Competitive -elimination

In all cases examined the ( )-isomers of the allylic alcohols reacted satisfactorily in the asymmetric epoxidation step, whereas the epoxidations of the (Z)-isomers were intolerably slow or nonstereoselective. The eryfhro-isomers obtained from the ( )-allylic alcohols may, however, be epimerized in 95% yield to the more stable tlireo-isomers by treatment of the acetonides with potassium carbonate (6a). The competitive -elimination is suppressed by the acetonide protecting group because it maintains orthogonality between the enolate 7i-system and the 8-alkoxy group (cf the Baldwin rules, p. 316). 

Mass spectra of the same diaziridines were reported later (74JOU1140). Whereas in longer alkyl side chains of diaziridines the typical amine degradation by n -1 carbon atoms predominates, successive elimination of NH and methyl was observed in (37a). The 1-methyl compound (37b) undergoes competitive elimination of NH and MeN. 

Nucleophile The nucleophile must be nonbasic to prevent a competitive elimination of HX (Section 11.7), but otherwise does not affect the reaction rate. Neutral nucleophiles work well. 

Tosylate seemed to be a better leaving group for minimizing competitive -elimination [60],... 

The primary advantage in the first step of the method described here (using 1-chlorobutane diluted in MeCN) is that it eliminates long reaction periods and allows the use of secondary alkyl halides without competitive elimination reactions. For example, the reaction of sec-butyl bromide with N-methylimidazole using the classical method (in neat alkyl halide) produces, along with the desired product, 20-30% of butenes and 1-methylimidazole hydrobromide. In the second step, the use of water as solvent allows the anion metathesis reaction to be quantitative in a very short time and allows the easy purification of the ionic liquids. Moreover, employing the potassium salt avoids the use of corrosive and difficult to handle hexafluorophosphoric add and the expensive silver tetrafluoroborate. ... 

Alternatively, acetaldehyde and acetic anhydride are fed directly to the cracking reactor where the same sulfonic acid can catalyze the condensation of the aldehvde-anhydride mixture to EDA and the subsequent thermal elimination forming vinyl acetate. The best results are obtained when acetic anhydride is present as solvent to inhibit the competitive elimination to acetaldehyde and anhydride (see reverse of equation 2). Aldehyde degradation reactions are minimal under these conditions. 

Scheme 2 Competitive / -elimination pathway from /soalkyirhodium complex to form 2-alkene instead of /so-aldehyde.

Jannasch, H. W., Competitive elimination of Enterobacteriaceae from seawater , Appl. Microbiol., 16,1616-1618 (1968). 

Alkylation of thienyllithium with 5-alkyl halide may lead to a competitive elimination reaction. This has now been overcome the reaction with trialkylboranes, followed by treatment with iodine, gives the corresponding alkyl derivatives in good yields (Scheme 172) (79BCJ1865, 81BCJ1587). 

FIGURE 1—20. Synapses are formed at a furious rate between birth and age 6. However, there is competitive elimination and restructuring of synapses, a phenomenon that peaks during pubescence and adolescence, leaving about half to two-thirds of the synapses present in childhood to survive into adulthood. 

S -Methylxanthates 62 derived from the same alcohols 50 are thermally less stable than the corresponding carbamates 57. They decompose during their synthesis according to two competitive elimination pathways, ft C—H and ft C—Si (3/2) eliminations14 (Scheme 12), giving rise to a lower metallole yield than from carbamates 57. 

Time-resolved i.r. emission from vibrationally excited 630 HQ and HF used to study competitive elimination channels in the IRMPD of C2H2F2Q2- Yield of HF shown to increase with increasing laser fluence that of HQ levels ofi and then decreases... 

In 1913, Kishner observed in one instance that under standard Wolff-Kishner reduction conditions, 2-hydroxy-2,6-dimethyloctan-3-one underwent eliminative reduction upon treatment with hydrazine hydrate and base at elevated temperatures to afford 2,6-dimethyloctan-2-ene (Scheme 7). This same reaction was later found to occur in the case of a-methoxy ketones and has since been referred to as the Kishner eliminative reduction. The reaction entails initial formation of the hydrazone and elimination of the a-substituent to afford the intermediate alkenyldiazene, which subsequently collapses to the desired alkene. Given the facile transformation of ketones into a-halo ketones, these conditions have been used to introduce alkenes regioselectively in the 2a-halocholestan-3-one series as shown in Scheme 8. Yields of 2-cholestene parallel the resistance of the a-halogen to undergo competitive elimination reactions. 

A competitive elimination of carbonyl ligands and AsBr2 occurs from 15. Bromine atom loss is also observed but only after the elimination of 4 CO groups, i.e. from (M — 4CO) ions. The mass spectrum of this compound exhibits (M — 4CO — 2As — Br)" ions resulting from the migration of three Br atoms from As to Fe followed by the loss of an As atom and a Br atom ... 

Taylor, G. Colthorpe, P. Farr, S.J. Pulmonary absorption of proteins influence of deposition site and competitive elimination processes. In Respiratory Drug Delivery /V Byron, P.R., Dalby, R.N., Farr, S.J., Eds. Interpharm Press, Inc. Buffalo Grove, IL, 1994 25-30. 

Alkylations with. r-alkyl halides give lower yields due to competitive elimination reactions. ... 

In contrast to the anion of diethyl phosphoramidate or trifluoromethanesulfonamide, which cannot be cleanly monoalkylated, - the anion of trifluoroacetamide (100) was monoalkylated by alkyl halides or alkyl methanesulfonate. The resulting A -alkylamides (101) were converted into primary amines by alkaline hydrolysis or reduction (NuBHa Scheme 42). Various primary amines were prepared from (100) with primary alkyl iodides or methanesulfonate, benzyl and allyl halides, a-bromocarbonyl compounds and 2,4-dinitrochlorobenzene. However, competitive elimination is a serious side reaction for less reactive primary alkyl chlorides and secondary halides or methanesulfonate. The synthesis of secondary amines from (100) has also been reported. ... 

---