Intermolecular asymmetric photoreaction

In comparison to the variety of intramolecular asymmetric photoreactions, inter-molecular asymmetric photoreactions have been scarcely reported. However, the reaction types do include [2 4- 2] photodimerization, photoaddition, photodecar-boxylative condensation, and photooxygenation. Herein the intermolecular reactions are more precisely described, including their reaction mechanisms and reaction paths in the crystals. 

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). 

When two different achiral molecules form a chiral cocrystal by spontaneous chiral cocrystallization, the occurrence of absolute asymmetric intermolecular photoreaction can be expected. In fact, we have achieved enantio- and diastereo-selective photodecarboxylative condensation, as well as absolute asymmetric pho-todecarboxylative condensation. The development of intermolecular photoreao tions leads to an extension of the scope of solid-state chiral photochemistry. Reactivity in a cocrystal is controlled by the crystal packing arrangement, so the key point is the preparation of photoreactive cocrystals. 

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. 

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