Polar elimination mechanisms

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

It should be mentioned, however, that the phosphoramidothioate 202 can undergo hydrolysis by another mechanism which becomes operative above all in polar solvents (e.g. aqueous KOH, and less so in methanol or acetone). P—O bond cleavage occurs, presumably via an addition/elimination mechanism, while the metaphosphorimidate pathway is characterized by P—S bond cleavage. 

A polar nucleophilic mechanism has also been advanced (86) (Fig. 4.34C). The mechanism is characterized by a nucleophilic attack of the amine on the 4a position of FAD to form the amine adduct followed by base-catalyzed elimination to the imine and FADH2. 

In a move in the opposite direction, the overlaps resulting from concatenation of different polarization transfer mechanisms in combined 2D experiments can be eliminated by reducing the dimensionality of an experiment. Similarly to the successful transformation of basic 2D NMR techniques into their ID counterparts [32-34], a conversion of combined 2D NMR techniques into their ID analogs is feasible and has been explored by several groups [35-40]. From a different perspective this process can be seen as a twofold reduction of the dimensionality in a 3D experiment. Equally, concatenation of three polarization transfer steps in a single ID experiment represents transformation of a possible 4D homonuclear experiment into its ID analog. 

Nucleophilic vinylic substitutions of 4//-pyran-4-onc and 2,6-dimethyl-4//-pyran-4-one with a hydroxide ion in aqueous solution were calculated by the density functional theory (B3LYP) and ab initio (MP2) methods using the 6-31+G(d) and 6-31G (d) basis sets. The aqueous solution was modelled by a supermolecular approach, where 11 water molecules were involved in the reaction system. The calculations confirmed a different addition-elimination mechanism of the reaction compared with that in the gas phase or non-polar solution. Addition of OH- at the C(2) vinylic carbon of the pyranone ring with an activation barrier of 10-11 kcalmol-1 (B3LYP) has been identified as the rate-determining step, in good quantitative and qualitative agreement with experimental kinetics. Solvent effects increase the activation barrier of the addition step and, conversely, decrease the barrier of the elimination step.138... 

Figure 7.16 Polarized multiple bond addition/elimination mechanisms in basic media. Reactants are in the upper left. The black route is an uncatalyzed addition. The gray route is the specific base catalyzed, and the dashed diagonal is the general base-catalyzed process.

Figure A.4 shows the usefulness of the reaction cube as a data structure. Additions to carbonyls often occur between different charge types, and frequently three-dimensional energy surfaces are used to clarify the various equilibria. We have seen two faces of this cube before as individual energy surfaces. The bottom faee of the cube is Figure 7.16, polarized multiple bond addition/elimination mechanisms in basic media. The back face of the cube is Figure 7.17, polarized multiple bond addition/elimination mechanisms in acidic media.

Cleavage of alkenylpentafluorosilicates by halogen or N-bromosuccinimide proceeds with retention of configuration, in contrast to the inversion observed under similar conditions in the reaction of alkenyltrimethylsilanes, in which the silicon is but tetracoor-dinate. Alkenyltrifluorosilanes react with inversion in carbon tetrachloride, and with retention in polar solvents135. Inversion is a consequence of the accepted trans-addition anti-elimination mechanism (Scheme 8), whereas retention is explained by direct electrophilic displacement of silicon by attack at the a-carbon atom (Scheme 9). 

Several modified INEPT and DEPT pulse sequences have recently been introduced (IS) (see Fig. 1). The new INEPT+, DEPT +, and DEPT + + sequences differ from the original INEPT and DEPT sequences only in that they employ additional refocusing and purging pulses. These serve to reduce or eliminate distortions inherent in the parent pulse sequences. The fundamental polarization transfer mechanism however remains unchanged. 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

This means that the carbon-hydrogen bond is more polar than the carbon-deuterium bond, and is therefore easier to break. The reaction that would support the addition-elimination mechanism is ... 

Activation of protective mechanisms occurs when the harmful substance penetrates into the body. The penetration rate of pollutants and their distribution in the body, however, as well as the range of detoxification or elimination mechanisms, is subject to anatomical, physiological, biochemical and other factors. Phase 1 biotransformation typically involves changes catalysed by hydrolases and oxidases, in which polar functional groups are... 

The unimolecular gas-phase elimination kinetics of 2-methoxy-l-chloroethane, 3-methoxy-l-chloropropane, and 4-methoxy-l-chlorobutane has been studied using density functional theory (DFT) methods. Results calculated for 2-methoxy-l-chloroethane and 3-methoxy-l-chloropropane suggest that the corresponding olefin forms by dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state. In the case of 4-methoxy-l-chlorobutane, in addition to the 1,2-elimination mechanism, anchimeric assistance by the methoxy group, through a polar five-centered cyclic transition state, provides 4-methoxybutene, tetrahydrofuran, and chloromethane. Polarization of the C-Cl bond is rate limiting in these elimination reactions. 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

Certain alkyl halides and tosylates undergo E2 eliminations faster when treated with such weak bases as Cl in polar aprotic solvents or PhS than with the usual E2 strong bases such as RO in ROH. In order to explain these results Parker et al. proposed that there is a spectrum of E2 transition states in which the base can interact in the transition state with the a carbon as well as with the p hydrogen. At one end of this spectrum is a mechanism (called E2C) in which, in the transition... 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

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