The effect of steric hindrance on the relative rates of anchimerically assisted alcohol eliminations from MH + ions of 2-substituted 1,4-dialkoxybutanes upon CI and CID : Experiment and theory

Abstract

1,4-Dialkoxybutanes afford very abundant [MH–ROH] + ions upon isobutane chemical ionization (CI) and collision induced dissociation (CID), in contrast to other primary mono-ethers or 1, ω-diethers of long chain diols. This behavior suggests involvement of anchimeric assistance in the mechanism of alcohol elimination from the MH + ions of 1,4-dialkoxybutanes. This concerted mechanistic pathway finds support in the CI and CID behavior of the [MH–ROH] + ions, obtained from isomeric mixed 1,4-dialkoxybutanes (1-ethoxy-4-methoxy- and 1-methoxy-4-ethoxy) substituted at position 2 with alkyl groups or with deuterium atoms. Density functional (DFT) calculations at B3LYP/6-31G** level of theory also support the anchimerically assisted elimination mechanism. The isomeric mixed 1,4-dialkoxybutanes, substituted at position 2 with alkyl groups of variable bulkiness, exhibit preferential elimination of alcohol from position C-4 rather than from C-1, and this tendency increases with the size of the substituents. This steric effect is explained by a more hindered transition state involved in the anchimerically assisted elimination of alcohol from C-1 due to interaction of the substituent(s) at position 2 with the protonated alkoxy group at position 1. Calculations show the energies of transition states resulting in 4-elimination products are lower than those leading to the elimination of alcohol from position 1, and the difference significantly increases with the increase of the bulkiness of the 2-substituents.