The mechanism of methanol loss from the (M-H)⁻ ions of cis- and trans-4-methoxycyclohexanol. The application of experiment and theory in concert

Abstract

Deprotonation of cis- and trans-4-methoxycyclohexanol by HO– in the ion source of a mass spectrometer yields the (M – H)– alkoxide ions exclusively. Both of these ions, on collisional activation, form MeO–, MeO–(H2O) and eliminate MeOH. The loss of methanol forms the base peak of the spectrum, and the structure of this daughter anion is shown to be the alkoxide ion from cyclohex-3-enol for both isomers. Evidence (based on product ion, deuterium labelling and AM1 semiempirical computational studies) indicates that the loss of methanol from the trans isomer proceeds by an internal SN2 cyclisation of O– at the four position (through a 1,4-epoxycyclohexane species) followed by 3,4 elimination. A similar sequence may occur for the cis-isomer, but in this case the process is not as energetically favourable as that for the trans isomer.