Kinetics and Mechanism of Ketone Enolization Mediated by Magnesium Bis(hexamethyldisilazide)

Abstract

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)(2)Mg(2)(mu-HMDS){mu-OC(Ph)=CHCH(3)}], (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) (DeltaH++ = 17.2 +/- 0.8 kcal/mol, DeltaS++ = -11 +/- 3 cal/mol.K). The intermediate 2 has been characterized by low-temperature (1)H 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)(2)Mg{eta(1)-O=C(Ph)CH(2)CH(3)}. Further support for this structure is provided by the synthesis and structural characterization of two model ketone complexes, (HMDS)(2)Mg(eta(1)-O=C(t)Bu(2)) (3) and (HMDS)(2)Mg{eta(1)-O=C((t)Bu)Ph} (4). A large primary deuterium isotope effect (k(H)/k(D) = 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 alpha carbon of the bound ketone to the nitrogen of a bound hexamethyldisilazide.

Magnesium(II) Complexes of the dpp‐BIAN Radical‐Anion: Synthesis, Molecular Structure, and Catalytic Activity in Lactide Polymerization

“…Until now, seven monomeric magnesium species with a terminal bis(trimethylsilyl)amido ligand (L) have been reported: L 2 Mg,27 L 2 MgPy 2 ,28 L 2 Mg(O=C t Bu 2 ),29 (2,4,6‐ t Bu 3 C 6 H 2 S)MgL(thf) 2 ,30 LMg(BH 4 )[O(C 2 H 4 OMe) 2 ],31 [(2,6‐ i Pr 2 C 6 H 3 NCMe) 2 CH]MgL,32 and [(2‐MeOC 6 H 4 NCMe) 2 CH]MgL 33. The search in the CCDC for complexes with coordination of a TEMPO ligand to magnesium resulted in only [{(Me 3 Si) 2 N}Mg(μ‐TEMPO)] 2 ,34 which, in contrast to compound 2 , contains a bridging TEMPO group.…”

Advances in alkaline earth-nitrogen chemistry

“…Alkali-alkaline earth mixed metal amides have solubility in organic solvents and unique reactivity that is markedly different from that of a mixture of the homometallic counterparts [125][126][127][128][129][130][131][132]. Alkaline earth-alkaline earth mixed metal species are less well known and limited to [MgCa{N(SiMe 3 ) 2 } 4 ] [127].…”

Enolisation Kinetics of m-Nitro Acetophenone

“…The concentrations of the other reactants were kept constant, and different percentage concentrations of acetic acid and DMF with water were tried. The rate constants of first order [9,10] were calculated as per Equation 1. The values of dielectric constants were taken from literature, which were 6.2 for acetic acid, 36.71 for DMF and 80.4 for water [11].…”

“…318, 323, 328, and 333 K. From the iodometeric measurements, the first order rate constants were calculated [9,10] which were plotted against reciprocal of the absolute temperature. The values of the slopes and intercepts were further used for the calculation of different thermodynamic parameters such as energy of activation (Ea) [12], entropy of activation (ΔS≠), frequency factor (log Pz or A = 10 11 ) [13], enthalpy of activation (ΔH≠) and Gibbs free energy (ΔG≠), which were calculated as follows: …”