Norrish cleavage/cyclization ratio

There does not seem to have been much study of the stereoselectivity of cyclization of a-diketones. Scheffer and coworkers reported that 1,2-cyclodecanedione forms only the c/s-l-hydroxybicyclo-[6,2,0]decan-2-one when irradiated as crystals, but a 7 1 cis/trans ratio in benzene [59]. In solution the products proceed to cleave to a (3-ketoaldehyde, probably by Norrish type I cleavage. This problem can affect any a-hy dr oxy ketone photoproduct if light below 330 nm is used to irradiate the a-diketone. Since oc-diketones absorb above 400 nm, use of such wavelengths can avoid the problem. 

Substituents in a-Position. To rationalize the ratio between Norrish type II cleavage and cyclization (see preceding chapter) the influence of a-substituents on cyclization were studied exclusively on straight-chain aryl ketones with preferential y-hydrogen abstraction. 

Norrish Type I fission of the side chain carbonyl group again at C-4. - Laser flash irradiation has been used as a aethod for the production of n-butylkotene from cyclohexanone. The chemistry of this ketene was studied in detail. The cyclohexanones (9a) undergo both Norrish Type I and II processes on irradiation. The fluorinated compounds (9b) showed a preference for Norrish Type II behaviour. Within the Norrish Type II biradical fluorine substitution leads to a preference for cyclization rather than cleavage. The Norrish Type I biradical afforded a ketene rather than an alkenal. A study of the photochemical reactivity of the diones (10) has shown that both Norrish Typo I and Type II reactivity can take place. The Typo I Type II product ratio is dependent upon ring size. Thus dione (10a) affords the Type II products (11) and (12) while dione (10c) yields the Norrish type I products (I3c-15c) and low yields of the Norrish Type II products (11) and (12). Compound (10b) is intermediate between these results affording a Type I Type II ratio of 0.3. A mechanistic study of the reactions was carried out. - ... 

The Norrish Type II reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the 1,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation O-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the triplet state reaction arc illustrated in Scheme 1 (30). 

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