Dehalogenation olefin

The stereospeciflcity of dehalogenation of vicinal dibromides to olefins was examined for reducing agents including Cr(II), iodide, and Fe° (Totten et al. 2001). For dibromostilbene, the ( )-stilbene represented >70% of the total olefin that was produced, and for threo-dibromopentane reduction by Cr(Il) produced ca. 70% of ( )-pent-2-ene, whereas values for iodide and Fe° were <5% of this. 

On the other hand, benzylic polyhalides were converted to the corresponding olefins via vicinal dihalide intermediates. Metallic nickel was also shown to be useful for the dehalogenation of vicinal dihalides(36,43). 

The C-C coupling reaction between RMgX and R X is considered to proceed though an Ni(R)(R )Lm intermediate, and acceleration of the reductive elimination of R-R by coordination with olefinic or aromatic R X to Ni(R)(R )Lm is necessitated for a smooth catalytic reaction [15,16]. On these bases Ni-pro-moted dehalogenative polycondensation of dihalo organic compounds is suited to the preparation of 7i-conjugated aromatic and olefinic polymers. 

The preparation and dehalogenation of gem-dibromocyclopropanes to give allenes can be carried out in one step by the reaction an excess of olefins with... 

The intermolecular Heck reactions of oxazoles and thiazoles with olefins are not too common. They are rarely high yielding since in several cases they are biased by dehalogenation. Due to this reason the olefination of these systems is usually achieved through Stille coupling with vinylstannanes. 

Compounds 4 and 6 react with a-oleftns and 1,3-dienes on addition of excess trimethylaluminum, and especially on addition of triethylalu-minum and water. Dehalogenation takes place, presumably with formation of aluminoxanes (129), to give very active halogen-free homogeneous catalysts for the polymerization of a-olefins, particularly ethylene (see Section VII). 

A thermally induced, intramolecular dehalogenation of a (chloro)(diethylamino)-carbene ligand affords a diethylaminocarbyne complex quantitatively in aliphatic, olefinic, aromatic or ether solvents ... 

Miura et al. built on Ryu s previous work to achieve four-component radical cascades leading to diketones (Scheme 63) [174]. This outstanding result relies on initial carbonylation of alkyl radicals to form acyl radicals, such as 196. The nucleophilicity of acyl radicals allowed them to react with electron-deficient olefins to form ct-cyano radicals (197), whose phihcity is now reversed. Thus, they were able to add onto stannyl enolates and led to ketyl radicals such as 198. Those latter radicals underwent / -elimination of trib-utylstannyl radicals. This key elimination regenerated the mediator for the initial dehalogenation. This very fine tuning of the radical reactivities is the key element that makes the whole process work. 

Reaction with iam dihaUdes. Dicobalt octacarbonyl reacts at 50° with activated gem-dihalides in THF or benzene first to give a coupling product, which is then dehalogenated to the dimer olefin ... 

Beginning with reductions, we must remember that this class of reactions results in a decrease in the molecules oxidation state. In other words, there is a net loss of functionality on the molecule being reduced. This loss of functionality may be manifested in the conversion of a double bond to a single bond (olefins to alkanes and carbonyls to alcohols), or in the complete removal of heteroatom-based functionalities (deoxygenation, dehalogenation, deamination). In all cases, the net result of a reduction reaction is the formation of a simpler and less reactive structure from one... 

The reagent is exceptionally useful for dehalogenation of tx c-dihalides. even when the resulting olefin is highly strained.9 The yields noted were determined by V PC. Yields of isolated products were somewhat lower. 

Removal of halogen from a vicinal dihalide with zinc can be used to form olefins, although it is not a common method for alkene synthesis. Similarly, dehalogenation of >SiX-SiX< with alkali metals appears to be successful in a limited sense. When R2Si(X)Si(X)R2 (R = t-Bu X = C1, Br) or RR Si(Cl)Si(Cl)RR (R = t-Bu R = c-C6H,) are reacted with lithium (sodium or potassium), the radical anion of the disilene is formed which is particularly long-lived in the latter case181. However, several intermediate... 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

Dr. Fred Guengerich at Vanderbilt University has published mechanistic schemata for cytochrome P450 involvement in an extensive array of both common and uncommon oxidative reactions and reductive reactions. Some of those are exhibited later in this chapter in a brief consideration of reductive reactions. Mechanisms for carbon hydroxylation, heteroatom oxygenation, N-dealkylation, O-dealkylation, alcohol oxidation, arene epoxidation, phenol formation, oxidation of olefins and acetylenes, reduction of nitro compounds, reductive dehalogenation, and azo reduction, to name a few, are provided. 

The optimization of catalyst and reaction parameters allowed development of a high performance catalyst for the reaction of bromoarenes with olefins whose activity exceeds that of any heterogeneous palladium catalysts reported in the literature by at least one order of magnitude (Table 6.) With the Pd/C catalysts used in this study no dehalogenation of bromoarenes occurred. This is in contrast to Arai et al. (15) where up to 80 % of benzene formation by dehalogenation was observed. Bromobenzene can be converted to 90 % within 2 hours and catalyst concentrations of only 0.005 mol% palladium. Turnover numbers (TON) up to... 

In such reactions the ylide is not isolated but is consumed in situ for Wittig olefinations. However, the ylide was isolated after reaction of triphenylphosphine with a-bromo sulfones247 and with 2,3-dibromopropionamide (other 2,3-dibromo carboxamides were smoothly dehalogenated by triphenylphosphine to the dibromophosphorane and acrylamide) 248... 

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