Cyclobutane photoirradiation

The polymer = 8.19 dlg in hexafluoro-2-propanol, HFIP, solution) in Figs 1 and 2 is prepared on photoirradiation by a 500 W super-high-pressure Hg lamp for several hours and subjected to the measurements without purification. The nmr peaks in Fig. 1 (5 9.36, 8.66 and 8.63, pyrazyl 7.35 and 7.23, phenylene 5.00, 4.93, 4.83 and 4.42, cyclobutane 4.05 and 1.10, ester) correspond precisely to the polymer structure which is predicted from the crystal structure of the monomer. The outstanding sharpness of all the peaks in this spectrum indicates that the photoproduct has few defects in its chemical structure. The X-ray patterns of the monomer and polymer in Fig. 2 show that they are nearly comparable to each other in crystallinity. These results indicate a strictly crystal-lattice controlled process for the four-centre-type photopolymerization of the [l OEt] crystal. 

Photodimerization behavior of 4-formyl-, 3,4-dichloro-, and several other cinnamic acid derivatives is greatly influenced by other molecules outside of the crystal (9,10). For example, 4-formylcinnamic acid 1 crystallizes in two modifications, photoreactive and photostable forms. The photoreactive crystals of 1 (mp 249 °C), on photoirradiation at room temperature in the presence of even a trace of moisture, dimerize to crystalline dimer 2 containing one molecule of water. The continuous change of the x-ray diffraction pattern during the photodimerization indicates a typical crystal-to-crystal transformation process. On the other hand, the same crystal 1 photodimerizes into amorphous dimer 2 in the absence of water. The same cyclobutane derivative is produced in very high yield in both reactions. However, highly crystalline dimer 2 is obtained only by the photodimerization of 1 in the presence of water and is not regenerated by any attempted recrystallization procedures from various aqueous solutions of 2. 

This idea was realized using crown ether styryl dyes (CESD) lc,d, 4c (Scheme 1,4). The compounds lc,d, 4c having betaine structures form supramolecular dimers with a crossed arrangement of molecules (a h-head-to-tail) in the presence of ions, due to the intermolecular interaction between the sulfo group of one of the molecules and a ion located in the crown-ether cavity of the other molecule [20,21], It was shown that photoirradiation of solutions of dimer results in stereospecific PCA giving only one of the 11 possible derivatives of cyclobutane, which is expected in conformity with the concerted superficial (s,s) addition of the reactants (Scheme 5) [22,23], It is noteworthy... 

Most of the polymers are easily depolymerized photochemically and thermally in solution to the corresponding monomers, as is expected from the ring cleavage reaction of a number of cyclobutane derivatives yielding two olefins. For example, poly-DSP in solution is depolymerized to DSP nearly quantitatively upon photoirradiation for a relatively short period36 or by heating at above 200°C70). 

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). 

Thymine and six photoirradiation products generated in the presence of p-aminobenzoic acid (5,6-dihydrothymine, 5-hydroxymethyluracil, cis-anti-, cis-syn-, trans-anti- and the trans-s w-cyclobutane dimers of thymine) were separated on a Ci8 column (photodiode array detector, A = 200-400 nm) using a 30-min 0/100 50/50 acetonitrile/water [75 mM phosphate buffer at pH 4.4] gradient... 

---