H-abstraction reactions

Simple olefins do not react with eaq at an appreciable rate, but compounds with an extended 7t-system such as butadiene can also accommodate an additional electron (k = 8 x 109 dm3 mol-1 s 1 Hart et al. 1964). However, as in the case of benzene, the rate is often below diffusion controlled [reaction (23) k = 7.2 x 106 dm3 mol 1 s 1 (Gordon et al. 1977) in THF, the reaction of the solvated electron with benzene is even reversible (Marasas et al. 2003)], and the resulting radical anion is rapidly protonated by water [reaction (24)]. 

A rapid protonation by water of the electron adducts of spin traps such as DMPO or 2-methyl-2-nitroso-propane yields the same species as are expected for the reaction of H (Sargent and Gardy 1975). This prevents a distinction between eaq and 11 by using this technique. 

IP readily adds to C-C double bonds. Like OH, it is a pronounced electrophilic radical (p = -0.45 Neta 1972) and thus shows a high regioselectivity in its addition reactions. With eaq , it shares a fast reaction with 02 [reaction (25) k = 

11 also undergoes H-abstraction reactions, albeit with much lower rates than OH. This is also reflected in a higher H/D isotope effect [e.g with 2-PrOH/2-PrOH-d2 ku/ko 7.5, reactions (26) and (27) (Anbar and Meyerstein 1964) see also Vacek and von Sonntag (1969), vs. ku/kD = 1.5 for OH (Anbar et al. 1966)]. 

If there is competition between addition and H-abstraction, addition is always preferred. As a consequence, H-abstraction from the sugar moiety is a very minor process in DNA and related compounds (Das et al. 1985). 

---

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

Thus, the process of hydride ion abstraction from a primary position is approximately thermoneutral, and hence we must conclude that it is an energetically allowed process, although possibly with a relatively small reaction rate. A process competing with primary H abstraction (Reaction 13) is methide ion abstraction (Reaction 11, loss of CH4 from the... 

Dissociation of the gases SiH4 and H2 by electron impact will create reactive species (radicals) and/or neutrals (Si2H6 and even higher-order silanes [195-198]). Atomic hydrogen is an important particle because it is formed in nearly all electron impact collisions, and the H-abstraction reaction [199, 200] of (di)silane is an important process, as is seen from sensitivity study. Dissociation of SiHa can create different SiH (with x = 0, 1,2, 3) radicals. Only silylene (SiH2) and... 

The disappearance of the spectra of the biradicals was attributed to ring closure to form the corresponding bicyclobutanes. H-abstraction from the surrounding matrices was excluded because (a) rates of decay were much too fast below 65 K compared with known radical H-abstraction reactions, (b) no radical signals were observed to grow in the EPR spectra, and (c) no rate differences were observed in deuterated compared with protio matrices. NMR and GC analyses of the EPR samples showed... 

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

In a more general situation, the redox chain depends, among other factors, of the type of R produced by the fast H-abstraction (reaction 14). 

Additional clues confirm the radical nature of the reaction of BTNO with RH substrates. A Hammett correlation, obtained on plotting log A h for reaction of BTNO with p-substimted benzyl alcohols, gave a p value of —0.55 vs. a+. This small value, which is reasonable for a radical reaction, compares well with the p values ranging from —0.54 to —0.70 and obtained vs. the same substrates with the aminoxyl radicals generated from X-aryl-substituted HPIs (Table 5). In all cases better Hammett correlations were obtained vs. the values. Hence, a uniform pattern of selectivity emerges among these electrophilic >N—O" species in H-abstraction reactions. 

FIGURE 1. Evans-Polanyi plot for the H-abstraction reaction by BTNO with selected substrates (EL stands for fluorene). Reprinted with permission from Reference 134. Copyright (2005) American Chemical Society... 

It will be useful to evaluate and compare the reactivity of additional aminoxyl radicals in the H-abstraction reaction in future investigations. 

Although OH behaves mechanistically like H in its reactions with organic molecules, it is less selective and more reactive than H in H-abstraction reactions because the formation of the H—OH bond is 57 kJ mol more exothermic than that of H—H. Examples of the reactivities of H , OH, and 0 are listed in Table 4. 

If we assiime that there is no activation energy for the disproportionation or recombination, then fcj 109-5 liter/mole-sec. (see Table III). This is about a factor of 10 higher than the values to be expected of H-abstraction reactions of alkyl radicals. It is furthermore anomalous in having a negligible activation energy compared to the expected 8 3 kcal. Note that if we assign 1 kcal. of activation energy to the disproportionation then Ad 101 U liter/mole-sec. 

Once again it would be extremely difficult if not impossible to account for such large A factors in terms of the relatively tight 4-center transition state of reaction H for which if anything a very small A factor of the order of 106 to 10 liter/mole-sec. might be expected. Compared to the H-abstraction reactions of radicals (Tables I and II) for which A = 108-5 liter/mole-sec., the value A = 10 2 is high by about 103-6. 

Solvolysis of MC15 by ammonia was studied as early as 1924, but the products were not conclusively characterized.260,261 Halo monoalkylamides262 and dialkyl amides were prepared. The monoalkylamides undergo facile cr-H abstraction reactions to nitrenes, especially when labile substitutents are present (Section 34.2.3.5) dialkylamides (Table 12) have been extensively studied by Bradley.263-265... 

Reaction 7. The Arrhenius parameters for the H-abstraction reactions of peroxy radicals may be estimated from general considerations. Generally H-abstraction reactions by polyatomic radicals have A factors in the range 107 3 to CH3 (54), CF3 (3, 45), CH30 ( 27). The... 

When the temperature is too low for methoxy to dissociate thermally, it may react through a number of H abstraction reactions to form methanol. These include reaction with methane (R19) and with formaldehyde... 

The H-abstraction reaction, however, does not determine selectivity entirely. Depending on the experimental conditions, as discussed earlier, butadiene desorption could be one important step. In this case, the electronic properties of the transition metal ions that determine the interaction with the unsaturated hydrocarbon have to be considered. 

Activation energies and rate constants for the H-abstraction reaction by H from simple aliphatic ketones in water has been calculated by EPR FID attenuation measurements.121... 

This source of peroxyl radicals has been used to study the peroxyl radical reactions with nucleobases (Simandan et al. 1998) and thymidine (Martini and Termini 1997 Chap. 10). The slow H-abstraction reactions of peroxyl radicals prevents their reaction with DNA in dilute aqueous solution unless they are positively charged and thus bound to DNA by electrostatic forces (Paul et al. 2000). Otherwise, their competing bimolecular termination reactions are much faster (Chap. 8). 

The various ways of forming OH were discussed in Chapter 2. It is a very reactive, electrophilic (9 = -0.41 Anbar et al. 1966a) radical, and with most substrates it reacts at close to diffusion-controlled rates (for a compilation of rate constants, see Buxton et al. 1988). It undergoes mainly three types of reactions (1) addition to C-C and C-N double bonds, (2) H-abstraction and (3) ET. Addition and H-abstraction reactions will be discussed below in some detail, because they are relevant for an OH-attack at the nucleobases and at the sugar moiety in DNA. 

The HO-H bond dissociation energy (BDE) is 499 kj mol-1, while the C-H bonds in saturated hydrocarbons are much weaker (BDE = 376-410 kj mol-1 Berkowitz et al. 1994 for a compilation, see Chap. 6). Thus, there is a considerable driving force for H-abstraction reactions by -OH. On the other hand, vinylic hydrogens are relatively tightly bound, and an addition to the C-C double bond is always favored over an H-abstraction of vinylic or aromatic hydrogens. Hence, in the case of ethene, no vinylic radicals are formed (Soylemez and von Sonntag 1980), and with benzene and its derivatives the formation of phenyl-type radicals has never been conclusively established. 

Despite the considerable driving force for the H-abstraction reaction, there is some remarkable selectivity. Primary hydrogens (-CH3) are less likely abstracted than secondary (-CH2 ) and tertiary (-CH-) ones (Asmus et al. 1973). In addition, neighboring substituents that can stabilize the resulting radical by elec-... 

In the case of amines, protonation that withdraws electron density from the center of reaction lowers the rate of reaction by a factor of 30 (Das and von Sonntag 1986). Besides H-abstraction from carbon [reactions (18) and (21)], the formation of N-centered radical cations is observed [reactions (19)/(22) and (20) for amino acids see, e.g Bonifacic et al. 1998 Hobel and von Sonntag 1998]. Reaction (20) is also an H-abstraction reaction. The ET reaction (19)/(22) may proceed via a (bona-fide, very short-lived) adduct (Chap. 7). 

The reaction of OH with thiolate ions, taken as an overall reaction, is an ET reaction [reaction (36)]. One must, however, again take into account that a three-electron bonded intermediate is formed in the first step (Chap. 7). In semi-de-protonated dithiothreitol reaction (32) dominates over the H-abstraction reaction (33) (Akhlaq and von Sonntag 1987), although the rate constant for the reaction of OH with a thiol and a thiolate ion are both diffusion controlled (k = 1.5 x 1010 dm3 mol-1 s 1). This is another example of the potentially high regio-selectivity of OH reactions. 

---