Substitution, vinyl elimination-addition

Halogenovinyl sulphoxides 551 react with nucleophiles to give -substituted vinyl sulphoxides 552. The first step in the reaction is a Michael addition, followed by an elimination of a halide anion605,627 (equation 351). 

An attempt to study resolved (( )-18) as a probe for the detailed mechanism of the Adn—E vinylic substitution reaction has been complicated by intervention of a competing reaction route this is believed to involve a competing (ElcB elimination-addition, for which antiperiplanar orientation of H and Cl is not a requirement.7 a-Deuterated (ca 50%) E- and Z-substitution products (which do not themselves exchange deuterium) are obtained on reaction with MeS in 9 1 CD3CN-D2O but no incorporation of deuterium in unreacted ((/r)-18) occurs and neither does isomerism to ((Z)-18) precede elimination. 

In 1994, it was found that several reactions thought to proceed via the vinylic SflArl mechanism were contaminated by a nonradical, a, /t-ehrninalion/addition pathway (equation 40)178. However, this elimination/addition pathway becomes inaccessible when substrates without ft (or ft1) hydrogens are utilized. Thus, the reaction of pinacolone enolate (f-Bu(CO)CH2) with l-bromo-l,2,2-triphenylethylene was touted to be the first unequivocal example of vinylic substitution exclusively by the S l pathway. 

When a proton is available for expulsion in the vicinity of the leaving group, both may be eliminated. Consecutive addition of the nucleophile Nu and hydrogen to the elimination product finally yields a vinylic substitution product. These elimination-addition routes are the... 

Competition between the j8,y- and the a,/ -elimination-addition modes is possible in the systems studied by Bottini and coworkers. N-(2-Bromoallyl)alkylamine (199) with NaNH2 in liquid NH3 gives mainly l-alkyl-2-methyleneaziridine (200) together with a lower amount of N-alkylpropargylamine (201) (Pollard and Parcell, 1951 Bottini and Roberts, 1957). The driving force for this intramolecular substitution of the unactivated vinyl bromide (199) is probably the presence of the... 

In the addition-elimination routes, either via a carbanionic intermediate (I) or via a neutral adduct (II), the anionic nucleophile Nu or the neutral nucleophile NuH attacks the /3-carbon with the expulsion of X. In the a,/8-route (IV), the /9,/3-route (VI) and the /8, y- elimination-addition routes (VII), HX is eliminated in the initial step, and the nucleophile and hydrogen are then added to the intermediates. Substitution occurs also by heterolytic C—X bond cleavage in an SN1 process (X). Initial prototropy followed by substitution can also give vinylic substitution products (XII, XIV), as well as two consecutive Sn2 reactions (XV) where the leaving group leaves from an allylic position. 

Recently, evidence for the vinylic nucleophilic substitutions suggests the occurrence of an ionic elimination-addition along with the originally proposed S l route352. Thus, the reaction of /Lbromostyrene with pinacolone enolate ions and FeCl2 as catalyst in DMSO gave a mixture of products 362-366 in yields that depend on the reaction time, 10 min or (3 h). 

The palladium-catalysed addition of aryl, vinyl, or substituted vinyl groups to organic halides or triflates, the Heck reaction, is one of the most synthetically useful palladium-catalysed reactions. The method is very efficient, and carries out a transformation that is difficult by more traditional techniques. The mechanism involves the oxidative addition of the halide, insertion of the olefin, and elimination of the product by a p-hydride elimination process. A base then regenerates the palladi-um(0) catalyst. The whole process is a catalytic cycle. 

Vinylic halogen atoms in perhalogenated alkenes are easily substituted by an addition-elimination mechanism with secondary phosphanes. 

Table IV compares the reactivity ratios of a soft (PhS-) to a hard (MeO-) nucleophile in vinylic substitution. PhS is always more reactive, and ratios lower than unity, as for 4, X = Br (4), are certainly due to elimination-addition with MeO . The ratios change by >2000-fold and are sensitive to the geometry of the substrate. An important feature is that for (3-halo-p-nitrostyrenes the ratio decreases strongly with the increased hardness of the (3-halogen (38). The lowest ratios are for the (3-fluoro derivative, whereas the differences between the chloro and bromo compounds are not so large. This behavior is similar to that in SNAr reactions. This behavior can be rationalized by symbiotic effects, which favor the soft-soft PhS--Br interaction and the hard-hard MeO-F interaction. A reactivity-selectivity relationship for vinyl bromides of different electrophilicities does not exist.

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