Hydrogen Abstraction and Fragmentation Reactions

One of the most common reactions of photoexcited carbonyl groups is hydrogen atom abstraction from solvent or some other hydrogen donor. A second common reaction is cleavage of the carbon-carbon bond adjacent to the carbonyl group, which is called a-cleavage. 

The hydrogen atom abstraction can be either intramolecular or intermolecular. The intermediates that are generated are free radicals. If these radicals come to thermal equilibrium, they have the same structure and reactivity as radicals generated by 

These reactions usually occur via the triplet excited state Tj. The intersystem crossing of the initially formed singlet excited state is so fast k s ) that reactions of the 

Si state are usually not observed. The reaction of the benzophenone Tj state has been particularly closely studied. Some of the facts that have been established in support of the general mechanism outlined above are as follows  

For a given hydrogen donor S-H, replacement by S—D leads to a decreased rate of reduction, relative to nonproductive decay to the ground state. This decreased rate is consistent with a primary isotope effect in the hydrogen abstraction step. 

The photoreduction efficiency of ortho-zSkyl benzophenone derivatives is greatly reduced by intramolecular enolization process, known as photoenolization reaction. For example, orf/io-ethyl benzophenone 2 on photoirradiation in deuterated hydroxylic solvents gives deuterated ethyl benzophenone 3 by photoenolization without reduction [3]. 

For some aromatic ketones, the reactive dienols undergo electrocyclization to cyclobutenols [4]. 

The reactive enols 4 may be trapped as Diels—Alder adducts 5, for example, wifli dimethyl acetylenedicarboxylate [4]  

Another important photochemical reaction of both aliphatic and aromatic ketones is the fragmentation reaction. Unconjugated ketones on photoexcitation undergo a-cleavage followed by decarbonylation and subsequent reactions of alkyl radicals to give product(s). All these processes are collectively known as Norrish type-1 cleavage reactions [5]. These reactions take place both in gaseous and liquid phases. 

Cyclic ketones 8-10 also undergo similar a-cleavage, decarbonylation and hydrogen abstraction reactions to give products [6, 8]. 

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Give examples of simple radical and biradical reactions - combination, disproportionation, hydrogen abstraction and fragmentation. 

In contrast to aminyl radicals, alkoxyl (and acyloxy) radicals are highly reactive. As illustrated in equation (7), their cyclization reactions are extremely rapid and irreversible. However, the rapidity of such cyclizations does not guarantee success because alkoxyl radicals are also reactive in inter- and intramolecular hydrogen abstractions, and -fragmentations (see Section 4.2.S.2). This lack of selectivity may limit the use of alkoxyl radicals in cyclizations, but S-exo cyclizations are so rapid that they should succeed in many cases, and other types of cyclizations may also be possible. 

Under typical operating conditions, in the absence of self-heating from the reaction, the equilibrium for this step lies in favour of the product This species undergoes a series of intramolecular hydrogen-abstraction and further 02-addition steps before fragmentation of the carbon chain. This final step produces three radical... 

The applications of such electronic effects extend to radical chemistry. For example, reactions of electrophilic oxygen-centered radicals display excellent chemoselectivity for cyclization onto the electron-rich silyl enol ether when competing with terminal alkene cyclization, 1,5-hydrogen abstraction, and p-fragmentation pathways (Figure 6.115). ... 

The lowest-lying excited state of ketones most often corresponds to a o 7t c=o transition. The maximum of this band is around 280 nm with simple aldehydes or ketones and is shifted to the red for conjugated or aryl derivatives. As hinted above, the unpaired electron on the hq orbital gives to these states electrophilic properties similar to those of alkoxy radicals, and indeed the observed chemistry is similar in the two cases. Typical reactions are a-fragmentation, inter- or intramolecular (from the easily accessible y position) hydrogen abstraction and attack of alkenes (finally resulting in a formal 2h-2 cycloaddition to give an oxetane, the Paterno-Btichi reaction). 

Among the competing principal reactions of alkoxyl radicals, intramolecular hydrogen abstraction and (3-fragmentation have been extensively and successfully used in organic synthesis. This situation is in contrast to the case of carbon-centered radicals in which intramolecular radical addition has been most successfully exploited in synthesis.Some representative examples of the synthetic application are described below. The literature covers up to the beginning of 2002. 

The past half a century has witnessed that alkoxyl radicals are highly reactive, unique chemical species and are as important and useful as carbon-centered radicals in organic synthesis. The usefulness of intramolecular hydrogen abstraction and (i-fragmentation of alkoxyl radicals in synthesis is beheved to be comparable to the intramolecular addition of carbon-centered radicals. This chapter is an attempt to bring together the diverse photochemical reactions of hypohalites, which are the most powerful and convenient precursors of alkoxyl radicals. 

There is quite some evidence for a mechanism as formulated above,especially for the six-membered transition state—the Barton reaction is observed only with starting materials of appropriate structure and geometry, while the photolysis of nitrite esters in general seldom leads to useful products formed by fragmentation, disproportionation or unselective intermolecular hydrogen abstraction. 

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. 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

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