Intermediacy of radicals in rearrangement and decomposition of metal-alkyl species: Relevance to metal-mediated polymerization of polar vinyl monomers

Article abstract of DOI:10.1021/om060519p

The neutral compound [2,3-bis(2,6-diisopropylphenylimino)butanejPd(CH ₂CH₂CH₂CO₂Me)(X) (X = Cl, Br) undergoes reverse chainwaMngtoform[2,3-bis(2,6- diisopropylphenylimino)butanelPd(CH(CO₂Me)CH₂CH ₃)(X) through a conventional -hvdrogen elimination/readdition pathway. However, reversible Pd-alkyl bond homolysis occurs for both alky I complexes, and the resultant radicals can initiate the polymerization of acrylates.

Full text of DOI:10.1021/om060519p

4722
Organometallics 2006, 25, 4722-4724
Intermediacy of Radicals in Rearrangement and Decomposition of
Metal-Alkyl Species: Relevance to Metal-Mediated Polymerization
of Polar Vinyl Monomers
Megan Nagel and Ayusman Sen*
Department of Chemistry, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802
ReceiVed June 13, 2006
Scheme 1. “Chain-Walking” Rearrangement To Form the
Stable Six-Membered Chelate
Summary: Theneutralcompound[2,3-bis(2,6-diisopropylphenylimino)-
butane]Pd(CH2CH2CH2CO2Me)(X) (X ) Cl, Br) undergoes
“reVerse”chainwalkingtoform[2,3-bis(2,6-diisopropylphenylimino)-
butane]Pd(CH(CO2Me)CH2CH3)(X) through a conVentional
â-hydrogen elimination/readdition pathway. HoweVer, reVers-
ible Pd-alkyl bond homolysis occurs for both alkyl complexes,
and the resultant radicals can initiate the polymerization of
acrylates.
In recent years, there has been considerable interest in the
development of metal-mediated insertion (co)polymerization of
polar vinyl monomers, especially acrylates.1 The commonly
cited problem that has stymied the development of suitable
catalysts is the interaction of the oxygen functionality with the
metal center, which hinders coordination of the next incoming
monomer. This has led to the examination of newer systems
that are less oxophilic: those involving late transition metals
and those that are neutral.1,2 Unfortunately, with some notable
exceptions, there have been few successes.3 However, there is
a second, less appreciated reason for the failure of most metal-
based systems to polymerize acrylates through a nonradical
insertion mechanism. For electronic reasons, acrylates have a
strong preference for 2,1-insertion into metal-carbon bonds.4
This results in the formation of a metal-alkyl species that is
particularly prone to homolysis because of the enhanced stability
of the resultant alkyl radical, one that is essentially the same as
the propagating species in radical-initiated acrylate polymeri-
zation.5 This paper focuses on this phenomenon and shows that
metal-carbon bond homolysis can be facile for the relatively
electron-rich neutral metal alkyls and can compete with
â-hydrogen abstraction/readdition steps.6
One of the very few systems that copolymerize acrylates
through an insertion mechanism is by Brookhart and involves
cationic Pd(II)-based complexes of the general type [(N∧N)-
Pd(Me)(L)][B(Arf)4] (N∧N ) 2,3-bis(2,6-diisopropylphenylimi-
no)butane, Arf ) 3,5-(CF3)2C6H3, L ) Et2O).3a This system is
able to incorporate up to 15 mol % acrylate in copolymerizations
with ethene and 1-alkenes. A novel feature of this system is
the rearrangement that follows acrylate insertion, resulting in
the removal of the ester functionality from the position R to
the metal, eventually forming a six-membered chelate, 1
(Scheme 1). This unique feature of the system prompted us to
examine the stability of the complex upon opening of the six-
membered chelate by forming a neutral species.
One equivalent of tetraphenylphosphonium bromide was
added to complex 1 in CD2Cl2 to open the chelate ring, and the
reaction was monitored by 1H NMR spectroscopy. A complete
and rapid conversion from 1 to 2 was observed (Scheme 2).
Compound 2 is not stable at room temperature and rearranges
to 3 within minutes (66% overall yield). The driving force for
this rearrangement is presumably the same as that for the 2,1-
insertion of acrylates into Pd-C bonds; in the cationic Brookhart
system the isomerization proceeds in the opposite direction
because of the enhanced stability of the six-membered chelate
over the smaller chelate rings. Finally, 3 was found to
decompose on further standing at room temperature to yield
methyl crotonate (68% overall yield), together with methyl
butyrate (6% overall yield) and a trace amount of the diester,
dimethyl suberate. While the transformations, 2 to 3 and 3 to
methyl crotonate, can be explained by invoking the usual
â-hydrogen abstraction/readdition mechanism, the formation of
methyl butyrate suggested the possibility that the methyl butyrate
and at least some of the methyl crotonate arose through Pd-
carbon bond homolysis in 3 followed by the well-known
disproportionation of the resultant radical. Likewise, the frag-
mentation of the palladium-alkyl bond in 2 followed by
radical-radical combination would lead to the formation of
* To whom correspondence should be addressed. E-mail:
asen@psu.edu.
(1) (a) Boffa, L. S.; Novak, B. M. Chem. ReV. 2000, 100, 1479-1493.
(b) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100, 1169-
1203. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-
316.
(2) (a) Stibrany, R. T.; Schulz, D. N.; Kacker, S.; Patil, A. O.; Baugh,
L. S.; Rucker, S. P.; Zushma, S.; Berluche, E.; Sissano, J. A. Macromol-
ecules 2003, 36, 8584-8586. (b) Younkin, T. R.; Connor, E. F.; Henderson,
J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287,
460-462. (c) Nagel, M.; Paxton, W. F.; Sen, A.; Zakharov, L.; Rheingold,
A. L. Macromolecules 2004, 37, 9305-9307.
(3) (a) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am.
Chem. Soc. 1998, 120, 888-899. (b) Drent, E.; Rudmer, D.; Ginkel, R.;
Oort, B.; Pugh, R. Chem. Commun. 2002, 744-745.
(4) (a) Philipp, D. M.; Muller, R. P.; Goddard, W. A.; McAdon, M.;
Mullins, M. J. Am. Chem. Soc. 2002, 124, 10198-10210. (b) Michalak,
A.; Ziegler, T. J. Am. Chem. Soc. 2001, 123, 12266-12278.
(5) (a) Albe´niz, A. C.; Espinet, P.; Lo´pez-Ferna´ndez, R. Organometallics
2003, 22, 4206-4212. (b) Elia, C.; Elyashiv-Barad, S.; Sen, A. Lo´pez-
Ferna´ndez, R.; Albe´niz, A. C.; Espinet, P. Organometallics 2002, 21, 4249-
4256. (c) Tian, G.; Boone, H. W.; Novak, B. M. Macromolecules 2001,
34, 7656-7663. (d) Also see: Yang, P.; Chan, B. C. K.; Baird, M. C.
Organometallics 2004, 23, 2752-2761
(6) (a) Mohring, V. M.; Fink, G. Angew. Chem., Int. Ed. Engl. 1985,
24, 1001-1003. (b) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am.
Chem. Soc. 2001, 123, 11539-11555. (c) Waltman, A. W.; Younkin, T.
R.; Grubbs, R. H. Organometallics 2004, 23, 5121-5123.
10.1021/om060519p CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/31/2006

Communications
Organometallics, Vol. 25, No. 20, 2006 4723
Scheme 2. Proposed Reaction Pathways following Disruption of the Six-Membered Chelate
Table 1. Quantification of Decomposition Productsa
a Reaction conditions: 0.0136 mmol of 1, 0.032 mmol of halide salt, 1 mL of CH2Cl2, stirred at ambient temperature for 18 h.
dimethyl suberate. Further support for the intermediate formation
of radicals came from the observation that when the bromide
salt was added to compound 1 in the presence of excess of
methyl acrylate (MA) (350× excess, relative to Pd) at room
temperature, the homopolymerization of MA ensued (22%
conversion, Mw ) 60 760, and PDI ) 1.71). No polymer
formation was observed in the absence of the palladium species.
we verified that CBr4 by itself did not react with 1. Our
observations clearly suggest that the palladium-alkyl species
present in the system readily undergo bond homolysis to
generate alkyl radicals.
To examine whether radicals were also involved in the
conversion of 2 to 3, the kinetics of the rearrangement in
1
methylene chloride was monitored by H NMR spectroscopy.
To provide more convincing evidence for the presence of
radicals, 1 equiv of CBr4 was added to 1 and PPh4+Br- (see
Table 1). After several hours, the reaction mixture contained
methyl 2-bromobutyrate (20% overall yield), the trapped product
from homolytic cleavage of the palladium-alkyl bond in 3. A
second species, methyl 4-bromobutyrate, was also observed in
a much greater concentration (38% overall yield).
There is a possibility that the observed alkyl bromides arise,
not from radical trapping with CBr4 but through reductive
elimination from the corresponding Pd(alkyl)(Br) species, 2 and
3. To further clarify the origin of the alkyl bromides, NEt4+Cl-
was used to disrupt the chelate and CBr4 was used as the radical
trapping agent. In this case, alkyl radicals trapped by CBr4 would
generate bromides while reductive elimination would lead to
the formation of chlorides. Only alkyl bromides were observed
by GC and NMR spectroscopy, the yields being similar to those
observed previously (Table 1). As a final control experiment,
The rearrangement was found to follow a linear first-order
kinetic plot. An Eyring analysis of this reaction over the
temperature range of 5-29 °C gives ∆Hq ) 12.1 ( 1.1 kcal/
mol and ∆Sq ) -14.1 ( 7.0 e.u. If this rearrangement was
occurring by a migratory “chain-walking” mechanism, a slightly
negative entropy of activation would be expected, due to the
ordered transition state needed for â-hydrogen elimination.6b
On the other hand, a large positive activation entropy should
accompany a mechanism involving bond homolysis. The
dissociation of the diimine ligand does not appear to play a
role in the conversion of 2 to 3, since the addition of an excess
of the N∧N ligand had no effect on the rate. Likewise, the
dissociation of the bromide leading to ion pair formation does
not appear to occur, since the rate of the rearrangement did not
decrease on moving from methylene chloride to less
polar chlorobenzene (CH2Cl2, k ) 0.039 s-1; PhCl, k ) 0.093
s-1).

4724 Organometallics, Vol. 25, No. 20, 2006
Communications
Our results are most consistent with the pathways shown in
Scheme 2. The rearrangement of 2 to 3 appears to proceed via
the traditional â-hydrogen elimination/readdition pathway (path
A). On the other hand, both 2 and 3 appear to readily undergo
(reversible) Pd-C bond homolysis and the resultant radicals
get trapped as bromides in the presence of CBr4. Methyl
crotonate can form via either radical disproportionation or
â-hydrogen elimination, whereas methyl butyrate can only arise
by radical disproportionation. Consequently, the low observed
ratio of methyl butyrate to methyl crotonate suggests that Pd-C
bond homolysis is reversible and that the predominant route to
methyl crotonate is by traditional â-hydrogen abstraction from
3 rather than Pd-C bond homolysis. Nevertheless, sufficient
alkyl radicals can and do escape the solvent cage and are capable
of initiating traditional radical polymerization of polar vinyl
monomers. This possibility needs to be taken into account before
mechanistic claims can be made with respect to metal-mediated
polymerization of these monomers. For example, the formation
of copolymers whose compositions vary significantly from those
predicted from the radical reactivity ratios must be demonstrated
before a nonradical mechanism can be invoked.
coordination of the acrylate ester group, favors this pathway.
Thus, simply preventing the coordination of the functionality
present on the polar vinyl monomer may not necessarily result
in a viable system for insertion polymerization. Indeed, it
appears that the success of the Brookhart system in copolymer-
izing acrylates is due to stable chelate formation following
acrylate insertion (complex 1); as discussed, when 1 is forced
open by forming a neutral halide-coordinated complex, ho-
molysis of the Pd-C bond is observed. The price paid for
chelate formation in the Brookhart system is slow polymeriza-
tion and the placement of the functionality predominantly at
the branch ends. One possible solution to the issues discussed
above is to use a sufficiently sterically encumbered system that
forces 1,2-insertion of acrylate into Pd-C bond, as predicted
by Ziegler.7
Acknowledgment. This research was supported by the U.S.
Department of Energy and Rohm & Haas Co.
Supporting Information Available: Text and figures giving
experimental procedures and NMR data. This material is available
free of charge via the Internet at http://pubs.acs.org.
In conclusion, our findings illustrate the propensity of neutral
Pd-alkyl species to undergo Pd-C bond homolysis. In the
present instance, the blocking of one of the coordination sites
on palladium by the halide ligand, a possible strategy to prevent
OM060519P
(7) Michalak, A.; Ziegler, T. J. Am. Chem. Soc. 2001, 123, 12266-
12278.