Reactive intermediates photogenerated

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Another applieation of photogenerated metal coordinated ketenes is based on the addition of protie nucleophiles and has been exploited in the synthesis of amino aeids and peptides. [66] As usual, the reactive intermediate is generated by photolysis of an aminoearbene complex 46, which may be a-alkylated in a previous step. The oxazolidine auxiliary applied successfully inducing asymmetry in the P-laetam formation, allowed an enantioselective synthesis of amino aeids. Sinee both enantiomers of the auxiliary may be obtained from the corresponding phenyl glycine enantiomer, natural (5) and non-natural R) amino acid esters 47 are accessible via this route (Scheme 25). A recent review on synthetical applications of chromium carbene photochemistry has been published, [li]... 

The formation of highly reactive intermediates from the photolysis of aromatic azides favor their use for surface modification and the preparation of substrates for solid phase reactions. Thus, azide photochemistry has been exploited for facile modification of graphitic surfaces and N-5-azidonitrobenzoyloysuccinimide undergoes photoaddition in a solid state reaction. A review on the covalent functionalization of graphene includes examples via photogenerated nitrenes. ... 

An important question is what happens in the absence of a suitable substrate that can trap the photogenerated intermediate ) Are reactive species such as [Mo(dppe)2] and [Mo(PPh2 ... 

In early works, the reactivity associated widi TiO2 photocatalysis was assigned to free OH radicals, but more recently die oxidative species has been identified as bound HO radicals and photogenerated holes. Reaction of the substrate with these species would require either die direct adsorption of organic compounds to the semiconductor interface or Fickian diffusion of the substrate to the semiconductor surface under depletion conditions. However, these two pathways imply different mechanisms, and in some cases different reaction products or intermediates. 

We conclude that oxidative transformations of organic substrates can be readily understood as emanating from either photogenerated surface adsorbed radical cations or from radicals formed by activated oxygen radicals (surface oxides or adsorbed hydroxy, hydroperoxy or peroxy radicals). Photoelectrochemical methods not only generate the reactive species by interfacial electron transfer, but also control the subsequent activity of the surface adsorbed intermediate. 

While it has been assumed (Galardy et al., 1973) that the reactive species is the mr excited state this may not always be the case. For acetophenones the nature of the photogenerated triplet is highly solvent dependent (Turro, 1979). In more polar environments the species formed is the jot state which is less reactive than the mr state and in certain cases it may prove useful to substitute the molecule with an electron withdrawing group (e.g. CFr) on the aromatic ring to ensure formation of the desired T, (mt ) intermediate. 

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