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

作者：Maxwell Christopher I; Neverov Alexei A; Mosey Nicholas J^*; Brown R Stan

来源：Journal of Physical Organic Chemistry, 2014, 27(5): 419-429.

DOI：10.1002/poc.3279

• DFT
• methanolysis
• mechanism
• transesterification
• Zn-complex promoted

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 (s)(s)pK(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)(s)pK(a)(HOLG)=(s)(s)pK(a)(HOCH3). 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 (s)(s)pK(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)(s)pK(a)(HOLG)=(s)(s)pK(a)(HOCH3)) for which the second step of metal ion assisted departure of methoxide becomes partially rate-limiting. The BrOnsted 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.

• 出版日期2014-5