Abstract
The reaction of an ethyl Grignard reagent with a 4-(4-pyridyl) coumarin was found to yield a 4-ethyl-4-(4-pyridyl)-3,4-dihydrocoumarin as well as the expected 2,2-diethyl-4-(4-pyridyl)-2H-chromene. A series of substituted coumarins was prepared to investigate the mode of addition of Grignard reagents. The novel 1,4-addition reaction to give a dihydrocoumarin was found to be enhanced when the 4-substituent of the coumarin was strongly electron-withdrawing, and depressed when the substituent was electron-donating. The Grignard reagent itself was found to affect the type of products obtained. Since the reactions of dialkylmagnesiums with coumarins were found to give high yields of the 1,2-addition products almost exclusively, it was concluded that this was the reactive species present in a Grignard reagent in those reactions which led to 1,2-addition. Conversely, alkyl-magnesium halides were thought to be the reactive species from a Grignard reagent when 1,4-addition products were obtained. The difference in mode of addition of the two forms of the reagent suggested that the dialkylmagnesiums reacted with a coumarin via a cyclic intermediate to give the 1,2-addition product, whilst alkylmagnesium halides effectively provided the carbanion R- nucleophilic attack by this leading to 1,4-addition. The reaction of a phenyl Grignard reagent with a 4-methylcoumarin yielded a phenylbutenone by the novel single 1,2-addition of a Grignard reagent to a 4-substituted coumarin. Attempts to purify this butenone by crystallisation afforded the isomeric 2-hydroxychromene. Some of the 3,4-dihydroeouinarins have been treated further with Grignard reagents to give products dependent upon the reaction conditions. Chromanols, formed by the novel single 1,2-addition of Grignard reagents, and carbinols derived from double 1,2-addition could be isolated. In some instances the double addition reaction could be carried out as two separate stages, with isolation of the intermediate chromanols.