Dimerization intramolecular, anthracene

Similar formations of bisanthracenes have been studied with a variety of substituents. An interesting example of a cation-assisted intramolecular anthracene dimerization where the bisanthracene formation includes cycli-zation of a polyether chain to a crown ether is illustrated in (4.27). In the absence of Li+ the dimer reverts back to the open chein compound, but in the presence of Li+ the crown ether is stabilized so that the product itself also becomes more stable 430). 

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). 

In this system, however, intermolecular dimerization may take place competitively with intramolecular dimerization. To rule out this possibility, compound 5, in which two anthracenes are linked by two polyether chains, was synthesized.171 It was found that intramolecular photodimerization proceeds rapidly in the presence of Na+ as the template metal cation. Compound 6 was also synthesized.181 Although this compound has not been applied in a photoswitch system, it displays a remarkable fluorescence change upon binding with RbC104 or H3N+(CH2)7NHj.[81 Yama-shita et al.[9] also synthesized 7, in which intermolecular photodimerization of anthracene is completely suppressed. The photochemically produced cyclic form 8 displayed excellent Na+ selectivity. 

Figure 14 Schematic representation of anthracene dimers with no parallel molecular planes (a) 55° dimer (see Ref. 88) (b) zero overlap twisted intramolecular charge-transfer type pair and (c) large overlap excimer-type conformation of anthracene units in l,l-di(9-anthryl)alkane (see Ref. 89).

Photodimers have been reported as the principal products from irradiation of numerous arene substrates. While there is little selectivity to be considered in most of these reactions, it has been reported that almost all photolyses of 9-substituted anthracenes give head-to-head dimers as depicted in U10). ° An intramolecular version of these photodimerizations has also been reported. Irradiation of di(a-2-naphthylmethyl) ether gave a mixture of the endo cyclomer (111) and the exo cyclomer (112). ° The photodimerization of 2-pyridones has been the subject of extensive investigation for a number of years." "" In most instances the head-to-tail or trans-anti isomer was the major or exclusive product in these reactions. Irradiation of A -methyl-2-pyridone for 15 h provided the trans-anti dimer (113) in 51% yield. This material was accompanied by much smaller quantities of other dimeric species. Vari-... 

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). 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

In many cases photophysical effects are much influenced by photochemical reaction. The fluorescence of naphthacene is affected by dimerization and oxidation. Interaction with anthracene and quinones also occurs. The adiabatic photolytic cycloreversion of substituted lipidopterenes into intramolecular exciplexes shows an example involving anthracene derivatives. A series of very detailed papers on conformational effects on the fluorescence and photochemistry of [2/j] 9,10-anthracenophanes have been published by Ferguson and coworkers.It is not possible in this review to summarize this very detailed work... 

Photodimerization reactions involving aromatic rings of two naphthalene units are not as frequently encountered as those involving anthracene. A study of the effect of high pressure on the photodimerization of methyl 3-methoxy-naphthalene-2-carboxylate shows that the rate can be doubled (at 2000 bars), but this represents a much lower activation volume than for similar thermal reactions. An intramolecular dimer is formed when the a f/-[3.3]-(l,4)naphthalenophane (182) is irradiated the cycloadduct reverts to (182) on heating or on further irradiation. The syn isomer of (182) does not undergo this reaction. 

Interest continues in the intramolecular photodimerization reactions of polynuclear aromatic moieties joined by an alkane chain. The absorption and exciplex emission spectra of the naphthalene-anthracene sandwich pair have been previously studied this species was generated by the light-induced (254 nm) cleavage of the intramolecular photo dimer (139) of l-(9-anthryl)-3-(l-naphthyl)-propane at low temperatures in a rigid matrix.173 The generation and properties of the sandwich pair have been examined by other workers, using single crystals of the intramolecular cycloadduct (139).174 The exciplex fluorescence is broad and... 

Thus, the cycloreversion of a biplanemer appeared to depend linearly on the laser intensity, whereas the formation of anthracene from its photo-dimer was proportional to the cubic of laser intensity (see Scheme 1 la). The unusual result with the biplanemer was explained as combination of a three-photon intramolecular cycloreversion and the conversion back to the reactant by a two-photon intramolecular cyclodimerization. The fact that multiphotonic processes of different order could occur within the same laser pulse was justified by the inhomogeneous spatial distribution of the laser intensity. At the center of the laser focus, the intensity is higher and a three photon process is favored, at the wings of the laser focus it is the two-photon process that predominates. ... 

The dipole moments of dimers of 9-substituted anthracenes indicate that they possess head to tail structures. Dimers having the head to head structure have been obtained only from intramolecular reactions, e.g., Eq. (185). The 9,10-disubstituted anthracene dimers which have been obtained, Eq. (184), are in general quite unstable and revert to the monomers at temperatures below the melting points of the monomers. 

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