Functional Metathesis Catalyst Through Ring Closing Enyne Metathesis: One Pot Protocol for Living Heterotelechelic Polymers

Article abstract of DOI:10.1021/jacs.7b12805

Enyne ring closing metathesis has been used to synthesize functional group carrying metathesis catalysts from a commercial (Ru-benzylidene) Grubbs' catalysts. The new Grubbs-type ruthenium carbene was used to synthesize living heterotelechelic ROMP polymers without any intermediate purification. Olefin metathesis with a mono substituted alkyne followed by ring closing metathesis with an allylic ether provided efficient access to new functional group carrying metathesis catalysts. Different functional benzylidene and alkylidene derivatives have been investigated in the synthesis of heterotelechelic polymers in one pot.

Full text of DOI:10.1021/jacs.7b12805

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Communication
Functional Metathesis Catalyst Through Ring Closing Enyne
Metathesis: One Pot Protocol for Living Hetero-telechelic Polymers
Subhajit Pal, Fiorella Lucarini, Albert Ruggi, and Andreas F.M. Kilbinger
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12805 • Publication Date (Web): 19 Feb 2018
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Functional Metathesis Catalyst Through Ring Closing Enyne Metath-
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Subhajit Pal, Fiorella Lucarini, Albert Ruggi, Andreas F.M. Kilbinger*
Department of Chemistry, University of Fribourg, Chemin du Musée 9, CH-1700 Fribourg, Switzerland
Pre-functionalization agents (PFA) can be used in excess to pre-func-
tionalize the ruthenium carbene complex without the need for purifica-
tion as the excess is rendered metathesis inactive (or less active than the
monomer that is introduced afterwards). Previously reported PFAs
worked particularly well with G1 but were limited to functional group
bearing benzylidene complexes. [34]
ABSTRACT: Enyne ring closing metathesis has been used to synthe-
size functional group carrying metathesis catalysts from a commercial
(Ru-benzylidene) Grubbs' catalysts. The new Grubbs-type ruthenium
carbene was used to synthesize living hetero-telechelic ROMP polymers
without any intermediate purification. Olefin metathesis with a mono
substituted alkyne followed by ring closing metathesis with an allylic
ether provided efficient access to new functional group carrying metath-
esis catalysts. Different functional benzylidene and alkylidene deriva-
tives have been investigated in the synthesis of hetero-telechelic poly-
mers in one pot.
Here, we report a new PFA which can be used with G1 and G3 yield-
ing functional group carrying benzylidene and alkylidene complexes.
N
N
P(Cy)3
Cl
Cl
Ru
Ru
N
N
R1
R2
Ph
Today, ruthenium and molybdenum-based metathesis cata-
lysts [1], [2], [3] are extensively used to construct carbon-carbon double
bonds. [4] The high functional group tolerance of the ruthenium based
catalysts make them not only a popular choice in organic chemistry for
ring-closing and cross-metathesis reactions but also in polymer chemis-
try for the polymerization of functional monomers. [5], [6] The first and
third generation Grubbs' catalysts are the most common choice for pol-
ymer chemists due to their high initiation to propagation rate ratio (Fig-
ure 1, G1 and G3), which allows their use in living polymerizations. [7]
Ph
Cl
Cl
n
Br
P(Cy)3
R3
Br
G1
G3
Figure 1. Left and center: first generation (G1) and third generation
(G3) Grubbs' catalyst. Right: An end-functional polymer (here a poly-
norbornene derivative) is called homotelechelic (for R1=R2) or hetero-
telechelic (for R1¹R2) assuming both groups R1 and R2 can undergo
post-polymerization reactions.
In addition to main-chain functionality, polymers in which the chain
ends can be addressed with high specificity are of great importance in a
number of disciplines bordering chemistry, such as materials science, bi-
ochemistry or medicine. [8], [9], [10], [11]
The reactivity of alkynes towards olefin metathesis has been de-
scribed. [35], [36], [37], [38] and enyne metathesis has been used in organic
chemistry for many years.[39] Recently, the Choi group reported a tan-
dem ring-opening ring-closing cross metathesis polymerization using an
enyne sequence. [40], [41], [42] We hypothesized that the reactivity of a
mono-substituted alkyne towards olefin metathesis would be higher
than that of di or tri substituted allylic ethers. As a consequence, G1 or
G3 would react faster with a terminal alkyne than an allylic olefin. The
newly formed carbene (Scheme 1, A) would ideally undergo an intra-
molecular ring closing reaction (Scheme 1, kRCM) rather than an inter-
molecular propagation reaction with another alkyne. This cyclization
yields the new functionalized ruthenium carbene (Scheme 1,
Ru=CR1R2) and a 2,5-dihydrofuran derivative (DHF, Scheme 1, B). As
PFA 1-6 would be used in excess, the newly formed ruthenium carbene
would undergo further enyne metathesis with excess 1-6 (Scheme 1, k2),
thereby producing a new DHF derivative (Scheme 1, C) and regenerat-
ing itself in the process. At the end of the cycle, the reaction mixture
would only contain the two DHF derivatives (Scheme 1, B and C) and
a functionalized ruthenium carbene. The DHF derivatives should ideally
show lower metathesis reactivity due to a lack of ring strain and, there-
fore, a subsequent polymerization could be carried out by simply adding
a strained monomer to the same reaction vessel without any purification
or workup.
Mono end-functional polymers can be made by functionally termi-
nating living ring opening metathesis polymerizations. Many methods
have been described to prepare polymers with alcohol, thiol, carboxylic
acid, aldehyde or amine end groups in addition to the introduction of
entire groups and moieties. [12], [13], [14], [15], [16], [17], [18], [19], [20]
Sacrificing the living character of the polymerization allows the syn-
thesis of homo-telechelic polymers (Figure 1), typically from monomers
[21], [22], [23], [24]
such as cyclooctene or cyclooctadiene.
Exploiting the
propagation and cross-metathesis kinetics of the Grubbs' 3rd generation
complex, homo-telechelics can also be prepared in a living manner. [25],
[26]
Hetero-telechelic polymers (Figure 1) are, by nature, more difficult
to synthesize. The sacrificial block copolymer method was used to pre-
pare polymers carrying alcohol and either aldehyde or carboxylic acid
end groups. [27] In one case, a regio-selective chain transfer agent was ex-
ploited to prepare hetero-telechelic polymers. [28] Functional polymer
initiation followed by functional termination will allow the synthesis of
hetero-telechelic polymers. Functional initiators can be prepared by
cross-metathesis reactions using functional acyclic olefins. [29], [30], [31], [32],
[33] However, the olefin is typically used in excess and the newly prepared
initiator needs to be purified prior to polymerization.
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Scheme 1. Pre-functionalization of commercial G1 and G3 com-
Page 2 of 5
1
2
3
plexes using PFA 1-6. For G1 the reaction was carried out at rt. For
G3 the reaction was carried out at -10°C or at rt in the presence of 3-
bromopyridine (30 equiv).
G1
+
2
Ru CH
OTIPS
16h
rt
a
yield (88% ) conv. (92 %)
4
5
6
7
8
9
8h
(80% )
(20% )
5
0
m in
m in
k1
(0% )
R1
H
R1
Ru
20.5
20.0
+
19.5
19.0
18.5
18.0
17.5
17.0
Ru
O
R2
R2
Chemical Shift (ppm)
O
1 - 6
A
G1, G3
G1
3
Ru CH
Br
kRCM
rt
R1
b
9h
4h
yield (92% ) conv. (97 %)
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R1
Ru
k2
R2
(88% )
(30% )
(0%)
R2
O
G1-1...G1-4, G1-6
G3-1...G3-6
B
C
5
m in
O
0 min
R1
R2
R1
R2
20.5
20.0
+
19.5
19.0
18.5
18.0
17.5
17.0
Chemical Shift (ppm)
OCH3
1:
2:
3:
H
4:
H
G3
2
Ru CH
OTIPS
180 m in yield (92% ) conv. (100 %)
rt
OTIPS
H
H
CH2CH2CH2OTBDPS
CH3
5: CH3
6: CH3
c
90 m in
(92% )
(89% )
Br
5
0
m in
m in
(0% )
To verify our hypothesis, compounds 1-6 were synthesized
20.5
20.0
19.5
19.0
18.5
18.0
17.5
17.0
(Scheme 1). As an initial test, 1 was subjected to 10 mol% of G1 in di-
chloromethane-d2 and the expected formation of 3-styryl-2,5-dihydro-
furan (Scheme 1, B) could be observed immediately (see supporting in-
formation (SI) for characterization of E and Z isomers).
Chemical Shift (ppm)
-10°C
G3
+
3
Ru CH
Br
d
90 min yield (92%) conv. (100 %)
45 m in
(87% )
(70% )
The pre-functionalization of G1 using 3 equiv of either 2-4 was fol-
lowed by 1H-NMR spectroscopy in dichloromethane-d2. The disappear-
ance of the benzylidene carbene signal (20.03 ppm) and the formation
of the respective new carbene signals in excellent yields could be ob-
served (Figure 2 and SI Figure S1). As a proof of principle that the syn-
thesis of hetero-telechelic polymers in one pot can be achieved following
this route, exo-N-methyl norbornene imide (MNI, 17 equiv, 3h) was
added to the pre-functionalized ruthenium carbene complexes G1-2
and G1-3 without any intermediate purification (Scheme 2). The G1-2
initiated poly(MNI) was terminated with chain transfer agent (CTA)
10 (20 equiv, 1h, see Scheme 2), yielding a heterotelechelic polymer
with a TIPS protected phenolic alcohol on one chain end and an alde-
hyde on the other (MnGPC= 5100 g mol-1, Đ=1.2, poly i, see SI). The
G1-3 initiated poly(MNI) was terminated with CTA 9 resulting in a
heterotelechelic polymer with an aryl bromide on one chain end and a
BOC protected allylic amine on the other (MnGPC= 6300 g mol-
1, Đ=1.2, poly h, see SI).
5
0
m in
m in
(0% )
20.5
20.0
+
19.5
19.0
18.5
18.0
17.5
17.0
Chemical Shift (ppm)
G3
3
Ru CH
Br
rt
e
180 min yield (97%) conv. (100 %)
90 m in
(97% )
(76% )
5
0
m in
m in
(0% )
20.5
20.0
+
19.5
19.0
18.5
18.0
17.5
17.0
Chemical Shift (ppm)
G3
4
Ru CH
OMe
rt
f
180 m in yield (94% ) conv. (98 %)
90 m in
(90% )
5
0
m in
m in
(88% )
(0% )
The synthesis of ruthenium alkylidene initiators is of interest because
of their faster rate of initiation compared to ruthenium benzylidenes. Re-
ports for one pot syntheses of alkylidene initiators without the need for
purification or workup are rare.[25],[26]
20.5
20.0
19.5
19.0
18.5
18.0
17.5
17.0
Chemical Shift (ppm)
rt
Monomer
G3
+
6
Propagating carbene
Ru
g
After addition of m onom er
yield (90% )
We therefore reacted 3 equiv of 6 with G1 in dichloromethane-d2.
Unfortunately, only 36% of the benzylidene signal (20.03 ppm) disap-
peared over 3 h indicating that the 1H-NMR silent ruthenium iso-prop-
ylidene complex could not have formed in high yield. We believe that
the newly formed ruthenium iso-propylidene is faster at reacting with
propargyl ether 6 (Scheme 1, k2) than the G1-benzylidene (Scheme 1,
k1) resulting in incomplete conversion of the G1-benzylidene. To
achieve higher conversions, the amount of 6 was increased (10 equiv).
Unfortunately, only 64% conversion of G1-benzylidene into the iso-
propylidene complex could be observed. (see SI). Similar observations
had previously been made with a norbornene-based PFA and G1. [34]
10 m in conv. (100 %)
0
m in
(0% )
20.5
20.0
19.5
19.0
18.5
18.0
17.5
17.0
Chemical Shift (ppm)
Figure 2. 1H NMR spectra (CD2Cl2, 400 MHz) of reactions: a) G1 with
2 forming G1-OTIPS (19.49 ppm), b) G1 with 3 forming G1-Br
(19.99 ppm), c) G3 with 2 forming G1-OTIPS (18.55 ppm) at rt in
presence of 30 equiv of 3-bromopyridine, d) G3 with 3 forming G3-Br
(19.05 ppm) at -10°C, e) G3 with 3 forming G3-Br (19.05 ppm) at rt in
presence of 30 equiv of 3-bromopyridine, f) G3 with 4 forming G3-
OMe (18.57 ppm) at rt in presence of 30 equiv of 3-bromopyridine and
g) reaction of G3 with 6 forming G3-isopropylidene followed by addi-
tion of MNI gives the propagating carbene (18.50 ppm) at rt in the pres-
ence of 30 equiv of 3-bromopyridine. (conv. = conversion)
Next, we investigated the pre-functionalization of G3 (Figure 1) with
PFA 1-6 (Scheme 1).
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Carrying out the reaction at rt in dichloromethane-d2, we observed
GPC analysis showed narrow molecular weight distributions and
MALDI-ToF mass spectrometry confirmed the presence of both func-
1
2
3
4
5
6
7
8
rapid catalyst decomposition by 1H-NMR spectroscopy. Model experi-
ments indicated that the G3 benzylidene complex decomposes in the
presence of 3-styryl-2,5-dihydrofuran as evidenced by new carbene spe-
cies formed in the region of 18-19 ppm (Scheme 1 B and SI Figure S3).
The mechanism of this decomposition is under investigation.
1
tional end groups in all cases (see SI). A detailed H-NMR end group
analysis was carried out for poly j (see SI).
Scheme 2. One pot synthesis of hetero-telechelic polymers.
To avoid this decomposition, a RCEYM of G3 with 3 was carried out
at -10°C in dichloromethane-d2 (Scheme 1). The reaction was followed
by 1H NMR spectroscopy. Within 45 min, the original G3 benzylidene
signal (19.06 ppm) shifted to a new carbene signal (19.05 ppm) with
little loss of signal intensity (87%, Figure 2 d). The reaction of 2 and 4
with G3 under identical conditions (Scheme 1) yielded new carbenes in
high yields (90% for 2, 96% for 4, see SI).
O
n
N
R3
R1
H
R1
R1
R2
[Ru]
O
nRu
R2
+
[Ru]
R2
O
9
O
1 - 6
O
N
R3
10
11
12
13
14
15
16
17
18
19
20
21
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60
O
O
R4
R4
(10)
The RCEYM of G3 either with 5 or 6 was investigated by 1H-NMR
spectroscopy in dichloromethane-d2 at -10°C (see SI Table 1). Unlike in
the case of G1 (see above), the G3 benzylidene signal quantitatively
shifted to an alkylidene carbene (complete disappearance of the benzyl-
idene signal for 5 and 6) using only 5 equiv of the corresponding PFAs
5 or 6 at -10°C. We assume that the rate of reaction between G3 benzyl-
idene and 5 or 6 (Scheme 1, k1) is higher than the rate of ring closing
metathesis (Scheme 1, kRCM), leading to a fast consumption of the G3
benzylidene complex. The newly formed carbenes G3-5 or G3-6 are,
therefore, no longer in competition with the original G3 benzylidene for
the PFA substrates. The minimum excess of PFA was investigated for
the reaction between G3 and 4. 1.5 equiv. of 4 (0.06 M) were sufficient
to give 91% conversion into the new carbene species in the presence of
30 equiv. of pyridine (see SI).
R1
R1
R4
O
n
n
R2
O
R2
O
O
O
N
R3
N
R3
monomers
MNI: R3 = CH3
chain transfer agents (CTA)
7: R4 = OH
8: R4 = Cl
9: R4 = NHBOC
EHNI: R3 = 2-ethylhexyl
NBE:
To prove the presence of two different end functional groups, a polymer
was prepared carrying a Coumarin 343 and Rhodamine B dye on oppo-
site chain ends (see SI). Förster resonance energy transfer (FRET) from
the excited Coumarin dye to the Rhodamine fluorophore proved the
presence of both chromophores in close proximity (see SI).
To lower its reactivity, we carried out the RCEYM of G3 with 3 in the
presence of 30 equiv of 3-bromopyridine in dichloromethane-d2 at rt.
1H-NMR spectroscopy revealed that the G3 benzylidene signal shifted
completely to the new benzylidene signal without appreciable loss of
catalyst (19.05 ppm, 97%, Figure 2 e). The remaining substrates were
subjected to G3 at identical reaction conditions and generated the cor-
responding new carbene complexes in high yields (see Figure 2).
In conclusion, we have successfully developed new PFAs for both G1
and G3. Methoxy, bromo, triisopropylsiloxy and substituted alkylidene
catalysts were synthesized in excellent yields. Hetero-telechelic poly-
mers were prepared using these PFAs, G1 and G3 in one pot reactions
without intermediate purification. End-functionalization was achieved
using symmetrical olefin chain transfer agents to install allylic hydroxy,
chloro or amine end groups or 2-methoxy-3,4-dihydro-2H-pyran to in-
stall aldehyde end groups. 1H-NMR, FRET and GPC analyses show an
excellent degree of end functionalization and good control over molec-
ular weight confirming the living character of the polymerizations. This
method provides an easy and efficient approach to functional Grubbs'
metathesis catalysts using PFAs that can be prepared in few straightfor-
ward steps. The heterotelechelic polymers that are accessible via this
route can serve as building blocks for preparing a wide range of new ma-
terials from functional bio-conjugates and well-defined co-polymer ar-
chitectures to reactive surface coatings.
Heterotelechelic polymers were synthesized using both methods, i.e.
low temperature pre-functionalization and 3-bromopyridine attenua-
tion. G3 was pre-functionalized with 4 at -10°C (120 min) followed by
polymerization of MNI (30 equiv, 3 h) and functional termination with
7 (20 equiv, 1h)giving a methoxyphenyl initiated polymer with an allylic
alcohol end group (MnGPC= 6100 g mol-1, Đ=1.07, poly a, see SI). Sim-
ilarly, heterotelechelic poly(MNI)s were prepared at -10°C via pre-func-
tionalization with 3 and termination with 7 (MnGPC= 4500 g mol-1,
Đ=1.06, poly b, see SI), pre-functionalization with 2 and termination
with 10 (MnGPC= 5200 g mol-1, Đ=1.06, poly c, see SI) and pre-func-
tionalization with 5 and termination with 10 (MnGPC= 7300 g mol-
1, Đ=1.06, poly e, see SI). Using norbornene as monomer at -10°C pre-
functionalization with 3 and termination with 7 (MnGPC= 3900 g mol-1,
Đ=1.24, poly g, see SI) gave a poly(NBE) with an aryl bromide at one
chain end and an allylic alcohol on the other.
ASSOCIATED CONTENT
Carrying out the polymerization of MNI at rt in the presence of 30
equiv of 3-bromopyridine, pre-functionalization with 6 (10 min),
polymerization of MNI (60 equiv, 3h ) and termination with 7 gave poly
d (MnGPC= 11300 g mol-1, Đ=1.06), pre-functionalization with 3 and
termination with 8 gave poly j (MnGPC= 11300 g mol-1, Đ=1.06) and
pre-functionalization with 5 and termination with 9 gave poly k
(MnGPC= 11300 g mol-1, Đ=1.06). Using exo-N-(2-ethylhexyl)nor-
bornene imide as monomer, pre-functionalization with 3 and termina-
tion with 7 gave heterotelechelic poly(EHNI) (poly f, MnGPC= 5300 g
mol-1, Đ=1.06, see also SI for all polymers).
Supporting Information. The Supporting Information is available free
of charge on the ACS Publications website.
Full experimental details, NMR spectra, MALDI ToF MS and GPC
data, description of the FRET experiments (PDF).
AUTHOR INFORMATION
Corresponding Author
* E-mail: andreas.kilbinger@unifr.ch
ORCID
Andreas F.M. Kilbinger: 0000-0002-2929-7499
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ACKNOWLEDGMENT
1
The authors thank the National Center of Competence in Research
(NCCR Bio-inspired Materials, Swiss National Science Foundation) for
support.
2
3
4
5
6
7
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