A Thermoresponsive Core-Shell Bifunctional Polymeric Catalyst: An Aldol-Michael-ATH Cascade Process

Key words

thermoresponsive polymer - PNIPAMA - DPEN ruthenium - asymmetric catalysis - transfer hydrogenation

Significance

A poly(N-isopropylmethacrylamide) (PNIPMAM)-based core-shell structured bifunctional composite (CSBC DBU/Ru*) bearing a DBU unit and a homochiral DPEN-ruthenium complex was prepared through a stepwise precipitation–poly­merization procedure. CSBC DBU/Ru* promoted the aldol condensation and the subsequent intramolecular oxa-Michael addition of 2-hydroxyaceto­phenones (1) and 4-bromobenzaldehyde at 70 °C to give 2-aryl-4-chromanone 2a (see Reaction I). CSBC DBU/Ru*also catalyzed the asymmetric transfer hydrogenation (ATH) of 4-chromanone 2a with HCOONa to give 3a (99% ee) (see Reaction II). When the reaction of 1 and 4-bromobenzaldehyde was carried out with CSBC DBU/Ru* at 70 °C followed by additional stirring at 40 °C, the aldol/­Michael/ATH sequence took place in one pot to give 99% ee of 3a (see Reaction III).

Comment

CSBC DBU/Ru* exists as a hydrogel in aqueous acetonitrile at 70 °C, where the inner core-tethered Ru* species is most inaccessible to show little catalytic performance, and the outer shell-tethered DBU unit promoted the aldol/oxa-Michael reactions. Ru*-catalyzed ATH smoothly proceeded in a solution of CSBC DBU/Ru* at 40 °C, where the organic substrates can diffuse into the core region of the polymer matrix. 4-Chromanone 2 is in a state of equilibrium with the retro-Michael product 2' to ­realize the ATH with the dynamic kinetic resolution (DKR). Thus, the thermoresponsive property of PNIMAM and the site-isolated core/shell immobilization of a DBU unit and a DPEN-Ru* complex ­enabled the direct access to chiral chromanols through the temperature-tuned aldol/Michael/ATH-DKR multi-step sequence.

Publication History

Article published online:
08 December 2023