Cationic Transfer Reactions

Mention should be made of a process first described by Watanabe et al. 62) in which cationic polymerization of alkylene oxides was initiated by Lewis acids and carried out in the presence of methacrylic acid or 2-hydroxyethyl methacrylate (HEMA). The products obtained were characterized and found to contain one terminal methacrylic ester function per chain  

1 Attempts to synthesize the same type of compounds by anionic polymerization of oxiranes in the presence of transfer agents snch as HEMA failed because anionic polymerization requires temperatures at which thermally initiated polymerization of the methacrylic double bond occurs 

The advantage of this procedure is that it yields monofunctional poly(alkylene oxide) macromonomers whereas a drawback is the low degree of polymerization (p 20)1. 

It seems that instead protonated species (anhydride or ester molecules) play a major role in the process. The protons originate from some added acid (e.g. acrylic or methacrylic acid). The characterization of the formed macromonomers revealed that the number of ester functions per molecule is close to 2. The role of the protons is evidenced by the increase of the reaction rate with increasing amount of methacrylic acid in the system. In the absence of a protonic acid high molecular weight poly-THF is produced, no anhydride is consumed and reshuffling does not take place. This mechanism which remains to be confirmed is in any case completely different from the inifer -type cationic transfer which may occur with unsaturated monomers. It is discussed in the next section. 

The preparation of PDMS by Katz (see formation of vinylsilane ) should be mentioned here. 

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The experimental values for kp and h , at different temperatures are summarized in Table 1. In every case hj, is much lower than kp and kp . Moreover, with temperature, the polymerization rates increase and simultaneously kp/hm becomes greater for both cations. Transfer reactions to monomer are more numerous at lower temperatures and also when I.i+ is used At - 40 C, the values of the kp/hm ratio are 47 for Li+ and 140 for Na+. 

This double injection of ions and electrons maintains electroneutrality of the system. The majority of intercalation compounds are known to undergo electron/cation transfer reactions, but NiO, for example, also supports reversible electron/anion transfer processes. In particular, an increase in the specific surface area is generally expected to significantly reduce the characteristic diffusion length of intercalation ions, while simultaneously increasing the number of accessible intercalation sites [16]. 

Molecules with acid-base properties can also be classified as a FIT reaction. For instance, the protonated base BH+ (neutral B) can be regarded either as a simple cation transfer reaction or as a proton transfer facilitated by the conjugated acid (67,68). Finally, the FIT by ion-pair formation can also be classified into this category (6). 

In 1982, Samec et al. studied the kinetics of assisted alkali and alkali-earth metal cation-transfer reactions by neutral carrier and conclnded that the kinetics of transfer of the monovalent ions were too fast to be measured [186]. In 1986, Kakutani et al. published a study of the kinetics of sodium transfer facilitated by di-benzo-18-crown-6 using ac-polarography [187]. They concluded that the transfer mechanism was a TIC process and that the rate constant was also high. Since then, kinetic studies of assisted-ion-transfer reactions have been mainly carried out at micro-lTlES. In 1995, Beattie et al. showed by impedance measurements that facilitated ion-transfer (FIT) reactions are somehow faster than the nonassisted ones [188,189]. In 1997, Shao and Mirkin used nanopipette voltammetry to measure the rate constant of the transfer of K+ assisted by the presence of di-benzo-18-crown-6, and standard rate constant values of the order of 1 cm-S were obtained [190]. A more systematic study was then published that showed the following sequence,, which is not in accordance with... 

The radical cation of 1 (T ) is produced by a photo-induced electron transfer reaction with an excited electron acceptor, chloranil. The major product observed in the CIDNP spectrum is the regenerated electron donor, 1. The parameters for Kaptein s net effect rule in this case are that the RP is from a triplet precursor (p. is +), the recombination product is that which is under consideration (e is +) and Ag is negative. This leaves the sign of the hyperfine coupling constant as the only unknown in the expression for the polarization phase. Roth et aJ [10] used the phase and intensity of each signal to detemiine the relative signs and magnitudes of the... 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

A mild and effective method for obtaining N- acyl- and N- alkyl-pyrroles and -indoles is to carry out these reactions under phase-transfer conditions (80JOC3172). For example, A-benzenesulfonylpyrrole is best prepared from pyrrole under phase-transfer conditions rather than by intermediate generation of the potassium salt (81TL4901). In this case the softer nature of the tetraalkylammonium cation facilitates reaction on nitrogen. The thallium salts of indoles prepared by reaction with thallium(I) ethoxide, a benzene-soluble liquid. 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

So far, as in Equation (3.33), the hydrolyses of ATP and other high-energy phosphates have been portrayed as simple processes. The situation in a real biological system is far more complex, owing to the operation of several ionic equilibria. First, ATP, ADP, and the other species in Table 3.3 can exist in several different ionization states that must be accounted for in any quantitative analysis. Second, phosphate compounds bind a variety of divalent and monovalent cations with substantial affinity, and the various metal complexes must also be considered in such analyses. Consideration of these special cases makes the quantitative analysis far more realistic. The importance of these multiple equilibria in group transfer reactions is illustrated for the hydrolysis of ATP, but the principles and methods presented are general and can be applied to any similar hydrolysis reaction. 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

In general, the activation energies for both cationic and anionic polymerization are small. For this reason, low-temperature conditions are normally used to reduce side reactions. Low temperatures also minimize chain transfer reactions. These reactions produce low-molecular weight polymers by disproportionation of the propagating polymer ... 

Chain growth occurs through a nucleophilic attack of the carbanion on the monomer. As in cationic polymerizations, lower temperatures favor anionic polymerizations by minimizing branching due to chain transfer reactions. 

Of course, these conclusions do not rule out completely the occurrence of other reactions such as those listed above, but their contribution to the overall mechanism must be very small in the production of the oligomers. The dark colour of these products was attributed to hydride transfer reactions, similar in nature to those encountered in the cationic polymerization of 2-vinyl furan [see Section III-B-l-c)]. The subsequent process which transforms these oligomers into cross-linked resins was not investigated. 

The role of divalent cations in the mechanism of enzyme catalysed phosphoryl and nucleotidyl transfer reactions. A. S. Mildvan and C. M. Grisham, Struct. Bonding (Berlin), 1974,20,1-21 (88),... 

J.ll In each of the following salts, either the cation or the anion is a weak acid or a weak base. Write the chemical equation for the proton transfer reaction of this cation or anion with water (a) NaC6H50 (b) KCIO (c) C,HSNHCI ... 

J.12 C6HsNH3C1 is a chloride salt with an acidic cation, (a) If 50.0 g of C6H5NH3C1 is dissolved in water to make 150.0 mL of solution, what is the initial molarity of the cation (b) Write the chemical equation for the proton transfer reaction of the cation with water. Identify the acid and the base in this reaction. 

J.I3 Na As04 is a salt of a weak base that can accept more than one proton, (a) Write the chemical equations for the sequential proton transfer reactions of the anion with water. Identify the acid and the base in each reaction, (b) If 35.0 g of Na3As04 is dissolved in water to make 250.0 ml. of solution, how many moles of sodium cations are in the solution ... 

The competing reactions are isomerization of the cationic chain end, transfer reactions to monomer, counterion and solvent, and also termination reactions. The actual process of propagation depends on the concrete interactions between the reactants present in the polymerizing system. A synopsis of interactions expected is given in Table 7. For the most important of them quantum chemical model calculations were carried out. 

The stable triphenylcyclopropenium cation (81) undergoes an electron-transfer reaction when photolyzed in acidic medium (van Tamelen et al., 1968, 1971). Irradiation of 81 for 4 hours in 10% aqueous sulfuric acid resulted in a 49% yield of hexaphenylbenzene (82). The reaction is presumed to proceed by initial charge transfer to produce the cyclopropenyl radical 83, which then couples to give 84. This compound in... 

These reactions are postulated to proceed by electro-transfer to give the radical cation of alkoxynaphtalene, which either undergoes reaction with copper(II) bromide or dimerizes (ref. 15). That is, one-electron transfer from the electron-rich alkoxynaphtalene to Cu(II) results in generation of the corresponding radical cation. The radical cation reacts with bromide anion leading to the brominated compound, whereas the radical cation undergoes reaction with another alkoxynaphtalene leading to the binaphtyl (eqns. 2-4). 

The ion-molecule reaction between thiirane and its radical cation to form a thiirane sulfide radical cation and ethylene has been studied by Qin, Meng and WiUiams [134]. ESR studies using a low-temperature sohd-state Freon radiolysis technique provided compeUing evidence that the hemibonded dimer radical cation of thiirane is an intermediate in this so-called sulfur-transfer reaction see Scheme 2. 

A hydrogen cation is a hydrogen atom that has lost its single electron, leaving a bare hydrogen nucleus. A bare hydrogen nucleus is a proton. Thus, any reaction in which H moves from one species to another is called a proton-transfer reaction. Protons readily form chemical bonds. In aqueous solution, they associate with water molecules to form hydronium ions. 

Because of their basic properties, aikaioids were among the first naturai substances that eariy chemists extracted and purified. Morphine was isoiated from poppies in 1805 and was the first aikaioid to be characterized. When treated with aqueous strong acid, aikaioids accept protons to produce water-soiubie cations. The protonated aikaioids dissoive, ieaving the rest of the piant materiais behind. Adding strong base to the aqueous extract reverses the proton-transfer reaction, converts the aikaioid back to its neutrai base form, and causes pure aikaioid to precipitate from the soiution ... 

Metallic iron is made up of neutral iron atoms held together by shared electrons (see Section 10.7). The formation of rust involves electron-transfer reactions. Iron atoms lose three electrons each, forming Fe cations. At the same time, molecular oxygen gains electrons from the metal, each molecule adding four electrons to form a pair of oxide anions. As our inset figure shows, the Fe cations combine with O anions to form insoluble F 2 O3, rust. Over time, the surface of an iron object becomes covered with flaky iron(ni) oxide and pitted from loss of iron atoms. 

PBE dendrons bearing a focal bipyridine moiety have been demonstrated to coordinate to Ru + cations, exhibiting luminescence from the metal cation core by the excitation of the dendron subunits [28-30]. The terminal peripheral unit was examined (e.g., phenyl, naphthyl, 4-f-butylphenyl) to control the luminescence. The Ru +-cored dendrimer complexes are thought to be photo/redox-active, and photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions are reported. 

Each of the heterogeneous charge-transfer reactions (32.3) and (32.4) can be conpled to a homogeneons chemical reaction. Often, an ion association or complex formation occnrs, for example, transfer of the cation facilitated by the formation of the complex with valinomycine (Val) in the potassinm-selective electrode. 

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