Formylation of aromatic hydroxy and alkoxy compounds by formic acid

By treatment of ethers of phenol, resorcinol, 2-napththol, alkylanisoles 10, 13, 15, 17, 20, 22, 24, 28 with formic acid/borontrichloride the aromatic aldehydes 11, 14, 16, 18, 21, 23, 25, 26, 29 can generally be prepared in low yields. The formylation reactions proceed between -20 and -10 degrees C within 20 min in 1,2-dichloroethane, chlorobenzene, or methylene chloride as solvents when formic acid and borontrichloride are used in excess [molar ratio of aromatic compound to HCOOH to BCl3 =1:1.2-1.7:1.6-1.8]. The more strongly activated 3,5-dimethoxyphenol (33) is formylated by formic acid/borontrichloride to give the aldehyde 34 with an acceptable yield. The reactions of resorcinol dimethylether with formic acid in the presence of super acids (mainly trifluormethansulfonic acid) in chlorobenzene or nitromethane deliver the aldehyde 21 only in small amounts.