Photochemical intermediates, generation ground state reactions

Hypervalent sulfonium radicals, H3S [188] and (CH3)2SH [189], have been shown to be metastable on the microsecond time scale when generated as partially deuterated species and studied by variable-time NR mass spectrometry. The metastability was attributed to the formation of excited electronic states, as the ground doublet states were calculated to be unbound and predicted to dissociate exothermically and without a barrier by S-H bond cleavage [188,189]. Hypervalent sulfonium radicals of the R3S type have been the long sought-after intermediates of radical substitution in sulfides in solution, but have never been detected [190]. The NRMS studies, combined with ab initio calculations, show that such intermediates most likely do not exist on the ground state potential energy surface, but may be of importance in photochemical reactions. 

In several instances, Mannich-type cyclizations can be carried out expeditiously under photochemical conditions. The photochemistry of iminium ions is dominated by pathways in which the excited state im-inium ion serves as a one-electron acceptor. The photophysical and photochemical ramifications of such single-electron transfer (SET) processes as applied to excited state iminium ions have been expertly reviewed. In short, one-electron transfer to excited state iminium ions occurs rapidly from one of several electron donors electron rich alkenes, aromatic hydrocarbons, alcohols and ethers. Alternatively, an excited state donor, usually aromatic, can transfer an electron to a ground state iminium ion to afford the same reactive intermediates. Scheme 46 adumbrates the two pathways that have found most application in intramolecular cyclizations. Simple alkenes and aromatic hydrocarbons will typically suffer addition processes (pathway A). However, alkenic and aromatic systems with allylic or benzylic groups more electrofugal than hydrogen e.g. silicon, tin) commonly undergo elimination reactions (pathway B) to generate the reactive radical pair. 

Square-pyramidal metal carbonyls [M(CO)s] (M = Cr, Mo, or W) are generated on u.v. photolysis of metal hexacarbonyls and related compounds and appear as intermediates in photochemical replacement reactions of the six-co-ordinate species. The reaction step relevant in this context is an internal rearrangement of the square-pyramidal [M(CO)6] via a trigonal-bipyramidal intermediate, and back to the square-pyramidal ground state. This spy - tbp spy process represents an inverse Berry mechanism and also accounts for many other aspects of the photochemistry of these compounds. ... 

One final reaction in this series is interesting in that the zwiterionic intermediate (33) can be generated either photochemically or by a ground-state path " (6.33b). In both cases the ratios of products show the same solvent dependence ... 

In a few cases alkyl complexes can be synthesized by the photoaddition of dichloromethane or chloroform to a low-valent transition metal complex, but the usual products from these halogen abstraction reactions are the dichloro complexes. Photoinduced oxidative addition reactions are not always the result of the generation of an excited state that is more reactive than the ground state. An alternative photochemical pathway can involve the initial dissociation of a ligand to generate a coordinately unsaturated intermediate, which is then activated toward oxidative addition of the substrate. An example of such a reaction is... 

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