Singlet vs. Triplet Reactivity

The relative reactivities of n,n singlets in hydrogen abstraction reactions has not yet been defined as precisely as for n,n triplets. There is little doubt that n,n singlets do abstract hydrogen atoms, since the unquenchable type II cycli-zation of compounds such as cyclohexenyl acetone involve allylic rearrangement 89,90). 

The singlet state type II reaction of aliphatic ketones proceeds very inefficiently 9°) and in a highly stereospecific fashion 91 , as opposed to the highly efficient 9 , nonstereospecific triplet reaction 91 . Although an extremely shortlived singlet diradical has been invoked 90-91 , it has been clear for some time that the situation may be more complicated 9h 

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