Elimination substituent effects

Substituent effects have a significant influence only on the carbanion intermediate. Since the reaction is basically an E2 elimination, substituent effects will result in a failure of the re activity-selectivity principle. 

N-chemical shifts of enamines and closely related amines have been determined using 15N of natural abundance62. In order to eliminate substituent effects, differential chemical shifts A<5(N) are defined as <5N(amine)-<5N(enamine). This parameter is shown to correlate well with the free energy of activation AG for restricted rotation about the N—C bond of enamines with extended conjugation. The experimental results suggest that the differential 15N shifts are a useful probe to study n-n interactions in enamines. 

Substituent effects are in some instances predictable and/or easily rationalized, but are more subtle in other cases. iV-Oxides generally show an abundant M—16 peak which is sometimes the base peak, and di-iV-oxides show successive elimination of two atoms of... 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

Fig. 6.5. Representation of changes in transition-state character in the variable transition state E2 elimination reaetion, showing displacement of transition-state location as a result of substituent effects (a) substituent Z stabilizes catfaanion character of Elcb-like transition state (b) substituent R stabilizes carbocation character of El-like transitions state.

There are two opposing substituent effects on this reaction. Electron-attracting aiyl substituents favor the deprotonation but disfavor the elimination step. The observed substituent effects are small, and under some conditions the Hammett plot is nonlinear. 

Also as noted above any substituents present have little effect upon such oxidations. In 2,2 -methylenedifuran (118) the rings are attacked simultaneously giving a tetramethoxy derivative.297 Even the bulk of the fert-butyl group has little effect.298 The only marked substituent effect is that exerted by an aromatic (benzene, thiophene, furan) residue which, if directly attached at the 2-position, promotes elimination instead of the addition of another methoxy group. The net process then becomes one of arylation, as when 2-(2-thienyl)furan (119) is oxidized to 120.298 There are reports that acetyl and carboxy groups can be ejected during oxidation, but that ester groups are usually retained.287... 

Although understanding the effect of the a-substituent on relative alkyl hydride stabilities is straightforward, understanding the effect on the catalyst-enamide diastereomers is not. To clarify this matter we performed a series of calculations on a variety of substituted enamides [78], To eliminate the effect of the amide oxygen, we examined frontier orbitals of the [Rh(PH3)2(formamide)]+ fragment (15) and those of model enamides (16). 

The transfer of iodine to the organic substrate represents a formal reductive elimination at tellurium(lV) to give tellurium(ll) as well as oxidation of the alkene. In a series of diaryltellurium(lV) diiodides, iodination of organic substrates is accelerated by electron-withdrawing substituents and is slowed by electron-donating substituents, which is consistent with the substituent effects one would expect for... 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

The addition-elimination mechanism is also very common in the monocyclic oxygen and sulfur heterocycles (e.g. equation 20), a fact frequently cited as evidence for their low aromaticity. Pyran-2-ones can react with electrophiles at the 3- and 5-positions and pyran-4-ones at the 3-position (they also react at the carbonyl oxygen atom, but this is classified as a substituent reaction). Moreover, while the position of substitution can often be predicted on the basis of charge distribution and substituent effects, the choice of experimental conditions can also profoundly affect the outcome of the reaction, as illustrated in Schemes 2 and 3. 

The initiation by aromatic tertiary amines is affected by the type of substituent and its position in the ring. A series of meta-substituted pyridines show a higher activity than ortho- or para-substituted derivatives with electron-releasing substituents. 3-Aminopyridine is much more reactive than 2-aminopyridine and 3-methylpyridine is more effective than 2- or 4-methylpyridine 19). The strong electron-withdrawing substituent of 2-bromopyridine and its steric hindrance eliminate the effect of this amine. A 2,6-Dimethylpyridine was also found to display a steric effect. Thus, in the presence of this amine, the rate of copolymerization decreases by a factor of 4 compared with 3-methylpyridine 69). 

It is now clear why substituent effects on k2bs are so complicated, (see Table 21), since any given alkyl substituent may affect the values of no less than three rate coefficients. If, however, the relative rates of alkyl-tin cleavages in an asymmetrical tetraalkyltin are considered, any variations in k2 and k are eliminated and only the relative values of kt are significant, e.g. 

Dias RMB, Vieira AJSC (1997) Substituent effect on superoxide elimination from peroxyl radicals of adenine and methylated derivatives. J Photochem Photobiol AChem 117 217-222 Didenko YT, Nastish DN, Pugach SP, Polovinka YA, Kvochka VI (1994) The effect of bulk solution temperature on the intensity and spectra of water sonoluminescence. Ultrasonics 32 71-76 Dillon D, Combes R, McConville M, Zeiger E (1992) Ozone is mutagenic in Salmonella. Environ Mol Mutagen 19 331-337... 

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