The Oxa‐di‐π‐methane Rearrangement of 1‐Phenyl‐3‐acetylcyclopentene: Identification of the Reactive Triplet as the Localized Styrene π, π*‐State. Preliminary Communication

Abstract

Dedicated to Professor V . Prelog on his 70th anniversary (16. 111. 76)Sunznzury. 1-Phenyl-3-acet.ylcyclopentene1) (3) undergoes triplet-sensitized oxa-di-n-methane rearrangement to a cu. 3 : 1 mixture of e d o -and exo-l-phenyl-5-acctylbicycio~.l.0;pentane2) (4). The reactive triplet of 3 has been identified as the localized styrene n.,n*-state of ET = 59 kcal/rnol by comparison of its phosphorescence at 77K in a rigid glass with that of I-phenyicyclopentene, and by sensitization quantum yields at room temperature in liquid solution. These quantum yields are independent of sensitizer energy in the range of 61-74 kcal/mol and reach zero for product formation with a sensitizer of Er = 59 kcal/mol. Although S1-t TI intersystem crossing is appreciable under phosphorescence conditions (fDD = 0.05), direct irradiation of 3 at 20" does not lead to triplet rearrangement, and the typical singlet 1,3-acetyl shift to 3-acctyl-3-phcnylcyclopentenes) (8) is observed only.

Synthetic Aspects of the Di-π-methane Rearrangement

“…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.…”

Photochemical studies on the mechanism of the oxadi-.pi.-methane rearrangement. An example of a stereospecific rearrangement

“…Chromatography of 1.185 g of this material on silica gel gave 42.0 mg of 2a and 396.0 mg (55%) of 6a. Distillation in a microstill gave pure 6methyltricyclo[5.3.0.0l'6]decan-2-one (6a) as a clear liquid: IR 2970,1670,1475,1455,1395,1342,1315,1275,1180,1130,1055,920 cm-'; NMR 2.50-2.90 (m,2H), 1.20-2.30 (br m, 11 ), 1.07 ppm (s, 3 H); UV (isooctane) Xmax 278 (26); mass spectrum m/e 164 (M+), 149,146,136,135,131,121,108,107,93 (base peak), 91, 79, 77, 67. Anal. Caled for Cj j Hj^O: C, 80.44; H, 9.82.…”

1971–1976 – Profiting from a New Beginning in Geneva

“…[17,18] Alternatively, an oxadi-π-methane rearrangement analogous to 19 → 20 is not competitive with the more efficient recombination/disproportionation and dissociation in geminate singlet and triplet radical pairs of aroyl analogs such as summarized in Scheme 5 for 21 and 22. [19] In Geneva, [20] and subsequently in Mülheim, [8] the 1,3-acetyl shift and oxadi-π-methane rearrangement of compounds of type 19 were analyzed eventually in great detail.…”