Recyclable Heterogeneous and Low-Loading Homogeneous Chiral Imidazolidinone Catalysts for α-Alkylation of Aldehydes

A supramolecular bifunctional iridium photoaminocatalyst for the enantioselective alkylation of aldehydes

The construction of a hybrid metal-organo-photoredox catalyst based on the conjugation of an imidazolidinone organocatalyst and Ir(ppy)2(bipy) (ppy = 2-phenylpyridine, bipy = bipyridine) is described. The introduction of the desired organocatalyst into the bipyridine moiety is quite modular, allowing the preparation of different hybrid photocatalysts, and is realized though a simple click reaction. The hybrid photocatalysts obtained were employed in the benchmark photoredox alkylation of aldehydes. Remarkably, the conjugation of a first-generation MacMillan catalyst produces an active and stereoselective hybrid photoredox catalyst.

Ionic, Core-Corona Polymer Microsphere-Immobilized MacMillan Catalyst for Asymmetric Diels-Alder Reaction

The improvement of the catalytic activity of a heterogeneous chiral catalyst is one of the most critical issues, as are its recovery and reuse. The design of a heterogeneous chiral catalyst, including the immobilization method and the support polymer, is of significance for the catalytic activity in asymmetric reactions. An ionic, core-corona polymer microsphere-immobilized MacMillan catalyst (ICCC) was successfully synthesized by the neutralization reaction of sulfonic acid functionalized core-corona polymer microsphere (CCM–SO3H) with a chiral imidazolidinone precursor. We selected the core-corona polymer microsphere as the polymer support for the improvement of catalytic activity and recovery. The MacMillan catalyst was immobilized onto the pendant position of the corona with ionic bonding. ICCC exhibited excellent enantioselectivity up to 92% enantiomeric excess (ee) (exo) and >99% ee (endo) in the asymmetric Diels-Alder (DA) reaction of (E)-cinnamaldehyde and 1,3-cyclopentadiene. ICCC was quantitatively recovered by centrifugation because of the microsphere structure. The recovered ICCC was reused without significant loss of the enantioselectivity.

Advances in Organic and Organic-Inorganic Hybrid Polymeric Supports for Catalytic Applications

In this review, the most recent advances (2014-2016) on the synthesis of new polymer-supported catalysts are reported, focusing the attention on the synthetic strategies developed for their preparation. The polymer-supported catalysts examined will be organic-based polymers and organic-inorganic hybrids and will include, among others, polystyrenes, poly-ionic liquids, chiral ionic polymers, dendrimers, carbon nanotubes, as well as silica and halloysite-based catalysts. Selected examples will show the synthesis and application in the field of organocatalysis and metal-based catalysis both for non-asymmetric and asymmetric transformations.

A catalytic reactor for the organocatalyzed enantioselective continuous flow alkylation of aldehydes

The use of immobilized metal-free catalysts offers the unique possibility to develop sustainable processes in flow mode. The challenging intermolecular organocatalyzed enantioselective alkylation of aldehydes was performed for the first time under continuous flow conditions. By using a packed-bed reactor filled with readily available supported enantiopure imidazolidinone, different aldehydes were treated with three distinct cationic electrophiles. In the organocatalyzed α-alkylation of aldehydes with 1,3-benzodithiolylium tetrafluoroborate, excellent enantioselectivities, in some cases even better than those obtained in the flask process (up to 95% ee at 25 °C), and high productivity (more than 3800 h(-1) ) were obtained, which thus shows that a catalytic reactor may continuously produce enantiomerically enriched compounds. Treatment of the alkylated products with Raney-nickel furnished enantiomerically enriched α-methyl derivatives, key intermediates for active pharmaceutical ingredients and natural products.