Vinyl halides reactivity

The order of halide reactivity is I > Br > Cl > F and alkyl halides are more reac tive than aryl and vinyl halides Indeed aryl and vinyl chlorides do not form Grignard reagents m diethyl ether When more vigorous reaction conditions are required tetrahy drofuran (THF) is used as the solvent... 

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... 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

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. 

Besides differences in reactant stability as well as differences in the stability of the intermediate ions, there may also be other reasons, such as differences in solvation and differences in backside nucleophilic assistance, for the large discrepancy in reactivity between alkyl halides and vinyl halides. 

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. 

In the course of our successful synthesis, we identified several limitations of our new method and associated strategy (1) the harsh conditions of the bicyclization reaction do not tolerate base-sensitive functionality such as vinyl halides (2) post-cyclization manipulations such as iododesilylation reactions are complicated by the sensitive/ reactive functionality of the products (a,p-unsaturated aldehyde, indoline, etc.) and (3) the incorporation of the required functionality into the Zincke aldehyde requires the synthesis of a complex tryptamine derivative, resulting in a lengthy, non-convergent route. In order to develop a concise route to strychnine, we would have to address each of these issues, and a straightforward solution to obviate all of these is described below. 

Oxidative addition occurs readily with allylic halides. Donor ligands (tertiary phosphines, bipyridyl, halide ions) and anionic complexes are required for activation of aromatic and vinyl halides (4, 70). Certain aliphatic halides are also reactive. The intermediate species R—Ni—X... 

Furthermore, arylthiophenes have been prepared using the Stille coupling of hypervalent iodonium salts [96] or organolead compounds [97, 98] as electrophiles in place of aryl or vinyl halides and triflates. Hypervalent iodonium salts are sufficiently reactive to undergo coupling at room temperature. 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

Finally, the hybridization of the carbon atom also has a marked effect on its willingness to attach to the transition metal. Allyl or benzyl halides undergo oxidative addition faster than aromatic or vinyl halides. The least reactive are alkyl halides which require the use of nickel(O)9 complexes or highly active catalyst systems.10 If we start from an optically active substrate, then the oxidative addition usually proceeds in a stereoselective manner. 

Vinylic halides are less reactive than simple alkenes, but generally afford clean addition products. The addition of HBr to vinyl chloride125 or vinyl bromide61,122,126,127 in the presence of iron(III) chloride or bromide affords primarily the geminal dihalides (equation 78). 

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