Trapping of protons

At elevated temperatures, where the electron lifetime was much shorter than the pulse lengths of a few nanoseconds used, a second mobile species could be observed as a slowly decaying after-pulse conductivity component for large pulses. This was attributed to proton conduction with a proton mobility of 6.4 x 10 cm /Vs in H,0 ice and a somewhat lower value in D2O ice. ° In the case of the proton, the mobility was found to have an apreciable negative activation energy of 0.22 eV. The motion and trapping of protons was tentatively explained in terms of an equilibrium between free protons and a proton complexed with an orientational L-defect. °... 

Kimst M, Warman JM. (1983) Nanosecond time-resolved conductivity studies of pulse-ionized ice 2. The mobility and trapping of protons. J Phys Chem 87 4093 095. 

Proton Traps. 2,6-Di-tert-butyl-4-methylpyridine (DBMP) and 2,6-di-tert-butyIpyridine (DtBP) are hindered bases incapable of reacting with electrophiles other than protonic acids. Consequently, they can be successfully used for the trapping of protons during their transfer to monomer (174. 175). At the same time they can disseminate between the two major initiation mechanisms encountered in cationic polymerization, that is, protonic initiation or carbenium initiation (176). 

A three-component Mannich-type reaction of a diazo compound, Ph-C(=N2)-C02Me, a carbamate, Bn02CNH2, and an imine, PhCH=NPh, gives access to both syn- and anP-Q ,jS-diamino acid derivatives (20). Co-catalysed by Rh2(OAc)4 and BINAP-derived phosphoric acids, the reaction involves diastereoselectively switch-able enantioselective trapping of proton carbamate ammonium ylide intermediates. (J) High levels of chemo-, diastereo-, and enantio-selectivities were achieved... 

An important function of the kidney in regulating the acid-base balance is ammonium secretion. Ammonia probably forms in the collecting tubes. The tubule cells form ammonia by deaminating glutamine. The ammonia reacts with protons to yield ammonium ions. It is not known whether these ammonium ions are formed within the tubular cell or within the tubular fluid. In any event, the trapping of protons by ammonia and the excretion of the ammonium ion into the tubular fluid confer buffering properties to the tubular fluids, reduce the difference between the pH of the tubular and the intracellular fluid, and further facilitate the excretion of hydrogen ions. 

Unless working with superdried systems or in the presence of proton traps, adventitious water is always present as a proton source. Polymeriza tion rates, monomer conversions, and to some extent polymer molecular weights are dependent on the amount of protic impurities therefore, weU-estabHshed drying methods should be followed to obtain reproducible results. The importance is not the elimination of the last trace of adventitious water, a heroic task, but to estabhsh a more or less constant level of dryness. 

The stereochemistry observed in hydrogen-exchange reactions of carbanions is very dependent on the conditions under which the anion is formed and trapped by proton... 

The use of a second base to trap all proton acids generated during the course of the rearrangement In most cases, a two-phase system of solid potassium carbonate as a suspension in dichloromethane or chloroform gave the best results [58 a-e]. 

Aminolysis of simple esters is snrprisingly difficnlt, despite the greater thermodynamic stability of amides than esters the problem is that the initial tetrahedral intermediate preferentially reverts to starting material (not only is the amine the better leaving gronp, bnt loss of alkoxide would lead to an A-protonated amide), and only trapping of this intermediate by proton transfer allows the reaction to proceed. ... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The first kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

In carbonyl addition reactions, a commonly occurring and important mechanistic step is the transfer of a proton from one site to another in a reactive intermediate (proton switch). If the proton switch occurs sufficiently rapidly compared with the rate of collapse of the intermediate to reactants, the overall reaction may be facilitated by trapping of the unstable intermediate by the proton switch (Jencks, 1976). For example, in the formation of oximes from the reaction of benzaldehyde with O-methylhydroxylamine shown in (87H89) (Sayer and Jencks, 1973 Rosenberg et al., 1974), the first unstable intermediate (It) on the reaction pathway is converted by a proton switch (88) to the intermediate (I2) which has less tendency than It to... 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

As in the reductive ring-opening, titanocene—oxygen bonds have to be protonated. Here, a titanium enolate, which is generated after reductive trapping of an enol radical, has to be protonated, in addition to a simple titanocene alkoxide. As before, 2,4,6-collidine hydrochloride constitutes a suitable acid to achieve catalytic turnover, but here zinc dust turned out to be the reductant of choice [31c], The features of the stoichiometric reaction are preserved under our conditions. Acrylates and acrylonitriles are excellent radical acceptors in these reactions. Methyl vinyl ketone did not yield the desired addition product. Under the standard reaction conditions, a-substituted acceptors are readily tolerated, but (3-substitution gives the products only in low yields. 

The key features of the catalytic cycle are trapping of the radical generated after cycliza-tion by an a,P-unsaturated carbonyl compound, reduction of the enol radical to give an enolate, and subsequent protonation of the titanocene alkoxide and enolate. The diaster-eoselectivity observed is essentially the same as that achieved in the simple cyclization reaction. An important point is that the tandem reactions can be carried out with alkynes as radical acceptors. The trapping of the formed vinyl radical with unsaturated carbonyl compounds occurs with very high stereoselectivity, as shown in Scheme 12.21. 

Besides protonation, a variety of other electrophiles have been employed for the trapping of allenyllithium intermediates 105, e.g. aldehydes and ketones, oxiranes and carbon dioxide [69]. Scheme 2.38 shows a selection of functionalized allenes obtained by this method. 

The ready protonation of radical anions under conditions of proton availability causes other problems to appear, as for example shown by the stepwise cathodic reduction of PBN to the corresponding imine and amine [reactions (59) and (60)] during which the intermediate radicals [21] and [22] appear and become trapped by PBN (Simonet et al., 1990). 

Complex exo-60 is then protonated to give the 773-vinylcarbene complex exo-64, which subsequently inserts carbon monoxide in the well-established manner (see Sections II,B, V,B, VI,B, VI,C, VI,E, VI,J, and VII), affording the 16-electron species endo-65. Anion trapping of the unsaturated species finally yields the vinylketene complex endo-62. 

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