Isomerization, configurational photochemical

Figure 22. Possible mechanisms for the configurational change of the exocyclic C=C double bond. A (nonprotonated chromophore) direct E Z isomerization (/tv) or coupling of two proton shifts ([H] formal I,5-H shifts) with the rotation around a C—C single bond (r>) B (protonated chromophore) direct E Z isomerization (either photochemical or thermal).

The i j -configuration of the 6,7-double bond in pre-vitamin D is critical to its subsequent thermal rearrangement to the active vitamin. A photochemical isomerization of pre-vitamin D to yield the inactive trans-isoTnen occurs under conditions of synthesis, and is especially detrimental if there is a significant short wavelength component, eg, 254 nm, to the radiation continuum used to effect the synthesis. This side reaction reduces overall yield of the process and limits conversion yields to ca 60% (71). Photochemical reconversion of the inactive side product, tachysterol, to pre-vitamin D allows recovery of the product which would otherwise be lost, and improves economics of the overall process (70). 

Especially detailed study of the mechanism of photochemical configurational isomerism has been done on Z- and A-stilbene." Spectroscopic data have established... 

The benzo[a Jquinolizinium ion and some of its derivatives (162) can be prepared by UV irradiation of styrylpyridinium salts (160 Scheme 91) (66JOC2616) in the presence of iodine (Table 6). It is believed that the styryl salt (160) is first isomerized to the cis configuration (161), then bond formation takes place, followed by loss of hydrogen. In view of the publication of an improved method for making styrylpyridinium salts (160) (69JMC1079) the photochemical method appears to offer an advantageous route to the parent compound (2). 

We now report the photochromic reaction of an azobenzene in the nanoporous silica film. Since the photochromic behavior is environmentally sensitive, photochromism of organic substances in solid matrices has been investigated to understand as well as to modify the photochromic behavior.[21] Photochromism of azobenzene and its derivatives due to cis-trans isomerization (Scheme I) has widely been investigated. Photocontrol of chemical and physical functions of various supramolecular systems has vigorously been studied by using photochemical configurational change of azobenzene derivatives.[22,23]... 

As mentioned above, the photochemically induced Pr - Pfr transformation manifests itself in the bilatriene chromophore, when still attached to the undecapeptide fragment, solely by the Z,E isomerism of the C(15) double bond in the Pr and Pfr froms (cf. Figure 5) [42,43], It is not yet known whether the chromophore in the intact Pfr chromoprotein differs additionally from the Pr chromophore with regard to the configuration of any of the other two exocyclic double bonds and/or the conformation. 

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

The 5,6,7-cis-triene configuration of vitamin D3 is important for its biological activity as the 5,6-trans form has very low activity. Exposure to iodine in non-polar solvents under diffuse light (39,40) or to mild acidic conditions (41) affords formation of the 5,6-trans isomer. The reverse transformation occurs photochemically (42). The 5,6-trans isomer can undergo further isomerizations upon exposure to heat (43) or acids or treatment with antimony trichloride. The cis and the trans forms of vitamin D3 display characteristic ultraviolet properties. The cis form as shown earlier has a UV maxima at 265 nm. The trans form has a UV maxima at 273 nm. 

The mechanism of the photoresponse was tentatively explained as follows. When azo units are in the planar, apolar, trans configuration, they merge into the hydrophobic core of the micelles, forcing the polypeptide chains to assume a coil conformation. Isomerization of the azo units to the skewed, polar, ds configuration inhibits hydrophobic interactions and causes the azo units to retreat from of the micelles, thus allowing the polypeptide chains to adopt the a-helix structure favored in the absence of micelles. In other words, the primary photochemical event is the trans-ds isomerization of the azobenzene... 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

Configurational dynamics, by photochemical and thermal cis—trans isomerizations, as well as... 

The initial photochemical step, (211) to (212), can be most simply viewed as a perturbed 6e electrocy-clic process, suggesting a trans configuration via conrotation for Ae dihydrophenanthrene intermediate (212). In support of this hypothesis, the stabilization of (216) by tautomerism to (217) in the photolysis of diethylstilbestrol (214), followed by ozonolysis, afforded only the racemic form of 1,2,3,4-buta-netetracarboxylic acid (218). The majority of dihydrophenanthrenes, however, are thermally unstable and undergo conversion to phenanthrenes (under oxidative or non-oxidadve conditions) or 1,/-H shifts to isomeric 9,10-dihydrophenanthrenes. ... 

Demonstration of cis addition with acetylene is unequivocal, in that the stereochemistry of the resulting bis(dihaloboryl)ethylene has been shown directly by spectroscopic evidence (90) and by conversion of the initially produced cis isomer to the trans compound by photochemical isomerization (27). Stereochemical inferences in the olefin addition reactions are based on the assumption that hydrolysis and oxidation of the bis(boryl) compounds to the corresponding diols occurs with retention of configuration, as is the case in closely related systems 19). The NMR spectrum of the addition product of B2CI4 with 1,3-cyclohexadiene is also consistent with cis addition (120). 

Stiver and Yates have studied the photochemical reactions of some hydroxy-keto steroids (28, 29). Irradiation of the isomeric compounds (28a, 29a) showed that the products obtained, (30) and (31) respectively, had retained the configuration of the carbon to which the hydroxy group is attached. The use of deuteriated derivatives (28b, 29b) has identified the hydrogen abstraction processes involved in the conversion of these ketones into the lactones (30b) and (31b) respectively. The authors " propose that there are two major factors which control the stereospecificity of the reactions. These are the shape of the hydroxy-bearing C-atom and the hydrogen transfer within the biradical formed on Norrish Type I fission. The stability of the biradical intermediate clearly plays an important part in determining the outcome of the reactions. 

The third mechanism (3) is the simplest one. When trans-cis photoisomerizable chromophores are incorporated into the polymer backbone, the photoinduced configuration change of the chromophores is expected to induce a conformation change of the polymer chain. Azobenzene (4) is the most widely used as the trans-cis photoisomerizable photoreceptor molecule. It undergoes isomerization from the trans to the cis form under ultraviolet irradiation, while the cis form can return to the trans form either thermally or photochemically [11]. 

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