Photochemical intermediates, generation

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

The thermal benzannulation of Group 6 carbene complexes with alkynes (the Dotz reaction) is highly developed and has been used extensively in synthesis [90,91]. It is thought to proceed through a chromium vinylketene intermediate generated by sequential insertion of the alkyne followed by carbon monoxide into the chromium-carbene-carbon double bond [92]. The realization that photodriven CO insertion into Z-dienylcarbene complexes should generate the same vinylketene intermediate led to the development of a photochemical variant of the Dotz reaction (Table 14). 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

We consider first how surfaces which are themselves not photosensitive can perturb chemical reactivity. First, the surface can influence diffusional motion of adsorbed substrates, intermediates or products. With preadsorbed substrates, one can probe the nature of motion of intermediates generated on the surface and search for differences in reactivity caused by surface confinement . When several photochemical precursors to benzyl radicals, e.g., benzyl phenylacetate, a dibenzyl ketone, or a dibenzyl sulfone, are irradiated as adsorbates on dry silica gel, singlet and triplet radical pairs are generated, Eq. (7). The extent of radical recombination observed requires... 

Firth S, Klotzbuecher WE, Poliakoff M, Turner JJ. Generation of Re2 (CO)9 (N2) from Re2(CO)10 identification of photochemical intermediates by matrix isolation and liquid-noble-gas techniques. Inorg Chem 1987 26(20) 3370-3375. 

The competition of nucleophilic attack by solvent at the ketene sp center has been shown to be useful for obtaining aza analogues of dienylketenes, namely dienylketenimines (297) fixHn ketene intermediates generated from (295). Upon dehydration, the addition product (296) afforded directly the Af-alkylated cy-clohexadienimines (298) in go( yield. The expected dienylketenimines (297) were not isolated. Regeneration of spectroscopically detectable (297) can be achieved by photochemical irradiation of (298), its stability above 0 C being solvent depen nt. ... 

The Visual Transduction Process. Picosecond absorption spectroscopy which utilizes OMCDs also has provided important mechanistic information that previously was not available by means of other techniques. Detailed pathways of a number of reactions which are important from a physical, chemical, and/or biological viewpoint have been elucidated by means of this technique. A recent picosecond spectroscopic study by Spalink et. al. (30) has demonstrated that an experimental criterion, which has been used to support the hypothesis that cis-trans isomerization (31) is the primary event in the visual transduction process, is not true. This criterion is based on the commonly occurring statement that both the naturally occurring 11-cis-rhodopsin and the synthetic 9-cis-rhodopsin lead to the same primary photochemical product, bathorhodopsin. Of course, the existence of a common intermediate generated from either 11-cis- or 9-cis-rhodopsin would support the commonly proposed mechanism of cis-trans isomerization as the primary event in the visual transduction process. However, the data obtained by Spalink et. al. (30) indicate that a common intermediate is not generated from both rhodopsins. 

In this chapter, the focus will be on the application of high pressure techniques in the study of the photochemical behavior of transition metal complexes (coordination, organometallic and bio-inorganic) in solution. We will present a systematic treatment of pressure effects on the nature of excited states (ES) and on the photophysical and photochemical processes that lead to ligand substitution, electron or energy transfer and thermal reactions of reactive intermediates generated by ES reactions. Selected examples will be presented in detail to illustrate how pressure effects can provide valuable mechanistic insight when combined with other quantitative studies. 

According to the Woodward-Hoflfmann rules, concerted thermal [2+2] cycloadditions are symmetry-forbidden, but should proceed via supra-antarafacial attack of the reactants. [2+2] cycloadditions of ketenes and related reactive intermediates generated in situ proceed by a stepwise mechanism. " Photochemical [2+2] cycloadditions are symmetry-allowed. Asymmetric [2+2] cycloadditions leading to 4-membered heterocycles, e.g. Staudinger reactions or Patemo-BUchi reactions, have been extensively studied in the past. 

No full paper has been published yet, but this report suggests that the triplet states of coordinatively unsaturated species might be formed and trapped by photochemical excitation in low-temperature matrices. Although these reactions would be of limited synthetic usefulness, the important point is that the ability to stabilize intermediates at low temperature has proved to be an invaluable method for obtaining spectroscopic data about suspected photochemical intermediates. In fact, as this example demonstrates, low-temperature generation and spectroscopic characterization of unstable species at low temperature, used in conjunction with DFT calculations, is a powerful combination for the mechanistic interpretation of reactions. 

In the case of an enol ether derivative (eq 5), reaction with triethylsilyl hydrotrioxide produced an intermediate dioxetane which was cleaved to give 3-phenylpropionaldehyde upon warming. In contrast, reaction of the same enol ether with photochem-ically generated singlet oxygen proceeded via an ene pathway. After hydrolysis, this gave an a , -unsaturated aldehyde in 37% yield. ... 

Sato, X, Niino, H., and Yabe, A, Reactive intermediates formed by the consecutive photolyses of naphthalenetetracarboxylic dianhydrides direct observation of reactive intermediates generated by laser-induced reaction in low-temperature argon matrices, /. Photochem. Photobiol. A Chem., 145, 3, 2001. 

Martinez, L. J. and Scaiano, J. C., Characterization of the transient intermediates generated from the photoexcitation of nabumetone a comparison with naproxen, Photochem. Photobiol, 68, 646, 1998. 

Photochemical elimination of carbon dioxide from suitable precursors has given a variety of reactive intermediates at low temperatures where they are often stable and can be studied further. This approach has been utilized in attempts to generate new 1,3-dipolar species, and photolysis of (515) gave an azomethine nitrene intermediate (516) (see Section 4.03.6)... 

Photochemical reaction of the ester 114 afforded the alkene 115 and three products derived from 115. A mechanism, involving dimerization of 114 leading to a dithietane intermediate 116, was proposed. Trapping of active sulfur species, generated from 116, with dienes was also described (75CB630). 

Tile following photochemical conversions also involve 1,2-dithietes as intermediates whose chemical trapping was reported in most cases. Tlie formation of the dithiin 249 from 250 may best be explained by the formation of the dithiete dimer 251 and the loss of S2 (73ZC424).Tlie formation of 252 and 253 from 254 (78NJC331) should be compared with the sulfuration of the acetylene 182 with elemental sulfur (93BCJ623).Tlie photolysis of 255 provides a rare example when the ejection of a nitrile was employed for the generation of a 1,2-dithiete (73ZC431). 

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

In related work, the reactions of hydrogen peroxide with iron(II) complexes, including Feu(edta), were examined.3 Some experiments were carried out with added 5.5"-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent fa so-called spin trap) for HO. These experiments were done to learn whether HO was truly as free as it is when generated photochemically. The hydroxyl radical adduct was indeed detected. but for some (not all) iron complexes evidence was obtained for an additional oxidizing intermediate, presumably an oxo-iron complex. 

Merlic developed a new variation of the thermally induced benzannulation reaction. The dienylcarbene complex 132 was reacted with isonitrile to give an orf/zo-alkoxyaniline derivative 135 [76] (Scheme 56). This annulation product is regiocomplementary to those reported from photochemical reaction of chromium dienyl(amino)carbene complexes. The metathesis of the isocyanide with the dienylcarbene complex 132 generates a chromium-complexed di-enylketenimine intermediate 133 which undergoes electrocyclisation. Final tau-tomerisation and demetalation afford the orf/zo-alkoxyaniline 135. 

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