Photocycloadditions anthracene

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). 

If the starting carbonyl compound or olefin has other chromo-phores which can undergo photochemistry, this may lead to complications. In the case of the photocycloaddition of 9-anthraldehyde to 2-methyl-2-butene (pp. 305, 324), prolonged irradiation of the oxetane 53 leads to anthracene-type dimers 87.25 When fluorenone is irradiated... 

Pyridone on irradiation in concentrated solution gives the dimer (437) 2-aminopyridine behaves similarly. The acridizinium ion (see Scheme 39) like anthracene undergoes [4 + 4] photocycloaddition... 

The photocycloaddition of cyclic and acyclic 1,3-alkadienes or furans to aromatic rings has been shown in some cases. Not only symmetrically allowed (4 + 4) photocycloaddition but also (4 + 2) photocycloaddition to benzene, naphthalene, and anthracene rings have been reported. These results are also shown in the following subsections. 

Photocycloadditions of more condensed aromatic hydrocarbons such as benz[fl]anthracene, naphthacene, di ben z a, / an(hracene, dibenz[aj]anthracene, dibenz[a,cjanthracene, with 1,3-cyclohexadiene, gave (4 + 4), (4 + 2), and (2 + 2) cycloadducts [176,177,221,222] (Scheme 47). 

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). 

Intramolecular photocycloaddition of naphthalene and anthracene has been studied by Chandross and Schiebel [327], At concentrations above 10 3 M, bi-molecular photodimerization of the anthracene occurs in deaerated methylcyclo-hexane solution. In contrast, irradiation of much more dilute ( 2 X 105 M) solutions resulted in the formation of intramolecular adduct 342 (Scheme 94). Bouas-Laurent et al. showed that the CH2—O—CH2 link is more efficient than the (CH2)3 chain in bridging the two chromophores [328], Irradiation of diethyl ether or methylcyclohexane solution of 343 (5 X 10 5 M) with a high-pressure mercury lamp and liquid filter (X > 335 nm) gave a single photoadduct 344, which was isolated quantitatively. The quantum yield of 344 is 10 times higher than that of 342. 

Photocycloaddition of A-M -A. In general, photoirradiation of 9-substi-tuted anthracene in organic solution results in [4TT + 4tt] cycloaddition of the aromatic rings at the 9,10-positions to yield mainly head-to-tail (h-t) rather than head-to-head (h-h) photocyclomers (Fig. 9) [85-91], This regioselectivity was rationalized in terms of electrostatic and/or steric effects of the substituents [92,93],... 

Intramolecular Photocycloaddition of N—P4—A. Although the photocycloaddition of anthracene [85-100] and that of naphthalene [65-67,80-82] have been extensively studied, until recently relatively little has been reported on the cross-photocycloaddition between an anthracene and a naphthalene moiety [101-110], The main reason for this is the large difference in the quantum yields between the photocycloaddition of anthracene and the cross-photocycloaddition of anthracene and naphthalene. Thus, bichromophoric molecules with anthryl as one chromophore and naphthyl as the other generally undergo intermolecular an-thryl-anthryl cycloaddition rather than intramolecular cross-cycloaddition when irradiated. 

It is possible to form cyclobutene adducts from a quinone which has a lowest (n,n ) excited state by a sequence involving protection of the quinone as a Diels-Alder adduct with anthracene (equation 95) . The enedione which undergoes photocycloaddition with alkyne reacts at the C=C bond rather than at the C=0 bond (see also equation 48). 

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. 

Photo-Diels Alder ([4 + 2]) photocycloadditions (Scheme 6.79) are rare.809,816 The asymmetric intramolecular cyclization of anthracen-9-ylmethyl (—)-menthyl fumarate (229) producing 230 in 56% diastereomeric excess832 is one example (Scheme 6.89). 

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. 

Albini, A., Fasani, E., Faiardi, D., Charge transfer and Exciplex Pathway in the Photocycloaddition of 9 Anthracenecarbonitrile with Anthracene and Naphthalenes, J. Org. Chem. 1987, 52, 155 157. 

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... 

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