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 S_N2 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.