Photodimerisation reaction

The solid state [2+2] photochemical reaction of olefins is an attractive transformation for the generation of C-C bonds. However, this type of reaction can only take place when the olefins to be dimerized crystallise in the appropriate relative orientation. For several decades chemists have strived to design molecules that will predictably crystallize in such orientations. In spite of these efforts, to date there are not general and reliable methods to align olefins in the solid state so that the photodimerisation reactions can take place. In this context, an approach that has started to emerge as a potentially useful alternative to orient olefins in the solid state is by templated processes. Some examples where hydro gen-bonding templates have been used in the photodimerisation of olefins have appeared and are discussed herein. 

Stoddart has employed the anion-directed [3]pseudorotaxane assemblies described above to control the outcome of solid-state photodimerisation reactions of olefins [116]. A combination of supramolecular interactions (one of them being hydrogen-bonding to PF,) has been employed to pre-organize bis(dialky-lammonium) salts containing fraws-stilbenoid units (89) into the [3]pseudoro-taxane assembly 90 shown in Scheme 45. 

A further example of a photodimerisation reaction is provided by the formation, in poor yield, of 1,4-naphthoquinone photodimer (50) on irradiation of 1,4-naphthoquinone in benzene solution (Expt 7.26). The dimerisation may be effected by sunlight or by means of a mercury arc lamp. 

Photodimerisation reactions in cholesteric phases have been examined (Nerbonne and Weiss, 1978, 1979). The high viscosity of such systems probably precludes the participation of excited singlet states in bimolecular reactions. 

Concomitant [2 - - 2] photodimerisation reactions in the solid state from supramolecular H-bonded arrays self-assembled via mechanochemistiy... 

Figure 4.24 (a) The self-assembly of a dicarboxylic acid to bring two double bonds into close proximity and (b) the before and after crystal structures of a supramolecularly templated [2 -1-2] photodimerisation reaction 4.6, templated by resorcinol. 

The [tc s+ti s] photodimerisation reactions proceeds via involvement of 2re-electrons from each enone. These two n-bonds converted into intermolecular CF-bonds. The photodimerisation of cyclopentenone is an example of cycloaddition. 

In the presence of a proper second molecule bimolecular photochemical processes occur. Obviously such reactions can also occur in an intramolecular fashion in bifunctional molecules containing both reactive centres. These reactions are a) hydrogen abstraction by the excited molecule if the second molecule (or reactive centre) is a hydrogen donor RH (1.14) b) photodimerisation (1.15) c) photoaddition or photocycloaddition (1.16) d) electron transfer (1.17), if no bonding takes place between the reactants (or reactive centres). ... 

The reaction types which includes 2 + 2 cycloaddition and carried out by photochemical means have also great synthetic potentiality. The photodimerisation of cinnamic acids were among the earliest photochemical reactions to be studied. These compounds give good yield of dimers when irradiated in the crystalline state. In solution, cis-trans isomerisation is the dominent reaction. 

A crystalline sample of this supramolecular assembly was irradiated with UV light and the formation of the corresponding cyclobutane 91 with syn-anti-syn stereochemistry was observed. In contrast, the photodimerisation of trans-stilbenoid-bis(dialkylammonium) salts does not take place in the absence of the macro cycle, indicating the importance of pre-organizing the stilbenoid units (which requires the presence of the anion) for this solid-state reaction to occur. 

For the preparation of larger amounts of the phenanthrene the reaction should be repeated photolysis of more concentrated solutions is to be avoided in order to minimise the possibility of photodimerisation to give a cyclobutane derivative. 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

The cyclopent-2-enone required for the photodimerisation is prepared by the hydrolysis and oxidation of 3-chlorocyclopentene, which is obtained by the low temperature addition of hydrogen chloride to cyclopentadiene. The latter is obtained by heating dicyclopentadiene. This depolymerisation is an example of a reverse (or retro) Diels-Alder cycloaddition reaction the diene readily reforms the dicyclopentadiene on standing at room temperature. 

For the case of stilbenecarboxylate (SBC, Fig. 9.9b), by comparison, both syn head-to-head and syn head-to-tail cyclodimers were formed. The product selectivity may also be controlled by the presence of co-absorbed photo-inactive molecules e.g. phenylbenzoate (Fig. 9.9c).The nature of the photodimerisation products can to some extent be controlled by changing the M2+/M3+ ratio, and hence the packing density of the incorporated monomer [113]. Norrish type II reactions of aromatic ketocarboxylates within the interlayer of MgAl-LDHs have also been studied [114]. 

Photodimerisation of Alkenes. When alkenes are irradiated, they often dimerise to give cyclobutanes. In a dimerisation reaction, the set of orbitals on the left of each pair in Fig. 8.1 and the set on the right will have identical... 

Sfen c factors. An interesting example of steric effects upon excimer formation is demonstrated by the anthracenes. Many anthracenes exhibit selfquenching of fluorescence and these results are shown in Table 1 (from Bouas-Laurent, et a ., 1980). As one moves from anthracene to 9-methylan-thracene and then to 9,10-dimethylanthracene one finds that the rate constant for self-quenching of fluorescence ( jq) decreases and so does the rate constant for photodimerisation For anthracene, the latter reaction is very... 

Photodimerisation of (61) affords the cw,a t/,cw-head-to-tail product (62). The position of the fluoro substituent appears to play some part in the outcome of the reaction since irradiation of the isomer (63) affords the cis,syn,cis head-to-head dimer. Further studies of the influence of fluoro substituents on the dimerisation of the styrylcoumarins (64) have also been reported. ... 

The unsaturated 6-lactone (84) undergoes intramolecular photochemical (2 + 2)-cycloaddition to give (85)." The scope of the process has been evaluated and the 5-lactones (86-88) all behave similarly, affording the products shown in Scheme 2. The lactones (86) and (87) both cyclise in the two possible modes. Intramolecular cycloaddition reactions within polymethyldia-minebis(4-methyl-7-coumarinyl)oxyacetamides have been described. Zhu and Wu have reported that a biscoumarin system linked by a phenanthroline unit does not undergo photodimerisation. Instead, intramolecular addition of a coumarin unit to the phenanthroline occurs. 

Single crystals of thymine derivatives with long alkyl-chain substituents are photochemically reactive and undergo (2 + 2) photodimerisation to yield solely the trans-anti dimer. In solution, however, the photoreaction affords the usual four cycloadducts. Irradiation of the bis-thymine PNA dimer (142) brings about intramolecular cycloaddition to give the adduct (143) in 50% yield. The reaction is carried out in water using irradiation at 254 nm. The results from a study of the photochemical cycloaddition within the thymidilyl system (144) has been reported. Photoadducts have been obtained from the... 

---