Abstract:Summary2-Cyclopentenyl and 3-phenyl-2-cyclopentenyl methyl ketones (15-18, 30, 31) undergo a 1,3-acetyl shift on direct irradiation, and the oxa-di-7c-methane rearrangement to photochemically non-interconverting endo and exo bicyclo-[2.l.O]pentyl methyl ketones on triplet sensitization. Exceptions include the 2-methyl-3-phenyl-2-cyclopentenyl methyl ketone 32 and the I-phenyl-2-cyclopentenyl methyl ketone 44 which are unreactive on direct irradiation and on triplet sensitization, respectively, and the 2-phenyl… Show more
“…The problem of the nature of the excited states involved in the ODPM rearrangement has also deserved a lot of attention. The first examples reported on the photochemical reactivity of β,γ-unsaturated ketones fit a pattern in which direct irradiation yielded products resulting from 1,3-acyl migration while the triplet-sensitized irradiation brought about the formation of ODPM products. 1c-f On the basis of these results, together to CNDO-MO calculations, phosphorescence studies, ,− quenching and sensitization experiments, ,,,, the excited state responsible for the rearrangement was considered to be the alkene T 1 state. The fact that the 1,3-acyl shift was not observed on triplet-sensitized irradiation and was not quenched by triplet quenchers, , in addition to nuclear polarization effects (photo-CIDNP) 79a,b favored the assignment of a keto S 1 (n,π*) excited state for the 1,3-acyl shift reaction.…”
Section: B the Nature Of The Excited State Involved
In
The Odpm Rearr...mentioning
confidence: 96%
“…The acetone-sensitized irradiation of the enantiomerically pure cyclic ketone 31 gives 32 , in high chemical yield, and with 95% optical purity. , The direct irradiation of 33 brings about the formation of the endo isomer 34 in quantitative yield . A series of studies on the ODPM rearrangement of cyclopentenyl ketones 35 show that all of them yield preferentially the endo product 36 . − The stereochemical results observed for the compounds collected in Scheme suggest concertedness or slow conformational equilibration of intermediates. 22 …”
“…The problem of the nature of the excited states involved in the ODPM rearrangement has also deserved a lot of attention. The first examples reported on the photochemical reactivity of β,γ-unsaturated ketones fit a pattern in which direct irradiation yielded products resulting from 1,3-acyl migration while the triplet-sensitized irradiation brought about the formation of ODPM products. 1c-f On the basis of these results, together to CNDO-MO calculations, phosphorescence studies, ,− quenching and sensitization experiments, ,,,, the excited state responsible for the rearrangement was considered to be the alkene T 1 state. The fact that the 1,3-acyl shift was not observed on triplet-sensitized irradiation and was not quenched by triplet quenchers, , in addition to nuclear polarization effects (photo-CIDNP) 79a,b favored the assignment of a keto S 1 (n,π*) excited state for the 1,3-acyl shift reaction.…”
Section: B the Nature Of The Excited State Involved
In
The Odpm Rearr...mentioning
confidence: 96%
“…The acetone-sensitized irradiation of the enantiomerically pure cyclic ketone 31 gives 32 , in high chemical yield, and with 95% optical purity. , The direct irradiation of 33 brings about the formation of the endo isomer 34 in quantitative yield . A series of studies on the ODPM rearrangement of cyclopentenyl ketones 35 show that all of them yield preferentially the endo product 36 . − The stereochemical results observed for the compounds collected in Scheme suggest concertedness or slow conformational equilibration of intermediates. 22 …”
On '71, n*-excitation the 6-keto-enones 5-8 are isomerized to compounds B (18, 22, 26, 28) via 1,3-acyl shift and to compounds C (19, 23, 27, 29) via 1,2-acyl shift, whereas the p, y , 6, E-unsaturated ketone 9 gives the isomers 32 and 33 by 1,2-and 1,5-acyl shift, respectively. Furthermore, isomerization of 6 to 24, dimerization of 8 to 30 and addition of methanol to 8 (8 -+ 31) is observed. Unlike 7 and 8 the acyclic ketones 5, 6 and 9 undergo photodecarbonylation on '71, 71"-excitation Evidence is given, that the conversion to B as well as the photodecarbonylation of 5, 6 and 9 arise from an excited singulet state, but the conversion to C as well as the dimerization of 8 from the T,-state.
“…had not been taken into account, and the comparatively small effects observed at 25" had been judged mechanistically insignificant. As a consequence, the results were erroneously interpreted in favour of a concerted 1,3-acyl-shift mechanism [3]. The finding that the photoracemization of (R)-(+)-l a and the 1,3-acyl-shift in a suitably deuteriated derivative of l:a occur with equal rates…”
mentioning
confidence: 99%
“…The formation of the thermally labile [34] oxadi-n-methane-rearrangement products on direct irradiation, which remained below reliable detection limits in the first photochemical study of Ib [3], has now been demonstrated by the use of improved analytical conditions. Figure 3.…”
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".
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