Electrostatic and electrophilic catalysis in the reductive cleavage of alkyl aryl ethers. The influence of ion pairing on the regioselectivity
Electrophilic and electrostatic catalysis have been identified as distinct contributions that affect the reactivity of radical anions in the reductive cleavage of alkyl aryl ethers. Two modes of mesolytic scission of these radical anions are possible: homolytic (dealkylation, a thermodynamically favored but kinetically forbidden process) and heterolytic (dealkoxylation). From our studies (alkali metal reductions, electrochemical studies, use of substrates with a preformed positive charge in certain positions of their structure) it can be concluded that the heterolytic scission is very much dependent on the electrophilic assistance by the counterion and it is only observed in contact ionic pairs with unsaturated cations (electrophilic catalysis). On the other hand, the homolytic scission is observed in solvent-separated ionic pairs, and it is especially efficient when the pair has a controlled topology with a tetralkylammonium cation (saturated cation) near the oxygen atom. The effect of the cation has, in this case, electrostatic origin (electrostatic catalysis), probably lowering the barrier of the intramolecular pi-sigma electron transfer process and thus reducing the kinetic control of the reaction in such a way that the thermodynamically more favorable process is produced.
Casado
F
Departament de Quimica, Universitat Autonoma de Barcelona, 08193 Bellaterra, Barcelona, Spain, Dipartimento de Chimica, Universita di Sassari, Via Vienna 2, I-07100, Sassari, Italy.
Pisano
L
Farriol
M
Gallardo
I
Marquet
J
Melloni
G