Isomeric Effect on H/D Exchange of Resveratrol Studied by NMR Spectroscopy

Resveratrol (3,5,4′‐trihydroxylstilbene), a phytoalexin in response to injury or fungal attack, is found in grapes and other food products. It has been well documented that the compound has beneficial effects as hypolipidemic, anticancer, antiviral, neuroprotective, antiaging, and anti‐inflammatory natural active principle. It was observed that both trans‐ and cis‐resveratrols undergo hydrogen‐deuterium (H/D) exchange at H‐2,6 and H‐4 positions of the A‐ring. The exchange rates were determined by ¹H NMR spectroscopy. The results reveal that the exchange rates are configuration and pH dependent. Derivative of 2‐O‐β‐D‐glucoside can significantly speed up the H/D exchange reactions for both isomers. The trans‐resveratrol experiences faster H/D exchanging than the cis‐resveratrol. Such isomeric effect is possibly due to the factor that the trans‐resveratrol is in favor of forming larger/super conjugative system and has less spatial interference. Theoretical calculation shows that electronegativity at these positions is in the order of H‐2,6>H‐4, which is in consistent with the exchange rates observed by NMR. The results may be of help in understanding the properties of resveratrol, and in analysis of resveratrol in natural products or body fluids using mass spectroscopy that occasionally requires stable deuterium isotope labeling.     

Aromatic H／D Exchange Reaction Catalyzed by Groups 5 and 6 Metal Chlorides

Groups 5 and 6 metal chlorides such as MoCl5, WCI6, NbCl5 and TaCl5 were found to be simple and very efficientcatalysts for the aromatic H/D exchange reactions. Compared with other metal chlorides such as ZnCl2, SnCl4 and TiCl4, groups 5 and 6 metal chlorides showed better catalytic activity in the H/D exchange reaction of naphthalene with C6D6. Deuteration of anthracene using MoC15 as a catalyst proceeded within 24 h at room temperature. Other aromatic compounds such as toluene, diphenylmethane and 1,1,2-triphenylethane were also deuterated smoothly in C6D6 within 24 h at room temperature.     

环丙烷衍生物在D2O离子体系中的H/D交换反应

在D2O化学反应气条件下研究了环丙烷衍生物的H/D交换反应特性.发现了三种新的产物离子[M+1]+、[M+2]+和[M+3]+.应用碰撞诱导碎裂(CID)技术研究了这些离子的碎裂反应特性.实验结果表明三种新的产物离子是由反应物与试剂离子之间发生H/D交换反应生成的.并获得了环丙烷衍生物结构中活泼氢位置及其数量的信息.     

Palladium(II)-catalysed H/D allylic exchange in alkenes: An intermediacy of palladium(IV)?

Evidence is presented that the dimeric π-allylic species [(η³-allyl)PdCl]₂ is not intermediate in the Li₂Pd₂Cl₆-catalysed allylic H/D exchange in alkenes. Neither H/D exchange in α-methylstyrene, nor enrichment of [(η³-2-PhC₃H₄)PdCl]₂, was observed when the latter complex was incubated at 100°C in D₃CCOOD either in the presence or in the absence of PhC(CH₃)?CH₂, respectively. The kinetics of H/D exchange in α-methylstyrene catalysed by Li₂Pd₂Cl₆ were studied in some detail. The exchange proceeds at highest rates when reduction of palladium(II) takes place and is much slower in the presence of 1,4-benzoquinone as a palladium reoxidant. The exchange rate is directly proportional to the alkene and catalyst concentrations and independent of the reoxidant concentration. It is suggested that the palladium(II)-catalysed exchange involves an intermediate hydrid-allyl species where palladium has a formal oxidation state of IV.     

Mechanistic considerations on catalytic H/D exchange mediated by organometallic transition metal complexes

The purpose of this review is to analyze the different reaction mechanisms of the H/D exchange on organic substrates catalyzed by transition metal complexes in homogeneous phase. The metal-catalyzed H/D exchange is a multifaceted reaction whose mechanism depends strongly on the reaction conditions and on the metal complex used as a catalyst. It is possible to group the different mechanisms into three main families depending on the “role” and behavior of the catalyst: (i) Lewis acid–base catalysis; (ii) CH activation (iii) insertion/β-elimination. For each macro-group, several representative examples are discussed and critically evaluated in order to provide the reader with keys to the understanding of how the different catalytic systems act and how their modification may affect their performance in terms of activity and selectivity. This knowledge is fundamental for designing improved organometallic H/D catalysts for labeling organic products in greener conditions with more cost-effective processes.     

Improving real-time measurement of H/D exchange using a FTIR biospectroscopic probe

We describe the improvement of a novel approach to investigating hydrogen/deuterium (H/D) exchange kinetics in biomolecules using transmission infrared spectroscopy. The method makes use of a Fourier transform infrared spectrometer coupled with a microdialysis flow cell to determine exchange rates of labile hydrogens. With this cell system, the monitoring of exchange reactions has been studied here as a function of some cell characteristics such as: (a) dialysis membrane surface contacting both the H₂O and D₂O compartments; (b) molecular cutoff of dialysis membrane; and (c) distance between the cell-filling holes. The best improvement has been obtained by increasing the dialysis membrane surface followed by increase of molecular cutoff. However, not significant differences were found using various distances between filling holes. The fastest exchange rate which can be measured with the cell system used here is found to be k = 0.41 ± 0.02 min⁻¹, that is, about threefold greater than the one got in a previous work. This microdialysis flow cell has been used here for the study of H/D exchange in nucleic acids with subsequent structural analysis by 2D correlation spectroscopy.     

Mechanistic Investigations of Two Bacterial Diterpene Cyclases: Spiroviolene Synthase and Tsukubadiene Synthase

The mechanisms of two diterpene cyclases from streptomycetes—one with an unknown product that was identified as the spirocyclic hydrocarbon spiroviolene and one with the known product tsukubadiene—were investigated in detail by isotope labeling experiments. Although the structures of the products were very different, the cyclization mechanisms of both enzymes proceed through the same initial cyclization reactions, before they diverge towards the individual products, which is reflected in the close phylogenetic relationship of the enzymes.     

The investigation on the NH reactivity of pentacoordinate spirophosphoranes by H/D exchange and NMR experiments

Different pentacoordinate spirophosphoranes were investigated by H/D exchange and NMR tracking experiments. The H/D exchanged site of spirophosphoranes was confirmed to be the NH bond instead of the PH bond according to the integration of the signal changes in proton-decoupled, proton-coupled ³¹P NMR and ¹H NMR tracking experiments. The reactivity of NH bond at the spiroring of spirophosphoranes was explored by the rate constant of H/D exchange of different spirophosphoranes. The results demonstrated that the reactivity of NH bond of spirophosphoranes was influenced by steric hindrance and the species of substituent at phosphorus atom, and the electronic effect of the substituent was the main effect factor on the reactivity of the NH bond. The strong electron-withdrawing group at phosphorus resulted in more reactive NH bond of spirophosphoranes. These results are beneficial to further understand and explore the characteristics of pentacoordinate spirophosphoranes.     

Room‐Temperature Alkane Reactivity in Zeolites: An H/D Exchange Study

As evidenced by H/D exchange with acidic zeolites, isoalkanes react readily at room temperature whereas linear alkanes do not. The observed regioselectivity of the exchange process demonstrates that the main factor controlling the reaction is not the accessibility to the acid sites, but the intrinsic reactivity of the alkane. The mechanism is best rationalized by classic organic chemistry involving carbocationic intermediates including the Markovnikov rule.     

环丙烷衍生物在CD3OD离子体系中的H/D交换反应

研究了环丙烷衍物生物在CD3OD离子体系中的H/D交换反应产物离子[M 1]^*,[M 2]^ 和[M 3]^*的碰撞诱民碎裂(CID)反应特征,实验结果表明这些产物离子是由反应物与试剂离子之间生H/D交换反应生成的,获得了环丙烷衍生物结构中活泼氢位置及其数量的信息.     

Iridium‐Catalyzed H/D Exchange: Ligand Complexes with Improved Efficiency and Scope

Hydrogen isotope exchange (HIE) is one of the most attractive tools for the introduction of deuterium or tritium to an organic compound. Herein, iridium complexes with N,P‐ligands, highly active catalysts for asymmetric double bond reductions, have been tested for their HIE capabilities. Electron‐rich ligands, containing dicyclohexylphosphines or phosphinites, have been identified as excellent ligands for efficient deuterium incorporation. Substrates with strong directing groups, that is, pyridines, ketones, and amides, as well as weak ligating units, such as, nitro, sulfones, and sulfonamides, could be labeled efficiently. With the addition of tris(pentafluorophenyl) borane to the reaction mixture, also highly deactivating nitrile substituents were well tolerated in the reaction. Based on the excellent results obtained with the chiral ThrePhox ligand, a structurally simpler, achiral ligand was developed. The iridium complex containing this ligand, proved to be a powerful catalyst for HIE reactions.     

Hydride‐Rhodium(III)‐N‐Heterocyclic Carbene Catalysts for Vinyl‐Selective H/D Exchange: A Structure–Activity Study

A series of neutral and cationic Rh^III‐hydride and Rh^III‐ethyl complexes bearing a NHC ligand has been synthesized and evaluated as catalyst precursors for H/D exchange of styrene using CD₃OD as a deuterium source. Various ligands have been examined in order to understand how the stereoelectronic properties can modulate the catalytic activity. Most of these complexes proved to be very active and selective in the vinylic H/D exchange, without deuteration at the aromatic positions, displaying very high selectivity toward the β‐positions. In particular, the cationic complex [RhClH(CH₃CN)₃(IPr)]CF₃SO₃ showed excellent catalytic activity, reaching the maximum attainable degree of β‐vinylic deuteration in only 20 min. By modulation of the catalyst structure, we obtained improved α/β selectivity. Thus, the catalyst [RhClH(κ²‐O,N‐C₉H₆NO)(SIPr)], bearing an 8‐quinolinolate ligand and a bulky and strongly electron‐donating SIPr as the NHC, showed total selectivity for the β‐vinylic positions. This systematic study has shown that increased electron density and steric demand at the metal center can improve both the catalytic activity and selectivity. Complexes bearing ligands with very high steric hindrance, however, proved to be inactive.     

Differentiation of sulfate and phosphate by H/D exchange mass spectrometry: application to isoflavone

Often phosphorylation or sulfation is an important step which occurs in the signal transduction and cascade of metabolic pathways. Some natural products and metabolites contain one or more sulfate or phosphate groups. Isoflavone sulfate has been identified from high-resolution mass spectrometry (HRMS) and enzymatic digestion by sulfatase. We previously reported the new water-soluble isoflavone analogs, daidzein 7-O-phosphate and genistein 7-O-phosphate, which were surprisingly hydrolyzed by sulfatase. In this previous study, we could not determine the phosphate from the results of HRMS and enzymatic digestion, that is, HRMS and enzymatic digestion did not provide clear evidence. In this case, we drew conclusions from NMR analysis. HRMS has been ineffective with a regular fast atom bombardment (FAB) mass spectrometer to distinguish between phosphate and sulfate since the mass difference is only 0.009 mass units. There was, however, no conventional method of microanalysis to distinguish phosphate from sulfate owing to the same nominal mass. It is still very difficult to determine by negative FABMS [--O--P(==O)(OH)(2)] = 80 and [--O--S(==O)(2)OH] = 80. In this paper, we report a method to distinguish between these groups by using a popular low-resolution mass instrument; thus, phosphate and sulfate were measured by H/D exchange mass spectrometry at the picomole level to differentiate [--O--P(==O)(OD)(2)] = 82 and [--O--S(==O)(2)OD] = 81, respectively. This method is applicable not only to the isoflavone, but also to other phospho and sulfo compounds.     

Room Temperature COad Desorption/Exchange Kinetics on Pt Electrodes—A Combined In Situ IR and Mass Spectrometry Study

The room temperature desorption and exchange of CO in a saturated CO adlayer on a Pt electrode, at potentials far below the onset of oxidation, was investigated by isotope labeling experiments, using a novel spectroelectrochemical setup, which allows the simultaneous detection of adsorbed species by in situ IR spectroscopy and of volatile (side) products and reactants by online mass spectrometry under controlled electrolyte flow conditions. Time‐resolved IR spectra show a rapid, statistical exchange of pre‐adsorbed ¹³CO_ad by ¹²CO_ad in ¹²CO containing electrolyte; mass spectrometric data reveal first‐order exchange kinetics, with the rate increasing with CO partial pressure. The increasing CO_ad desorption rate in equilibrium with a CO containing electrolyte is explained by a combination of an increasing CO_ad coverage upon increasing the CO pressure, and a decrease of the CO adsorption energy with coverage, due to repulsive CO_ad–CO_ad interactions.     

A Mechanistic Interpretation of Initialization Processes in RAFT‐Mediated Polymerization

Summary: The presence of strong chain length selectivity behavior during initialization in the RAFT process is an experimental observation. The mechanistic underpinning of the observation is, however, open to interpretation. The rates and concentration profiles for the cyanoisopropyl‐dithiobenzoate‐mediated polymerization of styrene at 70 °C can be relatively well reproduced by either the slow fragmentation or intermediate radical termination models. The use of modeling that provides a fit to the data is an important tool, but should be considered as additional evidence in support of experimental data rather than as experimental evidence in and of itself.

Comparison of the time dependencies of the concentrations of AD, AMD, and AM₂D by PREDICI modeling for the cyanoisopropyl‐mediated polymerization of styrene at 70 °C generated by the IRT model (lines) versus experimental data.