Highly Enantioselective Zirconium-Catalyzed Cyclization of Aminoalkenes

Aminoalkenes are catalytically cyclized in the presence of cyclopentadienylbis(oxazolinyl)borato group 4 complexes {PhB(C5H4)(Ox(R))2}M(NMe2)2 (M = Ti, Zr, Hf; Ox(R) = 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)(Ox(4S-iPr,Me2))2}Zr(NMe2)Cl or a cyclopentadienylmono(oxazolinyl)borato zirconium diamide {Ph2B(C5H4)(Ox(4S-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.