Hydrogen abstraction reactivity

Zipse H. Radical stability — a theoretical perspective. Top Curt Chem. 2006 263 163-189. Lai WZ, Li CS, Chen H, Shaik S. Hydrogen-abstraction reactivity patterns from A to Y the valence bond way. Angew Chem Int Ed. 2012 51 5556-5578. 

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. 

The relative amounts of double bond addition, hydrogen abstraction and 13-scission observed are dependent on the reactivity and concentration of the particular monomer(s) employed and the reaction conditions. Higher reaction temperatures are reported to favor abstraction over addition in the reaction of t-butoxy radicals with AMS413 and cyclopentadiene 417 However, the opposite trend is seen with isobutylene.2 1 24... 

The formation of dimethyl sulfide, dimethyl sulfone, and methane (by H-abstraction) observed in these photolyses is thus accounted for. Hydrogen abstraction by the methylsulfinyl radical affords methanesulfenic acid, CH3SOH, a very reactive molecule, which rapidly undergoes a series of secondary reactions to produce the methanesulfonic acid, methyl methanethiolsulfonate (CH3S02SCH3), and dimethyl disulfide which were also observed during these photolyses. 

Chung and coworkers tried to observe similar species in y-irradiated DMSO-h6 at 77 K, however, repeated attempts were unsuccessful. Besides no free -CH3 radicals were detected in the y-irradiated DMSO-h6. They suggested that this remarkable difference of an all-or-nothing deuterium effect might be connected with the very much larger reactivity of the methyl radical in a subsequent reaction of hydrogen abstraction due to the greater reactivity of the C—H over the C—D bond. 

The cancellation in GP effects in the state-to-state DCS are found [20-22, 26, 27, 29] at low impact parameters, when F(J) in Eq. (15) is chosen to include only contributions for which / < 9. It is well known [55,56] that most of the reactive scattering in this regime consists of head-on collisions, in which the reaction proceeds mainly by the H atom striking the H2 diatom at geometries that are close to linear. Most of the products are then formed by direct recoil in the backward (9 = 180°) region, this being typical behavior for a hydrogen-abstraction reaction. 

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

Addition reaction of peroxide-generated macroalkyl radicals with the reactive unsaturation in MA is shown in reaction scheme 4. The functionalised maleic-polymer adduct (II, scheme 4) is the product of hydrogen abstraction reaction of the adduct radical (I, scheme 4) with another PP chain. Concomitantly, a new macroalkyl radical is regenerated which feeds back into the cycle. The frequency of this feedback determines the efficiency of the cyclical mechanism, hence the degree of binding. Cross-linking reaction of I occurs by route c ( scheme 4). 

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