Optimisation of substitution at the 2- and 5- positions of 3,4-dialkoxythiophenes via the Mannich reaction: the influences of steric crowding, electrophile reactivity and temperature

Abstract

A number of 3,4-dialkoxythiophene compounds have been synthesised and their reactivities assessed via the Mannich reaction with secondary amines. These reactions surprisingly gave the bis-Mannich bases substituted at the 2- and 5-positions as well as the expected mono-Mannich bases substituted at the 2-position. Conditions were optimised to give symmetrical bis-2,5-Mannich bases in one step and asymmetrical bis-2,5-Mannich bases in two steps. Several bis(thien-2-ylmethyl)amines derived from 3,4-dialkoxythiophenes are reported, their synthesis being performed under both normal and high dilution conditions. Some syntheses also afforded the (thien-2-ylmethyl)amine oligomers. Further substitution of the bis(thien-2-ylmethyl)amines at the 5-position via the Mannich reaction also proved successful. The factors affecting the yields and substitution patterns are discussed, together with molecular modelling of the spatial requirements.