Structural requirements for palladium catalyst transfer on a carbon-carbon double bond

Article abstract of DOI:10.1021/jacs.5b03113

Intramolecular transfer of ^tBu₃PPd(0) on a carbon-carbon double bond (C=C) was investigated by using Suzuki-Miyaura coupling reaction of dibromostilbenes with aryl boronic acid or boronic acid esters in the presence of various additi

Full text of DOI:10.1021/jacs.5b03113

Communication
pubs.acs.org/JACS
Structural Requirements for Palladium Catalyst Transfer on a
Carbon−Carbon Double Bond
Masataka Nojima, Yoshihiro Ohta, and Tsutomu Yokozawa*
Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
S
* Supporting Information
thermodynamically preferred when the temperature was
t
raised.13 In accordance with this result, density functional
ABSTRACT: Intramolecular transfer of Bu3PPd(0) on a
theory calculation suggested that η2-CC coordination is more
carbon−carbon double bond (CC) was investigated by
using Suzuki−Miyaura coupling reaction of dibromostil-
benes with aryl boronic acid or boronic acid esters in the
presence of various additives containing CC as a model.
Substituent groups at the ortho position of CC of
stilbenes are critical for selective intramolecular catalyst
transfer and may serve to suppress formation of the
bimolecular CC−Pd−CC complex that leads to
stable than the free state.13b
In the present study, we investigated why tBu3PPd(0) did not
undergo intramolecular transfer during polymerization of
monomers containing CC by examining the Suzuki−Miyaura
coupling reaction of dibromostilbene derivatives with aryl
boronic acid or boronic acid esters in the presence and absence
t
of additives containing CC. We found that Bu3PPd(0) on
t
intermolecular transfer of Bu3PPd(0).
the π-face of the substrate readily undergoes intermolecular
transfer to the CC of another substrate or additive molecule.
Moreover, we found that this intermolecular transfer of the
catalyst can be suppressed by introduction of alkoxy groups at
the ortho positions of the CC. To demonstrate the effec-
tiveness of this strategy, we employed it to achieve selective
disubstitution of α,ω-dibromo polystilbene with arylboronic
acid ester via intramolecular transfer of tBu3PPd(0) on the long
conjugated backbone, which contains CC.
π-Conjugated polymers have received much attention due to
their potential applications in thin film transistors (TFTs),1
organic light-emitting diodes (OLEDs),2 and photovoltaic
cells.3 Catalyst-transfer condensation polymerization (CTCP)
is a powerful methodology for synthesis of well-defined
π-conjugated polymers with controlled molecular weight and
low polydispersity.4 We have proposed that this polymerization
involves intramolecular catalyst transfer on the polymer back-
bone, and therefore a critical aspect of successful CTCP is
selection of a catalyst with high activity and a high propensity
for intramolecular catalyst transfer. In CTCP with palladium
Suzuki−Miyaura coupling reaction of dibromostilbene 1
with equimolar arylboronic acid or boronic acid ester 2 was
t
carried out in the presence of 5 mol % of Bu3PPd(o-tolyl)Br
3,14 which generates Bu3PPd(0) by reaction with 2, to see
t
whether or not the Pd catalyst undergoes intramolecular
transfer (Scheme 1), in a similar manner to that employed by
McCullough and co-workers to demonstrate intramolecular
transfer of a Ni catalyst in Kumada−Tamao coupling reaction
of 2,5-dibromothiophene with a Grignard reagent.15 In the
reaction of 1 with 2, the C−Br bond of 1 first undergoes
t
(Pd) catalysts, Bu3PPd(Ar)Br is widely used as a catalyst for
the synthesis of a well-defined polyfluorene,5 polyphenylene,6
polythiophene,7 poly(fluorene-alt-benzothiadiazole),8 and poly-
(p-phenyleneethynylene).9 We have attempted to utilize Suzuki−
Miyaura10 and Mizoroki−Heck11 CTCP with this Pd catalyst
for synthesis of well-defined poly(phenylenevinylene) (PPV),
which has been intensively investigated especially for applica-
tion to OLEDs,2,12 in order to extend the range of CTCP for
synthesis of not only aromatic conjugated polymers but also
aromatic and carbon−carbon double bond (CC) conjugated
polymers. However, the molecular weight distribution of the
t
oxidative addition to Bu3PPd(0), followed by transmetalation
t
with 2 and reductive elimination to regenerate Bu3PPd(0). If
the Pd(0) diffuses into the reaction mixture, the main product
will be monosubstituted 4 (Scheme 1A). On the other hand, if
the Pd(0) inserts into the other C−Br bond of 4 via intra-
molecular transfer, the main product will be disubstituted 5
(Scheme 1B). We determined the preferred behavior of PdPtBu3
by measuring the product ratio of 4 to 5. To avoid issues with
the solubility of products, an alkoxy side chain was introduced
into 1 or 2. Several derivatives 4 and 5 were separately prepared
t
obtained PPV was broad, and the aryl group of the Bu3PPd-
(Ar)Br catalyst was not introduced at the polymer end,
indicating that tBu3PPd(0) generated by the reaction of the Pd
catalyst with the monomer underwent intermolecular transfer.
These results imply that a CC bond in the monomer or in
the polymer backbone may disturb intramolecular transfer of
tBu3PPd(0) during polymerization.
On the other hand, van der Boom and co-workers demon-
strated that the reaction of Pt(0) or Ni(0) with a bromostilbene
derivative resulted in selective η2-CC coordination, which
was kinetically preferred at low temperature, followed by intra-
molecular “ring walking” of the metal center and intramole-
cular oxidative addition of the aryl-bromide bond, which was
1
to aid in determining the ratio of 4 to 5 by means of H NMR
spectroscopy (see Supporting Information (SI)).
The reaction of trans-4,4′-dibromostilbene 1a with m-iso-
butoxyphenylboronic acid 2a was first carried out to afford 4
as a main product, implying that intramolecular transfer of
tBu3PPd(0) on CC did not take place (Table 1, entry 1).
Received: March 31, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b03113
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
t
We propose that the intermolecular transfer of Bu3PPd(0)
Scheme 1. Proposed Mechanisms for the Model Reaction
Occurring through (A) Intermolecular and (B)
Intramolecular Transfer of the Pd Catalyst
might occur on the CC of 4. Thus, styrene would approach
the η2 coordination complex of the CC of 4 to Bu3PPd(0),
t
which is the most stable coordination structure, and then the
catalyst would move to the CC of styrene (Scheme 2). This
Scheme 2. Proposed Mechanism of Intermolecular Transfer
of PdPtBu3
proposed mechanism is supported by the occurrence of selec-
tive disubstitution through intramolecular transfer of the catalyst
on a dibromo substrate without CC, 2,5-dibromothiophene,
with 2b even in the presence of styrene (see SI).
However, if all CC compounds disturb intramolecular
t
Table 1. Model Reactions of 1 with 2
transfer of Bu3PPd(0), 1b would also have acted as an inhi-
a
bitor. Thus, we examined the effects of stilbene and stilbene 6
(Chart 1) substituted with alkoxy side chains at the same
entry
1
2
additive
4:5
1
2
3
4
5
6
7
1a
2a
2b
2b
2c
2b
2b
2b
−
−
−
−
64:36
0:100
42:58
66:33
60:40
41:59
0:100
1b
1b′
1b
1b
1b
1b
Chart 1. Chemical Structures of Additives 6−13
styrene
stilbene
6
a
1
Determined by H NMR spectra.
However, the reaction of tetraalkoxy-substituted 1b with
phenylboronic acid ester 2b exclusively yielded 5 (entry 2). A
control experiment using diphenylethane derivative 1b′, which
was prepared by hydrogenation of the CC of 1b, afforded
a mixture of mono- and disubstituted products (entry 3). This
position as in 1b, using the reaction of 1b with 2b. In the case
of stilbene as an additive, 4 and 5 were formed (entry 6),
whereas additive 6 did not affect the intramolecular transfer of
the catalyst, and 5 was formed exclusively (entry 7). These
results suggest that alkoxy side chains, which were introduced
into dibromostilbene to increase solubility, also happened to
t
result strongly suggested that Bu3PPd(0) walked from one
t
benzene ring to the other benzene ring through CC by virtue
promote intramolecular transfer of Bu3PPd(0).
of coordination of the π-electrons of 1b.
We next investigated what substituents at which positions on
stilbene are necessary to avoid disturbance of the intra-
molecular transfer of the Pd catalyst in the reaction of 1b and
2b, using a variety of stilbenes as additives (Chart 1).
To investigate suitable conditions for polymerization of
phenylenevinylene monomers, 4-vinylphenylboronic acid ester
2c instead of 2b was reacted with 1b, resulting in a change of
the major product to 4 (entry 4). Therefore, we considered that
the alkoxy side chains of 1b would play an important role in the
First, electrical substituent effects at the para position of
stilbene were examined by the use of 7−9 (position B of
Chart 1). In all cases, both 4 and 5 were formed, but the ratio
of 4 was decreased in the order from an electron-withdrawing
to electron-donating group (Table 2, entries 1−3), implying
t
intramolecular transfer of Bu3PPd(0) through CC, whereas
tBu3PPd(0) would diffuse into the reaction mixture from the
CC of unsubstituted 1a or even the styryl 2c unit in 4,
despite the alkoxy side chains. To test this hypothesis, reaction
of 1b with 2b was carried out in the presence of styrene. In this
case, the added styrene should not disturb intramolecular
transfer of the Pd catalyst, which would afford 5 exclusively.
However, in fact, the main product was 4 (entry 5), and thus it
turned out that the added styrene acts as an inhibitor of
intramolecular transfer of the Pd catalyst. This result indicated
that intermolecular transfer of the catalyst takes place not by
spontaneous diffusion of the catalyst into the reaction mixture,
but by bimolecular ligand exchange reaction of the catalyst.
t
that the affinity of CC of stilbene for Bu3PPd(0) is pre-
sumably governed by back-donation from Pd(0) to the π*
orbital of CC,13c,16 probably because the LUMO level of
stilbene is lower than that of aromatics. However, substituents
at the meta position (positions A and C of Chart 1) did not
influence the ratio of 4 to 5: the same ratio was seen with both
10 (CF3) and 11 (CH3), and the selectivity for 5 was higher
than that in the cases of p-substituted stilbenes (entries 4
and 5). Accordingly, intermolecular transfer of the catalyst to
stilbene derivatives seems to be governed by the steric character
B
DOI: 10.1021/jacs.5b03113
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Journal of the American Chemical Society
Communication
Table 2. Model Reactions of 1b with 2b in the Presence of
Additives
a
entry additive
A
B
C
D
4:5
1
2
3
4
5
6
7
7
H
H
H
CF3
H
H
H
H
H
H
H
55:45
31:69
25:75
22:78
22:78
32:67
0:100
8
CH3
OC3H7
H
H
9
H
10
11
12
13
CF3
CF3
CH3
H
CH3
OC3H7
H
H
H
H
H
OC3H7
a
1
Determined by H NMR spectra.
around the CC of stilbene, rather than the electronic effect of
substituents.
We then examined the effect of the position of the alkoxy
group of stilbene by using 9, 12, and 13, which have a propoxy
group at the para, meta, and ortho positions, respectively. Para-
and meta-alkoxystilbene 9 and 12 resulted in formation of
a mixture of 4 and 5 (entries 3 and 6), whereas the ortho-
counterpart 13 gave exclusively 5 (entry 7). These results
support the idea that alkoxy groups at the ortho position of
Figure 1. MALDI-TOF mass spectra of products of (a) α,ω-dibromo
polystilbene (Mn = 4010, Mw/Mn = 1.66), (b) polystilbene (Mn
=
4290, Mw/Mn = 1.65) obtained by reaction of α,ω-dibromo poly-
stilbene with 14 in the presence of CsF/18-crown-6 and 10 mol %
P(tBu)3PdG2 in THF/H2O at room temperature for 7.5 h, and (c)
polystilbene (Mn = 6650, Mw/Mn = 1.60) obtained by reaction of α,ω-
dibromo polystilbene with 14 under the same conditions of (b) in the
presence of 10 mol % stilbene.
t
CC of stilbene suppress intermolecular transfer of Bu3PPd-
(0). This behavior might be attributed to difficulty in the ligand
exchange reaction of the η2 coordination Pd complex of 4 with
the CC of stilbene 13 by virtue of steric hindrance of the
alkoxy groups of 4 and 13 in the transition state (Scheme S2).
Consequently, the failure of Suzuki−Miyaura CTCP of phenyl-
enevinylene monomers10 is probably accounted for by insuffi-
cient steric bulkiness around the CC of the monomers.
Since we had established the structural requirement for
Scheme 3. Model Reaction of α,ω-Dibromo Polystilbene
with Equimolar 14
t
intramolecular transfer of Bu3PPd(0) on the CC, we next
examined intramolecular transfer on longer conjugated systems
containing CC. First, the same reaction was carried out with
alkoxy-substituted oligophenylenevinylene dibromide S3 having
two CC; this afforded the corresponding disubstituted com-
pound S5 exclusively (Scheme S3). We next examined intramo-
t
lecular transfer of Bu3PPd(0) on a π-conjugated polymer with
multiple CC. For this purpose, α,ω-dibromo polystilbene
was prepared by means of A2 + B2 type Suzuki−Miyaura poly-
condensation of dibromo monomer 1b and the corresponding
diboronic acid ester monomer having alkoxy side chains at the
same position as in 1b.
transfer of the Pd catalyst, the MALDI-TOF mass spectrum of
the products was dramatically changed, and the Br/Tolyl peaks
became major (Figure 1c). This result also supports the
occurrence of intramolecular transfer of the Pd catalyst on the
long conjugated polystilbene backbone in the Suzuki−Miyaura
coupling reaction of α,ω-dibromo polystilbene with 14.
The structural requirements for intramolecular transfer of
tBu3PPd(0) on CC were established by examination of the
Suzuki−Miyaura coupling reaction of dibromostilbenes with aryl
boronic acid esters as a model reaction of CTCP. Substituent
groups at the ortho position of CC are critical for selective
intramolecular transfer of the catalyst; they may serve to suppress
the formation of the bimolecular CC−Pd−CC complex
that leads to intermolecular transfer of PdPtBu3. We believe the
present findings will be useful in the design of CTCP reactions of
monomers containing CC and also represent an advance in
our understanding of organic/organometallic chemistry.
The matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrum of the obtained polystilbene
showed that it contained mainly bromines at both ends
(designated as Br/Br) (Figure 1a and SI). The number-average
molecular weight (Mn) was estimated to be 4000 by means of
gel permeation chromatography (GPC). The reaction of α,ω-
dibromo polystilbene with equimolar tolylboronic acid ester 14,
the amount of which was roughly determined based on the Mn
value of polystilbene, was carried out in the presence of 10 mol %
17
t
of P(tBu)3PdG2, which generates Bu3PPd(0) with a base,
and CsF/18-crown-6 for 7.5 h (Scheme 3). The MALDI-TOF
mass spectrum of the products contained two major series of
polystilbene molecules with Br/Br and Tolyl/Tolyl ends, as
well as a minor series of Tolyl/Br peaks (Figure 1b). Therefore,
it turned out that selective disubstitution with 14 occurred
through intramolecular transfer of PdPtBu3 on the longer conju-
gated π-faces including CC of polystilbene (10 benzene rings
and 5 CC’s are included in polystilbene with an Mn of 4000).
When the same reaction was carried out in the presence of
10 mol % stilbene as an additive to disturb intramolecular
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Detailed experiment and H NMR, 13C NMR, MALDI-TOF
MS. The Supporting Information is available free of charge on
the ACS Publications website at DOI: 10.1021/jacs.5b03113.
C
DOI: 10.1021/jacs.5b03113
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
AUTHOR INFORMATION
Corresponding Author
*yokozt01@kanagawa-u.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported by a Grant in Aid (No. 24550141)
for Scientific Research from the Japan Society for the
Promotion of Science (JSPS) and by the MEXT-Supported
Program for the Strategic Research Foundation at Private
Universities, 2013−2018.
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DOI: 10.1021/jacs.5b03113
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