Photochemical-mediated solid-state [2+2]-cycloaddition reactions of an unsymmetrical dibenzylidene acetone (monothiophos-dba)

A novel unsymmetrical dibenzylidene acetone (monothiophos-dba, compound 3), containing two chemically different alkene moieties, undergoes a selective photochemically-induced [2+2] cycloaddition solid-state transformation (single crystal and powder), as shown by X-ray diffraction methods. In the single crystal the rate constant for the cycloaddition reaction was determined {k = (2.3 +/- 0.1) x 10(-4) s(-1)}, with a 69% conversion attained, which compares well with a two-photon irradiation experiment using a Nd:YAG laser pumped dye laser at 605 nm radiation (66% conversion). Full conversion was attained using powdered 3 after 43 h. A similar conversion was noted between the single crystal and powder forms (59% and 66% conversion after 5 h irradiation). Irradiation of monothiophos-dba in solution using a 325 nm laser, and monitoring by H-1 NMR spectroscopy, showed that no cycloaddition occurred, but exclusive photochemically-induced trans -> cis isomerisation of the alkene bond connecting the unsubstituted aryl group (cis -> trans isomerisation in these electron-deficient and polarised C=C bonds occurs rapidly by thermal equilibration).