Pilar García-García

MOF catalysis in relation to their homogeneous counterparts and conventional solid catalysts

Metal–organic frameworks have recently attracted attention as heterogeneous catalysts due to their high content of metal centres and large surface area and pore volume, along with their impressive topological richness. Therefore, many studies describing the use of MOFs as heterogeneous Lewis acid catalysts have been published. In this regard, these efforts have been directed towards probing the catalytic activity. Further information is required in terms of kinetic parameters and comparison of performance with their homogeneous counterparts, or other conventional heterogeneous catalysts. Here we have attempted to put MOFs into perspective with respect to their homogeneous counterparts and more conventional heterogeneous catalysts to show their advantages and limitations. We have exemplified a number of reactions reported in the literature wherein MOFs have been used as catalysts, and we have carried them out using homogeneous counterparts, i.e. benzoates and acetates, and other well defined conventional solid catalysts. The activities and selectivities of the catalysts are compared and then put into perspective on the basis of kinetic parameters, such as turnover numbers and turnover frequencies. Additionally, we illustrate using selected examples what the potential advantages of MOF catalysts could be, and how they may outperform the potential of other solid catalysts.

Multisite Organic–Inorganic Hybrid Catalysts for the Direct Sustainable Synthesis of GABAergic Drugs

Multisite organic-inorganic hybrid catalysts have been prepared and applied in a new general, practical, and sustainable synthetic procedure toward industrially relevant GABA derivatives. The domino sequence is composed of seven chemical transformations which are performed in two one-pot reactions. The method produces both enantiomeric forms of the product in high enantiopurity as well as the racemate in good yields after a single column purification step. This protocol highlights major process intensification, catalyst recyclability, and low waste generation.

In situ preparation of a multifunctional chiral hybrid organic-inorganic catalyst for asymmetric multicomponent reactions

A chiral mesoporous organosilica material incorporating a urea based-cinchona derivative and propylamine groups was prepared by a co-condensation method. The multisite solid catalyst efficiently promoted the asymmetric multicomponent reaction of aldehydes, malonates and nitromethane.

Enantioselective Organocatalytic Reactions with Isatin

The authors gratefully acknowledge FSE, MICINN CTQ2009-11172BQU, Junta de Castilla and Leon (SA162A12-1) for financial support. MFF is grateful to the JCyL-ESF for her FPI fellowship. JP is grateful to MINECO for a FPU fellowship. P.G-G. is grateful for a JAE-DOC contract from CSIC co-funded by the European Social Fund (ESF)

Organic–inorganic supramolecular solid catalyst boosts organic reactions in water

Coordination polymers and metal-organic frameworks are appealing as synthetic hosts for mediating chemical reactions. Here we report the preparation of a mesoscopic metal-organic structure based on single-layer assembly of aluminium chains and organic alkylaryl spacers. The material markedly accelerates condensation reactions in water in the absence of acid or base catalyst, as well as organocatalytic Michael-type reactions that also show superior enantioselectivity when comparing with the host-free transformation. The mesoscopic phase of the solid allows for easy diffusion of products and the catalytic solid is recycled and reused. Saturation transfer difference and two-dimensional 1H nuclear Overhauser effect NOESY NMR spectroscopy show that non-covalent interactions are operative in these host–guest systems that account for substrate activation. The mesoscopic character of the host, its hydrophobicity and chemical stability in water, launch this material as a highly attractive supramolecular catalyst to facilitate (asymmetric) transformations under more environmentally friendly conditions.

Domino Elimination/Nucleophilic Addition in the Synthesis of Chiral Pyrrolidines

Polyhydroxylated pyrrolidines have been synthesized in a one-pot procedure by the addition of an organometallic reagent to isoxazolidines obtained by a 1,3-dipolar cycloaddition between nitrones and vinylsulfones. This method highlights sulfone reactivity and provides an easy approach for the preparation of chiral pyrrolidines using cyclic imines as key intermediates.

Catalytic Transfer Hydrogenation of Biomass-Derived Carbonyls over Hafnium-Based Metal–Organic Frameworks

A series of highly crystalline, porous, hafnium-based metal-organic frameworks (Hf-MOFs) have been shown to catalyze the transfer hydrogenation reaction of levulinic ester to produce γ-valerolactone by using isopropanol as a hydrogen donor. The results are compared with their zirconium-based counterparts. The role of the metal center in Hf-MOFs has been identified and reaction parameters optimized. NMR studies using isotopically labeled isopropanol provide evidence that the transfer hydrogenation occurs through a direct intermolecular hydrogen transfer route. The catalyst, Hf-MOF-808, can be recycled several times with only a minor decrease in catalytic activity. The generality of the procedure has been demonstrated by accomplishing the transformation with aldehydes, ketones, and α,β-unsaturated carbonyl compounds. The combination of Hf-MOF-808 with the Brønsted-acidic Al-Beta zeolite gives the four-step one-pot transformation of furfural to γ-valerolactone in good yield of 75 %.

Remarkable Acceleration of Benzimidazole Synthesis and Cyanosilylation Reactions in a Supramolecular Solid Catalyst

A solid metal–organic catalyst with hydrophobic pockets and Lewis acid centers strongly accelerates the reaction rate for organic reactions. This is exemplified for the cyanosilylation of ketones and for the synthesis of benzimidazoles, for which very high selectivities are obtained. The solid can be recovered and reused and its behavior approaches that of a functional enzyme mimic.

Chapter 2:Base Catalysis in Nonasymmetric Synthesis

This chapter focuses on the use of achiral bases such as alkyl or aryl amines, pyridine/imidazole derivatives and amidines, isothioureas and guanidines to promote a variety of reactions such as acylations, aldols, cycloadditions and Baylis–Hillman reactions. The aim of this chapter is to show the possibilities of Lewis-base catalysed reactions for the synthesis of very interesting nonasymmetric compounds. It also includes recently developed reactions using proline (and other chiral bases) as catalysts for reactions, which produce achiral products. The last part of this chapter deals with the use of nitrogen-heterocyclic carbenes (NHC) as catalytic systems for the synthesis of nonchiral compounds.

From isoxazolidines to tetrahydro-1,3-oxazines for the synthesis of chiral pyrrolidines

A novel approach for the synthesis of chiral tetrasubstituted pyrrolidines has been developed. The rearrangement of isoxazolidines into tetrahydro-1,3-oxazines using reactive organic bromides is herein described for the first time. The subsequent opening reaction of these tetrahydro-1,3-oxazines with nucleophiles probes the usefulness of the method for the synthesis of biologically active compounds.