Bimolecular asymmetric photoreaction

Since the concept of topochemically controlled reactions was established, various approaches to asymmetric synthesis using a solid-state reaction have been attempted, most actively by the research group at the Weismann Institute. Their studies have been concerned with the bimolecular reactions of chiral crystals in the solid state. In these studies, successful absolute asymmetric synthesis has been performed by using topochemically controlled four-centered photocyclodimerizations of a series of unsymmetrically substituted diolefin crystals. Research on reactivity in the crystalline state has been extended in recent years to a variety of new systems, and many absolute asymmetric syntheses have been provided. Successful examples of absolute asymmetric synthesis using chiral crystals are listed in Tables 2 to 4, which are divided into three categories intermolecular photoreaction in the solid state (Table 2), intramolecular photoreaction in the solid state (Table 3, A-D), and asymmetric induction in the solid-gas and homogeneous reactions (Table 4). 

The importance of asymmetric synthesis in organic technology was emphasized in Chapter 9. It is also possible to introduce chirality through a photochemical reaction by transferring it from a chiral auxiliary attached to the reacting molecule. If a bimolecular photoreaction is involved, the chiral auxiliary may be attached to any one of the reactants. Examples of such asymmetric induction are cyclobutanes, oxetanes, and cyclohexenes by [4 + 2] photocycloaddition (Pfoertner, 1990). 

A variety of bimolecular photoreactions that are suppressed in the solution phase proceed smoothly and selectively upon complexation with supramolecular hosts. Intermolecular photochemical reactions promoted by an achiral host or template, such as crown ether [74], cucurbituril [75,76] as well as self-assembled coordination cages [77], can occur with remarkable efficiency and chemoselectivity, for which the close proximity and the highly regulated orientation and special arrangement between substrates in the supramolecular aggregate are jointly responsible. The main advantage of CDs over the above-mentioned hosts is the inherent chirality, which makes them good cradles for asymmetric induction in bimolecular photoreactions. 

---