inherently chiral cavity, which serves as a built-in source of chirality without
incorporating additional chiral auxiliary; (3) the ready and wide-ranging modifiability, which enables us to significantly alter the size, shape, hydrophobicity, and chiral
properties of the cavity; and (4) the environmentally benign approach, which
employs the biodegradable glucose-based macrocyclic hosts, aqueous media, and
photon as a traceless reagent. By virtue of these features, CDs have widely been
exploited as chiral supramolecular hosts for mediating various photochirogenic
reactions for three decades. In this chapter, we will review the studies on chiral
photochemical reactions mediated by CD hosts, mainly focusing on those reported in
the last decade; for earlier studies, see the previous reviews [13].
The main goal of supramolecular photochirogenesis is to transfer the host chirality to a guest substrate residing in the cavity through supramolecular interactions. In
this regard, the use of chiral hosts is theoretically triply beneficial, allowing the
thermodynamic chiral recognition upon complexation of prochiral substrate(s) in the
ground state, the spectroscopic differentiation of the resulting diastereomeric hostguest complex pair upon photoexcitation, and the kinetic stereocontrol of the
subsequent reaction in the excited state. To optimize the photochirogenic performance and gain upmost stereochemical outcomes, the amount of free substrate
remaining in the bulk solution should be minimized, as the photoreaction occurred
outside the host cavity inevitably affords a racemic product. However, the complexation of organic guests with CD hosts, relying mostly on the hydrophobic interaction, is generally rather modest with the association constants ranging from 10 to
10
5 M
À1 [13]. In order to supplement the modest binding affinity and to optimize the
stereochemical outcome, an excess amount of host is often employed in the
CD-mediated supramolecular photochirogenesis. This may however hinder the
catalytic use of CD host at least in the direct excitation strategy if no invention is
exploited to discourage the photoreaction outside the cavity.
Chiral photochemical reactions mediated by CDs can be driven by direct excitation and also by photosensitization. The direct excitation of a UV/vis-absorbing
substrate accommodated in CD cavity is more straightforward and frequently
employed, whereas the photosensitization strategy necessitates chromophoremodified CD as a sensitizing host but is intrinsically catalytic, as only the guest
substrate accommodated in the host cavity undergoes the highly efficient intracomplex energy or electron transfer leading to chiral product. We will discuss the
former type of unimolecular photochirogenic reactions in section “CD-Mediated
Unimolecular Photochirogenic Reactions” and bimolecular ones in section “CDMediated Bimolecular Chiral Photochemical Reactions,” while the latter type of
unimolecular reactions will be discussed in section “Catalytic Supramolecular
Photochirogenesis with Sensitizing CD Hosts.”
18.2 CD-Mediated Unimolecular Photochirogenic Reactions
Figure 2 summarizes the CD-mediated unimolecular chiral photoreactions reported
in recent years [14–26].
18 Supramolecular Photochirogenesis with Cyclodextrin
511
incorporating additional chiral auxiliary; (3) the ready and wide-ranging modifiability, which enables us to significantly alter the size, shape, hydrophobicity, and chiral
properties of the cavity; and (4) the environmentally benign approach, which
employs the biodegradable glucose-based macrocyclic hosts, aqueous media, and
photon as a traceless reagent. By virtue of these features, CDs have widely been
exploited as chiral supramolecular hosts for mediating various photochirogenic
reactions for three decades. In this chapter, we will review the studies on chiral
photochemical reactions mediated by CD hosts, mainly focusing on those reported in
the last decade; for earlier studies, see the previous reviews [13].
The main goal of supramolecular photochirogenesis is to transfer the host chirality to a guest substrate residing in the cavity through supramolecular interactions. In
this regard, the use of chiral hosts is theoretically triply beneficial, allowing the
thermodynamic chiral recognition upon complexation of prochiral substrate(s) in the
ground state, the spectroscopic differentiation of the resulting diastereomeric hostguest complex pair upon photoexcitation, and the kinetic stereocontrol of the
subsequent reaction in the excited state. To optimize the photochirogenic performance and gain upmost stereochemical outcomes, the amount of free substrate
remaining in the bulk solution should be minimized, as the photoreaction occurred
outside the host cavity inevitably affords a racemic product. However, the complexation of organic guests with CD hosts, relying mostly on the hydrophobic interaction, is generally rather modest with the association constants ranging from 10 to
10
5 M
À1 [13]. In order to supplement the modest binding affinity and to optimize the
stereochemical outcome, an excess amount of host is often employed in the
CD-mediated supramolecular photochirogenesis. This may however hinder the
catalytic use of CD host at least in the direct excitation strategy if no invention is
exploited to discourage the photoreaction outside the cavity.
Chiral photochemical reactions mediated by CDs can be driven by direct excitation and also by photosensitization. The direct excitation of a UV/vis-absorbing
substrate accommodated in CD cavity is more straightforward and frequently
employed, whereas the photosensitization strategy necessitates chromophoremodified CD as a sensitizing host but is intrinsically catalytic, as only the guest
substrate accommodated in the host cavity undergoes the highly efficient intracomplex energy or electron transfer leading to chiral product. We will discuss the
former type of unimolecular photochirogenic reactions in section “CD-Mediated
Unimolecular Photochirogenic Reactions” and bimolecular ones in section “CDMediated Bimolecular Chiral Photochemical Reactions,” while the latter type of
unimolecular reactions will be discussed in section “Catalytic Supramolecular
Photochirogenesis with Sensitizing CD Hosts.”
18.2 CD-Mediated Unimolecular Photochirogenic Reactions
Figure 2 summarizes the CD-mediated unimolecular chiral photoreactions reported
in recent years [14–26].
18 Supramolecular Photochirogenesis with Cyclodextrin
511