E → Z Photoisomerization of Cinnamamides Using a Mixed-Solvent System in a Recycling Photoreactor.

This study aimed to improve the conversion of E-alkenes to Z-alkenes using a recycling photoreactor. Specifically, E-alkenes bearing Weinreb amides or pinacolborane functionalities were examined, but yielded unsatisfactory results even after multiple reaction cycles using acetonitrile as the solvent. Importantly, the photoisomerization reaction proceeded in mixed solvents predominantly composed of n-hexane, a solvent commonly used in column chromatography. For Weinreb amides derived from cinnamic acid, a mixed solvent of ethanol or ethyl acetate in n-hexane accelerated the photoisomerization reaction and improved chromatographic separation by HPLC. These results indicate that the use of such mixed solvents enables the efficient production of Z-alkenes bearing Weinreb amides via photoisomerization in a recycling photoreactor. The finding that photoisomerization reactions proceed in mixed solvents containing n-hexane indicates potential for future applications in photoreactions.