Norrish-type I process

Das zur Diskussion stehende 7t, n-angeregte Keton geht eine a-Spal-tung (Norrish Type I Process n>) zum entsprechenden 1,5-, 1,6- oder 1,7-Alkyl/Acyl-Biradikal ein, das... 

The Norrish type I process is not important for the photolysis of diaryl ketones. A reason suggested... 

Now aryl free radicals are extremely unstable, they are not stabilised by resonance. Since the stability of a product can act as a driving force for the reaction to proceed along that path then we can say that the more unstable the product is, the less likely that path will be followed. Hence Norrish type I process is energetically unfavourable for diaryl ketones. 

A similar variation in the quantum yield of the Norrish type I process is illustrated in Figure 3 for solid copolymers of ethylene containing three different ketone structures. The ketone groups in the backbone of the polymer chain in ethylene- copolymers show much lower quantum yields than those from the secondary or tertiary structures induced by copolymerization of methyl vinyl ketone and methyl isopropenyl ketone with ethylene. (See Table I, structures I, II and III.) In the latter two cases, the Norrish type I cleavage produces a small radical and a polymer radical, and it seems likely that the small radical has a much greater probability of escaping the cage than when the radicals produced are both polymeric, as in the case of structure I. 

The / -scission of the tertiary radical IX so produced provides another, potentially efficient, method of causing main-chain scission in the polymeric solid phase. Similar high quantum yields for the Norrish type I process were... 

There is some contribution due to / -scission of the alkyl radical formed by the type I process, particularly in the MIPK and tBVK polymers. Loss of carbonyl occurs from photoreduction or the formation of cyclobutanol rings, and also from vaporization of the aldehyde formed by hydrogen abstraction by acyl radicals formed in the Norrish type I process. As demonstrated previously (2) the quantum yields for chain scission are lower in the solid phase than in solution. Rates of carbonyl loss are substantially different for the copolymers, being fastest for tBVK, slower for MIPK, and least efficient for MVK copolymers (Table I and Figure 1). 

The results of these experiments are summarized in Table V, which shows values of A0/A and Z for the ketone copolymer films. All of the polymers in the first section underwent significant damage and were very brittle. However, there was not much attenuation in the first section as indicated by the similar Z values of the PS-MIPK films placed at the beginning and the end of the section. It should be pointed out that the errors in determination of Mn by GPC are substantial — considerably greater than the FTIR measurements. Nevertheless, the Z values correlate quite well with A0/A, indicating the importance of the Norrish type I process in causing chain scission. 

The product-forming reactions, Equations 28-2 through 28-5, all depend on the primary photochemical event, Equation 28-1, which breaks the C-C bond to the carbonyl group. This cleavage has been termed a Norrish type I process after the eminent photochemist, R. G. W. Norrish 2... 

Bohne, C. (1995) Norrish type I processes of ketones selected examples and synthetic applications, in CRC Handbook of Organic Photochemistry and Photobiology (eds W.M. Horspool and P-.S. Song), CRC Press, Boca Raton, pp. 423-429. 

Triplet-State Radical Pairs from Norrish Type I Processes... 

Norrish Type II reactivity are discussed later.) Subsequently, irradiation of this product (13) yields the cyclopentanone (14), again via a Norrish Type I process followed by cyclopropane ring opening and rebonding within intermediate (15). The direct irradiation of the bicyclic ketone (16a) in methanol-ether yields the two products (17) and (18).Further experimentation showed... 

Feldman has reported two interesting exeunples of epoxide ring opening. The opening reaction in the epoxides (34a) and (34b) affords an ylide-like species that is intramolecularly trapped to yield the two adducts (35a) and (35b). The 3-substituted derivative (34b) also affords another product identified as (36). The formation of this product is thought to involve a Norrish Type I process proceeding via (37) to the lactone (38) which then undergoes [2-1-2] cycloaddition. 

More interest has been shown in the past year in the laser-induced processes involving organic molecules. One such study is the laser irradiation (193.3 nm) of the ketones (1-4). This study has shown that the Norrish Type I process is dominant resulting in a-fission and the formation of alkyl and acyl radicals. The ultimate products formed are alkanes and carbon monoxide. Norrish Type I reactivity is also observed in more complex molecules such as the carbohydrate derivatives (5) and (6). Irradiation of these in solution again brings about a-cleavage to give the isomeric radicals ]) ... 

The thymidine derivatives (146) and (147) undergo cleavage of a C-C bond on irradiation. These reactions are typical Norrish Type I processes and provide a route to study C-3 -DNA radicals. Hydrogen abstraction by the radicals yields a 1 1 mixture of the threo and erythro derivatives (148). The reactions from the P-isomers (146) are generally more efficient than from the a-isomer (147). A study of the photochemical reactivity of the deoxyuridine derivative (149) has been reported. This novel compound is an electron-accepting nucleo base. It has been used as a means of cleaving DNA. The photochemical fission occurs specifically at the 5 -G of 5 GG3 sequences. 

Photo-decarbonylation of the cyclohexanone (32a) is efficient with a quantum yield of 0.9. The reaction yields the two products (33) and (34) in a ratio of 1 2. The cyclopentanone (32b) also decarbonylates photochemically but is less efilcient with a quantum yield of 0.5. A laser flash study has been carried out on these systems and has identified the biradicals produced by the Norrish type I process. The lifetime of the biradicals (35a) and (35b) are O.Ojxs and 0.5 xs respectively. In a related study the photodecarbonylation of cIs- and frans-2,6-diphenylcyclo-hexanone has been shown to yield a mixture of cis- and 2-diphenyl-cyclopentane and cIs- and... 

Reactions (1) and (3) are believed to be the most important under atmospheric conditions. The Norrish Type I process results in fragmentation into free radicals, whilst the Norrish Type II process, which is common to molecules with a y-hydrogen atom, is an intramolecular rearrangement that results in no radical formation. Experiments performed at EUPHORE and in indoor photoreactors as part of the RADICAL project found that flic Norris Type I process is dominant for n-butanal and smaller straight chain aldehydes, but ftie Norrish Type II process is the major pathway for the photolysis of n-pentanal and -hexanal. 

Results of the photolyses of acetone, 2-butanone, and 2-pentanone adsorbed on Vycor glass are shown in Table 2. It is well known that alkyl ketones with / hydrogen atoms, such as 2-pentanone, undergo the Norrish Type II processes (intramolecular elimination) as well as the Norrish Type I processes (C -cleavage into radical pairs), as shown in the following reaction mechanisms. In the gas phase photolysis of 2-pentanone at room temperature, the amount of products derived from the Type I processes is less than 5-15% of that derived from the Type II process (26). As seen in Table 2, the rate of CgHg formation is more than 75% that of C2H formation. 

In parallel to hydrogen abstraction, the primary radicals formed by the Norrish type I processes may recombine together before or after decomposition through decarbonylation or decarboxylation. The hydrogen abstraction on the phenyl rings leads to meta-biphenyl groups and accounts for one of the recombination pathways. 

The phosphonates (50) are photochemically reactive and lead to products dependent upon the nature of the substituents. Irradiation of the phosphonate (50a) yields the product (51a) as a result of a photoreaction analogous to a Norrish Type I process. An analogous product (51b) is also encountered in the photolysis of phosphonate (50b). In this instance however other products (52b) and (53b) are produced as a result of Norrish Type II reactivity and fission of the resultant biradical. Norrish Type II fission dominates the photoreactions of (50c) and (50d) yielding the monoester (52c, d) and the olefin (53c, d). Fission of the C—bond to afford the radical (54) and products derived from it is the result of irradiation of the phosphine (55). ... 

The pyrrolinone (144) undergoes photochemical ring contraction in Bu OH to yield ultimately the cyclopropyl carbamate (HS). Formally this is a reaction akin to the Norrish Type I process in ketones in that initial fission of a C—C bond results in a biradical which recombines to yield a ring-contracted isocyanate. The irradiation of the thioisoxazolinones (146) in methanol results in the extrusion of CO2 and the formation of products (147). The products are thought to be... 

The Norrish type I process is considered to be the mechanism by which radicals are formed. The regio-chemical outcome of Norrish type I cleavage depends on the wavelength of light employed. For example, photolysis of 2-butanone at 313 nm (91.4 kcal moT, 382.5 kJ mol" ) gave a 40 1 mixture of 40 and ethyl radical (CH3CH2 ). Irradiation at 253.7 nm (112.7 kcal mok 471.9 kJ moT ), however, gave a 2.4 1 mixture of methyl radical and acyl radical 21. 

It undergoes backbone scission as a secondary process following the cleavage of a C—C bond adjacent to carbonyl, in a Norrish-type I process as shown in Scheme 7.18. " ... 

Although the above scheme for the Norrish type I process is written for acylic compounds, they can be applied to cyclic compounds as well. In cyclic confounds, cleavage of the a-bond results in a diradical, instead of two separate radicals. We must remember that the above scheme is to be used only as a guideline. We have to... 

The photochemical reactions of the ketones (12) which are used as sunscreens in cosmetics has shown that degradation is considerable when they are irradiated as thin films. Norrish Type I reactions dominate affording benzoic acid derivatives." A study of the biradicals formed on flash photolysis of the ketone (13) has been reported. The reaction involves a Norrish Type I process and yields an acyl-ketyl biradical that transforms into an alkyl-ketyl biradical by decarbonyla-tion. 

P,7-Uiisaturated Systems - As was mentioned earlier in this Chapter, Norrish Type I processes can occur with P,y-unsaturated enones. This process is also observed in the formation of a ketene on irradiation at X, > 230 nm of cyclopenten-3-one in an argon matrix. The presence of the ketene intermediate was detected by IR spectroscopy. Irradiation at 300 nm of the enones... 

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