Stereoselective photodimerization

In studies where cholesteric phase order appears to influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dlmethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered glasses, the cis-syn dimer is formed almost exclusively in liquid-crystalline media. Furthermore, selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO). Since all of the... 

T. Tamaki, T. Kokubu, K. Ichimura, Regio- and stereoselective photodimerization of anthracene derivatives included by cyclodextrins. Tetrahedron, 1987, 43, 1485-1494. 

Yang J, Dewal MB, Shimizu LS (2006) Self-assembling bisurea macrocycles used as an organic zeolite for a highly stereoselective photodimerization of 2-cyclohexenone. J Am ChemSoc 128(25) 8122-8123... 

FICU RE 3.17 CB[8]-templated stereoselective photodimerization of reactant 33 to yield predominantly dimer product 34, but hydration product 35 in the absence of CB[8]. (Adapted from Ref. [129] with permission of American Chemical Society. Copyright 2006.)... 

The relative orientation of olefins within metal complexes provides an alternative approach to induce stereoselectivity in the cycloadditions of olefins in the solid state. Irradiation of SnCl4 complexes of cyclopentenones 14a-c in the solid state leads to stereoselective photodimerization to give a ri-HT cyclodimers 15a-c in high yields. ... 

Rao, V. P. and Fech, J., Stereoselective photodimerization of SnCl4 complexes of cycHc a,P-unsat-urated ketones in the solid state, /. Photochem. Photobiol. A Chem., 67, 51-56,1992. 

Cyclic a,P-enones photodimerize via the Tt-state, as in (4.23)426) and (4.24)427). By chosing the appropriate substituents the reactions proceed regio- and stereoselectively. In contrast, pyrimidine bases dimerize from the first excited singlet state to afford head to tail products (4.25) 428). 

Rao et al. observed that photoirradiation of tran.v-stilbene in crystalline y-cyclodextrin inclusion complexes yields a single isomer of. svn-tctraphenylcy-clobutane (119) [109] stereoselectively in high yield (70%). In contrast, the photodimerization of stilbene in solution is very inefficient, and no photodimer was observed even after prolonged irradiation of pure stilbene crystals. 

Fujita, M., Yoshizawa, M., Takeyama, Y., Kusukawa, T., Cavity-directed, highly stereoselective [2+2] photodimerization of olefins within self-assembled coordination cages. Angew. Chem. Int. Ed. 2002, 41, 1347-1349. 

Photodimerization of 2-pyridone (46) in the presence of the 2,2/-biphenyldi-carboxylic acid host (45) also proceeded via a catalytic process. First, irradiation of the 1 2 inclusion complex of 46 and 45 in the solid state gave the trans-anti dimer (47) in 92 % yield [27], The mechanism of this stereoselective photoreaction was investigated through X-ray analysis of this complex. In the complex, two 46 molecules are arranged in ideal positions for yielding 47 by dimerization [27], Secondly, a catalytic dimerization reaction of 46 was carried out. Photoirradiation for 20 h of a 1 4 mixture of powdered 45 and 46 under occasional mixing in the solid state gave 47 in 81 % yield. These data clearly show that molecules of... 

The photodimerization of 2-pyridones is an efficient, regiospecific, and stereoselective [4+ 4]-cycloaddition [56] that converts two achiral aromatics into a highly functionalized tricyclic cyclooctadiene with four stereogenic centers (Scheme 9.34). For tethered pyridones, the trans isomer is usually the major product when one or both pyridine nitrogens are methylated. By contrast, in the unsubstituted systems,... 

Photochemists are well aware that solid state photodimerizations may be highly stereoselective, but they are not frequently stereospecific. Long-range molecular transports connected to phase rebuildings provide an obvious explanation. However, there are also solid state photoprocesses which yield... 

Stilbene and its derivatives, which had been widely investigated in CDs and CAs, can be included in CB[8] cavity to form 2 1 complexes. Two ( )-diaminostilbene dihydrochloride 68 (Scheme 1.13) can insert into CB[8] cavity to give a 2 1 host-guest complex which has a good solubility in water. Photoirradiation of the complex gave [2-h2] photodimerization product la,2o,3)3,4)3-telrakis(4-aminophenyl)cyclobutane 69 as major product, along with a trace amount of la,2)3,3a,4j8-tetrakis(4-aminophenyl)cyclobutane 70. No formation of the isomerization product (Z)-68 was observed, which is significantly different from the photoreaction of ( )-68 in the absence of CB[8]. The stereoselectivity of photodimerization in the presence of CB[8] (synlanti = 95/5) is much better than that obtained with y-CD (synlanti = 80/20). 

Water soluble p-sulfonato calbc[w]arenes (w = 8, la-lb and n=6, 2a-2b) was employed as host to control the outcome of photodimerization and photoisomerization of 4-stilbazoles [17]. Novel macrocydic and medium-size stilbenophanes tethered by silyl chains were synthesized, and their photochemical and photophysical properties were examined (Figure 5.5) [18]. Direct irradiation of macrocydic stilbenophanes stereoselectively gave intramolecular photocydoadducts, and the efficiency increased with decreasing distance between the two stilbene units. The triplet-sensitized photoreaction of stilbenophanes caused cis-trans photoisomerization. Photoreactions of as-fixed stilbenophanes under an oxygen atmosphere selectively led to phenanthrenophanes. Fluorescence quantum yields increased with the introduction of silyl substituents, and hence those of silyl-tethered stilbenophanes were larger than that of unsubstituted fraws-stilbene. Intramolecular excimer emis-... 

Macartney et al. used CB[7] to mediate and template the stereoselective [4 + 4] photodimerization of 2-aminopyridine hydrochloride 36 [129]. In the presence of CB[7], 1 2 hosbguest inclusion complexes were formed, and photolysis gave exclusively the anti-trans addition photoproduct 37. In the absence of CB[7], both the anti-trans and the s)m-trans photoproduct are formed. Furthermore, they found that the formation of the photoproduct 37 inside the CB[7] nanoreactor stabilizes it significantly, preventing rearomatizalion back to the monomer reactants, which occurs thermally at room temperature in the absence of CB[7], This system is illustrated in Fig. 3.18. 

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