Rearrangement singlet

Whilst 1-methylcyclopropene is inert towards singlet oxygen, the photosensitized oxidation of cyclopropenes 288 rapidly produces numerous products which include enones. This latter free radical oxidation is thought to provide 289 and 290 which afford products by secondary rearrangement singlet oxygen is not involved in these reactions. 

When the valence tautomeric mixture of oxepin and benzene oxide is treated with singlet oxygen, the primary product is the 1,4-endoperoxide 3 which has proven to be too labile for isolation.219 Its formation can be rationalized by a 1,4-addition across the diene system of the benzene oxide structure 3 then rearranges to ba s-3,6,9-trioxatetracyclo[6.1.0.02 4.05 ]nonane (transbenzene trioxide, 4). 

If a charge exchange process, A + + B- A -f- B +, occurs when the distance between the two particles is large, we expect that no transfer of translational energy takes place in the reaction and that the same selection rules govern the ionization as in spectroscopic transitions. This means that if the molecule B is in a singlet state before the ionization, the ion B + will be formed in a doublet state after ionization of one electron without rearrangements of any other electrons, at least for small molecules. 

Scheme 5 Photo-Arbuzov rearrangement of arylethylphosphites via a short-lived singlet proximate radical pair mechanism. Reprinted with permission from [22]. Copyright 2001 American Chemical Society...

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

Interestingly, it was possible to probe the spin-forbidden component of the tunneling reaction with internal and external heavy atom effects. Such effects are well known to enhance the rates of intersystem crossing of electronically excited triplets to ground singlet states, where the presence of heavier nuclei increases spin-orbit coupling. Relative rates for the low-temperature rearrangements of 12 to 13 were... 

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