Macromolecular effect: Synthesis of a ferrocenylmethylphosphine-containing polymer as highly efficient ligands for room-temperature palladium(0)-catalyzed Suzuki cross-coupling reactions of aryl chlorides

Article abstract of DOI:10.1021/ja029021e

The design and synthesis of a ferrocenylmethylphosphine-containing polymer and its application as efficient ligands for the room-temperature Pd(0)-catalyzed cross-coupling reaction of aryl chlorides with arylboronic acids, for which corresponding monomeric monophosphines are totally inactive, are described. Our work demonstrated that rather small monophosphine moieties such as RPPh2 can be used as highly efficient ligands when appropriately incorporated into a rigid and sterically regular polymer network and using monophosphine-containing polymers as ligands is a feasible approach to access highly active (monophosphine)-palladium(0) complexes. The macromolecular approach described here may open a new avenue to other coordinatively unsaturated (monoligand)-transition metal complexes which are potentially useful in organic synthesis. Copyright

Full text of DOI:10.1021/ja029021e

Published on Web 02/19/2003
Macromolecular Effect: Synthesis of a
Ferrocenylmethylphosphine-Containing Polymer as Highly Efficient Ligands
for Room-Temperature Palladium(0)-Catalyzed Suzuki Cross-Coupling
Reactions of Aryl Chlorides
Qiao-Sheng Hu,* Yong Lu, Zhen-Yu Tang, and Hong-Bin Yu
Department of Chemistry, College of Staten Island and the Graduate Center of the City UniVersity of New York,
Staten Island, New York 10314
Received October 20, 2002 ; E-mail: qiaohu@postbox.csi.cuny.edu
Palladium-catalyzed carbon-carbon and carbon-heteroatom
bond formations are among the most powerful transformations in
organic synthesis.1,2 Coordinatively unsaturated palladium com-
plexes such as 14-electron (R3P)2Pd(0), formed through either ligand
dissociation or displacement, are generally accepted as the catalyti-
cally active species in most palladium(0)-catalyzed reactions. As
compared to the extensively studied 14-electron (R3P)2Pd(0)
The synthesis of polymer 1 is based on our recent finding of the
one-step, high yield conversion of ferrocenylmethyl alcohol 4 to
ferrocenylmethylphosphine 2 (Schemes 1 and 2).10 As shown in
Scheme 2, reaction of 1,3,5-tribromobenzene with n-BuLi (1 equiv)
followed by treatment with ferrocenecarboxaldehyde generated
ferrocenylmethyl alcohol 5 in 80% yield.11 Polymerization of 5 with
diboronic acid 612 was carried out by using the Suzuki coupling
polymerization condition (Pd(PPh3)4, THF/2 M K2CO3).12 Treatment
of the generated polymer 7 with excess HPPh2 in HOAc/CH2Cl2
at room temperature afforded polymer 1 in 86% yield. The 1H NMR
spectrum of 1 showed complete conversion of -OH to -PPh2 as
evidenced by the disappearance of peaks of the methylene group
that connects to the OH group at 5.50-5.57 ppm and the OH peaks
at 2.4 ppm. 31P NMR of 1 in CDCl3 (H3PO4 as standard) shows
one major peak at 1.78 ppm along with a minor peak at 2.50 ppm,
corresponding to the phosphine moieties at the end of the polymer
chain. 1 is an air-stable solid and soluble in common organic
solvents such as CH2Cl2, THF, and toluene. Gel permeation
complexes, coordinatively unsaturated (R3P)Pd(0) complexes which
are believed to be more active remain “elusive”. Recently, insight
into the importance of (R3P)Pd(0) complexes as catalytically active
species has been gained.3-6 For example, through the kinetic study,
Hartwig demonstrated that the monocoordinated (o-toly3P)Pd
complex acts as a catalytically active intermediate for various
coupling reactions.3 Fu proposed that Pd(0)/P(t-Bu)3-catalyzed
Suzuki coupling reactions of aryl chlorides with arylboronic acids
involve (t-Bu3P)Pd(0) species as the catalytically active species,
although an isolated complex has not been obtained.4 Coordinatively
unsaturated (monophosphine)-Pd(0) complexes remain largely
unexplored because of the lack of a general method to access them,6
mainly due to the tendency of monophosphines, even very bulky
ones, to form (R3P)2Pd(0) complexes. It is therefore fundamentally
interesting and synthetically useful to develop general approaches
to coordinatively unsaturated (monophosphine)-Pd(0) complexes.
Mechanistically, means that can prevent two monophosphines
from coordinating to one Pd(0) center should favor the formation
of coordinatively unsaturated (R3P)Pd(0) complexes.1 We envision
that using rigid and sterically regular monophosphine-containing
polymers as ligands could be a general approach to (R3P)Pd(0)
complexes which are difficult or impossible to be accessed by using
monomeric monophosphines or flexible, sterically irregular mono-
phosphine-containing polymers.7 We reason that the rigidity and
stereoregularity of rigid and sterically regular polymers with suitable
monophosphine installation could make the second monophosphine
ligand unavailable for coordination, and thus only one, rather than
two, monophosphine moiety will coordinate to one palladium(0)
center when they are used as ligands. This macromolecular approach
may allow for the use of relatively small monophosphine moieties
as ligands for reactions that typically require the use of bulky,
electron-rich phosphines. On the basis of this hypothesis, we
designed and synthesized monophosphine-containing polymer 1
based on the rather small ferrocenylmethylphosphine 2 and
employed 1 as ligands for room-temperature palladium(0)-catalyzed
Suzuki cross-coupling reactions of aryl chlorides,4,5,8,9 for which
monophosphines 2 and 3 are ineffective ligands. Our study showed
that using sterically regular monophosphine-containing polymer as
ligands is a feasible approach to access highly active transition metal
catalysts. Herein our results are reported.
chromatography shows that the molecular weight of 1 is Mw
7700, Mn ) 3100 (PDI ) 2.49) (THF, polystyrene standards).
)
The room-temperature Pd(0)-catalyzed Suzuki cross-coupling
reaction of aryl chlorides with arylboronic acids has only been
realized recently and represents a notable advance in Pd(0)-catalyzed
coupling reactions.4,5,9 Among a few phosphine systems developed
for this objective, only Buchwald’s dialkylphosphines are effective
for couplings of both activated and unactivated aryl chlorides.5 We
have employed 1 as ligands for this transformation. We found that
1/Pd(0) complexes can achieve this objective. As shown in Table
1, both activated and unactivated aryl chlorides can be coupled with
arylboronic acids in the presence of 5% 1/Pd(OAc)2 (1:1) com-
plexes. In contrast, no reaction was observed when monophosphine
2/Pd(OAc)2 or 3/Pd(OAc)2 complexes (2:1 or 1:1) were used for
the reaction.
The ability of 1/Pd(0) complexes to effect the room-temperature
Suzuki cross-coupling of unactivated aryl chlorides indicates that
the catalytically active species in 1/Pd(0) complexes are highly
active and are different from those in 2/Pd(0) or 3/Pd(0) systems.
We have excluded the possibility of Pd(0) itself as the catalytically
active species because no reaction was observed when the cross-
coupling reaction of p-chloroanisole with phenyl boronic acids was
carried out without a phosphine ligand.9b Previous catalyst systems
9
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J. AM. CHEM. SOC. 2003, 125, 2856-2857
10.1021/ja029021e CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Direct Conversion of Ferrocenylmethyl Alcohol 4 to 2
Scheme 2. Synthesis of Monophosphine-Containing Polymer 1
ligands for room-temperature cross-couplings of aryl chlorides with
arylboronic acids, for which corresponding monomeric monophos-
phines are totally inactive. Our work not only showed that using
rigid and sterically regular monophosphine-containing polymers as
ligands is a feasible approach to access highly active palladium
catalysts, but it also holds future promise for the development of
unprecedented highly efficient palladium catalyst systems, as many
options to improve the efficiency of 1 may be envisioned. The
macromolecular approach described here may open a new avenue
to other coordinatively unsaturated mono(ligand)-transition metal
complexes. The synthesis of monophosphine-containing polymers
that can avoid the interchain cross-linking and related polymers
including optically active ones, and their application as ligands for
transition metal catalysis, are currently under active investigation,
and the results will be reported in due course.
Acknowledgment. This work was supported by the Department
of Chemistry, College of Staten Island-City University of New York
(CUNY). Partial support from the Petroleum Research Fund
administered by ACS and PSC-CUNY Research Award Program
is gratefully acknowledged. This work is dedicated to Professor
Chang-Ming Hu on the occasion of his 70th birthday.
Table 1. Room-Temperature Palladium(0)-Catalyzed
Cross-Couplings of Aryl Chlorides with Arylboronic Acidsa
Supporting Information Available: Synthesis and characterization
of 1, 2, 3, 5, and 7, and the general procedure for 1/Pd(0)-catalyzed
cross-coupling of aryl chlorides (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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to the polymer chain, some of the monophosphine moieties are
expected to remain as noninterchain cross-linking sites. Coordina-
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catalysts because it will limit the formation of (R3P)Pd(0) complexes
and make it more difficult to access these catalytically active sites.
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