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(t)) to the radical pair R · · R ′¹ and intersystem crossing (k_isc) to the T₂ state are among the competing S₁ deactivation processes. The T₂ state in turn cleaves (k) to R · · R ′³ A ‘low-temperature range’ with k_isc ? k(t) and a ‘high-temperature range’ with k(t) ? k_isc exhibiting preferential reactivity from the T₂ and S₁ states, respectively, can be defined for all three β,γ-unsaturated ketones 1a – c .