Intermolecular Hydrogen-abstraction Reactions

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

In each case the weakest C-H bond in the R -H molecule is broken. A consequence of this reaction is that the range of suitable organic solvents for carbonyl photochemistry is somewhat limited. Cyclohexane is a relatively poor hydrogen donor although it has limited success in dissolving polar compounds, so acetonitrile and tertiary alcohols may be used, as is benzene. 

The pair of radicals that is initially generated can react in a number of ways  

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Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Triplet Carbene Intermolecular Hydrogen Abstraction Reactions 434... 

Predict the products of the intermolecular hydrogen abstraction reactions of ketones and discuss the mechanisms of these reactions. 

The photoreaction investigated to test the above model is the well-known electron-transfer-initiated intermolecular hydrogen abstraction reaction of carbonyl compounds [277]. Under the conditions employed, one of the guest mole-... 

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. 

Rubin, M. B., Photoinduced Intermolecular Hydrogen Abstraction Reactions of Ketones. In Horspool, W. M., Song, P. S. (eds), CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1995, pp. 430 436. 

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

Although such alkoxides have never been isolated it is assumed that with bulky alcohols such as steroidal alcohols, the main contributing structure in such an equilibrium (especially when excess lead tetraacetate is present) is the one in which n = 1. An advantage of this procedure held in common with the hypoiodite reaction is the fact that the alcohol derivative is formed in situ. Intermolecular hydrogen abstraction e.g., reaction with solvent)... 

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. 

The above reaction is a convincing example of an intermolecular hydrogen abstraction leading essentially to the same result as obtained in the pyrolysis of alkyl-substituted thiirane oxides through an intramolecular /(-elimination of hydrogen. 

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

Waals radii of two atoms (2.90 A). Nevertheless, each reaction site is far apart the hydrogen abstraction from the benzyl group proceeds and the product was obtained in optically active form. In this case, another mechanism besides the direct intramolecular hydrogen abstraction from the benzyl group may be involved, like intermolecular hydrogen abstraction or hydrogen shift. 

The photolysis of n-octyl nitrite in n-heptane provides a favorable situation for comparing the tendency of an alkoxyl radical to undergo the Barton-type intramolecular reaction or, alternatively, the intermolecular reaction as indicated in eqs. 1 and 2. Calculations23 reveal that both intramolecular and intermolecular hydrogen abstraction involving a second-... 

Similarly, the products derived from the photolysis of steroidal nitrites (60 and 11/8) in toluene were mainly the result of intramolecular hydrogen abstraction (the Barton reaction). Kxothermicity calculations23 predict intermolecular hydrogen abstraction process (AH = 25 kcal./mole) to predominate over intramolecular hydrogen abstraction (AH = —3 to 5 kcal./mole). 

However, in the presence of a suitable substrate as illustrated by RH in reaction 4, an intermolecular hydrogen abstraction by the intermediate alkoxy radical favorably competes with the decomposition reaction (2). The resulting intermediate alkyl radical, R, normally combines with... 

Aromatic ketones, which undergo intersystem crossing with great efficiency, give Type II (and Type I) reactions solely from the triplet state. Like the products of intermolecular hydrogen abstraction, Type II products are largely suppressed if the lowest triplet is 77,77. 91... 

When the alkoxyl radical and the hydrogen to be abstracted are not properly disposed for the Barton reaction, the reactions of the alkoxyl radical, for example -fragmentation, intramolecular addition to the double bond, disproportionation or a-hydrogen fission, and intermolecular hydrogen abstraction, compete with the Barton reaction or result in an exclusive reaction. Among these reactions, /l-frag-... 

The typical reactions of nitrenes generated from aliphatic azide precursors are shown below and include isomerization to imines (a), intramolecular insertion into C—H bonds (b) has been questioned (see 1. c. n> p. 57) and intermolecular hydrogen abstraction (c). 

In contrast to the number of intramolecular asymmetric hydrogen abstraction reactions using supramolecular approaches (Sec. IV.D.), intermolecular asymmetric hydrogen abstraction reactions have been scarcely reported. Only two examples are presented these occur in inclusion crystals of deoxycholic acid with ketones and crystals of cyclodextrin with acetophenone. 

For R = hydrogen, methyl or ethyl, the e.s.r. spectrum of the corresponding vinyl radical was observed, showing that no reaction had occurred. This lack of reaction is not surprising as the radicals cannot adopt a suitable configuration for reaction. It also shows that the vinyl radicals are sufficiently isolated from unreacted vinyl halide molecules to prevent the occurrence of intermolecular hydrogen abstraction. However, when R = propyl, a six-membered cyclic transition state can be formed and in fact the spectrum of the vinyl radical is completely replaced by that of the alkyl radical formed by abstraction from the terminal methyl group (reaction 32). 

A variety of intramolecular hydrogen-abstraction reactions from amino groups by carbonyl groups have been studied. There is no direct evidence that these reactions involve an intramolecular electron-transfer reaction. However, on the basis of studies on intermolecular systems, this is likely to be the case when the carbonyl group is part of an electron-accepting system, e.g. a quinone or phthalimide group. Quinone [153] was found to give [154], [155] and [156] (Scheme 35) (Maruyama et al., [911 Falci el al., 1977). 

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