Diametrically opposite trends in alkene insertion in late and early transition metal compounds: Relevance to transition-metal-catalyzed polymerization of polar vinyl monomers

Article abstract of DOI:10.1021/ja026550+

Variable-temperature ¹H NMR studies of the reaction of cationic (α-diimine)Pd-alkyl complexes with alkenes are presented. The studies reveal that vinyl bromide coordinates to the Pd(II)-Me complex followed by migratory insertion and β-bromo elimination, to generate free propene. Propene further reacts to give β-agostic Pd(II)-tert-butyl species. From the reactions with vinyl bromide, stable chloro-bridged dicationic Pd complex was isolated and characterized. For a series of alkenes (CH₂=CHX), the rate for migratory insertion decreases as follows: X = CO₂Me > Br > H > Me. Copyright

Full text of DOI:10.1021/ja026550+

Published on Web 09/19/2002
Diametrically Opposite Trends in Alkene Insertion in Late and Early
Transition Metal Compounds: Relevance to Transition-Metal-Catalyzed
Polymerization of Polar Vinyl Monomers
Myeongsoon Kang,† Ayusman Sen,*,† Lev Zakharov,‡ and Arnold L. Rheingold‡
Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and
Department of Chemistry, UniVersity of Delaware, Newark, Delaware 19716
Received April 16, 2002
Scheme 1. Proposed Reaction Pathway8
Metal-catalyzed insertion copolymerization of polar vinyl mono-
mers with nonpolar alkenes remains an area of great interest in
synthetic polymer chemistry, because the addition of functionalities
to a polymer which is otherwise nonpolar can greatly enhance the
range of attainable properties.1 For vinyl monomers with pendant
coordinating functionalities, such as acrylates, the principal problem
has been catalyst poisoning through functional group coordination.1-3
Interestingly, vinyl halides, which do not possess any strongly
coordinating functionality, are also not polymerized by any known
transition-metal-catalyzed systems. Recently, Jordan4 and Wolc-
zanski5 have reported the reaction of vinyl halides with rac-(EBI)-
ZrMe and (tBu3SiO)3TaH2, respectively. It was demonstrated that,
following 1,2-insertion of the alkene, â-halide elimination occurs
to generate a metal-halide bond. Because the halophilicity of the
transition metal ions tends to decrease on going from left to right
in the periodic table, we undertook an examination of the reactivity
of vinyl halides toward a late transition metal complex (palladium).
While we have also observed â-halide elimination following
insertion, one surprising result that has emerged is a linear positiVe
Hammett correlation between the rate of insertion and the increasing
electron-withdrawing effect of the substituent on the alkene. This
stands in stark contrast with that observed for the above tantalum-
based system where the second-order rate constants for vinyl halide
insertions are significantly smaller than those for ethylene or vinyl
ethers.5 In addition, the generally slower rate of propene versus
ethene insertion in metal complexes6 is usually attributed to the
steric bulk of the methyl group of the former. Our results suggest
that even for the sterically encumbered Brookhart-type system, it
is the donating ability of the methyl group, rather than its size,
which results in slower rate for propene insertion. These results
are clearly important in the context of alkene polymerizations
proceeding by an insertion mechanism.
form 7. The cationic Pd(II)-halide species arising from 4 by
propene loss converts to the chloro-bridged dimer, 6. The structure
of 6 as a dicationic complex with two aluminum tetrachloride
counteranions was established by an X-ray crystal structure
determination (Supporting Information). The identity of the halide
ligand in 4 has not been established, but the formation of 6 opens
up the possibility of an aluminum-assisted â-bromo abstraction
pathway shown in Scheme 1.
The starting point of our investigation was the Brookhart-type
cationic Pd(II)-methyl species, 2, generated in our case by the
addition of 2 equiv of AlCl3 to the corresponding neutral Pd(II)-
methyl chloride, 17 (Scheme 1). Several equivalents of vinyl
bromide was added to a CD2Cl2 solution of 2 at -90 °C, and the
Once formed, 7 undergoes 1,2-insertion of propene to form the
known â-agostic Pd(II)-tert-butyl compound 8.9 The complexes 7
and 8 were also independently formed by the addition of propene
to a CD2Cl2 solution of 2. The initially formed 7 was found to
convert to 8 when the solution was warmed to -36 °C. At ambient
temperature, the three methyl groups of the tert-butyl complex
exchange rapidly on the NMR time scale and appear as a singlet at
-0.28 ppm. Upon lowering the temperature to -86 °C, a static 1H
NMR spectrum is observed, and the agostic proton appears as a
singlet at -8 ppm.
The migratory insertion rates of bound vinyl bromide and propene
in 3 and 7, respectively, were directly measured by monitoring the
disappearance of the corresponding Pd-CH3 resonance. For pro-
pene, our value was in close agreement with that reported by
Brookhart.10 For vinyl bromide, an Arrhenius plot was constructed
1
reaction mixture was monitored by H NMR spectroscopy as it
was gradually warmed (Scheme 1). The coordination of vinyl
bromide to the metal center in 2 was observed even at -86 °C,
resulting in the formation of 3. Warming the reaction mixture to
-74 °C resulted in the formation of the propene coordinated species,
4, suggesting 1,2-alkene insertion followed by â-bromo elimination.
Propene is gradually lost from 4 and is trapped by unreacted 2 to
* To whom correspondence should be addressed. E-mail: asen@
chem.psu.edu.
† The Pennsylvania State University.
‡ University of Delaware.
9
12080
J. AM. CHEM. SOC. 2002, 124, 12080-12081
10.1021/ja026550+ CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Kinetic Data for Insertion of Alkenes into the Pd(II)-Me
propene insertion and, hence, polymerization. This argument applies
even for the sterically encumbered Brookhart-type system.
What is the implication of our work with respect to the metal-
catalyzed polymerization of polar vinyl monomers? First, for the
late metal compounds, the polar vinyl monomers can clearly
outcompete ethene and simple 1-alkenes with respect to insertion.
However, the ground-state destabilization of the alkene complex
that favors the migratory insertion of the polar vinyl monomers is
a two-edged sword because it biases the alkene coordination toward
ethene and 1-alkenes. Indeed, we have observed the near quantita-
tive displacement of vinyl bromide by propene to form 7 from 3.
Thus, the extent of incorporation of the polar vinyl monomer in
the polymer will depend on the opposing trends in alkene
coordination and migratory insertion. The above discussion does
not take into account the problem of functional group coordination
for acrylates or â-halide abstraction for vinyl halides. With respect
to the latter, we are currently exploring approaches to suppress this
“termination” step, for example, decreasing the electrophilicity of
the metal center.
Bonda
q
q
k (×103 s-1
)
∆H (kcal/mol)
∆S (cal/K‚mol)
propene
0.54
1.9
ethene6
14.2 ( 0.1
11.9 ( 0.1
12.1 ( 1.4
-11.2 ( 0.8
-16.8 ( 0.1
-14.1 ( 7.0
vinyl bromide
22.0
55.0
methyl acrylate10
a Measured or extrapolated to 236.5 K.
Acknowledgment. This research was supported by a grant from
the U.S. Department of Energy, Office of Basic Energy Sciences.
Supporting Information Available: Experimental, NMR spectra
(PDF), and crystal structure data for compound 6 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 1. Hammett plot for migratory insertion of alkenes into the
palladium(II)-methyl bond.
References
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A Hammett plot of the relative insertion rates of substituted
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(Figure 1). Of note is that the line encompasses values obtained by
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