Density functional theory study of methoxide promoted and Zn(II)-complexed methoxide promoted cleavages of aryl- and alkyl acetates in methanol. Transition from concerted to stepwise processes as a function of leaving group ability

Density functional theory calculations were performed for the methanolysis reactions of a set of aryloxy and alkoxy acetates (1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1i, 1j, 1k, 1l, 1m) promoted by methoxide and a 1,5,9‐triazacyclododecane‐complexed Zn(II)‐methoxide [2(OCH3)]+ in order to give free energies and structural data for the various intermediates and transition states along the reaction pathway. The methoxide‐promoted reactions experience a transition of pathways from enforced‐concerted addition of CH3O− to the C = O unit for substrates having a good aryloxy leaving groups (LGs) with strong electron withdrawers (1a, 1b, 1c, 1d, 1e) to a two step process with rate‐limiting CH3O‐ attack on aryloxy acetates having higher sspKaHOLG (the pKa of the parent phenol of the LG in methanol) values. Only in the case of the substrates 1i‐m having alkoxy LGs is there an observed change in rate‐limiting step that occurs at the quasi‐symmetrical point where the sspKaHOLG=sspKaHOCH3. The methanolysis process for the 2,4‐dinitrophenoxy substrate (1a) promoted by [2(OCH3)]+ involves transient binding of the substrate to the metal complex followed by a rate‐limiting, enforced‐concerted attack of Zn(II)‐coordinated –OCH3, with fast breakdown of an addition intermediate that does not have a significant lifetime. For substrates 1b,c having slightly less electron withdrawing substituents, the reaction has two steps with rate‐limiting attack and an unassisted LG departure. As the sspKaHOLG increases, the reaction still has two steps with rate‐limiting attack, but departure of the LG is now assisted by its coordination to the metal ion. For alkoxy containing substrates, a change in rate‐limiting step occurs centered at methoxy acetate, 1j, (when sspKaHOLG=sspKaHOCH3) for which the second step of metal ion assisted departure of methoxide becomes partially rate‐limiting. The Brønsted plots computed for the methoxide‐promoted and [2(OCH3)]+‐promoted methanolyses are compared with the previously determined experimental data and are analyzed as arising not from a common line attributable to all substrates but rather in terms of separate, but intersecting, plots for aryl‐ and alkyl acetates. Copyright © 2014 John Wiley & Sons, Ltd. Density functional theory computations reveal that a downward break in the Brønsted plot for methanolysis of esters promoted by a Zn‐methoxide complex stems from inclusion of esters having both aryloxy leaving groups (LGs) and alkoxy LGs in the same relation