Elimination-vinylation reaction

Following the success of the elimination-allylation and propargylation sequences, Madsen and coworkers investigated the corresponding elimination-vinylation reaction [37]. The vinylation, however, cannot be carried out directly under Barbier conditions because vinyl bromide does not insert zinc under the conditions for the reductive elimination preformed divinyl zinc was used instead [51,52], Treatment of iodo furanoside 55 with zinc dust and divinyl zinc in THF gave diene 56 in a good yield (85%) and excellent diastereoseleetivity (10 1) (Scheme 3.18). 

As another possibility for introducing a double bond in the alkylation step, the intermediate aldehyde can be olefinated in the presence of carbon diiodide [57]. When iodo mannopyranoside 64 was treated with zinc, CH2I2, TMSCl, and PbCl2, diene 56a was obtained (Scheme 3.22) [37]. Since there are no stereocenters generated in this transformation, it is a better method for preparing diene 56a than the elimination-vinylation reaction (Section 1.2, Scheme 3.18). 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Lewis acid-free catalysts for acyclic diene metathesis obviate the formation of carbocations, which in turn completely eliminates competing reactions, mostly involving cationic oligomerisation via 1-alkene bonds. Thus, metathesis polycondensation occurs quantitatively to yield high molecular weight poly(l-alke-nylenejs with vinyl end groups and ethylene as a byproduct. 

Alkenyl(aryl)iodonium salts 4 are similarly efficacious for the alkenylation of nucleophiles. Such reactions proceed by various mechanisms, including addition-elimination, -elimination, vinylic S 1 and S 2, and ligandcoupling (LC) pathways (89RHA92, 95MI3, OOJOM494, 00RCR105, 02ACR12). 

Reduction of the vinylic gem-disulfide (XIV) in the presence of CO2 yields the carboxylated compound (XV) [130] described as an addition-elimination ECEC reaction. 

An alternative possibility in accord with the above labeling results is a nucleophihc addition elimination. The reaction of phenylhthium and vinyl-lithium with S-phenyldibenzothiophenium fluoroborate addresses itself to this problem. 20) This salt reacts with phenyllithium to 5deld 2-phenylthio-o-terphenyl exclusively in nearly quantitative pelds. If an addition elimina-... 

Reactions of poly(vinyl chloride) with aromatic amines, amino alcohols, or aliphatic amines in DMF solution result in both substitutions and in eliminations [168]. Reactions with aniline yield the following structure [168] ... 

This divergent oxidative reactivity of alcohols and carboxylic acids with olefins is illustrated in Equation 16.103. Bofli reactions generate products from p-hydrogen elimination of an alkoxyalkyl or acetoxyalkyl complex. However, the vinyl ether product generated by p-hy-drogen elimination undergoes reaction with a second equivalent of alcohol to form the acetal in a process catalyzed by the acidic medium or the action of palladium(II) as Levris add. 

A variety of mechanisms have been show to operate in alkenylation of nucleophiles including ligand coupling, addition-elimination and elimination-addition reactions for anionic nucleophiles and vinylic Sjv2, vinylic Sjvl and elimination-addition reactions for neutral nucleophiles. The mechanisms so far elucidated have been extensively reviewed by Ochiai and Okuyama and herein we provide a summary of these processes. 

The vinylic Sjy2 reaction of reagents 3 has been proposed to proceed through reversible ligand exchange followed by an in-plane bimolecular substitution process (Scheme 24). These reactions are characterised by exclusive inversion of configuration and can be complicated by elimination side reactions to form alkynes. The ratio of Sj 2 substitution versus elimination depends on the nature of the... 

Intramolecular Friedel rafts vinylation reactions of aryl-tethered ( )-alkenyl-(aryl)iodanes 49 to afford 1,2-dihydronaphthalenes and 2/f-chromenes 50 have also been reported to occur via an addition-elimination process (Scheme 30) or alternatively an intramolecular Sat2 displacement at the vinylic centre. The intramolecular displacement was deemed energetically unstable on steric grounds but could not be ruled out. In these cases the presence of a vinylic cation was ruled out as the ionising power of the solvent has little effect on the cyclisation and the corresponding (Z)-isomers do not undergo ring closure. 

As for various catalytic activity of alkali metal hydroxides, it should be noted that this effect is not unique for this reaction and is observed in almost all base-catalytic processes involving alkalis, for example, vinylation reaction [5,6,109,152,166], nucleophilic substitution and elimination [153], Favorsky reaction [167], synthesis of divinyl sulfide from acetylene and alkali metal sulfides [168], and cyclization of cyanoacetylenic alcohols [169]. For instance, in vinylation of 2-ethoxyethanol with acetylene in the presence of different hydroxides, the following relative reaction rates are observed [109,152] ... 

Polymers can be classified as addition polymers and condensation polymers. Addition polymers are formed by iiitermolecular reactions of the monomeric units without the elimination of atoms or groups. An example is vinyl chloride, which can be made to combine with itself to yield polyvinyl chloride ... 

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