Photochemical reactions cycloaddition: aromatics

The major classes of photochemical reaction for aromatic compounds are nucleophilic substitution and a range of processes that lead to non-aromatic products—valence isomerization, addition or cycloaddition reactions, and cyclization involving 6-electron systems. These five general categories of reaction will be described in the following sections, together with a few examples of more specific processes. 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Three types of cycloaddition products are generally obtained from the photochemical reaction between aromatic compounds and alkenes (Scheme 31). While [2 + 2] (ortho) and [3 + 2] (meta) cycloaddition are frequently described, the [4 + 2] (para or photo-Diels-Alder reaction) pathway is rarely observed [81-83]. Starting from rather simple compounds, polycyclic products of high functionality are obtained in one step. With dissymmetric alkenes, several asymmetric carbons are created during the cycloaddition process. Since many of the resulting products are interesting intermediates for organic syntheses, it is particularly attractive to perform these reactions in a diastereoselective way. 

Among the photochemical reactions of aromatic compounds, the photocycloadditions are most frequently applied to the synthesis of complex polycyclic compounds [6, 9]. The [2+3] or meta photocycloaddition of aromatic compounds and alkenes is the most prominent example [10]. This transformation also demonstrates complementarities between photochemical and ground state reactions since such reactions are almost impossible using conventional activation. A [2+2] ot ortho photocycloaddition between carbocyclic aromatic compounds and alkenes is observed as well. It is often competitive with other cycloaddition modes, in particular the [2+3] mode [11]. Many of these reactions are reversible, and photostationary equilibria are involved. This reaction was much less applied to organic synthesis. Recently, it was found that an acidic reaction medium may have an influence on the outeome of the reaction. The intramolecular photocycloaddition of resorcinol derivatives such as 1 is difficult due to its reversibility (Scheme 29.1). However, in an acidic reaction medium, the cycloadducts 2a,b are protonated at the oxygen atom of the tetrahydrofuran moiety... 

This chapter deals with the main photochemical reactions of aromatic compounds, including photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement and related photoreactions. 

The irradiation of benzenes with alkenes provides a fascinating array of photochemical reactions, not least because it converts the aromatic substrates into polycyclic, non-aromatic products. In principle, benzene can undergo reaction across the 1,2-(ortho). 1,3-(meta), or 1,4-(para) positions the 1,3-cycloaddition is structurally the most complex, but it is the predominant mode of reaction for many of the simplest benzene/alkene systems. The products are tricyclic compounds with a fusion of two five-membered rings and one three-membered ring, and an example is the reaction of benzene with vinyl acetate (3.411. For monosubstituted benzenes there can be a high... 

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

The photochemical reaction of tetranitromethane with aromatic compounds leads in low yields to 1,3,2-dioxazoles, as products of the nitroalkene cycloaddition of the trinitromethyl intermediate <1995ACS482, 1996ACS29, 1996ACS735>. The cycloaddition products 22a and 22b (0.2-1.8%) were isolated from 1,3-dimethylnaphthalene <1995ACS482>, compound 23a (20%) from 1,2,3-trimethylbenzene <1996ACS29>, and compound 23b (3%) from 1,2,3,4-tetramethylbenzene <1996ACS735>. The structure of all these compounds has been proved by single crystal X-ray analysis (see Section 6.02.3.1). 

Consideration of the HOMO-LUMO interactions also indicates that the [2Tr+2Tr] addition is allowed photochemically. The HOMO in this case is the excited alkene tt orbital. The LUMO is the tt of the ground state alkene, and a bonding interaction is present between both pairs of carbons where new bonds must be formed. Similarly, the concept of aromatic transition states shows that the reaction has an antiaromatic 4tt combination of basis set orbitals, which predicts an allowed photochemical reaction. Thus, orbital symmetry considerations indicate that photochemical [2tt- -2tt] cycloaddition of alkenes is feasible. 

The photochemistry of aromatic compounds is classified into the same categories adopted in the previous reviews in the series. The photoisomerization of arylalkenes, photoaddition and cycloaddition to aromatic rings, photosubstitution, photorearrangement reactions have less appeared in the period (2010-2011) considered. On the other hand, the photo-chromism including photoisomerization of azobenzenes and intramolecular photocyclization and cycloreversion of 1,2-diarylethenes, and the photodimerization have been widely developed. Supramolecular Photochemistry as a series of Molecular and Supramolecular Photochemistry was edited by Ramamurthy and Inoue in the period. In addition, it should be noteworthy that so many photochemical reactions in solid and/or crystalline states have appeared and developed. 

Table 20.2 summarizes all An and 4m + 2 reactions. Other 4m processes will follow the rules for the 2 + 2 dimerization of a pair of alkenes, and 4m + 2 processes will resemble the 4 + 2 cycloaddition we know as the Diels-Alder reaction. Perhaps you can see the relationship to aromatidty (4m + 2) that plays a role in this analysis. The transition state for these cycloaddition reactions is cyclic and will be allowed only in the cases where the number of electrons makes the transition state aromatic, 4m + 2 electrons for thermal processes and 4m for photochemical reactions. 

The photochemical cycloadditions of alkenes and alkynes with aromatic compounds have received by far the most attention. Yields of [2+2] cydoadducts can be good, but reaction times are often long and secondary rearrangement products are common [139, 140, 141,142, 143,144, 145,146] (equations 63-65). The pioneering mechanistic and synthetic work on aromatic photocycloadditions has been reviewed [147],... 

Several examples of [5C+1S] cycloaddition reactions have been described involving in all cases a 1,3,5-metalahexatriene carbene complex as the C5-syn-thon and a CO or an isocyanide as the Cl-synthon. Thus,Merlic et al. described the photochemically driven benzannulation of dienylcarbene complexes to produce ortho alkoxyphenol derivatives when the reaction is performed under an atmosphere of CO, or ortho alkoxyanilines when the reaction is thermally performed in the presence of an isonitrile [111] (Scheme 63). In related works, Barluenga et al. carried out analogous reactions under thermal conditions [36a, c, 47a]. Interestingly, the dienylcarbene complexes are obtained in a first step by a [2+2] or a [3S+2C] process (see Sects. 2.3 and 2.5.1). Further reaction of these complexes with CO or an isonitrile leads to highly functionalised aromatic compounds (Scheme 63). 

Photocycloaddition of thiones to alkenes is the most popular and fruitful method for the preparation of the thietane system. In analogy to the formation of the oxetanes by cycloaddition of the electronic excited ( ,tc ) carbonyls, thietanes can be expected to arise photochemically from aromatic thioketones and substituted olefins as well as 1,2- and 1,3-dienes. ° Thiobenzophenone serves as a source of a sulfur atom and, because of its blue color, which disappears on photocycloaddition, permits exact control over the reaction time. A mixture of thiobenzophenone and a-phellandrene must be irradiated for 70 hr before the blue color disappears (Eq. 2) and... 

Trimethyloxazole 257 undergoes photochemically induced [2 + 2] cycloaddition with aromatic and aliphatic aldehydes to provide bicyclic oxazolines 258 with excellent regiochemical and stereochemical control. Diastereoselec-tivities from 75-99% can be achieved, which is the first reported example of a Paterno-Biichi reaction involving an oxazole. The oxetane cycloadducts can be hydrolyzed to a-amino-(3-hydroxy ketones. Other oxazoles have not been evaluated to determine if they undergo the photochemical cycloaddition (Scheme 8.71). 

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