EZ-Photoisomerization

In contrast to cyclization and rearrangement as the unimolecular reaction, the EZ isomerization of olefins is difficult due to a drastic and unenviable change in the size and shape of the occupied space by substituents on the double bond during isomerization in the crystalline state. Some (Z,Z)-muconic derivatives provide a geometrical isomer as the photoproduct in a high yield, but not a polymer, under UV irradiation in the crystalline state, as is described in the Introduction (Scheme 1 and Table 1). This isomerization is a crystal-to-crystal reaction with an excellent selectivity, which is completely different from ordinary photoisomerizations. 

Thus, the solid-state photoisomerization of 10 proceeds via a topotactic reaction mechanism, while some (Z,Z)-muconic esters (e.g., 13 and 15) can pho-toisomerize to the corresponding ( , )-muconic esters (14 and 16) without any change in the space group, i.e., proceed via topochemical EZ isomerization (Scheme 13, Fig. 15) [115]. 

The direct photoisomerizations are singlet-state reactions, as shown by their independence of oxygen and the addition of triplet quenchers. Ilge et a/.31 103 demonstrated that no potential barriers in the photoisomerizations, EE - EZ, and EZ — ZZ, were found. In contrast, there was a small barrier in the photoisomerization of ZZ - EZ or ZE a veiy weak fluorescence was detected at 77 K, and the fluorescence of the ZZ isomer disappeared at temperatures above 130 K. 

Several years ago, we found that the isomerization of n-butylammonium ( Z,Z)-muconate produces the corresponding EE-isomer in a high yield in the crystalline state under photoirradiation [41]. This solid-state photoisomerization was re e e< t0 he a one-way reaction and no EZ-isomer was formed during the reaction, unsaturated compounds such as olefins, polyenes, and azo compounds generally undergo reversible one-bonded photoisomerization to form a mixture of omers. Previously, we pointed out the possibility that the isomerization of the but thni(i er Va ves the s°hd state f°ll°ws the bicycle-pedal reaction mechanism, et al [4 eta s °ffhe molecular dynamics ofthe reaction had not been clarified. Saltiel react an< fl have independently discussed volume-conserving... 

---