Anthracenes photocycloaddition reactions

There are several reports this year of photocycloaddition reactions between polycyclic aromatic hydrocarbons and 1,3-dienes. Anthracene (58 R = H) or 9-cyanoanthracene (58 R = CN) give [4 -I- 4] and [4 -I- 2]adducts on irradiation with buta-1,3-diene, but the different product ratios and temperature effects are used to support the previously proposed biradical mechanism for the reaction. Another report from the same group deals with anthracene-hexa-2,4-diene and 9-phenylanthracene-penta-1,3-diene or cyclohexa-1,3-diene systems in the case of 9-phenylanthracene (59) and cyclohexa-1,3-diene, two [2 -I- 2]cycloadducts involving a terminal aromatic ring are isolated, as well as the more usual [4 + 2] and [4 -f- 4]adducts involving the central ring. Irradiation of substituted anthracenes... 

The marked excess-energy dependence of the fluorescence-decay characteristics of tetracene vapour has again been interpreted in terms of the Sx - S0 internal-conversion process being enhanced by population of higher vibrational levels of the Sx state.67 The process has been observed directly in the case of pentacene by the recognition of transient absorption bands ascribed to hot SQ levels produced in the internal conversion.68 Non-linear absorption in the laser photolysis of anthracene,69 the intramolecular photocycloaddition reaction of l-(9-anthryl)-3-(l-naphthyl)propane,80 and the quenching of fluorescence of aromatic hydrocarbons by caesium chloride81 have been reported. 

Figure 44 Reversible photocycloaddition reactions of bis(anthracene)-functionalized crown ether. (Reproduced from Ref. 84. Elsevier, 1980.)...

An example of the effect of CDs on the photocycloaddition reaction is the photodimerization of water-soluble 2-anthracene sulfonate 114 (Scheme 5.37). In sole water, the cycloadducts 115,116,117, and 118 were obtained in relative percentage of 45,35,18, and 2%, respectively. The presence of a- and y-CD did not change the isomeric distribution, while the /3-CD allowed only the dimer 115 to be obtained. The results were explained by the formation of different guest/host inclusion complexes. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

Photochemical (4 + 2) cycloaddition of anthracene and dimethyl fumarate or maleate have been reported by Kaupp [301], The reaction requires a high concentration of dimethyl fumarate or maleate owing to the short lifetime of the excited singlet state of anthracene and its facile dimerization. However, irradiation of a benzene solution of (9-anthryl)methyl methyl fumarate or maleate resulted in the intramolecular (4 + 2) photocycloaddition efficiently [302], Asymmetric induction in this intramolecular photocycloaddition of 291b-c was also investigated (Scheme 83). 

An important aspect of photocycloaddition consists of its regio- and stereoselectivity. Thus, dimerization of substituted olefins generally yields head-to-head adducts 61a and 61b in a regiospecific reaction, while head-to-tail dimers 63 are obtained from 9-substituted anthracenes (62) in polar solutions (Applequist et al., 1959 cf. Kaupp and Teufel, 1980). 

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. 

Although intermolecular photocycloaddition of two benzene rings in the condensed phase has not been observed, this reaction is common for polycyclic aromatic hydrocar-bons. For example, anthracene-9-carbonitrile (231) in acetonitrile undergoes efficient [4 + 4] photocycloaddition with anthracene to give an adduct 232 in 94% chemical yield (Scheme 6.90).833 This process is thermally reversible. 

The anthracenes (188), which have a 3,5-dialkoxybenzyloxymethyl substituent on the 9-position, undergo quantitative (4jt + 4ji) intramolecular photocycloaddition to yield (189). " The process is thermally reversible, and (189) is readily converted to the diketone (190) on treatment with acid. In the presence of acid, the linked naphthyl and resorcinyl moieties in (191) undergo (2ji -I- 2jt) photoaddition with 300 nm radiation to give the tetrahydrofuran derivatives (192) by the route outlined in Scheme 3." The reaction also occurs in the absence of acid for (191) with R = -0-(CH2)2-0Me, but the quantum efficiency is reduced by 35-fold. The products (192) are labile under 254 nm radiation and undergo a novel photoextrusion of acetaldehyde to yield (193) by the pathway... 

Several reviews have been published within the year which are of general relevance to the photoreactions of aromatic compounds. The subjects of these reviews include photochemistry in ionic liquids and in isotropic and anisotropic media, organic synthesis utilizing photoinduced electron-transfer reactions," heteroatom-directed photoarylation processes, photochromism, and photochemical molecular devices. Reviews more directly pertinent to the sections in the present chapter include those of the photoisomerization of five-membered heteroaromatic azoles, the photocycloaddition of benzene derivatives to alkenes, Diels-Alder additions of anthracenes, advances in the synthesis of polycyclic aromatic compounds, diarylethene-based photochromic switches, the photo-Fries rearrangement, and the application of Diels-Alder trapping of photogenerated o-xylenols to the synthesis of novel compounds. " A number of chapters in the two recently published handbooks of photochemistry and photobiology and in the revised edition of the text on photochromism are also pertinent to the current subject matter. 

The reactions of polynuclear aromatics can also be rationalized by simple models such as those shown for benzene in Figures 12.48 and 12.49. Two notable reactions are (i) the photodimerizations of anthracenes (equation 12.71) and (ii) the photoisomerization of frans-stilbene (99) to the cis isomer (100), followed by intramolecular photocycloaddition to a dihydrophenanthrene (101), which can then be oxidized to phenanthrene (102) ... 

Kobayashi investigated a self-assembled boronic ester cavitand capsule for photochemical reaction of 2,6-diacetoxyanthracene. Tsuda found a self-assembled helical anthracene nanofiber in a vortex. Chou examined rotational behaviors and fluorescence energy transfer of N-l- and N-2-anthryl succinimide derivatives. " Reversible photoinduced twisting of molecular crystal microribbons via [4 - - 4] photocycloaddition of 9-anthra-cenecarboxylic acid. Reversible single walled carbon nanotubes of 1,3-bis(9-anthracenylmethyl)imidazolium chloride was examined as a functionalized anthracene salts.Karatsu reported the intramolecular photodimerization of 9-substituted anthracene derivatives (253) tethered by oligosilanes giving [4 + 4] and [2 - - 4]cycloadducts (254), (255), and (256). ... 

Anthracene and its derivatives undergo a [4-F4] photocycloaddition to cyclooctane-containing dimers. This reaction can be reversed with mechanical forces, 3delding fluorescent anthracenes. Poly(vinyl alcohol) was crosslinked with anthracene dimers to give polymers that could report microcracks in a similar way as the tricinnamate-containing polymers described above (Figure 11.3). 

The photocycloaddition of cis-1,2-dihydrophthalic anhydride to benzene, naphthalene, or anthracene affords in each case products arising from ( 4j + 4J reaction, a type of addition unknown for cyclohexa-1,3-diene itself. Energy transfer from benzene to the diene anhydride still provided the main reaction pathway, namely electrocyclic closure to give the cis-dihydroDewarbenzene anhydride. The naphthalene photo-adduct (393) on xanthone-photosensitized irradition gave the cage compound (394) similar reaction also occurs with the benzene photo-adduct. 

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