Photoreactions with Aromatics Cycloaddition

The first investigations of the photochemical reaction of MA with benzene can be credited to three groups of workers.Thus, when a benzene solution of MA was irradiated with UV light, a 2 1 MA benzene adduct was formed. The adduct has been assigned the structure 1. Schenck and Steinmetz have 

This reaction is quite general with aromatics although earlier reports claimed that only polymeric material was produced from benzene derivatives. 

A number of substituted benzene derivatives have since been reported to undergo formation of the diadduct. Table 6.1 summarizes the adduct yields from various substituted benzene derivatives obtained by Bradshaw.  

It has generally been assumed that the first step in the photoreaction is the formation of a cyclobutane adduct 2. The latter being a diene undergoes a Diels-Alder reaction to give the final adduct 1. That the diene 2 does undergo the required DA reaction has been shown.(See Sec. 6.2.2 for the mechanism.) 

In the case of substituted benzenes, the position of attack is quite important. Thus, as illustrated below, initial 1,2, 2,3, or 3,4 attack will give final adducts 3, 4, or 5 respectively. Bryce-Smith and Gilbert,based on spectroscopic evidence, have assigned structure 5 to the adduct formed with diphenyl (R = Ph). Bradshaw also studied the reaction of alkylbenzenes with MA sensitized with acetophenone. He confirmed that the adduct forms with an initial attack at positions 3,4. However, Bryce-Smith and Gilbert later showed that toluene, (7-xylene, and p-xylene each gave two thermally stable isomers on benzophenone-sensitized reaction with MA. Furthermore, the composition of the isomers changed with temperature as illustrated in Table 6.2. Under ambient conditions for toluene and (7-xylene, it appears that the preferred attack is at positions 3,4 with smaller amounts of product due 

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Photo-[4+4] cycloaddition of 2-pyridone is not restricted to reaction with other pyridones. Sato, Ikeda, and Kanaoka found that cyclic and acyclic-1,3-dienes photoreact with 2-pyridones (Figure 8), ° reactions similar to the [4+4] photocycloaddition of 1,3-dienes with other aromatic molecules. Cyclopentadiene is abetter substrate than cyclohexadiene, as it is for the Diels-Alder reaction. A mixture of trans (24) and cis (25) isomers is formed in all cases. Use of acyclic... 

This chapter deals with the photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement reactions of aromatic compounds and related photoreactions. 

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