Substitution, homolytic, bimolecular

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

Redox and homolytic substitution reactions almost never directly form C—C, C—N and C—O bonds. Such bonds are generated in radical addition reactions (Scheme 14). Intermolecular addition reactions are presented in this chapter. Cyclization reactions have important similarities with, and differences from, bimolecular additions, and they are presented in Chapter 4.2 of this volume. Falling under the umbrella of addition reactions are radical eliminations (the reverse of addition) and radical migrations (which are usually, but not always, comprised of an addition and an elimination). 

BIMOLECULAR HOMOLYTIC SUBSTITUTION (SH2) AT THE METAL CENTER OF AN ORGANOMETALLIC SUBSTRATE... 

There are two common types of radical reactions of organoboranes bimolecular homolytic substitution (5h2 equation 4) and a-abstraction processes (equation 5). Both of these can lead to final products by chain processes, as illustrated for the a-abstraction reaction by the continuation shown in equation (6). The radicals produced in these reactions are unlikely to retain complete stereochemical integrity except in special circumstances. The nature of the further reactions, such as equation (6), determines whether or not the products are such as to be included in this section. ... 

H atoms (H ) tend to abstract hydrogen atoms from C-H bonds, particularly if these are activated by neighboring functionalities such as sulfur atoms. The sulfur atom in thioethers was not usually considered as the primary target of H atom despite some, mostly recent, reports showing that bimolecular homolytic substitutions with H atom might occur with thioethers. Because these sulfur atoms are hypervalent, they can form adducts with H atoms. Radical chemistry studies on 1,3,5-trithiane (1,3,5-TT), performed by pulse radiolysis,... 

The energetics of the H abstraction reaction [reaction (27)], AG= -11.4 kcal moH, and the /3-scission [reaction (28)], AG = -1-8.1 kcal mol are from DFT calculations. The two-step mechanism was excluded because of the endothermic step in reaction (28). However, a concerted reaction [combining reactions (27) and (28)] would be spontaneous, but with only a small exothermicity, AG ncerted = -3.3 kcal mol Therefore, the source of these radicals appears to be a bimolecular homolytic substitution (Sjj2) of the acetamide, by hydrogen atoms [reaction (29)] ... 

Wisniowski PB, Bobrowski K, Carmichael I, Hug GL. (2004) Bimolecular homolytic substitution (Sjjj) reaction with hydrogen atoms. Time resolved ESR detection in the pulse radiolysis of a-(methylthioacetamide). J Am Chem Soc 126 14668-14474. 

A136. K. U. Ingold and B. P, Roberts, Free Radical Substitution Reactions Bimolecular Homolytic Substitutions (Sh2 Reactions) at Saturated Multivalent Atoms. Wiley (Interscience), New York, 1971. 245 pp. There is a section on Sa2 reactions in organo-group IV compounds,... 

A139. J. K. Kochi, ed., Free Radicals. Wiley (Interscience), New York, 1973. Vol. 1, 683 pp. Vol. 2, 852 pp. Volume 1, Chapter 6 contains a useful summary of organo-metallie-alkyl halide reactions, mainly involving RLi species. Chapter 10 by A. G. Davies and B. P. Roberts is entitled Bimolecular Homolytic Substitution at Metal Atoms." Volume 2 contains a chapter of 67 pp. (266) by H. Sakurai on group IVB radicals. 

In the first type, the radical combines with a neutral molecule to form a new, larger radical in the second, one radical attacks a molecule and abstracts a hydrogen atom to form a new radical in the third, there is a fragmentation reaction, which is similar to the reverse of the first type while in the fourth, there is a rearrangement of the radical into another radical. Propagation steps like type two are given the label SH2, which stands for bimolecular homolytic substitution reactions, while initiation reactions are called SHI reactions. 

Radical substitution reactions involve an initiation step, in which the number of radicals increases a number of propagation steps, which are also called SHI reactions, in which the number of radicals remains constant and a number of termination steps, in which the number of radicals decreases. A very common propagation step is the abstraction of a hydrogen atom from a carbon/ hydrogen bond, which is an example of a bimolecular homolytic substitution reaction, SH2. 

Free radicals, such as t-butoxyl radicals, react with hydrocarbons by bimolecular homolytic substitution (SH2) at hydrogen centres (equation 3-1, M = H), for example, equation 3-2. 

The accidental observation in 1957 that allyl halides reacted with tin hydrides not by addition across the double bond, but by replacement of the halogen by hydrogen, provided the basis for the extensive use which the tin hydrides (Section 15.3.5), distannanes (Section 18.2.3), allylstannanes (Section 9.1.3.3), and related compounds now find in organic synthesis. The reaction involves bimolecular homolytic substitution (Sh2) at the halogen centre, and ab initio calculations indicate that, when R = H, R = Me, and X = Cl, Br, or I, the transition state is colinear, as illustrated in equation 20-18.58... 

Ingold, K.U. Roberts, B.P. Free Radical Substitution Reactions, Bimolecular Homolytic Substitution (Sg2 Reactions) at Saturated Multivalent Atom.s, Wiley-Interscience New York, 1971. 

The second publication is a review article by Ingold [390] on rate coefficients for free radical reactions in solution which includes comprehensive coverage of radical—molecule reactions. Metathetical reactions are usually referred to as Sn 2 reactions, i.e. substitution, homolytic and bimolecular, by organic chemists. Most quantitative kinetic studies of this class of solution reactions involve H atom transfer but halogen-transfer reactions have also been studied. 

Davies, A. G., Roberts, B. P., Bimolecular Homolytic Substitution at a Metal Center, Accounts Chem. Res. 5 [1972] 387/92. 

Relative reactivity studies of homolytic substitution of monosubstituted benzene derivatives reveal that the 1-adamantyl radical has more pronounced nucleophilic properties than have other, more strained, bridgehead radicals. With benzo-nitrile 21.1) almost exclusive para-substitution is observed, whereas with anisole 0.65) the three possible sites are attacked almost equally. Kinetic studies on the reaction of adamantanethione with adamantane-2-thiol indicate that the rate-controlling chain-propagation step is hydrogen abstraction from the thiol (AdHSH) by the carbon-centred radical AdHSSAd-, and that the main mode of termination involves the diffusion-controlled bimolecular self-reaction of these radicals. ... 

Of four possible processes considered it was suggested that the reactions proceed either by a concerted transfer of a carbanion from cobalt to chromium accompanied by electron transfer from chromium to cobalt or alternatively by an Sb2 mechanism involving bimolecular homolytic substitution at saturated... 

Additions. Homolytic bimolecular addition reactions of carbon-centered radicals to unsaturated groups have been studied in detail because these are the reactions of synthesis and polymerization. Within this group, radical additions to substituted alkenes are by far the best understood. An excellent compilation of rate constants for carbon radical additions to alkenes is recommended for many specihc kinetic values. ... 

SF2 SHI See SEi. Homolytic cleavage resulting in substitution. SH2 Bimolecular radical abstraction. 

Radical-Molecule Interaction Sh2 (Substitution Homolytic Bimolecular) Reactions... 

There are at least two basic mechanisms for chain transfer (1) atom or group transfer and (2) addition-fragmentation (169). The two mechanisms differ mainly in the nature of the intermediate that is formed by the two participating molecules. The atom transfer mechanism—the most common mechanism—proceeds via an S—2 mechanism (substitution, homolytic, bimolecular) as depicted in equation 26. 

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