End-Group-Confined Chain Walking within a Group 4 Living Polyolefin and Well-Defined Cationic Zirconium Alkyl Complexes for Modeling This Behavior

Article abstract of DOI:10.1021/ja0496975

Living polymers derived from the polymerization of 1-butene using the cationic zirconium initiator, {Cp*ZrMe[N(Et)C(Me)-N(^tBu)]}[B(C₆F₅)4] (Cp* = η⁵-C₅Me₅) (1), have been shown to undergo e

Full text of DOI:10.1021/ja0496975

Published on Web 03/23/2004
End-Group-Confined Chain Walking within a Group 4 Living Polyolefin and
Well-Defined Cationic Zirconium Alkyl Complexes for Modeling This Behavior
Matthew B. Harney, Richard J. Keaton, and Lawrence R. Sita*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
Received January 16, 2004; E-mail: lsita@umd.edu
One of the last remaining mysteries associated with the Ziegler-
Natta polymerization of propene by zirconocene-based catalysts is
the mechanism by which chain-end epimerization of the last
generated stereocenter occurs under conditions of low monomer
1
1f
concentration. In this regard, Landis and co-workers recently
reported the direct observation of chain-end epimerization of living
zirconocene polypropylene by low-temperature NMR spectroscopy.
Simple model complexes for this process are still required, however,
as they should greatly facilitate mechanistic studies. Unfortunately,
given the low kinetic stability of alkyl substituents bearing
â-hydrogens within neutral and cationic zirconocene complexes,2
such model systems do not exist, and they may prove extremely
3
difficult to come by. Herein, we now report our direct observation
of end-group-confined chain walking, which is an integral part of
the proposed Busico mechanism for chain-end epimerization,1
within living polyolefins derived from higher R-olefins and the
cyclopentadienyl zirconium acetamidinate initiator, {Cp*ZrMe-
a
Figure 1. 1H NMR (400 MHz) spectra of olefinic region for living polymer
t
5
4,5
[
N(Et)C(Me)N( Bu)]}[B(C
further report that the well-defined cationic zirconium complexes,
and 3, bearing alkyl groups with strong â-hydrogen agostic
6
F
5
)
4
] (Cp* ) η -C
5 5
Me ) (1). We
of isotactic poly(1-butene) (DP ≈ 20) kept at -10 °C and (bottom)
immediately quenched after formation and (top) quenched after 72 h.6
2
Scheme 1
interactions, can serve as models for chain walking; however, these
complexes are surprisingly more stable toward â-hydride elimina-
tion/chain release than the living system derived from 1-butene
polymerization (LPB) where such â-agostic interactions appear to
be absent. These results should prove useful in better defining the
steric and electronic factors that govern chain walking and chain-
end epimerization within early transition-metal propagating species.
of the terminal trisubstituted alkene II (δ 5.18 and 5.27 ppm) and
for the vinyl end group III (δ 4.90, 4.96, and 5.68 ppm) also appear
at later time intervals (see Figure 1). Noticeably absent within the
detection limits of NMR, however, are resonances for the trisub-
stituted olefin IV (see Scheme 1). As shown in Scheme 1, it is
reasonable to assume that II and III are derived from the
intermediates VI-VIII that arise from V through end-group-
confined chain walking involving a series of successive â-hydride
Polymerizations of 1-butene and 1-hexene proceed in a living
and highly stereoselective manner when 1 is used as the initiator
at -10 °C in chlorobenzene, with propagation occurring in a strict
,6
1
,2-fashion.5 Further, if living polymers with a degree of polym-
erization (DP) g 100 are acid quenched at -10 °C immediately
1
1,3
after formation, virtually no alkene H NMR resonances resulting
elimination/olefin rotation/reinsertion steps. Importantly, this chain
from termination by â-hydride elimination are discernible using
standard acquisition parameters. However, as the two selected H
walking, which is assumed to be intramolecular in nature, must be
competitive with chain release after each â-hydride elimination step.
It is further assumed that IV is inaccessible from the tertiary
intermediate VI for steric reasons. Unfortunately, attempts to
spectroscopically observe 13C NMR resonances for the R-carbons
of either VI or of the secondary and primary species, VII and VIII,
respectively, have not yet been productive due to either low
abundance or transient lifetimes. By employing the isobutyl initiator
1
NMR spectra in Figure 1 reveal, when a low-molecular weight
isotactic poly(1-butene) living polymer (DP ≈ 20) is allowed to
incubate at -10 °C for varying periods of time before quenching,
alkene resonances for end groups resulting from â-hydride elimina-
6
tion do grow in at a steady rate. Unexpectedly, however, in addition
to the 1H NMR resonances at δ 4.69 and 4.76 ppm for the
vinylidene moiety I that originates from direct â-hydride elimination
of the initially formed living polymer, those for a cis,trans mixture
5
13
2 and (1- C, 50%) 1-decene, however, it could be established that
chain-end epimerization of the resulting living polymer does not
4536
9
J. AM. CHEM. SOC. 2004, 126, 4536-4537
10.1021/ja0496975 CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Scheme 2
3-methyl-2-pentene (cis and trans), and 3-methyl-1-pentene which
is in agreement with a chain-walking process that is competitive
with chain release. Unfortunately, for both 2 and 3, it has not yet
proven possible to detect any zirconium tertiary alkyl intermediates
that are presumed to be part of the isomerization mechanism, nor
has it been possible to observe zirconium secondary alkyl products
derived from chain walking of 3. Finally, it should be mentioned
that as with decomposition, at -10 °C, structural isomerizations
involving 2 and 3 appear to be significantly retarded relative to
the chain-walking process observed for the living polymer LPB.
The unexpected result that decomposition through chain release
and chain walking appear to be accelerated by an increase in the
steric bulk of the alkyl substituent bound to zirconium (i.e., 2 < 3
appear to occur in the present system as evidenced by a single
diastereomeric C-labeled methyl end group being observed at 20.4
ppm by 13C NMR analysis after acid quenching at various times
13
[cf., the two methyl end group resonances observed at 20.4 and
<
LPB) can be reconciled by assuming a ground-state destabiliza-
1
9.9 ppm that arise from the first insertion of 1-hexene into 1 that
tion by steric effects. It is also possible that, as originally proposed
by Bercaw and co-workers,11 the strong â-agostic interactions
observed for both 2 and 3 increase the barrier for â-hydride
elimination through stabilization of the ground state relative to that
of the living polymer in which such a strong interaction is absent.
Clearly, these observed differences between model compounds and
the living polymer, which are likely the cause of subtle steric and
electronic factors, suggest that caution should be exercised in
extrapolating the properties of the latter from those of the former.
Additional mechanistic studies are now in progress to define further
the factors controlling â-hydride elimination and chain walking in
the present system, including the potential observation or trapping
of zirconium tertiary alkyl species that are proposed as intermediates
in the latter process.
occurs with 95% diastereoselectivity in the formation of isotactic
poly(1-hexene)].6
Well-defined model complexes that can mimic the chain walking
behavior of living polymers derived from 1 should lead to a better
understanding of the factors governing this process. Accordingly,
in addition to complexes 2 and 3 that were prepared through
previously reported procedures,5 the isotopic single- and double-
labeled derivatives, 2′ and 2′′, respectively, were prepared according
to Scheme 2 using a recently reported hydrozirconation procedure
that provides products that are kinetically stable toward alkyl group
isomerizations, thus preserving the location of the isotopic label
prior to formation of 2′ and 2′′ through chemoselective demeth-
ylation as shown.6
-7
,8
As previously reported, a series of H and 13C 1- and 2D (COSY,
J-resolved HSQC) NMR experiments revealed that, at 25 °C and
below, the â-hydrogen of 2 is engaged in a strong agostic
5
1
Acknowledgment. Funding for this work was provided by the
NSF (CHE-0092493) for which we are grateful. We also thank the
ACS Division of Organic Chemistry for a graduate fellowship (to
R.J.K.) sponsored by the Proctor and Gamble Company.
9
interaction with the metal center, and this was once again found
1
13
1
â â
to be true for 3 as well [cf., δâH -0.27 ppm, J( C - H ) 92 Hz
for 2 and 0.06 ppm, 86 Hz for 3]. Additional support for this agostic
interaction was obtained in the case of 2 through single-crystal X-ray
analysis that provided a molecular structure in which the crystal-
lographically located â-hydrogen is clearly bound to the metal in
Supporting Information Available: Experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
an agostic fashion [Zr-H
â
distance of 2.25(3) Å].10 In contrast, a
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(
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(
6) Details are provided in the Supporting Information.
(
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6
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8
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(
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a,d,e
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1
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J. AM. CHEM. SOC.
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