Photo‐CIDNP. Investigation of β,γ‐Unsaturated Ketones: Evidence for Temperature‐dependent S₁vs. T₂ Reactivities of Cyclopent‐2‐enyl Methyl Ketones

Abstract

On ultraviolet irradiation of the cyclopent-Zenyl methyl1 ketones la-c at -5 4 " s t I 139", photo-CIDNP. effects of the starting ketones, the 1,3-acetyl shifted isomers (2), and radical disproportionation and corn bination 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 ratilonalized in terms of Schemes 2 and 3.a-Cleavage is a major excited-state reaction of 1 a-c on direct irradiation.Temperature-activated a-cleavage (k:(t)) to the radical pair R . . R" and intersystem crossing (&) to the T, state are among the competing s, deactivation processes. The T, state in turn cleaves (kz2) to R.-R'3. A 'low-temperature range' with k& k,S ( t ) and a 'high-temperature range' with k,S ( t ) % kisc exhibiting preferential reactivity from the T, and S 1 states, respectively, can be defined for all three p , y-unsaturated ketones la-c.Introduction. -Most 8, y-unsaturated ketones predominantly undergo two characteristic photoreactions: an allylic 1,3-acyl shift on direct irradiation, and a triplet sensitized oxadi-n-methane rearrangement (Scheme These results clearly showed from the beginning that at least two excited states are reactive. It has been universally accepted that the oxadi-n-methane rearrangement occurs from the lowest-lying triplet (TJ, and that this state is predominantly 7z, n+& in nature3). The I) 2, 3, Present address: Wissenschaftliches Hauptlaboratorium WHZ, BASF AG, D-6700 Ludwigshafen. See 111 for comprehensive reviews of P,y-unsaturated ketone photochemistry, and 121 for a progress report on cyclopent-2-enyl methyl ketones. For experimental identifications of Tl(rc,rc*) as the oxadi-rc-methane-reactive state, see 13-51, 0018-019X/79/6/ 1952-14$01.00/0 0 1979 Schweizerische Chemische Gesellschaft 5b. .Y = H b , The denotations (a)-(e) refer to the signal assignments in Figure I . Cf: Figure I A . d)CJ Figure 1B. e, A=enhanced absorption, E=emission. Negligible CIDNP. effect. g) in CD30D at -54".

Mechanism of the Oxadi-π-methane and [1,3]-Acyl Sigmatropic Rearrangements of β,γ-Enones: A Theoretical Study

“…The vertical excitation energies for the various states had to be calculated using state-averaged orbitals at the ground state equilibrium geometry (Figure a), and for this system the 3 (nπ*) vertical excitation energy lies about 3 kcal mol -1 above that of the 3 (ππ*) which is in acceptable agreement with experiment . Notice that the existence of a minimum and a barrier on the 1 (nπ*) surface close to the vertical excitation energy is consistent with the observed temperature dependence of the fluorescence. , …”

“…However, this selectivity is not universal, and the reasons for this are not fully understood. In some cases the [1,3]-acyl shift product is also formed on triplet sensitization, while the ODPM rearrangement product has been observed on direct photolysis of certain enones. − Nowadays it is generally accepted that the [1,3]-acyl shift occurs from an nπ* excited state in either the singlet (S 1 ) or triplet (T 2 ) manifold, while the ODPM rearrangement originates from the slightly lower energy triplet ππ* state (T 1 ) where the excitation is localized on the alkene moiety. , The energy difference between the two triplet states is thought to be as little as 2 kcal mol -1 . Both rearrangements have been the subject of several reviews, 12a,, and the possible reaction mechanisms have been summarized in Scheme .…”

“…The existence of this process has been confirmed by the formation of side products that result from the recombination of free radicals which are sometimes observed on the photolysis of acyclic β,γ-enones and electron-poor olefins. However, in the case of some cyclic β,γ-enones, for example, cyclopentenyl methyl ketones (CPMK), the stereochemistry of the products indicates that the reaction appears to take place at least partly by a quasi-concerted mechanism involving the biradical D , , where no scrambling of the substituent groups on the α and γ carbon atoms would be possible.…”

Effects of diffusion on the self‐termination kinetics of isopropylol radicals in solution

“…As other a-methyl and hydroxy substituted ketones, 2,4-dihydroxy-2,4-dimethylpentan-3-one is expected to undergo type I photocleavage from the n7r* triplet state to yield alkyl and acyl type radicals [20,22,27]. The acyl radical Electron spin resonance spectrum of (CH&COH in benzene at 20OC.…”

“…With the aid of Kaptein's rules [32] they are easily interpreted in terms of the expected reactions and are therefore discussed here only briefly. In reaction (5) the radical pair is formed from a triplet precursor [20], combines to ketone, disproportionates to 2-hydroxy-2-methylpropanal and the enol of acetone, and decays to free radicals. Decarbonylation is fast, and the isopropylol radicals form secondary pairs which react via reactions (7)-(9).…”