Subject photodimerization

The subject of the first section of this chapter is the photodimerization and photocycloaddition reactions of olefins. In this category we include only those compounds in which the photoreactive olefin or polyene is not part of an aromatic system although it may bear an aromatic substituent, such as in styrene. 

As mentioned briefly in Chapter 5, the photodimerization of acenaphthylene is subject to a very interesting heavy-atom solvent effect. The results of the photolysis of acenaphthylene in some heavy-atom solvents are given in Table 10.6.<4a) The data in Table 10.6 show that the heavy-atom solvents n-propyl bromide and ethyl iodide yield product ratios similar to that obtained in the sensitized photolysis, indicating a greater role of the triplet state in... 

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

Some derivatives have also been subjected to photodimerization reactions [69JCS(C)1729 82JHC1319]. 

The stereochemistry of photodimerization in the solid state and solution has been reported for several halogenated derivatives of t-1 (Table 2) (59-62). Solid state photodimerization of stilbenes, like other alkenes, is subject to topochem-ical control viz, the two reactive double bonds must be parallel and separated by < 4.2 A (63). The photostability of t-1 in the solid state (39b,59) is consistent with its reported crystal packing (64). The halogenated stilbenes 15-20 serve to illustrate the variety of stereochemical outcomes observed for solution and solid state dimerization (eq. 11). 

It is well established, in a qualitative sense, that chemical reactions occurring in crystals are subject to restrictive forces, not found in solution, which limit the allowable range of atomic and molecular motions along the reaction coordinate. This often leads to differences, either in the product structures or the product ratios, in going from solution to the solid state. This was first demonstrated in a systematic way by Cohen and Schmidt in 1964 in their studies on the solid state photodimerization of cinnamic acid and its derivatives (1 ). This work led to the formulation of the famous topochemical principle which states, in... 

As a second example of intersystem crossing mechanism in biochromophores we include here the case of the DNA pyrimidine nucleobases, starting by the uracil molecule [91]. In previous sections we presented a model for the rapid internal conversion of the singlet excited rationalizes the ultrafast decay component observed in these systems, both in the gas phase and in solution. Despite the short lifetimes associated to this state, which is the main contributor to the photophysics of the system, formation of photodimers PyroPyr has been observed for the monomers in solution, as well as in solid state, for oligonucleotides, and DNA [92], Since the sixties, the determination of the mechanism of the photoinduced formation of cyclobutane dimers has been the subject of numerous studies [92, 93-97], One of the most classic models that has been proposed for the photodimerization of Pyr nucleobases in solution invokes photoexcitation of a molecule to a singlet state followed by population of a triplet state by an intersystem crossing mechanism... 

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-... 

On the subject of spontaneous generation of chirality, it is of interest to know that spontaneous formation of chiral aggregates from nonchiral monomers is known to occur, e.g. the assembly of tetra-alkyl benzimidocyanins 3 as monitored by CD (circular dichroism). Formation of chiral crystals from achiral monomers is also reported, e.g. by photodimerization in the solid state. " In a recent example, chiral crystals of acridine 4 and diphenylacetic acid 5 give excess of the (.S)-product 6 upon a photodecarboxylating condensation reaction. Symmetry breaking is also known to occur for supramolecular complexes of achiral components e.g. glu-tarimide 7 and the diaminopyridine 8, and, as will be discussed below, in monolayers at the air-water interface. ... 

Interestingly, in the absence of external olefins as cycloaddition partners, indoles appear to undergo photodimerization upon being subjected to ultraviolet light irradiation. Both head-to-head (33, major) and head-to-tail (34, minor) dimerization products have been observed [24] (Scheme 7). 

In this review article, after a description of the historical background of the present subject, some general features of reactions in amorphous polymer solids are summarized and then specific features of photo-physical processes, photoisomerization, photodimerization, chain scission, and crosslinking reactions are discussed separately. Typical theoretical treatments of solid-state reactions are reviewed in relation to the interpretation of experimental results. 

Coumarins have historically been the subject of intense photochemical interest. Photodimerization of coumarin 103 (0.01 m) in sole water or in aqueous micellar media (SDS, CTAB, Triton X-100) afforded only the adduct 104 (Scheme 5.34). In MeOH at the same concentration a mixture of 104 and 105 in a 1 13 ratio was obtained, while in benzene (0.31 m) only the adduct 105 was detected. 

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