Vinylic halides unreactivity

A key step in the reaction mechanism appears to be nucleophilic attack on the alkyl halide by the negatively charged copper atom but the details of the mechanism are not well understood Indeed there is probably more than one mechanism by which cuprates react with organic halogen compounds Vinyl halides and aryl halides are known to be very unreactive toward nucleophilic attack yet react with lithium dialkylcuprates... 

Although nor shown in the preceding reactivity order, vinylic halides (R2C=CRX) and aryl halides are unreactive toward Sn2 reaction. This lack of reactivity is probably due to steric factors, because the incoming nucleophile... 

Vinylic halides are unreactive (pp. 428, 433), but they can be hydrolyzed to ketones at room temperature with mercuric trifluoroacetate, or with mercuric acetate... 

The second reason for the lack of early investigations into vinyl cations was the seemingly extreme unreactivity of vinyl halides in solvolytic processes. The unreactivity of vinyl chloride, for instance, even in the presence of silver nitrate, has been almost a legend in organic chemistry (102). This lack of reactivity of simple alkylvinyl halides has been attributed to the low stability of simple vinyl cations or to the very strong carbon-halogen bond, or both. 

This seeming unreactivity of vinyl halides in solvolytic processes and the lack of availability of more reactive precursors, such as sulfonate esters, until recently has discouraged early attempts at mechanistic investigations of vinyl cations generated by solvolyses. However, vinyl cations have been generated via vinyl diazonium ions derived from various precursors. 

Vinylic halides and phenyl halides are generally unreactive in SnI or SnI reactions. 

The presence of halogen atoms appears to exert little, if any, effect on catalyst activity, but it can influence the course of the metathesis reaction. Vinylic halides are unreactive, as exemplified by the ring-opening polymerization of l-chloro-l,5-cyclooctadiene, which afforded a perfectly alternating copolymer of butadiene and chloroprene (7/2) via polymerization exclusively through the unsubstituted double bond. 

Vinylic halides are virtually unreactive and a high selectivity is to be found in the preferential cleavage of aliphatic carbon-halogen bonds of haloalkanoic amides and esters, and of nitro- and cyanoaryl derivatives. Activated haloarenes, e.g. 1-chloro-2,4-dinitrobenzene, however, give a complex mixture of products [7]. 

By contrast, vinyl halides such as chloroethene, CHj CHCl, and halogenobenzenes are very unreactive towards nucleophiles. This stems from the fact that the halogen atom is now bonded to an sp hybridised carbon, with the result that the electron pair of the C—Cl bond is drawn closer to carbon than in the bond to an sp hybridised carbon. The C—Q is found to be stronger, and thus less easily broken, than in, for example, CH3CH2CI, and the C—Q dipole is smaller there is thus less tendency to ionisation (8 1) and a less positive carbon for OH to attack Sf l) the n electrons of the double bond also inhibit the close approach of an attacking nucleophile. The double bond would not help to stabilise either the 8 y2 transition state or the carbocation involved in the 8 1 pathway. Very much the same considerations apply to halogenobenzenes, with their sp hybridised carbons and the tt orbital system of the benzene nucleus their reactions, which though often bimolecular are not in fact simply 8 2 in nature, are discussed further below (p. 170). 

Allyl chloride and bromide afford high yields of HI addition products (equation 131).176,177 Even vinylic halides add HI readily, though 1-bromo-l-propene gives a mixture of products (equations 132-135).125,131,177 However, perfluoroalkenes have proven unreactive towards HI addition. 

Among the halides that react through this process are unactivated aromatic and heteroaromatic halides, vinyl halides, activated alkyl halides [nitroalkyl, nitroallyl, nitro-benzyl and other benzylic halides substituted with electron-withdrawing groups (EWG) as well as the heterocyclic analogues of these benzylic systems] and non-activated alkyl halides that have proved to be unreactive or poorly reactive towards polar mechanisms (bicycloalkyl, neopentyl and cycloalkyl halides and perfluoroalkyl iodides). 

The authors point out that, unlike vinyl halides, the vinyl chloronium ion 23a could hardly show double bond character in the C-Cl bond because of unfavourable charge repulsion in 23b. The unreactivity of vinyl halides may therefore primarily be a consequence of the strength of the a bond to the sp2, carbon. However if opening of 23a is assumed to be due to nucleophilic attack of the solvent (or any other nucleophile) to either C5 or C2, the greater amount of shifted products obtained from alkynes than from alkenes is a direct consequence of the relative facility of nucleophilic attack at the sp3 carbon and sp% carbon (Rappoport, 1969 Modena, 1971). 

The introduction of a halogen atom into the nucleus of an unsaturated heterocycle serves at once as a valuable synthetic route to heterocyclic derivatives and as a revealing probe of substitution processes at unsaturated carbon.1 From a synthetic standpoint, aromatic and heterocyclic halides have become more attractive starting materials in recent years. Traditionally considered to be rather unreactive, these vinylic halides had been found suitable only in certain reactions... 

For R = hydrogen, methyl or ethyl, the e.s.r. spectrum of the corresponding vinyl radical was observed, showing that no reaction had occurred. This lack of reaction is not surprising as the radicals cannot adopt a suitable configuration for reaction. It also shows that the vinyl radicals are sufficiently isolated from unreacted vinyl halide molecules to prevent the occurrence of intermolecular hydrogen abstraction. However, when R = propyl, a six-membered cyclic transition state can be formed and in fact the spectrum of the vinyl radical is completely replaced by that of the alkyl radical formed by abstraction from the terminal methyl group (reaction 32). 

For the most part, vinyl halides are unreactive however, a few have been converted to vinyl-type cyanides under conditions employed for aromatic halogen compounds. Thus, sym-diiodoethylene has been converted by cuprous cyanide with an amine promoter to fumaronitrile (74%). The halogen atom in certain triarylvinyl bromides has also been replaced by the cyano group under these conditions. ... 

Vinyl halides and aryl halides are unreactive. The reaction does not occur on benzene rings substituted by meta deactivating groups or NHg groups (18.1 OB). 

With vinyl halides they react in the presence of cobaltous salts, affording low yields of alkylation products . Although unreactive towards saturated alkyl halides, they are... 

The parallel between aryl and vinyl halides goes further both are unreactive toward nucleophilic substitution and, as we shall see, for basically the same reason. Moreover, this low reactivity is caused—partly, at least—by the same structural feature that is responsible for their anomalous influence on electrophilic attack partial double-bond character of the carbpn-halogen bond. 

This aUows the cross-coupling of an aryUialide (2) with an aryltrialkyl stannane in presence of a boronic ester. The latter remains unreactive because it would need to be hydrolyzed to a boronate to be suited for Pd-mediated couplings. Progression in a couphng cycle would further require OH , CO , or F , not present under the used StiUe couphng conditions [178,179]. The Stille conditions proved very useful for affording many aryl-aryl [1, 4, 180, 181, 182], vinyl-aryl [9, 161, 183, 184] and also aUcyl-aryl [185] compounds bearing hydrolytically labile moieties. The commonly used reactivities of aryl or vinyl halide components are like those in the... 

As a reactive monomer forms a stable free radical, the radical reactivity will be the reverse order of the series given above. This means that monomers containing conjugated systems (styrene, butadiene, acrylates, acrylonitriles, etc.) will be highly reactive monomers but will form stable and so relatively unreactive radicals. Conversely, nonconjugated monomers (ethylene, vinyl halides, vinyl acetate, etc.) are relatively unreactive toward free radicals but will form unstable and highly reactive adducts. 

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