Electrofugal group proton

The presence of a good electrofugal group (L + ), such as H+ or Me3Si +, makes elimination the predominant path (Scheme 10.14, pathways a and b). When R is an aromatic ring, pathway (a) is favorable and a benzylic proton is lost to form styrenes this results in a photochemical process analogous to the Heck reaction (Scheme 10.15) [12],... 

Electronically excited phthalimides can act as good electron acceptors and carboxylic acids are documented to serve as electron donors in photoinduced SET reactions. It is likely then, that in the excited state, an electron transfer process between the phthalimide system and the carboxylic acid would occur, leading to charge-separated diradical 6. Proton transfer from the carboxylic acid function (iT" is an electrofugal group) would form a carboxy radical 7, which could undergo rapid decarboxylation to azomethyne ylide 8. This reactive species is transformed into the final decarboxylated product 3 by intramolecular proton transfer (Scheme 16.4). 

Amines form relatively stable radical cations, and back electron transfer often overrides chemical reactions, except in the presence of good electrofugal groups in the a—position. Nevertheless, a-deprotonation (or alternatively a proton or hydrogen transfer process within an exciplex) may be fast enough to cause addition to unsaturated esters and ketones, " aromatics, and arylalkenes." " Deprotonation and further oxidation of some tertiary amines appear to produce aminium salts that are then aUylated by aUyl radicals simultaneously produced by fragmentation of allylsilane. Both processes are brought about by photosensitized electron transfer." ... 

In addition to protons, other electrofugic leaving groups such as SO3 (i. e., anions of sulfonic acids), Cl, Br, I, C02, and others can also be displaced in azo coupling reactions with aromatic substrates. The mechanism of such substitutions is in principle the same as that of dehydrogenation (see Fischer and Zollinger, 1972). 

Replacement of electrofugic leaving groups other than protons is called //750-substitution. For a discussion of o-complexes see Sec. 12.8. 

CO2 as the leaving group. Evidently, the order of electrofugal ability is CO2 > H + > COOH, so that it is necessary, at least in most cases, for the COOH to lose a proton before it can cleave. 

Electrophilic substitution allows replacement of a proton or another electrofuge with a different group and is the most-used process for functionalizing aromatic rings, whereas nucleophilic substitution is often considered to be more difficult or to require special substituents or reaction conditions. Prior to 1970, only two significant classes of nucleophilic substitution processes on aromatic rings had been... 

Due to their ready isomerization simple cyclopentenones present a particular challenge in the Nazarov cyclization. In all of the cases studied in a- and -monosubstituted and a, -disubstituted systems the cy-clopentenone product contained the double bond in the less substituted position, as required by loss of the silicon electrofuge (Scheme 17). The relative conBguration of substituents in disubstituted cases is controlled by kinetic protonation and weakly favors the cis isomers. Substituent effects in rate were particularly noted in these cases where substitution with a- and -alkyl groups greatly accelerated and decelerated the reactions, respectively. 

Proton Transfer to a Lone Pair ionization of a Leaving Group Trapping of an Electron-Deficient Species Electrophile Addition to a Multiple Bond Electrofuge Loss from a Cation to Form a Pi Bond The S m2 Substitution The E2 Elimination The AdeS Addition... 

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