Carbonyl compounds Norrish type II elimination

Aliphatic ketones have n,Jt lowest singlets that intersystem cross to their n,Jt lowest triplets at rates below 108s which is slow enough to allow singlet reactions to occur.933,948 Structural changes in an alkyl chain can affect the photokinetic behaviour the faster the hydrogen abstraction, the less efficient intersystem crossing (Table 6.11). 

The cleavage/cyclization ratio in straight-chain ketones is usually relatively high ( 5 1). Some bulky a-substituents may increase the probability of cyclization 

The type II reaction of n,jt excited ketones is largely influenced by the reaction environment and experimental conditions. The initial step requires close 

Photodegradation of polymers (photoageing), involving chain scission and/or cross-linking, occurs by exposure to solar or artificial radiation and causes structural modifications, usually accompanied by a dramatic deterioration of the physical and mechanical properties of the polymer.954 Typically, radical intermediates are formed upon excitation, which initiate subsequent (dark) degradation. The presence of other species, such as oxygen, water, organic solvents or additives, and also mechanical stress and heat, may enhance the efficiency of these processes. 

Case Study 6.21 Asymmetric synthesis in crystals - application of chiral auxiliaries 

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