Norrish type I photoreaction

Keywords 1-phenylcyclopentyl ketone, Patemo-Buchi reaction, Norrish type I photoreaction, oxetane... 

With that background, we are now ready to discuss the first class of carbonyl photochemical reactions. Eq. 16.55 shows the a cleavage or Norrish type I photoreaction. After excitation, the reaction involves reformation of the carbonyl double bond with concomitant cleavage of a C-C bond a to the carbonyl and expulsion of a carbon radical. Also formed is an acyl radical that, based on the bond dissociation energies of Table 2.2, is a fairly stable radical. 

Jimenez, M. C, Leal, R, Miranda, M. A., and Tormos, R., Norrish type-I photoreaction in the presence of phenols. An intermolecular photo-Fries rearrangement, /. Chem. Soc., Chem. Commun., 2009,1995. Grimme, S., Theoretical investigation on the photodissociation of carbon oxygen bonds in aromatic-compounds, Chem. Phys., 163, 313, 1992. 

Most aldehydes and ketones in inert solvents or in the gas phase undergo one or two photoreactions, called Norrish Type I and Norrish Type II processes.81 The Norrish Type I reaction, shown in Equation 13.62, may originate... 

Keywords a-diketone, silica gel surface, Norrish type I reaction, photoreaction... 

Intramolecnlar.—The photochemical reactivity of dienone (1) has been shown to be associated with the Si or the T2 state, since the photoreactions undergone cannot be quenched or sensitized. The reactions are wavelength dependent such that at 313 nm initial Norrish Type I fission is followed by radical recombination (as was established by deuterium labelling studies) to give the... 

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). ... 

Reasonable yields of the adducts (103) are obtained when acetone is irradiated in the presence of 2,3-dihydro- and 2-methyl-2,3-dihydro-furan. Compounds of this structure can also be obtained by the photocyclization of the vinyloxy ketones (104a) and (104b),. which yields the adducts (105a) and (105b), respectively." Intramolecular cycloaddition is also encountered in the photoreactions of the cyclic alkanones (106)." However the reaction favours the formation of the crossed adduct (107) in all the cases studied. The accompanying products are the alternative oxetan (108) and the products from Norrish Type I fission. 

Single-component positive photoresists have also been developed, and an example is shown in Figure 17.25 C. The polysulfone is sensitive to electron beam or deep UV irradiation, and it produces direct backbone scission of the poljnner in a photoreaction similar to the Norrish type I process (see Section 16.3.7). This renders the exposed region soluble. 

The classic photoreaction within photo-oxidative degradation is the photolysis of hydrogen peroxides. Also known in organic photochemistry are photo-induced reactions such as Norrish type I and Norrish type II reactions that result in cleavage of the carbon structure and thus to a loss in molecular weight, Figure 5.12 [381. 

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