Facilitation by protonation

The unsubstituted cyclopropane radical cation has received considerable experimental [72] and eoretical [73] attention and is thought to exist as three equivalent 2A partially-ring-opened structures. These three equivalent structures are able to interconvert relatively easily, via three equivalent 23 2 structures. Although the symmetry is reduced in the methyl-substituted system, we are able to observe the appropriate 1,2-shift operating by a mechanism analogous to that 

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The reaction catalyzed by hexokinase. Attack on the terminal phosphorus atom of ATP is probably facilitated by proton removal by a negatively charged group in the catalytic site of the enzyme ( B in the figure). It is also facilitated by the fact that the terminal phosphate of ATP is an excellent leaving group. 

The reaction is facilitated by protonation of the enolate produced in the initial conjugate-addition step. The resultant enol rapidly rearranges to the more stable ketone form. Since the base is regenerated, a catalytic amount may be used. The Michael reaction is reversible. Hence, efficient protonation (EtOH, H2O) of the adduct is necessary. 

Departure of urea leaving group during turnover of creatine by creatinase is facilitated by proton donation by the imidazolium group of His232, the same residue that extracted the proton from water to promote nucleophilic attack [66]. 

A direct substitution mechanism was indicated for the 2-pyridone catalysis of aminolysis of methyl acetate by methylamine." This mechanism is represented in Figure 7.9. It avoids a tetrahedral intermediate and describes a concerted displacement process that is facilitated by proton transfer involving 2-pyridone. Two very closely related TSs involving either the 2-hydroxypyridine or 2-pyridone tautomers were found. These TSs show extensive cleavage of the C-0 bond (2.0-2.2 A) and formation... 

Scheme 10.4 Propagation of acetylene metathesis by a metal carbene chain carrier, facilitated by proton transfer.

Several general conclusions may be made regarding synthesis of reduced pterin complexes of molybdenum. The stability of these complexes depends on formation of a Mo=N bond, which, electronically, replaces one of the two Mo=0 groups of the reagent as detailed in Scheme 2.12. Loss of an oxo ligand is facilitated by protonation by the two equivalents of HCl associated with... 

Acid catalysis is observed when the leaving group is an anion derived from a weak acid (Ns", CN", F", N02", S0, HPO. SO. oxalate malonate COj and various acetates). Cleavage of the metal-ligand bond can be facilitated by protonation of the leaving group. In the case of [Co(NH3)jF], the reaction can be represented. 

On the other hand, the rapid and extensive association facilitated by protonation of kraft lignin phenoxide moieties as the pH of the aqueous solution is lowered, or when they are present in non-aqueous solvents, is presumably mediated by hydrogen bonding and/or dipolar interactions, with non-bonded orbital interactions remaining the primary intermolecular forces. It has been shown that the absolute molecular weights of associated kraft lignin complexes encountered in /V,/V-dimethylformamide (DMF) could exceed values 1000 times larger than those observed in aqueous alkaline solutions. ... 

Chloride and CIF then interact to give CI2 again, the process is probably facilitated by protonation of the F. 

The rate constant for the reaction of penicillin with the monocation of 1,2-diaminoethane is ca 100-fold greater than that predicted from the Bronsted plot for a monoamine of the same basicity. The rate enhancement is attributed to intramolecular general acid catalysis of aminolysis by the protonated amine (Morris and Page, 1980a Martin et al., 1979). Breakdown of the tetrahedral intermediate, T, is facilitated by proton donation from the terminal protonated amino group to the P-lactam nitrogen [59]. It is not known whether proton transfer and carbon—nitrogen bond fission are concerted or occur by a stepwise process. 

Loss of the EPR signal during Fe(CN)g (hcf) oxidation of imidazolidine nitroxides having a hydrogen at position 4 of the heterocycle (HIMD) supports the assumption that the HIMD fragmentation is facilitated by proton abstraction from the position 4 of the oxoammonium cation formed as a result of the initial one-electron HIMD oxidation. ... 

Figure 7.1 illustrates the essential components of an MFC microbes, anode, cathode, and optional separator. Microbes oxidize organic or inorganic matter and release electrons and protons. The electrons flow through the circuit and combine with protons and oxygen (or other oxidants) on the cathode to form water and generate electrical current. The protons travel through the electrolyte solution, typically facilitated by proton carriers, such as phosphate or carbonate... 

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