10.1021/ja064927w.s001
Xuyang He
Xuyang
He
J. Jacob Morris
J. Jacob
Morris
Bruce C. Noll
Bruce C.
Noll
Seth N. Brown
Seth N.
Brown
Kenneth W. Henderson
Kenneth W.
Henderson
Kinetics and Mechanism of Ketone Enolization Mediated by
Magnesium Bis(hexamethyldisilazide)
American Chemical Society
2006
Ketone Enolization Mediated
1 H NMR
intramolecular proton transfer
deuterium isotope effect
rate studies
proton transfer
isotope effect displays
temperature dependence
enolate group
ambient temperature
α carbon
HMDS
solution NMR studies
Mg
toluene solution
UV
characterization
substoichiometric amounts
data support
rate constants
enolization proceeding
model ketone complexes
295 K
IR spectroscopy
2006-10-18 00:00:00
Dataset
https://acs.figshare.com/articles/dataset/Kinetics_and_Mechanism_of_Ketone_Enolization_Mediated_by_Magnesium_Bis_hexamethyldisilazide_/3052996
Magnesium bis(hexamethyldisilazide), Mg(HMDS)<sub>2</sub>, reacts with substoichiometric amounts of
propiophenone in toluene solution at ambient temperature to form a 74:26 mixture of the enolates (<i>E</i>)- and
(<i>Z</i>)-[(HMDS)<sub>2</sub>Mg<sub>2</sub>(μ-HMDS){μ-OC(Ph)CHCH<sub>3</sub>}], (<i>E</i>)-<b>1</b> and (<i>Z</i>)-<b>1</b>, which contain a pair of three-coordinate
metal centers bridged by an amide and an enolate group. The compositions of (<i>E</i>)-<b>1</b> and (<i>Z</i>)-<b>1</b> 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, <b>2</b>,
between the ketone and magnesium, which undergoes first-order decay with rate constants independent
of the concentration of excess Mg(HMDS)<sub>2</sub> (Δ<i>H</i><sup>⧧</sup> = 17.2 ± 0.8 kcal/mol, Δ<i>S</i><sup>⧧</sup> = −11 ± 3 cal/mol·K). The
intermediate <b>2</b> has been characterized by low-temperature <sup>1</sup>H NMR, diffusion-ordered NMR, and IR
spectroscopy and investigated by computational studies, all of which are consistent with the formulation of
<b>2</b> as a three-coordinate monomer, (HMDS)<sub>2</sub>Mg{η<sup>1</sup>-OC(Ph)CH<sub>2</sub>CH<sub>3</sub>}. Further support for this structure is
provided by the synthesis and structural characterization of two model ketone complexes, (HMDS)<sub>2</sub>Mg(η<sup>1</sup>-OC<i><sup>t</sup></i><sup></sup>Bu<sub>2</sub>) (<b>3</b>) and (HMDS)<sub>2</sub>Mg{η<sup>1</sup>-OC(<i><sup>t</sup></i><sup></sup>Bu)Ph} (<b>4</b>). A large primary deuterium isotope effect (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> =
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 <b>2</b> via intramolecular proton transfer
from the α carbon of the bound ketone to the nitrogen of a bound hexamethyldisilazide.