Isomerization, photo

Isomerization of ( /Z) isomers is another important transformation. Isomerization of ( ) and (Z-) conjugated amides is effected photochemically " (photo-isomerization " ). There is a rather high energy barrier for the excited state required for (E/Z) isomerization. Isomerization of the C=C units in dienes is also induced photochemically. " Isomerization of cyclic alkenes is more difficult but cyclooctene is isomerized photochemically. " Conjugated aldehydes have been isomerized... 

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

The direct photolysis of cis- or ftmr-stilbene in solution gives rise to photo-isomerization. In addition, dihydrophenanthrene is formed from the cis isomer ... 

Photo-isomerism is also found in this group of compounds. Cis-Pt(NH3)2(H20)l+ photo-isomerizes to trans with a quantum yield of about 0.1 at 363 nm (40). Photoisomerism of Pt (glycine) 2 from cis to trans, but not its reverse is also reported. 

Figure 9.20 The photo-activated process that accounts for vision the 11-cis — 11-trans photo-isomerism of retinal...

The active compound within the bacillary layer is retinal. To simplify the photo-physics within the rods and cones hugely, absorption of a photon initiates a series of conformational changes that lead ultimately to photo-isomerization of retinal from the 11-cis isomer to the 11-trans isomer see Figure 9.20. The uncoiling of the molecule following photo-excitation triggers a neural impulse, which is detected and deconvoluted by the brain. The photochemical reaction is breakage and, after rotation, re-formation of the C=C bond. 

The tricyclic antidepressives, doxepin (394) and dothiepin (395), were photo-isomerized when aqueous solutions of their hydrochlorides were irradiated with a medium-pressure mercury lamp [195]. Doxepin was photodegraded by... 

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. 

Robinson, J., Richardson, A., Bush, B., and Elgar, K.E. A photo-isomerization product of dieldrin. Bull. Environ. Contam. Toxicol, 1(4) 127-132, 1966. 

Nitro-substitution especially at the 6-position of BIPS opens up a triplet pathway for photo-isomerization. This pathway runs in parallel to the singlet manifold. This increases the yield and, in turn, may lead to photo-aggregation that is observed for these compounds. Photochemical ring closure to the spiropy-ran form is more efficient for these 6-nitro-substituted compounds. The photochemistry of 6-nitro-BIPS merocyanine is similar to that of unsubstituted BIPS(s) however, the 6,8-dinitro compound efficiently cyclizes upon excitation to form the spiropyran closed form via a singlet manifold. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

The photo-isomerization of certain pyridines to Dewar pyridines is described in Section 3.2.1.2.2, and the formation of bridged ring 6H-1,2-dihydro-3-pyridones in Section 3.2.1.10.4. 

W. M. Gelbart and S. A. Rice, Some Formal Results in a Theory of Molecular Rearrangements Photo-Isomerism, in J. Chem. Phys. (in press). 

Trifluoromethyl)-3//-diazirin-3-yl]arenes, e.g. 36 (R1 = CF3), were introduced as photophores by Brunner et alJ116 to provide a more favorable carbene than the one produced from diazoesters[117 or from aryldiazirines. 118 Both 3//-diazirin-3-ylarenes I8-1201 and the [(3-tri-fluoromethyl)-3//-diazirin-3-yl]arenes (e.g., 36, R = H, CF3, respectively) 116 are more stable in acid than most diazoesters, and photolyze at around 360 nm to generate highly reactive carbenes such as 37. However, these diazirines can photo-isomerize to some extent to the... 

The phase separation temperature shift by photo-isomerization implies that in the temperature range between 19.4 and 26.0 °C, ultraviolet irradiation solubilizes the polymer or expands the polymer chain, while visible irradiation decreases the solubility, or shrinks the chain. Figure 3 shows the photo-stimulated phase separation at 19.5 °C accompanied by the conformation change of the polymer chain. Upon exposure to UV light (350 <%< 410 nm), the opaque solution became transparent, while visible irradiation (>. > 470 nm) again decreased the transmittance of the solution. The polymer chain conformation changed concomitantly with the phase separation. 

Vegetable juice p-carotene isomers, astaxanthin, zeaxanthin, canthaxanthin, echinenone Extraction with MTBE, photo isomerization C-30 MeOH-MTBE HPLC/DAD (450 nm)/MS/ APCI(+) 1 pmol for P-carotene 70... 

Aron and Fidanza 1982 has described a photochemical method for determination and separation of chloroquine at nanogram level covering a range between 4 and 104 ng. They showed that reversible photo isomerization of chloroquine takes place and yields a fluorescent photoproduct. The chloroquine solution are spoted on silica gel plates and developed in alcohol ammonia mixture. The dried plates are irradiated with the image of mercury are focused on chloroquine spots. The fluorescence intensity was recorded. For photolysis studies chloroquine zone were scraped from the plates and mixed well with the 4 ml of water. The clear supernatant decanted and studied by absorption photometry and spectrofluoro-metry (36). 

Scheme 5 Feringa s unidirectional rotor [32], The 360° unidirectional rotation is achieved in a four-step process involving alternative photo-isomerization of the double bond and thermal interconversion of helicity...

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

Cis-stilbenes, which are easily obtained from their trans-isomers by trans, cis-photo-isomerization, can give rise to dihydrophenanthrene derivatives upon direct irradiation (Scheme 9.17). The latter compounds are easily converted into phenanthrenes under oxidizing conditions (oxygen, I2, or rc-electron acceptors such as tetracyano-ethene) [24]. 

The full paper on the photo-isomerization of cis -verbanone has been published (Vol. 4, p. 61) trans -verbanone gives the same products but in opposite proportions.308 Direct, and sensitized, photochemical rearrangement of 4-methylverbene (229) gives 3-methylenelimonene, from the corresponding triplet biradical, in... 

The conversions of various steroidal l,4-dien-3-ones (109) into 5-ene-la,3/3-diols (111) via deconjugation to the l,5-dien-3-one, reduction to the l,5-dien-3/8-ol (110), and hydroboration have been described in a series of papers.125 The corresponding 5,7-diene-la,3j8-diols (118), required for photo-isomerization as a key step in the preparation of la-hydroxylated derivatives of vitamin D, have been obtained either by allylic bromination-dehydrobromination of the 5-ene-la,3/3-diols or by the alternative route outlined in Scheme 2. The key step in this reaction sequence is the protection of the 5,7-diene system as the adduct (116) with 4-phenyl-... 

In such a situation, reliable theoretical studies on the absorption spectra would provide useful information on the relationship between the structure and the absorption spectrum. As shown in Figure 4-3, three models, Al, A2, and B, were examined for the photo-isomerization. The Models Al and A2 were based on the Resonance Raman study by Kneip et al [59], For Model A2, we also referred to a study by Lippitsch et al. [60] in which a rotation around a single bond (C14-C15) was also suggested (Hula Twist). Model B was based on the Resonance Raman study by Andel III and co-workers [56],... 

Figure 4-3. Possible mechanisms for the photo-isomerization of phytochromobilin...

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