Halogen elimination

Although homonuclear cyclosilanes have been known for a long time, it is only recently that the smallest members of this series, the cyclotrisilanes, have become available [1], Cyclotrisilanes 1 are obtained by reductive halogen elimination from ort/zo-disubstituted diaryldichlorosilanes [2, 3] or, in particular cases, from hexaalkyl-l,3-dichlorotrisilanes [4]. 

Other cases of hidden H-transpositions preceding halogen elimination were found for the molecular ions of some of the halosuccinates 98-103. 

The halogen azides XN3 (X = Cl, Br, I) react with element chlorides (e.g. BX3, AII3, SnCLi, SbCls, TiCLi, NbCls, TaCls, CrCls, WCle, ReCls, UCle) under halogen elimination to form the element halogenide azides. 

To understand the carbanion mechanism in flavocytochrome 62 it is useful to first consider work carried out on related flavoenzymes. An investigation into o-amino acid oxidase by Walsh et al. 107), revealed that pyruvate was produced as a by-product of the oxidation of )8-chloroalanine to chloropyruvate. This observation was interpreted as evidence for a mechanism in which the initial step was C -H abstraction to form a carbanion intermediate. This intermediate would then be oxidized to form chloropyruvate or would undergo halogen elimination to form an enamine with subsequent ketonization to yield pyruvate. The analogous reaction of lactate oxidase with jS-chlorolactate gave similar results 108) and it was proposed that these flavoenzymes worked by a common mechanism. Further evidence consistent with these proposals was obtained by inactivation studies of flavin oxidases with acetylenic substrates, wherein the carbanion intermediate can lead to an allenic carbanion, which can then form a stable covalent adduct with the flavin group 109). Finally, it was noted that preformed nitroalkane carbanions, such as ethane nitronate, acted as substrates of D-amino acid oxidase 110). Thus three lines of experimental evidence were consistent with a carbanion mechanism in flavoenzymes such as D-amino acid oxidase. 

The [l,2,l]-eliminations as induced by bases or nucleophiles also succeed in the family of the polyhalogen compounds, generally under the influence of organo-lithiums at low temperatures. This suggests initial mctalation. In this way, with a yield of 68-96% the educts 15 react stereoselectively to the (Z)-isomer 16 11 and the alkene 17 opens up an interesting synthesis of the cyclopentyne (18)12), which can be trapped, e.g., to 19 (36%). In the case of several alkenes 20, the reductive [1,2,1]-halogen elimination can be achieved by electrolysis in DMF. Alkynes 21 form in good yields 13). 

Ate-complexes of 1-boraadamantane containing a halogen atom in the y-position of either the side chain (132) or of the adamantane framework (133) undergo y-elimination at room temperature leading to the formation of cyclopropanes (134) and (135), respectively (Scheme 53). These 1,3-elimination reactions presumably proceed synchronously, i.e., simultaneously with the halogen elimination the B—C bond is cleaved and the cyclopropane ring is created. Moreover, the elimination proceeds with the inversion of both of the reaction centers (Scheme 53) <88IZV415>. 

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 is well-established in organic chemistry that acids increase rates of halogen elimination. There is still no agreement on the mechanism of thermal dehydrochlorination of PVC but ionic mechanisms prevail in disputes. More detailed description is available in monographic sources which suggests polarization of H-C and C-Cl bonds to form carboanion or carbonium ion. If snch conrse of reaction is accepted then participation of acids is well-explained by the mechanism of thermal degradation. 

Synthesize the compound shown below from methylcyclopentane and 2-methylpropane using those compounds as the source of the carbon atoms and any other reagents necessary. Synthetic tools you might need include Markovnikov or anti-Markovnikov hydration, Markovnikov or anti-Markovnikov hydrobromination, radical halogenation, elimination, and nucleophilic substitution reactions. 

Use of quinones like chloranil and DDQ has also been employed for this dehydrogenation. Halogenation-elimination with bromine or NBS has been a chemical means to promote the dehydrogenation. However, with the bromination/elimination protocol, one must be careful as aromatic halogenation can occur. 

A second reaction typical of alkyl halides is elimination of the halogen to regenerate the C=C double bond. Halogen elimination occurs to some extent whenever alkyl haUdes react with base ( OH ), but tends to become the main reaction if the alkyl halide is tertiary ... 

The CF2 energy distribution for elimination of Br2 and CI2 from CF2Br2 and CF2CI2 also has been studied by laser fluorescence. In these cases, halogen elimination is the minor channel relative to Br or Cl rupture. For these halogen eliminations the distributions were Boltzmann with < i(CF2)> 1.5, < y(CF2)> 1.8-2.9, and < r> 1.6 kcal mole"These... 

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