Photoexcited enones

It is widely believed that enone-alkene photoadditions proceed through an exciplex (excited complex). For cyclopentenone and 7, the exciplex forms from the photoexcited enone in its triplet state and the glycal in its ground-state. The regiochemistry of addition probably reflects a preferred alignment of the addends in the exciplex. Because a photoexcited enone probably has considerable diradical character, it is reasonable to assume that the more electrophilic a-keto radical would prefer to bind to the more nucleophilic portion of the alkene, since this would maximise attractive interactions within the complex. The reaction of 7 with cyclopentenone would thus favour the formation of diradical 22, which would then ring-close to cyclobutane 14 (Scheme 6.7). 

As mentioned in the previous section, Corey15 considered an oriented jt-complex between the photoexcited enone and the ground state alkene. In the case of cycloaddition of methoxy-ethylene or acrylonitrile to 2-cyclohexenone the preferred orientation is shown in the scheme. Assuming that the triplet excited state of the enone is n,7t in nature15, calculations of charge distribution indicate that is fairly negative relative to Ca consequently the excited enone has a polarization complementary to its ground state. The more favorable orientation of the k complex is that in which the dipole of the olefin is opposed to that of the excited enone. 

Stereochemistry of Ring Fusion of Cycloadducts Stereochemistry of the Alkene Component in Enone Photocycloadditions Regiochemistry in Enone-Alkene [2+2]-Photocycloadditions Reactivity of Alkenes Toward Photoexcited Enones... 

Exploitation of time-resolved spectroscopy allows the direct observation of the reactive intermediates (i.e., ion-radical pair) involved in the oxidation of enol silyl ether (ESE) by photoactivated chloranil (3CA ), and their temporal evolution to the enone and adduct in the following way.41c Photoexcitation of chloranil (at lexc = 355 nm) produces excited chloranil triplet (3CA ) which is a powerful electron acceptor (EKelectron-rich enol silyl ethers (Em = 1.0-1.5 V versus SCE) to the ion-radical pair with unit quantum yield, both in dichloromethane and in acetonitrile (equation 20). 

There is considerable current interest over the question of what factors govern the a versus p regioselectivity of hydrogen atom abstraction by the carbon-carbon double bonds of photoexcited a,p-unsaturated ketones (12-14). In solution, abstraction by the p-carbon is normally preferred, and this is the regioselectivity followed in six of the eight enones studied in this work. This... 

Bischof and Mattay have shown that radical anion cyclization leading to spirocyclic products compete effectively with intramolecular [2 -f 2]-cycloaddition on photoexcitation of olefinic enones in the presence of triethylamine [332, 333]. The [2 -f 2] cycloadducts could be converted to the corresponding spiro compounds under PET conditions (Scheme 75) [307]. 

The influence of the sulfur substituent on the addition of radicals to the enones (265) has been assessed. The radicals are produced by hydrogen abstraction from the alcohols using photoexcited benzophenone and the products formed were identified as (266) and (267). 

Photoexcitation of 3-(5-cyclopropylpent-4-enyl)cyclohex-2-enones, e.g. 25, yielded diradicals which took part in hex-5-enyl type radical cyclizations prior to coupling. The cyclopropylmethyl-containing diradicals, formed in this cyclization process, either coupled directly to afford the major products, or first rearranged by jS-scission. The minor products were obtained by intramolecular coupling of the ring-opened diradicals. 

These data were accepted for a long time as proper measures of alkene reactivity in photocycloadditions and were critical elements in the formulation of Corey s famous exciplex mechanism to explain enone photocycloadditions.However, as is now well known, product ratios in photochemical processes reflect relative quantum efficiencies for disappearance of starting materials and/or formation of products and rarely reflect relative rates of reaction of the photoexcited state, particularly when it is a triplet state. ° ... 

On the basis of the regiochemistry and the relative rate factors observed in his studies of aUcene additions to photoexcited CH (see above), Corey proposed in 1964 that the first step of the [2-1-2]- photocycloaddition of enones to alkenes involved interaction of a polarized enone triplet excited state with the ground-state alkene to give an oriented Tt-complex (illustrated in structure 27) for the case of addition of CH to methoxyethene. 

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