Proton migrations

The last part of the chapter summarizes two specific reactions that can be viewed as a proton migration from an achiral sp3 carbon to a prochiral sp2 carbon, both reactions being catalyzed by cinchona derivatives. 

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Because of this extreme sensitivity, attention shifted to an acidic system, the phosphoric acid fuel cell (PAFC), for other applications. Although it is tolerant to CO, the need for liquid water to be present to facilitate proton migration adds complexity to the system. It is now a relatively mature technology, having been developed extensively for stationary power usage, and 200 kW units (designed for co-generation) are currently for sale and have demonstrated 40,000 hours of operation. An 11 MW model has also been tested. 

In this reaction, the C atom of C02, the Lewis acid, accepts an electron pair from the O atom of a water molecule, the Lewis base, and a proton migrates from an H20 oxygen atom to a C02 oxygen atom. The product, an H2C03 molecule, is a Bronsted acid. 

Hydroxy Benzophenone N(CH3)2 3-N,N -Dimethylamino Phenoxy Electron Abstraction, Followed by Proton Migration Photocrosslinking" ... 

Until recently the products of all nitrile cyclizations by the Thorpe reaction had been formulated as imines, although the products were found in 1955 to be better written as the enamine structure. In order to verify the reaction mechanism of the Thorpe reaction, the solid-state reaction of 84 and Bu OK was monitored by measurement of IR spectra in Nujol mulls. As the reaction proceeds (Scheme 14), the CN absorption of 84 at 2250 cm" decreases and a new CN absorption of the imine intermediate (87) arises at 2143 cm As 87 is converted into 88 by a proton migration, the CN absorption of 87 at 2143 cm" disappears, and only the CN absorption of 88 at 2189 cm remains finally [13]. 

Ovchinnikov 234 237) has shown that bovine rhodopsin, although quite different in amino acid sequence (348 residues), also forms seven transmembrane helices. This structural similarity between bacterial and mammalian light activated membrane proteins is remarkable. Since the two amino acid sequences have little in common it would appear that the necessary requirement is seven transmembrane helices to form a channel which is specific for proton migration. For example it has been suggested that a similar arrangement and function is performed by the lactose permease of E. coli237). 

Jensen, M. O., Rothlisberger, U., Rovira, C., Hydroxide and proton migration in aqua-porins, Biophys. J. 2005, 89,1744-1759... 

The passive film itself could not be treated as purely insulating since the known thickness of the film should give rise to a capacitive effect that would easily be seen in the impedance diagram. In fact, the film is apparently relatively conducting, probably through proton migration. 

Although Sah etal. (1983) and Gale etal. (1983) have demonstrated that H can be introduced into Si by electron injection into the oxide layer of metal-oxide-silicon devices, there has been no report of hydrogen penetration with an applied bias of opposite polarity. This may suggest electric-field-induced proton migration through the oxide. 

A-(Hydroxymethyl) derivatives of amines undergo acid- and base-catalyzed decomposition, plus an uncatalyzed reaction in the pH range of ca. 4-9. The postulated mechanism of this last reaction is shown in Fig. 8.20,c and involves intramolecular proton migration to form a zwitterion. It is estimated that the breakdown of /V-(hydroxymclhyl) derivatives of amines with pKa > 2 should be < 3 min at pH 7 and 25° [214],... 

Fuel cell operation entails (1) coupled proton migration and water fluxes in the PEM, (2) circulation and electrochemical conversion of electrons, protons, reactant gases, and water in CLs, and (3) gaseous diffusion and water exchange via vaporization/condensation in pores and channels of CLs, GDLs, and EEs. All components of an operating cell have to cooperate well in order to optimize the highly nonlinear interplay of these processes. It can be estimated that this optimization involves several 10s of parameters. 

Most electrophilic carbene complexes with hydrogen at Cjj will undergo fast 1,2-proton migration with subsequent elimination of the metal and formation of an alkene. For this reason, transition metal-catalyzed cyclopropanations with non-acceptor-substituted diazoalkanes have mainly been limited to the use of diazomethane, aryl-, and diaryldiazomethanes (Tables 3.4 and 3.5). 

The outcome of reactions involving carbonyl ylides is not always as easy to predict, as the examples in Table 4.20. Depending on the basicity of the intermediates, proton migrations might occur and unexpected results can be obtained. Two examples of such peculiar conversions and the proposed mechanisms are sketched in Figure 4.16. 

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