Radical chain reactions addition

This comparison suggests that of these two similar reactions, only the former is likely to be a part of an efficient radical-chain reaction. Addition to a carbonyl group, in contrast, is endothermic. Radical additions to carbon-carbon double bonds can be further facilitated by radical-stabilizing groups. A similar comparison can be made for abstraction of... 

The surface-mediated reaction enables the addition of HBr to 1-octene to proceed at room temperature without the formation of 1-bromooctane by free radical chain reaction addition. Brown, H. C. Liu, K.-T. J. Am. Chem. Soc. 1975, 97, 600. 

The ultraviolet lamps used in the photochlorination process serve to dissociate the chlorine into free radicals and start the radical-chain reaction. Other radical sources, such as 2,2 -a2obisisobutyronitrile, have been used (63,64). Primary by-products of the photochlorination process include 1,1,2-trichloroethane (15—20%), tetrachloroethanes, and pentachloroethane. Selectivity to 1,1,1-trichloroethane is higher in vapor-phase chlorination. Various additives, most containing iodine or an aromatic ring in the molecule, have been used to increase the selectivity of the reaction to... 

The present method offers several advantages over earlier methods. The use of carbon tetrachloride instead of diethyl ether as solvent avoids the intrusion of certain radical-chain reactions with solvent which are observed with bromine and to a lesser degree with chlorine. In addition, the potassium bromide has a reduced solubility in carbon tetrachloride compared to diethyl ether, thus providing additional driving force for the reaction and ease of purification of product. The selection of bro-... 

The decomposition of tri- and tetrasulfane in CCI4 solution (0.2 mol 1 ) at 70 °C and in the absence of oxygen has been studied by H NMR spectroscopy [64]. Initially, tetrasulfane decomposes to a mixture of tri- and pentasul-fane but slowly and after an induction period hydrogen sulfide and disulfane are formed in addition. These results have been interpreted in terms of a radical-chain reaction. The initial step is assumed to be the homolytic cleavage of the central SS bond which has by far the lowest dissociation enthalpy of the molecule ... 

In addition to oxidation, many other reactions occur as free radical chain reactions polymerization, decomposition, fluorination, chlorination, etc. All chain reactions have a few important general peculiarities [1—3]. 

Curran2 has reviewed recent applications of the tin hydride method for initiation of radical chain reactions in organic synthesis (191 references). The review covers intermolecular additions of radicals to alkenes (Giese reaction) as well as intramolecular radical cyclizations, including use of vinyl radical cyclization. 

Radical cyclizations of this type can be also achieved in chemical radical chain reactions [124, 125], often in a wider scope. The anodically initiated cyclization, however, has advantages. It avoids tin hydride, which is mostly used as coreagent in chemical radical chain cyclizations and because the toxicity of tin organics makes these reactions less attractive for the synthesis of pharmaceuticals. In chemical radical chain reactions, which involve in most cases an addition and an atom-transfer reaction, one C,C- and one C,H- or C,X bond is being formed, while in anodic addition followed by heterocoupling two C,C bonds are being formed, where the second one is established simply and in wide variety by the appropriate choice of the coacid. 

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

Carbonylation procedures have been successfully used for C—C bond forming radical strategies. Alkyl halides could be carbonylated under moderate pressure of CO (15-30 atm) in benzene at 80 °C in the presence of (TMS)3SiH and AIBN [16], Reaction (7.8) shows the effect of the CO pressure on the carbonylation of a primary alkyl bromide. These radical chain reactions proceed by the addition of an alkyl radical onto carbon monoxide, which generates... 

Two examples of three-components coupling reaction are shown in Reactions (7.77) and (7.78) [27,87]. These radical chain reactions proceeded by the addition of an alkyl or vinyl radical onto carbon monoxide, generating an acyl radical intermediate, which, in turn, can further react with electron-deficient olefins to lead, after reduction, to a formal double alkylation of carbon monoxide. These three-components coupling reactions require the generation of four highly disciplined radical species, which have specific functions during the chain reaction. 

A regiospecific concerted syn addition (path (a)) competing with a radical chain reaction (path (b)) has been proposed to rationahze the addition of TeCl4 to olefins. 

Investigations conducted by the same group using laser flash photolysis techniques elucidated details of the PET-reductive activation of selenosilanes and the application of this chemistry to a bimolecular group-transfer radical reaction and intermolecular radical chain-transfer addition [59], Based on this new concept, a catalytic procedure utilizing PhSeSiRs for radical reactions such as cycliza-tion, intermolecular addition and tandem anellation was designed (Scheme 39) [60],... 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

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