Reusable Silica-Supported Ammonium BINSate Catalysts for Enantio- and Diastereoselective Friedel–Crafts-Type Double Aminoalkylation of N-Alkylpyrroles with Aldimines

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Abstract

Silica-supported ammonium (R)-BINSate catalysts for the enantio- and diastereoselective Friedel–Crafts-type double aminoalkylation of N-benzyl- or N-methylpyrrole with aldimines were developed. The present heterogeneous catalysts showed high catalytic activity compared to our previous homogeneous (R)-BINSA ammonium catalysts, which were effective for single aminoalkylation. Simple aldimines could be used, and the corresponding pyrrole-derived chiral C2-symmetric triamines were obtained in the dl-form with good to extremely high enantioselectivities. The heterogeneous catalyst could be easily recovered and reused three times without any loss of catalytic activity or enantiocontrol. An XPS analysis supported precise preparation of the catalysts in situ and with good quality after recycling three times. From the perspective of modern green chemistry with fine asymmetric organocatalysis, the development of such chiral strong Brønsted acid catalysts might be useful for both laboratory and industrial applications.

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