Campus Units

Chemistry, Ames Laboratory

Document Type

Article

Publication Version

Published Version

Publication Date

4-30-2013

Journal or Book Title

Journal of the American Chemical Society

Volume

135

Issue

19

First Page

7235

Last Page

7250

DOI

10.1021/ja4000189

Abstract

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 (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.

Comments

Reprinted (adapted) with permission from Journal of the American Chemical Society 135 (2013): 7235, doi: 10.1021/ja4000189. Copyright 2013 American Chemical Society.

Copyright Owner

American Chemical Society

Language

en

File Format

application/pdf

Included in

Chemistry Commons

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