Photoresponsive Azobenzene-Functionalized Shell Cross-Linked Micelles for Selective Asymmetric Transfer Hydrogenation.

We describe the substrate-selective asymmetric transfer hydrogenation of aromatic ketones using rhodium complexes immobilized on a photoresponsive nanoreactor. The nanoreactor switches its morphology upon light irradiation in a wavelength-selective manner. Kinetic studies show that the gated behavior in the cross-linking layer is key to discriminating among substrates and reagents during catalysis. Under ultraviolet light irradiation, the nanoreactor displays substrate selectivity, converting smaller ketone substrates faster to the corresponding secondary alcohols.

Compartmentalisation of molecular catalysts for nonorthogonal tandem catalysis.

The development of nonorthogonal tandem catalysis enables the use of a combination of arbitrary catalysts to rapidly synthesize complex products in a substainable, efficient, and timely manner. The key is to compartmentalise the molecular catalysts, thereby overcoming inherent incompatibilities between individual catalysts or reaction conditions. This tutorial review analyses the development of the past two decades in the field of nonorthogonal tandem catalysis with an emphasis on compartmentalisation strategies. We highlight design principles of functional materials for compartmentalisation and suggest future directions in the field of nonorthogonal tandem catalysis.

Compartmentalization and Photoregulating Pathways for Incompatible Tandem Catalysis.

This contribution describes an advanced compartmentalized micellar nanoreactor that possesses a reversible photoresponsive feature and its application toward photoregulating reaction pathways for incompatible tandem catalysis under aqueous conditions. The smart nanoreactor is based on multifunctional amphiphilic poly(2-oxazoline)s and covalently cross-linked with spiropyran upon micelle formation in water. It responds to light irradiation in a wavelength-selective manner switching its morphology as confirmed by dynamic light scattering and cryo-transition electron microscopy. The compartmental structure renders distinct nanoconfinements for two incompatible enantioselective transformations: a rhodium-diene complex-catalyzed asymmetric 1,4-addition occurs in the hydrophilic corona, while a Rh-TsDPEN-catalyzed asymmetric transfer hydrogenation proceeds in the hydrophobic core. Control experiments and kinetic studies showed that the gated behavior induced by the phototriggered reversible spiropyran to merocyanine transition in the cross-linking layer is key to discriminate among substrates/reagents during the catalysis. The smart nanoreactor realized photoregulation to direct the reaction pathway to give a multichiral product with high conversions and perfect enantioselectivities in aqueous media. Our SCM catalytic system, on a basic level, mimics the concepts of compartmentalization and responsiveness Nature uses to coordinate thousands of incompatible chemical transformations into streamlined metabolic processes.

Reversible Photoswitching in Poly(2-oxazoline) Nanoreactors.

This contribution reports light responsive catalytic nanoreactors based on poly(2-oxazoline) diblock copolymers. The hydrophobic block of the copolymer is a random copolymer consisting of a spiropyran functionalized 2-oxazoline (SPOx) and 2-(but-3-yn-1-yl)-4,5-dihydrooxazole (ButynOx), while the hydrophilic block is based on 2-methyl-2-oxazoline (MeOx). The block copolymer is terminated with tris(2-aminoethyl) amine (TREN) that serves as catalyst in a Knoevenagel condensation. Four block copolymers with different ButynOx/SPOx and hydrophilic/hydrophobic ratios are synthesized and self-assembled through solvent exchange. Micelles and vesicles of various sizes are observed by TEM, which undergo morphological and size changes in response to irradiation with UV light. We hypothesize that these transformations in the nanostructures are caused by increases in the hydrophilicity of the hydrophobic block when spiropyran (SP) isomerizes to merocyanine (MC) in the presence of UV light. The reversible transition from micellar to vesicular nanoreactors resulted in increased reaction kinetics through improved substrate accessibility to the catalytic site, or termination of the catalytic reaction due to polymer precipitation. These nanoreactors present a promising platform towards photoregulating reaction outcomes based on changes in nanostructure morphology.

Rh(I)-Catalyzed Coupling of Conjugated Enynones with Arylboronic Acids: Synthesis of Furyl-Containing Triarylmethanes.

Conjugated enynones can be used as carbene precursors to couple with arylboronic acids in the presence of Rh(I) catalyst. This reaction shows good functional compatibility, and a range of furyl-containing triarylmethanes can be smoothly synthesized from easily available starting materials under mild reaction conditions. Mechanistically, the formation of Rh(I) (2-furyl)carbene species and the subsequent carbene migratory insertion are proposed as the key steps in this reaction.

Palladium-catalyzed carbene migratory insertion using conjugated ene-yne-ketones as carbene precursors.

Palladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones lead to the formation of 2-alkenyl-substituted furans. This novel coupling reaction involves oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, ß-hydride elimination, and catalyst regeneration. Palladium (2-furyl)carbene is proposed as the key intermediate, which is supported by DFT calculations. The palladium carbene character of the key intermediate is validated by three aspects, including bond lengths, Wiberg bond order indices, and molecular orbitals, by comparison to those reported for stable palladium carbene species. Computational studies also revealed that the rate-limiting step is ene-yne-ketone cyclization, which leads to the formation of the palladium (2-furyl)carbene, while the subsequent carbene migratory insertion is a facile process with a low energy barrier (<5 kcal/mol).

Palladium-catalyzed diarylmethyl C(sp3)-C(sp2) bond formation: a new coupling approach toward triarylmethanes.

Palladium-catalyzed reductive coupling reactions between N-tosylhydrazones and aryl halides lead to the formation of C(sp(3))-C(sp(2)) bonds. This approach provides a general route for the synthesis of triarylmethanes.

Catalyst-free intramolecular formal carbon insertion into σ-C-C bonds: a new approach toward phenanthrols and naphthols.

The different reactivity of two kinds of carbonyl groups in keto aldehyde substrates has been exploited for the synthesis of phenanthrols, naphthols, and their heteroatom-containing analogues. Key to this highly efficient and robust methodology is the catalyst-free intramolecular formal diazo carbon insertion of N-tosylhydrazones into keto C-C bonds.