%0 Generic
%A He, Xuyang
%A Morris, J. Jacob
%A Noll, Bruce C.
%A Brown, Seth N.
%A Henderson, Kenneth W.
%D 2006
%T Kinetics and Mechanism of Ketone Enolization Mediated by
Magnesium Bis(hexamethyldisilazide)
%U https://acs.figshare.com/articles/dataset/Kinetics_and_Mechanism_of_Ketone_Enolization_Mediated_by_Magnesium_Bis_hexamethyldisilazide_/3052990
%R 10.1021/ja064927w.s003
%2 https://acs.figshare.com/ndownloader/files/4758658
%K Ketone Enolization Mediated
%K 1 H NMR
%K intramolecular proton transfer
%K deuterium isotope effect
%K rate studies
%K proton transfer
%K isotope effect displays
%K temperature dependence
%K enolate group
%K ambient temperature
%K α carbon
%K HMDS
%K solution NMR studies
%K Mg
%K toluene solution
%K UV
%K characterization
%K substoichiometric amounts
%K data support
%K rate constants
%K enolization proceeding
%K model ketone complexes
%K 295 K
%K IR spectroscopy
%X Magnesium bis(hexamethyldisilazide), Mg(HMDS)2, reacts with substoichiometric amounts of
propiophenone in toluene solution at ambient temperature to form a 74:26 mixture of the enolates (E)- and
(Z)-[(HMDS)2Mg2(μ-HMDS){μ-OC(Ph)CHCH3}], (E)-1 and (Z)-1, which contain a pair of three-coordinate
metal centers bridged by an amide and an enolate group. The compositions of (E)-1 and (Z)-1 were
confirmed by solution NMR studies and also by crystallographic characterization in the solid state. Rate
studies using UV−vis spectroscopy reveal the rapid and complete formation of a reaction intermediate, 2,
between the ketone and magnesium, which undergoes first-order decay with rate constants independent
of the concentration of excess Mg(HMDS)2 (ΔH⧧ = 17.2 ± 0.8 kcal/mol, ΔS⧧ = −11 ± 3 cal/mol·K). The
intermediate 2 has been characterized by low-temperature 1H NMR, diffusion-ordered NMR, and IR
spectroscopy and investigated by computational studies, all of which are consistent with the formulation of
2 as a three-coordinate monomer, (HMDS)2Mg{η1-OC(Ph)CH2CH3}. Further support for this structure is
provided by the synthesis and structural characterization of two model ketone complexes, (HMDS)2Mg(η1-OCtBu2) (3) and (HMDS)2Mg{η1-OC(tBu)Ph} (4). A large primary deuterium isotope effect (kH/kD =
18.9 at 295 K) indicates that proton transfer is the rate-limiting step of the reaction. The isotope effect
displays a strong temperature dependence, indicative of tunneling. In combination, these data support the
mechanism of enolization proceeding through the single intermediate 2 via intramolecular proton transfer
from the α carbon of the bound ketone to the nitrogen of a bound hexamethyldisilazide.
%I ACS Publications