Yang hydrogen abstraction

Explain how intramolecular hydrogen abstraction in carbonyl compounds can lead either to cleavage (Norrish type 2 reaction) or to the formation of cyclic compounds (Yang cyclisation). 

The most common bimolecular reaction of triplet ketones in solution is hydrogen abstraction from a suitable substrate to yield alkyl radicals and semipinacol radicals. The radicals so formed perform their own characteristic reactions depending upon the actual conditions. Yang has recently proposed that in high-intensity irradiations, these... 

The latter example (reaction 36) already indicates that the Yang cydization can also be used to synthesize four-membered heterocycles. After light absorption, the a,(3-unsaturated carbonyl compound 84 undergoes intramolecular hydrogen abstraction at the a-position of the carbonyl moiety (reaction 37), leading to the 1,4-biradical intermediate XXX [87]. A radical combination then efficiently yields the spirocyclic P-lactam derivative 85, and only one stereoisomer is formed in this case. In this transformation, the a,P-unsaturated carbonyl function can be considered as being vinylogous to a simple ketone. 

The Norrish-Yang reaction [63] is a widely used photochemical reaction, which consists of an intramolecular hydrogen abstraction by a photoexcited ketone, followed by carbon-carbon bond formation in the (l,n)-biradical intermediates (Scheme 9.38). 

The first three chapters by Wagner, Wessig, and Griesbeck deal with typical carbonyl chemistry Norrish type II reactions followed by Yang-cyclization, homologous Norrish type II reactions (i.e. hydrogen abstractions from non y-positions), and Paterno-Buchi [2+2]-photocycloadditions. The enantiomerically pure (S-am ido-cycl o butanol 1 is formed from a chiral... 

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. 

One of the first examples of a transannular hydrogen abstraction is Padwa s bicyclo[l.l.l]pentane synthesis published in 1967 [56]. By virtue of the considerable ring strain of bicyclo[l.l.l]pentanes the formation of 64 from 63 impressively demonstrates the synthetic scope of the Norrish-Yang reaction (Sch. 23). 

Two other examples of the synthesis of bicyclic compounds whose skeleton consists only of carbon atoms are shown in Sch. 24, each of them is instructive in its own way. Upon irradiation of the cyclohexane derivative 65, not the expected product from a hydrogen abstraction from the allylic position, but the bicyclo[3.3.1]heptane 67 was obtained by cyclization of the less stable biradical 66-B [57]. The reason for this is the exclusive hydrogen back-transfer of the more stable biradical and it underlines the importance of this process in regioselectivity phenomena of the Norrish-Yang reaction. 

As discussed in Sec. 3.3.1.4, besides ketones also imides can be used as reactants in the Norrish-Yang reaction. For clarity, photochemical reactions of imides, which are initiated by a hydrogen abstraction both... 

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