Mn-salen catalytically active moiety has been incorporated into dynamic covalent imine gel via the reaction of a chiral Mn-salen dialdehyde unit 61 with a
tetraamine linker (50) [49]. This Mn-salen imine gel is a good catalyst for asymmetric kinetic resolution of secondary alcohols. Moreover, the imine gel can be
coated onto a functionalized capillary and displayed improved catalytic ability.
After changing the –OH on the wall of fused-silica capillary to NH 2 , the gel is
readily immobilized on the fused-silica capillary reactor via in situ gelation
(Fig. 4.16). The assembled microfluidic gel reactor is proven effective for the
enantioselective kinetic resolution of secondary alcohols, giving good conversions
and enantiomeric excess (ee) within shorter reaction time (15 min) compared to the
batch process and can be reused at least eight times without loss of activity or
enantioselectivity (Scheme 4.18).
Metalloporphyrin imine gel Pd-tapp-50 is also supported on the inner surface of
a functionalized capillary and used to assemble catalytic gel capillary reactor [48]. It
achieves similar yields in a much shorter time in Suzuki–Miyaura cross-coupling of
4-bromoanisole and phenylboronic acid compared to the batch processes.
4.2.2 Calix[4]Arene-Derived Acylhydrazone Gels
Jaworski, Jung and co-workers reported dynamic covalent polymer gels based on
acylhydrazone bonding [50]. The gels are prepared from calix[4]arene derivative 62
bearing four hydrazide groups and diphenyl terephthalate-derivative 63 bearing two
aldehyde groups in DMSO via hydrazone linkage and non-covalent assembly like
H-bond on hydrogen-bonding sites (Scheme 4.19). This calix[4]arene-derivative
gels show ductile and high tensile strength. Reversible sol–gel transition suggests
that the resulting gel consists of discrete multimeric gelator species rather than
extended polymer structure. Interestingly, the mechanical and ductile properties of
the gels can be controlled by catalytic HCl. The tensile strength of the gel formed
without HCl approaches over 40 MPa. The most remarkable aspect of this study is
the mechanical property of the hydrogel. After the solvent DMSO was exchanged
with water, the resulting hydrogel showed 7000- to 100,000-fold enhanced
mechanical properties. The hydrazone reaction produces –OH groups under
non-acidic conditions or with addition of water, offering enhanced intermolecular
hydrogen-bonding interactions and thus increased elastic properties (Fig. 4.17).
This represents an interesting moulding method from small molecules to retain the
gel shape after processing. The gel also shows good ionic conductivities as gel
electrolytes.
Acylhydrazone gels have been developed based on calix[4]arene derivative 62
and photosensitive stilbene derivative 64 (Scheme 4.19) [51]. Calixarene-derived
networks can provide robust supramolecular gels, and introducing a photoreactive
stilbene moiety makes it possible that its aggregation state is tuned by heating and
UV exposure. [2+2] cycloaddition of stilbene moieties occurred for stilbene
H-aggregates after UV exposure result in the effect of their fluorescence by
4.2 Dynamic Covalent Polymer Gelators
145
tetraamine linker (50) [49]. This Mn-salen imine gel is a good catalyst for asymmetric kinetic resolution of secondary alcohols. Moreover, the imine gel can be
coated onto a functionalized capillary and displayed improved catalytic ability.
After changing the –OH on the wall of fused-silica capillary to NH 2 , the gel is
readily immobilized on the fused-silica capillary reactor via in situ gelation
(Fig. 4.16). The assembled microfluidic gel reactor is proven effective for the
enantioselective kinetic resolution of secondary alcohols, giving good conversions
and enantiomeric excess (ee) within shorter reaction time (15 min) compared to the
batch process and can be reused at least eight times without loss of activity or
enantioselectivity (Scheme 4.18).
Metalloporphyrin imine gel Pd-tapp-50 is also supported on the inner surface of
a functionalized capillary and used to assemble catalytic gel capillary reactor [48]. It
achieves similar yields in a much shorter time in Suzuki–Miyaura cross-coupling of
4-bromoanisole and phenylboronic acid compared to the batch processes.
4.2.2 Calix[4]Arene-Derived Acylhydrazone Gels
Jaworski, Jung and co-workers reported dynamic covalent polymer gels based on
acylhydrazone bonding [50]. The gels are prepared from calix[4]arene derivative 62
bearing four hydrazide groups and diphenyl terephthalate-derivative 63 bearing two
aldehyde groups in DMSO via hydrazone linkage and non-covalent assembly like
H-bond on hydrogen-bonding sites (Scheme 4.19). This calix[4]arene-derivative
gels show ductile and high tensile strength. Reversible sol–gel transition suggests
that the resulting gel consists of discrete multimeric gelator species rather than
extended polymer structure. Interestingly, the mechanical and ductile properties of
the gels can be controlled by catalytic HCl. The tensile strength of the gel formed
without HCl approaches over 40 MPa. The most remarkable aspect of this study is
the mechanical property of the hydrogel. After the solvent DMSO was exchanged
with water, the resulting hydrogel showed 7000- to 100,000-fold enhanced
mechanical properties. The hydrazone reaction produces –OH groups under
non-acidic conditions or with addition of water, offering enhanced intermolecular
hydrogen-bonding interactions and thus increased elastic properties (Fig. 4.17).
This represents an interesting moulding method from small molecules to retain the
gel shape after processing. The gel also shows good ionic conductivities as gel
electrolytes.
Acylhydrazone gels have been developed based on calix[4]arene derivative 62
and photosensitive stilbene derivative 64 (Scheme 4.19) [51]. Calixarene-derived
networks can provide robust supramolecular gels, and introducing a photoreactive
stilbene moiety makes it possible that its aggregation state is tuned by heating and
UV exposure. [2+2] cycloaddition of stilbene moieties occurred for stilbene
H-aggregates after UV exposure result in the effect of their fluorescence by
4.2 Dynamic Covalent Polymer Gelators
145