Stilbenes, photochemical conversion

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

Laarhoven et al. report that irradiation of (130) in methanol or hexane yields (131) by the route illustrated in Scheme 10. A mechanism involving radicals is proposed to account for the transformation of (i-methy 1-2-vinyl stilbene into l-(l-ind2myl)-l-phenylethene on irradiation. Calculations have been carried out in an analysis of the photo isomerization of naphthvalene into naphthalene. Photocyclization of the 2-vinylbiphenyls (132) under both direct and xanthone-sensitized irradiation conditions has been studied. The tetraene (133) is isomerized into (134) on irradiation. Tochtermann et al. have reported the photochemical conversion of the oxepine derivatives (135) into the tricyclic aldehydes (136). ... 

To summarize, in the present section, we have demonstrated that the stilbene photoisomerization is a training area, a relatively simple and convenient model reaction for a thorough investigation of detailed mechanisms of photochemical reactions and factors affecting the photochemical conversion rate. Theoretical and experimental data in this area vdll pave the way for practical application of stilbenes as switching materials and biophysical probes (Chapter 10). 

Irradiation rraws-2-[3-(7V-methylamino)propyl] stilbene 89 results in the formation of 7V-methyl-l-benzyltetrahydro-2-benzazepine 90 as the only significant primary photoproduct (equation 26), which in turn undergoes secondary photochemical N-demethylation. The final mixture contains 90 (38%) and 91 (25%) at high (>95%) conversion. Intramolecular photoadditions of these (equations 24-26) secondary (aminoalkyl)-stilbenes are highly regioselective processes24. 

There are a large number of photochemical cydizations of aromatic compounds that lead initially to polycyclic, non-aromatic products, although subsequent rearrangement, elimination or oxidation occurs in many instances to form aromatic secondary products. The archetype for one major class of photocydization is the conversion of stilbene to phenanthrene by way of a dihydrophenanthrene (3.60). 

Photocyclization of 1-benzylideneisoquinolines to the Aibexao[d,e,g -quinolines is a well-studied example of the above stilbene-phenanthrene cyclization and therefore has been extended to the development of new photochemical methods for the synthesis of aporphine alkaloids (11-14). In this section, photocyclization of 2-acyl-1 -benzylideneisoquinolines to dehy-droaporphines and their subsequent conversion to aporphines and oxo-aporphines are summarized. 

Pd-C, or iodine/ " The reaction is a photochemically allowed conrotatory conversion of a 1,3,5-hexatriene to a cyclohexadiene, followed by removal of two hydrogen atoms by the oxidizing agent. The intermediate dihydrophenanthrene has been isolated. The use of substrates containing heteroatoms (e.g., PhN=NPh) allows the formation of heterocyclic ring systems. The actual reacting species must be the c/i-stilbene, but frani-stilbenes can often be used, because they are isomerized to the cis isomers under the reaction conditions. The reaction can be extended to the preparation of many fused aromatic systems, for example, ... 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

Silver ions cause perturbation of the (E)-(Z) photoisomerization pathway for both stilbene and azobenzene . The efficiency of silver ions in this respect is compared with the effect of Nal which can only induce a heavy atom effect. Ag+ clearly forms complexes with both compounds. Observation of cis-trans conversion in olefin radical cations shows that electron transfer can bring about isomerization of stilbene derivatives. The efficiency of such processes obviously depends on the presence and nature of any substituents. Another study deals with photochemical generation, isomerization, and effects of oxygenation on stilbene radicals. The intermediates examined were generated by electron transfer reactions. Related behaviour probably occurs through the effect of exciplex formation on photoisomerization of styrene derivatives of 5,6-benz-2,2 -diquinoyE. ... 

A key basis for modern helicene cyclization chemistry occurred in 1964 when Wood and Mallory effectively prepared phenanthrene via photochemical induced cyclization of stilbene [81]. The new method effectively provided efficient pathways to substituted phenanthrene derivatives, easier access to existing and new helicene structures, as well as sufficient quantities for property studies. In the mid 1960s, Martin and coworkers successfully isolated the previously made hexahelicene as well as new hepta- 99, octa- 100, and nonahelicenes 101 via highly efficient syntheses (Scheme 1.22) [82]. These preparations culminated in photo-induced cyclization of bisarylethylenes with an I2 oxidant and Hg-lamp radiation source. The key improvements over previous efforts included production of a phenanthrene derivative via photo-induced cyclization of intermediate 102 and subsequent conversion to Wittig ylide 103, a common intermediate to the four helicenes. 

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