EZ Isomers

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... 

Mn2(CO)jQ with 1,3,5r-hexatriene yields (166), together with its (EZ) isomer.Complexes (167) and (168) have been structurally characterized as products from the photoreaction of HgRe2(CO) 2 with PhC=CPh. ° Photolysis of CpMo(CO) 2 (u-H) (ii-PPh2)Mn(CO) with butadiene yields (169) which reversibly loses CO to give... 

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. 

When Con,-EDDA-complexes are synthesized under conditions favoring equilibrium state, the cis-a-compound is essentially obtained72. Cis-(3-complexes can also be obtained, most favorably as mixed ligand complexes with a bidentate ligand, e.g. carbonate, oxalate or 1,2-diamines73-75. But in all cases known, it is the cis-a-isomer which is formed in a greater amount. Reilley and co-workers conclude from the simplicity of the NMR-contact shift spectra of [Ni(EDDA)(H20)2] that the compound is present in only one isomeric form, and assume that this is the ezs-a-form28. ... 

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. 

In compounds 77 (R = H, Me) with a second electron-withdrawing group, four planar isomers 77a (EE), 77b (EZ), 77c (ZE) and 77d (ZZ) are theoretically possible. However 77a, which a priori is expected to be less stable, has never been detected127,128. The presence of the other three isomers, with strong intramolecular hydrogen bonds, has been demonstrated for compound 77, R = H (equation 8). In compound 77, R = Me,... 

Three isomeric forms of ArF are available EE, EZ, and ZZ. These three isomers are capable of photochemical intertransformations and cyclization. Cyclization only occurs from a sterically strained E form, and its efficiency depends on the structure of the F-isomer. 

Figure 12.11 Separations of three branched chain C14 hydrocarbons, three dimethylenaphthalene isomers, and C13, C14, C15 n-alkanes on 30 m x 0.25 mm x 0.25 p,m PDMS column at (a) 100°C, (b) 120°C, (c) 150°C and (d) programmed 100°C/1 min-10°C/min-140°C/4 min, late eluting peaks. (Separations simulated using EZ-Chrom GC , Restek Corp. Inc.)...

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