Olefins dihalides

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

With a 16,17-dihalide, the initial product would be expected to be the A -olefin, which in the presence of alkali isomerizes to the conjugated ester. 

Monoalkylthallium(III) compounds are unstable (73, 79), and very few examples of this class have been isolated. A number of alkylthallium diacetates have been obtained either from oxythallation of olefins with thallium-(III) acetate (see below) or from exchange reactions such as that shown in Eq. (11) (74, 75). Only four alkylthallium dihalides have been isolated so far, namely a neopentylthallium dihalide (60) [Eq. (12)] and the isomeric 2-, 3-, and 4-pyridiomethylthallium dichlorides (20) [Eq. (13)]. Monoaryl-and monovinylthallium(III) derivatives are considerably more stable than... 

Dihalides are reduced by Cr(II) to the corresponding olefin ". Allylic and benzylic halides and polyhalides are reduced more readily than simple alkyl halides, but even the latter are readily reduced by an ethylenediamine complex ofCr(ri) . ... 

There is on the other hand a great deal of evidence showing that the electrochemical reduction of 1,2-dihalides to olefins can occur via a concerted pathway, i.e., via a transition state (39) in which both carbon-halogen bonds are partially broken and the carbon-carbon double bond is partially formed. An important, indeed critical, point of evidence supporting the conclusion that reduction is concerted lies in the remarkable ease with which vicinal dihalides are reduced. For example, the half-wave potentials of ethyl bromide and 1,2-dibromoethane are -2.08 V and -1.52 V (vs. s.c.e.), respectively 15 >46) those of ethyl iodide and /J-chloroethyl iodide are -1.6 V and -0.9 V, respectively 47). These very large differences must reflect the lower energy of delocalized transition state 39 relative to the transition state for reduction of an alkyl monohalide. 

Low intensity ultrasound has also been applied to the Simmons-Smith cyclopropanation of olefins with zinc-diiodomethane (237). This reaction normally will not occur without activation of mossy Zn with I2 or Li, and was difficult to scale-up due to delayed initiation. Yields upon sonication are nearly quantitative, activation of the Zn is unnecessary, and no delayed exotherms are observed. In reactions with another class of organic dihalides, ultrasonic irradiation of Zn with a,a -dibromo-o-xylene has proved a facile way to generate an o-xylylene-like species [Eq. (49)],... 

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). 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

When the metallic additive to the intermediate 374 was zinc dihalide (or another Lewis acid, such as aluminum trichloride, iron trichloride or boron trifluoride), a conjugate addition to electrophilic olefins affords 381 . In the case of the lithium-zinc transmetallation, a palladium-catalyzed Negishi cross-coupling reaction with aryl bromides or iodides allowed the preparation of arylated componnds 384 ° in 26-77% yield. In addition, a Sn2 allylation of the mentioned zinc intermediates with reagents of type R CH=CHCH(R )X (X = chlorine, bromine) gave the corresponding compounds 385 in 52-68% yield. ... 

The preparation of three-, five- and six-membered rings in modest yield by the elec-trochemically induced coupling of activated olefins with dihalides has been reported (equation 35)98. Use of an aluminum anode was critical for the formation of cyclized product. Application of this technique to the preparation of four-membered rings was unsuccessful. 

A widely exploited procedure for bringing about carbenoid reactions of organic mono- and fifem-dihalides is by use of lithium alkyls. Examples are given in equations (11) and (12). Dimeric olefin formation, stereospecific cyclopropane formation from olefins, and insertion into carbon-hydrogen bonds have all been observed in suitable cases, together with further reactions of these products with excess of the lithium alkyl. 

The solution of the gem-dihalide and the olefin in DMF CH2C12 (v/v = 1 9) containing Bu4NX (X = I, Br) was electrolysed at 40°C with a zinc anode and a carbon fibre or a nickel foam cathode. The method compares favourably with the usual route involving CH2I2 and Cu/Zn. 

The high yield reduction of 1,2-dihalides to produce olefins has been employed to advantage to prepare reactive olefins. Electron transfer in electrochemistry is proportional to the diffusion coefficient, which is related in a much less sensitive way to temperature changes than is chemical reactivity. Thus it may become possible to synthesize and study electro-chemically species whose chemical reactivity is high by working at low temperatures. Electroreduction of 1,2-dibromobenzocyclobutene (144) in acetonitrile or butyroni-trile/TEAP or chemical reduction using the biphenyl radical anion resulted in the formation of benzocyclobutadiene (145)128. Efforts to observe the electrochemically generated anion radical or dianion of benzocyclobutadiene indicated that dimerization to 146 was faster than further reduction (equation 84). 

Aryl tellurium trichlorides and tribromides react with terminal and internal linear olefins and with cycloalkenes to produce aryl 2-chloroalkyl tellurium dihalides. The halide ion and the aryldichlorotelluro group approach the olefin from opposite sides of the plane defined by the atoms bonded to the olefinic carbon atoms. These arm-additions yield threo-addition products from (Z)-olefins, and erythro-products from (E)-olefins4 6. 

Phenyl tellurium trihalides react with linear olefins and cycloalkenes in methanol or other alcohols. In contrast to reactions in inert organic solvents, in alcoholic media the intermediate adduct between the olefin and the positive phenyldihalotelluro group is attacked by the alcohol and not by the halide ion. Therefore, the product is a 2-alkoxyalkyl phenyl tellurium dihalide. The reactions are regiospecific (tellurium bonding to the less hindered carbon atom) and highly stereospecific (anti-addition). 

Diorgano tellurium compounds that contain a carbon-carbon double or triple bond in the molecule react with equimolar amounts of bromine or iodine to form the diorgano tellurium dihalides. Under these conditions, the halogens do not attack the carbon-carbon multiple bonds. The following diorgano tellurium dihalides with olefinic or acetylenic organic groups were obtained in this manner ... 

The dihalides dissolved in dichloroinethane had not reacted with olefins after 48 h at 20" in the dark. Irradiation of the reaction mixtures yielded small amounts of allylic halides1. 

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