Intermolecular photoreactions

The trans-to-cis photoisomerization of stilbene-like organic species can be carried out upon their coordination to the Re1 centre to form the fac-[Re(CO)3(NN)( rans-L)]+ complexes [26]. Another spectacular example of photoisomerization is generation of several different isomers by photochemical rearrangement of norbornadiene in the presence of different metal catalysts, such as Cu1 complexes or carbonyls of Fe, Ni, and Cr [27, 28]. 

The deactivation of an excited molecular entity through a non-radiative process can also occur as a result of an external environmental influence. A molecular entity that deactivates (quenches) an excited state of another molecular entity, by energy transfer, electron transfer, or a chemical mechanism is called a quencher [29]. Photophysical processes (energy or electron transfer) were described in Chapter 4 the present discussion is confined to the chemical consequences of quenching. 

The intermolecular systems represent situations in which excitations are used to promote reactions that are not possible for the ground electronic states. Their main types can be specified photosubstitution, photoaddition (photocycloaddition), 

Metal centred (MC) Ligand centred (LC) Charge transfer (CT) 

A polymerization process requiring a photon for the propagation step is called photopolymerization or photoinduced polymerization, when polymerization of a monomer by a free radical or ionic chain reaction is initiated by photoexcitation. 

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C-P4-C can undergo intra- and intermolecular photoreactions. Irradiation of <5 X 10 4 M C-P4-C in an organic solvent such as benzene results primarily in intramolecular syn head-to-tail cyclomer (Fig. 12). Upon irradiation of more concentrated solutions, a large amount of oligomeric material was formed. For example, at concentration of 1 X 10-3 M, -20% of oligomers were present in the product mixture. As observed in the case of the N-P -N, irradiation of 1 X 10 2 mol/g-film C-P4-C in LDPE resulted in conversion to intramolecular cyclomer. As in the case of irradiations in homogeneous solutions, only the syn head-to-tail... 

Intermolecular photoreaction of an aryl halide with another aromatic compound may lead to the formation of biaryls. In this section several examples of such reactions will be discussed. In some cases, information concerning the reaction mechanism is available but the depth to which mechanisms have been investigated varies greatly. In many cases aryl radicals formed by homolysis of the carbon-halogen bond are the reactive species. Such radicals may also be produced via electron transfer, followed by departure of halide anion. In some cases aryl cations have been proposed as intermediates. Intermolecular bond formation may also be preceded by charge transfer within an exciplex or by formation of radical ion pairs. 

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). 

We have used chiral cocrystals composed of two different molecules [10]. When one of the two components is chiral, a cocrystal of a chiral space group is necessarily obtained. Combination of an electron donor molecule with an electron acceptor molecule, or a photoinert molecule with a photosensitizer molecule, in a cocrystal can induce new photoreactivity as well as intermolecular photoreaction. 

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. 

In contrast to these efficient intramolecular reactions most intermolecular photoreactions involving acyclic imines proceed with very low quantum yields. This is probably due to the rapid deactivation of the excited imine chromophore by C-N bond rotation. Cyclic imines, however, which lack this possibility, can undergo photoaddition to alkenes. 

Equivalent intermolecular photoreactions have been reported. The approach is particularly useful in photoarylation 199-202 photoheteroarylation, and 2-(methylthio)-... 

Finally, it has been reported that the intermolecular photoreaction of aromatic aldehydes with indole 83 in the solid state leads to the formation of bis-indole... 

Takechi H, Machida M et al (1988) Photochemistry of the phthalimide system. XLII. Intermolecular photoreactions of phthalimide-alkene systems. Regio- and stereoselective oxetane formation from N-methylphthalimide and N-acetylindole derivatives. Chem Pharm Bull 36 3770-3779... 

Kim et al. reported that CB[8] could accommodate two aromatic guest molecules to form 1 2 host-guest complexes or 1 1 1 ternary complexes. The solubility of CB[8] is quite poor in water and organic solvents. Fortunately, by adding some metal cations, the solubility of CB[8] could be considerably increased in water. This observation enabled scientists to investigate intermolecular photoreactions in CB[8] cavity. 

Photochemistry of alkenes may be studied under two categories-(l) Intramolecular photoreactions and (2) Intermolecular photoreactions of Alkenes. The second is mainly the study of intermolecular addition. The photolysis products from an intermolecular reaction of an alkene depend on the nature of alkene, the number of sites of unsaturation in the molecule and their structural relationship to one another. While the intramolecular reactions of Alkenes is governed by life-time of excited states mainly. 

Numerous reviews on the photochemistry and photophysics of ketones [18,23,25,78,79] and azoalkanes [38,43,80,81] are already available. This chapter focuses on intermolecular photoreactions of azoalkanes, which are compared with known data for ketones (see Structure 3.1). Unimolecular reactions such as the Norrish type-1 a-cleavage reaction of ketones [17,82-87] and their Norrish type-II reactions [20,24,73,83,88-91] as well as denitrogenation [8,9,43,68,92,93] and cis-trans isomerization of azoalkanes [43,92,93] are not discussed. The anphasis lies, besides data compilation, on mechanistic understanding, such that classical applications of azo compounds as dyes [94] or more recent apphcations of azo compounds in photochromic materials [10-12], or ketones as radical initiators in polymerization [95], are omitted as well. 

Quenching rate constants of representative n,jt -excited stales are compiled in Table 3.4 through Table 3.12. The focus of the discussion is on the intermolecular photoreactivity of n,n -excited azoalkanes, using DBO and DBH-T as representative singlet- and triplet-excited states. Different classes of quenchers are discussed first, including olefins and dienes, alcohols and ethers, sulfides, amines, and aromatic compounds. The photoreactivity of azoalkanes is then compared with that of representative n,7t -excited ketones, i.e., acetone, benzophenone and biacetyl. Furthermore, alkoxyl radicals as ground stale models of n,7t -excited states are... 

Garcia, H., Marttnez-UtriUa, R, Mirada, M. A., and Roquet-Jalmar, M. F., Intra- and intermolecular photoreactions of o-benzoyloxyacetophenone derivatives,/. Chem. Res., (S), 350,1982. 

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