European Journal of Organic Chemistry p. 2020 - 2028 (2019)
Update date:2022-08-17
Topics:
Ciogli, Alessia
Capitani, Donatella
Di Iorio, Nicola
Crotti, Simone
Bencivenni, Giorgio
Donzello, Maria Pia
Villani, Claudio
A heterogeneous, silica-based catalyst containing 9-amino-9-deoxy-epi-quinine (or quinidine) and a derivative of benzoic acid was synthesized through radical thiol-ene click reaction. The acid component allows the in situ activation of cinchona amino group, acting as a bifunctional catalyst. The heterogenized catalysts efficiently promoted the reaction of ketones with trans-β-nitrostyrene, with diastereo- and enantioselectivity comparable to those of the homogeneous counterparts (dr up to 90:10 and 90 % ee). In addition, the catalyst retained a constant activity for at least four cycles. Finally, the supported catalyst (9-amino-9-deoxy-epi-quinine/achiral acid) was employed under continuous-flow conditions. Two enantioselective Michael reactions were in sequence performed with the same homemade packed-bed reactor. The addition of cyclohexanone to trans-β-nitrostyrene provided the evaluation of optimal residence time with high level of stereoselection (2 μL/min flow rate, 83 % ee). Furthermore, the flow reactor well performed in the preparation of warfarin (isolated yield 95 %, 78 % ee. in 16 h at room temperature). The dual (chiral amine/achiral acid) solid supported system, making an even easier work-out, represents a valuable tool for green chemistry and is attractive for large scale applications.
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