Norrish type II reactions

Photochemical reaction of aldehydes or ketones bearing y-hydrogens 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9  

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

There are only a few examples for a preparative use of this reaction of more importance have so far been mechanistic aspects.  

Halton, Organic Photochemistry, Cambridge University Press, London, 1974, p. 58-78. 

In general however the various possible reaction pathways give rise to formation of a mixture of products. The type I-cleavage reaction is only of limited synthetic importance, but rather an interfering side-reaction—e.g. with an attempted Paterno-Buchi reaction, or when an aldehyde or ketone is used as sensitizer in a [2 + 2]-cycloaddition reaction. 

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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. 

A Norrish type I reaction for side-chain carbonyl functionahty is equation 7, and a Norrish type II reaction for side-chain carbonyl functionahty is equation 8. 

Reactions are known where both Norrish Type I and Norrish Type II reactions compete, and the substituents on and nature of the substrate will determine which leads to the major product." ... 

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) ... 

The photolysis of an azetidine has been found to yield pyrole derivatives by the Norrish type II reaction/1075... 

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

The process of intramolecular abstraction of an hydrogen atom from the y carbon atom by the excited carbonyl group is commonly referred to as Norrish type II reaction. The resulting diradical can close to a cyclo-... 

An interesting application of this Norrish type II reaction consists in the photochemical oxydation of alcohols to carbonyl compounds by irradiation of their pyruvic esters (4.11) 412). 

Phthalimides undergo a variety of photochemical reactions416). The sequence of a Norrish type II reaction has been utilized in the synthesis of medium ring thiacyclols (4.15)417). 

Photo-oxidation of protoporphyrin IX and its dimethyl ester Norrish type II reaction of amphiphilic ketoacid Quenching of naphthalene fluorescence by Ni2 +... 

Priebe et al. [79] investigated the chemical stabiHty of iodixanol under accelerating cleavage of the central bridge under ultraviolet irradiation by a Norrish Type-II reaction. Basic conditions (pH 14) combined with heat (60 °C) initiated a cyclisation reaction. On the other hand, less than 1 % iodixanol decomposed in solution heated to 140 °C for 2 days or under both basic conditions (pH 11,20°C, 5 days) and acidic conditions (pH 0.4,80 °C, 5 days) or under an oxygen atmosphere (100°C,3 days). 

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

Photolysis of JV-aryl amides of /3-ketocarboxylic acids 135 proceeds in a different way.93 Thus, phenylisocyanate 136 is formed in 60% yield when in 135 R = phenyl. When R = methyl or propyl, the yield decreases to 18%. Consequently, this reaction belongs only formally to the scope of the discussed rearrangements. It seems that this is a special example of Norrish Type II reaction.324 This assumption is supported by the yield dependence of... 

Norrish Type I and Norrish Type II Reactions in the Isolated Molecule... 

Table 1.1 clearly shows that the major pathway in the photochemistry of pentanal is the y-H transfer, followed by the C—C cleavage. The H detachment is only a minor pathway. A high percentage of trajectories are unreactive in this timescale. The relative yield of Norrish type I versus Norrish type II reaction from this table is 66% Norrish type II reaction and 34% Norrish type I reaction. This compares well to the observed experimental yield of 80% for Norrish type II reaction [16, 70]. 

To compare with. Figure 1.4 shows the first step of the Norrish type II reaction, namely, the y-H transfer to the C=0 group. 

The comparison timescale of the Norrish type I versus Norrish type II reactions is very interesting and is summarized in the histogram in Figure 1.5. 

Norrish type I reaction occurs on two timescales one is ultrafast and below 10 ps and the second is slower at 45 ps. On the other hand, Norrish type II reaction... 

Another important effect on the Norrish type I/II ratio is the occurrence of intramolecular vibrational energy redistribution (IVR). For short timescale processes shorter than 10 ps (such as the Norrish type I reaction), IVR is yet far from completed as assumed by statistical theories such as RRKM. The opposite is true for Norrish type II reaction. The reaction only starts after 20 ps, pointing out that IVR seems to be necessary for the reaction. The longer the cai bon chain (the larger... 

Figure 1.5 Histogram of (a) Norrish type I reactions and (b) Norrish type II reactions in Pentanal in the timescale of 100ps. Reprinted with permission from Ref. [31]. Copyright (2013) American Chemical Society.

Finally, the discrepancy between experiment and theory on the ratio between Norrish type I reaction and Norrish type II reaction can be explained by considering the following factors. Experimental conditions in the gas phase allow for collisions between different molecules, a factor that has not been taken into account by the theoretical simulation. In addition, the presence of O2 or N2 in the experiment might additionally affect the ratio. 

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