Protonation, reversible

As seen from Scheme 1.19, the cation-radicals transform into radicals that are more or less stable and can be protonated reversibly. If the radicals formed are unstable, they perish before protonation. If the initial cation-radicals have no hydrogen atoms, their stability appears to be higher. Deprotonation is typical for cation-radicals that contain proton-active hydrogen atoms and form, after deprotonation, either quite stable or, the reverse, quite unstable radicals. 

Di-t-butylcyclopenta-l,2,3-dithiazole (52) is protonated reversibly on nitrogen by trifluoro-acetic acid in CDCI3 to produce a violet solution of salt (53) (Scheme 3) <85TL189>. 

On the basis of more detailed results White and Olah30", as well as Borsch30", agree that this conformation is also that of the predominant isomer of protonated methyl formate 5b). The minor component will be the conformer with the ester methyl group and added proton reversed (Sc), viz. 

Systems. As already observed for methane (vide supra), with decreasing acidity it becomes more and more difficult to protonate reversibly C—H bonds. Nevertheless, when alkanes with more than two carbon atoms are used as starting material, carbenium ions are generated by competitive protolytic and oxidative processes. Depending on the strength of the superacid system, proton exchange can take place by two competitive reactions (i) directly via reversible protonation and (ii) via deprotonation of the carbenium ion and reprotonation of the alkene. 

This invariance of the core is illustrated by the iron complexes [Fe(CO)2(S4)] and [Fe(CO)2(NHS4)], which have been investigated particularly thoroughly. In CH2CI2 solution, the thiolate donors of both [Fe(CO)2(S4)] and [Fe(CO)2 (NhS4)] can be protonated reversibly and in single steps by HBF4, as shown in Eq. 20. 

The electrochemical behavior of fuchsone (1) has been studied [62] in some detail. An unusual pathway is followed in the presence of acids (in this case phenols). The first-formed radical anion is protonated reversibly at oxygen and the triphenylmethyl radical so formed is not further reduced at the first reduction potential (Scheme 21). However,... 

Amino-derived BDPP (2,4-bis[diphenylphosphino]pentane) has been used in asymmetric hydrogenation catalysis [15-17] (cf. Sections 6.2 and 6.9). NMR analysis showed that a ten-fold excess of HBF4 is sufficient to protonate reversibly all four amino groups in the [Rh(diene)(BDPP)]BF4 complex. Recycling of the catalyst after enantioselective hydrogenation of dehydroamino acid derivatives in methanol is achieved by acidification with aqueous FIBF4 followed by extraction of the product with Et20. Immobilization of the protonated BDPP rhodium complex on a Nafion support has been studied as well [18]. 

The existence of surface hydroxyls is evidenced in the above schemes. These surface groups may be protonated (reverse of reaction(l)) or deprotonated (reaction (2)), according to the solution pH ... 

The fluorenylidene derivative can be sublimed unchanged at 150- 160° /O.OOl mm. It is unaffected by perchloric or tetrafluoroboric acids but is protonated in concentrated sulphuric acid the cyclopenta-dienylidene derivatives are more readily protonated (reversibly) at the a-position of the five-membered ring, resulting in the formation of substituted undecahoxaenium cations. N.m.r. spectra suggest little if any delocalisation in these fulvalcnes. 

If the associated calculated cumulative carbon mass losses are examined (Fig. 11.3 la) and C corrosion-driven Pt surface area decrease (Fig. 11.31b) for the 25 pm case calculated, it can be concluded that PEM side-chain degradation mitigates the cathode carbon corrosion This is because of the calculated decrease of PEM proton conductivity (from the PEM degradation model), which contributes to decreasing the proton reverse current (from the cathode to the anode) and thus mitigates the carbon corrosion in the cathode. These results provide some ejqrlanation of some experimental data showing that cathode carbon support corrodes less with PEMs that have some facility to degrade their conductivity. ... 

Some oxoalkyl complexes protonate reversibly in an analogous manner. 

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