Nucleophilic substitution allylic elimination

The method is quite useful for particularly active alkyl halides such as allylic, benzylic, and propargylic halides, and for a-halo ethers and esters, but is not very serviceable for ordinary primary and secondary halides. Tertiary halides do not give the reaction at all since, with respect to the halide, this is nucleophilic substitution and elimination predominates. The reaction can also be applied to activated aryl halides (such as 2,4-dinitrochlorobenzene see Chapter 13), to epoxides, " and to activated alkenes such as acrylonitrile. The latter is a Michael type reaction (p. 976) with respect to the alkene. 

Alkyl halides can be hydrolyzed to alcohols. Hydroxide ion is usually required, except that especially active substrates such as allylic or benzylic types can be hydrolyzed by water. Ordinary halides can also be hydrolyzed by water, if the solvent is HMPA or A-methyl-2-pyrrolidinone." In contrast to most nucleophilic substitutions at saturated carbons, this reaction can be performed on tertiary substrates without significant interference from elimination side reactions. Tertiary alkyl a-halocarbonyl compounds can be converted to the corresponding alcohol with silver oxide in aqueous acetonitrile." The reaction is not frequently used for synthetic purposes, because alkyl halides are usually obtained from alcohols. 

These TT-allyl complexes are moderately electrophilic 101 in character and react with a variety of nucleophiles, usually at the less-substituted allylic terminus. After nucleophilic addition occurs, the resulting organopalladium intermediate usually breaks down by elimination of Pd(0) and H+. The overall transformation is an allylic substitution. 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

The neutral species formed by abstraction of protons located laterally at C3 and C5 in 1,2-type and at C2 of 1,3-type N-oxyazolium salts discussed in Section 1.5.2.1 are prone to react with nucleophiles in an allylic type substitution with elimination of -OR. The reaction is facilitated by the easy cleavage of the weak N-O bond (Scheme 19). The global reaction is displacement of specific lateral protons with a nucleophile. The entire sequence can be run in one pot. 

Mechanism 6-1 Allylic Bromination 228 Summary Methods for Preparing Alkyl Halides 229 6-7 Reactions of Alkyl Halides Substitution and Elimination 231 6-8 Second-Order Nucleophilic Substitution The Sn2 Reaction 232 Key Mechanism 6-2 The S j2 Reaction 233 6-9 Generality of the SN2 Reaction 234... 

Nucleophilic substitution with the phenolate anion derived from EC-20 and K2C03 induced 30% substitution, along with elimination of hydrogen bromide.331 Phosphonium groups can be introduced at the polystyrene and poly(MA) terminal via reaction with EC-21,355 and methyl groups with Me3Al (EC-22).356 An allyl terminal is obtained also via an ionic pathway, where the polystyryl carbocation generated... 

In most cases, treatment of allylic halides containing one ASG with a nucleophile does not result in formation of electrophilic cyclopropanes (MIRC product) instead, other reaction pathways are followed, e.g. addition, substitution, rearrangement and elimination reactions.However, with certain alkenes or nucleophiles or under the appropriate conditions a conjugate addition-nucleophilic substitution pathway is favored, resulting in cyclopropanes substituted with one ASG. Representative examples are compiled in Tables 20 and 21 where organometallic compounds or active methylene compounds are used as the nucleophilic species in combination with allyl bromides containing an ester or a sulfone as ASG. 

The formation of rearranged products during nucleophilic substitution in a-bromo-ketones probably involves enolisation, followed by allylic (5n2 ) substitution. The ready conversion of4, 5i8-epoxy-3-ketones (45) into 2a-acetoxy-or 2a-hydroxy-4-en-3-ones is similarly rationalised.When the nucleophile is dimethyl sulphoxide, the resulting 2-oxysulphonium ion (46) breaks down with elimination of dimethyl sulphide to give the 4-ene-2,3-dione (47), or its A -enolic equivalent (48). 

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