Norrish-I cleavage

It is also possible that stretched (high pH) versus coiled (low pH) polymer chains may exhibit different photophysics. This was commented by Chou and Jellinek in their early report on PMAA photochemistry, and a detailed TREPR study of pH-dependent cage effect manipulation using initiator-labeled PMAA was reported by Maliakal et al. " It has been reported by Mittal and coworkers that for small molecule analogs of PAA and PMAA, the quantum yield for Norrish I cleavage drops to zero for the completely deprotonated structure. For PMAA, we also observe a pH dependence in the TREPR signal intensities (data not shown). The data in Fig. 14.7 are the hrst experimental evidence for such an effect in PAA radicals. 

Norrish-I cleavage of 2-methylpropanoyltripropylstannane Pr3SnC(0)zPr affords the expected spin-polarized products. A much more complex... 

The photochemistry of Py-epoxy cyclic ketones has also been examined, and the influence of the alkyl radical centre, formed in the initial Norrish I cleavage, upon the possible subsequent scission of the adjacent epoxide C—O bond has been discussed in the light of observed substituent and/or skeletal factors. 

Norrish Type I cleavage of cyclic ketones necessarily yields biradicals, and in certain cases (e.g., cycloheptanone, camphor) strong emissions due to T i S mixing have been reported (Gloss and Doubleday, 1972). 

When applied to ketones, this is called Norrish Type / cleavage or often just Type I cleavage. In a secondary process, the acyl radical R —CO can then lose CO to give R radicals. Another example of a category 1 process is cleavage of CI2 to give two Cl atoms. Other bonds that are easily cleaved by photolysis are the 0—0 bonds of peroxy compounds and the C—N bonds of aliphatic azo compounds R—N=N—R. The latter is an important source of radicals R , since the other product is the very stable N2. 

The mechanism probably involves a Norrish type I cleavage (p. 318), loss of CO from the resulting radical, and recombination of the radical fragments. 

The photolysis of the following steroid system resulted in two products corresponding to the Norrish type II reaction and one product due to a-cleavage (Norrish type I cleavage)<101) ... 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

Finally, a nice combination of a light-induced Wolff reaction of a diazoketone (e. g., 5-108) with a thermal Cope rearrangement, a light-induced Norrish type I cleavage and a recombination was developed by Stoltz and coworkers (Scheme 5.22) [38]. Here, irradiation of 5-108 at 254 nm in a photoreactor afforded the bicy-... 

Light induces formation of a 1,2-diradical. Norrish type I cleavage to give the stabler of the two possible 1,5-diradicals then occurs. 

Photoexcitation of the ketone gives a 1,2-diradical, which undergoes Norrish type I cleavage of the C1-C2 bond to give a 1,5-diradical. The cyclopropylcarbinyl radical opens up to give a 1,3-diradical, which finally loses CO to give the observed diene. Some of these steps may be concerted. 

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. 

Further examples of the formation of cyclobutanes via Norrish type I cleavage of cyclopen-tanones are given in Table 4. 

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. 

Among the most widely used photofragmenting initiators are alkoxyaceto-phenones and hydroxy-alkylacetophenones (Figure 1). The primary reaction of these initiators is a Norrish Type I cleavage leading to the formation of a benzoyl radical and a fragment radial moiety both of which may initiate polymerization. Sander and Osborn (15) have shown that 2,2-dimethyoxy-2-... 

NORRISH TYPE I CLEAVAGE REACTION OF CARBONYL COMPOUNDS 215... 

Figure 15.6. (a) Frontier orbitals of the fragments of a Norrish Type I cleavage (b) electronic configurations with two electrons (c) order of the energies of the electronic states. 

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