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In 2004, the Jacobsen group demonstrated that an anion‐binding catalyst could be used to affect an enantioselective acyl‐Pictet‐Spengler reaction (Scheme 4.51) [170]. A variant that features activation of a hydroxylactam by trimethylsilyl chloride (TMSCl) was later demonstrated as well [171]. Mechanistically this reaction proceeds by formation of an acyl iminium cation, where the chloride counteranion is hydrogen bound to the chiral urea catalyst. This keeps the chiral information close to the cationic center, allowing selective nucleophilic attack by the indole.

Scheme 4.51. Enantioselective acyl‐Pictet‐Spengler reaction enabled by anion‐binding catalysis.

Source: Based on [170].

Scheme 4.52. Enantioselective reactions enabled by anion‐binding catalysis proceeding through oxocarbenium and carbocation intermediates.

Source: Based on [172].

In addition to iminium reactivity, this anion‐binding strategy was applicable to the generation and enantioselective transformations of oxocarbeniums. In the first report by the Jacobsen group, chloroisochromans were activated by anion‐binding of the chloride, generating an oxocarbenium ion‐paired to a chloride‐bound chiral catalyst (Scheme 4.52) [172]. The oxocarbenium is then trapped enantioselectively by silyl ketene acetals in good yield and enantioselectivities. In 2016, the Seidel group demonstrated the first highly enantioselective, catalytic oxa‐Pictet‐Spengler via anion‐binding catalysis [173]. A strong counteranion effect on the enantioselectivity of the reaction reveals the importance of cooperativity between the anion and hydrogen‐bond donor. The ionization of alkyl halides could also be accomplished without the aid of an adjacent heteroatom donor. By utilizing neryl chloride analogues, the Jacobsen lab demonstrated an enantioselective tail‐to‐head cyclization proceeding in high yields and enantioselectivities [174].