Norrish-type II process

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. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

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. 

A recent development in the synthesis of 2-benzazepine-l,5-diones is the photoaddition of alkenes to A-alkylphthalimides. A-Alkylphthalimides capable of undergoing a Norrish Type II process follow the route outlined in Scheme 35 (path a) to yield hydrobenz-azepinediones (264) by ring opening of the azacyclobutanol (262) (78ACR407). 

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. 

In this pathway (Norrish type II process), cleavage occurs at the Ca-Cfi bond to give, as the major product, a ketone of shorter chain length and an alkene. Thus for 2-pentanone ... 

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 II processes from the triplet state involve a biradical even when concerted reaction would be exothermic. Photolysis of 60 gives the products in Equation 13.65. Concerted reaction with the formation of triplet stilbene (a process corresponding to Equation 13.29) would be exothermic, but triplet stilbene is not formed The 7 state of stilbene decays to a 60 40 cis-trans mixture, but photolysis of 60 gives 98.6 percent tra/u-stilbene.94... 

The photocleavage of phenylglyoxylate esters is a simple Norrish type II process with a secondary mechanistic twist, namely that continued... 

PET cyclizations of donors-substituted phenylgloxylates have been intensively studied by the groups of Neckers and Hasegawa. For the thioether derivatives 1 the efficiency of the cyclization depended on the linker chain length (Sch. 5) [18]. With increasing carbon tether (>7 = 2-10) the yield dropped steadily and secondary reductive dimerizations or Norrish Type II processes became competitive. Consequently, the Cn-linked substrate solely underwent dimerization and cleavage [18b]. 

The photochemistry of these aliphatic ketones is distinguished from that of other aliphatic ketones by the occurrence of an intramolecular primary process which gives an olefin and a methyl ketone. This process is classified as a Norrish Type II process to distinguish it from the primary process which leads to free radicals. It occurs in the photolysis of many high aliphatic ketones and a similar process also occurs in the photolysis of many aldehydes 101 and esters.4 Primary processes which give rise to free radicals also occur. 

Methylpentanal. Early studies of propanal and 1-butanal in the gas phase have been reviewed by Cundall and Davies (61). Only a few other saturated aldehydes have been studied recently, and 3-methylpentanal is one of them. Rebbert and Ausloos (195) have compared the direct and the triplet-sensitized photolysis of 3-methylpentanal, since this molecule can undergo two kinds of intramolecular rearrangement process / Norrish type II process 18, a primary or a secondary a-hydrogen atom transfer to the carbonyl oxygen, giving 1-butene or trans/cis-2-butene, respectively. 

Radiationless Processes. The primary processes of 2-pentanone are similar to those of the other ketones. The dominant pathway for the depopulation of the first excited singlet state at lower energies is the S —intersystem crossing. Both a-cleavage (type I) and Norrish type II processes can occur from the singlet state and from the triplet state. The intersystem crossing yield has only been measured in solution (see ref. 247) With 313-nm excitation, a value of isC = 0.81 was measured by... 

Photodecomposition. The photochemistry of 2-pentanone is distinguished by the possibility of "Y-hydrogen abstraction by the carbonyl oxygen (Norrish type II process). The biradical intermediate thus formed allows the possibility of cyclobutanol formation. 

The solvent dependency of the photochemistry of the dione (268) has been studied.The diketones (269a-f) undergo Norrish Type II processes to yield... 

Reactions I, II and III were first suggested by Leighton et al., and step IV by Blacet and Calvert . Step III, generally known as the Norrish type II process, only occurs in the photolysis of aldehydes having a hydrogen atom (or atoms) in the y position. The mechanism of the Norrish type II process will be dealt with, in detail, in section 6. Blacet and Crane assumed a further primary step, analogous... 

Steps I and II are often referred to as Norrish type I and Norrish type II processes, respectively. Free radicals are formed in the former, while the latter leads to stable molecules without the participation of free radicals. The terms, type I and type II, were suggested by Bamford and Norrish following the discovery of primary decomposition mode II by Norrish and Appleyard in the photolysis of 2-hexanone. 

The amide (38a) is photochemically inert on irradiation in ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type II process to yield a mixture of products.The results of a study of the enantioselective photodeconjugation reactions of the lactones (39) have been published. The behaviour of the ketones (40) and (41) in the isotropic and the two solid phases of heneicosane (CcxH ) has been evaluated. The influence of the various phases on the ratio of elimination to cyclization products of the ketones was discussed. The modification of the photoreactivity of ketones (42) in cyclodextrin has been... 

A study of the photodeconjugation reaction of the ester (136) in the presence of a variety of optically active amines such as (IR,2S)-l-phenyl-2-isopropylamino propanol has shown that the product can be obtained with an enantiomeric excess of about 70 X. Irradiation of the alkynylketones (137) in alcohols (methanol, ethanol or propanol) affords the furan derivatives (138) by a free radical hydrogen abstraction path. Excitation of the cyclohexenone derivatives (139) yields the cyclized products (140) the structures of which were verified by X-ray crystallography. The formation of the amide products is reminiscent of a Norrish Type II process. 

The dihydro dithiosuccinimides (333) are photochemically reactive and yield the cyclized adducts (334) by a Norrish Type II process. A variety of ring sizes can be synthesized by this approach. The irradiation of the thiophthalimide... 

Hydrogen Abstraction.- Xanthone-sensitized irradiation of the dihydroisoquinoline (19) affords a 6X yield of the spiro derivative (20). The reaction is akin to a Norrish Type II process but in this instance hydrogen abstraction by the imine nitrogen is involved. The resultant biradical cyclizes to yield the observed product (20). 

A number of related photochemical processes resulting in ring closure of the 3,4-bond of /3-lactams have been reported. Irradiation of iV,iV-dialkyl-a-oxoamides (150), especially in the solid state, gives /3-lactams by a Norrish type II process (79JA5343,79SC179,78JOC419). There are several examples of bicyclic /3-lactams produced in low yield by this route (78T1731 p. 1739). A related reaction occurs with a-thioxamides (151) (79[Pg.258]

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