Z-E-Photoisomerization

The first example of a counterthermodynamic one-way E - Z photoisomerization based upon intramolecular hydrogen bonding was reported for the 7V-methyl-3-(2-pyridyl)-propenamide systems. 

Quantum yields for direct E,Z-photoisomerization of conjugated polyenes... 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The E,Z-photoisomerization of previtamin D to tachysterol has also received recent attention. Jacobs and coworkers examined the process in various solvents at 92 K and found evidence for the formation of a triene intermediate which converts thermally (Ea ca 6.5 kcal mol 1) to the more stable tEc rotamer of tachysterol (tEc-T equation 58)230. The rate of this conversion is viscosity dependent. They identified this intermediate as the cEc rotamer, produced by selective excitation of the cZc rotamer of previtamin D. In a re-examination of the low temperature ,Z-photoisomerization of previtamin D as a function of excitation wavelength, Fuss and coworkers have suggested an alternative mechanism, in which tEc-1 is produced directly from cZc-P and cEc-T directly from tZc-P (equation 59)103. This mechanism involves isomerization about both the central double bond and one of its associated single bonds—the hula-twist mechanism of Liu and Browne101 — and involves a smaller volume change than the conventional mechanism for ,Z-isomerization. The vitamin D system has also been the subject of recent theoretical study by Bemardi, Robb and Olivucci and their co workers232. 

This cross-sectional mismatch between the small anchor and the large dendritic component modified the intermolecular packing in the Langmuir-Blodgett mono-layer and film so that sufficient mobUity was present near the azobenzene group. This mobility greatly facihtated efficient molecular reorganization in response to E Z photoisomerization. 

The high diastereoselectivity in the addition of i-PrOH, t-BuOH and EtOH (at low concentration) suggests that E Z photoisomerization of (E)- or (Z)-16 does not occur in solution at room temperature or that the trapping of (E)- or (Z)-16 by alcohols proceeds faster than the E Z isomerization. In addition, the results show that proton transfer in the intermediate adduct formed by the disilenes and alcohols occurs much faster than rotation around the Si—Si bond. However, in the reaction with ethanol, an appreciable amount of the anti addition product was formed. Thus, the diastereoselectivity remarkably depended on the concentration of ethanol. 

Scheme 12 Photochemically driven shuttling movement of an a-cyclodextrin in an azobenzene-containing thread through reversible E/Z photoisomerization [61]...

In the rather short history of organic photochemistry, the geometrical E-Z photoisomerization has been exceptionally intensively studied for half a century and a number of reviews have been published [11-18], Although the geometrical isomerization of alkenes can be effected thermally, catalytically, and photochemically, one of the unique features of photoisomerization is that the photostationary EfZ ratio is independent from the ground-state thermodynamics but is instead governed by the excited-state potential surfaces, which enables the thermodynamically less-stable isomers... 

In the diastereodifferentiating E-Z photoisomerization of (Z)-3-benzoyloxycyclo-octene 86aZ, the diastereomeric excess (de) of the photoproduct 86E displays a unique concentration dependence (Sch. 30). Thus, when 86aZ was irradiated at a low substrate concentration (1 mM) in... 

Glaze el al. reported that fulgide 35 is a highly effective photochromic compound. But when the R is hydrogen atom 37a, no photocyclization reaction occurs,41 and only E —> Z photoisomerization takes place when 37a is irradiated by UV light, as shown in Scheme 8. 

Scheme 29. E<- Z photoisomerization of phenyl fulgides (R, R1 are hydrogen or alkyl groups ...

Table 4.27. Quantum Yields of E Z Photoisomerization of Indolyl Fulgides 85 in Toluene (Irradiated at 405 nm) ...

Quaternary salts of l,2-bis(pyrazin-2-yl)ethylene (372) underwent (E — Z) photoisomerization the quantum yield for the chloride salt was better than that for the iodide salt.1165... 

The reversible E/Z photoisomerization of azobenzene upon UV/VIS irradiation, in methanolic solution, has been utilized to develop the first reusable chemical... 

The photochromism and thermochromism of methyl 12-aminodehydroabiatate Schiff bases have been studied in crystalline states, rigid glasses, and in solution. The E-Z photoisomerization of benzophenone-9-anthraldehyde... 

Rearrangements.—Various examples of Z,E-photoisomeri7ation about the carbon-nitrogen double bond have been reported. The E-esters (1) have been prepared by irradiation of the Z-isomers (2) in pentane-benzene, and quantum yields for the photochemically induced interconversions of the four stereoisomers of l-(methoxyimino)-3-phenylprop-2-ene have been determined. Particular interest has been shown in the isomerization of azines the E — Z photoisomerization of benzophenone-9-anthraldehyde azine is quenched by dyes with the appropriate singlet energy," with the quantum yield for this transformation being influenced by solvent polarity." The composition of the photostationary state has been determined for a number of arylazines "... 

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