Hydrogen abstraction carbonyl compound

Basic Principles Practical Photochemistry General Considerations Carbonyl Compounds a-Cleavage Carbonyl Compounds Hydrogen Abstraction ... 

The radical derived from the carbonyl compound may abstract a second hydrogen, either from a molecule of RH or from R ... 

In contrast to excited carbonyl compounds, which abstract hydrogen efficiently from the n,Jt state (Section 6.3.1), thiocarbonyl compounds react from both n,Jt and n,n states and the hydrogen atom can be added to either sulfur or carbon atom of the C=S bond.1259,1269,1270 The sulfur atom is larger than oxygen, therefore the reaction can occur... 

Probably in analogy to the well-known hydrogen abstraction reactions of carbonyl and nitro compounds, hydrogen abstraction has been proposed as a primary process of sulfoxide excited states. It is safe to say that while some of these suggestions appear quite reasonable, the acmal evidence for hydrogen abstraction is much thinner than for a-cleavage. 

Most of the characteristics that allow excited carbonyl compounds to abstract hydrogen atoms have been determined by studies of bimolecular photoreductions and Norrish type II elimination reactions (see Chapter 52). The following sections provide abbreviated descriptions of various aspects that are more fully covered in Chapter 52. 

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. 

Aldol reactions, Like all carbonyl condensations, occur by nucleophilic addition of the enolate ion of the donor molecule to the carbonyl group of the acceptor molecule. The resultant tetrahedral intermediate is then protonated to give an alcohol product (Figure 23.2). The reverse process occurs in exactty the opposite manner base abstracts the -OH hydrogen from the aldol to yield a /3-keto alkoxide ion, which cleaves to give one molecule of enolate ion and one molecule of neutral carbonyl compound. 

Like aldehydes and ketones, the a-hydrogens of acid and acid derivatives are acidic and can be abstracted with base to generate the carban-ions, which can then react with various electrophiles such as halogens, aldehydes, ketones, unsaturated carbonyl compounds, and imines, to give the corresponding products. Many of these reactions can be performed in aqueous conditions. These have been covered in related chapters. 

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... 

TV-Substituted amides and lactams possess potentially reactive C—H bonds on carbon atoms alpha to the nitrogen and carbonyl group. These hydrogen atoms are easily abstracted by excited carbonyl compounds (e.g., acetone or benzophenone) to produce radicals which undergo olefin addition <9a,98 97) ... 

P. J. Wagner, B. -S. Park, Photoinduced Hydrogen Atom Abstraction by Carbonyl Compounds, in Organic Photochemistry, Vol. 11. A. Padwa (ed.), Marcel Dekker, New York, 1991, pp. 227-365 and references therein. 

Allylic hydrogens can easily be abstracted by excited carbonyl compounds. Recombination of the two radicals afford products which are apparently formed by RH addition to the carbonyl group (4.17)419). 

The traditional chain oxidation with chain propagation via the reaction RO/ + RH occurs at a sufficiently elevated temperature when chain propagation is more rapid than chain termination (see earlier discussion). The main molecular product of this reaction is hydroperoxide. When tertiary peroxyl radicals react more rapidly in the reaction R02 + R02 with formation of alkoxyl radicals than in the reaction R02 + RH, the mechanism of oxidation changes. Alkoxyl radicals are very reactive. They react with parent hydrocarbon and alcohols formed as primary products of hydrocarbon chain oxidation. As we see, alkoxyl radicals decompose with production of carbonyl compounds. The activation energy of their decomposition is higher than the reaction with hydrocarbons (see earlier discussion). As a result, heating of the system leads to conditions when the alkoxyl radical decomposition occurs more rapidly than the abstraction of the hydrogen atom from the hydrocarbon. The new chain mechanism of the hydrocarbon oxidation occurs under such conditions, with chain... 

In this study, the metal centred radicals were formed indirectly by the photolysis of (Me3CO)2 yielding Me3CO radicals which then abstracted a hydrogen atom from the (n-Bu)3MH (M = Ge or Sn). The resulting metal based radical reacts with the carbonyl compound by adding to the oxygen atom (reaction 3). 

The photoreductions of a number of carbonyl compounds with either lowest mr or jiji triplet states in the presence of tributyltinhydride are reported24. The carbonyl compounds include cyclohexanone and acetone which possess nrr lowest-energy triplets, and 2-acetonaphthone, 1-naphthaldehyde and 2-naphthaldehyde which possess lowest-energy mr triplets. In the case of the two njr triplets, a simple mechanism is proposed which involves the abstraction of a hydrogen atom from the tributyltinhydride by the triplet state... 

When considered as a part of the photochemistry of carbonyl compounds, irradiations of esters constitute a minor component. The more frequent photolyses of other carbonyl compounds, in particular ketones, is not surprising, as, even though parallels exist between ester and ketone photochemistry (for example, both experience a-cleavage and hydrogen abstraction-reactions), esters require radiation of higher energy for reaction, and typically produce more-complex mixtures of products. In addition to their similarity to other carbonyl compounds in their reactivity, esters also experience reactions that are uniquely their own. 

When the esters listed in Table IX are irradiated, typical carbonyl reactions result that is, each of these compounds (22-27) experiences both a-cleavage and hydrogen-abstraction reactions. For the esters of l,2 3,4-di-0-isopropylidene-a-D-galactopyranose (22-26), the hydro-gen-abstraction reaction is internal, and leads69 to a Type II reaction... 

Three types of photochemical reaction of carbohydrate acetals have been investigated. Early studies centered on the photochemical fragmentation of phenyl glycosides, and the photolysis of o-nitrobenzyli-dene acetals. (The latter reactions will be discussed with the photolysis of other nitro compounds see Sect. VII,1.) Later experiments were concerned with hydrogen-abstraction reactions from acetal carbon atoms by excited carbonyl compounds. 

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 second principal reaction of the (n,jr) excited state of carbonyl compounds is the abstraction of a hydrogen atom from another molecule ... 

The polymerisation of vinyl monomers can be initiated by radicals formed by a-cleavage or hydrogen abstraction of a carbonyl compound. 

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