1285536-16-4Relevant academic research and scientific papers
Highly enantioselective zirconium-catalyzed cyclization of aminoalkenes
Manna, Kuntal,Everett, William C.,Schoendorff, George,Ellern, Arkady,Windus, Theresa L.,Sadow, Aaron D.
supporting information, p. 7235 - 7250 (2013/06/27)
Aminoalkenes are catalytically cyclized in the presence of cyclopentadienylbis(oxazolinyl)borato group 4 complexes {PhB(C5H 4)(OxR)2}M(NMe2)2 (M = Ti, Zr, Hf; OxR = 4,4-dimethyl-2-oxazoline, 4S-isopropyl-5,5- dimethyl-2-oxazoline, 4S-tert-butyl-2-oxazoline) at room temperature and below, affording five-, six-, and seven-membered N-heterocyclic amines with enantiomeric excesses of >90% in many cases and up to 99%. Mechanistic investigations of this highly selective system employed synthetic tests, kinetics, and stereochemistry. Secondary aminopentene cyclizations require a primary amine (1-2 equiv vs catalyst). Aminoalkenes are unchanged in the presence of a zirconium monoamido complex {PhB(C5H 4)(Ox4S-iPr,Me2)2}Zr(NMe2)Cl or a cyclopentadienylmono(oxazolinyl)borato zirconium diamide {Ph2B(C 5H4)(Ox4S-iPr,Me2)}Zr(NMe2) 2. Plots of initial rate versus [substrate] show a rate dependence that evolves from first-order at low concentration to zero-order at high concentration, and this is consistent with a reversible substrate-catalyst interaction preceding an irreversible step. Primary kinetic isotope effects from substrate conversion measurements (k′obs(H)/ k′obs(D) = 3.3 ± 0.3) and from initial rate analysis (k2(H)/k2(D) = 2.3 ± 0.4) indicate that a N-H bond is broken in the turnover-limiting and irreversible step of the catalytic cycle. Asymmetric hydroamination/cyclization of N-deutero-aminoalkenes provides products with higher optical purities than obtained with N-proteo-aminoalkenes. Transition state theory, applied to the rate constant k2 that characterizes the irreversible step, provides activation parameters consistent with a highly organized transition state (ΔS? = -43(7) cal·mol-1 K -1) and a remarkably low enthalpic barrier (ΔH ? = 6.7(2) kcal·mol-1). A six-centered, concerted transition state for C-N and C-H bond formation and N-H bond cleavage involving two amidoalkene ligands is proposed as most consistent with the current data.
A highly enantioselective zirconium catalyst for intramolecular alkene hydroamination: Significant isotope effects on rate and stereoselectivity
Manna, Kuntal,Xu, Songchen,Sadow, Aaron D.
supporting information; experimental part, p. 1865 - 1868 (2011/04/22)
Zirconium catalysts sparkle: A new chiral zirconium complex has been used to catalyze hydroamination reactions to cyclize aminopentenes into 2-methylpyrrolidines (see scheme). The rate law supports a mechanism involving a reversible substrate-catalyst interaction that precedes the rate-determining step. A new mechanism for zirconium-catalyzed hydroamination has been proposed based on kinetic isotope effects and the significant effect of isotopic substitution on enantioselectivity.
