18.5 Conclusion and Future Perspective
We have reviewed and discussed the recent developments and achievements in the
supramolecular photochirogenesis employing native and modified cyclodextrins as
chiral hosts, which certainly provide us with valuable mechanistic insights and
practical tools for precisely controlling the diastereomeric complex formation and
the subsequent enantiodifferentiating photochemical transformation. Indeed, a near
perfect stereocontrol is achieved to yield a single enantiomeric photoproduct in some
specific cases, which is highly encouraging in further advancing the study but is not
always the case in reality. A difficulty in using CDs as chiral hosts lays in the
axisymmetric cavity of native CD with fairly smooth interior wall surface, where the
chirality transfer from CD to prochiral substrate guided by the nondirectional
hydrophobic interaction is not very stereoselective in general, often resulting in
modest results. However, the recent studies have revealed that the chemical modification of CD is highly effective in breaking or distorting the axisymmetric cavity of
native CD and thus improving the stereoselectivity of supramolecular photochemical
reaction. Experimentally, the fairly sophisticated tuning of the size, shape, flexibility,
and chiral environment of CD cavity is feasible to optimize the chemo-, regio-, and
stereoselectivities of chiral photoreactions, as discussed above.
Supramolecular photochirogenesis in the solid state, e.g., crystal and zeolite
supercage, may appear more straightforward to achieve a higher level of chiral
induction than that in solution (though accompanies low conversion and no host
turnover). This is true as far as a given pair of chiral host and prochiral guest
substrate forms a single diastereomeric complex that is photoreactive in the solid
state, which is however not always the case. In solution, the photoreaction of free
unbound substrate to racemic product as well as the conformational freedoms
remaining in the complexed substrate often deteriorates the stereochemical outcomes. Nevertheless, the undesirable photoreaction of free substrate in the bulk
solution can be suppressed by adding salts carrying heavy metal or spin (e.g., Cs
+
and Cu
2+ ) [27, 37], which quench the excited state by accelerating the intersystem
crossing through the spin-orbit coupling interaction. The conformational flexibility
of the substrate accommodated in chiral host is not necessarily a difficulty or
disadvantage but rather provides us with an indispensable handle to critically
manipulate and optimize the stereochemical outcomes by modifying the host structure and also by changing the external entropy-related factors (e.g., temperature,
solvent, and pressure), as discussed above [40, 53, 55, 61, 62]. In some extreme
cases, the antipodal products are obtained by manipulating the entropy-related
variant, while using the identical host [33, 40].
The supramolecular photochirogenesis through higher-order 2:2 complex, which
has been found for the photocyclodimerization of AC mediated not only by β-CD
[27] but also by chiral hydrogen-bonding template (2-prolinol) [70], provides a new
opportunity to precisely regio- and enantio-control the ground-state complexation
and the subsequent photoreaction. However, more striking is the fact that the higherorder complexation enforces the encapsulated ACs to exhibit a novel reactivity to
afford the otherwise-inaccessible photoproducts, which has never been realized in
530
J. Yao et al.
We have reviewed and discussed the recent developments and achievements in the
supramolecular photochirogenesis employing native and modified cyclodextrins as
chiral hosts, which certainly provide us with valuable mechanistic insights and
practical tools for precisely controlling the diastereomeric complex formation and
the subsequent enantiodifferentiating photochemical transformation. Indeed, a near
perfect stereocontrol is achieved to yield a single enantiomeric photoproduct in some
specific cases, which is highly encouraging in further advancing the study but is not
always the case in reality. A difficulty in using CDs as chiral hosts lays in the
axisymmetric cavity of native CD with fairly smooth interior wall surface, where the
chirality transfer from CD to prochiral substrate guided by the nondirectional
hydrophobic interaction is not very stereoselective in general, often resulting in
modest results. However, the recent studies have revealed that the chemical modification of CD is highly effective in breaking or distorting the axisymmetric cavity of
native CD and thus improving the stereoselectivity of supramolecular photochemical
reaction. Experimentally, the fairly sophisticated tuning of the size, shape, flexibility,
and chiral environment of CD cavity is feasible to optimize the chemo-, regio-, and
stereoselectivities of chiral photoreactions, as discussed above.
Supramolecular photochirogenesis in the solid state, e.g., crystal and zeolite
supercage, may appear more straightforward to achieve a higher level of chiral
induction than that in solution (though accompanies low conversion and no host
turnover). This is true as far as a given pair of chiral host and prochiral guest
substrate forms a single diastereomeric complex that is photoreactive in the solid
state, which is however not always the case. In solution, the photoreaction of free
unbound substrate to racemic product as well as the conformational freedoms
remaining in the complexed substrate often deteriorates the stereochemical outcomes. Nevertheless, the undesirable photoreaction of free substrate in the bulk
solution can be suppressed by adding salts carrying heavy metal or spin (e.g., Cs
+
and Cu
2+ ) [27, 37], which quench the excited state by accelerating the intersystem
crossing through the spin-orbit coupling interaction. The conformational flexibility
of the substrate accommodated in chiral host is not necessarily a difficulty or
disadvantage but rather provides us with an indispensable handle to critically
manipulate and optimize the stereochemical outcomes by modifying the host structure and also by changing the external entropy-related factors (e.g., temperature,
solvent, and pressure), as discussed above [40, 53, 55, 61, 62]. In some extreme
cases, the antipodal products are obtained by manipulating the entropy-related
variant, while using the identical host [33, 40].
The supramolecular photochirogenesis through higher-order 2:2 complex, which
has been found for the photocyclodimerization of AC mediated not only by β-CD
[27] but also by chiral hydrogen-bonding template (2-prolinol) [70], provides a new
opportunity to precisely regio- and enantio-control the ground-state complexation
and the subsequent photoreaction. However, more striking is the fact that the higherorder complexation enforces the encapsulated ACs to exhibit a novel reactivity to
afford the otherwise-inaccessible photoproducts, which has never been realized in
530
J. Yao et al.