Highly Enantioselective Zirconium-Catalyzed Cyclization of Aminoalkenes

Aminoalkenes are catalytically cyclized in the presence of cyclopenta­dienyl­bis(oxazolinyl)borato group 4 complexes {PhB(C5H4)(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(C5H4)(Ox4S‑iPr,Me2 )2}Zr(NMe2)Cl or a cyclopenta­dienyl­mono(oxazolinyl)borato zirconium diamide {Ph2B(C5H4)(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 (k 2 (H)/k 2 (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 k 2 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.