Carbonyl compounds intramolecular hydrogen abstraction

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

With compounds containing C—H bonds y to the carbonyl moiety, one often observes intramolecular hydrogen abstraction, in contrast to the in-termolecular hydrogen abstraction by aryl ketones discussed earlier in this... 

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). 

An example where this abstraction from the carbonyl compound competes with photocycloaddition was discussed in the previous section (p. 312). In that case, 2-methylbenzophenone underwent intramolecular hydrogen abstraction, yielding the tautomeric enol,46 a process which completely inhibited the photocycloaddition reaction.37... 

P.. Wagner, in P. de Mayo (ed.). Rearrangements in Ground and fxc/ted States, vol. 3, Academic Press (1980). The photorearrangements of a simple carbonyl compounds that occur by way ol biradicals are the subject of this account, including reactions that involve intramolecular hydrogen abstraction. 

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. 

One of the most common photochemical reaction pathways of carbonyl compounds is the formation of a diradicaloid excited state which is able to abstract a hydrogen atom at the y (or, more rarely, e) position, followed by either fragmentation or recombination. This process, which is known as the Norrish type II reaction, has a parallel in the photochemistry of nitro groups the intramolecular hydrogen abstraction of excited ortho-nitrotoluene is actually one of the very early synthetic photochemical transformations [9]. It has been exploited in a family of photolabile protecting groups, most prominent among which are derivatives of ortho-nitrobcnzyl alcohol, as introduced in 1966 by Barltrop et al. (Scheme 13.1) [10, 11],... 

Alternative intramolecular hydrogen abstractions are possible in a, 3-unsaturated carbonyl compounds. Irradiation of the 2-(N-acyl-N-alkyl-amino)cyclohex-2-enones (115), for example, gave the lV-alkyl-1-azaspiro[3.5]nonane-2,5-diones (116), presumably via 1,4-biradical intermediates (117), The 2-(dialkylamino)-3-acetylenyl-1,4-naphthoquinones... 

One of the earliest photoreactions to be studied was the photoreduction of benzophenone (Ciamician and Silber, 1900)—that is, the conversion of a carbonyl compound into an alcohol by an intermolecular hydrogen abstraction reaction. Intramolecular hydrogen abstraction by the carbonyl group, usually from the y site, is referred to as a Norrish type II reaction. Hydrogen abstraction by olefins and heterocycles has also been observed. 

Alkyne complex 37 is prepared by intramolecular hydrogen abstraction followed by methane elimination from the methyl metallocene compound 36 in the presence of a donor ligand (Scheme 6.8). 37 reacts with alkenes, alkynes, carbonyl compounds, and nitriles to give 5-membered metallacycles [89, 116-117]. In the absence of any donor ligand, metallacyclopentadiene 38 is formed. Complex 37 is also isolated by trapping 30 with an alkyne [89]. 

In those cases where a hydrogen atom is located on a carbon gamma to a carbonyl group, intramolecular H-abstraction can occur and radical coupling then results in formation of a four membered ring. This process occured with high efficiency with certain aliphatic diones 163-164)) the only compounds investigated to date in which such intramolecular H-... 

Photolysis of a carbonyl compound having y-hydrogen commonly gives a shorter chain carbonyl compound, an alkene, and a cyclobutanol, which are formed from a 1,4-biradicaP produced via y-hydrogen abstraction by the excited carbonyl. The photoreactions derived from the 1,4-biradical are termed the Norrish type 11 reaction, and those involving intramolecular hydrogen abstraction from y- and other positions are generically referred to as the type II family. The chemical behavior of the 1,4-biradical determines the type 11 reactivity of carbonyl compounds. In this chapter, the influence of environment on the formation and behavior of the 1,4-biradical from simple alkyl aryl ketones will be described. Environmental effects on the type 11 reaction have been reviewed. ... 

A wide variety of carbonyl compounds undergo photoinduced intramolecular hydrogen atom abstraction to form 1-hydroxy-IvJc-biradicals, which then undergo two common competing reactions (1) coupling to produce cycKc alcohols and (2) disproportionation back to ketone or to various enols. The overall process closely parallels the well-known bimolecular photoreduction of ketones, the most common products of which are formed by radical coupling. These intramolecular hydrogen abstraction processes have been widely studied, and several reviews are available. ... 

In 2008, Sakamoto and coworkers have reported the synthesis of optically active (3-lactams via photochemical intramolecular y-hydrogen abstraction reaction of thioimides [177]. This reaction provides the first example of a chiral-memory effect for the photochemical y-hydrogen abstraction reaction of thiocarbonyl or carbonyl compounds, and a useful synthetic methodology for preparing optically active (5-lactams (Scheme 78). 

Hydrogen abstraction by triplet carbonyl compounds has been the most widely studied excited state reaction in terms of structural variations in reactants. Consequently, the most detailed structure-reactivity relationships in photochemistry have been developed for hydrogen abstraction. These correlations derive from studies of both bimolecular reaction and intramolecular reactions. The effects of C—H bond strength and the inductive and steric effects of substituents have been analyzed. The only really quantitative comparisons between singlets and triplets and between n,n and 71,71 states have been provided by studies of photoinduced hydrogen abstractions. 

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