Proton transfer cyanide+hydrogen

Intensive studies using NMR methods, kinetic experiments, and computational calculations were conducted to elucidate the catalytic mechanism and observed stereoinduction [22]. The data revealed that the hydrocyanation catalyzed by 33 presumably proceed over an initial amido-thiourea catalyzed proton transfer from hydrogen isocyanide to imine 32 to generate a catalyst-bound diastereomeric iminium/cyanide ion pair. Thereby, hydrogen isocyanide, as the tautomeric form of HCN, is stabilized by the thiourea moiety of 33. The stabilization degree of the formed iminium ion by the catalyst is seen as the basis for enantioselectivity. Subsequent collapse of the ion pair and bond formation between the electrophile and the cyanide ion forms the a-amino nitrile. It should be emphasized that the productive catalytic cycle with 33 does not involve a direct imine-urea binding, although this interaction was observed both kinetically and spectroscopically in the Strecker reaction catalyzed by 25 (see above) [19],... 

One after the other, step through (or animate) the sequence of structures depicting the SN2 and proton transfer reactions shown above. Compare the two. From what direction does cyanide approach the hydrogen in HCl From the same side as Cl ( frontside ), or from the other side ( backside ) Does the Sn2 reaction follow a similar trajectory ... 

Another example of an acid is hydrogen cyanide, HCN, which transfers its proton to water when it dissolves to form the solution known as hydrocyanic acid, HCN(aq). However, only a small fraction of the HCN molecules donate their protons, and so we classify HCN as a weak acid in water. We write the proton transfer reaction with equilibrium half-arrows ... 

Fig. 6. Mechanisms for the reduction of compounds I and II of HRP C by ferulic acid, after Henriksen et al. 195). This scheme is based on new information from the 1.45 A resolution crystal structure of the ternary complex of ferulic acid and cyanide-ligated HRP C 195). The direction of proton transfer is indicated by the dotted arrows. The mechanism is discussed in Section IV,B,2, and the crystal structure data in Section IV,F,4. Note that a distal site water molecule makes an important hydrogen bond with the backbone carbonyl group of Prol39 (a residue conserved in all members of the plant peroxidase superfamily).

Studies of proton transfers from carbon with f1/2 less than one microsecond are quite rare. The ionization of hydrogen cyanide in aqueous alkaline solution has been studied [20] using the ultrasonic stationary method which is applicable to reactions with tl/2 in the range 10 5 to 10 9 sec. Several reactions of benzyl carbanion having f 1/2 in the range 10-6—5 x 10-8 sec have been studied in tetrahydrofuran. The carbanion was generated by pulse radiolysis of solutions of dibenzyl mercury and its subsequent reaction with water and alcohols was followed spectrophotometrically [21]. 

It is thought that the first amino acids were synthesized from formaldehyde, hydrogen cyanide, ammonia and water in the primitive atmosphere. A possible synthesis involves a series of nucleophilic attacks and proton transfers. Propose a mechanism for the synthesis of glycine using the above mentioned compounds. 

The largest difference between the G2 methods and experimental proton affinities occurs for methyl vinyl ether (-18.8 to —21.6 kJ mol" ). The proton affinity of methyl vinyl ether is known to be very clo.se to that of ammonia, consistent with the G2 predictions. The experimental proton affinity of 868 kJ mol" is probably too high by 15-20 kJ mol". The G2 proton transfer energy of ethyl cyanide with methyl formate is in close agreement with experiment and suggests the G2 value is the more reliable estimate of the proton affinity of ethyl cyanide. The discrepancy between G2 and experiment over the proton affinity of hydrogen i.socyanide can be traced to the difference in the heats of formation of FINC, 191.5 kJ mol" (G2) and 201 8 kJ mol" (experiment). The G2 heat of formation of FINC is likely to be the more reliable. 

In the second step the hydrogen cyanide ionizes to form H+ and cyanide ion. The proton is transferred to ammonia,... 

---