Norrish type-1 mechanism Cleavage

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

Since the photochemistry of many compounds that have been used as triplet sensitizers has been well studied, we will not attempt to cover these reactions in detail. Unless the investigator is unaware of them, common photochemical processes such as the Norrish Type II cleavage are not ordinarily a complication and as will be mentioned later, they can actually serve as mechanistic probes. A discussion of the mechanisms of triplet energy transfer1,3,9 is beyond the scope of this review as are other specific reactions which have been recently covered elsewhere. 

There are at least three types of mechanisms discussed in order to explain the ODPM rearrangement. The first mechanism (Sch. 6) is radicaloid in nature in involves a Norrish type I cleavage leading to the formation of acyl and allyl radical and recombination of these radicaloid species to the ODPM rearrangement product in two ways. 

A mechanism involving Norrish type I cleavage followed by radical cyclization has been proposed to explain this photolytically induced rearrangement.An alternative possibility might involve bonding from the carbonyl carbon to the 6-carbon followed by alkoxy radical expulsion and ring closure. 

The Norrish type I process is considered to be the mechanism by which radicals are formed. The regio-chemical outcome of Norrish type I cleavage depends on the wavelength of light employed. For example, photolysis of 2-butanone at 313 nm (91.4 kcal moT, 382.5 kJ mol" ) gave a 40 1 mixture of 40 and ethyl radical (CH3CH2 ). Irradiation at 253.7 nm (112.7 kcal mok 471.9 kJ moT ), however, gave a 2.4 1 mixture of methyl radical and acyl radical 21. 

The fact that the presence of I2 reduces carbon monoxide formation is indicative of intermediates that can react with I2 to give different products. The Norrish type I cleavage mechanism is consistent with this ... 

The photochemistry of aldehydes and ketones has been extensively examined (Turro, 1978). In high concentrations or in the absence of oxygen, a ketone such as acetone can fragment homolytically to an alkyl-acyl radical pair by the well-known Norrish type I cleavage mechanism ... 

Various mechanisms for the 1,3-acyl shift have been proposed, the following three pathways being the most widely discussed a) a Norrish type I cleavage to an acyl and an allyl radical followed by recombination either to starting ketone or to the rearranged ketone [Eq. (19)]. i i>,21,25,33, 37,59,96,96) For some cases it has been pointed out that the radical centers do not become independent of each other but exist as a radical pkir in a solvent cage. 21,37,59) b) a concerted process as depicted in Eq. (20) and c) a concerted n2 -f <,2 cydoaddition of the C—1,C—2 bond with the C—3,C—4 double bond [Eq. (21)]. > Mechanisms (b) and (c) have not always been properly distinguished. Circumstantial evidence has been reported which supports the nonconcerted and/or... 

Studies of the photolysis of carbonyl-containing polymers (See e.g. Guillet, 1985.) have revealed two main mechanisms of degradation. The first, the Norrish Type 1 cleavage, involves direct scission of the bond a to the excited carbonyl, with the production of two radicals. 

Several of the more common commodity polymers like the polyolefins are susceptible to photo-oxidation. For a polymer like polyethylene, photo-oxidation leads to increasing amounts of carbonyl compounds. In-chain ketone groups act as sensitisers by UV light absorption. Through the well-known Norrish type I and II degradations radicals, end-vinyl and ketone groups are formed. Other products often observed in photo-oxidised low-density polyethylene (LDPE) are esters [5]. Scheme 1 shows one mechanism for abiotic ester formation. By Norrish type I cleavage the radical formed can react with an alkoxyl radical on the polyethylene (PE) chain. 

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. 

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. 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

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