Abstract
Metal–organic frameworks (MOFs) have become one of the versatile solid materials used for a wide range of applications, such as gas storage, gas separation, proton conductivity, sensors and catalysis. Among these fields, one of the more well-studied areas is the use of MOFs as heterogeneous catalysts for a broad range of organic reactions. In the present review, the employment of MOFs as solid catalysts for the Henry reaction is discussed, and the available literature data from the last decade are grouped. The review is organized with a brief introduction of the importance of Henry reactions and structural properties of MOFs that are suitable for catalysis. The second part of the review discusses the use of MOFs as solid catalysts for the Henry reaction involving metal nodes as active sites, while the third section provides data utilizing basic sites (primary amine, secondary amine, amides and urea-donating sites). While commenting on the catalytic results in these two sections, the advantage of MOFs over other solid catalysts is compared in terms of activity by providing turnover number (TON) values and the structural stability of MOFs during the course of the reaction. The final section provides our views on further directions in this field.
Highlights
Among the synthetic methodologies available for creating C-C bonds in organic molecules, acid-base catalysed aldol condensations are traditionally the most widely employed
The Henry reaction consists of the nitroaldol condensation between a nitroalkane, with a α-H to the nitro group, and a carbonyl group, usually an aldehyde (Scheme 1)
The urea containing the V-shaped dicarboxylate linker reacted with Cu(II) to generate 2D Metal–organic frameworks (MOFs) (Figure 5) with catalytic activity for C–C-bond-forming reactions, such as the Henry reaction between benzaldehyde and nitromethane in methanol at 60 ◦C, resulting in a turnover number (TON) of 55 and a TOF of 2.3 h−1
Summary
Among the synthetic methodologies available for creating C-C bonds in organic molecules, acid-base catalysed aldol condensations are traditionally the most widely employed. The nitroalcohol formed during the C-C coupling usually dehydrates into the nitroalkene, depending on the catalyst and reaction conditions (basicity, solvent, temperature, etc.). This reaction originally employed stoichiometric amounts of inorganic bases, such as hydroxides or hydrogen carbonates [1], which present important drawbacks associated with: (i) the formation of stoichiometric amounts of salts, (ii) the impossibility of recovering and recycling the base, and (iii) the tedious work-up with large amounts of organic solvents in order to isolate the product. The activity of [{Cd2(Lglu)2(bpe)3(H2O)}·2H2O] was assessed in the Henry reaction of benzaldehyde with nitroethane using 10 mol% of Zn(II) salt as a co-catalyst to obtain the corresponding βnitroalcohol in 89.3% yield after 72 h in methanol at room temperature. Powder XRD of the recycled solid matched with the fresh solid, suggesting the retainment of structural integrity during the reaction
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