Elimination reactions, comparison

Claisen rearrangement, 1194-1195 dehydration, 622 elimination reactions, 393 oxidation, 625-626 radical reactions, 243-244 characteristics of, 162-164 comparison with laboratory reactions, 162-164 conventions for writing, 162. 190 energy diagram of, 161 reduction, 723-725 reductive animation, 932 substitution reactions, 381-383 Biological reduction, NADH and, 610-611... 

The quantitation of products that form in low yields requires special care with HPLC analyses. In cases where the product yield is <1%, it is generally not feasible to obtain sufficient material for a detailed physical characterization of the product. Therefore, the product identification is restricted to a comparison of the UV-vis spectrum and HPLC retention time with those for an authentic standard. However, if a minor reaction product forms with a UV spectrum and HPLC chromatographic properties similar to those for the putative substitution or elimination reaction, this may lead to errors in structural assignments. Our practice is to treat rate constant ratios determined from very low product yields as limits, until additional evidence can be obtained that our experimental value for this ratio provides a chemically reasonable description of the partitioning of the carbocation intermediate. For example, verification of the structure of an alkene that is proposed to form in low yields by deprotonation of the carbocation by solvent can be obtained from a detailed analysis of the increase in the yield of this product due to general base catalysis of carbocation deprotonation.14,16... 

A detection of p-isomer HCH in water in relatively larger quantities in comparison with other isomers shows a high degree of insecticide transformation in the soil (mainly by microorganisms), and hence loss or leaching of insecticide residues deposited some decades ago. It is known that p-isomer HCH is the most stable compound among others of HCH isomers, i.e., it is not or very weakly exposed to elimination reaction—dehydrochlorination (Cristol, 1947). High persistence of HCH p-isomer is... 

In the reaction of group 13 element halides with metal carbonyl dianions, the analysis is more complex than observed for the reactions with metal monoanions. Upon addition of metal dianions to EX3 or REX3, either one or two halide ions may be eliminated. When only one halide ion is eliminated per added metal dianion, the complexes may still be viewed as E3+ derivatives (Equations (33)-(36)).19 This may be controlled to some extent by the stoichiometry of the reaction. Comparison of Equations (33)19 and (34)19 shows that the electron demand at the main group element can be satisfied by coordination either to an electron-rich metal center 26 or formation of a halide bridge 27. Ligand-stabilized forms may also be prepared in this fashion (Equation (36)).19... 

A rel = 1.2 x 10 6) (Ingold, 1957). Neopentyl halides are known in fact to be extremely stable towards nucleophilic displacement. There are few examples in the gas phase that allow for a full comparison since alkyl halides with P-hydrogens may be prone to undergo elimination reactions simultaneously with displacement. For example, the reaction of F with C2H5C1 proceeds with a rate constant krel = 0.86, compared to that of CH3C1 (Jose, 1976), whereas (CH3)3CCH2C1 reacts with k[cl = 0.61 (Olmstead and Brauman,... 

For similar reactions, comparison of die p values can be used to determine which reaction has a greater charge development. Comparison of the olefin-forming eliminations below reveals which reaction has greater charge development at die benzylic position and thus which has a greater degree of proton removal in die activated complex. 

There are other instances, however, where unusually large extents of elimination are encountered which cannot be explained in this way. An example is provided by a study of the nucleophilic and elimination reactions of the dipentamethyl benzhydryl cation 69.247 Comparisons of pA"R values (from extrapolations based on the HR acidity function) with those of the unmethylated and partially methylated benzhydryl cations shown below, indicate that methylation cumulatively stabilizes the cation relative to the alcohol (possibly in part because the latter is destabilized by steric congestion). 

As an example, comparison of the experimental rate of this elimination reaction... 

Comparison of SN2 and acyl addition-elimination reactions with methoxide as the leaving group. In the concerted SN2, methoxide leaves in a slightly endothermic step, and the bond to methoxide is largely broken in the transition state. In the acyl substitution, methoxide leaves in an exothermic second step with a reactant-like transition state The bond to methoxide has just begun to break in the transition state. 

The /3-methyl elimination reactions are rare by comparison, examples being the decomposition of zirconium neopentyl complexes 109... 

This is the second chapter of a two-part review concerned with insertion reactions of transition metal-carbon a-bonded compounds. The first chapter, which appeared in Volume 11 of this series (137), provided a broad introduction to the subject of insertion reactions in general and a detailed treatment of the carbon monoxide insertion and decarbonylation. Presented herein are the insertion and elimination reactions of sulfur dioxide and of a few other unsaturated molecules. The reactions of sulfur dioxide are accorded a complete literature coverage, whereas those of the other inserting species are treated selectively. Metal-carbon a-bonded compounds of the main group elements are discussed only in the context of comparisons with their transition metal analogs. 

A comparison of E2 elimination reactions using alkyl halides and amines... 

The stereochemistry of dehydrohalogenation with DBU has been studied by several authors. Wolkoff (82JOC1944) studied in detail the stereochemical consequences of dehydrohalogenation of secondary and tertiary alkyl and cycloalkyl halides with DBU. A comparison of the product distributions obtained in the elimination reactions of alkyl halides with DBU, with weak bases, and with anionic bases indicated that the elimination reactions with DBU very probably follow an E2C-like mechanism. 

Selenoxide elimination occurs under relatively mild conditions in comparison to the elimination reactions described above. Selenoxides undergo spontaneous yn-elimi-nation at room temperature or below and thus have been used for the preparation of a variety of unsaturated compounds. The selenide precursors can be obtained by displacement of halides or sulfonate esters with PhSeNa. Oxidation of the selenides with hydrogen peroxide or tert-huiyX hydroperoxide, sodium periodate, or peroxycar-boxylic acids furnishes the corresponding selenoxides. Their eliminations usually favor formation of the less substituted olefin in the absence of heteroatom substituents or delocalizing groups. Since selenium compounds are toxic, they should be handled with care. 

Scheme 18 illustrates the proposed stages in 6-MSA biosynthesis in which the first and second condensation steps proceed with inversion to give the triketide (63). Ketoreduction gives the alcohol (64) and then elimination followed by a final malonyl condensation generates the tetraketide (65) which cyclises via an intramolecular condensation and enolises to give the aromatic nucleus of (66). In the first set of experiments (J )- and (S)-[l- C, H]nialonales were incubated separately with 6-MSA synthase purified from Penicillium patulum [56]. Isotope incorporations were determined by mass spectrometry. All the possible isotope patterns for retention or loss of the pro-J or pro-S hydrogens from C-3 and C-5 were permutated. Comparison with the actual spectra obtained demonstrated that opposite prochiral hydrogens were eliminated. The absolute stereochemistry was established in an analogous experiment [57] where the chiral malonates were incubated with acetoacetyl CoA rather than acetyl CoA. Subsequent mass spectral analysis showed that it is the Hr proton that is retained at C-3 of 6-MSA and so it can be deduced that the hydrogen at C-5 must be derived from the opposite prochiral hydrogen, Hg. The overall result is summarised in Scheme 18. In a recent collaborative study we have synthesised the triketide alcohol (64) as its NAC thioester and shown that it is indeed a precursor as, on incubation with 6-MSA synthase and malonyl CoA, 6-MSA production is observed [unpublished results]. Current work is aimed at synthesis of both enantiomers of (64) to study the overall stereochemistry of the ketoreduction and elimination reactions.

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