Norrish type fragmentation

Even simple enols have substantial lifetimes, provided that bases or acids are completely excluded173. Thus, an aromatic enol 4 is prepared in situ by Norrish-type fragmentation of 2. If (-)-ephedrine is present in the reaction mixture, the enol reverts enantioselectively to (/ )-2-rnethy 1 -1 -indanone (3). With as little as 0.01 mol % catalyst, 45% ee is obtained176. The crucial enol 4 has also been generated from either the benzyl enol ester 5 by palladium on charcoal and hydrogen or from the allyl ester 6 by palladium acetate, triphenylphosphine and ammonium formate. In the presence of a chiral 1.2-hydroxyamine, e.g., ephedrine, substantial stereogenic induction in 2-methylindanone 3 was observed175. 

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. 

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. 

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... 

Frequently B will also undergo a back hydrogen transfer which regenerates the parent ketone, as well as cyclization (in most cases a minor reaction) as a result of this competition the quantum yields of fragmentation are typically in the 0.1-0.5 range in non-polar media. When the Norrish Type II process takes place in a polymer it can result in the cleavage of the polymer backbone. Poly(phenyl vinyl ketone) has frequently been used as a model polymer in which this reaction is resonsible for its photodegradation, reaction 2. 

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

The alkoxy radical of Scheme 18.3 (upper reaction) could scission to produce the same carboxyl radical as seen in the Norrish type 1 path (Scheme 18.1, path A) discussed above. As such, it is an additional source of CO2 but not taken into account in the report by Day and Wiles [25], Not reported but still obvious, the other fragment of this scission is an aliphatic aldehyde that could also have been one of the aldehyde carbonyl IR signals reported [11, 25], Hydrolysis of this chain end would yield the reported glyoxal [21],... 

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. ...

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

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. 

Rather than fragmentation of Norrish Type IT biradicals, cyclization can occur. This is called the Yang Cyclization. ... 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

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