Chlorination proton sponges

Phenyltrichlorostannane reacts with 1 surplus amine to yield the hexacoordinate tin compound 2c only. An elimination of HCl from 2c seems to be impossible. Even reaction with a proton sponge, Ai, A, A, A -tetramethylnaphthalene-1,8-diamine, failed. 2c crystallizes in a non-centrosymmetric monoclinic space group (Cc). The molecule involves the all-d5 configuration of the salen type ligand. The angle (93.9°) of C19-Sn-Cl indicates a real cis arrangement of the chlorine atom and the phenyl group (Fig. 2) [2]. 

In one instance, the proton sponge derivatives 61 were prepared by nucleophilic displacement of the activated chlorine atoms in pen-dichloroacenaphthene 60 on heating with excess of dimethyl- or diethylamine (equation 2)53. Regrettably, the structure of all compounds reported in this work was elucidated without NMR analysis, utilizing only combustion analysis and IR spectroscopy. 

From the preparative point of view, most valuable are the chlorination and bromination of proton sponge 1 at positions 2 and 7 with, respectively, V-chlorobenzolriazole (CBT)189 and Af-bromosuccinimide98 (Scheme 30). The method is useful for the preparation of ortho-dihalides 101, 102 and monochloride 175. The monobromide 115, even using one equivalent of NBS, is rarely formed free of significant amounts of the disubstituted 102, which are hard to separate. Consequently, an alternative way to 115 is monolithiation of the corresponding dibromide with butyllithium followed by quenching the reaction mixture with water. 

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. 

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