478
K. Tanaka
product purification, and catalyst reusability. Because of the numerous benefits of
homochiral MOF catalysts, a variety of chiral MOFs have been designed and investigated as asymmetric catalysts. A large number of chiral stationary phases based
on crown ethers, polysaccharides, and cyclodextrins have been reported for separation of enantiomers. In addition, chiral MOFs show great potential as the chiral
stationary phase for enantiomer separation by HPLC. The advantages of chiral MOFs
over other chiral stationary phases are their well-ordered frameworks with defined
chiral pores that are accessible to potential substrates and their controllable functionalities by varying the chiral linkers and metal ions. This review highlights the
recent successful applications of chiral MOFs in the fields of asymmetric catalysis
and chromatographic enantiomer separation from 2012 until the present.
23.2 Asymmetric Catalysis
23.2.1 Ring-Opening Reaction of Meso-Epoxides
with Aromatic Amines
In 2008, Tanaka and co-workers [4] reported synthesis of (R)-MOF-1 by
treating (R)-2,2’-dihydroxyl-1,1’-binaphthalene-5,5’-dicarboxylic acid 1 with
Cu(NO 3 ) 2 under solvothermal conditions. The resulting two-dimensional infinite [Cu 2 (1) 2 (H 2 O) 2 ]·MeOH·H 2 O framework contains copper paddle wheels as
secondary building units (SBUs) stacked along the b axis with an interlayer Cu–
Cu distance of 15.6 Å (Fig. 23.1). Using (R)-MOF-1 as a catalyst for asymmetric
ring-opening reactions of cyclohexene oxide with some aniline derivatives to furnish
the corresponding optically active β-amino alcohols resulted in good yields with
excellent enantioselectivities (Table 23.1) [5]. Hirao et al. [6] reported the mechanism of this catalytic reaction by QM/MM computational study. The QM/MM
calculations showed that the reaction consists of two major steps. In the first step,
ring-opening of the epoxide occurs to form an intermediate with an alkoxide ion,
and the strong binding of the alkoxide ion to the Cu(II) center results in cleavage
of one of the four coordination bonds of the copper with carboxylate ligands. In
the second step, proton transfer occurs from the aniline to the alkoxide oxygen
atom to form the β-amino alcohol. The transition state for the ring-opening step in
formation of the (R,R)-isomer is stabilized by CH–π interactions, whereas such
interactions are absent in the transition state for formation of the (S,S)-isomer.
Tanaka et al. [5] synthesized (R)-MOF-2 by solvothermal reaction of the chiral
organic linker (R)-2,2’-diethoxy-1,1’-binaphthyl-4,4’-di(5-isophthalic acid) 2 and
Zn(NO 3 ) 2 ·6H 2 O in a mixed solvent (N,N-dimethylacetamide–EtOH) at 110°C for
2 days. X-ray diffraction analysis revealed that a rigid MOF was constructed around
two pentacoordinated Zn(II) cations with a nearest Zn···Zn distance of 3.384(2)
Å (Fig. 23.2), which were bridged by three carboxyl groups from three different
K. Tanaka
product purification, and catalyst reusability. Because of the numerous benefits of
homochiral MOF catalysts, a variety of chiral MOFs have been designed and investigated as asymmetric catalysts. A large number of chiral stationary phases based
on crown ethers, polysaccharides, and cyclodextrins have been reported for separation of enantiomers. In addition, chiral MOFs show great potential as the chiral
stationary phase for enantiomer separation by HPLC. The advantages of chiral MOFs
over other chiral stationary phases are their well-ordered frameworks with defined
chiral pores that are accessible to potential substrates and their controllable functionalities by varying the chiral linkers and metal ions. This review highlights the
recent successful applications of chiral MOFs in the fields of asymmetric catalysis
and chromatographic enantiomer separation from 2012 until the present.
23.2 Asymmetric Catalysis
23.2.1 Ring-Opening Reaction of Meso-Epoxides
with Aromatic Amines
In 2008, Tanaka and co-workers [4] reported synthesis of (R)-MOF-1 by
treating (R)-2,2’-dihydroxyl-1,1’-binaphthalene-5,5’-dicarboxylic acid 1 with
Cu(NO 3 ) 2 under solvothermal conditions. The resulting two-dimensional infinite [Cu 2 (1) 2 (H 2 O) 2 ]·MeOH·H 2 O framework contains copper paddle wheels as
secondary building units (SBUs) stacked along the b axis with an interlayer Cu–
Cu distance of 15.6 Å (Fig. 23.1). Using (R)-MOF-1 as a catalyst for asymmetric
ring-opening reactions of cyclohexene oxide with some aniline derivatives to furnish
the corresponding optically active β-amino alcohols resulted in good yields with
excellent enantioselectivities (Table 23.1) [5]. Hirao et al. [6] reported the mechanism of this catalytic reaction by QM/MM computational study. The QM/MM
calculations showed that the reaction consists of two major steps. In the first step,
ring-opening of the epoxide occurs to form an intermediate with an alkoxide ion,
and the strong binding of the alkoxide ion to the Cu(II) center results in cleavage
of one of the four coordination bonds of the copper with carboxylate ligands. In
the second step, proton transfer occurs from the aniline to the alkoxide oxygen
atom to form the β-amino alcohol. The transition state for the ring-opening step in
formation of the (R,R)-isomer is stabilized by CH–π interactions, whereas such
interactions are absent in the transition state for formation of the (S,S)-isomer.
Tanaka et al. [5] synthesized (R)-MOF-2 by solvothermal reaction of the chiral
organic linker (R)-2,2’-diethoxy-1,1’-binaphthyl-4,4’-di(5-isophthalic acid) 2 and
Zn(NO 3 ) 2 ·6H 2 O in a mixed solvent (N,N-dimethylacetamide–EtOH) at 110°C for
2 days. X-ray diffraction analysis revealed that a rigid MOF was constructed around
two pentacoordinated Zn(II) cations with a nearest Zn···Zn distance of 3.384(2)
Å (Fig. 23.2), which were bridged by three carboxyl groups from three different