The role of hydride migration in the mechanism of alcohol elimination from protonated ethers upon chemical ionization: Experiment and theory

Abstract

An enhanced elimination of methanol under isobutane-chemical ionization (CI) conditions, resulting in highly abundant [MHCH 3OH] + ions, has been observed in several primary and secondary methyl ethers having a tertiary β-position (methine), as compared with those with β-methylene. This elimination is stereospecific in stereoisomeric 2-methyl-1-methoxycyclohexanes and in other ethers affording significantly more abundant [MHCH 3OH] + ions in the cis-isomers than in their trans-counterparts. These findings suggest involvement of a 1,2-hydride migration from the β- to α-position in the course of the alcohol elimination from the MH + ions of the above cis-ethers, resulting in stabilized tertiary carbocation structures. The possible pathways of methanol elimination from protonated cis-2-methyl-1-methoxycyclohexane were explored by density functional calculations at the B3LYP/6-31+G(d,p) level of theory. The transition states for MeOH elimination involving 1,2-hydride migration were located and the activation energy of the process was evaluated. The activation barrier of the alcohol elimination assisted by 1,2-hydride migration is lower by ∼10 kcal/mol than the simple bond cleavage (9.6 kcal/mol vs. 20.5 kcal/mol). These computational results support the mechanistic pathway involving the 1,2-hydride transfer. A step-wise mechanistic pathway is proposed for the less efficient elimination of methanol from protonated trans-2-methyl-1-methoxycyclohexane.