Alkene and halogen

Particnlarly extensive effort has been directed to the anaerobic dechlorination of alkenes and halogenated phenols that are discussed in Chapter 7, Part 3 and Chapter 9, Part 2. As a resnlt of this, the range of organisms that are able to carry this ont is considerable and inclndes... 

The nature of the interaction between alkenes and halogens (complex 26 of Scheme 10) preceding the reactions of addition of the halogens to the C=C double bond is labelled according to Mulliken s classification71 as a CT complex. The nature of the carbonium... 

The idea that a cyclic bromonium ion was an intermediate was a novel concept at the time of its proposal in 1937. Much additional evidence, including the isolation of a stable cyclic bromonium ion, has been obtained since then to support it. Similarly, cyclic chloronium ions are believed to be involved in the addition of chlorine to alkenes. In the next section we shall see how cyclic chloronium and bromonium ions (halonium ions) are intermediates in a second reaction involving alkenes and halogens. 

For the reaction of OH radicals with vinyl chloride and vinyl bromide the halogen elimination reactions are thermo-chemlcally favorable, the overall reactions being <11 kcal mole and <24 kcal mole exothermic for X = Cl and Br, respectively (203). The elimination of Br atoms from activated chloro-bromoalkyl radicals (206, 207), and of H, Cl, or Br atoms from activated fluoroalkyl radicals (208-213), have been studied using molecular beam techniques, these Intermediate radicals being produced by the reaction of Cl atoms with bromlnated alkenes (206, 207) or of F atoms with alkenes and halogen-substituted alkenes (208-213). For the elimination of Br atoms In the reactions... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Markovnikov regiochemislry occurs, with H adding to the less highly substituted alkene carbon and halogen adding to the more highly substituted carbon. 

Naphthalene dioxygenase from P. putida strain FI is able to oxidize a number of haloge-nated ethenes, propenes, and butenes, and d5 -hept-2-ene and cis-oct-2-ene (Lange and Wackett 1997). Alkenes with halogen and methyl substituents at double bonds form allyl alcohols, whereas those with only alkyl or chloromethyl groups form diols. 

A range of mechanisms has been found for the biodegradation of halogenated alkanes, alkenes, and alkanoates ... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

The addition of halogen hydracids to simple alkenes is found to be somewhat less stereoselective than was the addition of halogens, being rather more dependent on the particular alkene, and on the reaction conditions. 

In the first step, the k electrons of the alkene double bond attack the halogen. (In the absence of oxygen, some reactions between alkenes and chlorine proceed... 

Similar intramolecular hydroarylations of alkynes and alkenes, which obviate the need for a halide or triflate group on the aryl ring, are now well established. Sames group screened over 60 potential catalysts and over 200 reaction conditions, and found that Ru(m) complexes and a silver salt were optimal. This process appears to tolerate steric hindrance and halogen substrates on the arene (Equations (175)—(177)). The reaction is thought to involve alkene-Ru coordination and an electrophilic pathway rather than a formal C-H activation of the arene followed by alkene hydrometallation, and advocates the necessary cautious approach to labeling this reaction as a C-H functionalization... 

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