CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION IX (l): PHOTOREACTIONS OF SOME KETONES

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".

mentioning

confidence: 99%

“…[21] [33] except in CC1, where exciplex mechanisms have been proposed [42]. From the lack of temperature-dependent fluorescence [38] [43] of several aliphatic ketones with long S1 lifetimes Turro et al [38] recently concluded that S , a-cleavage either proceeds with zero activation energy or does not occur in the temperature range -5"s t I 80".…”

mentioning

confidence: 99%

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

Henne¹,

Siew²,

Schaffner³

1979

“…Neben dieser Keaktion wird, insbesondere bei u-Arylaldehyden, u , P-ungesattigten Aldehyden und bei rein aliphatischen Aldehyden fur A 5 270 nm eine intramolekulare Decarbonylierung nach HCR ---+ H R + CO (2) die nicht uber freie Radikale ablauft [9] [17] aus den Intensitaten A1 = I-I, der Signale wahrend und I,(t) nach Bestrahlungsende zur Zeit t bestimmt. Die Relaxationszeitlronstanten T, wurden nach der nrapid passage)) Methode [19] …”

Section: )unclassified

“…-In den letzten Jahren haben wir und andere in einer Reihe von Arbeiten [l- ------+ K-C. + * R R = CH,, R' = C(CH,), [2] R = R' -= CH(CH,), [4] K == R' = C(CH,), [6! an die sich Sekundarprozesse der entstehenden Acyl-und Alkylradikale anschliessen. Rei photochemischen Reaktionen in Fliissigkeiten und in der Gasphase nachgewiesene PI-odukte 112-141 lassen erkennen, dass G1.…”

unclassified

Photochemische Primärreaktionen α‐verzweigter aliphatischer Ketone und Aldehyde in lösung

Blank

Henne

Fischer

1974

I. 74)Swmzary. UV.-Irradiation of methyl t-butyl ketone, di-t-butyl ketone, pivalaldehyde and isobutyraldehyde in benzenc solution leads to chemically induced dynamic nuclear polarization (CIYNP.) of the parent compounds and of various reaction products. C1DNP.-effects and product distributions establish type I a-cleavage predominantly of triplet state molecules as 'the major primary photochemical step for all the four a-branched carbonyl compounds. I n chlorinated solvents singlet reactions interfere with the triplet processes.

Elektronenspinresonanz freier Radikale bei photochemischen Reaktionen von Ketonen in Lösung

Paul

Fischer

1973

Self Cite

186

Szlnwaary. Transient alkyl and acyl radicals are detected and identified by electron spin resonance during UV.-Irradiation of dialkyl and alkyl aryl ketones in liquid solutions. They are formed by thc primary processes of a-cleavage, intermolecular photoreduction or a-chloro elimination. For sevcral ketones a temperature dependent competition between cr-cleavage and photoreduction is observed indicating a higher energy of activation for the cleavage than for the reduction process. The results are discussed with respect to the photochemical behaviour of the ketones.Electron spin resonance parameters are reported for a variety of radicals. In particular 13C coupling constants support a sigma type structure of acyl radicals and require a nearly planar radical ccnter of t-butyl and isopropyl radicals.1 . Einleitung. -Das photochemische Verhalten vieler Ketone in Losung ist in den letzten Jahren eingehend untersucht und auf das Eintreten einer oder mehrerer der folgenden Primarreaktionen nach Anregung des nn*-ubergangs der Carbonylp p p e n zuruckgefuhrt worden [I41 :a-Spaltung (Typ I) hv RI-CO-R,Rl-CO-( CH,) ,-CHR,R,RI-CO-CHC1-R,