The spectroscopic properties and photoisomerization reactions of several (E)- and (Z)-cinnamamides have been investigated in the absence and presence of the strong Lewis acid BF3. The (E)-cinnamamides are essentially planar and exist predominantly in the enone s-cis conformation, except in the case of the α-methyl tertiary amide which adopts the s-trans conformation in order to minimize nonbonded repulsion. The (Z)-cinnamamides exist predominantly in the highly nonplanar s-trans conformation. This unusual conformational preference is attributed to intramolecular charge transfer from the aromatic to amide functionality. Photoisomerization efficiencies are dependent upon N-alkylation, aromatic substitution, α-alkylation, solvent, and excitation wavelength. These effects are attributed to the existence of two lowest energy π, π singlet states (one reactive and one nonreactive) whose relative energies are dependent upon substitution. The cinnamamides form 1:1 complexes with BF3 with equilibrium constants >103. Complexation alters both the electronic structure and photochemical behavior of the cinnamamides. Quantitative E → Z isomerization has been observed for the BF3 complexes of two tertiary amides.
ASJC Scopus subject areas
- Organic Chemistry