Hydrogen-abstraction Reactions

2 Hydrogen Abstraction Reactions - The taxine derivatives (101) undergo smooth photochemical conversion into the tetracyclic derivatives (102) in a reaction which involves hydrogen abstraction by the a-carbon of the excited state enone from the transannular site labelled a . The resultant biradical ring closes to afford the final products. 

p-Unsaturated Systems. 2.1.1 Hydrogen Abstraction Reactions - The photochemical behaviour of the unsaturated enone (89) has been investigated. Irradiation of (89) yields the demethylated ketone (90) by way of a Norrish Type II hydrogen abstraction reaction from the MeO substituent by the excited carbonyl group. This product is accompanied by the aldehyde (91) which arises by a Norrish Type I fission. The fission of this bond affords both an acyl radical and a stabilized allyl radical. The ketone (89) is also reactive in the di-rc-methane mode and affords a bicyclopentene product. 

The enone (92) is reactive in its triplet state and when irradiated in methylene chloride solution is converted into the tetracyclic compound (93). The reaction involves a step-wise process in which the biradical (94) is involved. This process is reminiscent of (2-i-2)-cycloaddition reactions where bonding occurs at the P-atom of the enone, and rather than completing the cyclization, hydrogen (or deuterium) abstraction occurs. A detailed stereochemical analysis of the system was carried out and proof of the stereochemistry of the final product (93) has been presented. 

Photoreduction of steroids (95), (96) and (97) in zeolites has been studied. The results indicate that hydrogen abstraction by the enone system is enhanced in NaY zeolites.  

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

R = CH2CN, CHMeCN, CMe2CN, CH2C02Et, CH2C02H, COEt, CH2COMe 

Ground-state and lowest excited-state energy surfaces for stilbene have been calculated by a semiempirical procedure.12 Calculations show that in the first 

Bortolus and Galiazzo18 have studied the photochemical trans-cis-isomerization of the styrylnaphthalenes (12) and (13), by both direct and anthraquinone-sensitized irradiation. The influence of oxygen upon the irradiation has shown 

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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 +... 

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

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Intramolecular hydrogen abstraction reactions have also been observed in mediumsized rings. The reaction of cyclooctene with carbon tetrachloride is an interesting case. As shown in the equation below, whereas bromotrichloromethane adds to cyclooctene in a completely normal manner, carbon tetrachloride gives some 4-chloro-l-trichloromethyl-cyclooctane as well as the expected product ... 

A striking illustration of the influence of polar factors in hydrogen abstraction reactions can be seen in the following examples (Figure 1.8) where different sites on the molecule are attacked preferentially according to the nature of the attacking radical.67... 

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

FIGURE 1.7. The potential energy surface of the CH4 + C1 supersystem for the collinear hydrogen abstraction reaction CH4 + Cl—> CH3 + HC1. The counter lines are given in spaces of 10 kcal/mol and the coordinates in angstroms. 

Abstraction reactions, see Hydrogen abstraction reactions Activation energy, see Free energy, of activation... 

Hydrogen abstraction reactions potential surfaces for, 25-26,26,41 resonance structures for, 24 Hydrogen atom, 2 Hydrogen bonds, 169,184 Hydrogen fluoride, 19-20, 20,22-23 Hydrogen molecules, 15-18 energy of, 11,16,17 Hamiltonian for, 4,15-16 induced dipoles, 75,125 lithium ion effect on, 12... 

Similarly, energy-transfer processes, together with electron transfer and hydrogen abstraction reactions could be induced in poly(organophosphazenes) in an intramolecular way by preparing POPs geminally substituted at the same phosphorus with two different substituent groups. 

Table 28 Hydrogen abstraction reactions in phosphazene polymers and copolymers photosensitized by external reagents...

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. 

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. 

Scheme 3.1 Possible reaction steps in the hydrogen abstraction reaction of fluoranil with 2-propanol. Note FA= fluoranil, (CH3)2CHOH = 2-propanaol, FAH = fluoranil ketyl radical, FA = fluoranil radical anion.

Triplet Carbene Intermolecular Hydrogen Abstraction Reactions 434... 

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. 

Bernardi, F., Bottom, 1997, Polar Effect in Hydrogen Abstraction Reactions from Halo-Substituted Methanes by Methyl Radical A Comparison Between Hartree-Fock, Perturbation, and Density Functional Theories , J. Phys. Chem., 101, 1912. 

Johnson, B. G., Gonzales, C. A., Gill, P. M. W., Pople, J. A., 1994, A Density Functional Study of the Simplest Hydrogen Abstraction Reaction. Effect of Self-Interaction Correction , Chem. Phys. Lett., 221, 100. 

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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