"Syn effect" in nucleophilic addition of amines to 1,3-dienyl sulfone and ethyl (E)-2,4-pentadienoate

The stereochemistry of nucleophilic addition of amines to (E)-1-tosyl-1,3-butadiene was investigated. The Z/E ratios of the resulting allylic sulfones varied with amines, solvents, temperature, and concentration. When diethylamine was reacted in low concentration at high temperature, the corresponding sterically unfavorable (Z)-4-amino-2-butenyl sulfone was preferentially obtained. The stereochemistry of nucleophilic addition of amines to ethyl (E)-2,4-pentadienoate, which possesses an ester group as a conjugated electron-withdrawing group instead of a p-toluenesulfonyl (Ts) group, was also found to realize similar high Z selectivity. The predominant formation of Z isomers in both cases was rationalized by a “syn effect,” which might be mainly due to n⁄σ→π* interaction and/or 6π-electron homoaromaticity.