Abstract
Transition metal-catalyzed addition of an aromatic C-H bond to a C-C multiple bond provides an elegant, new atom-economic process that is widely applicable for a variety of valuable aromatic compounds. If coumarin can be synthesized from phenol and propiolic acid, this process will give coumarin and water and the most atom-economic process. When phenylpropiolic acid was reacted with 3-methoxyphenol in the presence of Pd(OAc)2 in trifluoroacetic acid, 7-methoxy-4-phenylcoumarin was obtained in 62% yield. Similarly, the reaction of 2-octynoic acid with 3-methoxyphenol gave 7-methoxy-4-pentylcoumarin in 52% yield. However, the reaction of propiolic acid gave 5- and 7-methoxycoumarins in 19 and 9% yields. The formation of the regioisomeric coumarins in the case of propiolic acid is considered to be attributed to relief of the steric congestion. The reaction with other phenols will be discussed along with the mechanistic aspect.
