Evidence for singlet state β cleavage in the photoreaction of α-(2,6-dimethoxyphenoxy)-acetophenone inferred from time-resolved CIDNP spectroscopy

The photochemistry of α-(2,6-dimethoxyphenoxy)-acetophenone (1) was studied using CIDNP spectroscopy; an XeCl excimer laser (308 nm) and a 250 MHz ¹H nuclear magnetic resonance (NMR) spectrometer were employed. The results are compared with the photoreactions of α-hydroxyacetophenone (2) and α-methoxyacetophenone (3). The CIDNP spectra of 1 can be interpreted as a result of a β cleavage from the singlet state. With time-resolved CIDNP spectroscopy it is possible to detect the enol of acetophenone in a second reaction path and to observe the tautomerization to acetophenone. Evidence is presented that this reaction path starts with the photoreduction of 1 from the triplet state. Compound 2 shows α cleavage from the triplet state and no β cleavage. As expected no CIDNP spectra are observed from 3 which is known to react via a Norrish type II mechanism.