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

摘要

Density functional theory calculations were performed for the methanolysis reactions of a set of aryloxy and alkoxy acetates (1a-m) promoted by methoxide and a 1,5,9-triazacyclododecane-complexed Zn~((II))-methoxide [2(OCH_3)]~+ in order to give free energies and structural data for the various intermediates and transition states along the reaction pathway. The methoxidepromoted reactions experience a transition of pathways from enforced-concerted addition of CH_3O~- to the C =O unit for substrates having a good aryloxy leaving groups (LGs) with strong electron withdrawers (1a-e) to a two step process with rate-limiting CH_3O~- attack on aryloxy acetates having higher _s~spK_a~(HOLG) (the pK_a 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 _s~spK_a~(HOLG)=_s~spK_a~(HOCH3). The methanolysis process for the 2,4-dinitrophenoxy substrate (1a) promoted by [2(OCH_3)]~+ involves transient binding of the substrate to the metal complex followed by a rate-limiting, enforced-concerted attack of Zn~((II))-coordinated ~–OCH_3, 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 _s~spK_a~(HOLG) 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 _s~spK_a~(HOLG)=_s~spK_a~(HOCH3)) for which the second step of metal ion assisted departure of methoxide becomes partially rate-limiting. The Br?nsted plots computed for the methoxide-promoted and [2(OCH_3)]~+-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.