34949-25-2Relevant academic research and scientific papers
Pb(II)-promoted amide cleavage: Mechanistic comparison to a Zn(II) analogue
Elton, Eric S.,Zhang, Tingting,Prabhakar, Rajeev,Arif, Atta M.,Berreau, Lisa M.
, p. 11480 - 11492 (2013)
Two new Pb(II) complexes of the amide-appended nitrogen/sulfur epppa (N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl) methyl)amine) chelate ligand, [(epppa)Pb(NO3)2] (4-NO 3) and [(epppa)Pb(ClO4)2] (4-ClO4), were prepared and characterized. In the solid state, 4-NO3 exhibits κ5-epppa chelate ligand coordination as well as the coordination of two bidentate nitrate ions. In acetonitrile, 4-NO3 is a 1:1 electrolyte with a coordinated NO3-, whereas 4-ClO4 is a 1:2 electrolyte. Treatment of 4-ClO4 with 1 equiv Me4NOH·5H2O in CH3CN:CH 3OH (3:5) results in amide methanolysis in a reaction that is akin to that previously reported for the Zn(II) analogue [(epppa)Zn](ClO 4)2 (3-ClO4). 1H NMR kinetic studies of the amide methanolysis reactions of 4-ClO4 and 3-ClO4 as a function of temperature revealed free energies of activation of 21.3 and 24.5 kcal/mol, respectively. The amide methanolysis reactions of 4-ClO 4 and 3-ClO4 differ in terms of the effect of the concentration of methanol (saturation kinetics for 4-ClO4; second-order behavior for 3-ClO4), the observation of a small solvent kinetic isotope effect (SKIE) only for the reaction of the Zn(II)-containing 3-ClO4, and the properties of an initial intermediate isolated from each reaction upon treatment with Me4NOH·5H2O. These experimental results, combined with computational studies of the amide methanolysis reaction pathways of 4-ClO4 and 3-ClO4, indicate that the Zn(II)-containing 3-ClO4 initially undergoes amide deprotonation upon treatment with Me4NOH·5H2O. Subsequent amide protonation from coordinated methanol yields a structure containing a coordinated neutral amide and methoxide anion from which amide cleavage can then proceed. The rate-determining step in this pathway is either amide protonation or protonation of the leaving group. The Pb(II)-containing 4-ClO4 instead directly forms a neutral amide-containing, epppa-ligated Pb(II)-OH/Pb(II)-OCH3 equilibrium mixture upon treatment with Me4NOH·5H2O in methanol. The rate-determining step in the amide methanolysis pathway of 4-ClO4 is nucleophilic attack of the Pb(II)-OCH3 moiety on the coordinated amide. Overall, it is the larger size of the Pb(II) center and the availability of coordination positions that enable direct formation of a Pb(II)-OH/Pb(II)- OCH3 mixture versus the initial amide deprotonation identified in the reaction of the Zn(II)-containing 3-ClO4.
Concave reagents: Part 40 - The copper(ii) complex of a concave reagent as a selective catalyst for ester methanolysis
Cacciapaglia, Roberta,Di Stefano, Stefano,Fahrenkrug, Frank,Luening, Ulrich,Mandolini, Luigi
, p. 350 - 355 (2007/10/03)
We have shown that the CuII complexes of the concave ligand 1 and its model compound 2 are efficient catalysts of ester methanolysis under conditions close to neutrality. Turnover catalysis without product inhibition was demonstrated by the clean first-order release of a greater than stoichiometric amount of product. Compared with background methanolysis, the metal catalysts give greater rate accelerations for methyl acetate methanolysis than for the p-nitrophenyl acetate methanolysis. Analysis of electronic and steric effects on rates of metal-mediated vs metal-free methoxide addition to the esters has provided compelling evidence that transfer of methoxide from the metal to the carbonyl carbon is accompanied by extensive Lewis acid activation of the carbonyl via a four-membered chelate transition state that includes the metal ion. Copyright
