LASER PHOTOISOMERIZATION OF METHYL ALPHA-ARYLCINNAMATES, EFFECT OF CHLORO SUBSTITUTION

An investigation of the 308 nm laser photochemistry of methyl alpha-ph enylcinnamate (1a) and a series of p-chloro/substituted derivatives is reported. Dilute solutions of (E)-1a or (Z)-1a undergo photoisomeriza tion, but also form ring closure products analogous to those observed during the photolysis of cis-stilbene. The photostationary state (PSS) compositions (E/Z) were determined by analytical high performance liq uid chromatography (HPLC). The PSS values are not dependent on the las er power or the presence of oxygen. However, they are unusually rich i n the E isomer compared with the PSS of alkyl cinnamates (over 70% E c ompared with about 50% E for alkyl cinnamates). p-Chloro substitution on the a ring causes a small increase in the E content of the PSS, whe reas p-chloro substitution on the beta ring leads to a dramatic reduct ion in the E content of the PSS (28% E). Possible explanations for the se results are discussed.