Bronsted catalyzed electron transfer

Bronsted acid catalysis in electron transfer described in Section 1.3.1 has also been effective for redox reactions via the electron transfer step. As shown in the case of metal ion-catalyzed hydride transfer reactions (see above), hydride transfer reactions from an NADH analogue to /7-benzoquinones also proceed via Bronsted acid-catalyzed electron transfer [255, 256]. Since NADH and ordinary NADH model compounds are subjected to the acid-catalyzed hydration [98, 257, 258], an acid-stable NADH model compound, 10-methyl-9,10-dihydroacridine (AcrH2), was used as a hydride donor to / -benzoquinone (Eq. 24) ... 

S, and S interconvert photochemically in basic emimCVAlCl3 and bpCl/AlCl, but by different mechanisms. The photoisomerization could not be studied in the Lewis acidic ionic liquids because this results in the very large Bronsted acidity of traces of HCl in the media S for instance, underwent an acid-catalyzed dimerization in acidic [emim] [Clj/AlClj. In basic [bp] [ClJ/AlClj, both and S, photoi-somerize cleanly to give a PS containing -99% S,. In addition, the electron transfers from S, and to bp+ are quite exothermic (electron transfers from the triplet states are endothermic). Thus, the isomerizations in [bp] [Cl]/AlCl3 occur by the Electron Transfer mechanism. 

Abstract. Alkoxide ion departure from Meisenheimer complexes of the 1,l-dialkoxy-2,6-dinitro-4-X-cyclohexadienate type is catalyzed by pyridinium ions and by the hydronium ion. Bronsted a values which vary between 0.35 and 0.65 decrease when the alkoxy group becomes more electron withdrawing and increase when the X-substituent is made more electron withdrawing. This is inconsistent with a mechanism where proton transfer is rate limiting but in complete agreement with a concerted mechanism of acid catalysis as shown with the help of More O Ferrall-Jencks energy diagrams. 

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