The unimolecular chemistry of the enol of ionized methyl glycolate: Formation of the hydrogen‐bridged radical cation [CH3O(H)…︁H…︁OCH].+

Abstract

AbstractDissociative ionization of methyl 2‐hydroxy‐isovalerate and dimethyl tartrate cleanly generate, via McLafferty rearrangements, the 1‐methoxy‐ethene‐1,2‐diol ion HOCHC(OH)OCH, 2. The unimolecular chemistry of 2, the enol form of ionized methyl glycolate, HOCH2C(O)OCH, 1, was investigated by a variety of tandem‐mass spectrometry‐based techniques using D‐ and 18O‐labelled precursor molecules. The enol ion undergoes four major dissociations viz. loss of CH, CO, CH3OH and C2HO. Loss of CH involves isomerization of 2, via a 1,4 H shift, into the distonic ion HC(O.)C(OH)O+(H)CH3, 4, followed by direct bond cleavage yielding the product ion HC(O)C(OH). A second 1,4 H shift yields the hydroxyketene/methanol ion–dipole complex which serves as the precursor for the losses of CH3OH and C2HO, yielding HO(H)CCO+· and CH3OH respectively. A further isomerization step leads to the loss of CO, yielding the O…︁H ⃛O bridged ion [CH3O(H) ⃛H…︁ OCH]+·, one of the most stable isomers on the C2H6O potential energy surface. Ionized methyl acetate, CH3C(O)OCH and related aliphatic esters, readily interconvert with their enol isomers prior to dissociation, but no such tautomerization occurs in 1. This is because the HOCH functionality opens up facile rearrangement/dissociation pathways in 1 and 2 whose energy requirements lie below the tautomerization barrier 1→2.