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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
Author(s) -
Maxwell Christopher I.,
Neverov Alexei A.,
Mosey Nicholas J.,
Stan Brown R.
Publication year - 2014
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3279
Subject(s) - methoxide , chemistry , aryl , reductive elimination , medicinal chemistry , methanol , sodium methoxide , reaction rate constant , transition state , density functional theory , alkoxy group , rate determining step , alkyl , catalysis , computational chemistry , stereochemistry , organic chemistry , kinetics , physics , quantum mechanics
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(OCH 3 )] + 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 CH 3 O − 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 CH 3 O ‐ attack on aryloxy acetates having highers s p K 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 thes s p K a HOLG = s s p K a HOCH 3. 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 thes s p K a HOLGincreases, 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 , (whens s p K a HOLG = s s p K a HOCH 3) 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(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. Copyright © 2014 John Wiley & Sons, Ltd.