Base-induced elimination reaction

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

In the formation of the carbon-to-carbon double bond, the Br atom, together with an H atom on an adjacent carbon atom, has been removed or eliminated from the haloalkane and not replaced. This reaction is often referred to as a base-induced elimination reaction (see p. 62). 

This reaction is achieved by heating the monohaloalkane under reflux with ethanollc potassium (or sodium) hydroxide, i.e. potassium (or sodium) hydroxide dissolved in ethanol. The reaction is referred to as a base-induced elimination reaction and to demonstrate more clearly the role of the base in the reaction, it is worth considering the mechanism involved. This is illustrated for the reaction between 2-bromopropane and ethanolic potassium hydroxide ... 

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. " The... 

Density functional theory and a high-level cib initio procedure (G2+) have been used to explore the potential energy surface for the base-induced elimination reaction of fluoride ion with ethyl fluoride.11 The DFT barriers are smaller and looser than those predicted by the ab initio method but the nature of the transition state cannot be defined with confidence since the predictions are unusually sensitive to the choice of functional and basis set. The results suggest that improvement in density functional methods will require fundamental change in the functionals themselves. 

The elimination reactions discussed above for diethyl ether are suggested to proceed via a transition state with a periplanar geometry. This hypothesis has been tested by studying the stereoelectronic control in the base-induced elimination reactions of cis- and /ra s-4-t-butylmethoxycyclohexane (de Koning and Nibbering, 1987). Both compounds give free and water-solvated methoxide by reaction with OTP, as summarized in (39a) and (39b). 

Describe the base induced elimination reactions of the deuterated compounds represented by the following formulae and assign appropriate stereodescriptors to the resulting olefins. (Note that as well as the hydrogen atom the deuterium atom can also be abstracted from these compounds.)... 

The first step is to convert the Fischer projection formulae into sawhorse projections. Note that for a base-induced elimination reaction of the E2 type, the tosyl group must be antiperiplanar to the proton or deutron being abstracted from the neighbouring carbon atom. As a consequence of this reaction mechanism, the configuration of the resulting double bond can be predicted. Because of the higher steric hindrance in the transition states leading to the Z isomers, in both cases the E isomer should be expected as the major product. 

The gas-phase base-induced elimination reaction of halonium ions was thoroughly investigated in radiolytic experiments22. Radiolytically generated acids C/JH5+ (n = 1,2) were allowed to react at 760 Torr with selected 2,3-dihalobutanes to form the halonium intermediates which, in the presence of trimethylamine, undergo base-induced bimolecu-lar elimination as shown in Scheme 6. This elimination reaction occurs in competition with unimolecular nucleophilic displacement to the cyclic halonium ion and subsequent rearrangement. Isolation and identification of the neutral haloalkenes formed and kinetic treatment of the experimental results indicated that 3-halo-1 -butene is formed preferentially with respect to the isomeric 2-halo-2-butenes and that the bimolecular elimination process occurs predominantly via a transition state with an anti configuration22. 

Based on mechanistic considerations, 171 has been suggested as an intermediate in the base-induced transformation of 170 to benzocyclopropene (172) (150). On treatment of 173 with lithium dimethylamide at — 75°C, the fulvene derivative 175 could be isolated (151). The mechanism by which the fulvene arises was elucidated by several isotope labeling experiments. These results require the formation of triene 174 as an intermediate in the base-induced elimination reaction of 173 (151). 

The factors which may influence the transjcis olefin ratios arising from base-induced elimination reactions have been discussed and are summarised in a recent paper by Feit and Gitlin". These factors include, in addition to eclipsing effects, steric effects of unsymmetrical leaving groups and bases, and the incursion of 5y/i-elimination (c/. Section 2.2.2.(d)). 

HCFC-123 (CF3CHCI2) and HFC-134a (CF3CH2F) are replacements for chlorofluorocarbons (CFCs), which are known to deplete the stratospheric ozone layer, as described in the A Word About...CFCs, the Ozone Layer, and Trade-Offs on page 200. Predict the products from the base-induced elimination reactions of HCFC-123 and HFC-134a. 

In a series of studies Bickelhaupt et al. [64, 87, 88] investigated base-induced elimination reactions experimentally with Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectroscopy and theoretically with DF calculations. They applied the computational scheme of Baerends et al. [5], using the gradient corrections of Becke [89] for the exchange potential. They studied the fundamental class of reactions... 

The p-quinodimethane system can be formed in situ through a base-induced elimination reaction. This is onfy possible if the a,[Pg.31]

The reaction of an alkyl halide or tosylate with a nucleophile/base results either in substitution or in elimination. The resultant nucleophilic substitution and base-induced elimination reactions are two of the most widely occurring and versatile reaction types in organic chemistry, both in the laboratory and in biological pathways. 

The possible reaction mechanism for a cascade olefination-hydrogenation reaction is illustrated in Scheme 1.21. First, the reaction of proline with ciis-isomer 67 generates the iminium cation 68, which reacts with electrophile 64 via a Mannich-type reaction to generate Mannich product 69. A retro-Mannich or base-induced elimination reaction of amine 69 would furnish active olefin 70. The subsequent hydrogen-transfer reaction is dependent on the electronic nature of the in situ-generated conjugated system or, more precisely, the HOMO-LUMO gap of reactants 65 and 70. 

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