Regioselectivity elimination reactions

The reaction of isoprene with MeOH catalyzed by Pd(acac)2 and Ph3P is not regioselective, giving a mixture of isomers[37]. However, l-methoxy-2,6-dimethyl-2,7-octadiene (35), the head-to-tail dimer, was obtained in 80% yield, accompanied by the tail-to-tail dimer (15%) using 7r-allylpalladium chloride and BU3P. On heating, 35 was converted into 2.6-dimethyl-1,3,7-octatriene (36) by an elimination reaction[38]. 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

The nucleophilic attack of nitrogen bases leads to a variety of products as the result of addition or addition-elimination reactions The regioselectivity resembles that of attack by alcohols and alkoxides an intermediate carbanion is believed to be involved In the absence of protic reagents, the fluorocarbanion generated by the addition of sodium azide to polyfluonnated olefins can be captured by carbon dioxide or esters of fluonnated acids [J 2, 3] (equation I)... 

As one might expect, most of the reported cases of Camps quinoline syntheses involve reactions in which only one of the carbonyl groups is enolizable, thus eliminating the regioselectivity problem. 

For the regioselectivity similar rules as for the ester pyrolysis do apply. With simple, alkylsubstituted amine oxides a statistical mixture of regioisomeric olefins is obtained. On the other hand with cycloalkyl amine oxides the regioselectivity is determined by the ability to pass through a planar, five-membered transition state. This has been demonstrated for the elimination reaction of menthyl dimethylamine oxide 10 and neomenthyl dimethylamine oxide 11 ... 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

A-Alkylhydroxylamines react with substituted allyl acetates (e.g. 15, equation 11) in palladium catalyzed addition-elimination reactions giving the corresponding A-alkyl, A-allylhydroxylamines 16. The reaction proceeds with high regioselectivity but complete racemization. A similar reaction with 0-acyl hydroxamic acids has been carried out using allylic a-alkoxycarbonyloxyphosphonates. ... 

The thermal iyn-elimination reactions of X-oxides have assumed great importance in synthesis because of their mildness and regioselectivity. 

Moderate yields of 3-cinnamylchroman-4-one are achieved upon heating a mixture of 2-allyloxybenzaldehyde and ( )-P-nitrostyrene in the presence of benzoyl peroxide. This radical induced addition-elimination reaction proceeds in a regioselective manner (Equation 433) <2004JOC3961>. 

To circumvent elimination reactions, fluorides have been introduced by nucleophilic ring opening of epoxides.15 The regioselective opening of aldosyl epoxides proceeds ira r-diaxially.16 Epoxides can also be opened with chloride, bromide and iodide ions. The chemistry of epoxides is discussed in detail in Section 3.6. 

As mentioned before, alkyl radicals and acyl radicals have a nucleophilic character therefore, radical alkylation and acylation of aromatics shows the opposite reactivity and selectivity to polar alkylation and acylation with the Friedel-Crafts reaction. Thus, alkyl radicals and acyl radicals do not react with anisole, but may react with pyridine. Eq. 5.1 shows the reaction of an alkyl radical with y-picoline (1). The nucleophilic alkyl radical reacts at the 2-position of y-picoline (1), where electron density is lower than that of the 3-position. So, 2-alkyl-4-methylpyridine (2) is obtained with complete regioselectivity. When pyridine is used instead of y-picoline, a mixture of 2-alkylpyridine and 4-alkylpyridine is obtained. Generally, radical alkylation or radical acylation onto aromatics is not a radical chain reaction, since it is just a substitution reaction of a hydrogen atom of aromatics by an alkyl radical or an acyl radical through the addition-elimination reaction. Therefore, the intermediate adduct radical (a complex) must be rearomatized to form a product and a hydrogen atom (or H+ and e ). Thus, this type of reactions proceeds effectively under oxidative conditions [1-6]. 

As shown in Scheme 42, selenoxides are crucial intermediates in the selenoxide elimination reactions. These are syn-eliminations which proceed via an intramolecular mechanism to yield alkenes as reaction products. The regioselectivities of these eliminations are dependent on the nature of the substituent Y in the / -position as shown in Scheme 44.286 The mild reaction conditions for these elimination reactions make them highly useful in organic synthesis and theoretical studies on this reaction have been carried out as well.286... 

These elimination reactions are stereoselective and regioselective, so the most stable alkene is usually formed as the major product. 

In 1913, Kishner observed in one instance that under standard Wolff-Kishner reduction conditions, 2-hydroxy-2,6-dimethyloctan-3-one underwent eliminative reduction upon treatment with hydrazine hydrate and base at elevated temperatures to afford 2,6-dimethyloctan-2-ene (Scheme 7). This same reaction was later found to occur in the case of a-methoxy ketones and has since been referred to as the Kishner eliminative reduction. The reaction entails initial formation of the hydrazone and elimination of the a-substituent to afford the intermediate alkenyldiazene, which subsequently collapses to the desired alkene. Given the facile transformation of ketones into a-halo ketones, these conditions have been used to introduce alkenes regioselectively in the 2a-halocholestan-3-one series as shown in Scheme 8. Yields of 2-cholestene parallel the resistance of the a-halogen to undergo competitive elimination reactions. 

Further exploration of the regioselectivity of alkene formation in elimination reactions (ElcB anc E2). 

Nonaka and coworkers [177,178] found that a stereoselective one-electron reductive elimination reaction of cyclic sulfates of 1,2-diols proceeded to provide the tra/ .s -olefins and disulfate dianions of the parent diols. This reaction may be not only stereoselective but also stereospecific. An epoxide was also stereoselectively and regioselectively reduced to the monoalcohol [179,180]. 

The regiochemistiy of this reaction is reminiscent of the related selenoxide elimination since it mainly leads to the less-substituted alkene (Scheme 63). However, striking differences exist in the case of 1-octylcyclobutyl methyl selenide, where the reaction regioselectively produces octylidenecyclobutane via the ylide route (Scheme 64, a), whereas a 1 1 mixture of octylidenecyclobutane and 1-octylcyclo-butene is formed via the selenoxide route (Scheme 64, b). ... 

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