Hydrogens abstraction

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. 

Several explanations have been advanced to explain the radical-like reactivity of excited ketones with 71,71 lowest triplets. Vibronic mixing of the two proximate 

Inasmuch as naphthyl ketones are not totally unreactive at hydrogen abstractions 54,60), there must be a small amount of reactivity intrinsic to n,n states. Such reactivity is actually known for 71,71 olefin triplets 81 and for enone triplets 6 2). In the case of phenyl ketones the carbonyl and aryl jr-systems are mixed, such that a locally excited carbonyl n,n triplet contributes a small fraction to the overall wave function for the triplet. Given this intrinsic reactivity, which is some 10-4—10-6 that of an n,n triplet, the observed reactivity of 71,71 triplets with nearby n,7t triplets can be expressed as follows, where k% is the rate constant 

The only studies specifically designed to differentiate between vibronic mixing and equilibration as the source of ,jr -reactivity in ketones with 71,71 lowest triplets have been performed here at Michigan State University. Perhaps the best evidence that equilibration is the major mechanism involves a comparison of intramolecular triplet state hydrogen abstraction by a series of phenyl ketones hi,71 lowest) and an analogous series of p-methoxyphenyl ketones (37i,n lowest). 

The observed rate constants for y-hydrogen abstraction are decreased in parallel fashion by electron-withdrawing substituents near the y-carbon for both the benzoyl and anisyl compounds 58 59). Such a result is expected for equilibration, as long as the substituent does not change the energy gap between n,n and 7i,7i triplets. However, it is difficult to rationalize how a 71,71 triplet, even with a small fraction of n,7t character, could become electron deficient at oxygen. 

Hydrogen Abstraction.- Xanthone-sensitized irradiation of the dihydroisoquinoline (19) affords a 6X yield of the spiro derivative (20). The reaction is akin to a Norrish Type II process but in this instance hydrogen abstraction by the imine nitrogen is involved. The resultant biradical cyclizes to yield the observed product (20). 

A study of the photochemical behaviour of the 1,1-diphenyl alkenes (21-24) in the presence of the electron-accepting sensitizers 1,4-dicyanobenzene and 1-cyanonaphthalene results in deconjugation to afford products isomeric with starting 

Sensitized irradiation of the allene (28) in xylene-methanol afforded the photochemically stable reduced compound (29) as the only monomeric product. In this instance excitation of the alkene moiety results. The allene (30) reacted differently under the same conditions affording the isomeric trienes (31). The formation of these products is presumed to arise by excitation of the allene. This gives rise to the biradical (32) which cyclizes to yield (33) in a process analogous to a Cope reaction.Collapse of the biradical (33) yields (31). 

Miscellaneous Reactions.- A photochemical 1,3-migration is reported in the conversion of the thiophenyl sulphone (34) into the isomer (35). Irradiation of the fluoroalkene (36) results in the formation of the reduced-dechlorinated product (37) and the product (38), the result of a 1,2-phenyl migration. The compounds produced in this reaction are influenced by solvent and by the nature of the second halogen. Radical and cationic intermediates are thought to be involved. 

The alkene (44) is photochemically reactive on irradiation in a matrix at low temperature. An analysis of the i.r. spectrum of the photolysate showed that the ketene (45) and acetaldehyde are formed.Other experiments in a variety of media failed to give evidence for the formation of a perpendicular alkene which had been predicted following thermal equilibration of the singlet state. Extended irradiation of (44) in argon led to the formation of a new compound which was identified as the oxirane (46) formed by addition of the carbene (47), produced by secondary irradiation of (45). to acetaldehyde. 

Early J-block metal complexes containing one or two a-hydrogen atoms (see 23.38) may undergo a-hydrogen 

Many of the reaction t5 pes discussed in Section 23.7 are represented in the catal5dic processes described in Chapter 26. Unsaturated (16-electron) metal centres play an important role in catal5dic cycles selected catalysts or catalyst precursors are summarized below. 

Pd(PPh3)4 Many laboratory applications including the Heck reaction 

HRh(CO)4 is more active than HCo(CO)4 in hydroformylation, but shows a lower regioselectivity (see equation 26.5 and discussion). 

Abstraction of a second a-H atom gives a carbyne complex (e.g. reaction 23.40). Other routes to carbenes and carbynes are described in Section 23.12. 

Any type of free radical may participate in the hydrogen abstraction from a polymer macromolecule. Depending on the polymer chain structure, the hydrogen atoms can be abstracted in the order of primary secondary tertiary C—H sites, and this process is independent of the nature of the attacking radical [1743]. 

Low molecular free radicals formed from the photocleavage of external impurities (RH) may abstract hydrogen from the polymer molecule (PH) giving polymer alkyl radicals (P )  

Polymer alkyl oxy (PO ) and polymer peroxy (POO ) radicals easily abstract hydrogen from the same and/or neighbouring macromolecule giving hydroxyl (OH) and hydroperoxy (OOH) groups, respectively  

Hydrogen abstraction occurs principally from the tertiary carbon atoms R R 

However, it may also occur from the secondary carbon atoms in methylene groups  

Alkanes.—Hydrogen Substitution. Whereas thermal hydrogen atoms react with alkanes exclusively by hydrogen abstraction, tritium atoms goierated by nuclear recoil also undergo the energetic substitution reaction (52) in high yield.  

Chou and Rowland demonstrated that photodionically generated tritium atoms are capable of promoting both substitution (S3) and hydrogm abstraction (54) in reaction with methane, the ratio of the yields of processes (53) and (54) being 0.27 for atoms of initial energy 2.8 eV. produced by photolysis of TBr at 185 nm. The thresliold for substitution of T for D in CD is about 1.5 eV, comparable with a rough value for T-for-H replacement in cyclohexane and appreciably 

The reaction of hot tritium with methane has been the subject of several theoretical studies. These include compute simulation using a hard-sphere model, an examination of the nuclear displacements on the reaction co-ordinate in terms of the potential energy gradient, as evaluated from the electron density, and computations of the potential energy surface. Several investigations of the reaction by trajectory analysis have also been carried out.  

Raff used a six-body potential energy surface based on the results of semi-empirical and ab initio calculations and the thermodynamic data for reactants and products, but not adjusted to fit kinetic data for the reaction. Cross-sections for reactions (53) and (54) and for the corresponding reactions (55) and (56) of T  

Ill T Valencich and D. L. Bunker, Chem. Phys. Letters, 1973, 20, 50 J. Chem. Phys., 1974, 

This remarkably low reactivity of triplet oxygen is in sharp contrast with the reactivity of other oxygen-centered radicals. Hydrogen peroxide (D(O-H) =87.1 kcal/mol) or aliphatic alcohols such as methanol (D(O-H) = 104kcal/mol), for instance, have much stronger O-H bonds than the hydroperoxyl radical, and the corresponding oxyl radicals will usually quickly and irreversibly abstract hydrogen atoms from alkanes to yield alkyl radicals (Table 3.1). 

The direct catalyzed or uncatalyzed oxidation of alkanes with oxygen is an important reaction in the industrial production of carboxylic acids, hydroperoxides (for production of epoxides from alkenes), alcohols, ketones, or aldehydes [60], 

Heteroatoms or functional groups can either increase or diminish the rate of autoxidation of alkyl groups. Haloalkanes and alkanes substituted with electron-withdrawing groups are usually more resistant toward homolytic C-H bond cleav- 

Autoxidation can lead to deterioration of food, drugs, cosmetics, or polymers, and inhibition of this reaction is therefore an important technical issue. The most important classes of autoxidation inhibitors are radical scavengers (phenols, sterically demanding amines [65, 66]), oxygen scavengers (e.g. ascorbic acid), UV-light absorbers, and chelators such as EDTA (to stabilize high oxidation states of metals and thereby suppress the metal-catalyzed conversion of peroxides to alkoxyl radicals) [67]. 

Early -block metal complexes containing one or two Qt-hydrogen atoms (see 24.47) may undergo a-hydrogen abstraction to yield carbene (alkylidene, 24.49) or carbyne alkylidyne, 24.50) complexes. The solid state structure of the product of reaction 24.45 confirms differences in the Ta—C bond lengths 225 pm for Ta—Caikyi and 205 pm for Ta Ccarbene- 

A basic knowledge of the reaction types described in this section allows us to proceed to a discussion of the chemistry of selected organometallic complexes and (in Chapter 27) catalysis. Oxidative additions and alkyl migrations in particular are very important in the catalytic processes used in the manufacture of many organic chemicals. Selected important organometallic compounds used as catalysts are summarized in Box 24.3. 

PhjCC T]. Subsequent studies by Kuntz (43, 44) showed that Ph3C— was not an endgroup of the polymer. Kuntz (43) demonstrated that 

Bawn et al. (19), and Kuntz (43) suggested that after hydride ion abstraction the resulting species reacts with another molecule of THF to form a growing polymer molecule which has an acetal end group  

Initiation by p-ClC6H4N3 PFjj also seemed to occur via a hydride ion abstraction. Dreyfuss and Dreyfuss (25) showed that the expected product of hydride ion abstraction, chlorobenzene, is formed in the decomposition of p-ClC6H4N PF3 in 2-MeTHF and the product of thermal decomposition, p-chlorofluorobenzene, was absent. In this case 

The implication is that where hydride ion abstraction is indicated, the probable true initiator is the add HSbCl6 or HPFS. Dreyfuss et al. further point out that one cannot make the apriori assumption that the reactivity of the dialkyl oxonium ion (THF HSbCle) is comparable to the reactivity of the trialkyl oxonium ion (i.e. the propagating species). In fact, it may be slower (46). Thus any assumptions about the rate of initiation, or about the number of active centers formed, or any attempts to correlate the degree of polymerization of the resulting polymer with the amount of carbonium ion salt or aryl diazonium salt initiators charged should be made with extreme caution. Again, for theoretical studies of the polymerization, the use of preformed trialkyloxonium salts is to be preferred. 

There have been numerous claims (16, 18, 21, 26, 31, 48, 49) that the polymerization of THF in the presence of certain catalysts proceeds without termination. A number of authors hasten to add that termination and transfer are not important under the conditions of their experiments but that slow termination or transfer reactions may exist. Several kinds 

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Xi M and Bent B E 1993 Reaction of deuterium atoms with cyclohexane on Cu(111)—hydrogen abstraction reactions by Eley-Rideal mechanisms J. Phys. Chem. 97 4167... 

Maeda K, Terazima M, Azumi T and Tanimoto Y 1991 CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the... 

Hemmi N and Suits A G 1998 The dynamics of hydrogen abstraction reactions crossed-beam reaction Cl +... 

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

The percentage of cyclohexylation is given in Fig. 1-20. (411,412). Hydrogen abstraction from the alkyl side-chain produces, in addition, secondary products resulting from the dimerization of thiazolylalkyl radicals or from their reaction with cyclohexyl radicals (Scheme 68) (411). 

As a class of compounds, the two main toxicity concerns for nitriles are acute lethality and osteolathyrsm. A comprehensive review of the toxicity of nitriles, including detailed discussion of biochemical mechanisms of toxicity and stmcture-activity relationships, is available (12). Nitriles vary broadly in their abiUty to cause acute lethaUty and subde differences in stmcture can greatly affect toxic potency. The biochemical basis of their acute toxicity is related to their metaboHsm in the body. Following exposure and absorption, nitriles are metabolized by cytochrome p450 enzymes in the Hver. The metaboHsm involves initial hydrogen abstraction resulting in the formation of a carbon radical, followed by hydroxylation of the carbon radical. MetaboHsm at the carbon atom adjacent (alpha) to the cyano group would yield a cyanohydrin metaboHte, which decomposes readily in the body to produce cyanide. Hydroxylation at other carbon positions in the nitrile does not result in cyanide release. 

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

Butane. The VPO of butane (148—152) is, in most respects, quite similar to the VPO of propane. However, at this carbon chain length an important reaction known as back-biting first becomes significant. There is evidence that a P-dicarbonyl intermediate is generated, probably by intramolecular hydrogen abstraction (eq. 32). A postulated subsequent difunctional peroxide may very well be the precursor of the acetone formed. 

Reaction 36 may occur through a peroxy radical complex with the metal ion (2,25,182). In any event, reaction 34 followed by reaction 36 is the equivalent of a metal ion-cataly2ed hydrogen abstraction by a peroxy radical. 

Methyl ethyl ketone, a significant coproduct, seems likely to arise in large part from the termination reactions of j -butylperoxy radicals by the Russell mechanism (eq. 15, where R = CH and R = CH2CH2). Since alcohols oxidize rapidly vs paraffins, the j -butyl alcohol produced (eq. 15) is rapidly oxidized to methyl ethyl ketone. Some of the j -butyl alcohol probably arises from hydrogen abstraction by j -butoxy radicals, but the high efficiency to ethanol indicates this is a minor source. 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

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). 

Without other alternatives, the carboxyalkyl radicals couple to form dibasic acids HOOC(CH)2 COOH. In addition, the carboxyalkyl radical can be used for other desired radical reactions, eg, hydrogen abstraction, vinyl monomer polymerization, addition of carbon monoxide, etc. The reactions of this radical with chloride and cyanide ions are used to produce amino acids and lactams employed in the manufacture of polyamides, eg, nylon. 

It is usually postulated that the final product in the accepted mechanism, the alkoxyl radical (4), cleaves (eqs. 14 and 15) before or after hydrogen abstraction, and that this accounts for the drop in molecular weight of the... 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Hydrogen-abstraction reactions-. Kadical-decomposition reactions-. 

Photochemical Reactions. Increased knowledge of the centraUty of quinone chemistry in photosynthesis has stimulated renewed interest in their photochemical behavior. Synthetically interesting work has centered on the 1,4-quinones and the two reaction types most frequentiy observed, ie [2 A 2] cycloaddition and hydrogen abstraction. Excellent reviews of these reactions, along with mechanistic discussion, are available (34,35). 

Fig. 6. Coupling of polymer chains via (a) photoinduced hydrogen abstraction free-radical reactions and (b) nitrene insertion/addition reactions.

Ca.ta.lysis, The mechanism of hydrogen abstraction from alcohols to form aldehydes (qv) over silver has been elucidated (11). Silver is the principal catalyst for the production of formaldehyde (qv), the U.S. production of which was 4 x 10 metric tons in 1993. The catalytic oxidation of... 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

Surprisingly little is known about the attack of radicals on small and large heterocycles. Hydrogen abstraction from the heteroatom of small rings leads to ring opening, and in the... 

Oxa-2-azaspiro[2.5]octane A7-alkylation, 7, 204 hydrogen abstraction, 7, 26 NH-transfer, 7, 209... 

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