Asymmetric synthesis, MOFs

Ionic liquids (room-temperature molten salts) with chiral cations were efficiently used as reaction media to prepare homochiral MOFs [33], But the use of homochiral ligands as precursors in the synthesis is even more efficient For instance, Lin et al. [34] prepared MOFs with BINOL (l,l -bi-2,2 -naphthol) and BINAP (2,2 -his(diphenylphosphino)-1,1 -binaphthyl) as chiral linkers. Unfortunately, attempts to use such chiral catalysts in asymmetric catalysis showed some but not high enantiomeric excess in the studied reactions, perhaps the flexibility of the ligands in the MOF framework is not sufficient to induce the chirality effect in the reactions. The most illustrative example was presented hy Lin [35] the reaction of asymmetric hydrogenation of aromatic ketones demonstrated a 99.2% ee for Ru-BINAP/MOF systems. 

Nevertheless, photoactive MOFs also show unique photocatalytic properties that other materials cannot compete with, especially in organic synthesis applications. MOFs create the opportunity to combine photocatalyst with organocatalyst. One example is the chiral MOF, namely, Zn-PYIl, which exhibits high selectivity for photocatalytic asymmetric a-alkylation of aldehydes, as demonstrated in Fig. 4.13c. The Zn-PYll has also been synthesised via a PSM process of the parent MOF Zn-BClPl (top of Fig. 4.13c), which has been synthesised via solvothermal reaction from L-N-tert-butoxycarbonyl-2-(imidazole)-l-pyrrolidine (l-BCIP) [58]. The key point of the PSM process is the removal of the protective tert-butoxycarbonyl (Boc) moiety to expose active sites, which are likely to be the N — H of pyrrolidine of the L-BCIP molecules that is located within the channels according to dye adsorption test. This has been realised by microwave irradiation in dry lV,lV-dimethyl-formamide solution. The activated Zn-PYIl shows a high reaction efficiency (74 % in yield) and excellent enantioselectivity (92 % ee) in photocatalytic a-alkylation of aliphatic aldehydes compared to that of other MOFs. 

Scheme 10.6 Chemical structures of chiral ligands or chiral catalysts (Lj-Ljj) incorporated in to chiral MOFs through either direct incorporation or post-synthesis modification for asymmetric catalysis.

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