Photochemical cycloadditions with aromatic compounds

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],... 

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. 

Aromatic compounds undergo photochemical cycloaddition with a few classes of addend other than those that react by way of a... 

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). 

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. 

Photochemical cycloaddition of aromatic carbonyl compounds to vinylcyclo-propanes gives the products (761) and (762), and in some cases (763) is also formed by thermal or acid-catalysed decomposition of (761). The proportion of (762) obtained increases with the temperature at which the irradiation is conducted, and the results seem best accounted for by a mechanism in which the biradical derived by addition of the photoexcited triplet of the carbonyl compound to the vinylcyclopropane undergoes two competing processes. Compound (761) is formed by simple cyclization and... 

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. 

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). 

The photocycloadditions of readily accessible aromatic compounds lead to highly functionalyzed products in only one step. They can be used in a flexible manner for the synthesis of numerous target structures. The total synthesis of these products is considerably shortened and simplified by applying the photochemical cycloadditions as key steps which was particularly demonstrated with the transformation of many intramolecular meta cycloadducts [66]. 

Among these reactions, the photochemical cycloadditions of C=C bom which can create up to four asymmetric carbons during the photochemical sti are particularly interesting, and numerous synthetic applications of this react have been reported. Advances in the understanding of the origin of asymmefa induction, during addition of alkenes with carbonyl derivatives, cyclic enom and aromatic compounds, will be discussed in detail. 

Alkynes react photochemically with aromatic aldehydes or ketones to give a,p-unsaturated carbonyl compounds (equation 81). This occurs by way of cycloaddition to give an oxete, followed by thermal ring-opening of this intermediate, and the orientation of addition is in accord with a two-step cycloaddition via the more stable biradical intermediate (equation 82). 

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. ... 

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... 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

Alkynes react with the excited states of aromatic nitro compounds (equation 98) , perhaps in part by way of cycloaddition to give an unstable dioxazole by analogy with the photoaddition of nitrobenzene to alkenes. However, [2 + 2] cycloaddition is an attractive alternative, since photochemical transformation of nitrones (33) to amides via oxaziridines is well documented. 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

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