Mechanisms of elimination reactions. XV. Effect of base, solvent, and structure on product ratios in elimination reactions of some quaternary ammonium salts

Abstract

The effect of changing from an n-to a sec-to a f-alkoxide-alcohol medium on the products of the E2 reactions of 2-and 3-pentyltrimethylammomum ion, 2-methyl-3-pentyltrimethylammonium ion, and 2,5-dimethyl-3-hexyltrimethylammonium ion has been studied. The reaction of phenoxide bases with two of the salts was also examined. The most important effect of changing the base-solvent system was found to be that of the strength of the base on the transition-state structure. Stronger bases cause an increase in the proportion of the Hofmann rule olefin by increasing the reactant-like nature of the transition state. The effect of the base on the trans: cis ratios depends on the stereochemistry of the reaction. If the reaction is purely anti the change to a stronger base will cause an increase in the proportion of the cis-ene. If syn elimination competes with anti elimination the change to a stronger base will cause an increase in the proportion of trans-ene. In the former case the "normal" pattern of an increase in the Hofmann rule product and a decrease in the trans: cis ratio will be observed. In the latter case, the "abnormal" pattern of an increase in both the Hofmann rule product as well as the trans: cis ratio will be observed. The occurrence of the "normal" or "abnormal" pattern gives qualitative information about the stereochemistry. The first example of an acyclic quaternary ammonium salt following the Saytzeff rule is described and is attributed to a steric effect. The effect of the alkyl structure on the trans: cis ratios is shown to follow the pre-

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Flueggenines A and B, Two Novel C,C-Linked Dimeric Indolizidine Alkaloids from Flueggea virosa

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