ja064927w_si_003.cif (21.44 kB)
Kinetics and Mechanism of Ketone Enolization Mediated by Magnesium Bis(hexamethyldisilazide)
dataset
posted on 2006-10-18, 00:00 authored by Xuyang He, J. Jacob Morris, Bruce C. Noll, Seth N. Brown, Kenneth W. HendersonMagnesium 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.
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Ketone Enolization Mediated1 H NMRintramolecular proton transferdeuterium isotope effectrate studiesproton transferisotope effect displaystemperature dependenceenolate groupambient temperatureα carbonHMDSsolution NMR studiesMgtoluene solutionUVcharacterizationsubstoichiometric amountsdata supportrate constantsenolization proceedingmodel ketone complexes295 KIR spectroscopy
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