Group Transfer Reactions Research Articles

Theoretical studies on thiocarbonyl group transfer reactions

The gas-phase thiocarbonyl transfer reactions X− + RC(S)Z ⇌ RC(S)X + Z− have been investigated with X, Z = Cl, Br and R = H, CH3 at the MP2 and G2(+) levels using the MP2/6-311+G** optimized geometries. The thiocarbonyl transfers proceed by a stepwise mechanism through a triple-well potential energy surface involving a tetrahedral intermediate, T−. The strong proclivity toward the stepwise path is caused mainly by the low-lying πCS* level coupled with the high energy σC–Z* orbital. The reaction barriers, ΔG≠, are higher than the corresponding values for the carbonyl transfers, excepting for the X = Z = Cl case at the G2(+) level. The major factors for these elevated barriers, despite the low deformation energies (ΔEdef), are lower proximate σ–σ* charge transfer energies (δΔEσ–σ*(2)) and the relatively high electrostatic repulsion (ΔEes > 0) in the transition state due to the highly polarized structure with a large negative charge on the S atom. The occurrence of a well-defined intermediate, T−, in the reaction coordinate leads to a relatively early transition state with a low degree of bond making and breaking in the transition state.

Nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to amines. Intrinsic and apparent reactivity.

We have studied the nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to primary and secondary amines, observing that the rate of the reaction increases as a consequence of the presence of electron withdrawing groups on the aromatic ring of the nitrosating agents. The rate constants determined for the nitroso group transfer, ktr, give good Bronsted-type relationships between log ktr (rate constant for nitroso group transfer) and pKaR2NH2+ and pKaleaving group. The study of the nitrosation processes of secondary amines catalyzed by ONSCN and denitrosation catalyzed by SCN-, in combination with the formation equilibrium of ONSCN, has enabled us to calculate the value of the equilibrium constant for the loss of the NO+ group from a protonated N-nitrosamine (pKNOR2N+HNO), which can be defined by analogy with pKaR2NH2+. The value of pKNOX-NO for the loss of the NO+ group from an N-methyl-N-nitrosobenzenesulfonamide was obtained in a similar way. By using values of delta pKNO = pKNOR2N+HNO - pKNOX-NO, we were able to calculate the equilibrium constant for the nitroso group transfer and characterize the transition state. On the basis of Bronsted-type correlations, we have obtained values of beta nuclnorm and alpha lgnorm approximately equal to 0.55, showing a perfectly balanced transition state. In terms of the Marcus theory, the calculation of the intrinsic barriers for the nitroso group transfer reaction shows that the presence of electron withdrawing groups on the aromatic ring of the N-methyl-N-nitrosobenzenesulfonamides does not cause these barriers to vary.

Nonionic hydrogels by copolymerization of vinylene carbonate and oligo(ethylene glycol) vinyl ethers

Bulk polymerization of vinylene carbonate (VCA) with ethylene glycol monovinyl ether (EMVE), diethylene glycol monovinyl ether (DMVE) and methyl triethylene glycol vinyl ether (MTVE), respectively, was carried out. Compared with DMVE and MTVE, the yield of the VCA copolymer with EMVE was low due to a side reaction of acetalization. The copolymerization of MTVE with VCA has been shown to produce linear copolymers and alternating copolymers have been found to form at a 1:1 molar monomer ratio. The copolymerization of EMVE and DMVE with VCA, on the other hand, generally yielded crosslinked copolymers. The reason may be a radical transfer reaction that occurred at methylene groups adjacent to hydroxyl groups of pendent oxyethylene moieties, as inferred from studying the swelling behavior of hydrolyzed copolymers in acidic aqueous solution. The copolymers and their hydrolysis products, especially copoly(MTVE/VCA) and copoly(DMVE/VCA), exhibited low glass transition temperatures, which is apparently due to flexible pendent groups on polymer chains. The copolymerization in the presence of oligo(ethylene glycol) divinyl ethers as crosslinking agents followed by hydrolysis with a 5wt.% NaOH solution gave rise to the formation of three different nonionic hydrogels. The swelling behavior of all the gels was found to be independent of the presence of some electrolyte salts at various concentrations in an aqueous solution. However, the hydrogels based on copoly(EMVE/VCA) and copoly(DMVE/VCA) were shown to be unstable under acidic conditions, whereas those based on copoly(MTVE/VCA) were found to be only slightly influenced by the pH value of an aqueous medium.

Atom transfer reactions of (TTP)Ti(eta 2-3-hexyne): synthesis and molecular structure of trans-(TTP)Ti[OP(Oct)3]2.

Atom and group transfer reactions were found to occur between heterocumulenes and (TTP)Ti(eta 2-3-hexyne), 1 (TTP = meso-5,10,15,20-tetra-p-tolylporphyrinato dianion). The imido derivatives (TTP)Ti=NR (R = iPr, 2; tBu, 3) were produced upon treatment of complex 1 with iPrN=C=NiPr, iPrNCO, or tBuNCO. Reactions between complex 1 and CS2, tBuNCS, or tBuNCSe afforded the chalcogenido complexes, (TTP)Ti=Ch (Ch = Se, 4; S, 5). Treatment of complex 1 with 2 equiv of PEt3 yielded the bis(phosphine) complex, (TTP)Ti(PEt3)2, 6. Although (TTP)Ti(eta 2-3-hexyne) readily abstracts oxygen from epoxides and sulfoxides, the reaction between 1 and O=P(Oct)3 did not result in oxygen atom transfer. Instead, the paramagnetic titanium(II) derivative (TTP)Ti[O=P(Oct)3]2, 7, was formed. The molecular structure of complex 7 was determined by single-crystal X-ray diffraction: Ti-O distance 2.080(2) A and Ti-O-P angle of 138.43(10) degrees. Estimates of Ti=O, Ti=S, Ti=Se, and Ti=NR bond strengths are discussed.

Crosslinking during radical polymerization of dodecyl methacrylate

A much more efficient formation of crosslinks was observed in the free-radical polymerization of dodecyl methacrylate with respect to the amount of decomposed peroxide than it would correspond to the additional peroxide crosslinking of formed poly(dodecyl methacrylate). Polymer crosslinking also proceeds after using 2,2´-azoisobutyronitrile as initiator of the polymerization of dodecyl methacrylate, although with a substantially lower efficiency compared to the initiation by peroxide under comparable conditions. The efficient formation of crosslinked structures can be explained by branching and copolymerization of monomer with multifunctional dead polymer. Multifunctionality of the formed macromolecules is a result of transfer and addition reactions of the present free radicals with the formed polymer. The difference in the influence of the initiator follows from the higher reactivity of oxy radicals in transfer reactions with monomer dodecyl methacrylate which results in a greater number of polymerizable double bonds built in the polymer chain.

Crosslinking during radical polymerization of dodecyl methacrylate

A much more efficient formation of crosslinks was observed in the free-radical polymerization of dodecyl methacrylate with respect to the amount of decomposed peroxide than it would correspond to the additional peroxide crosslinking of formed poly(dodecyl methacrylate). Polymer crosslinking also proceeds after using 2,2´-azoisobutyronitrile as initiator of the polymerization of dodecyl methacrylate, although with a substantially lower efficiency compared to the initiation by peroxide under comparable conditions. The efficient formation of crosslinked structures can be explained by branching and copolymerization of monomer with multifunctional dead polymer. Multifunctionality of the formed macromolecules is a result of transfer and addition reactions of the present free radicals with the formed polymer. The difference in the influence of the initiator follows from the higher reactivity of oxy radicals in transfer reactions with monomer dodecyl methacrylate which results in a greater number of polymerizable double bonds built in the polymer chain.

Decavanadate Inhibits Catalysis by Ribonuclease A

Pentavalent organo-vanadates have been used extensively to mimic the transition state of phosphoryl group transfer reactions. Here, decavanadate (V10O6−28) is shown to be an inhibitor of catalysis by bovine pancreatic ribonuclease A (RNase A). Isothermal titration calorimetry shows that the Kd for the RNase A · decavanadate complex is 1.4 μM. This value is consistent with kinetic measurements of the inhibition of enzymatic catalysis. The interaction between RNase A and decavanadate has a coulombic component, as the affinity for decavanadate is diminished by NaCl and binding is weaker to variant enzymes in which one (K41A RNase A) or three (K7A/R10A/K66A RNase A) of the cationic residues near the active site have been replaced with alanine. Decavanadate is thus the first oxometalate to be identified as an inhibitor of catalysis by a ribonuclease. Surprisingly, decavanadate binds to RNase A with an affinity similar to that of the pentavalent organo-vanadate, uridine 2′,3′-cyclic vanadate.

PhSeSiR(3)-Catalyzed Group Transfer Radical Reactions.

A novel design for initiating radical-based chemistry in a catalytic fashion is described. The design of the concept is based on the phenylselenyl group transfer reaction from alkyl phenyl selenides by utilizing PhSeSiR(3) (1) as a catalytic reagent. The reaction is initiated by the homolytic cleavage of -C-Se- bond of an alkyl phenyl selenide by the in situ generated alkylsilyl radical (R(3)Si(*)), obtained by the mesolysis of PhSeSiR(3)](*)(-)( )()(1(*)(-)). The oxidative dimerization of counteranion PhSe(-) to PhSeSePh functions as radical terminator. The generation of 1(*)(-) is achieved by the photoinduced electron transfer (PET) promoted reductive activation of 1 through a photosystem comprising of a visible-light (410 nm)-absorbing electron rich DMA as an electron donor and ascorbic acid as a co-oxidant (Figure 1). The optimum mole ratio between the catalyst 1 and alkyl phenyl selenides for successful reaction is established to be 1:10. The generality of the concept is demonstrated by carrying out variety of radical reactions such as cyclization (10, 15-18), intermolecular addition (25), and tandem annulations (32).

An acyltransferase catalyzing the formation of diacylglucose is a serine carboxypeptidase-like protein

1-O-beta-acyl acetals serve as activated donors in group transfer reactions involved in plant natural product biosynthesis and hormone metabolism. However, the acyltransferases that mediate transacylation from 1-O-beta-acyl acetals have not been identified. We report the identification of a cDNA encoding a 1-O-beta-acylglucose-dependent acyltransferase functioning in glucose polyester biosynthesis by Lycopersicon pennellii. The acyltransferase cDNA encodes a serine carboxypeptidase-like protein, with a conserved Ser-His-Asp catalytic triad. Expression of the acyltransferase cDNA in Saccharomyces cerevisiae conferred the ability to disproportionate 1-O-beta-acylglucose to diacylglucose. The disproportionation reaction is regiospecific, catalyzing the conversion of two equivalents of 1-O-beta-acylglucose to 1, 2-di-O-acylglucose and glucose. Diisopropyl fluorophosphate, a transition-state analog inhibitor of serine carboxypeptidases, inhibited acyltransferase activity and covalently labeled the purified acyltransferase, suggesting the involvement of an active serine in the mechanism of the transacylation. The acyltransferase exhibits no carboxypeptidase activity; conversely, the serine carboxypeptidases we have tested show no ability to transacylate using 1-O-acyl-beta-glucoses. This acyltransferase may represent one member of a broader class of enzymes recruited from proteases that have adapted a common catalytic mechanism of catabolism and modified it to accommodate a wide range of group transfer reactions used in biosynthetic reactions of secondary metabolism. The abundance of serine carboxypeptidase-like proteins in plants suggests that this motif has been used widely for metabolic functions.

Kinetics of ampicillin synthesis catalyzed by penicillin acylase from E. coli in homogeneous and heterogeneous systems. Quantitative characterization of nucleophile reactivity and mathematical modeling of the process.

Kinetic regularities of the enzymatic acyl group transfer reactions have been studied using ampicillin synthesis catalyzed by E. coli penicillin acylase as an example. It was shown that ampicillin synthesis proceeds through the formation of an acylenzyme-nucleophile complex capable of undergoing hydrolysis. The relative nucleophile reactivity of 6-aminopenicillanic acid (6-APA) is a complex parameter dependent on the nucleophile concentration. The kinetic analysis showed that the maximum yield of antibiotic being synthesized depended only on the nucleophile reactivity of 6-APA, the ratio between the enzyme reactivities with respect to the target product and acyl donor, and the initial concentrations of reagents. The parameters characterizing the nucleophile reactivity of 6-APA have been determined. The algorithm of modeling the enzymatic synthesis has been elaborated. The proposed algorithm allows the kinetics of the process not only in homogeneous, but also in heterogeneous ("aqueous solution-precipitate") systems to be quantitatively predicted and described based on experimental values of parameters of the reaction. It was shown that in heterogeneous "aqueous solution-precipitate" systems PA-catalyzed ampicillin synthesis proceeds much more efficiently compared to the homogeneous solution.

Double Group Transfer Reactions of an Unsaturated Tantalum Methylidene Complex with Pyridine N-Oxides

The electrophilic nature of the benzamidinate tantalum methylidene complex [TolC(NSiMe3)2]2Ta(CH2)CH3 (1) allows for reactivity with pyridine N-oxides, which leads to the formation of the tantalum oxo complex [TolC(NSiMe3)2]2Ta(O)CH3 (5) and methylpyridines; treatment of 1 with the related nitrone species N-tert-butyl-α-phenylnitrone leads to overall different products. Mechanistic studies show the reactions proceed via regioselective methylene group transfer to the pyridine N-oxides.

Radical transfer reactions in polymers

Free-radical transfer in polymers have been studied by pulse radiolysis and product analysis with the water-soluble polymers poly(vinyl alcohol), poly(acrylic acid), poly(methacrylic acid), polynucleotides and DNA. When OH radicals react with polymers the lifetime of the polymer radical thus created strongly depends on the number of radicals per polymer chain. Moreover, in negatively charged polymers the increased stiffness at high pH results in a remarkable increase of the lifetime of the radicals with respect to recombination. This allows a number of radical transfer reactions to occur (e.g. intramolecular H-transfer, β-fragmentation, depolymerization, reactions with additives).

Thermal decomposition of polymers modified by catalytic effects of copper and iron chlorides

Effects of copper and iron chlorides were studied on the thermal decomposition reactions of polymers using pyrolysis-gas chromatography/mass spectrometry. Copper and iron chlorides found to depress the radical transfer reactions of lower probability in polypropylene and polystyrene at 500°C through the interaction of the radicals and the transition metal ions. Cu(I) and Fe(II) chlorides enhance the production of aromatic and polyaromatic compounds from polyethylene at 1000°C promoting hydrogen elimination. Partial decomposition of the methyl ester side groups takes place in poly (methyl methacrylate) at 450°C, leading to chloromethane and carbon dioxide, when Cu(II), Fe(II) or Fe(III) chloride is present. Heterolytic cleavage of the methoxy group is presumably promoted by the electrophyl transition metal ions. Similar effect was proposed explaining the enhanced phenol evolution from phenol-formaldehyde resin, polycarbonate and epoxy resin at 500–600°C in the presence of Cu(II) and iron chlorides. The rupture of phenolic rings and the reduction of the phenolic hydroxyl groups leading to polyaromatic pyrolysis products from phenol-formaldehyde resin at 1000°C was reduced by Cu(II) and iron chlorides presumably due to a protective interaction of the phenol rings and the transition metal ions. Pyrolysis of polymers was performed also in the presence of PVC and copper or iron. The results lead to the conclusion that the chlorides of lower oxidation state are forming from PVC and copper or iron when co-pyrolysed with polyethylene or phenol-formaldehyde resin.

The Interaction of Carbonyl Groups with Substituents

The carbonyl group is probably the most important functional group in organic chemistry, and its properties are strongly affected by substituents. This Account will be concerned with the modes of interaction between carbonyl groups and substituents, and how these interactions are manifested in the properties of the compounds containing this group. A comparison will be made between carbonyl groups and related groups, such as thiocarbonyl. The interaction of carbonyl groups with substituents has usually been described in terms of π electron interactions such as “amide resonance”. However, substituents may affect carbonyl groups in several different ways. They may withdraw or donate electron density via the σ bond, depending on their electronegativity. One of the important features of the carbonyl group is the difference in electronegativity between carbon and oxygen, which leads to both σ and π electron transfer from C to O and to a positively charged carbon. As a result, substituents that have lone pairs may also transfer π electron density to the electron-deficient carbon. Finally, if the atom of the substituent that is attached to the carbonyl group bears a partial charge, it will lead to a Coulombic interaction with the positively charged carbon. All of these modes of interaction have been found. There are three basic ways in which the energetics of the interaction of a carbonyl group with a substituent may be examined. The first makes use of a group-transfer reaction such as

Formation of Long-Lived Radicals on Proteins by Radical Transfer from Heme Enzymes—A Common Process?

Incubation of Fe(III)myoglobin (Fe(III)Mb) with H2O2in the presence of bovine serum albumin (BSA) has been shown previously to give albumin-derived radicals as a result of radical transfer from myoglobin to BSA. In this study the occurrence of similar processes with peroxidases has been investigated using horseradish peroxidase (HRP)/H2O2, in the presence and absence of added tyrosine. Incubation of HRP with H2O2and bovine or human serum albumins, in the presence and absence of tyrosine, gave long-lived albumin-derived radicals as detected by EPR spectroscopy. Evidence has been obtained for these albumin radicals being located on buried tyrosine residues on the basis of blocking experiments. The effect of protein conformation on radical transfer has been investigated using partial proteolytic digestion prior to protein oxidation. With HRP/H2O2/BSA and Fe(III)Mb/H2O2/BSA increased radical concentrations were observed after limited digestion, although this effect was less marked with the HRP/H2O2/BSA system than with Fe(III)Mb/H2O2/BSA, consistent with different modes of radical transfer. More extensive digestion of BSA decreased the radical concentration to levels below those detected with native albumin, indicating that the tertiary structure of the target protein plays an important role in determining the rate of radical transfer and/or the stability of the resultant species. These results are consistent with a mechanism for the HRP/H2O2/no free tyrosine system involving radical transfer to the albumin via the heme edge of the peroxidase. In contrast, albumin radical formation by the HRP/H2O2/free tyrosine system was only marginally affected by proteolysis, consistent with free tyrosine phenoxyl radicals being the mediators of radical transfer, without significant protein–protein interaction. These protein-to-protein radical transfer reactions may have important consequences for understanding protein oxidation in biological systems.

Synthetic, structural and electrochemical studies of cationic chromium(VI) and molybdenum(VI) bis(imido) complexes supported by 1,4,7-triazacyclononane macrocycles

Treatment of [Cr(NtBu)2Cl2] or [Mo(NtBu)2Cl2(dme)] with 1,4,7-triazacyclononane (tacn) in THF, followed by metathesis using NaClO4 in aqueous medium, readily afforded [M(tacn)(NtBu)2Cl]ClO4 (M = Cr 1 or Mo 2); [M(Me3tacn)(NtBu)2Cl]Cl (M = Cr 3 or Mo 4) were similarly prepared using the metal precursors and 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn). Complexes 1–4 are resistant to hydrolysis, but do not mediate imido group transfer reactions. Distinctive UV-vis absorption bands are tentatively assigned to imido-to-metal LMCT transitions. The crystal structures of 1 and 2·CH3CN show that the 3 nitrogen atoms of tacn are not equidistant from the respective metal centres. A severely distorted imido moiety (imido angle 151.8(4)°) has been observed in the structure of 1 but not 2·CH3CN, and is attributed to steric and crystal packing factors rather than electronic effects. Metathesis of both chloride groups in 3 using AgOTf (OTf = SO3CF3) yielded [Cr(Me3tacn)(NtBu)2(OTf)]OTf 5. The cyclic voltammograms of 1–5 in acetonitrile consist of a quasi-reversible couple (+0.86 to +1.20 V vs. Cp2Fe0/+) and an irreversible wave (–1.04 to –2.08 V vs. Cp2Fe0/+); the former couple is assigned to an imido-centred oxidation.

ChemInform Abstract: Generation and Mesolysis of PhSeSiR3]×‐: Mechanistic Studies by Laser Flash Photolysis and Application for Biomolecular Group Transfer Radical Reactions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds

A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R1COR2→>C=O+R1R2C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH+R1CH=CH2→>C=O+R1C·HCH3; >R1CH=CH2+R2C·HCH2R3→R2C·HCH3+R2CH=CHR3. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (adelhydes, ketones, and quinones) from the C−H and O−H bonds were compared.

Synthesis of arsenic containing polymethylmethacrylate using p-acetyl benzylidene triphenyl arsonium ylide as photoinitiator

Arsenic containing syndiotactic polymethylmethacrylate (PMMA) using p-ABTAY (p-acetyl benzylidene triphenyl arsonium ylide)in benzene as the photoinitiator at 30 ± 0.2°C has been synthesized. The system follows non-ideal kinetics ( R p ∝ [ I] 0.25 [M]) due to both primary radical termination and degradative chain transfer reaction. The glass transition temperature of PMMA is 145°C. Analysis of the polymer by SEM technique shows the presence of arsenic in it. The polymer has been characterized by Infra-red spectroscopy.

Polypropylene Modified with Sulphur Dioxide

The bonding of sulphate and sulphonate groups to atactic and isotactic polypropylene (PP) by the reaction of sulphur dioxide and hydroperoxides (ROOH) have been investigated. The oxidized PP and the blends of PP and low molecular cumylhydroperoxide (CHP) and t-butylhydroperoxide (TBHP) were used. The oxidized PP were prepared by low temperature oxidation of polymer foils, powder, and PP fabric at 50°C using ozone-oxygen mixture. The sulphoxy groups concentration in the atactic PP treated by SO2 exceeds a concentration of hydroperoxides from 4 to 23 times as a temperature increases from 30 to 70°C. The overall activation energy 40 kJ/mol was determined for this process. The increasing stoichiometry of SO2/ROOH, with raising a temperature and determined temperature coefficient, show on an increasing importance of hydrogen transfer reactions of free radicals on the extent of PP reaction.