Photochemical product formation

Here c > = R2/(k 4 kR2) is the photochemical product formation yield. Note that the absorbance and e values are those at the exciting wavelength. In contrast to the case of photo deracemization, in photodestruction reactions the absorbance [and consequently the photokinetic factor F, Eq. (12)] is not constant therefore the integral... 

Here 4> = + A r2) is the photochemical product formation yield. Note that... 

Photochemical product formation can arise from other reactive intermediates as well. Figure 12.36 shows the products from direct irradiation of 2,3-dimethylbut-2-ene (20). Excitation is thought to produce the Rydberg state 21 and then the carbene 22, which rearranges to products 23 and 24. There is apparently another mechanism responsible for the formation of the isomerization product 25, and a pathway involving a n,cr state has been suggested. ° ... 

There is a great deal of flexibility in the choice of laser radiation in the production of thin Aims by photochemical decomposition, and many routes for achieving the same objective can be explored. In most reactions of indusuial interest the reaction path is via tire formation of free radicals as intermediates, and the complete details of the reaction patlrs are not adequately defined. However, it may be anticipated that the success of the photochemical production of new materials in tlrin fllms and in fine powder form will lead to considerably greater effort in the elucidation of these kinetics. 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

The photochemical reaction of 2,3,4,6-tetra-0-acetyl-/3-D-gluco-pyranosyl sulfone (57) in benzene under nitrogen has been carefully studied, and a number of products identified116 (see Scheme 22). A mechanism that involves a photochemically initiated series of free-radical processes has been proposed that is consistent not only with product formation but also with the extent of incorporation of deuterium found in the various products following irradiation of 57 in benzene-d6. The mechanism shown in Scheme 22 is compatible with proposals offered to explain sulfone photochemistry in noncarbohy-... 

Gel electrophoresis of P-end-labelled oligonucleotides irradiated with visible light in the presence of RufTAP)] showed that the principal photochemical product is a less electrophoretically-mobile species [96]. This is consistent with the formation of a photo-adduct and it is clear that the yield of this reaction is... 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

In contrast with the sensors described elsewhere in this Chapter, the device proposed by the authors group uses no reagent, but photons, to induce a photochemical reaction, and involves electrochemical detection of the photochemical product, which allows one to continuously monitor the formation of the electroactive product. Kinetic monitoring increases the selectivity of determinations by eliminating matrix effects and the contribution of side reactions, whether slower or faster than the main reaction. The electrochemical system chosen for implementation of this special sensor was the Fe(II)/C204 couple, which was used for the kinetic determination of oxalate ion based on the following reaction ... 

Irradiation of thiatriazole-5-thiol (18) and its anion (5) at 313 nm is reported to give rise to formation of nitrogen, sulfur, and SCN (HSCN). This scheme is in accordance with the otherwise known photochemical reactions of thiatriazoles but analytical data for product formation is not given. Quantum yields were calculated for the reaction in the pH range 0.0 to 11.8 <86POL2l 19>. 

The processes leading to product formation from radicals 65-68 (Path A) and from 70 (Path B) were added arbitrarily32b to the primary photochemical processes. 

Since water is required for the reaction, the primary photochemical product is thought to be a surface bound hydroxy radical. The observed chemoselectivity for radical formation from the adsorbed ether, however, is thought to be governed by adsorption phenomena since a free hydroxyl radical in homogeneous solution is much less selective than the photoirradiated catalyst system. 

Photolysis of the 1,2,3,5-oxathiadiazine (132) in the presence of ethanol leads to the quinazolone (133). The product is not formed in the absence of ethanol, and the proposed mechanism involves electrocyclic ring opening, addition of ethanol, photochemical diradical formation and closure on to the C-6 phenyl ring followed by aromatization as shown in Scheme 8 (79JOC4435). 

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