier, C. Coutanceau, The potency of γ-valerolactone as bio-sourced polar aprotic organic medium for the electrocarboxlation of furfural by CO2. J. Electroanal. Chem. 848 (2019) 113257

Synthesis, molecular docking, dynamic simulations, kinetic mechanism, cytotoxicity evaluation of N-(substituted-phenyl)-4-{(4-[(E)-3-phenyl-2-propenyl]-1-piperazinyl} butanamides as tyrosinase and melanin inhibitors: In vitro, in vivo and in silico approaches.

In the current research work, different N-(substituted-phenyl)-4-{(4-[(E)-3-phenyl-2-propenyl]-1-piperazinyl}butanamides have been synthesized according to the protocol described in scheme 1. The synthesis was initiated by reacting various substituted anilines (1a-e) with 4-chlorobutanoyl chloride (2) in aqueous basic medium to give various electrophiles, 4-chloro-N-(substituted-phenyl)butanamides (3a-e). These electrophiles were then coupled with 1-[(E)-3-phenyl-2-propenyl]piperazine (4) in polar aprotic medium to attain the targeted N-(substituted-phenyl)-4-{(4-[(E)-3-phenyl-2-propenyl]-1-piperazinyl}butanamides (5a-e). The structures of all derivatives were identified and characterized by proton-nuclear magnetic resonance (1H NMR), carbon-nuclear magnetic resonance (13C NMR) and Infra-Red (IR) spectral data along with CHN analysis. The in vitro inhibitory potential of these butanamides was evaluated against Mushroom tyrosinase, whereby all compounds were found to be biologically active. Among them, 5b exhibited highest inhibitory potential with IC50 value of 0.013±0.001µM. The same compound 5b was also assayed through in vivo approach, and it was explored that it significantly reduced the pigments in zebrafish. The in silico studies were also in agreement with aforesaid results. Moreover, these molecules were profiled for their cytotoxicity through hemolytic activity, and it was found that except 5e, all other compounds showed minimal toxicity. The compound 5a also exhibited comparable results. Hence, some of these compounds might be worthy candidates for the formulation and development of depigmentation drugs with minimum side effects.

The potency of γ-valerolactone as bio-sourced polar aprotic organic medium for the electrocarboxlation of furfural by CO2

The reductive electro-carboxylation of furfural at a Pt/C electrode was studied in a bio-sourced γ-valerolactone (GVL) solvent using linear scan voltammetry, in situ infrared spectroscopy (in situ FTIRS) under electrode potential control and by chrono-amperometry (CA) with analysis of reaction products. In situ FTIRS measurements indicated that GVL reacted at the electrode in the absence of furfural and/or of CO2, while the solvent became much less reactive in the presence of furfural and/or of CO2. It was also found that CO2 was activated from the high cathodic potential of −0.8 V vs SCE (saturated calomel electrode) and that it reacted with furfural. CA measurements at a cell voltage of 2.0 V, corresponding to a cathode potential higher than −1.66 V vs. SHE (standard hydrogen electrode), were performed for 24 h in the presence of CO2 at the cathode. Gas chromatography coupled with mass spectroscopy (GC–MS) analyses revealed the formation of 2-hydroxyvaleric acid (reaction between GVL and residual water) and oxalic acid (coupling reaction of CO2) as main reaction products in the absence of furfural in the reaction medium. In the presence of furfural in the reaction medium, 2-hydroxyvaleric acid and oxalic acid were detected as traces, whereas a new carboxylated specie, 2-furyl(hydroxy)acetic acid, corresponded by far to the main reaction product (ca. 82%). 1H NMR and LC-MS analyses confirmed the formation of this last compound. Under the experimental conditions described in this article, the electro-activation of CO2 could be performed at high cathode potential in GVL, leading in presence of furfural to the formation of the α-hydroxy-acid.

A spectroscopic exploration of the influence of charge donor group on ESIPT process and its consequences in a salicylimine

Photophysical properties of a newly designed synthetic compound DYMDAP have been investigated in combination with computational calculations. DYMDAP shows a solvent dependent emissive behavior which in non-polar solvents shows two distinct peaks along with a shoulder but with increasing polarity as also proticity of the solvents the intensity of each band varies significantly. Along with the two bands, a new band is observed in polar protic solvent due to the proton transfer (PT) in the excited state of the compound. Only in polar protic solvents, intensity of the PT band increases significantly whereas in non-polar or polar aprotic medium intensity is sparse. By comparing the spectral observations of DYMDAP with two control compounds DYMP and DYMDAB the proton transfer process is assigned to be due to solvent mediated and the proton transfer process is enhanced by the presence of charge transfer moiety.

Synthesis and characterization of low dimensional bioactive monetite by solvent exchange method

A method is demonstrated to synthesize high purity low-dimensional monetite (CaHPO4, dicalcium phosphate anhydrous) for biomedical applications by solvent exchange in a polar aprotic medium. Stoichiometric aqueous solution containing calcium nitrate and phosphoric acid was prepared and the solvent was exchanged with acetone, thereby to obtain the new phase. The precipitate contained phase-pure monetite particles having 10–20μm size with burr-like nano surface growth features. The material obtained was found bioactive in immersion test using simulated body fluid and cell compatible in cell adhesion studies using L929 cell lines. It will be ideal for the use in bioceramics and bone cements.

An unusual deprotonation trend in 2-(2'-hydroxyphenyl)pyridoimidazoles.

The anion sensitivity and the deprotonation nature of the nitrogenous analogues of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) are investigated in a polar aprotic medium. It is observed that the substitution of pyridyl nitrogen enhances the anion sensitivity. However, despite the enhanced sensitivity of the nitrogenous analogues the deprotonation of these molecules in the presence of strong anions is less favored as compared to HPBI. This anomalous trend observed for the nitrogenous analogs is discussed and explained using theoretical calculations and experimental findings. It is also found that the sensitivity towards anions and the formation of anions also depend on the position of the pyridyl nitrogen.

Estimation of the pKa for various Brønsted acids in polar aprotic media using electrochemical measurements of chromium (III) with picolinic acid

The electrochemical reduction of Crpic and the effect of various proton-donors in DMF were investigated in this work. Measurements showed important electrochemical changes in the presence of proton-donors such as ascorbic acid, benzoic acid and acetic acid. Dissociation constants and acid strength are estimated using the Nernstian behavior of the chromium (III) complex and the proportional relation between ΔEp vs pKa are assistance to estimate the dissociation constants and the acid strength.

Solution phase photophysics of 5,7-dimethoxy-2,3,4,9-tetrahydro-1H-carbazol-1-one: Analysing the lineaments of a new fluorosensor to probe different micro-environments

We present here a detailed photophysical study of a newly synthesised fluorophore 5,7-dimethoxy-2,3,4,9-tetrahydro-1H-carbazol-1-one (KTHC-57). Spectroscopic investigation of the compound has been carried out in 14 different protic and aprotic solvents, as well as in different binary solvent mixtures, using absorption, steady-state and time-resolved fluorescence techniques. The spectral behaviour of this compound is found to be extremely sensitive to the polarity and hydrogen bonding nature of the solvent. When considered in micelles, reverse micelles and β-cyclodextrin, KTHC-57 behaves as a reporter of its immediate microenvironment. Extent of hypsochromic shift varies as we move from cationic surfactant cetyltrimethylammonium bromide (~40nm) to anionic surfactant sodium dodecyl sulphate (~23nm) to non-ionic surfactant Triton X-100 (~37nm). In case of anionic reverse micellar nanocavities composed of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/heptane with increasing water contents, a bathochromic shift of ~50nm is observed. A quantitative assessment of the emission intensity data on Benesi–Hildebrand equation reveals a 1:1 stoichiometry for KTHC-57:β-CD complex. Triethylamine (TEA) is a simple aliphatic organic molecule that interacts with KTHC-57 in polar aprotic medium. Within β-CD environment, fluorescence quenching takes place along with a bathochromic shift. Interaction of (TEA) with KTHC-57 in β-CD nano-confinement is studied using absorption spectroscopy, steady-state as well as time-resolved fluorescence spectroscopy and laser flash photolysis in conjunction with an external magnetic field.

Antioxidant Properties of Kanakugiol Revealed Through the Hydrogen Atom Transfer, Electron Transfer and M2+ (M2+ = Cu(II) or Co(II) Ion) Coordination Ability Mechanisms. A DFT Study In Vacuo and in Solution

DFT studies on the conformational and antioxidant properties of kanakugiol were performed to investigate factors that contribute to its conformational preferences and to elucidate its antioxidant properties and mechanisms. The antioxidant activity has been considered through the ability of kanakugiol to scavenge free radical species and through its ability to chelate metal ions. The antiradical activity was performed using the B3LYP and the B3P86 and by using both the 6-31+G(d,p) and the 6-311+G(d,p) basis sets. The gas-phase interaction of kanakugiol with Cu (II) and Co (II) ions were studied by means of G96LYP/6-311+G(3df,2p)//G96LYP/6-31G(d) calculations. The results show that the preferred neutral conformers are stabilised by the presence of intramolecular hydrogen bonds (IHBs) and the orientation of both the OCH3 groups and the prop-2-en-1-one chain. The neutral and cationic radical species are stabilised by both IHBs and the spin density delocalisation of the unpaired electron. A comparison of the OH bond dissociation enthalpy and the ionisation potential values across media suggests that each parameter has the lowest value in acetonitrile and highest value in vacuo, which indicates that kanakugiol may have an optimal tendency to scavenge radical species in polar aprotic media. Among the metal ion complexes, the preferred geometry is dependent on the media; in vacuo the preferred complex is one in which the metal ion is dentated to the π system of the aromatic ring while in water solution, the preferred complex is one in which the metal ion is dentated between O atoms. An estimation of metal ion affinity in aqueous solution shows a remarkable decrease with respect to the results in vacuo. The charge on metal ions decreases on interaction with kanakugiol, suggesting that both metal ions are reduced while kanakugiol is oxidised. An analysis of the spin density distribution and the electron delocalisation index elucidates the difference in the type of bonding between the Co(II) and Cu(II) complexes.

Fluorescence quenching of (dimethylamino)naphthalene dyes Badan and Prodan by tryptophan in cytochromes P450 and micelles.

Fluorescence of 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes Badan and Prodan is quenched by tryptophan in Brij 58 micelles as well as in two cytochrome P450 proteins (CYP102, CYP119) with Badan covalently attached to a cysteine residue. Formation of nonemissive complexes between a dye molecule and tryptophan accounts for about 76% of the fluorescence intensity quenching in micelles, the rest is due to diffusive encounters. In the absence of tryptophan, fluorescence of Badan-labeled cytochromes decays with triexponential kinetics characterized by lifetimes of about 100 ps, 700–800 ps, and 3 ns. Site mutation of a histidine residue in the vicinity of the Badan label by tryptophan results in shortening of all three decay lifetimes. The relative amplitude of the fastest component increases at the expense of the two slower ones. The average quenching rate constants are 4.5 × 108 s–1 (CYP102) and 3.7 × 108 s–1 (CYP119), at 288 K. Cyclic voltammetry of Prodan in MeCN shows a reversible reduction peak at −1.85 V vs NHE that becomes chemically irreversible and shifts positively upon addition of water. A quasireversible reduction at −0.88 V was observed in an aqueous buffer (pH 7.3). The excited-state reduction potential of Prodan (and Badan) is estimated to vary from about +0.6 V (vs NHE) in polar aprotic media (MeCN) to approximately +1.6 V in water. Tryptophan quenching of Badan/Prodan fluorescence in CYPs and Brij 58 micelles is exergonic by ≤0.5 V and involves tryptophan oxidation by excited Badan/Prodan, coupled with a fast reaction between the reduced dye and water. Photoreduction is a new quenching mechanism for 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes that are often used as solvatochromic polarity probes, FRET donors and acceptors, as well as reporters of solvation dynamics.

Conformational Preferences of N,N-Dimethylsuccinamate as a Function of Alkali and Alkaline Earth Metal Salts: Experimental Studies in DMSO and Water As Determined by 1H NMR Spectroscopy

The fraction of gauche conformers of N,N-dimethylsuccinamic acid (1) and its Li+, Na+, K+, Mg2+, Ca2+, and N(Bu)4+ salts were estimated in DMSO and D2O solution by comparing the experimental vicinal proton–proton couplings determined by 1H NMR spectroscopy with those calculated using the Haasnoot, de Leeuw, and Altona (HLA) equation. In DMSO, the gauche preferences were found to increase with decreasing Ahrens ionic radius of the metal counterion. The same trend was not seen in D2O, where the gauche fraction for all of the metallic salts were estimated to be approximately statistical or less. This highlights the importance of metal chelation on the conformation of organic molecules in polar aprotic media, which has implications for protein folding.

Selective Synthesis of Poly(m-phenylene oxides) over Oxacalixarenes

We report a high yielding AA−BB type step-growth synthesis of poly(m-phenylene oxides) (mPOs) by nucleophilic aromatic substitution (SNAr) reactions. Previously, poly(mPOs) have proven difficult to synthesize, especially by SNAr, due to competing cyclooligomer (oxacalixarene) formation. We have found that the linear (polymeric) vs cyclic (oligomeric) product distribution is strongly dependent on the substitution pattern of the electrophilic monomer, and a model is described that predicts whether the reacting monomers will kinetically favor formation of poly(mPOs) or oxacalixarenes. The polycondensation reaction tolerates a range of functional groups on both reactive monomers, and the synthesized poly(mPOs) have high thermal and aqueous stability but are readily depolymerized in polar aprotic media.

Diameter and Metallicity Dependent Redox Influences on the Separation of Single-Wall Carbon Nanotubes

Recently, it has become possible to separate and/or enrich fractions of single-wall carbon nanotubes (SWNTs) according to type (or otherwise termed "metallicity") and diameter (d(t)). Exposure of acid-treated SWNTs to amines has shown such separation. In this contribution, we describe the underlying mechanism for this separation and provide a better description of the physicochemical properties of charge-stabilized SWNT dispersions in polar aprotic media, such as N,N-dimethylformide (DMF). With the establishment of the reversible nature of the redox chemistry, SWNT(n+) + (n/2)H2O <==> SWNT + nH(+) + (n/4)O2, amine-induced pH changes as well as variations in H2O and O2 concentration in DMF are shown to cause differential partial-reduction trends according to d(t) and metallicity. At a pH of 10, the (n,m)-SWNTs that resist complete reduction to their undoped state remain in suspension while the rest that lose their charges populate the precipitate. These d(t)- and metallicity-dependent redox and separation trends are modeled based on the Gibbs free energy and charge loss as it pertains to the (n,m)-dependent SWNT integrated density of states (I(DOS)) across the corresponding pH-induced redox jump. At a given redox potential, the relative placement of the van Hove singularities and continuum determines the amount of charge left on various (n,m)-SWNTs that governs their relative dispersion stability in DMF.

Excited state intermolecular proton transfer and caging of salicylidine-3,4,7-methyl amine in cyclodextrins

The excited state intermolecular proton transfer (EIMPT) of salicylidine-3,4,7-methyl amine (SMA) has been studied using absorbance, steady state and time resolved emission spectroscopy in aqueous and polar aprotic media both in presence and absence of cyclodextrins (CDs). In the ground state, both the closed cis enol form and zwitterionic species coexist in all the solvents used. After encapsulation by β- and γ-CD, only intramolecularly H-bonded cis enol form exists in all the solvent media, while in α-CD both the species coexist. In the excited state, intermoleculer proton transfer to water efficiently leads to the formation of anion in absence of CDs. Caging by CDs provokes a 15 (in β-CD) and 10 (in γ-CD) nm blue shift of the emission along with a decrease in intensity of the band, whereas no change is observed in presence of α-CD and glucose. The emission lifetime of anion decreases from 11.8 to 3.4 ns due to the formation of H-bonded complex. Analysis of the spectroscopic data by Benesi–Hildebrand plot shows that SMA forms 1:1 complex in the ground state with β- and γ-CD. Stern–Volmer plot reveals that β-CD is the stronger quencher of SMA anion compared to γ-CD.

Picosecond kinetics of excited-state charge separation in 4-(dimethylamino)pyridine

4-(Dimethylamino)pyridine (DMAP) shows solvent-dependent dual fluorescence from the initially excited state B* and a highly polar TICT state A*. Room-temperature time-resolved picosecond fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction B*-->A* in polar aprotic media. In medium polarity solvents (such as ethyl acetate) two emitting states of DMAP are shown to reach equilibrium within 50 ps. Both emitting states originate from the same ground state. The rate of excited-state charge separation depends on polarity and proton donating ability of the surrounding medium. The effects of temperature on the quantum yields of both fluorescences of DMAP in polar aprotic media indicate the transition from the kinetic regime (at low temperatures) to the equilibrium regime (at high temperatures). The kinetic behaviour of the dual luminescence of DMAP in protic solvents is more complex than in aprotic ones. In alcohols an efficient nonradiative channel competes with excited-state charge separation.

Kinetics and mechanism for CO(2) scrambling in a N-carboxyimidazolidone analogue for N(1)-carboxybiotin.

The N-carboxyimidazolidone anion, 2(-), was prepared as an analogue for N(1)-carboxybiotin, and the kinetics of its CO(2)-dependent chemistry were studied in polar aprotic media. The objective was to assess the viability of unimolecular CO(2) elimination from N(1)-carboxybiotin as a microscopic step in biotin-dependent carboxyl transfer enzymes. The anionic 2(-) was prepared as its lithium salt by first deprotonating 2-imidazolidone with phenyllithium, followed by direct reaction with carbon dioxide. This procedure also permitted isolation of the (13)C enriched derivative 2(-)[(13)C] by reaction with (13)CO(2). Proton and (13)C NMR and isotope-dependent FTIR measurements confirmed that carboxylation had occurred at the nitrogen atom of 2-imidazolidone to give 2(-). Time-dependent FTIR spectroscopy showed that Li2 undergoes carboxyl exchange with free carbon dioxide, with kinetics indicative of rate-limiting unimolecular dissociation of the N-CO(2) bond. Under these conditions, the weak Lewis acid Mg(2+) catalyses the exchange of 2(-) with free CO(2), which appears to be related to the ability of the metal ion to coordinate to 2(-). Reaction of Li2 with carboxylic acids in DMSO results in acid-dependent decarboxylation of 2(-) with a rate that is dependent on the concentration of the acid and its pK(a). A common mechanistic framework is presented for both Lewis acid catalyzed carboxyl exchange and acid-dependent decarboxylation that involves initial interaction at the carbonyl oxygen and which has the effect of polarizing the nitrogen lone pair toward the imidazolidone ring rather than the carboxyl group. Lewis acid interaction with the carbonyl oxygen thus weakens the N-CO(2)(-) bond and functions as a trigger for dissociation of CO(2). In the context of biotin-dependent enzymes, this suggests a means by which the kinetically stable N(1)-carboxybiotin cofactor intermediate might be triggered for dissociation of CO(2).

Reaction of Internal Fluoroolefin Oxides with Camphor Thiosemicarbazone

Epoxy derivatives of internal fluoroolefins (both cis and trans isomers) react in a stereospecific manner with (1S)- or rac-camphor thiosemicarbazone in a polar aprotic medium to give the corresponding 5-fluoro-4-hydroxy-4,5-bis(polyfluoroalkyl)thiazol-2-ylhydrazones. From unsymmetrical 2,3-epoxydodecafluorohexane a mixture of regioisomeric hydrazones is formed. According to the 1H and 19F NMR data, the resulting trans-hydrazones in (CD3)2SO and CDCl3 exist as mixtures of diastereoisomers occurring in a dynamic equilibrium.