Sensitizer cleavage

The fragmentation of Ca -ATPase by proteolytic enzymes [42,85,235,236] and by vanadate-catalyzed photocleavage [104,105] occurs at well defined and conforma-tionally sensitive cleavage sites that delineate functional domains within the Ca -ATPase. The functional changes that follow the cleavage of the polypeptide chain provide useful hints about the role of various domains in the mechanism of Ca transport. 

The 1,4-bifunctional intermediate is implicated further by the electron transfer sensitized isomerization of cis- to Irons-1,2-diphenoxylcyclobutane [115], and by the unique CIDNP effects observed during the electron transfer sensitized cleavage of the anti-head-to-head dimer of 3,3-dimethylindene (vide infra) [121]. 

Four-membered rings may be generated in a less systematic manner by a variety of photochemical transformations, many of which are multistep reactions. In the field of radical chemistry, the photochemical generation of radical intermediates is frequently applied such that, when the stereoelectronic requirements are fulfilled, the cyclization steps lead to four-membered rings. The oxime derivative 99 undergoes photochemical sensitized cleavage at the fragile N—O bond (Scheme 5.19, reaction... 

The photochemistry of these salts is, however, rather complicated [7, 118-120], Both direct and energy-transfer sensitized cleavages are observed with heterolysis and homolysis products detected, but the details are not yet completely understood. Many of these salts are able to rapidly generate Bronsted acids (by rather complicated pathways) and are used to initiate... 

Energy transfer is rather seldom encountered [2j] due to the obvious difficulty to have the S triplet-state level higher than that of I. The most typical case of energy transfer is encountered between a thioxanthone derivative and a morpholino ketone derivative (10.60) the excitation moves from the thioxanthone triplet state to the morpholino ketone triplet state. With other thioxanthone derivatives, electron transfer followed by proton transfer with the amine moiety can compete to some extent [212]. The sensitized cleavage in covalently linked thioxanthone derivative/cleavable photoinitiator was recently investigated and discussed [213],... 

The product of deoxyribose oxidation at Cl, 5, could be observed by HPLC. However, it was noted that a minor amount of C4 hydroxylation may be observed as well at some residues in the loops. The oxidation of the C4 -H bond of some deoxyribose units in the single-stranded loops was due to an hydroxylation (formation of 4 -hydroxylated abasic site, 23), no 3 -phosphogly-colate were detected. The C4 oxidation mechanism was evidenced, on polyacrylamide gels, by the protection of NaBH4 toward piperidine sensitive cleavage at some T residues. 

Multiple ether oxygen atoms can be present in the molecule. Cleavage and coupling reactions occur with direct fluorination although to a lesser extent than with ECF. This allows the direct fluorination of acid-sensitive materials, such as the formal shown below, which would not survive ECF (8). 

The oxidation may be carried out with an inert solvent thermally (35), with a sensitizer such as bromine (36), with uv radiation (37), or over a suitable catalyst (38). Principal by-products of all these oxidation processes are the acyl fluoride products derived from oxidative cleavage of the perfluoroaLkene (eq. 

Almost all organic peroxides are thermally and photolyticaHy sensitive owing to the facile cleavage of the weak oxygen—oxygen bond, ie, the range of AHis about —84 to —184 kJ/mol (—20 to —44 kcal/mol) (9—11) ... 

Initiation. Free-radical initiators are produced by several processes. The high temperatures and shearing stresses required for compounding, extmsion, and molding of polymeric materials can produce alkyl radicals by homolytic chain cleavage. Oxidatively sensitive substrates can react directly with oxygen, particularly at elevated temperatures, to yield radicals. 

Dibismuthines are very sensitive to oxidation. Thus tetramethyldibismuthine fumes ia air, and tetraphenyldibismuthine ia toluene solution rapidly absorbs oxygen. Under controlled conditions, dibismuthines react with chalcogens resulting ia cleavage of the bismuth—bismuth bond and iasertion of a chalcogen atom (105,138,140—142) ... 

Fig. 14. Schematic cross section of Spectra integral film. The 600 Plus film has a similar stmcture. In these films the yellow image is formed by silver-assisted cleavage of a yellow dye releaser. A colorless developer reduces exposed silver hahde in the blue-sensitive emulsion in unexposed areas dissolved silver diffuses to the dye releaser layer and triggers the release of the yellow image dye.

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

Most diaziridines are not sensitive towards alkali. As an exception, diaziridines derived from 2-hydroxyketones are quickly decomposed by heating with aqueous alkali. Acetaldehyde, acetic acid and ammonia are formed from (162). This reaction is not a simple N—N cleavage effected intramolecularly by a deprotonated hydroxy group, since highly purified hydroxydiaziridine (162) is quite stable towards alkali. Addition of small amounts of hydroxybutanone results in fast decomposition. An assumed reaction path — Grob fragmentation of a hydroxyketone-diaziridine adduct (163) — is in accord with these observations (B-67MI50800). 

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