Norrish type II photoelimination

There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

Whenever a carbonyl is in or adjacent to the backbone of a polymer chain, the main cause of molecular weight breakdown will be the Norrish Type II photoelimination reaction. TTie quantum efficiency of the reaction in polymers appears to be nearly independent of temperature and physical state (i.e., solid or liquid). 

The groups of Pete and Rau also employed chiral amino alcohols for the enantioselective protonation of simple enols 23a-c that were photochemically generated from 2-/-butyl indanones and tetralones 22a-c by a Norrish type II photoelimination (Scheme 9) [41,42]. Best enantioselectivities were obtained at — 40°C in acetonitrile with 0.1 equivalent of the chiral amino alcohol. In the case of indanone 22a, the selectivity reached 49% ee with (— )-ephedrine ent-18) and could not be further enhanced by the camphor derived inductor 20. With this amino alcohol, enantioselectivities over 80% ee were induced in the case of tetralone 22b. A benzyl substituent in place of the methyl group led to substantial decrease of the selectivity to 47% ee. Linear ketones gave low yields and enantioselectivities around 9% ee. 

The formation of an olefin by the Norrish type II photoelimination was not much affected by the presence of an unconjugated olefinic group, but the formation of propylene (and acrolein) in 5-hexenal requires a new photochemical transformation mechanism. It has been suggested that an olefinic, triplet precursor which can undergo intramolecular H-atom abstraction reaction is responsible for this process. 

P. J. Wagner, P. Kian, Chapter 52 Norrish Type II Photoelimination of Ketones Cleavage of 1,4-Biradicals Formed by y-Hydrogcn Abstraction, in CRC Handbook of Organic Photochemistry and Photobiology, W. M. Horspool, F. Lengi, (Eds.), CRC Press, Boca Raton, 2003, p. 1-31. 

UV and y-radiation of poly(methyl isopropenyl ketone) produced random chain scission at 23 °C. The presence of air increases unexpectedly the main chain scission of the polymer under y-radiation [377]. In a series of publications [378] the radiolysis and photolysis of poly(phenyl vinyl ketone), poly(vinyl benzophenone), and poly(/-butyl vinyl ketone) [357] were described. The authors stated that photodegradation of poly (phenyl vinyl ketone) occurred by the abstraction of a hydrogen in the y-position to a carbonyl group, followed by chain scission by a Norrish type II photoelimination mechanism. 

A number of papers on secopenicillins and related compounds have been published during the year under review. Treatment of the ester (350) with phenacyl bromide and potassium t-butoxide gave the seco derivative (351) which, on photolysis, underwent a Norrish type II photoelimination to give the thione (352). The phenoxyacetyl analogue (353) of the latter was cyclized in 71 % yield by triethyl-amine, giving the DL-5,6-dehydropeniciIlin (354). Benzyl-6-phthaIimido-5-... 

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