Beta-elimination of alkyl

The method of zeolite synthesis, the Si/Al ratio in the synthesis gel, and the crystallite size of Beta zeolite are factors to be considered for optimum alkylation performance [123,128,129[. For both USY and Beta zeolites, the amount and type of extraframework Al species (EFAL) remaining in the zeolite after calcination and/or post-synthesis treatments also affected the catalyst performance [130,131], For both zeolites, elimination of highly dispersed cationic-type EFAL formed under mild (hydro)thermal treatments decreased the alkylation... 

The Hofmann eliminations (19-21) give very low isotope effects, excepting the controversial intramolecular isotope effects for the 2-p-nitro-phenylethyl-trimethylammonium ion. The conditions vary widely and comparisons are thus difficult. The enhanced acidity of this substrate, however, should promote extensive proton transfer and the small tritium isotope effects would seem most plausibly interpreted as indicative of highly carbanionic transition states for these eliminations. The alkyl ammonium salts, possessing much more weakly acidic beta hydrogen, probably react via less carbanion-like E2 transition states . 

Additional to the polar factors, the steric requirements of the alkyl substituents may play a dominating role by hindering approach of the base to a beta proton or alternatively causing a divergence from the requisite coplanarity of reacting bonds in the transition state for elimination. Often, along a series of alkyl substituents, both steric and inductive effects vary in the same direction and the interpretation of the salient orientating features is a matter of personal preference. 

The direction of elimination in the reaction of a series of alkyl bromides (118f changes from Saytzeff to Hofmann as the size of a beta alkyl substituent is increased, indicating a gradual increase in importance of steric interaction. 

The mechanisms of base-induced beta-eliminations leading to alkynes are highly dependent on the type of base employed, and for this reason the dehydrohalogenations induced by metal alkoxides are considered separately from those initiated by metal amides or metal alkyls. 

The configurational stability of alkenyl anions is obviously remarkably high compared with that of alkyl anions ". The order of stability of the carbanions of the chloroethylenes , (207), reflects that a/i//-beta-elimination occurs more easily than syn-beta-elimination or alpha-elimination ". 

The active site is a cationic metallocene alkyl generated by reaction of a neutral metallocene formed from reaction with excess MAO or other suitable cocatalysts such as a borane Lewis acid. This sequence is shown in Figure 5.1 employing MAO with ethylene to form PE. Initiation and propagation occur through pre-coordination and insertion of the ethylene into the alkyl group polymer chain. Here, termination occurs through beta-hydride elimination... 

All beta hydrogen elimination occurs from the 14-electron species Cp2ZrR+ where, by definition, R is branched. In contrast, beta hydrogen elimination from Cp2ZrR,+ is slow (because the alkyl group, n-hexyl, is linear) and the rate of this reaction is taken to be zero. 

Carboalkoxy acyl halides are made from mono esters of dibasic acids and thionyl chloride or phosphorus pentachloride. Examples are numerous. Halides with the ester group in the beta position are unstable to prolonged heating. Alkyl halide is eliminated with the formation of an anhydride. Under certain conditions a rearrangement occurs in the preparation of ester acid chlorides. The product obtained is a mixture of the expected compound and its isomer in which the ester and acid chloride groups are interchanged, viz., ROjCCHR (CI ) CO,H — RO,C(CH,) CHR COQ. The cyclic anhydride is a likely intermediate. ... 

Production of alpha-terpinyl alkyl ethers Beta Excellent yields in continuous reactor Eliminates use of HCl, H2SO4, AICI3, toluene, sulfonic acid, boron trifluoride etherate, and acidic cation resins as catalysts... 

---