Mechanism protonation

A number of investigations concerning the effect of added electrolytes and change in the solvent polarity have been reported (see ref. 25 pp. 17-19 for a more detailed account). The addition of neutral salts causes a large increase in the rate of reaction in both the one and two-proton mechanisms. In addition, the effect of increasing the water content in a dioxan-water solvent, provided that the concentration of water is above a certain threshold value, also produces a large increase in... 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

Scheme 16 are easily distinguished by the different products obtained, and by the breaks in the LFER plots of the log ko values against mechanism changes.287 The O-protonation mechanism of Scheme 16 is preferred over the more obvious JV-protonation one on the basis of the observed m values.287 The further decomposition of the first-formed benzamide, benzene-sulfonamide and jV-nitroamine reaction products takes place as previously discussed. 

Micellar rate enhancements of bimolecular, non-solvolytic reactions are due largely to increased reactant concentrations at the micellar surface, and micelles should favor third- over second-order reactions. The benzidine rearrangement typically proceeds through a two-proton transition state (Shine, 1967 Banthorpe, 1979). The first step is a reversible pre-equilibrium and in the second step proton transfer may be concerted with N—N bond breaking (17) (Bunton and Rubin, 1976 Shine et al., 1982). Electron-donating substituents permit incursion of a one-proton mechanism, probably involving a pre-equilibrium step. 

Anionic micelles strongly favor the two-proton mechanism, because of the increased concentration of hydronium ions at the micellar surface (Bunton and Rubin, 1976 Bunton et al., 1978b). 

Clearly, in the related catalysts containing just simple bidentate phosphines, dipyridines, or bis-oxazolines the concerted, heterolytic transfer cannot take place in the same way, unless we invoke an alkoxide or other anion as the proton-receiving moiety. In Figure 4.28 we have presented a simplified scheme for the hydride/proton mechanism for hydrogen transfer using an external base. 

The reference 48 authors used an UB3LYP unrestricted DFT method. Jaguar 5.5 and Gaussian 03 software, beginning calculations with an LACVP (Fe)6-31G (rest) basis set. See Section 4.4.3.The researchers assumed a direct protonation mechanism involving a framework of amino acid residues... 

Figure 13.18 Protonation mechanism for i-butyiene over a Bronsted acid site-tertiary carbenium ion or an alkoxide.

Figure 13.19 Protonation mechanism for /-butylene over a Bronsted acid site-primary carbenium ion.

The protonation mechanism includes Coulomb electrostatic forces resulting from charged surfaces. The development of surface acidity by the solid phase of the subsurface offers the possibility that solutes having proton-selective organic functional groups can be adsorbed through a protonation reaction. 

A very interesting and complex protonation mechanism has been snggested for the hydride cluster [W3S4H3(dmpe)3]PF6 in CH2CI2 solutions. In the presence of an excess of HCl, a careful kinetic study of the process in eq. (10.4) by the stopped-flow technique [9] has revealed three kinetically distinguishable steps very fast, fast, and slow, with rate constants A 1, ki, and k3. The kinetic order in the initial hydride cluster in the slow step has been measured as 1. At the same time, rate constants k and A 2 have corresponded to a second-order dependence on acid concentration, while the third step has shown a zero kinetic order on HCl. The rate constants have been determined as A i =2.41 x 10 M-2/s, k2 = 1.03 X 10 M /s, A 3 = 4 X 10 s . Note that the protonation process becomes simple at lower concentrations of HCl. Under these conditions it shows a single step with a first kinetic order on the acid. 

The /3-pinene fraction was used as a reference to determine the isomerization activity of the supports. Results given in Table 4 show that carbon VII is particularly inert with respect to /3-pinene. This behaviour is certainly related to the high content of this carbon in potassium (0.5 wt.-%). On the contrary, the CaO impurities present in carbon V seem to increase the isomerization activity of this carbon. It is well-known that the double bond shift isomerization of hydrocarbons can proceed via carbocation intermediates (protonic catalysis) or via allylic carbanion intermediates (acido-basic or purely basic catalysis) [Ref.7]. The results obtained with potassium-doped carbons show that in /3-pinene isomerization during HDS, the protonic mechanism predominates. 

Both assimilatory and dissimilatory nitrate reductases are molybdoenzymes, which bind nitrate at the molybdenum. EXAFS studies1050 have shown that there are structural differences between the assimilatory nitrate reductase from Chlorella vulgaris and the dissimilatory enzyme from E. coli. The Chlorella enzyme strongly resembles sulfite oxidase1050,1053 and shuttles between mon-and di-oxo forms, suggesting an oxo-transfer mechanism for reduction of nitrate. This does not appear to be the case for the E. coli enzyme, for which an oxo-transfer mechanism seems to be unlikely. The E. coli enzyme probably involves an electron transfer and protonation mechanism for the reduction of nitrate.1056 It is noteworthy that the EXAFS study on the E. coli nitrate reductase showed a long-distance interaction with what could be an electron-transfer subunit. 

O or N protonation mechanism was the lowest energy pathway, the authors preferred the O protonation mechanism, via (78). Steric effects at N(l) and C(2) were found to have a significant effect on the rate of reaction. 

Linear amino polymers containing basic nitrogen atoms are critically reviewed with regard to their synthesis, protonation and complex formation in solution with metal ions. Cross linked resins having essentially the same structure as linear polymers, are also mentioned. As far as the proto-nation is concerned, special care has been given to thermodynamic aspects, and to the most probable protonation mechanism. Complexing abilities of these polymers have been evaluated through stability constants and spectroscopic parameters. Practical implications of the properties have been considered. 

A considerable amount of data on the protonation, and complex formation with metal ions of polymeric amines have been reported. A critical insight leads to the conclusion that much has to be done in order to reach a clear vision of the chemical properties of many polymers of this kind. Most protonation studies deal with the determination of basicity constants with potentiometric techniques, which alone give little information on the protonation mechanism only few studies have been substantiated by spectroscopic (nmr) and calorimetric measurements. 

When the catalyst coordination center is highly electrophilic and the metal carbon bonds (if present) are of low reactivity, then easily polarized monomers may be converted into carbonium ion species and undergo coordinated cationic polymerization. Although non-protonic mechanisms have been considered, it seems most probable that water and other protonic impurities are involved in the initiation step. 

Scheme 2). Initial calculations indicated that C5-protonation is favored over C6-protonation, by about 10 kcal mol-1 at MP2/6-31 + G7/HF/6-31 + G (see above) the authors therefore turned their focus to the C5-protonation mechanism.

Results. Siegbahn and coworkers considered three mechanisms direct (C6)-protonation (mechanism v, 1 — 10, Scheme 2), 02-protonation (mechanism ii, 1 — 4 — 5, Scheme 2), and 04-protonation (mechanism iii, 1 — 6 — 7, Scheme 2).59 The direct protonation mechanism was calculated using several different models of the active site wherein some combination of methylamines, aspartates, and/or water was used. The lowest barriers were found for models that involve chains of residues spanning the methylammonium involved in protonating C6 and either 02 or 04 (for example, 14). 

The 04-protonation mechanism was explored only briefly, due to the fact that the crystal structures do not show any acidic residues in the vicinity of 04 (Fig. 2). The authors did calculate an 04-protonation mechanism using a model in which methylammonium protonates 04 via a bridging water molecule as proposed previously by Houk et al. (16).23... 

So far, three computational studies of isotope effects related to the ODCase mechanism have been published Singleton, Beak and Lee used 13C isotope effects to elucidate the mechanism by which the uncatalyzed decarboxylation of orotic acid takes place.46 Phillips and Lee calculated 15N isotope effects and compared them to known experimental values to show that oxygen-protonation mechanisms are viable for the enzyme-catalyzed process.47 Kollman and coworkers focused on the 15N isotope effect associated with C5-protonation.27 Each study is described further below. 

Results. Kollman and coworkers computed 15N EIEs for C5-protonation of orotate (la) and 1-methylorotate (lb) are 0.994 and 0.995, respectively. These inverse IEs indicate that there is some bond order change at N1 upon C5-protonation. The authors point out, however, that this isotope effect must be multiplied by the isotope effect for decarboxylation, which they expect to be normal and large enough to compensate for the inverse IE associated with the C5-protonation step. This would result in a normal IE overall, consistent with the experimental value of 1.0068 measured by Cleland et al. (see previous section), and therefore not ruling out their C5-protonation mechanism. 

---