Triplet ground state intramolecular reactions

Mechanistic evidence indicates 450,451> that the triplet enone first approaches the olefinic partner to form an exciplex. The next step consists in the formation of one of the new C—C bonds to give a 1,4-diradical, which is now the immediate precursor of the cyclobutane. Both exciplex and 1,4-diradical can decay resp. disproportionate to afford ground state enone and alkene. Eventually oxetane formation, i.e. addition of the carbonyl group of the enone to an olefin is also observed452. Although at first view the photocycloaddition of an enone to an alkene would be expected to afford a variety of structurally related products, the knowledge of the influence of substituents on the stereochemical outcome of the reaction allows the selective synthesis of the desired annelation product in inter-molecular reactions 453,454a b). As for intramolecular reactions, the substituent effects are made up by structural limitations 449). 

It is possible that some of these photochemical cycloadditions take place by a [,2S + 2S] mechanism (which is of course allowed by orbital symmetry) when and if they do, one of the molecules must be in the excited singlet state (St) and the other in the ground state.980 The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,981 making it likely that the [ 2S + 2S] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain cases, the diionic) mechanism is taking place. In one intramolecular case, the intermediate diradical has been trapped.982 Photosensitized 2ir + 2-rr cycloadditions almost always involve the triplet state and hence a diradical (or diionic) mechanism. 

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

The intramolecular proton transfer (IPT) reaction is unfavorable in the ground state from both the thermodynamic as well as kinetic reasons for both compounds. However, both factors favor the ESIPT process in the lowest excited singlet and triplet states. 

Fig. 20. Molecular orbital description for the electron transfer reactions within ECL processes of intramolecular donor-acceptor systems A-D to give (A) excited singlet intramolecular charge-transfer state (B) excited triplet intramolecular charge-transfer state (C) locally excited triplet and (D) ground state. Adapted from [138],...

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