Norrish Type I a-Cleavage Reaction

The photochemical decarbonylation of ketones can be traced back to 1910 when acetone was photolysed in the gas phase to yield ethane and carbon monoxide. A radical process involving a-cleavage (Norrish type I reaction) and decarbonylation as two separate steps was proposed a few years later by Norrish and Appleyard (Scheme 1). Each of the two cleavage reactions has been the subject of numerous theoretical and mechanistic studies that have been covered in several reviews. 

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) ... 

In the thermolysis of cyclohexanone virtually all of the products arise from the (8-cleaved structure 5.23) Jwo effects are important here. In the ion the a-cleavage structure is specifically stabilized (see below) and the jff-cleavage structure 9, would be specifically destabilized by the heteroatom which is at an active site in the odd alternant n system associated with the carbonyl. The primary photolysis products of cyclohexanone 24) are much more closely correlated to its mass spectrum than the thermolysis products are. This is because n- n excitation can be relaxed by an a-cleavage (Norrish type I) analogous to 7. The analogy for the photochemical reactions is, however, far from perfect because of the stabilizing effect of the oxygen atom on the even alternant even electron ions, e.g. 10. The photolysis of... 

Homolytic cleavage of the a-bond (a-cleavage Norrish type I reaction917), often followed by decarbonylation of the acyl radical intermediate thereby formed909 (Scheme 6.109), is one of the most common reactions of excited ketones.903,905 This reaction can be accompanied by competing processes, such as the Norrish type II reaction (Section 6.3.4) or photoreduction (Section 6.3.1). 

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. 

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 ... 

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. 

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]. 

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. 

When salt crystals of the aryl 1-phenylcyclopenty 1 ketone carboxylic acid 40 with chiral amines such as (+ )-bomylamine or (—)-1-phenylethylamine were irradiated, the optically active exo- and endo-oxetanes 41 or 42 were formed in low to moderate enantiomeric excesses (Scheme 10) [57]. The formation of the oxetanes is believed to occur through Norrish type 1 cleavage and hydrogen abstraction, producing an alkene and an aldehyde, followed by a Paterno-Buchi reaction within the crystal lattice cage. In contrast, solution photolysis of 40 in acetonitrile afforded product 43 as the only isolable product via a typical Norrish type I a-cleavage followed by radical coupling. 

The classical photochemical free-radical source is a compound that is photoexcited and then undergoes bond cleavage to yield active radicals. Benzoin and its ethers are efficient radical sources and are the most commonly employed photoinitiators in industrial photopolymerization processes (42,43). The reaction is a simple Norrish type I cleavage ... 

A = acetone, B= benzophenone, ox = oxetane see also Table 7 for regioselectivity of CC bond cleavage in Norrish type I reactions. Column C refers to experimental value and column D to calculated values. 

To the extent that studies have been done, most of the results can be interpreted on the basis that siloxycarbene formation is a rapid reversible process in which the short-lived siloxycarbene must be trapped quickly by a kinetically acidic reagent. Competitive with this is a slower Norrish type I cleavage reaction, which only predominates when there is no good trapping agent present to siphon off the simultaneously formed siloxycarbene (equation 67). 

Singlet excited molecules are usually relatively short-lived and, therefore, are not very likely to undergo bimolecular reactions. In many cases, however, chemical bond cleavage competes with physical monomolecular deactivation paths. For example, singlet excited carbonyl groups contained in a polyethylene chain can undergo the Norrish type I reaction, resulting in a free radical couple [see Eq. (1-17)]. 

Equations 12.46 and 12.47 do not show the spin states of the unpaired electrons. In general, a cleavage can occur from both the singlet and triplet n,n states, but quantum yields are much greater for the triplet state reactions. Zewail and co-workers reported femtosecond studies and theoretical calculations of the Norrish type I reaction of acetone. They found a barrier of ca. 18 kcal/mol for a cleavage from the Si state of acetone but a barrier of only 5 kcal/mol for dissociation from the Ti state. Thus, they concluded that the photodissociation occurs from the triplet state and that intersystem crossing from Si to Ti is the rate-limiting step in the Norrish type I reaction of acetone. ... 

For each of the following reactions, propose a mechanism in which the first step is a Norrish type I reaction (a cleavage). 

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