Alkene photoisomerization

The next three chapters are by Inoue and Mori, Albini, and Rossi, and deal with alkene photoisomerization reactions, the modification of benzylic positions and photochemical aromatic substitution reactions. (E)-2-cyclo-heptenone is produced upon irradiation of the Z-isomer at — 50 °C and can be trapped by cyclopentadiene to afford the adduct 13 [13]. Benzyl-substituted dihydroisoquinolinium derivatives can be used for the photochemical synthesis of tetrahydroisoquinolines. The corresponding... 

Figure 32 The 124 , 124Z, 124 , and 124Z equilibrium system in which intermolecular host-guest complexation of 1-adamantol competes with cinnamate self-complexation in 124Z and E to Z isomerization about the amide bond occurs while the E stereochemistry about the cinnamate alkene bond is retained. Photoisomerization by irradiating at 300 nm switches the stereochemistry about the cinnamate alkene bond to Z to produce the 125 , 125Z, 125 , and 125Z system in which no competitive cinnamate self-complexation occurs. The cinnamate alkene photoisomerization is reversed by irradiation at 254 nm to complete an on-off photoswitching process.

The sensitized photoisomerization of alkenes is complex, with a variety of sensitizer-olefin interactions operative.(78 83)... 

SCHEME E The biradical model for the direct ,Z-photoisomerization of (a) alkenes and (b) conjugated dienes... 

Diphenylethene (Stilbene). This molecule has been the subject of many photophysical and photochemical investigations and the subject of several reviews. It is the prototypical alkene for studies of photoisomerization. Transient spectroscopic measurements in the picosecond time domain have been performed on electronically excited tran -stilbene in a wide range of environments. Selections from these studies are described here. 

The loss of stereochemical memory in the non-vertical excited state implies that stereospecific, concerted reactions of an alkene singlet state may take place from the vertical state. Of particular importance is that the change from cis or trans geometry to something in between opens up a route for converting one geometrical isomer of an alkene to another, and this is a photoisomerization reaction that will be described in the next section. 

Photoisomerization of alkenes via the triplet excited state is known to be possible by triplet sensitization, usually efficient in conjugated C=C bonds that fulfill the requirement of possessing triplet excited energies below those of the typical triplet sensitizers such as acetone, acetophenone, benzophenone, etc. (Table 2). Sensitization with the opposite order of triplet excited energies is possible in cases with strong electronic or strong... 

A major advantage is the potential to lock (and protect) written information in the photobistable material. A number of chemical gated systems involving mutual regulation of the photochromic event and, for instance, fluorescence, ion binding, or electrochemical properties have been reported.1501 Scheme 19 illustrates a chiral gated response system based on donor-acceptor substituted alkene 17.[511 The photochemical isomerization process of both the M-ds and the P-trans form was effectively blocked by the addition of trifluoroacetic acid. Protonation of the dimethyl-amine donor unit of M-rfs-17a and P-trons-17b resulted in an ineffective acceptor-acceptor (nitro and ammonium) substituted thioxanthene lower half. Since the stereoselective photoisomerization of 17 relies on the presence of both a donor and acceptor unit, photochemical switching could be restored by deprotonation by the addition of triethylamine. 

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... 

The reactions of alkenes and related compounds are grouped here into nine sections. The first five deal essentially with photoisomerization processes—geometrical isomerization about a carbon—carbon double bond, concerted (electrocydic) cycfization, concerted shifts (usually of hydrogen) along the -system, thedi-jr-methane reaction. 

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