Halogenation proton sponges

The ortao-halogenation probably proceeds via participation of the neutral proton sponge, rather than its cation. According to calculations, the n -electron density at positions 2 and 7 of 1 is greater than at the para- and meta-positions. Again, the radical mechanism of substitution cannot be ruled out190. 

The 1/BQ system alone or with Lewis acid additives was also employed for asymmetric catalysis in the synthesis of /3-amino acids235. During an elaboration of the tandem catalytic asymmetric chlorination/esterification process, Lectka and coworkers found that proton sponge 1 competes with ketenes in the reaction with halogenating agents, such... 

In contrast to a-fluorination, direct a-chlorination and a-bromination were developed on the basis of ketene intermediate mechanism in the presence of O-benzoylquinine (122) by Lectka and coworkers in 2001 (Scheme 6.36) [64, 65], Ketenes derived from acid chlorides 120 with either BEMP resin or proton sponge are added to 122 to form zwitterionic enolates, which are a-halogenated by perha-loquinones 123, 124. Finally, O-benzoylquinine moiety is substituted by haloaro-matic phenolate anions, generated from 123 or 124, to afford chiral a-halocarbonyl products 121 up to 99% ee. 

Darabantu, M. Lequeux, T. Pommelet, J.-C. Pie, N. Turck, A. Selective fluorination by halogen exchange of chlorodiazines and chloropyridines promoted by the proton sponge -triethylamine tris(hydrogen fluoride) system. Tetrahedron 2001, 57, 739-750. 

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