COP-Catalyzed Asymmetric Synthesis of Allylic Esters
A R T I C L E S
Scheme 3. Two Possible Stereochemical Courses of
Antarafacial-SN2′ Displacement
fragment (4). Furthermore, a geometric comparison of bond lengths
and angles confirmed 28 as a reasonable surrogate of crystallo-
graphically characterized COP complexes.8,29,30
Three possible reaction pathways were investigated (Figure
4). The first involved an anti-oxypalladation pathway, wherein
external acetate attacks a COP-alkene complex having an
acetate ligand (I, Figure 4). In this reaction pathway, the imidate
nitrogen is protonated. The second anti-oxypalladation mech-
anism investigated involved external acetate attack on a cationic
palladium complex in which the imidate nitrogen is bonded to
the palladium center (II). The third reaction simulation involved
syn oxypalladation of a palladium complex wherein the acetate
nucleophile is delivered internally (III). Although the possibility
of a concerted oxypalladation/deoxypalladation pathway was
initially pursued, transition states corresponding to this reaction
mode were not found.31 We reasoned that syn-oxypalladation
processes in which the nucleophile is not bound to palladium
would be sterically improbable because of the necessity of the
nucleophile to pass by either the tetraphenylcyclobutadiene or
oxazoline fragments of the COP framework to interact with the
activated π-bond.
which of the allowable reaction pathways is most plausible by
investigating computationally derived reaction coordinate
diagrams.
The TURBOMOLE V6.223 quantum chemistry program using
the B3-LYP24 hybrid functional was utilized throughout this
study. All atoms were represented by the triple-ꢁ def2-TZVP
basis set.25 A polarizable continuum solvation model (COSMO)
was implemented to model the bulk effects of the dichlo-
romethane solvent.26 Full geometry optimization and numerical
frequency calculations were performed on all structures. Transi-
tion-state structures were characterized by exactly one imaginary
vibrational mode along the elemental step studied; intermediates
were confirmed by the absence of imaginary vibrational
modes.27 All energies are reported as electronic energies plus
unscaled zero-point corrections.
Figure 4. Three reaction pathways investigated.
Coordination Geometry of Palladium in a COP Complex.
The coordination geometry at the palladium center was exam-
ined first (Figure 5). Each of the three pathways required slightly
different coordination geometries of the starting complex. For
the anti-oxypalladation pathway I, monodentate complexes, in
which the palladium was bound to the alkene Si-face, were
investigated (29-32). Monodentate complexes (33-36) with
For ease of calculation, palladacyclic fragment 28 was chosen
as a model for the COP ligand. Natural population analysis (NPA)28
indicated that 28 was a reasonable electronic substitute for the COP
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(27) Because the COSMO solvent model was used, analytical second
derivatives were unavailable; as a result, numerical differentiation was
performed (TURBOMOLE program NumForce) using a step size of
0.02 bohr on 3N coordinates.
(23) (a) TURBOMOLE V6.2, Turbomole GmbH: Karlsruhe, Germany, 2009;
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(28) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
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(29) See the Supporting Information for details.
(30) (a) Kirsch, S. F.; Overman, L. E.; Watson, M. P. J. Org. Chem. 2004,
69, 8101–8104. (b) The four Pd-C distances in cyclooctadienylpal-
ladium(II) chloride were determined to be 2.200, 2.209, 2.211, and
2.259 Å by X-ray crystallography. These compare favorably to the
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(31) All attempts to locate a concerted transition state converged on the
transition structure 49 (Figure 7).
9
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