Norrish type I reactions

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. 

With unsymmetrical ketones two different bonds are available for photolytic cleavage the actual cleavage pathway depends on the relative stability of the possible radical species R and R.  

The acyl radical 2 can abstract a /3-hydrogen from the radical 3, to give an aldehyde 10 and an alkene 11  

Since the quantum yield of the Norrish type I reaction is generally low, it has been assumed that the initial homolytic cleavage is a reversible process. Evidence came from an investigation by Barltrop et al. which has shown that erythro-2,3-dimethylcyclohexanone 12 isomerizes to t/zreo-2,3-dimethylcyclohexanone 13 upon irradiation  

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15  

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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. 

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. 

The behaviour of triplet acyl-diphenylmethyl biradicals 0=C -(CH2) 2-C Ph2, generated from the Norrish type-I reaction of 2,2-diphenylcycloalkanones (CK) with various ring sizes, n = 6, 7, 9, 11, 12, 13, was the subject of a study. Eor 2,2-diphenylcycloalkanones where n = 6 and 7 an intramolecular disproportionation takes place giving rise to a diphenylalkenal (94). The primary products in the photolysis... 

As far as we are aware, these observations are the first that show that the well-known Norrish Type I reactions of p,7-unsaturated carbonyl compounds can take place by excitation of the alkene moiety rather than the carbonyl group. This unusual reactivity may be due to the fact that the TiC-ir, -ir ) excited states of 53 and 55 possess sufficient energy to promote the homolytic allylic bond fission to form the stabilized pentadienyl radical 57. As a result, photodecarbony-lation competes favorably with the ODPM rearrangement. 

Norrish type-I reaction, has been studied over the years in extreme detail, with every imaginable physical and theoretical method at hand. Data gathered through studying such reactions on the femtosecond time scale, together with new theoretical work prompted by the dynamics observed, have provided a detailed picture of the processes involved and a fresh perspective on nonconcerted ot-cleavage events. 

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]. 

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. 

The statistics of Norrish type I and Norrish type II reactions occurring in one pentanal molecule in the cluster (i.e., not followed by subsequent reactions) can be compared with the previously discussed statistics in the bare pentanal, in di-pinonic acid, and hydrated c/i -pinonic acid Norrish type I reaction is most pronounced in c/Y-pinonic acid, with 37% of yield, much more than for the bare pentanal (14% only). The water and the pentanal cluster reduce in both systems the percentage of Norrish type I reaction (PA-H2O - 18%, PA-(H20)5 - 24%, pentanal cluster - 9%). On the contrary, Norrish type II reaction is mostly observed in the bare pentanal (27%), compared to 10% for the bare pinonic acid. Hydration of the cA-pinonic acid further reduces the percentage of Norrish type II reactions. The existence of the pentanal cluster also decreases the percentage of Norrish type II reaction (PA-H2O - 2%, PA-(H20)5 - 3%, pentanal cluster - 1 %). 

The orientation of the molecules has a large effect on the reactions observed here. Only certain orientations allow an H atom transfer to neighboring molecules. In addition, the cross-molecular processes, especially Norrish type I reaction, are less affected by the initial orientation of the molecules. The cross-reactions observed in the cluster are distributed almost equally over the whole simulation timescale. 

For example, in Part I of this paper it was shown that the Norrish type I reaction could be quite efficient in the solid phase, and that a reduction in polymer molecular weight could be achieved through /3 scission via the reaction sequence ... 

Photochemistry. One might deduce that since the lowest electronic transition corresponds to transfer of an electron from an oxygen atom to a carbon atom, the nn state should have substantial diradical character and should react also by a McLafferty-type rearrangement or a cleavage, as in the mass spectrometer. This is indeed the case. The photochemical a cleavage is called the Norrish type I reaction, and the rearrangement is called the Norrish type II reaction. Both are discussed in Chapter 15. 

Figure 15.7. Dauben-Salem-Turro analysis of the photochemical step of the Norrish Type I reaction for (a) saturated carbonyls and (b) conjugated carbonyls. The reaction is most efficient on the 3( 7 ) surface to yield triplet diradical products. It is also efficient on the 3(nn ) surface since IC permits formation of products in their ground state.

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... 

Norrish Type I reactions occur from 7 only if the state is 3(rc,7r ). Compound 53 has a phosphorescence lifetime of 10 3 sec, and there is a mirror-image relationship between its closely spaced phosphorescence and absorption spectra. Compound 54, however, has a phosphorescence lifetime of 5.5 sec, and there is a large Stokes shift between its dissimilar absorption and phosphorescence spectra. Apparently both compounds have a lowest (n, r ) singlet,... 

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