Substituent effect in the excited and ground states of cinnamonitriles

Abstract

Cinnamonitrile derivatives, which have a double bond conjugated with benzene ring and cyano group were selected to study the substituent effect on the reactivity of excited state and on the variation of chemical shift of vinyl protons of ground state. Chemical shift differences of vinyl protons of $\mbox{\underline{cis}}$- and $\mbox{\underline{trans}}$-cinnamonitrile derivatives are well correlated with $(σ_I,σ_R^˚)$, $σ_p^+$, and (F, R) (r=0.9996-0.8946), and found much better correlation than the case of methyl cinnamates. $\mbox{\underline{Para}}$-substituted and $\mbox{\underline{trans}}$-cinnamonitrile derivatives have larger resonance contribution than $\mbox{\underline{meta}}$-substituted and $\mbox{\underline{cis}}$-derivatives. Six (2+2) photoadducts obtained between the first excited singlet state of cinnamonitriles (cinnamonitrile, m-bromo-, m-chloro-, m-methoxy-, p-methyl, and pchlorocinnamonitrile) and tetramethylethylene (TME) are separated by column chromatography and found the reaction to be stereospecific. Fluorescence quenching of mmethoxycinnamonitrile by TME provides an additional support for the reactive state to be the first excited singlet state. The quantum yields in neat TME are calculated by the least square method; 0.586 for cinnamonitrile, 0.457 for m-chlorocinnamonitrile, 0.426 for m-bromocinnamonitrile, 0.524 for p-chlorocinnamonitrile, 0.229 for p-methylcinnamonitrile. The quantum yields of $\mbox{\underline{trans}}$-cinnamonitrile derivatives are decreased when the concentration of TME is decreased.