Multiple Hydrogen Transfer

Equation (6.22) may not only be used in the case of a single proton transfer but also in the case of concerted multiple proton transfers, according to Fig. 6.10, characterized by a single barrier. Only some minor changes are necessary. 

Ae is symbolized schematically in Fig. 6.10 by the different spacings of H and D levels in the ground state and the transition states. 

The tunneling mass of a given isotopic reaction is written as 

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ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

The scope of this chapter is, therefore, (i) to review the Bell-Limbach tunneling model in comparison with other models and its use for describing single steps of multiple hydrogen transfer networks and (ii) to review applications of this approach in a number of cases which have been studied mainly by NMR. A description of the techniques used for the determination of rate constants of Id-transfer will not be included in this chapter readers interested in this problem are referred to a recent review [4]. 

In this section first different models of single H-transfer will be reviewed, including primary kinetic H/D isotope effects, where the focus is on the Bell-Limbach model. Then formal kinetics will be used to describe multiple hydrogen transfers and their kinetic isotope effects. 

Intermolecular Multiple Hydrogen Transfer in H-bonds of Medium Strength... 

The focus of this chapter is on enzyme mechanisms that employ multiple hydrogen transfers, where both transfers are mechanistically central steps. The exchange of hydrons with solvent often presents both challenges and opportunities to the kinetic analysis of enzyme systems that undergo multiple hydrogen transfers. The... 

The stereoinversion of carbon centers catalyzed by racemases and epimerases is an archetypal enzyme catalyzed reaction for studying multiple hydrogen transfers. 

Volume 2 concludes in Part VII with contributions on the variational transition state theory approach to hydrogen transfer in various contexts (Truhlar and Garrett, Ch. 27), on experimental evidence of hydrogen atom tunneling in simple systems (Ingold, Ch. 28), and finally on a theoretical perspective for multiple hydrogen transfers (Smedarchina, Siebrand and Fernandez-Ramos, Ch. 29). 

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