Elimination reactions salt effects

In aqueous solution it is not always easy to establish with certainty the existence of catalysis by species other than hydrogen or hydroxyl ions, and some of the early conclusions in this field were based on insufficient evidence. The safest method is to use as catalysts a series of buffer solutions of equal ratios but varying concentrations, using the principle of constant ionic strength to eliminate secondary salt effects, as described in the last sub-section. If the observed reaction velocity increases with increasing buffer concentration in such a series, this is proof that one or... 

On the other hand, the provision of vast numbers of minute nuclei assists the phosphate coating reaction to start at a multitude of centres, resulting in a finely crystalline coating. This effect can be obtained chemically by a predip in a solution of sodium phosphate containing minutely dispersed traces of titanium or zirconium salts or in weak solution of oxalic acid. This type of pre-dip entirely eliminates any coarsening effect due to previous treatment in strong alkalis or acids. 

Only low yields of the azide ion adduct are obtained from the reaction of simple tertiary derivatives in the presence of azide ion 2145 46 and it is not possible to rigorously determine the kinetic order of the reaction of azide ion, owing to uncertainties in the magnitude of specific salt effects on the rate constants for the solvolysis and elimination reactions. Therefore, these experiments do not distinguish between stepwise and concerted mechanisms for substitution reactions at tertiary carbon. 

An unusual one-pot intramolecular sulfoxide alkylation-elimination reaction was found by Gibson et al. <2001SL712>. These authors found that treatment of 459 with potassium bis-trimethylsilylamide resulted in a ring closure to 460 in acceptable yield. Furthermore, Batori and Messmer found an effective method for preparation of [l,2,3]triazolo[l,5- ]pyrimidinium salts <1994JHC1041> oxidative cyclization of hydrazones 461 by 2,4,4,6-tetrabromo-2,5-cyclohexadienone gave rise to the quaternary salts 462. Under certain reaction conditions, the formation of 6-bromo-salts 462 (R6 = Br) was also experienced. As neither the starting compound nor the quaternary triazolopyridinium salt underwent bromination in this position, the authors assumed that this bromination process occurred on one of the intermediates in the course of the above-mentioned cyclization reaction. 

Salt elimination reactions proved to be very effective for the synthesis of closo-heteroborates with the heavy elements of this group. Todd and co-workers prepared for the first time the lead, tin and germanium cluster dianions [EBnHn]2 as shown in Scheme 3.3-5 [17]. 

In contrast to the reactivity of the nitroethylenes, acrylonitrile generally reacts with indoles to form the l-(2-cyanoethyl) derivatives (B-70MI30500,79MI30501), whereas Michael addition at the 3-position requires the catalytic effect of copper(II) salts. The addition-elimination reaction of pyrroles and indoles with l,l-dicyano-2-ethoxyethylene proceeds in low yield (<30%) to give the dicyanovinyl derivatives, which can be converted by standard procedures into the formyl compounds (81H(16)1499). Tetracyanoethylene forms charge transfer complexes with indoles, which collapse to the Michael adduct anions and subsequently eliminate a cyanide ion with the formation of the tricyanovinylindole (B-70MI30500). 

The search for electrophilic catalysis could in principle be aided by use of tertiary amines as nucleophiles, in which case an intramolecular or solvent assisted proton shift, formulated in [13] and [14], is eliminated. Thus Ayediran, Bamkole and Hirst (1974) studied salt effects on the reactions of 4-fluoro- and 4-chloronitro-benzene with trimethylamine in DMSO, which proceed according to equations (29) and (30). It had been noted (Suhr, 1967) that the... 

Hofmann elimination (Section 24.7) a method for effecting the elimination reaction of an amine to yield an alkene. The amine is first treated with excess iodomethane, and the resultant quaternary ammonium salt is heated with silver oxide. 

Detailed studies of the mechanism of these reactions have been performed by Mattay and by Kochi . The former has shown that the endo/exo regiochemistry of the ring closure reaction can be controlled either by variation of the silyl group or by addition of polar molecules such as alcohols (probably the source of hydrogen in equations 37a-c). Based on solvent and salt effects, Kochi has proposed that the oxidation of enols to ketones in the presence of activated chloranil proceeds via photoactivation of chloranil which reacts with the silyl enolate through two competing pathways, namely oxidative elimination to the ketone and oxidative addition to the adduct 51 (equation 38). Non-polar solvents such as dichloromethane favour the oxidative eliminations, while polar solvents such as acetonitrile direct the reaction towards the oxidative addition. More strikingly. 

Wirth and co-workers used various chiral nitrogen-containing diselenides 20 - 24, which worked effectively as procatalysts for diethylzinc addition to aldehydes (see Sect. 3.1) [13] and for the catalytic oxyselenenylation-elimination reaction of frans- -methylstyrene (Scheme 26) [30]. Under the reaction conditions reported by Iwaoka and Tomoda [19], the diselenide 20 yields the product with highest enantioselectivity (up to 56% ee). Potassium peroxodisulfate seems to be superior to sodium and ammonium analogues. Effect of metal salts on stereoselectivity in the catalytic reaction using the diselenide 20 was investigated since it is known that metal ions can accelerate the decomposition of peroxo-... 

Saunders, W. H., Jr., Ashe, T. A. Mechanisms of elimination reactions. XII. Hydrogen isotope effects and the nature of the transition state in eliminations from alicyclic quaternary ammonium salts. J. Am. Chem. Soc. 1969, 91,4473-4478. 

Before leaving our discussion of elimination reactions, the distinction which has been drawn between E2 reactions of alkyl halides and onium salts should be emphasized. When onium salts arc decomposed it has been assumed that the most important step in the process is the formation of a reactive complex between the cation and a base. Once this transition state is attained, olefin formation occurs directly so that the process is effectively irreversible. Consequently, if there is more than one point within a molecule where such complexes can be formed, reaction will proceed predominantly through the one which can be attained most easily. 

The accelerating effect of cupric ions on the ferric ion catalysis which was observed by Bohnson and Robertson is considered by Barb el al. to be due to reaction (5 ), as was the analogous effect of cupric salts on the ferrous ion catalysis. For conditions in which Scheme A applies reaction (4 ) is in effect catalyzed by (5 ). At high cupric ion concentrations (5 ) will eliminate (3), since effectively all the radical HO2 will react in (5 ). In these conditions the enhancement reaches a limit as was observed by Bohnson and Robertson. However (1) now becomes the operative chain-terminating step, and hence the kinetics of Scheme B should apply (Eq. g). Unfortunately no data on the peroxide dependence is available, but analysis of the data (43,66) shows the rate to be proportional to [Fe+++]w as required by (g). There is the same discrepancy in hydrogen ion dependence as was found in the simple ferric reaction. 

The water species involved in this reaction must be neutral (and not OH") because of the fact that the rate of uracil photohydrate formation is independent of NaCl concentration up to 1M, and is the same in unbuffered water as in 0.1M phosphate buffer. The rate constant for photohydrate formation in CU was also observed, in a series of runs all made in the same day with the same initial CU concentration, to be 0.0418 0.010 at NaCl concentrations of 0, 0.001M, 0.01M, 0.1 M, and 1M. The lack of salt effect is consonant, according to Debye-Huckel theory (3) with the reaction of a charged species (UH+) with an uncharged species, as written in Reaction f, and eliminates reaction between two charged species in the product-forming process. 

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