Molecular orbital model for the photochemistry of .beta.,.gamma.-unsaturated ketones

Houk, K. N.; Northington, D. J.; DUKE, R. E. JUN.

doi:10.1021/ja00772a073

Cited by 28 publications

(9 citation statements)

References 2 publications

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“…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 .…”

Section: Introductionmentioning

confidence: 99%

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

et al. 1996

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The oxadi-π-methane ([1,2]-acyl shift) and [1,3]-acyl shift rearrangements of a simple β,γ-enone (but-3-enal) have been investigated using MC-SCF computations in a 6-31G* basis set. The excited state reaction pathways and decay funnels for this model compound can be used to explain the direct and triplet-sensitized photochemistry of β,γ-enones in general. Our calculations show that the “classical” biradical intermediates proposed for both reactions correspond to decay funnels at which four states (S1(nπ*), T1(ππ*), T2(nπ*), and S0) are degenerate. Both efficient internal conversion (IC) and efficient intersystem crossing (ISC) can occur at these points, and the ground state reaction path is therefore the same regardless of the state initially populated. The ratio of products formed on photolysis is governed by the relative heights of the barriers leading to these decay funnels, and these will be sensitive to substituent effects on the reactant molecule. The oxadi-π-methane rearrangement is found to occur via a three-step process, where the four-level decay funnel corresponds to the first of two floppy intermediates on S0. There are two possible mechanisms leading to the [1,3]-acyl shift product: one involving the four-level decay funnel which corresponds to a “tight” intermediate in a quasi-concerted pathway, and a second which involves dissociation and recombination.

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Section: Introductionmentioning

confidence: 99%

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

et al. 1996

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“…Schustnf & Usderwood [4] proposed a rationalization based on spin density distribution of the 3(%,n*)-state, whereas a CNDO/S calculation with configuration interaction b:y Houk [5] predicts n,n* character of the ODPM reactive triplet. In the rearrangement of the cyclopentadiene-tropone [4 + 61 adduct it was argued that either the triplet n,z* or charge-transfer state is responsible for reaction [6].…”

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confidence: 99%

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

Tegmo‐Larsson¹,

Gonzenbach²,

Schaffner³

1976

Helvetica Chimica Acta

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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.

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“…The Hn,x*) excited state is represented in Figure 2 as a resonance hybrid Of structure 50 and the charge-transfer (CT) structure 51, in accord with recent MO calculations on /3,7-unsaturated ketones. 46 The importance of 51 increases with the polarizability of the olefin x elections, which in turn is reflected by a lower x ionization potential. By analogy with previous studies,41 •45 this CT structure presumably lies on the reaction pathway to oxetane but ring closure requires rotation of the olefin p orbital into nearly the same plane as the oxygen n orbital.…”

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confidence: 99%

Systematic structural modifications in the photochemistry by .beta.,.gamma.-unsaturated ketones. I. Cyclic olefins

Engel¹,

Schexnayder²

1975

J. Am. Chem. Soc.

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The photochemical behavior of six 0,7-unsaturated ketones (5a-e, 19), whose double bond is incorporated into rings of four to eight members, has been studied under acetone sensitization and direct irradiation. In the triplet manifold, the four and five-membered rings undergo the oxadi-ir-methane (ODPM) rearrangement, but the larger rings do not. Only the four-membered compound (5a) shows a cleavage upon direct irradiation, but all of the others give cyclobutanols. The seven-and eight-membered compounds yield oxetanes, a reaction which is favored by high olefin electron density and the ability of the olefin to twist in the excited state. Several thermal rearrangements of the products to starting ketone and to one another have been noted.

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Molecular orbital model for the photochemistry of .beta.,.gamma.-unsaturated ketones

Cited by 28 publications

References 2 publications

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

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

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

Systematic structural modifications in the photochemistry by .beta.,.gamma.-unsaturated ketones. I. Cyclic olefins

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