Intramolecular photodimerization

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

In this type of process an excited molecule adds to a second — identical — molecule in its ground state, usually with formation of a ring. These dimerizations are thus most commonly intermolecular reactions, but obviously the two reactive moieties can also be linked together, e.g. by an alkyl chain. Such intramolecular photodimerization reactions have been studied in detail422). 

Photochemical reactions of the pyrimidine polymers in solution were studied to determine the quantum yields of the intramolecular photodimerization of the pyrimidine units along the polymer chains. Photoreactions of the polymers were carried out in very dilute solutions to avoid an intermolecular(interchain) photodimerization. Quantum yields determined at 280 nm for the polymers (1-6 in Figure 1) are listed in Table I. The quantum yield of the 5-bromouracil polymer [poly(MAOU-5Br)] could not be determined because of side reactions of the base during the irradiation. 

On irradiation in acetonitrile solution A. A -tethered 2,4-dioxotetrahydropyridines 7 undergo (intramolecular) photodimerization.11 The stercospecific synthesis of a c/.v-.syn-pyrimidine photodimer has been achieved using a cleavable linker between nitrogen atoms.12... 

In contrast to the intramolecular photodimerizations reported above, inlermoleeular [2 + 2] photocyclodimerizations very often suffer from the fact that the reactions are neither regio- nor stereospecific, affording product mixtures, and are, therefore, less useful for synthetic purposes. 

Intramolecular photodimerization in P1VN and oratDNP was studied for cyclohexane, dichloromethane, and benzene solvents117>. The conversion to photodimer was... 

Intramolecular excimer formation of 1, -dinaphthylal kanes has been investigated in detail [319,320], Chandross and Dempster reported the first example of intramolecular photodimerization of l,3-di(l-naphthyl)propane (328), which was discovered during their intramolecular excimer study [321,322], Upon irradiation of 328 in methylcyclohexane, 328 formed 329, which is thermally unstable to give 330 (Scheme 92). Both 329 and 330 are converted substantially to 328 upon irradiation with 254-nm light. Similar irradiation of l,3-di(2-naphthyl)propane did not give any photocycloadduct. The authors assumed that the intramolecular cyclom-erization occurred via an intramolecular excimer. Davidson and Whelan reported further evidence for this assumption [320],... 

Table 9 Intramolecular Photodimerization and Photocycloreversion Quantum Yields for 351a-f and 352a-f in Methylcyclohexane at 20°C...

Table 11 Quantum Yields for Intramolecular Photodimerization Compound3 Odlmer...

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. 

The intramolecular photodimerization and [2 + 2]-photocycloaddition in DNA involves thymine or cytosine as the chromophore. This chemistry has been intensively investigated with regards to DNA damage and repair [131]. Despite the fact that the area is of continuous interest [132], the synthetic applications are limited and are not covered here in detail. However, some preparative aspects of 4-pyrimidinone photocycloaddition chemistry will be addressed. Aitken et al. have prepared a plethora of constrained cyclobutane P-amino acids by intra- or intermolecular [2 + 2]-photocycloaddition to uracil and its derivatives [133, 134]. In a chiral adaptation of this method, the uracil-derived enone 140 was employed to prepare the diastereomeric cyclobutanes 141 in very good yield (Scheme 6.49). The compounds are easily separated and were - despite the relatively low auxiliary-induced diastereoselectivity - well suited to prepare the as-2-aminocyclobutanecarboxylic acids 142 in enantiomerically pure form. Enantioselective access to the corresponding trans-products was feasible by epimerization in a-position to the carboxyl group [135],... 

Intramolecular photodimerization of 2-pyridones provides a series of primary photoproducts, the structures of which were a function of the chain length comecting the two reactive centers." The initially formed dimers were characterized as being derived from a photochemically allowed [2 + 2] cycloaddition process in each case examined. A particularly interesting example from this study in terms of [4 + 4] cycloaddition chemistry is the conversion of dipyridone (114) into Ae net [4 + 4] dimer (115) via a facile thermal rearrangement. " Further irradiation of the [4 + 4] species (115) produced a second [2 + 2] adduct that was isomeric with the initially formed metastable species. 

The intramolecular photodimerization of linked arenes, particularly dianthracenes, continues to be a topic of considerable interest. The aza-crown ether (181) with pendant 9-anthracenyl groups binds Group I metal cations, giving 1 1... 

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

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... 

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). ... 

Kita et al studied the photodimerization of synthetic polymers containing thymine bases the study was carried out on the polymers and on models. Intramolecular photodimerization of the polymer was seven times faster than that of the corresponding models, however, intermolecular photodimerization of the polymers did not occur under the experimental conditions used. 

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