Alkenes from 1,2-dihalo compounds

A variety of sulfur reagents have been used for the dehydrohalogenation of wc-dihaloalkanes to alkenes. Aqueous sodium trithiocarbonate in the presence of a phase transfer catalyst was reported to give high yields under very mild conditions [217]. A radical mechanism involving a one-electron transfer was proposed as the first step in this reductive elimination. 

To a solution of mew-1,2-dibromo-l,2-diphenylethane (2mmol) in benzene (5 ml) was added a 30% aqueous Na2CS, solution (6 mmol) and trioctylmethylamroonium chloride (0.08 mmol). The mixture was stirred 

The Corey-Winter olefin synthesis is based on the stereospecific loss of carbon dioxide or carbon disulfide from the carbenes formed by treatment with trivalent organophosphorus compounds of thionocarbonates or trithiocarbonates [218-220]. 

Vic-diols can thus be easily converted to alkenes through their reaction with /V,A -thiocarbonyldiinidazole. The reported synthesis of irans-cyclooctene is illustrative of the method [219]. It should be noted that continuous elimination of /ram-cyclooctene by a stream of argon was necessary to avoid isomerization to the cis isomer. The conversion of cis-cyclooctene to fram-cyclooctene through a trithiocarbonate is described in the same paper. 

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Cyclopropanation of alkenes carrying electron-withdrawing groups with dihalo-carbenes does not take place under the normal reaction conditions unless the dihalocar-bene is generated from trihalomethylphenylmercury compounds. By this procedure acrylonitrile was converted into l-cyano-2, 2-dichlorocyclopropane in 78% yield and other acrylic derivatives are transformed into dichloromethyl cyclopropane carboxylates (186) (equation 46). Treatment of electron-deficient olefins with dibromomethane in the presence of Ni(0) complex/Zinc/Lewis acid seems to be very effective for cyclopropanation. ... 

These dihalo-compounds presumably coordinates to iron(O) and then the excess iron(O) species removes the halogen atoms from the coordinated dihalo-alkenes. 

A terminal alkyne can be synthesized from a l,2-dihalo compound. We recall that this starting material can be synthesized in an addition reaction of a halogen to a monosubstituted (terminal) alkene. For example, adding chlorine to 1-hexene yields 1,2-dichlorohexane. The double dehydrohalogena-tion of 1,2-dichlorohexane yields 1-hexyne. 

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

Even the weak nucleophile CF3SO2O can participate in the second step The addition of CI2 or Br2 to alkenes in the presence of this ion resulted in the formation of some p-haloalkyl triflates. There is evidence that the mechanism with CI2 and H2O is different from that with HOCl. HOCl and HOBr can be added to triple bonds to give dihalo carbonyl compounds —CX2—CO—. 

Oxidative addition of alkenyl halides, triflates, and other esters to zerovalent palladium compounds has been long known as a viable route to palladium(ii)-alkenyl complexes. Stereospecific coupling reactions involving mono- and (E)- or (Z)-dihalo-alkenes with palladium-copper catalysis under modified Sonogashira conditions are quite versatile and useful. These proceed through oxidative addition of the alkenyl halide via palladium(ii) alkenyl complexes, followed by coupling with a nucleophile (usually an alkynylcopper reagent obtained in situ with co-catalytic copper(i) from terminal alkynes in the presence of, in this case, a base like piperidine instead of diethylamine). ... 

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