Substituent-dependent reactivity in aldehyde transformations: 4-(phenylethynyl)benzaldehydes versus simple benzaldehydes

Effects of substituents on transformations of 4-(phenylethynyl)benzaldehydes and related benzaldehydes were analyzed in aldol and thiazolidine formation reactions. The aldol reaction of 4-cyanobenzaldehyde was 54-fold faster than that of 4-methoxybenzaldehyde. In contrast, the aldol reaction of 4-(4-cyanophenylethynyl)benzaldehyde was only 1.4-fold faster than that of 4-(4-methoxyphenylethynyl)benzaldehyde. Electronic features of substituents are significantly less influential in a diphenylacetylene system than in a simple benzene system.