Tandem Aldol Reaction from Acetal Mixtures by an Artificial Enzyme with Site-Isolated Acid and Base Functionalities

Design and engineering of microenvironments of supported catalysts toward more efficient chemical synthesis

Catalytic species such as molecular catalysts and metal catalysts are commonly attached to varieties of supports to simplify their separation and recovery and accommodate various reaction conditions. The physicochemical microenvironments surrounding catalytic species play an important role in catalytic performance, and the rational design and engineering of microenvironments can achieve more efficient chemical synthesis, leading to greener and more sustainable catalysis. In this review, we highlight recent works addressing the topic of the design and engineering of microenvironments of supported catalysts, including supported molecular catalysts and supported metal catalysts. Six types of materials, including oxide nano/microparticle, mesoporous silica nanoparticle (MSN), polymer nanomaterial, reticular material, zeolite, and carbon-based nanomaterial, are widely used as supports for the immobilization of catalytic species. We summarize and discuss the synthesis and modification of supports and the positive effects of microenvironments on catalytic properties such as metal-support interaction, molecular recognition, pseudo-solvent effect, regulating mass transfer, steric effect, etc. These design principles and engineering strategies allow access to a better understanding of structure-property relationships and advance the development of more efficient catalytic processes.

Improving the Selectivity of the C-C Coupled Product Electrosynthesis by Using Molecularly Imprinted Polymer─An Enhanced Route from Phenol to Biphenol

We developed a procedure for selective 2,4-dimethylphenol, DMPh, direct electro-oxidation to 3,3',5,5'-tetramethyl-2,2'-biphenol, TMBh, a C-C coupled product. For that, we used an electrode coated with a product-selective molecularly imprinted polymer (MIP). The procedure is reasonably selective toward TMBh without requiring harmful additives or elevated temperatures. The TMBh product itself was used as a template for imprinting. We followed the template interaction with various functional monomers (FMs) using density functional theory (DFT) simulations to select optimal FM. On this basis, we used a prepolymerization complex of TMBh with carboxyl-containing FM at a 1:2 TMBh-to-FM molar ratio for MIP fabrication. The template-FM interaction was also followed by using different spectroscopic techniques. Then, we prepared the MIP on the electrode surface in the form of a thin film by the potentiodynamic electropolymerization of the chosen complex and extracted the template. Afterward, we characterized the fabricated films by using electrochemistry, FTIR spectroscopy, and AFM, elucidating their composition and morphology. Ultimately, the DMPh electro-oxidation was performed on the MIP film-coated electrode to obtain the desired TMBh product. The electrosynthesis selectivity was much higher at the electrode coated with MIP film in comparison with the reference nonimprinted polymer (NIP) film-coated or bare electrodes, reaching 39% under optimized conditions. MIP film thickness and electrosynthesis parameters significantly affected the electrosynthesis yield and selectivity. At thicker films, the yield was higher at the expense of selectivity, while the electrosynthesis potential increase enhanced the TMBh product yield. Computer simulations of the imprinted cavity interaction with the substrate molecule demonstrated that the MIP cavity promoted direct coupling of the substrate to form the desired TMBh product.

Supramolecular Regulation of Catalytic Activity in Molecularly Responsive Catalysts

Some enzymes switch between an open form and a closed form. We report a molecularly tuned catalyst that accommodates a substrate and a signal molecule simultaneously. Binding of the signal molecule helps direct the reactive group of the substrate to the catalytic group and enhances the catalytic activity. Subtle structural changes in either the substrate or the signal molecule are readily detected. The switching mechanism also allows the catalytic reaction to be turned on and off reversibly by specific molecular signals.

Multistep Cascade Catalytic Reactions Employing Bifunctional Framework Compounds

Multistep cascade reactions are important to achieve atom as well as step economy over conventional synthesis. This approach, however, is limited due to the incompatibility of the available reactive centers in a catalyst. In the present study, new MOF compounds, [Zn₂(SDBA)(3-ATZ)₂]·solvent, I and II, with tetrahedral Zn centers as good Lewis acidic sites and the free amino group of the 3-amino triazole ligand as a strong Lewis base center were shown to perform 4-step cascade/tandem reaction in a facile manner. Effective conversion of benzaldehyde dimethyl acetal in the presence of excess nitromethane at 100 °C in water to 1-(1,3-dinitropropan-2-yl) benzene was achieved in 10 h with yields of ∼95% (I) and ∼94% (II). This 4-step cascade reaction proceeds via deacetalization (Lewis acid), Henry (Lewis base), and Michael (Lewis base) reactions. The present work highlights the importance of spatially separated functional groups in multistep tandem catalysis─the examples of which are not common.

Synergistic Hydrolysis of Cellulose by a Blend of Cellulase-Mimicking Polymeric Nanoparticle Catalysts

Enzyme-like catalysts by design have been a long sought-after goal of chemists but difficult to realize due to the challenges in the construction of multifunctionalized active sites with accurately positioned catalytic groups for complex substrates. Hydrolysis of cellulose is a key step in biomass utilization and requires multiple enzymes to work in concert to overcome the difficulty associated with hydrolyzing the recalcitrant substrate. We here report methods to construct synthetic versions of these enzymes through covalent molecular imprinting and strategic postmodification of the imprinted sites. The synthetic catalysts cleave a cellulose chain endolytically at multiple positions or exolytically from the nonreducing end by one or three glucose units at a time, all using the dicarboxylic acid motif found in natural cellulases. By mimicking the endocellulase, exocellulase, and β-glucosidase, the synthetic catalysts hydrolyze cellulose in a synergistic manner, with an activity at 90 °C in pH 6.5 buffer more than doubled that of Aspergillus niger cellulase at pH 5 and 37 °C and 44% of that of a commercial cellulase blend (from Novozyme). As robust cross-linked polymeric nanoparticles, the synthetic catalysts showed little changes in activity after preheating at 90 °C for 3 days and could be reused, maintaining 76% of activity after 10 reaction cycles.

Imprinted Polymeric Nanoparticles as Artificial Enzymes for Ester Hydrolysis at Room Temperature and pH 7

Natural esterases hydrolyze esters under physiological pHs but chemists often have to use strongly acidic or basic conditions for the same hydrolysis. We report synthetic nanoparticle catalysts that hydrolyze nonactivated alkyl esters at room temperature and neutral pH, with enzyme-like catalytic mechanisms and exquisite substrate selectivity. Unlike natural enzymes that denature easily at elevated temperatures, the synthetic catalysts become more active at higher temperatures.

Environmental Modulation of Chiral Prolinamide Catalysts for Stereodivergent Conjugate Addition

Synthetic chiral catalysts generally rely on proximal functional groups or ligands for chiral induction. Enzymes often employ environmental chirality to achieve stereoselectivity. Environmentally controlled catalysis has benefits such as size and shape selectivity but is underexplored by chemists. We here report molecularly imprinted nanoparticles (MINPs) that utilized their environmental chirality to either augment or reverse the intrinsic selectivity of a chiral prolinamide cofactor. The latter ability allowed the catalyst to produce products otherwise disfavored in the conjugate addition of aldehyde to nitroalkene. The catalysis occurred in water at room temperature and afforded γ-nitroaldehydes with excellent yields (up to 94%) and ee (>90% in most cases). Up to 25:1 syn/anti and 1:6 syn/anti ratios were achieved through a combination of catalyst-derived and environmentally enabled selectivity. The high enantioselectivity of the MINP also made it possible for racemic catalysts to perform asymmetric catalysis, with up to 80% ee for the conjugate addition.

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.