Complicated photoreactions

This can be demonstrated using the following example of a consecutive 

It is assumed that the partial photochemical quantum yields and p are constant. Then 

If this equation is integrated in the same way as eq. (3.107), one finds a result which corresponds to eq. (3.109) 

The inner integral has to be solved within the limits, (z,0) to, (2,t). By these means it depends on two degrees of advancement. Thus 2 can be calculated using eq. (2.90). One finds 

In Fig. 3.19 T is given as a function of 7 for constant turnover =0.1. The ratio pjpj is chosen as a parameter, x = 0-5 is used. The depth of penetration becomes larger with increasing ratio of p, to pj- 

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The method presented here can be applied to many but not all complicated photoreactions. The derivations discussed above can be used in principle if the photochemical quantum yields of all partial steps of reactions do not depend on the amount of light absorbed, or if they depend on this amount of light absorbed in the same way. An example for the latter condi-... 

H. Mauser, The evolution of complicated photoreactions in viscous media, Z. Naturforschg. 34c(1979)1264. 

Epoxy-amine cured coatings based on di-, tri- and tetra-functional epoxydes and aliphatic or aromatic amines (Table 4.2) undergo complicated photoreactions when exposed to UV radiation. 

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

Each one of the two types of photoreactions to be discussed has several other possible complicating features, and these will be examined in turn in this section. 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

The metalloproteases (MPs) and matrix metalloproteinases (MMPs) are a class of metallohydrolases of particular interest to the pharmaceutical industry due to their role in a number of pathological processes [81-83], The lack of an enzyme-bound nucleophilic residue in the metallohydrolases complicates the design of ABPP probes for this class of enzymes. Rather than mechanism-based and electrophilic probes for ABPP, photoreactive variants of reversible inhibitors of metallohydrolases have been developed [84-86]. These reversible inhibitors usually contain a hydroxamate moiety that is capable of chelating the catalytic zinc ion in a bidentate manner [79, 80]. The hydroxamate moiety was incorporated into the first generation of metallohydrolase ABPP probes along with a benzophenone group capable of covalent bond formation upon UV irradiation (Scheme 4). 

All of the photoreactions of aliphatic carbonyl compounds result from just four primary reactions of n,n states. More complicated carbonyl compounds can undergo various rearrangement reactions and reactions characteristic of 71,31 states but also undergo the four basic carbonyl reactions. A brief summary of these reaction types is presented in this section the remainder of this review is devoted to a critical summary of how structure affects the rates and efficiencies of these basic reactions. Since each of the primary photoreactions produces another... 

Another example of such complications are the differing photoreactions of para - and meta -methoxy benzoin acetates 194>. The ara-isomer undergoes predominately a-cleavage and a little cyclization, both apparently from the triplet. 

The photoreactions of aliphatic amines with aromatic hydrocarbons have also been reported by several groups. With tertiary amines, deprotonation occurs from the radical cations of amines at the a-carbon to generate carbon radicals which react with the radical anion of aromatic hydrocarbons. With secondary amines, deprotonation from the radical cations of amines occurs both at the a-carbon and at the nitrogen atom, so that the reaction becomes complicated [64-65]. 

A recent synthesis of 3-substituted furan derivatives illustrates an important application of the furan-carbonyl photocycloaddition. Zamojski has reported the rearomatization of oxetane (115) in the presence of p-toluenesulfonic acid to 3-furylmethanol derivative (116). Synthesis of (117), itself a substrate for the intramolecular photocycloaddition reaction (Section 2.4.6), involved a similar rearomatization process (PPTS/CHjClj) and capitalized upon the chemoselectivity observed in the ketone-furan photocycloaddition. Similarly, a synthesis of perillaketone (118) by Kawanisi involved irradiation of a carbonyl compound and furan. A complication in the rearomatization is that acid also catalyzes the reversion of the photoadduct to starting materials to circumvent this problem the photoreaction was run in the presence of acid, so that rearomatization would occur in situ and the products of competitive reversion would promptly recombine. 

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