Water-Participation in the Crystalline-State Photodimerization of Cinnamic Acid Derivatives. A New Type of Organic Photoreaction

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Abstract
A new type of organic photoreaction, a water-participation reaction in the crystalline state, has been investigated. The photoirradiation of p-formylcinnamic acid (p-FCA) crystals in the presence of water leads to 4,4′-diformyl-β-truxinic acid (p-FCA dimer) which has one molecule of crystal water. A comparison of the photodimerization carried out in the presence/absence of water revealed the effects of the water molecule on the reaction. That is, the water molecule is incorporated into the p-FCA dimer during the photodimerization process. Furthermore, the water accelerates the reaction rate of photodimerization, leading to the formation of the crystalline dimer, whereas only the water-free amorphous dimer results in the absence of water. In addition, by heating at 100–110 °C, the p-FCA dimer with a water molecule is converted into a water-free crystalline dimer, which is not attainable from a water-free amorphous dimer. The photodimerization of several β-form crystals of cinnamic acid derivatives has been examined from the viewpoint of water-participation: p-nitro, p-chloro, and 3,4-dichlorocinnamic acid were found to photodimerize in the same manner as p-FCA.