Covalent Species Research Articles

Dynamic NMR as a Nondestructive Method for Determination of Rates of Dissociation. XV. Dissociation of Thioether and Ether Ligands in Tin(IV) and Boron Complexes and Significance of Entropy of Activation Therein

Abstract The rates of dissociation of thioether ligands which are imbedded in organotin(IV) compounds, such as trichloro[o-(ethylthiomethyl)phenyl]stannane, were determined by the dynamic NMR method. The dynamic processes of the corresponding ether ligands was too fast to detect as the change in line shapes in 1H NMR spectra. All the boron compounds examined, which possessed similar structures to the stannane and were of the boronate type, failed to show dissociation of a thioether or ether ligand due to their large rates of dissociation. The rates of dissociation of a thioether–tribromoboron complex were determined by the same technique. The feature of the activation parameters was that the compounds examined exhibited fairly large positive entropy of activation in accordance with other results that involve dissociation of ionic species into covalent species.

Evidence for a covalent surface KCN species on Pd(100) from inter- atomic Auger transitions.

The He i and Ne ii uv spectra and ${\mathrm{He}}^{\mathrm{*}}$ Penning ionization electron spectra of coadsorbed K and CN annealed to above 800 K show extra features, not present in Ne i and ${\mathrm{Ne}}^{\mathrm{*}}$ spectra, that are observed at constant kinetic electron energies. Using the observed valence levels of ${\mathrm{KCN}}_{\mathrm{ads}}$ and the excitation energies, the extra features are shown to result from interatomic Auger transitions involving a K(3p) hole and mixed K-CN states. This strong interadsorbate interaction is attributed to the formation of molecular orbitals of a more covalently bonded KCN molecule.

Covalent and ionic species in cationic polymerizations: Rates of exchange

AbstractThe general kinetic scheme of simultaneous propagation on covalent and ionic species is discussed. There are a few ways of interconversion of covalent and ionic species, and at least two different modes of covalent propagation. This is either a direct inclusion with reformation of covalent species or reaction of covalent species with a monomer, giving ionic species, collapsing then into the covalent species and/or propagating ionically.Significance of these processes for the overall kinetics is discussed; rates of exchange covalent ⇌ ionic and covalent + monomer ⇌ ionic are discussed in more detail on the basis of trityl chloride ionization and ester‐ion equilibria in the polymerization of THF and oxepane.

A new conceptualization in relation to ionic polymerizations

AbstractA new systematization is presented for the polymerization reactions in relation to the so‐called “Ionic Polymerizations'. The basis of the new systematization, is the concept of the reaction between “Electrophile” and “Nucleophile”. The reactants are either ions (cation or anion) or dipoles. Polymerization reactions except free radical polymerization and polycondensation are classified into three categories depending upon the reactivity characteristics (electrophilic or nucleophilic) of the propagating species.a. Polymerization via electrophilic propagating speciesb. Polymerization via nucleophilic propagating speciesc. Polymerization via zwitterionsEmphasis is placed on the polymerization reactions which propagate through covalent species having nucleophilic or electrophilic reactivity. By this systematization, several polymerization systems which have been treated as unusual or exceptional cases are thus reasonably categorized.

Identification of some carbocationic active species: Structure and stability relationship

AbstractThe nature and the stability of the various types of propagating species involved in cationic polymerization of ethylenic monomers are examined. After a short discussion of the evidence for and against the presence of covalent species in styrene polymerization, recent data about the identification of carbocationic active centres involved in the polymerization of para‐substituted α‐methyl‐styrenes are given. Carbenium ions were generated in methylene chloride either by reaction of the p‐substituted cumyl chloride with SbF5 or by reaction of the monomer with trifluoromethanesulfonic acid (HA). They are stable around −70°C, but some of them give secondary reactions at higher temperature. The various types of species were identified by 1H NMR correlated with UV spectroscopy.For the p‐tert‐butyl derivative, monomeric cations remain stable at −70°C (and even at higher temperature) while oligomeric ones are less stable, giving indanic derivatives. With p‐methoxy‐α‐methylstyrene, the monomeric, dimeric and trimeric cations are stable at −63°C but give indanic derivatives at −40°C and above. For both monomers, monomeric cations show a single maximum at 350 or 367 nm with a high absorption coefficient (≁ 30000). In the presence of an excess of monomer, a bathochromic shift of this maximum (≁ 15 nm) is observed, together with the appearance of a new (smaller) maximum around 480 nm. The presence of these two maxima could be correlated with that of “isomerized” species, some of which could be identified by NMR. The possible occurrence of a similar situation with other monomers and particularly with p‐methoxystyrene is discussed.

Activated esters in the cationic polymerization of styrenes

AbstractCationic polymerization of styrenes initiated by protonic acids can proceed via covalent intermediates. Calculations of the charge distribution in these species show low probability of transfer reactions. Macromolecular and model covalent species in the perchloric acid initiated polymerization of styrene have been directly observed by NMR. Esters formed with triflic acid are very reactive and participate in Friedel‐Crafts alkylation at −78 °C. Acetates become active only in the presence of protonic and Lewis acids. 1‐Phenylethyl trifluoroacetate activated with the parent acid reacts with styrene faster than the acid does. The racemization of the optically active ester is faster than its incorporation indicating an ionic mechanism of propagation in this system. Activation by Lewis acids proceeds by the reversible formation of complexes. Some systems yield polymers with low polydispersity.

Role of inflammatory cells in the metabolic activation of polycyclic aromatic hydrocarbons in mouse skin

Oxidants, such as those generated by activated polymorphonuclear leukocytes (PMNs) during inflammation, have been implicated in the metabolic activation of procarcinogens to their ultimate carcinogenic form. In this study we examined the effect of inflammation on the metabolic activation of (±)- trans-7,8-dihydroxy-7,8-dihydrobenzo[ a]pyrene (BP 7,8-dihydrodiol) to a covalent binding species in mouse epidermis. Interaction of BP 7,8-dihydrodiol with 12- O-tetradecanoylphorbol-13-acetate (TPA)-stimulated murine leukocytes resulted in the generation of both a chemiluminescent intermediate and one that covalently bound to the DNA of cocultured epidermal keratinocytes. Topical treatment of mouse skin with TPA led to an influx of PMNs into the skin beginning several hours after application. Myeloperoxidase activity, a marker for neutrophils, increased 15-fold in the skin by 16 hr after TPA treatment. Dual applications of TPA at both 16 hr before and concurrently with administration of [ 3H]BP 7,8-dihydrodiol led to a 50% enhancement of the level of carcinogen that was covalently bound to epidermal DNA. However, a single application of TPA, either 16 hr before or concurrently with BP 7,8-dihydrodiol administration, had no enhancing effect, suggesting that both initial recruitment of PMNs into the skin and subsequent stimulation of oxidant production by the PMNs were required to enhance carcinogen binding. By contrast, no enhancement of benzo[ a]pyrene binding was observed by TPA treatments in vivo. However, TPA-stimulated neutrophils did not activate this procarcinogen to a chemiluminescent metabolite in vitro. These results suggest that oxidants generated by metabolically stimulated PMNs can activate penultimate polycyclic aromatic hydrocarbons, such as BP 7,8-dihydrodiol, to potentially genotoxic metabolites in vivo and further define a role for inflammation in carcinogenesis.

Theoretical basis and kinetic sense of covalent propagation in cationic polymerization

AbstractPolymerization of several vinyl and heterocyclic monomers was described recently as proceeding by covalent propagation. In this paper the theoretical basis of purely covalent multcenter propagation is discussed in terms of the orbital symmetry theory and the parity rules. A mechanism is considered in which ionic and covalent species are present simultaneously. Covalent species may react with monomers, forming intermediate ions. The kinetic sense of the rate of this elementary reaction is described.

Nucleophilic aromatic substitution: effect of complexation on the alkaline hydrolysis of 4-aminophenylazobenzothiazolium dyes

Nucleophilic attack by hydroxide ion on 2-(4-dimethylaminophenophenylazo)-3,6-dimethylbenzothiazolium iodide to expel the dimethylamino function is inhibited in aqueous solution by aromatic carboxylate, aromatic sulphonate, and phenolate ions. The inhibition is demonstrated to involve complexation of dye with solute to to form an unreactive species. The dissociation constant of the complex is relatively insensitive to structure except in the case of phenolate ions where electron-withdrawing substituents favour stability. Both charge transfer and electrostatic effects are necessary for efficient complexing ability of the additive. Complexation of cyclodextrin with the dye is followed by reaction to yield a dye–cyclodextrin covalent species; the intermediate slowly decomposes to give quinone imine, the regular product of dye hydrolysis.

Are there covalent active species in the polymerization of styrene initiated by trifluoromethanesulfonic acid?

AbstractPolymerization of styrene by perchloric and trifluoromethanesulfonic acids has been assumed previously to proceed via a pseudo‐cationic mechanism. We found that for the latter system the trifluoromethanesulfonate ester is not stable. Attempts to observe the model 1‐phenylethylester were unsuccessful even at −78°C. In the reaction between 1‐phenylethyl bromide and AgSO3CF3 a very rapid alkylation of the aromatic rings occurs. In the reaction between 1‐phenylethanol and (CF3SO2)2O a very rapid reaction between the intermediate ester (or ion) and the alcohol leads to the formation of the corresponding ether. A reaction carried out with the optically active alcohol indicates that no more than 4% of the intermediate species might have reacted with the alcohol as ester before racemization (i.e. ionization). The macromolecular ester does not seem to be much more stable and in 19F NMR spectra only the acid was observed. An attempt at the formation of a block copolymer with N‐tert‐butylaziridine give zero or very low yield, contrarily to results previously observed with perchloric acid. An analysis of some former results on polymerization kinetics and of the new data given in this paper show no real evidence in favor of the pseudo‐cationic polymerization of styrene in CH2Cl2, initiated by CF3SO3H. The various data may be explained more satisfactorily by a more classical ionic mechanism. Triflic acid can reversibly protonate olefins due to the affinity of the triflate anion being higher for a proton than for a secondary or tertiary carbon atom; this is sufficient to explain the extremely low concentration of ionic species.

Ring‐opening polymerization of nitrogen– and phosphorus‐containing monomers: Polymerization mechanism and polymer properties

AbstractThe first part of this paper describes cationic polymerizations of cyclic imino ethers, 2‐oxazolines and 5,6‐dihydro–4H–1,3–oxazines, which proceed via cyclic onium propagating species or via covalently bonded alkyl halide species. In an extreme case, both ionic and covalent species are present in equilibrium and propagate concurrently. The propagation rate constants due to the respective species were determined. A poly(cyclic imino ether) becomes hydrophilic or lipophilic dependent on the substituent of the monomer. Based on this principle, various types of nonionic polymer surfactants have been prepared, e.g., diblock and triblock copolymers, graft copolymers, and surfactants having one 2‐oxazoline chain. The second part is concerned with ring‐opening polymerizations of new eight cyclic trivalent phosphorus monomers. These polymerizations produced phosphorus‐containing functional polymers such as a chelating resin. 31P NMR analyses of polymerization of cyclic phosphinite monomers led to propose a new mechanism of “Electrophilic Ring‐Opening Polymerization”.

Cyclic imino ethers — Polymerization chemistry and polymer characteristics

AbstractThis paper describes recent developments in polymerization of cyclic imino ethers, 2‐oxazolines and 5,6‐dihydro‐4H‐1,3‐oxazines, which have been performed mainly in our laboratory. Cationic ring‐opening polymerizations of these two kinds of monomers gave poly(N‐acylethylenimine)s and poly(N‐acyltrimethylenimine)s, whose hydrolysis produced linear poly‐ethylenimine and poly(trimethylenimine), respectively. The mechanism of the polymerization is mainly governed by the nature of monomer and of the counter anion derived from initiator. There are two types of stable propagating species; cyclic onium species and covalent alkyl halide species. In a specific case, e.g., the polymerization of 5‐methyl‐2‐oxazoline with MeI initiator, both ionic and covalent species are involved in equilibrium and propagate concurrently. Then, a new terminology of “Electrophile Polymerization” has been proposed as a broader expression including cationic and covalent propagating species. By utilizing living nature of the cyclic imino ether polymerization, block copolymerizations between these monomers were performed. A poly(cyclic imino ether) chain becomes hydrophilic or lipophilic depending on the monomer. Product AB and ABA type block copolymers were soluble in water and showed excellent surface properties reflected by values of surface tension (γ), when A (or B) and B (or A) are hydrophilic and lipophilic. Thus, various nonionic polymer surfactants of block and graft type have been prepared from cyclic imino ethers. Finally, poly(N‐acetylethylenimine) chain showed good compatibility with poly(vinyl chloride). Cellulose diacetate and poly(vinyl chloride) are not miscible, but cellulose diacetate g‐poly(N‐acetylethylenimine) is miscible with poly(vinyl chloride), the graft chain acting as a “compatibilizer”.

Kinetics of the reaction of thrombin and α2-macroglobulin

The kinetics of the reaction of alpha 2-macroglobulin (alpha 2M) with human thrombin were studied by recording the appearance of thiol groups spectrophotometrically and by measuring the distribution of protein species by denaturing non-reducing gel electrophoresis. The goals were to study the relation between the formation of various covalent enzyme-inhibitor complex species and the appearance of free thiol, and from the kinetic analysis, to try to characterize the chemical nature of the protein complexes. The kinetics of thiol-group release were observed to be biphasic, the early phase showing second-order behaviour, results consistent with previous reports in the literature. The observed second-order rate constant for thiol-group release was found to be faster than the second-order rate constant for the disappearance of the band corresponding to native alpha 2M on gel electrophoresis. This may be a reflection of the multiple products formed from the thioester. Alternatively, it is possible that covalent-bond formation is slower than some enzyme-induced change in the thioester centre, and this may be suggestive evidence for a reactive alpha 2M centre that does not contain an intact thioester. The kinetics of covalent-bond formation were found to be consistent with the internal cross-link of several alpha 2M chains by the bound proteinase, providing further evidence that the very-high-Mr species seen on gels may arise from dimers of the alpha 2M molecule held together by covalent bonds to the enzyme.

Cytochrome P-450 dependent metabolic activation of 1-naphthol to naphthoquinones and covalent binding species

1-Naphthol was metabolised by a fully reconstituted cytochrome P-450 system in the presence of NADPH to methanol-soluble and covalently bound products. The formation of 1,4-naphthoquinone, the major methanol-soluble product at early time points, showed an almost total dependence on cytochrome P-450, NADPH-cytochrome P-450 reductase and NADPH, and to a lesser extent on dilauroylphosphatidylcholine. The metabolism was rapid and detectable levels of 1,4-naphthoquinone were formed within 30 sec. 1,4-Naphthoquinone formation was dependent on the concentration of both cytochrome P-450 (0.05-0.04 μM) and 1-naphthol (5–50 μM). Whereas 1,4-naphthoquinone was the major product observed at early time points, additional products were observed after prolonged incubation. In the absence of NADPH and NADPH-cytochrome P-450 reductase, 1-naphthol was metabolised, in a cumene hydroperoxide- and cytochrome P-450-dependent reaction, to 1,2- and 1,4-naphthoquinone and covalently bound products. Glutathione and ethylenediamine inhibited both the NADPH- and cumene hydroperoxide-dependent formation of covalently bound products. These data show that cytochrome P-450 catalyses the activation of 1-naphthol to naphthoquinone metabolites and covalently bound species, the latter most likely being derived from naphthoquinones.

Metabolic activation of 1-naphthol by rat liver microsomes to 1,4-naphthoquinone and covalent binding species

1-Naphthol was metabolized by rat liver microsomes, in the presence of an NADPH-generating system, both to methanol-soluble metabolites including 1, 4-naphthoquinone and an uncharacterized product(s) (X) and also to covalently bound products. NADH was much less effective as an electron donor than NADPH. Metyrapone, SKF 525-A and carbon monoxide all inhibited the metabolism of 1-naphthol to 1,4-naphthoquinone and to covalently bound products suggesting the involvement of cytochrome P-450 in at least one step in the metabolic activation of 1-naphthol to reactive products. Ethylene diamine, which reacts selectively with 1, 2-naphthoquinone but not 1,4-naphthoquinone, did not affect the covalent binding whereas glutathione, which reacts with both naphthoquinones, caused an almost total inhibition of covalent binding. These and other results suggested that 1, 4-naphthoquinone, or a metabolite derived from it, was responsible for most of the covalent binding observed and that little if any of the binding was due to 1, 2-naphthoquinone.

Ionic and covalent species in the cationic polymerization – equilibria and reactivities

AbstractStructure and reactivities in the cationic polymerization of the heterocyclic monomers is reviewed. Particularly the equilibria between major kinds of active species, including the kinetics of interconversions between the covalent species and the corresponding macroion‐pairs are discussed. Reason for the higher reactivity of the ionic species than of the covalent ones in the chain growth, observed in the majority of studied systems, is confronted with the recently observed similarity in reactivities, found for oxepane. Further evidence is discussed, supporting the earlier formulated opinion that ion‐pairs and ions propagate with similar rate constants. These features of the cationic ring‐opening polymerizations were compared with the cationic vinyl polymerization. In the former process the covalent and ionic active species have separately been observed by NMR and the covalent propagation clearly demonstrated, whereas there is only a secondary and weak support for the covalent growth of vinyl monomers.

Kinetics and mechanism of cationic polymerization of 5-methyl-2-oxazoline. Concurrent propagation through ionic and covalent species with methyl iodide initiator

Kinetic analysis of the cationic polymerization of 5-methyl-2-oxazoline (5-MeOZO) was performed with methyl tosylate(MeOTs) and methyl iodide(MeI) initiators by using 1H NMR spectroscopy. With MeOTs the polymerization of 5-MeOZO proceeded through oxazolinium tosylate species 3 and 4 and the rate constant of propagation (kp) was determined. With MeI, on the other hand, the propagating ends are present in equilibrium between alkyl iodide (covalent) species 7, 9 and 11 and oxazolinium(ionic) species 6, 8 and 10. Two model reactions enabled to determine the equilibrium constant (K) of 6/7 and to evaluate the reactivity of covalent species 9 (reflected by kp(c)) and of ionic species 8 (reflected by kp(i)). Both species 8 and 9 propagate concurrently and contribute to the whole propagation at comparative extents.

Interaction between uranium pentafluoride and the pentafluorides of vanadium, arsenic, niobium, tantalum, and bismuth, and the tetrafluoride of sulphur

The interaction of UF5 with SF4, SF4O, and some Lewis-acid pentafluorides of various strengths has been studied. In anhydrous HF solutions, SF4 was shown to yield an adduct of composition 3UF5·SF4 in which both ionic and fluorine-bridged species are present. The pentafluorides of arsenic, tantalum, and niobium combine with UF5 to give adducts of composition 1.5 UF5·AsF5, UF5·2TaF5, and UF5·2NbF5, respectively. The arsenic derivative is stable at room temperature only under a pressure of AsF5, whereas the tantalum and niobium adducts decompose at higher temperature forming UF5·TaF5 and UF5·NbF5, respectively. Vibrational spectroscopic study of the pentafluoride adducts of UF5has shown that they consist of covalent fluorine-bridged species. Uranium pentafluoride is fluorinated at room temperature by VF5 and by BiF5 in anhydrous HF.

Structural and chromatographic heterogeneity of normal plasma fibrinogen associated with the presence of three γ-chain types with distinct molecular weights

Three different γ chains have been identified in fibrinogen isolated form normal human plasma with apparent molecular weights of 50000 (γ 50), 55000 (γ 55) and 57500 (γ 57.5), as shown by SDS-polyacrylamide gel electrophoresis. Plasma fibrinogen was separated by ion-exchange chromatography on DEAE-Sephacel into three populations of molecules, each differing in γ-chain composition. The first peak contained 87% of the total fibrinogen and was composed of molecules containing only γ 50 chains; the second peak included 3% of the fibrinogen and contained one γ 50 and one γ 55.5 chain and the third peak had 10% of the total which containedone γ 50 and one γ 57.5 chain, Cross-linked fibrin obtained from fibrinogen with only γ 50 chains contained γγ dimers exclusively of M r 100000, representing a uniform γ 50γ 50 dimer composition. The γγ dimers from purified fibrinogen of γ 50γ 55 type had molecular weights of 111000, 105000 and 100000, while dimers from γ 50γ 57.5 fibrinogen were M r 115000, 108000 and 100000. The relative proportions of γγ dimers from each purified fibrinogen population were consistent with random crosslinking of the γ monomers. The γ-chain identity of the variants was established by their conversion to covalent dimeric species after clotting by thrombin in the presence of Factor XIII, their incorporation of the fluorescent lysine analog dansyl cadaverine, by the staining intensity of monomeric and dimeric forms with the periodic acid-Schiff reagent after electrophoretic separation, and by plasmic degradation of all monomeric and dimeric forms in a pattern that was characteristic for γ chains. Individual γγ dimers were purified by preparative gel electrophoresis, and tyrosine was demonstrated to be the amino-terminal residue of all three γ chain types. We conclude that normal human fibrinogen can be separated into three populations of molecules due to molecular weight heterogeneity of their γ chains.

The determination of dialkyllead, trialkyllead, tetraalkyllead and lead(II) ions in water by chelation/ extraction and gas chromatography/atomic absorption spectrometry

The high polar and solvated dialkyllead, trialkyllead and lead(II) ions are quantitatively extracted into benzene from aqueous solution after chelation with dithiocarbamate. The lead species are butylated by Grignard reagent to the teraalkyl form, R n PbBu 4− n(R = CH 3 or C 2H 5) and Bu 4Pb 1all of which can be quantified by a gas chromatography—atomic absorption spectrometry method. Molecular covalent tetraalkyllead species, if present in the sample, are also extracted and quantified simultaneously. A detection limit of 0.1 μg l t-1 lead can be achieved with one litre of water. Other metals co-extracted by the chelating agent do not interfere.