Intramolecular photoreactions

The most important monomolecular photoreactions are photodissociation and photoisomerization. The former leads to a fragmentation in a molecular species by a photon of sufficient energy to rupture a bond, which may be of homo- or heterolytic character. The light-induced bond cleavage is often called photolysis, especially when flash techniques are used (flash photolysis). 

In more complex or large systems intramolecular redox photoreactions are also observed this pathway can be illustrated by photoinduced electron transfer in the compound containing Cu11 and MoIV centres connected by the CN bridges 

The photoinduced electron transfer (PET) is especially important in the case of large or giant molecules (supermolecules), ie systems made up of molecular components in the same way as molecules are made up of atoms [11-19], As the systems are made up of a number of discrete components held together by different but not always exactly specified forces (covalent bonds, electrostatic interactions, hydrogen bonds, or other intermolecular interactions), the photoinduced electron transfer or energy transfer in these systems may be formally treated as intermolecular [20], 

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Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Intermolecular addition of photochemically generated nitrenes and in particular acylnitrenes to alkenes provides a useful and widely applied route to aziridines.385 An analogous intramolecular photoreaction is thought to be involved in the conversion of the o-azidophenylethylfuran 461 into the pyrrolo[l,2-a]quinoline 462 as outlined in Scheme 13,386 and intramolecular addition to an azo group has been observed in the 8-azido-1-arylazonaphthalenes 463.387... 

The photochemistry of imides, especially of the N-substituted phthalimides, has been studied intensively by several research groups during the last two decades [233-235]. It has been shown that the determining step in inter- and intramolecular photoreactions of phthalimides with various electron donors is the electron transfer process. In terms of a rapid proton transfer from the intermediate radical cation to the phthalimide moieties the photocyclization can also be rationalized via a charge transfer complex in the excited state. 

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 host-guest and ionic chiral auxiliary approaches have been most intensively applied for solid-state asymmetric induction. A number of achiral organic compounds could be converted into chiral compounds in high enantioselectivities. However, all the photoreactions in themselves are well-known intramolecular photoreactions photocyclization, [2 + 2] photocyclization, Norrish type II photo-cyclization, di-ir-methane photorearrangement and photoisomerization. New types of asymmetric photoreactions have never been reported. 

Intramolecular photoreaction of dithioimides of type 147 yielded substituted azocanes 148 in low yields (Scheme 61 <2000H(53)2781 . 

Early reports by Kossanyi and coworkers illustrate the variety of strained products that can be obtained from intramolecular photoreactions. For example, irradiation of 2-allylcyclohexanone gave a mixture of (186 32%), (188 22%) and (189 14%), the latter two arising presumably from decomposition of the strained tricyclic oxetane (187). In a related azulene synthesis,Kossanyi obtained high yields of oxetanes (190) and (191), the latter comprising 80% of the reaction mixture. When thermolyzed, compound (191) formed a mixture of isomeric dihydroazulene compounds that could be dehydrogenated (Pd/AhOs) to form azulenes. 

In the intramolecular photoreaction of l-nitro-8-alkynylnaphthalcnes (equation 99) the product (34) is formally analogous to that obtained by cycloaddition of an... 

A higher chemoselectivity was obtained in the intramolecular photoreaction of a-silylamines (Scheme 59). Irradiation of 246 and 249 in methanol leads to the cyclized non-TMS products 247 and 250, respectively, whereas, TMS containing... 

The course of the intramolecular photoreaction of carbonyl compounds with electron-rich alkenyl- or aryl-substituents in the side-chain is dictated essentially by the thermodynamics of the electron-transfer step. This relationship has been intensively studied for phthalimides. When AG°et is positive, [n 2 + 2] cycloaddition reactions were observed with alkenyl substituents and classical Norrish II chemistry for aryl-substituted substrates. When AG°et was negative, electron transfer prod-... 

A recent investigation of the intramolecular photoreactions of 2-pentanone, 2-octanone and 5-methyl 2-hexanone in solution indicated that the intramolecular abstraction of a tertiary hydrogen atom from the y-position is faster than that of a secondary one, which in turn is faster than the abstraction of a primary H atom. This was found to be equally valid for the reactions of the triplet and the excited singlet states. 

Much information about the detailed mechanism of anthracene dimerization was gained in the study of intramolecular photoreactions of linked anthracenes such as a,(d-bis(9-anthryl)aikanes (66). It was shown that luminescence and cycloaddition are competing pathways for the deactivation of excimers. In compounds with sterically demanding substituents R and R that impair the cycloaddition reaction, the radiative deactivation is enhanced (H.-D. Becker, 1982). 

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

Intramolecular Photoreactions of Carbonyl Compounds and Related Processes. The intramolecular photoreactivity of aromatic ketones is subjected to major changes when reactions are carried out in the presence of CD. The formation of inclusion complexes was detected by NMR spectroscopy and X-ray powder diffractometry. 

Takechi H, Machida M et al (1988) Photochemistry of the phthalimide system. XLI. Intramolecular photoreactions of phthalimide-alkene systems. Oxetane formation of N-(w-indol-3-ylalkyl)phthalimides. Chem Pharm Bull 36 2853-2863... 

Since guest molecules are arranged close together and in one conformer in a host-guest complex, inter- and intramolecular photoreactions of the guest compound would proceed regio- and stereo-selectively and efficiently by irradiation of the complex in the solid state. When an optically active host compound is used, enantio-control of the reaction is expected. When the complexation is applied to an equilibrium mixture of tautomers, one labile tautomer can be isolated as a host-guest complex. By an irradiation of the complex in the solid state, stereo- and enantio-controls of the reaction of the labile tautomer can be achieved. 

Organic photochemistry has had an impact in almost every area of chemistry. Only limited data are available in the metal 7c-complex field the subject is nonetheless potentially stimulating. Inter- and intramolecular photoreactions apparently involve preformed metal olefin 7c-complexes. 

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. 

Shimo, X, Yasuda, M., Xajima, J. and Somekawa, K., Intramolecular photoreactions of 4-((o-aIkeny-loxy)-6-methyl-2-pyrones, J. Heterocyclic Chem., 28,745, 1991. 

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