Living Cyclocopolymerization through Alternating Insertion of Isocyanide and Allene via Controlling the Reactivity of the Propagation Species: Detailed Mechanistic Investigation

Article abstract of DOI:10.1021/jacs.9b07431

Living cyclocopolymerization through the alternating insertion of an isocyanide and allene into palladium-carbon bond was developed based on the controlling the reactivity of the propagation species using bidentate ligands. We revealed that the rate of the presented cyclocopolymerization was depended on the ligands of Pd-initiator. When the palladium-methyl complexes having appropriate cis-chelating ligand, such as 1,3-bis(diphenylphosphino)propane (dppp), were used as initiator, the cyclocopolymerization of bifunctional aryl isocyanides (1) that contain both isocyano and allenyl moieties polymerized to afford poly(quinolylene-2,3-methylene)s with controlled molecular weight and narrow molecular weight distributions. The resulting polymer was characterized by ¹H and ¹³C NMR analyses, which clearly showed that the terminal moiety of the polymer formed well-defined organopalladium complex as the resting state for the polymerization, which could undergo further polymerization; not only cyclocopolymerization with 1 but also homopolymerization of simple aryl isocyanide. In the analysis of the cyclocopolymerization mechanism, we conclusively demonstrated that the insertion reaction of isocyanide is the rate-determination step in the cyclocopolymerization, which proceeds via a five-coordinate intermediate with a geometrical change. The cis-chelating ligand controls the site interchange reaction, which dominates the reactivity of propagation species.

Full text of DOI:10.1021/jacs.9b07431

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Article
Living Cyclocopolymerization Through Alternating Insertion
of Isocyanide and Allene via Controlling the Reactivity of
the Propagation Species: Detailed Mechanistic Investigation
Naoya Kanbayashi, Taka-aki Okamura, and Kiyotaka Onitsuka
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07431 • Publication Date (Web): 02 Sep 2019
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Journal of the American Chemical Society
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Living Cyclocopolymerization Through Alternating Insertion of
Isocyanide and Allene via Controlling the Reactivity of the
Propagation Species: Detailed Mechanistic Investigation
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Naoya Kanbayashi,* Taka-aki Okamura, and Kiyotaka Onitsuka*
Department of Macromolecular Science Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
ABSTRACT: Living cyclocopolymerization through the alternating insertion of an isocyanide and allene into palladium–carbon
bond was developed based on the controlling the reactivity of the propagation species using bidentate ligands. We revealed that the
rate of the presented cyclocopolymerization was depended on the ligands of Pd-initiator. When the palladium–methyl complexes
having appropriate cis-chelating ligand, such as 1,3-bis(diphenylphosphino)propane (dppp), were used as initiator, the
cyclocopolymerization of bifunctional aryl isocyanides (1) that contain both isocyano and allenyl moieties polymerized to afford
poly(quinolylene-2,3-methylene)s with controlled molecular weight and narrow molecular weight distributions. The resulting
1
13
polymer was characterized by H and C NMR analyses, which clearly showed that the terminal moiety of the polymer formed well-
defined organopalladium complex as the resting state for the polymerization, which could undergo further polymerization; not only
cyclocopolymerization with 1 but also homopolymerization of simple aryl isocyanide. In the analysis of the cyclocopolymerization
mechanism, we conclusively demonstrated that the insertion reaction of isocyanide is the rate-determination step in the
cyclocopolymerization, which proceeds via a five-coordinate intermediate with a geometrical change. The cis-chelating ligand
controls the site interchange reaction, which dominates the reactivity of propagation species.
Introduction
afford various types of polyisocyanides, and these polymers
have been developed in many fields. Previously, we studied
5
The precise design of polymer structures has received
cyclocopolymerization systems involving the intramolecular
alternating insertion of isocyanides and unsaturated
hydrocarbons into a palladium–carbon bond using bifunctional
aryl isocyanides 1 bearing unsaturated hydrocarbons at the
significant attention because many factors influence the
chemical and physical properties the primary structure and its
constituent units, associated molecular weights, and the final
structure of the polymer. The development of precise synthesis
of novel polymer architectures is an important subject in
polymer chemistry. Cyclopolymerization of bifunctional
monomers is one of the efficient method of synthesizing new
6
7
ortho-position. Recently, we focused on an allene as the
unsaturated molecules during our polymerization, and aryl
isocyanide containing allenyl moiety at the ortho-position was
8
1
designed and synthesized. The molecules polymerized rapidly
types of polymer materials. The resulting polymer has a cyclic
3 2
within 10 min in the presence of (PPh ) PdMeCl through
completely alternating insertions of the isocyanide and allene
moieties into the palladium–carbon bond with perfect head-to-
structure in the backbone, exhibiting unique properties
compared to those of the corresponding linear polymers. To
date, various precise cyclopolymerization systems have been
developed using bifunctional monomers with unsaturated
hydrocarbons as substituents, such as non-conjugated dienes
tail selectivity. This is
a
novel example of
cyclocopolymerization involving completely different types of
substituents. The resulting polymers, poly(quinolylene-2,3-
methylene)s, are completely new architecture and hold potential
for a new polymer design. However, in this reaction system, the
molecular weight distribution of the resulting polymer is
2
and divinyl compounds. In most cyclopolymerization systems,
the same type of reactive site is used as a substituent on the
bifunctional monomer, resulting in mechanistically similar
reactions successively proceeded. Cyclocopolymerization via
the combination of different types of substituents by an
alternating intra- and intermolecular mechanisms is challenging
and has therefore hardly been achieved, despite the expanding
w n
relatively broad (M /M = 1.42–1.70), and control of the
polymerization reaction has not been achieved (Scheme 1). This
is because the propagation reaction is quite faster than the
initiation reaction, and the structure of the propagation species
is unclear due to the instability of the terminal palladium
complex, leaving the detailed reaction mechanism unclarified.
In order to control the polymerization reaction, it is necessary
to determine the terminal structure and mechanism, and to
regulate the reactivity of the propagation species.
3
range of polymer designs. Controlling the reactivity of each
substituent is generally difficult, which prevents intermolecular
homopolymerization and promotes the alternate reaction.
Additionally, it is necessary to regulate head-to-tail selectivity.
Isocyanide is an important monomer in polymer
chemistry, which is known to undergo polymerization in the
Scheme 1. Cyclocopolymerization Based on Alternating
Insertion of Isocyanide and Allene
presence of an organometallic catalyst through multiple
4
isocyanide insertions into
a
metal-carbon bond.
Polymerization systems with isocyanides have been designed to
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Page 2 of 13
wide variety of polymer design, such as block copolymers, via
the well-defined terminal structure. Herein, we report a novel
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living polymerization based on our cyclocopolymerization
system involving the alternating incorporation of isocyanides
and allene moieties. When the appropriate bidentate ligands
were used, the reactivity of the propagation species was
controlled to achieve living polymerization, and a well-defined
palladium complex was remained at the growth end. The
resulting propagation species maintained its reactivity, which
can be utilized for the block copolymerization. In the latter part
of this paper, the polymerization mechanism was investigated,
and we conclusively demonstrated that insertion reaction of
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isocyanide
is
rate-determination
step
in
the
cyclocopolymerization, which proceeds via the five-coordinate
intermediate with a conformational change to the appropriate
configuration depending on the bidentate ligands.
In the past decades, living polymerization systems have
been developed, which allow not only to control the molecular
weight and molecular weight distributions but also to realize a
Table 1. Cyclocopolymerization of Monomer 1 with Pd Complex Bearing Various Ligands a
b
entry
1
2
[1] / [2]
time
min.)
conv.
(%)
M
n
(DP
n
)
w n
M /M
(
Mn(SEC)
7800
7500
6700
3200
7800
6900
2700
6900
7500
7500
12 000
18 000
7200
7100
7500
b
Mn(NMR)
c
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
2a
2b
2c
2e
2e
2e
2c
2d
2d
2f
50
50
50
50
50
30
15
30
30
30
30
30
30
30
30
< 10
< 10
< 10
30
>99
>99
>99
45
15 200 (51)
15 000 (51)
15 000 (51)
-
1.44
1.52
1.65
1.11
1.15
1.06
1.20
- e
150
120
90
>99
>99
>99
10
15 000 (51)
9800 (30)
5200 (16)
- e
120
900
120
120
120
120
30
>99
96
- e
1.72
1.10
1.56
2.20
4.29
1.06
1.06
1
1
1
1
1
1
0
1
2
3
4
5
9500 (29)
- e
2g
2h
2i
96
82
- e
>99
>99
>99
- e
2j
10 000 (31)
10 000 (31)
2k
30
a
b
c
[
2 2
1] = 0.1 M ([1]/[2] = 30) in CH Cl (5.0 mL) at 25 °C , [1]/[2] = 30 or 50, Determined by SEC using polystyrene standards.
1
e
Determined by H NMR spectroscopy based on the terminal methyl group. The polymerization degrees (DP) is shown in parenthesis. Not
Determined.
Results and Discussion
chromatography (SEC) in CHCl
calibration. When mono phosphines, PPh
2a-2c) were used as a ligand, polymerization was completed
3
with polystyrene standard
3
, PPh Me, and PEt
2
3
Living Cyclocopolymerization Through Alternating
Insertion of Isocyanide and Allene. Initially, the
(
within only 10 min at 25 °C, and poly(quinolylene-2,3-
methylene)s (poly-1a-(2a–2c)) were obtained with broad
molecular weight distributions (M /M ) (entries 1–3). The use
w n
of cis-chelating ligands affected the polymerization reaction
significantly. When 1,3-bis(diphenylphosphino)propane (dppp)
was used as a ligand (2e), the reaction slowed down and the
resulting polymer exhibited a narrower molecular weight
polymerization of aryl isocyanide 1a (R
=
2-
ethylhexyloxycarbonyl) that contains isocyano and allenyl
moieties in the presence of several palladium complexes
LPdMeCl (2) with different types of ligands was examined in
2 2
CH Cl ([1] = 0.1 M, [1]/[2] = 50) at 25 °C (Table 1). The
resulting polymers were analyzed by size-exclusion
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distribution (M
w
/M
n
= 1.15 in entry 5) than poly-1a-(2a–2c).
resulting polymers were in good agreement with the ideal M
n
values calculated from [1]/[2] feed ratio, suggesting that all of
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
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2
2
2
2
2
3
3
3
3
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3
3
3
3
3
4
4
4
4
4
4
4
4
4
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5
5
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5
5
5
5
5
5
5
6
The resulting polymer was characterized by NMR spectroscopy
1
(Figure S9, see the Supporting Information (SI)). The H NMR
spectrum of poly-1a-(2e) exhibited signals associated with the
quinolylene-2,3-methylene backbone [δ 8.34 (s, 1H), 7.78 (s,
the initial palladium complex participated in the polymerization
1
5
(Figure 1). The Mn(SEC) values of poly-1b-(2), which were
calculated by size-exclusion chromatography (SEC) using
polystyrene standards, were different from the Mn(NMR) values,
although they exhibited a linear relationship. This phenomenon
was attributed to the difference in hydrodynamic radius
1
H), 7.62 (d, 1H), 7.41 (d, 1H)] that was generated by the
alternating insertion of isocyanide and allene, and the allenyl
protons of 1a [δ 6.53 (t, 1H), 5.33 (d, 2H)] had been clearly
consumed. The results indicate that the cyclocopolymerization
of 1a also proceeded via the alternating insertion of isocyanide
and allene when using dppp as a ligand. The molecular weight
(Mn(NMR)) of the resulting polymer was calculated from the H
NMR signal of the terminal methyl group and methylene
moieties on the backbone (Mn(NMR) = 15 000, n = 51), and was
in good agreement with the ideal M value calculated on the
n
8
1
between polystyrene and poly-1b-(2). Interestingly, in the H
NMR spectra of poly-1b-(2e), several small peaks assigned to
a quinolylmethyl-palladium complex were clearly observed
(Figure 2a and S10). The phosphine ligand (f-H, h-H, and g-H)
and methylene protons (a-H) adjacent to the palladium were
clearly observed, and two doublet peaks derived from dppp
ligand of the terminal palladium complex were also recorded in
0
1
2
3
4
5
6
7
8
9
0
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2
3
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5
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7
8
9
0
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9
0
1
3
1
basis of [1]/[2] ratio of 50. When 1b (R = decyloxycarbonyl)
and 1c (R = l-menthyl) were used as a monomer ([1] = 0.1 M,
P NMR spectrum (Figure 2b). Additionally, several units
1
adjacent to the Pd complex can be clearly distinguished by H
NMR (b-H, c-H, d-H). These peaks were assigned by H 2D-
1
0
1
1
b; [1]/[2] = 30, 1c; [1]/[2] = 15), the polymerization also
proceeded to yield poly-1b-(2e) and poly-1c-(2e) with narrow
/M (entries 6 and 7). Subsequently, other types of cis-
1
13
ROESY, and H, C-HSQC experiments of the resulting
polymers (Figure S17 and S18). The corresponding methylene
protons (b-H, c-H, and d-H) were assigned. These results
clearly indicate that palladium complex remained on the
terminal moiety of the polymer. This propagation complex
survives quantitatively based on a comparison of the integral of
M
w
n
1
1
chelating phosphine ligands with different bite angles were
investigated using the conditions in entry 6. The
cyclocopolymerization of 1 was affected by the bite angle.
When 1,3-bis(diphenylphosphino)ethane (dppe) (2d), which
has a narrower bite angle (85°) than 91° of 2e, was used as the
ligand, the polymerization rate markedly decreased with
increasing the reaction time and the molecular weight
distribution became broad (entries 8 and 9). In the case of 1,3-
bis(diphenylphosphino)ferrocene (dppf) (2f) which has larger
bite angle (95°) than that of 2e, a similar M
obtained with 2e (M /M = 1.10, entry 10), whereas, when the
complexes of 1,3-bis(diphenylphosphino)butane (dppb) (2g),
which has much larger bite angle (98°) was used, the molecular
weight distribution was broad (M /M = 1.56, entry 11). On the
w n
other hand, even when ligand with a similar bite angle to that of
2e but more bulky substituents on phosphorus or twist spacer of
cis-chelating
propylphosphino)propane
bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) (2i; 92°),
were used, the molecular weight distributions were broad
1
2
1
both terminal signals in the H NMR spectrum. Similar results
were also found in the cases of 2f and 2j (Figure S14 and S15).
Table 2. Cyclocopolymerization using Pd Complex 2e and
a
2j with the Different Initial Feed Ratios of Monomer 1b
w
/M was also
b
w
entry
2
[1b]/[2]
time
(min.)
M
M
n
(DP
n
)
w n
M /M
b
c
n(SEC)
M
n(NMR)
1
2
3
4
5
6
7
8
2e
2e
2e
2e
2j
2j
2j
2j
10
20
30
50
10
20
30
50
30
2100
4800
6900
13 000
2700
5100
7100
12 200
3400 (11)
7200 (22)
9800 (30)
17 000 (53)
3600 (10)
7200 (22)
10 000 (31)
17 300 (53)
1.20
1.09
1.06
1.15
1.16
1.08
1.06
1.06
90
120
150
30
ligand,
for
(d(i-Pr)pp)
example,
(2h)
1,3-bis(di-i-
or 2,2´-
30
30
(entries 11–13). These results indicate that not only the simple
40
bite angle of the ligand but also appropriate direction of the
phosphorous lone pair electrons toward the metal is crucial for
achieving narrow molecular weight distributions in the
cyclocopolymerization of 1. Next, we examined cis-chelating
nitrogen ligands. When 2,2´-bipyridine (2j) was used, the
polymerization reaction completed in 30 min, and the desired
a
b
[
1] = 0.1 M in CH
2
Cl
2
(5.0 mL) at 25 °C. Determined by SEC
Determined by H NMR
c
1
using a polystyrene standard.
spectroscopy based on the integral intensity of the terminal methyl
group. The polymerization degrees (DP) is shown in parenthesis.
polymer with a narrow molecular weight distribution (M
w n
/M =
1
3
1
.06) was obtained (entry 14), even though the bite angle (78°)
of 2j is smaller than that of 2d. A similar trend was also
observed in other rigid bidentate nitrogen ligands, such as
bis(arylimino)acenaphthene ligands (2k), which was used for
olefin polymerization (entry 15). The differences in the
tendency of the polymerization between phosphorous and
nitrogen ligands can be attributed to the coordination ability or
14
the trans effect.
The polymerization of 1b initiated by 2e (L = dppp) or 2j
(L = bpy) at different initial feed ratios ([1]/[2] = 10–50) was
examined (Table 2) under the conditions described above. In
all cases, after consumption of the monomer, all the polymer
exhibited a monomodal elution peak with a narrow molecular
weight distribution. Furthermore, the Mn(NMR) values of the
n w n
Figure 1. Relationship between M , M /M and feed ratio of
monomer 1 to initiator (a) 2e and (b) 2j in polymerization
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Because the living polymer presented herein contains a
Pd-σ alkyl bond at the growing polymer end, further
1
2
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5
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9
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1
1
1
1
1
1
1
1
1
2
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1
6
polymerization using a different monomer was conducted.
Thus, we attempted the block copolymerization of isocyanide
using poly-1b20-(2j) (Mn(SEC) = 5100, M /M = 1.09) as an
w
n
initiator (Scheme 2). According to a general method, the block
polymerization of menthyl-4-isocyanobenzoate 3 (20 equiv)
was performed in THF at 55 °C. Unfortunately, the
polymerization did not proceed, because the bpy ligand of 2j
1
7
was likely replaced by the excess isocyanide. Therefore, a
ligand exchange reaction was conducted using phosphine that
was previously in the polymerization of isocyanide. In the
presence of excess triphenylphosphine (5 equiv), the desired
block copolymer (poly-1b20-b-320) was successfully formed
with a narrow molecular weight distribution. The prepared
block copolymer exhibited a unimodal SEC trace (Figure 4),
0
1
2
3
4
5
6
7
8
9
0
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2
3
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5
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2
3
4
5
6
7
8
9
0
1
and the corresponding H NMR spectrum contained the
expected signals (Figure S16).
Scheme 2. Block Copolymerization with Isocyanide 3
Figure 2. (a) H NMR (400 MHz) and (b) 31P NMR spectra of poly-
1
1
b-(2e) ([1b]/[2e] = 20, entry 2 in Table 2 in CDCl
3
.
To investigate the living nature of the polymerization,
stepwise addition of the monomer was conducted. A five-step
addition of 10 equiv of 1b to a dichloromethane solution of
initiator (2e or 2j) resulted in the stepwise formation of the
corresponding polymers. The polymers exhibited a narrow
molecular weight distribution and a linear relationship between
n
M and cumulative addition of 1b (Table S1 and Figure 3).
These results indicated that each propagation chain end group
of poly-1b-(2e) or poly-1b-(2j) remained active after the
monomer was depleted, and all polymer chains grew at the same
rate without chain transfer and termination. Therefore, the
system is a living polymerization.
Figure 4. SEC curves of poly-1b20-(2j) and poly-1b20-b-320
.
Investigation of the Polymerization Mechanism
Kinetic Study of the Polymerization. n previous studies
of the cyclocopolymerization system, the polymerization
kinetic information was not obtained at all, because the
propagation reaction was too fast. To gain further insight into
Figure 3. (a) and (b) SEC trace of poly-1b following each addition
step using 2e and 2j as an initiator. (c) and (d) M (SEC) as a
n
function of the cumulative addition of 1b using 2e and 2j as an
the polymerization mechanism, we performed
a trace
experiment on the polymerization of 1b with 2e ([1] = 0.06 M,
1
[
2 2
2] = 0.002) in CD Cl at 25 ˚C using H NMR spectroscopy.
initiator.
When the concentration of 1b was plotted against reaction time,
a linear relationship was obtained, which indicated that the rate
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Journal of the American Chemical Society
–
of the present polymerization was independent of the
anion forms C-H···Cl hydrogen bonds from methylene and
phenyl protons of the dppp ligand, which is closed to the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
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5
5
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5
5
5
5
5
6
concentration of monomer 1b; the rate was found to be zero
order with respect to monomer concentration (Figure 5a, kobs(25
–
platinum center in the square plane without a Pt-Cl bond (dPt-
Cl– ≈ 4.3 Å). This result implies that the downfield shifts of the
methylene protons in 4e and 4e-Pt are caused by the C-H···Cl
hydrogen bonds, and the Cl anion is also located in the binding
-5 -1
°C) = 1.3 × 10 s ). Similar experiments were also conducted
using 2j. The zero-order polymerization of 1b using 2j also
–
-5
-1
–
proceeded at 10 °C (Figure 5b, kobs(10 °C) = 2.1 × 10 s ).
These results were in sharp contrast with the first-order kinetics
observed for the polymerization of aryl isocyanides with
3
pocket created by four protons of the dppp ligand in CDCl .
Additionally, the downfield shift of the methylene protons of
the dppp ligand is supported by the simulation of the chemical
shifts using DFT calculations based on the crystal structure of
4e-Pt (Figure S29).
4
i,4l
organopalladium complexes. Additionally, the rate constants
for polymerization were determined at several temperatures
(Table S2), and the thermodynamic activation parameters were
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
calculated from the Arrhenius plots of the rate constants. The
‡
-1
thermodynamic activation parameters are ΔH = 56.2 kJ mol ,
Scheme 3. Stoichiometric Reaction of 1d with (a) 2e and (b)
2e-Pt
‡
-1
-1
‡
-1
ΔS = –150.0 J K mol , and ΔG = 100.9 kJ mol at 25 ˚C on
‡
-1
‡
-1
-1
‡
2e and ΔH = 58.1 kJ mol , ΔS = –129.1 J K mol , and ΔG
= 94.6 kJ mol at 10 ˚C on 2j.
-1
Figure 5. Plots of the concentration of 1b. Initial concentration: [1]
= 0.06 M, [2] = 0.002 M; CD
= 2j).
2 2
Cl , (a) 25 °C (2 = 2e), (b) 10 °C (2
Stoichiometric Reaction. To understand the successive
insertion mechanism of the isocyanides and allenes, a series of
stoichiometric reactions was performed. Initially, 1d (R = 2-
methyloxycarbonyl) was allowed to react with 1.4 equiv of
3
Pd(dppp)MeCl (2e) in CDCl at –50 °C (Scheme 3a), which was
1
monitored by NMR spectroscopy. In the H NMR spectra, the
signals of 1d were quantitatively consumed, and new signals
arising from the coordinated isocyanide appeared (Figure 6c).
The resulting isocyanide-coordinated complex 4e was stable at
–
50 °C, and no further reaction was observed. The methylene
protons (h-H and f-H) of the phosphine ligand (–CH CH PPh
.19–3.12 ppm) are present in a lower magnetic field region
2
2
2
,
3
than those of other four-coordinate complexes bearing dppp [2e
(2.47 and 2.33 ppm)]. Yamamoto and co-workers reported five-
coordinate nickel complexes with isocyanide, chloride, and
1
8
dppp, formulated as [NiCl(dppp)(3,5-Me
and the chemical shifts of the methylene protons on the
phosphine ligand (–CH CH PPh , 3.21 ppm) are similar to
2 6 3 2 6
C H NC) ](PF ),
2
2
2
those of 4e. To determine the geometry of 4e experimentally, a
similar stoichiometric reaction was carried out using a more
inert platinum complex (2e-Pt: Pt(dppp)MeCl) (Scheme 3b).
The isocyanide-coordinated 4e-Pt could be prepared in a
quantitative yield by combining 2e-Pt (1.0 equiv) with 1e (1.0
2 2
equiv) in CH Cl at room temperature, and the resulting 4e-Pt
1
was stable at room temperature. The H NMR spectrum of 4e-
Pt resembles that of 4e showing a characteristic downfield shift
of the methylene protons of the phosphine ligand (–
2 2 2
CH CH PPh , 3.34-3.25 ppm) like that of 4e (Figure 6d).
Furthermore, X-ray quality single crystals were obtained by the
slow diffusion of diethyl ether into a chloroform solution. The
molecular structure of 4e-Pt is shown in Figure 7. This structure
–
has a square planar geometry around the Pt center. The Cl
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and S19–S23). The initial reaction, the insertion of isocyanide,
-4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
followed a first-order kinetic with a rate constant of 8.8 × 10
-1
s with respect to the concentration of 4e. At low conversions
of 4e, iminoacyl complex 5e and quinolylmethyl-palladium
complex 6e were observed (Figure 8b). Because 5e is consumed
as soon as it is produced, the insertion reaction of isocyanide is
the rate-determining step of the successive insertion reactions
of isocyanide and allene (Figure S23).
Scheme 4. Consecutive Insertion Reactions of Isocyanide
and Allene
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
Figure 6. H NMR (400 MHz) spectra of (a) 1d and (b) 2e in CDCl
3
3 3
at 25 °C, (c) 4e in CDCl at –50 °C, and (d) 4e-Pt in CDCl at
25 °C.
Figure 7. X-ray structure of the 4e-Pt complex viewed from (a) the
top, and (b) the side of the square plane. (c) Expansion of the region
relevant to the C-H···Cl hydrogen bonds with dppp. The
schematic drawings indicates selected bond distances (Å) of C-
–
–
H···Cl hydrogen bonds.
Upon warming the reaction mixture in Figure 6c, up to –
30 °C, the succeeding reactions of 4e slowly proceed (Scheme
4a), which was monitored by H NMR spectroscopy (Figures 8,
1
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reactions. In the initiation step, the isocyanide-coordination
1
2
3
4
5
6
7
8
9
complex 4 was rapidly formed from the reaction of initiator 2
with 1, and subsequently, the insertion reaction of isocyanide
proceeds to form iminoacyl complex 5 (Scheme 5a). Generally,
it has been proposed that the insertion of an isocyanide into Pd–
C
bonds involves the formation of
a five-coordinate
intermediate that facilitates the migratory insertion of
1
9
isocyanide (Scheme 5c). Based on of the stoichiometric
reaction (Scheme 4b), chloride as a coordinating anion is
needed to promote the insertion of isocyanide 1, allowing the
cyclocopolymerization to proceed via the five-coordinate
intermediate for isocyanide insertion. Presumably, after the
formation of the five-coordinate complex 4-Cl via coordination
of the chloride anion, conformational rearrangements involving
pseudorotation are required to achieve a suitable configuration,
such as trigonal bipyramidal, for alkyl migration to the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
0
coordinated isocyanide. Additionally, the polymerization
reactions were dependent on the type of phosphine ligands (vide
supra). These differences can be attributed to the
conformational rearrangements of 4-Cl required for isocyanide
insertion. For the monophosphine ligands, because the
conformational change is rapid, the propagation reaction cannot
be controlled. In contrast, because the rigid cis-chelating
phosphine ligands prevent ligand site-interchange reactions, the
reaction rate decreased. The ligands dppp and dppf exhibit a
much more flexible backbone and larger bite angle than those
of dppe, which likely allowed the site-interchange reaction to
occur, resulting in successful polymerization. The same trend
has also been reported in the context of CO insertion into an
organopalladium complex containing bidentate phosphine
1
Figure 8. H NMR spectra of stoichiometric reaction of 2e with 1d
producing 4e (a) at –50 °C in CDCl (identical to Figure 6c), (b)
3
21
ligands. However, when bpy was used as the ligand, the
after warming up to –30 °C for 30 min., and (c) 130 min.
polymerization proceeded smoothly, despite the narrower bite
2
2
angle in bpy (81°) than in dppe. This result suggests that the
successive insertion proceeds via a different mechanism in the
case of bpy. The insertion reaction of isocyanide likely occurs
via palladium–nitrogen bond dissociation and subsequent
Similar stoichiometric experiments were performed
using other palladium complexes. When the ionic complex
Pd(dppp)Me(MeCN)](BAr
F
–
F
F
[
4 4
) (2e') having BAr [Ar = 3,5-
(
CF ] as a non-coordinating counter anion instead of
3 2 6 3
) C H
2
3
conformational rearrangements of 4.
chloride was allowed to react with 1d at –50 °C, isocyanide
coordination complex 4e' was also formed (Scheme 4b);
however, the successive insertion reaction did not proceed at –
0 °C. Upon heating warming to –10 °C, 4e' slowly
decomposed, resulting in no insertion products. To reveal the
role of the chloride anion, a stoichiometric reaction to that
After the insertion of isocyanide, the iminoacyl complex
5 was formed. Subsequently, intramolecular insertion of the
allene moiety proceeded to form 6. Because the allene moiety
insertion is quite rapid compared to the isocyanide insertion
step, the reaction details remain unclear. Therefore, DFT
calculations were performed using B3LYP to gain further
3
shown in Scheme 4a was performed in the presence of NBu
1.4 equiv), but the reaction proceeded similarly without
4
Cl
(
2
4
changes in the reaction rate. Thus, it can be concluded that the
weakly interacting chloride anion of 4e, located near the
palladium center, is necessary to promote the insertion reaction.
For the monophosphine complexation 2c with 1b (Scheme 4c),
coordination complex 4c was also formed at –50 °C. When the
reaction temperature was increased to –30 °C, polymerization
proceeded slowly to produce the corresponding polymer (poly-
insight into the reaction mechanism of allene insertion. The
detailed procedures and the optimized structures are shown in
the Supporting Information (Figure S29). Initially, the model
structure of 4e (L = dppp) was constructed based on the X-ray
structure (4e-Pt). The iminoacyl model 5e was composed from
4e and subsequently optimized. Based on the optimized
structure of 5e, suitable constraints to constract the palladium-
allene coordination model (7e; L = dppp, R′ = Me), followed by
distance constraints to form the quinolyl moiety, resulted in
reasonable molecular structure. Finally, the removal constraints
1
w n
b-(2c); Mn (SEC) = 8500, M /M = 1.46). These results suggest
that 1b, generated by the dissociation from 2c, preferentially
reacted with the propagation chain end. The insertion reaction
of 1b with the resulting quinoline-methyl complex bearing
trans-phosphorus ligands 6c or poly-1b-(2c) was much faster
than that the reaction with 2c.
and structural optimization yielded 8e (L = dppp, R′ = Me) with
3
η
coordination of the quinolyl methylene moiety to the
palladium center, which was more stable than the optimized
structure of 4e (E8e–4e = –63.9 kcal/mol). The Pd–C(3) bond
(2.14 Å) is much shorter than Pd–C(1) (2.49 Å) and Pd–C(2)
Reaction Mechanism. The initiation and propagation
mechanisms of the above experiments are summarized in
Schemes 5a and 5b. In this cyclocopolymerization system, the
insertion reaction of isocyanide was determined to be the rate-
determining step of the successive isocyanide and allene
1
(2.31 Å), which may be due to the contribution of an η structure
1
(Figure 9a). Furthermore, the η -quinolyl methyl complex 6e,
formed by moving the chloride anion of 8e to form a square
planar complex, is slightly more stable compared to 8e (E =
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4
.8 kcal/mol) (Figure 9b). These results suggest that the consistent with the finding that the rate of the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
insertion of the allene occurs in coordination complex 7, and the
insertion reaction subsequently proceeds rapidly to form the
more stable η coordination complexes 8 followed by complex
6
cyclocopolymerization shows a zero-order dependence on the
monomer concentration and results in a negative entropy
activation value (Figure 5). After consumption of 1, the
3
25
(Scheme 5a).
chloride anion coordinates to the palladium center, resulting in
3
1
1
η –η isomerization to form η -quinolyl methyl complex (poly-
(n+1)). The complex represents the resting state for the
During the polymerization, excess isocyanide easily
1
3
1
coordinates to the vacant site produced by η –η isomerization
poly-1(n)) to form a square planar isocyanide-coordination
cyclocopolymerization, which corresponds to the well-defined
(
terminal structure of the resulting polymers (poly-1-(2)).
complex (poly-4(n)-(2)). Further polymerization occurs via a
five-coordinate intermediate with a geometrical change, as
shown in the initiation mechanism (Scheme 5b). This is
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 5. Proposed Mechanism in Cyclocopolymerization
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/M ) of the polymers were determined by size-exclusion
(M
w
n
1
2
3
4
5
6
7
8
9
chromatography (SEC) in chloroform at 40 °C with polystyrene
gel column [Tosoh; TSKgel GMHHR-M × 3 (exclusion
molecular weight = 4 × 10 ); flow rate 0.7 mL min ] connected
to Shimadzu LC6-AD and Shimadzu SPD-10A UV-vis
detectors. IR spectra were recorded on SHIMADZU IR
Prestige-21 using KBr tablet.
6
-1
Standard Method of the Cyclocopolymerization. To
a dichloromethane solution (3.0 mL) of 2e (0.01 mmol) was
added the dichloromethane solution (2.0 mL) of 1b (97.8 mg,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
.3 mmol) at 0 °C. The mixture was warmed to 25 °C and stirred
for 120 min. After the completion of polymerization reaction,
the reaction mixture was concentrated in vacuo to give yellow
solid (quant.).
Elongation of the Polymer Chain. To
a
dichloromethane solution (3.0 mL) of 2 (0.01 mmol) was added
a dichloromethane solution (2.0 mL) of 1b (32.6 mg, 0.10
mmol) at 0 °C. The mixture was warmed to 25 °C and stirred
for 30 min. The resulting solution was added a dichloromethane
solution (2.0 mL) of 1b (32.6 mg, 0.10 mmol) by syringe at 30
min interval for four times. The resulting solution was
concentrated in vacuo to give a yellow solid.
3
1
Figure 9. Optimized structures of (a) η and (b) η -coordination
quinolyl methyl complex.
Conclusions
In conclusion, a new living cyclocopolymerization
reaction through the alternating insertion of isocyanide and
allene into a palladium-carbon bond was demonstrated. When
bidentate ligands, such as dppp and bpy, were used, the
propagation reaction was controlled to afford the desired
polymer, poly(quinolylene-2,3-methylene)s, with a narrow
molecular weight distribution. The resulting polymer has a
well-defined palladium complex at the propagation terminal
and the polymer structure near the propagation terminal was
clearly distinguished. The terminal palladium complex
maintained its reactivity, which can be extended to block
copolymerization with isocyanide. Kinetic studies and the
results of stoichiometric reactions elucidated the mechanistic
aspects of polymerization: the insertion reaction of isocyanide
is the rate-determining step, and the reaction proceeds via a
five-coordinate intermediate with a conformation change to the
appropriate configuration. In the case of the cis-chelating
ligand, because the site interchange reaction can be controlled,
the reactivity of the propagation species is dominated. The
presented living polymerization reaction greatly expands the
possibility of cyclocopolymerization based on alternating
insertion of isocyanides and allenes, and further molecular
design becomes possible. Additionally, these results and
mechanistic investigation provide new insight for designing
other types of cyclocopolymerization system via the
combination of completely different types of substituents.
Studies focusing on the synthesis of new functional polymers
based on the resulting living polymerization and the
development of new cyclocopolymerization system are
currently in progress.
The procedure for the Kinetic Study. To a solution of
2j (0.62 mg, 2.0 µmol), 1,4-dimethoxybenzene (internal
2 2
standard, 1.38 mg, 10.0 µmol), in CD Cl (0.8 mL) was added
1b (19.6 mg, 60.0 µmol) in CD Cl (0.4 mL) at –78 °C. The
2
2
reaction mixture was warmed to reaction temperature, and the
1
reaction course was monitored by H NMR spectra.
Stoichiometric reaction of 2e and 1d. To a solution of
2e (7.1 mg, 12.5 µmol) in CDCl
3
(0.6 mL) was added 1d (2.0
mg, 10.0 µmol) in CDCl (0.2 mL) at –78 °C. The reaction
3
1
mixture was warmed to –50 °C, which was monitored by H
NMR spectroscopy. After 10 min. the reaction mixture was
warmed to –40 °C. The insertion reaction proceeded, and time
1
course was monitored by H NMR spectra.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
Experimental details and characterization data (PDF)
Crystallographic data for 4e-Pt (CIF)
AUTHOR INFORMATION
Corresponding Author
E-mail: naokou@chem.sci.osaka-u.ac.jp (N.K.)
onitsuka@chem.sci.osaka-u.ac.jp (K.O.)
Experimental Procedures
ORCID
General. All reactions were carried out under an Ar
atmosphere, whereas the workup was performed in air. NMR
Naoya Kanbayashi: 0000-0001-8934-2389
Taka-aki Okamura: 0000-0002-9005-4015
spectra were recorded in CDCl
ECS400, JEOL JNM-ECA500 and spectrometers. In H and C
NMR, SiMe was used as an internal standard, and an external
5% H PO
measurement was carried out on Thermo Fisher Scientific LTQ-
Orbitrap XL. The molecular weights (M ) and its distributions
3 6
and benzene-d on JEOL JNM-
1
13
Notes
4
The authors declare no competing financial interest.
3
1
8
3
4
reference was used for P NMR. HR-MS
ACKNOWLEDGMENT
n
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This work was supported by a grant-in-aid for JSPS KAKENHI
Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts. J.
1
2
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4
5
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7
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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3
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3
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3
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3
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4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Grant JP16K21154, 19K05582, and the financial support from the
Tokuyama Science Foundation and the TonenGeneral Sekiyu
Foundation. We thank Jun Ikegami for assistance of the synthesis
of 1c, and Manami Narukawa for assistance with some of the
experiments in block copolymerization with isocyanide.
Am. Chem. Soc. 2016, 138, 11227-11233. (p) Kang, E.-H.; Kang, C.;
Yang, S.; Oks, E.; Choi, T.-L. Mechanistic Investigations on the
Competition between the Cyclopolymerization and [2 + 2 + 2]
Cycloaddition of 1,6-Heptadiyne Derivatives Using Second-
Generation Grubbs Catalysts. Macromolecules 2016, 49, 6240-6250.
(q) Shimomoto, H.; Kikuchi, M.; Aoyama, J.; Sakayoshi, D.; Itoh, T.;
Ihara, E. Cyclopolymerization of Bis(diazocarbonyl) Compounds
Leading to Well-Defined Polymers Essentially Consisting of Cyclic
Constitutional Units. Macromolecules 2016, 49, 8459-8465.
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(
CH
2
2
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