Formate protonation

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

For reduced catalysts in solution, protonation competes with the reaction with CO2 (step 2 of Sch. 2). Protonation of the reduced metal results in the formation of a transition metal hydride, which may ultimately lead to the formation of hydrogen or formate. Protonation can be quite fast and favorable. For example, [Co (r c-Me6[14]4,ll-diene)]+ has binding constants of 2.5 x 10 and 2.5 x 10 pKa = 11.4) for the reaction with CO2 and protons, respectively [53]. Similarly, the rate constants for binding protons... 

Because of the high reactivity of Grignard reagents, calorimetric measurements require total exclusion of air and moisture and vacuum tight equipment must be used. The following three reactions (equations 3-5) have usually been studied formation, protonation and reaction with bromine. 

Grollmann U, Schnabel W (1980) On the kinetics of polymer degradation in solution, 9. Pulse radiolysis of polyethylene oxide). Makromol Chem 181 1215-1226 Hamer DH (1986) Metallothionein. In Richardson CC, Boyer PD, Dawid IB, Meister A (eds) Annual review of biochemistry. Annual Reviews, Palo Alto, pp 913-951 Held KD, Harrop HA, Michael BD (1985) Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars. Radiat Res 103 171-185 Hilborn JW, PincockJA (1991) Rates of decarboxylation of acyloxy radicals formed in the photocleavage of substituted 1-naphthylmethyl alkanoates. J Am Chem Soc 113 2683-2686 Hiller K-O, Asmus K-D (1983) Formation and reduction reactions of a-amino radicals derived from methionine and its derivatives in aqueous solutions. J Phys Chem 87 3682-3688 Hiller K-O, Masloch B, Gobi M, Asmus K-D (1981) Mechanism of the OH radical induced oxidation of methionine in aqueous solution. J Am Chem Soc 103 2734-2743 Hoffman MZ, Hayon E (1972) One-electron reduction of the disulfide linkage in aqueous solution. Formation, protonation and decay kinetics of the RSSR radical. J Am Chem Soc 94 7950-7957... 

Significant mechanistic insights into the DABCO-catalysed isomerization of y-hydroxy-o ,/3-alkynyl esters to y-oxo-a,p-trans-alkenyl esters have been reported.33 The reaction mechanism involves cumulene formation, protonation with the conjugate acid of the amine, and protonation of the resulting allenol with water. 

Thus tryptophan synthetase, which catalyzes the addition of serine to indole via an QE.P-unsaturated imine derivative, is inactivated by QE-cyanoglycine (33). In this case, QE-cyanoglycine, an analogue of the substrate serine, undergoes Schiff base formation. Proton abstraction then occurs and the resultant QE-cyano carbanlon is apparently reprotonated to generate a reactive keteneimlne which can alkylate a nucleophilic active site residue. 

W Schliephake, WJunge and FIT Witt (1968) Correlation between field formation, proton translocation, and the light reaction in photosynthesis. Z Naturforsch 23b 1571-1578... 

Earlier, we saw how deuterium replaces all a hydrogens in acetaldehyde. Why doesn t the a-iodo compound react further to add more iodines The way to answer a question such as this one is to work through the mechanism of the hypothetical reaction and see if you can find a point at which it is disfavored. In this case, that point appears right away. It is enol formation itself that is slowed by the introduction of the first halogen. The introduced iodine inductively withdraws electrons and disfavors the first step in enol formation, protonation of the carbonyl group (Fig. 19.35). 

Conversely, extra-framework tin(iv) oxide nanoparticles on Si-Beta as well as Zr-, Ti-, Hf- and Al-Beta samples did not catalyse the formation of the lactone to a significant extent. As sketched in Scheme 21.9, the proposed reaction pathway with the tin catalysts involves a set of successive steps tin-enolate and carbon-carbon bond formation, proton transfer, dehydration, and cyclisation to afford the lactone. 

Hydrogen bond formation, protonation, and solvents with a high dielectric constant shift n-ir transitions toward the blue but do not affect the tt-tt transitions. Deprotonation, breaking of hydrogen bonds, and aprotic solvents (hexane, etc.) induce red shifts. 

Acidic resins can be used as heterogeneous catalysts for in situ peracid formation. Protonic ion-exchange resins based on sulfonated polystyrene were used for epoxidations [119]. Acetic acid was, thus, used successfully as epoxidation catalyst in combination with resins such as Dowex 50W-X8 [104,120,121], Dowex 50X-12 [95], or Amberlite IR-120 [118,122] to epoxidize PBD. The formation of products from side reactions of the epoxides are to be expected and in some cases have been reported [122]. A more advanced approach uses the immobilized enzyme Novozym 435 to catalyze the in situ formation of peracetic acid from acetic acid and hydrogen peroxide [123]. 

---