Elimination reactions solvent-promoted

It is possible to take advantage of the differing characteristics of the periphery and the interior to promote chemical reactions. For example, a dendrimer having a non-polar aliphatic periphery with highly polar inner branches can be used to catalyse unimolecular elimination reactions in tertiary alkyl halides in a non-polar aliphatic solvent. This works because the alkyl halide has some polarity, so become relatively concentrated within the polar branches of the dendrimer. This polar medium favours the formation of polar transition states and intermediates, and allows some free alkene to be formed. This, being nonpolar, is expelled from the polar region, and moves out of the dendrimer and into the non-polar solvent. This is a highly efficient process, and the elimination reaction can be driven to completion with only 0.01 % by mass of a dendrimer in the reaction mixture in the presence of an auxiliary base such as potassium carbonate. 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

Effect of solvent on El vs. E2 vs. ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB mechanisms will be aided by increasing polarity of solvent and by increasing ionic strength. With certain substrates, polar aprotic solvents promote elimination with weak bases (the E2C reaction). 

The opposite of an addition to a double bond is a 1,2-elimination reaction. In solution, where the reaction is promoted by solvent or by base, the most common eliminations (and those to which we shall limit our discussion) are those that involve loss of HX, although loss of X2 from 1,2-dihalides and similar reactions are also well known. The mechanisms of eliminations of HX are of three main types (1) The Ex (elimination, first-order), shown in Equation 7.22, which is the reverse of the AdE2 reaction and which has the same first, and rate-determining,... 

It is noteworthy that the use of DMSO in this manner contrasts with its established usage in elimination reactions. DMSO has normally been used because of its ability to promote eliminations by weakly basic anions (Parker, 1971), its effectiveness in enhancing the basicity of alkoxides (Cram et al., 1961), and also because it allows variation of base without the usual variation of solvent (Dolman and Stewart, 1967). Full details of such studies and their importance in the development of an understanding of elimination mechanisms are given in a recent text (Saunders and Cockerill, 1973).8... 

Strong bases favor elimination rather than substitution and the 2 mechanism rather than 1, particularly if they are highly concentrated. Reaction with low concentrations of base or no base in ionizing solvents promotes 5n1 rather than 1. Strong nucleophiles or weak bases favor substitution, unless the solvent is polar aprotic, in which case an 2 reaction is preferred. 

There are some exceptions to this general observation. For example, //-elimination reactions have been found to be promoted by weakly ionising solvents, but only when the eliminated substituent is neutral. 

Several weakly basic anion-exchange resin catalysts were screened in laboratory-scale and the results showed that a very good product distribution with respect to aldol can be obtained. Extensive kinetic studies demonstrated that weakly basic anion-exchange resin catalysts promoted both the aldolization and elimination processes, but the product distribution can be steered by selection of the solvent and the ratio of the reactants. A kinetic model based on the aldolization and elimination reaction mechanisms was developed and sin5>lified. The model predictions were in good agreement with the... 

The c -alkene is formed alone in a stereospecific concerted reaction, analogous to an ctnti-E2 elimination, which is encouraged by electron-withdrawing substituents (Y) and a poorer ionising medium. However, electron-releasing substituents and a more ionising solvent promote benzylic carbonium ion character and a mixture of alkenes, in which the thermodynamically more stable trans product predominantes, is formed, e.g. 

In low-polarity media, specific interaction with protic species (water) dramatically affects the reactivity (nucleophilicity or basicity) of anions with high charge density (OH. F. oxanions. carbanions, etc.). Basicity of OH in the Hofinann elimination reaction of (hexyl)4 N 0H /7H20 (Eq. 9), carried out in a chlorobenzene-water two-phase system, increases 50.000 times by reducing the hydration number n of the anion from 11 to 3. The enhancement is extrapolated to be more than nine powers of 10 for the hypothetical anhydrous hydroxide. This indicates that the largely dehydrated hydroxide, extracted in a low-polarity solvent (chlorobenzene) from concentrated alkaline soluiions. is an extremely powerful base. Results account for the dramatic effect produced by an increase of base on the rate of reactions promoted by alkali hydroxides under LL-PTC conditions, such as carbanion formation and alkylation, alkene isomerization, H/D exchanges in carbon acids, and acid-base equilibria ... 

Solvent effects are important for substitution and elimination reaetions. Solvents can be categorized as polar or nonpolar and also as protie or aprotic. Protic solvents have an acidic proton. Water is assumed to favor ionization and promote Si l/El reaetions over S]y2/E2 reactions. Other protic solvents tend to favor bimoleeular reaetions, but slow ionization is possible with long reaction times. Aprotic solvents favor bimolecular reactions, particularly Sjy2 reactions. 

Strong bases promote S-elimination reactions. Strong bases that serve effectively in 8-eliminations of haloalkanes are OH , OR, NH2", and acetylide anions. Following are three examples of base-promoted S-elimination reactions. In the first example, the base is shown as a reactant. In the second and third examples, the base is a reactant but is shown over the reaction arrow. Note that the solvent used is commonly the conjugate acid of the base used in the elimination. 

Specific base-solvent combinations have been shown to control the orientation of 1,2-eliminations from AT-chloro-2-ethylpyrrolidine to give substantial proportions of the previously unreported 5-ethyl-l-pyrroline (39). The results clearly establish the utility of base-promoted elimination reactions for the preparation of previously unavailable cyclic imines. 

The addition of two o-methyl groups to ring-substituted cumyl derivatives to give derivatives (28) causes only a modest change (< 5-fold) in feobs for reaction in 50 vol.% CF3CH2OH-H2O 2,6-Me2/2,6-H2 rate ratios for X = MeO, Me, and H are 0.63, 1.7, and 4.4, respectively. However, for the sterically hindered cation (29, X = Me) there is a 70-fold decrease in and a 60-fold increase in fee the product rate-constant ratios for partitioning of (29) between substitution and elimination reactions promoted by solvent (feg and fee) and by azide ion and fes, respectively) have also been discussed. 

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