Cycloaddition Reactions of Aromatic Compounds

Photodimerizations are observed not only for olefins, but also for aromatic compounds, allenes, and acetylenes. The photodimerization of anthracene. 

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). 

Photocyclization of bis(9-anthryl)methane (67) and the corresponding photocycloreversion were shown by picosecond laser spectroscopy to have a common intermediate whose electronic structure is different in polar or nonpolar solvents as judged by different absorption spectra in various solvents (Manring et al., 1985). 

The [2 + 2] cyclodimerization of benzene has been studied theoretically (Engelke et al., 1984) and used practically in the synthesis of polycyclic hydrocarbons such as 69 (Fessner et al., 1983). This represents a first step in the synthesis of the undecacyclic hydrocarbon pagodane (70), which in turn can be isomerized to give dodecahedrane (Fessner et al., 1987). 

Mixed photocycloadditions of anthracene and conjugated polyenes yield products that correspond to a concerted reaction path, as well as others that are Woodward-Hoffmann-forbidden and presumably result from noncon-certed reactions. For example, the reaction of singlet-excited anthracene with 1,3-cyclohexadiene yields small quantities of the [ 4 +, 2J product 72 in addition to the allowed 4, +, 4J product 71. 

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Photo-Diels-Alder Cycloaddition Reactions of Aromatic Compounds... 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

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... 

The [2 + 2 + 2] cydotrimerization of alkynes and its analogous arynes [134] leads directly to aromatic hydrocarbons and can be considered as special cases of [2 + 2 + 2] cycloaddition reactions of unsaturated compounds [135, 136]. Especially the catalyzed cydotrimerization reactions are often highly regio- and stereoselective and have been established as valuable methods for the syntheses of highly substituted benzenes, biphenylenes, triphenylenes, and in the approach towards natural products. 

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

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... 

A (1/7-imidazol-l-yl) silver species (55) has been postulated as the key intermediate in the 3-l-2-cycloaddition reaction of diazoalkanes (54) with benzynes yielding 2-aryl-2H-indazoles (56) (Scheme 18). The 3-I-2-cycloaddition reaction of 3-trifluoromethyl-4-diazopyrazolinones with dialkyl acetylene dicarboxylates, in refluxing toluene, produced spiro 3/f-pyrazole adducts that rearranged to the trifluoromethyl-substituted pyrazolo[l,5-fi(][l,2,4]triazin-7-ones. ° The 1,3-dipolar cycloaddition reaction of aromatic thioketones (58) with 2-aza-1,3-dicarbonyl compounds (57), at 20-50 C, yielded thiadiazoline adducts (59) that readily eliminate nitrogen to produce oxathioles (60) in moderate yields (up to 70%) (Scheme 19). ... 

Diphenylcyclopropenone is the first stable molecule prepared which has a carbonyl group in a three-membered ring In a very real sense the compound has aromatic character and is fairly stable.4 An interesting cycloaddition reaction of enamincs with diphenylcyclopropenone has been reported.7... 

Dihydrovinylphenanthrenes are more reactive than the corresponding vinyl phenanthrenes and undergo Diels-Alder reactions easily. They have been used in the synthesis of polycyclic aromatic compounds and helicenes. Examples of cycloaddition reactions of the 3,4-dihydro-1-vinylphenanthrene (70), [61] 3,4-dihydro-2-vinylphenanthrene (71) [68] and l,2-dihydro-4-vinylphenanthrene (72) [69] are reported in Equation 2.22 and Schemes 2.27 and 2.28. 

Flamers R. J., Coulter S. K., Ellison M. D., Flovis J. S., Padowitz D. F., Schwartz M. P., Greenlief C. M., Russell J. N. Jr Cycloaddition Chemistry of Organic Molecules With Semiconductor Surfaces Acc. Chem. Res. 2000 33 617 624 Keywords carbonyi group, semiconductor materiais, surface reaction, aikenes, aromatic compounds, azo compounds, cycioaikadienes, isothiocyanates, unsaturated compounds... 

Another example of the preparing of aromatic compounds via the Diels-Alder reaction of nitroalkenes is presented in Eq. 8.16.26 Cycloaddition of methyl propiolate affords a high yield of the isomeric product. 

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. 

The extremely sterically hindered 1,2,4-diazaphospholes 139-142 were prepared via [2+3] cycloaddition reaction of 2-(2,4,6-tri-/,< +-butylphenyl)-l-phosphaethyne 138 with trimethylsilyldiazomethane derivatives (Scheme 11). Structures of 1,2,4-diazaphospholes 140 and 142 were investigated by NMR spectroscopy and X-ray diffraction. The experimental structural studies as well as theoretical calculations confirmed aromatic character of these 1,2,4-diazaphospholes. The crystal structure of compounds 140 and 142 showed remarkable hydrogen bonding character in relation to molecular aggregation due to the presence of the bulky aryl groups <2007EJI3491>. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

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. 

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