Norrish Type I fragmentation

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

Scheme 5.22. Domino Wolff/Cope rearrangement/Norrish type I fragmentation/recombination process.

The direct irradiation (A = 300 nm) of the ketone (5) in Bu OH yields (6) and (7), the two possible Norrish Type I fragmentation products. a-Cleavage does not occur on irradiation in Pr OH but rather reductive loss of fluorine yields cyclohexanone. The cyclohexenone (8) shows an analogous solvent-dependent selectivity. Sonawane et al. have shown that the photochemical... 

The enone (160) undergoes a Norrish Type I fragmentation in methanol to afford the product (161). ° The product is formed from the intermediate biradical (162) by fission of the 5,10-bond to give a trappable ketene. Irradiation of the... 

The Norrish Type I fragmentation of the cyclic ketone (178) has been used in a synthesis of grandisol (179), a major component of the male boll weevil pheromone.125 The cleavage reaction gave the aldehyde (180) which was subsequently decarbonylated to give the desired product. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

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

If there are hydrogen atoms in the y-position relative to the acyl group, irradiation of an imidazolide leads to a 1,2-shift of the acyl group (step one) followed by a Norrish type II or type I fragmentation (step two) [41,[51... 

Figure 20.5. A graphical representation of the time evolution of transients for the Norrish type-I a-cleavage 43 and 46 amu fragments from acetone and from acetone-de- The representative sets of data points ( for 43 amu, for 46 amu fragments) are modeled with simple buildup and decay response functions, I(t) = 4[exp(—t/t2) — exp(—f/x])] the time constants of buildup and decay are Ti and T2, respectively. A modest isotope effect on the characteristic time for formation of these acyl radicals (60 and 80 fs, respectively) and a more prominent —CH3/—CD3 effect on decays through loss of CO (420 and 670 fs, respectively) were recorded. ...

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-... 

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. 

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. 

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