Photocycloaddition of Arenes to Olefins

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441.  

2-Additions of benzene, not to olefins but to acetylenes have been utilized in the synthesis of cyclooctatetraenes (4.38)443). 

Naphthalene and substituted naphthalenes add to olefins 444) (4.39) and to acetylenes445) (4.40) to give 1,2-adducts. In the latter case the primary addition product undergoes a further [2 + 2]intramolecular cycloaddition. 

The addition of anisole to cyclopentene gives an 1,3-adduct which rearranges to tricycloundecenone in the presence of traces of acid (4.41)446). 

The uniqueness of the intramolecular 1,3-photocycloaddition of arenes to olefins with the developement of three new rings and up to six stereocentres has been efficiently demonstrated in the synthesis of (+)-a-cedrene from the 

---

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

A fascinating new route to ( )-hirsutene involves an arene-olefin photocycloaddition of the acetate (105) to form the alcohol (106), albeit in low yield, as a mixture of three isomers. Acid-induced cleavage of a cyclopropane ring then... 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

A schematic representation of the possible processes in arene-olefin photocycloaddition according to Leismann et al. [182] is given in Scheme 35. Increasing charge separation in the exciplex, caused by decreasing ionization potentials of the olefins, can lead to zwitterionic intermediates, which are able to form ortho adducts. Increasing solvent polarity can reduce the formation of adducts by propagating the formation of radical ion pairs. The possibility that the ortho adduct may be formed directly or via a different short-lived intermediate was not incorporated in the scheme. 

Yamamura has extensively explored the intramolecular [5 - - 2]-cycloaddition mode. This chemistry bears a remarkable similarity to the so-called arene olefin meta-photocycloaddition [64] reaction (e.g., 267 to 268) that has frequently been used in the synthesis of complex bioactive natural products. While different intermediates are involved, the photo-and electrochemical reactions provide access to many of the same basic ring systems. This is nicely demonstrated by comparing key steps and the outcome of an electrochemical approach to the total synthesis of pentalene (53) [65] and a photochemical pathway that ultimately led to cedrene (compare equations 48 and 49). 

The arene-olefin meta-photocycloaddition approach to triquinane natural products is further illustrated this year with a three-step total synthesis of ( )-silphinene (5) starting from 2-bromotoluene (1). Thus, irradiation of the arene-olefin (2) derived from (1) gave a 1 1 mixture of the photoadducts (3) and (4) in 70% yield. Regioselective reductive cleavage of the cyclopropane bond in (3),... 

---