Ion cyclotron resonance and deuterium kinetic isotope studies of the gas-phase reactions of the alkoxide negative ions with anhydrides and esters

Abstract

(i) The i.c.r. spectrum of the CD₃O^-/acetic anhydride system shows the occurrence of the negative ion McLafferty rearrangement [reaction (1)] and the characteristic elimination shown in reaction (2) (R = COMe)

CD₃O^-+ (MeCO)₂O → (CD₃O)(Me)(HO)C-O^-+ CH₂CO (1)

CD₃0^-+ MeCO₂R → -CH₂C0₂CD₃+ ROH (2)

(ii) The i.c.r. spectra of CD₃O^-/alkyl ester systems show major peaks due to [ester-H⁺]^- ions, together with small peaks corresponding to [ester+CD₃O^-]^- adducts. Carboxylate and alkoxide anions (derived from the ester) are also observed in certain spectra; peaks due to these ions increase in intensity with elaboration of alkyl substituents. Reaction (2) is characteristic of all esters studied (illustrated above for acetates) which have at least one hydrogen substituent on the carbon atom α to the carbonyl group. The reaction must occur by nucleophilic attack of the alkoxide anion at the carbonyl group of the ester. Negative ion McLafferty rearrangements do not occur in alkyl ester systems.

(iii) The H transfer reactions (1) and (2) show deuterium isotope effects k_H/k_D of 3.0 and 1.5 respectively (when the carbon adjacent to the carbonyl group contains one deuterium substituent). Isotope effect calculations suggest that reaction (1) proceeds through a 'near symmetrical' six membered transition state, whereas (2) goes through a 'reactant-like' but distorted four-centred transition state.