Alkenes photoexcited states

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. ° ... 

The observed products correspond to the formation of the most stable intermediate by the addition of the photoexcited triplet (diradical) state of acetophenone to the alkene. 

Cycloaddition of alkenes to the benzene ring does not occur when both molecules are in their ground electronic states. The reaction can only be brought about by photoexcitation of either of the two addends. Three types of photochemical cycloaddition of alkenes to benzene and its derivatives are presently known. Ortho photocycloaddition, also referred to as 1,2-photocycloaddition or [2 + 2] photocycloaddition, leads to bicyclo[4.2.0]octa-2,4-dienes. Meta photocycloaddition, also referred to as 1,3-photocycloaddition or [2 + 3] photocycloaddition, gives triyclo[3.3.0.02 8]oct-3-enes, also named l,2,2a,2b,4a,4b-hexahydrocyclo-propa[crf]pcn(alcnes. Para photocycloaddition, also referred to as 1,4-photocy-cloaddition or [2 + 4] photocycloaddition, results in bicyclo[2.2.2]octa-2,5-... 

Cu(I) triflate has been used as an olefin cycloaddition photoassistance agent as in reactions (42) and (43).127 Detailed study of reaction (43) shows that it is very likely that it is photoexcitation of a Cu(alkene)2 complex that results in the dimerization.127) This provides a second good example of the possible utility of transition metal photoassistance involving reactions between coordinated ligands. The nature of the excited state leading to reactions (40)-(43) is not known. 

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). 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, which can be lowered, however, by polar effects. [2 + 2] Photocycloadditions are common and usually involve diradical intermediates e.g., photoexcited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and triplet (n, 7t ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

There are a variety of photochemical reactions that non-conjugated dienes can undergo. One of these that is currently of considerable interest is the reactivity brought about by electron-accepting sensitizers such as the cyanoarenes. The photoreactivity of these systems involves the photochemical excitation of the sensitizer to an excited state. Thereafter, the reactivity is dependent on the ease of oxidation of the alkene or diene. With the transfer of an electron from the diene to the photoexcited sensitizer a radical cation is formed. It is this intermediate that brings about the various processes which occur within the diene systems under investigation. 

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. 

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