Norrish I intermediate

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

UV stabilisers protect polymers by restricting UV penetration to the surface and therefore confine the damage to surface layers. Protection is important because the energy possessed by UV radiation is sufficient to break chemical bonds. The initial breakage can either be by a radical (Norrish type I) or non-radical (Norrish type II) pathway. The effects are similar to degradation of the polymer by oxidation routes the radical intermediates can be neutralised by anti-oxidants. 

The photochemistry of cyclobutanone presents a special case since the Norrish type-I cleavage to give an acylalkyl diradical intermediate releases ring-strain energy. Thus the energy available for subsequent reactions is reduced correspondingly, compared to the energy retained in an acyl radical from an acychc ketone, or less strained cyclic ketones. 

In photolysis of ketones CIDNP studies have confirmed that the Norrish type I split occurs predominantly from a phototriplet state (32,38,118), although some of the reactions with aliphatic ketones exhibit polarization involving both the excited singlet and the triplet (47,118) states as well as the postulated exci-plex intermediates (71). An exciplex mechanism has also been postulated in the CIDNP observation of the photolysis of tri-fluoroacetophenone with dimethoxybenzene in acid solutions (117). 

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... 

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. - ... 

Stiver and Yates have studied the photochemical reactions of some hydroxy-keto steroids (28, 29). Irradiation of the isomeric compounds (28a, 29a) showed that the products obtained, (30) and (31) respectively, had retained the configuration of the carbon to which the hydroxy group is attached. The use of deuteriated derivatives (28b, 29b) has identified the hydrogen abstraction processes involved in the conversion of these ketones into the lactones (30b) and (31b) respectively. The authors " propose that there are two major factors which control the stereospecificity of the reactions. These are the shape of the hydroxy-bearing C-atom and the hydrogen transfer within the biradical formed on Norrish Type I fission. The stability of the biradical intermediate clearly plays an important part in determining the outcome of the reactions. 

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