Proton transfer from amines

A final explanation of the slow deswelling is that above pH 6.5, the gel must deprotonate by transferring its protons to hydroxyl ions and buffer species. Except at very high pH, hydroxyl ion concentrations are quite low. Neither citrate nor phosphate buffer can be very helpful. The highest pK for citrate is approximately 6.4, well below the pKa of the gel amines (7.8). Thus proton transfer from amine to citrate is not favored. Phosphate buffer has pK values 2.15,7.10, and 12.12. The first two pKs are lower than 7.8, so the corresponding phosphate groups are weaker bases than the gel amines. At and below pH 11.0, the phosphate group with pK = 12.12 will retain its acidic proton, and therefore will also not be able to extract a proton from the gel. 

The reaction was found to be first order with respect to amines and acrylamides and no base catalysis was observed. The reaction is believed to occur in a single step in which the addition of amine to Cp of acrylamide and proton transfer from amine to Ca of acrylamide take place concurrently with a four-membered cyclic transition-state structure. The Hammett (px) and Brpnsted (/3X) coefficients are rather small, suggesting an early transition state (TS). The sign and magnitude of the cross-interaction constant, pxy(= —0.26), is comparable to those found in the bond formation processes in the. S n2 and addition reactions. The normal kinetic isotope effect ( h/ d > 1.0) and relatively low A and large negative Avalues are also consistent with the mechanism proposed.192... 

The mechanism for this reduction, shown in the preceding box, involves successive electron transfers from lithium (or sodium) atoms and proton transfers from amines (or ammonia). In the first step, a lithium atom transfers an electron to the aikyne to produce an intermediate that bears a negative charge and has an unpaired electron, called a radical anion. In the second step, an amine transfers a proton to produce a vinylic radical. Then, transfer of another electron gives a vinylic anion. It is this step that determines the stereochemistry of the reaction. The trans-vinyWc anion is formed preferentially because it is more stable the bulky alkyl groups are farther apart. Protonation of the trans-v my ic anion leads to the nam-alkene. 

C. N. Rowley, T.-G. Ong, J. Priem, D. S. Richeson, T. K. Woo, Analysis of the critical step in catalytic carbodiimide transformation proton transfer from amines, phosphines, and alkynes to guanidinates, phosphaguanidinates, and propiolamidinates with Li and A1 catalysts. Inorg. Chem. 47, 2008 12024. 

Whenever possible, the chemical reactions involved in the fonnation of diastereomers and their- conversion to separate enantiomers are simple acid-base reactions. For example, naturally occurring (5)-(—)-malic acid is often used to resolve fflnines. One such amine that has been resolved in this way is 1-phenylethylarnine. Amines are bases, and malic acid is an acid. Proton transfer from (5)-(—)-malic acid to a racemic mixture of (/ )- and (5)-1-phenylethylarnine gives a mixture of diastereorneric salts. 

O Nucleophilic addition of a secondary amine to the ketone or aldehyde, followed by proton transfer from nitrogen to oxygen, yields an intermediate carbinolamine in the normal way. 

Since the rate was independent of acidity even over the range where H0 and pH differ, and the concentration of free amine is inversely proportional to the acidity function it follows that the rate of substitution is proportional to h0. If the substitution rate was proportional to [H30+] then a decrease in rate by a factor of 17 should be observed on changing [H+] from 0.05 to 6.0. This was not observed and the discrepancy is not a salt effect since chloride ion had no effect. Thus the rate of proton transfer from the medium depends on the acidity function, yet the mechanism of the reaction (confirmed by the isotope effect studies) is A-SE2, so that again correlation of rate with acidity function is not a satisfactory criterion of the A-l mechanism. 

The first experimental data for a reaction involving proton transfer from a hydrogen-bonded acid to a series of bases which were chosen to give ApK-values each side of ApK=0 are given in Fig. 15 (Hibbert and Awwal, 1976, 1978 Hibbert, 1981). The results were obtained for proton transfer from 4-(3-nitrophenylazo)salicylate ion to a series of tertiary aliphatic amines in aqueous solution, as in (64) with R = 3-nitrophenylazo. Kinetic measurements were made using the temperature-jump technique with spectrophoto-metric detection to follow reactions with half-lives down to 5 x 10"6s. The reciprocal relaxation time (t ), which is the time constant of the exponential... 

In the two examples of buffer catalysis of proton transfer from an intramolecularly hydrogen-bonded acid which have been discussed, it seems reasonably certain that the mechanism in Scheme 7 applies. The reactions are of the first order with respect to the catalyst B and it therefore follows that proton removal from the non-hydrogen-bonded species is rate-limiting k j > 2[B]- If this step consists of diffusion-controlled proton removal from a low concentration intermediate, the value k2 lx 109dm3 moP s-1 will apply for proton transfer to an amine. In the case of proton removal by hydroxide ion from 4-(3-nitrophenylazo)salicylate ion, the reaction was found to be of the first order in hydroxide ion up to the highest concentrations which could be studied (0.003 mol dm-3) with a rate... 

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Crampton24 has also demonstrated that for Meisenheimer complex formation, increased crowding at the reaction site caused by change from primary amines to piperidine results in rate reduction of proton transfer from the complex to the amine catalyst, and Hirst199... 

Methylamine dehydrogenase of Methylophilus methylotro-phus, proton transfer from the methylamine adduct of tryptophan tryptophylquinone (TTQ) CHs-amine vs. CDs-amine, transient kinetics studies of H-transfer step, H/D IE 5-40 °C, Ah/Ad = 16.8 0.5, AH 42.2 1.1 (H), 43.2 1.8 (D) kJ/mol, rate constant unchanged. 

From the first transition state (TSl, Fig. 1), the reaction path leads to the tetrahedral intermediate 1 (INTI). In the latter, the proton transfer from methanol to the tertiary amine function is completed (from 1.183 to 1.059 A), and the negative charge at the former carbonyl oxygen atom reaches its maximum. This charge is compensated by a further shortening of the bifurcated hydrogen bonds to 2.040 A (-0.103 A) and 1.765 A (-0.096 A) (Fig. 1). The thiourea moiety thus forms an oxyanion hole similar to the amide groups of the serine protease backbone [41]. 

The radical pair generated by proton transfer from tertiary amine radical cations to a,p-unsaturated ketone radical anions (e.g., 71) couple in the p position, forming... 

Mechanism. The neutral amine nucleophile attacks the carbonyl carbon to form a dipolar tetrahedral intermediate. The intramolecular proton transfer from nitrogen and oxygen yields a neutral carbinolamine tetrahedral intermediate. The hydroxyl group is protonated, and the dehydration of the protonated carbinolamine produces an iminium ion and water. Loss of proton to water yields the imine and regenerates the acid catalyst. 

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