Photochemical Cycloaromatization Reaction

Selective photochemical initiation is a viable option for inducing cycloaromatizations of enediynes and related compounds. Although increasing the efficiency of photochemical cycloaromatizations has been shown to be feasible, further research is needed to fully elucidate the electronic factors that control these processes. The presence of two orthogonal x-systems in alkynes is important for the understanding and control of photochemical cycloaromatization reactions since the two x-systems play complementary and important roles. 

Figure 9.9 Bergman cycloaromatization reactions for ltex-3-ett-l,5-diyne and its perfluorinated congener, as well as a photochemical reaction scheme for generating the perfluorinated diradical from an iodinated precursor. What spectral features would be expected to be most diagnostic of the different intermediates What levels of theory would be appropriate for predicting these spectral signatures (Note that equilibrium arrows of unequal length indicate which species predominates at equilibrium.)...

Bergman cycloaromatization reaction Thermal or photochemical cycloaromatization of enediynes to form substituted benzene rings. 56... 

Turro, N.J. Evenzahav, A. Nicolaou, K.C. Photochemical analogue of the Bergman cycloaromatization reaction. Tetrahedron Lett. 1994, 35, 8089-8092. 

Transition metal catalysts can also be used in photochemical organic synthesis. For example, the photo-Bergman reaction (cycloaromatization see also Scheme 6.62) of (Z)-hex-3-en-l,5-diyne [(Z)-589], which is obtained by photostationary E Z isomerization (Section 6.1.1) from ( )-589, occurs via transition metal-catalysed cycloaromatization reaction followed by photochemical dissociation of the arene ligand from the complex 590 (Scheme 6.289).1530 The product, 1,2-di( -propyl)benzene (591), is obtained in 91% chemical yield. 

In the absence of the competing coordination with arene, the Ru- and Fe-catalyzed reactions of nonbenzannelated enediynes proceed quickly and efficiently, although photochemical liberation of the arene ligand occurs more effectively with Fe due to its lower arenophihcity relative to ruthenium (Scheme 30.18). Facile photochemical dissociation of the prodnct-metal complexes can be used for the creation of catalytic cycles. A fascinating feature of this approach is that it allows incorporation of trans/cis isomerization in the catalytic cycle. The possibihty of using trans enediynes in cycloaromatization reactions has the potential of dramatically increasing the scope of these reactions. 

The major classes of nonchain free-radical reactions are photochemical reactions, reductions and oxidations with metals, and cycloaromatizations. 

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