Photo-CIDNP. Investigation of β,γ-Unsaturated Ketones: Evidence for Temperature-dependent S1 vs. T2 Reactivities of Cyclopent-2-enyl Methyl Ketones

On ultraviolet irradiation of the cyclopent‐2‐enyl methyl ketones 1a–c at −54° ⩽ t ⩽ 139°, photo‐CIDNP. effects of the starting ketones, the 1,3‐acetyl shifted isomers (2), and radical disproportionation and combination products (4–7) were observed. These effects show a unique dependence of the polarization phase on temperature which is a novel feature in photo‐CIDNP. studies. The results of the investigation, which also included experiments using triplet quenchers, triplet sensitizers and radical scavengers, are rationalized in terms of Schemes 2 and 3. α‐Cleavage is a major excited‐state reaction of 1a–c on direct irradiation. Temperature‐activated α‐cleavage (k aS(t)) to the radical pair R · · R′1 and intersystem crossing (kisc) to the T2 state are among the competing S1 deactivation processes. The T2 state in turn cleaves (k aT 2) to R · · R′3 A ‘low‐temperature range’ with kisc ≫ k aS(t) and a ‘high‐temperature range’ with k aS(t) ≫ kisc exhibiting preferential reactivity from the T2 and S1 states, respectively, can be defined for all three β,γ‐unsaturated ketones 1a–c.