Enolesters as chain end-functionalizing agents for the living ring opening metathesis polymerization

Article abstract of DOI:10.1002/pola.28468

Functional enolethers have previously been used to introduce functional end groups at the chain end of ruthenium carbene complex initiated living ring opening metathesis polymers. Here, we investigated whether the weaker π-donating enolesters could equally be used in regio selective reactions with ruthenium carbene complexes and thus as polymer end-functionalization reagents. Enolesters such as vinyl acetate, butenyl acetate, 3-(4-(tert-butoxy)phenyl)propenyl acetate and 6-(((benzyloxy)carbonyl)amino)hex-1-en-1-yl acetate were used as living ROMP terminating agents. All gave the expected end groups proving that enolesters are synthetically easily accessible targets for living ROMP end-functionalization.

Full text of DOI:10.1002/pola.28468

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Enolesters as Chain End-Functionalizing Agents for the Living
Ring Opening Metathesis Polymerization
€
Peng Liu, Mohammad Yasir, Helena Kurzen, Nils Hanik, Mark Schafer, Andreas F.M. Kilbinger
ꢀ
Chemistry Department, University of Fribourg, Chemin du Musee 9, Fribourg, CH-1700, Switzerland
Correspondence to: A. Kilbinger (E-mail: andreas.kilbinger@unifr.ch)
Received 3 November 2016; accepted 23 November 2016; published online 00 Month 2017
DOI: 10.1002/pola.28468
ABSTRACT: Functional enolethers have previously been used to
introduce functional end groups at the chain end of ruthenium
carbene complex initiated living ring opening metathesis poly-
mers. Here, we investigated whether the weaker p-donating
enolesters could equally be used in regio selective reactions
with ruthenium carbene complexes and thus as polymer end-
functionalization reagents. Enolesters such as vinyl acetate,
butenyl acetate, 3-(4-(tert-butoxy)phenyl)propenyl acetate and
6-(((benzyloxy)carbonyl)amino)hex-1-en-1-yl acetate were used
as living ROMP terminating agents. All gave the expected end
groups proving that enolesters are synthetically easily accessi-
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ble targets for living ROMP end-functionalization. 2017 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 00,
000–000
KEYWORDS: enolesters; functionalization of polymers; Grubbs
catalyst; metathesis; ROMP
polymer chain end forming a so-called Fischer-carbene.21
Substituted vinyl ethers have therefore been employed to
attach larger molecular fragments to the ends of polymer
chains.22,23 A high cis-content in these vinyl ethers is, however,
crucial to achieve rapid termination as the cis-vinyl ethers
react significantly faster with the ruthenium alkylidene than
the trans-vinyl ethers. A cis-selective synthesis of a functiona-
lizable chain terminating agent based on vinyl ethers was
recently reported.24
INTRODUCTION The living ring opening metathesis polymeri-
zation (ROMP) using ruthenium based carbene complexes
developed by Grubbs et al. is one of the most functional group
tolerant polymerization methods known today.1 Despite the
principal problem of functionalizing a functional group toler-
ant propagating species, many selective methods for introduc-
ing a number of functional groups have been reported over the
last ten years.2–4 Alcohols,5–7 thiols,8,9 and amine10 end groups
have been introduced via the sacrificial block copolymer syn-
thesis. Aldehydes11,12 and carboxylic acids11 could be prepared
via the reaction of the propagating ruthenium carbene complex
with unsaturated lactones, cyclic vinyl acetals, and unsaturated
cyclic carbonates. Terminal cross-metathesis with symmetrical
acyclic olefins is another method that allows the introduction
of functional groups at the polymer chain end.13,14 Substituted
vinyl ethers have been used to attach a variety of functional
groups or molecules to the end of polymer chains.15–17 The
group of Slugovc successfully used acrylates for the functional
termination of ROMP polymers.18 Combinations of termination
techniques allowed the preparation of hetero-telechelic ROMP
polymers.19 Furthermore, functional end-groups on ROMP
polymers were recently used to control molecular weight and
polydispersity in a living catalytic ROMP.20 Ethyl vinyl ether
(EVE) has long been known to selectively terminate living
ROMP polymers by transferring a methylene group onto the
Considering our previous findings that vinyl lactones reacted
rapidly and regioselectively with ruthenium carbene com-
plexes11 we hypothesized that acyclic enolesters and in partic-
ular functional enolesters could be suitable and readily
accessible terminating agents for living ROMP. Here, we report
the synthesis of enolester derivatives and their use as function-
al chain termination agents in living ROMP.
EXPERIMENTAL
Materials
Benzene, e-caprolactone, caesium iodide, Celite, Dess-Martin peri-
odinane, DIBAL-H, diethyl ether, dimethylsulfide, ethyl vinyl ace-
tate, Grubbs first generation catalyst, Grubbs third generation
catalyst, magnesium perchlorate, methyl 3-(4-hydroxyphenyl)-
propanoate, pyridinium chlorochromate, sodium carbonate,
Dedicated to Robert H. Grubbs on the Occasion of His 75th Birthday
Additional Supporting Information may be found in the online version of this article.
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2017 Wiley Periodicals, Inc.
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triethyl amine, and Benzyl (6-hydroxyhexyl)carbamate were pur-
chased from Sigma-Aldrich and used without further purification.
Butyraldehyde was purchased from Sigma-Aldrich, distilled, and
stored under an argon atmosphere before use. Potassium carbon-
ate was purchased from Sigma-Aldrich and was dried under vac-
uum before use. Acetic anhydride, carbic anhydride, methylamine
in cyclohexane, and methyl 3-(4-hydroxyphenyl)propanoate were
purchased from Acros Organics and used without further purifi-
cation. N-Chlorosuccinimide was purchased from Acros and
recrystallized from benzene. Magnesium sulfate, sodium chloride,
and sodium thiosulfate were purchased from rectolab and used
without further purification. Acetic acid, sodium acetate and sodi-
um hydroxide are purchased from Merck and used without fur-
ther purification. Solvents of analytical grade were purchased
from Honeywell, Acros Organics, Sigma Aldrich, Fischer Scientific,
and were used without further purification. Solvents of technical
grade were purified by distillation. Deuterated solvents (CDCl3,
C6D6 and CD2Cl2) were purchased from Cambridge Isotope Labo-
ratories. Exo-N-methyl-5-norbornene-2,3-dicarboximide (MNI),25
exo-N-hexyl-5-norbornene-2,3-dicarboximide (HNI)25 and exo-
N-methyl-7-oxanorbornene-2,3-dicarboximide were synthesized
according to the described procedure.26
1H NMR (300 MHz, CDCl3) d 7.05 (dt, J 5 12.4, 1.6 Hz, 0.6H,
trans), 6.96 (dt, J 5 6.4, 1.6 Hz, 0.4H, cis), 5.44 (dt, J 5 12.4,
7.1 Hz, 0.6H, trans), 4.86 (dd, J 5 13.8, 7.4 Hz, 0.4H, cis),
1.97-2.17 (m, 5H), 0.96-1.03 (m, 3H). 13C NMR (75 MHz,
CDCl3) d 168.32, 168.16, 134.99, 133.46, 116.66, 115.88,
20.71, 20.67, 17.88, 13.97, 13.85.
Methyl 3-(4-(Tert-butoxy)phenyl)propanoate (2)
Methyl 3-(4-hydroxyphenyl)propanoate (1 eq, 3.0 g, 16.6
mmol) and Mg(ClO4)2 (0.2 eq, 0.75 g, 3.3 mmol) were dis-
solved in dry DCM (15 mL) in a the purged flask with Argon.
Boc2O (2.3 eq, 8.35 g, 38.2 mmol) in dry DCM (8 mL) was
added by syringe pump at a rate of 8 mL/h and the reaction
mixture was stirred for 2 h at room temperature. Water
(15 mL) was added to quench the reaction and the reaction
mixture was stirred for 1 h. DCM was added and the reac-
tion mixture was extracted with DCM (30 mL‡2). The
organic phases were collected, dried over Na2SO4, filtered
and concentrated. The crude product was purified by
chromatography (Hexane: EtOAc 5 10:1) to afford methyl
3-(4-(tert-butoxy)phenyl)propanoate 2 (3.2 g, 81%) as colou-
less liquid.
1H NMR (400 MHz, CDCl3) d 7.07 (d, J 5 8.5 Hz, 2H), 6.90
(d, J 5 8.5 Hz, 2H), 3.66 (s, 3H), 2.88–2.93 (m, 2H), 2.40–
2.71 (m, 2H), 1.32 (s, 9H). 13C NMR (101 MHz, CDCl3)
d 173.44, 153.69, 135.34, 128.60, 124.22, 51.58, 35.85,
Instrumentation
ESI-MS analysis for synthesized compounds was carried out
on a Bruker 4.7T BioAPEX II. MALDI-ToF MS analysis of the
polymers was carried out on a Bruker ultrafleXtremeTM using
2-[(2E)-3-(4-tertbutylphenyl)-2-methylprop-2-enylidene]ma-
lononitrile (DCTB) as the matrix and silver trifluoroacetate or
sodium trifluoroacetate as the added salt. Relative molecular
weights and molecular weight distributions were measured by
30.31, 28.84. HR-MS (ESI) calcd. For
C
14H20O3Na1
[M 1 Na1]: 259.1305; Found: 259.1304.
3-(4-(Tert-butoxy)phenyl)propanal (3)
Methyl 3-(4-(tert-butoxy)phenyl)proponate 2 (1 eq, 3.1 g,
13.1 mmol) was dissolved in dry THF (25 mL) and cooled in
a Dewar flask by acetone to 278 8C with a cryostat. Then
DIBAL-H 1M in cyclohexane (1 eq, 13.1 mL, 13.1 mmol) was
added dropwise by syringe pump at a rate of 13 mL/h. The
stiring was continued for 90 min. Water (15 mL) was slowly
added to quench the reaction. The stiring was continued for
15 min then the reaction was allowed to warm up to 0 8C.
MgSO4 and NaCl were added and the reaction mixture was
decanted. The decanted solution was washed with brine. The
aqueous layer was then extracted with THF. The organic
layer was collected and concentrated under reduced pres-
sure. The crude reaction mixure was purified by chromatog-
raphy (Hexane:EtOAc 5 10:1) to afford 3-(4-(tert-butoxy)
phenyl)propanal 3 (1.8 g, 70%) as colorless liquid.
gel permeation chromatography (GPC) with
a Viscotek
GPCmax VE2001 GPC Solvent/Sample Module, a Viscotek UV
detector 2600, a Viscotek VE3580 RI detector, and two
Viscotek T6000 M columns (7.8 Å, 300 mm, 103–107 Da) at a
flow rate of 1 mL/min for samples measured in THF and with
a system consisting of a Duratec vacuum degasser, a JASCO
PU-2087 plus pump, an Applied Biosystems UV absorbance
detector 759A (set to 254 nm wavelength). Calibrations were
carried out using Malvern PolycalTM UCS-PS polystyrene (PS)
standards. NMR spectra were recorded on a Bruker Avance III
300 MHz NMR spectrometer (1H NMR 300 MHz, 13C-NMR
75 MHz) and Bruker Avance III 400 MHz NMR spectrometer
(1H NMR 400 MHz, 13C-NMR 101 MHz).
Syntheses of Terminating Agents
Butenyl Acetate (1)
1H NMR (400 MHz, CDCl3) d 9.82 (t, J 5 1.5 Hz, 1H), 7.07 (d,
J 5 8.6 Hz, 2H), 6.91 (d, J 5 8.5 Hz, 2H), 2.92 (t, J 5 7.5 Hz,
2H), 2.76 (t, J 5 7.4 Hz, 2H), 1.32 (s, 9H). 13C NMR (101
MHz, CDCl3) d 201.75, 153.73, 135.10, 128.62, 124.31, 78.29,
45.37, 28.83, 27.46. HR-MS (ESI) calcd. For C13H18O2Na1
[M 1 Na1]: 229.1200; Found: 229.1199.
Butyraldehyde (1 eq, 12.2 mL, 139 mmol), acetic anhydride
(2.3 eq, 30 mL, 314 mmol), potassium carbonate (0.12 eq,
2.3 g, 16.7 mmol), and sodium acetate (0.26 eq, 3.0 g, 36.6
mmol) were heated under reflux for 3 h. The reaction was
quenched with water and the organic layer was extracted
with DCM. The organic phase was washed with aqueous
solution of sodium bicarbonate until the pH of the aqueous
phase was higher than 8. The organic layer was dried over
magnesium sulfate and the product was distilled under vacu-
um (48 8C at 40 mbar) to obtain butenyl acetate (Bac; 4.1 g,
26%, cis:trans 5 60:40) as a colorless liquid.
3-(4-(Tert-butoxy)phenyl)propenyl Acetate (4)
3-(4-(Tert-butoxy)phenyl)propanal
3 (1 eq, 1.4 g, 6.78
mmol), acetic anhydride (2 eq, 1.3 mL, 13.6 mmol), potassi-
um carbonate (0.12 eq, 113.3 mg, 0.82 mmol) and sodium
acerate (0.3 eq, 167.3 mg, 2.04 mmol) were heated under
2
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reflux for 4 h. The reaction was quenched with water and the
organic layer was extracted with DCM. The organic phase was
washed with aqueous solution of sodium bicarbonate until the
pH of the aqueous phase was higher than 8. The organic layer
was dried over magnesium sulfate and the product was con-
centrated under reduced pressure. The crude product was
purified by chromatography (Hexane:EtOAc 5 10:1) to afford
3-(4-(tert-butoxy)phenyl)propenyl acetate 4 (893 mg, 53%,
cis:trans 5 57:43) as a light yellow liquid.
(m, 1H), 5.00 (s, 2H), 4.72–4.77 (m, 0.5H), 3.08–3.14 (m,
2H), 1.90–2.10 (m, 5H), 1.28–1.47 (m, 4H). 13C NMR (101
MHz, CDCl3) d 168.25, 168.11, 156.40, 136.63, 135.75,
134.42, 128.53, 128.12, 114.39, 113.44, 66.64, 40.86, 29.40,
26.90, 26.61, 26.22, 23.95, 20.74, 20.70. HR-MS (ESI) calcd.
For C16H21NO4Na1 [M 1 Na1]: 314.1363; Found: 314.1362.
Polymer 1 Terminated with Vinyl Acetate (VA)
Exo-N-methyl-5-norbornene-2,3-dicarboximide
(16.89
eq,
450 mg, 2.54 mmol) was dissolved in degassed DCM (2 mL) in a
Schlenk flask, which was evacuated and charged with argon
three times. The polymerization was initiated by quick addition
of G1 (1 eq, 105 mg, 0.13 mmol) in degassed DCM (2.5 mL).
After 1 h, 1.5 mL of the polymer solution was added to VA (50
eq, 186 mg, 2.17 mmol) and stirred for 12 h. A second portion
(1.5 mL) of the polymer solution was added to EVE (see poly-
mer 2, below) and a third portion to BAc (see polymer 3,
below). The solution was concentrated by a slight flow of nitro-
gen and the polymer recovered by precipitation into a vigorous-
ly stirred ten-fold volume of methanol. The mixture was filtered
to afford the respective polymer [143 mg, 95% yield, Mn(GPC,
THF) 5 5400 g/mol, PDI: 1.23] as a colorless solid.
1H NMR (400 MHz, CDCl3) d 7.13–7.19 (m, 1H, cis and
trans), 7.06–7.09 (m, 2H, cis and trans), 6.89–6.93(m, 2H, cis
and trans), 5.54–5.61 (m, 0.5H, cis), 5.06–5.11 (m, 0.38H,
trans), 3.46 (d, J 5 7.6 Hz, 0.78H, cis), 3.29 (d, J 5 7.6 Hz,
1.04H, cis), 2.17 (s, 1.12H, trans), 2.12 (s, 1.50H, cis), 1.33
(d, 9H, cis and trans). 13C NMR (101 MHz, CDCl3) d 168.20,
168.06, 153.75, 153.60, 136.22, 134.83, 134.57, 134.50,
128.68, 128.58, 124.26, 113.98, 112.77, 78.26, 78.23, 32.92,
29.98, 28.84, 20.76, 20.71. HR-MS (ESI) calcd. For
C
15H20O3Na1 [M 1 Na1]: 271.1305; Found: 271.1304.
Benzyl-(6-oxohexyl)carbamate (5)
PCC (1.5 eq, 1.3 g, 6 mmol) was suspended in dry THF
(32 mL) in a 100 mL round bottom flask and cooled to 0 8C
with an ice bath. Benzyl (6-hydroxyhexyl)carbamate (1 eq,
1 g, 40 mmol) in dry THF (8 mL) was added dropwise to
the PCC suspension by syringe pump at a rate of 8 mL/h.
After adding, the reaction was followed by TLC until the
reaction was finished. Silica was added and THF was evapo-
rated. The deposit on silica was washed with diethyl ether to
afford the crude product. The crude product was purified by
chromatography (Hexane:EtOAc 5 2:1) to afford pure benzyl
1H NMR (300 MHz, CDCl3) d 7.16-7.33 (m), 5.42–5.71 (m),
3.41 (s), 2.64–3.23 (m), 1.19–2.13(m), 0.76–0.83 (m). 13C NMR
(75 MHz, CDCl3) d 178.36, 133.41, 132.07, 131.88, 131.84,
131.70, 128.55, 128.32, 126.31, 52.67, 51.12, 51.02, 45.82,
45.62, 42.14, 41.97, 40.87, 29.70, 24.76. MALDI-ToF MS calcd.
For C148H162N14O28Na1 [M 1 Na1]: 2606.16; Found: 2606.18.
Polymer 2 Terminated with EVE
EVE (50 eq, 157 mg, 2.17 mmol) was added to a propagating
solution of polymer 1 (1.5 mL, see above) and the reaction
stirred for 12 h. The solution was concentrated by a slight
flow of nitrogen. The polymer was precipitated by slow
addition into a 10-fold excess of methanol. The mixture was
filtered to afford the respective polymer [146 mg, 97% yield,
Mn(GPC, THF) 5 5300 g/mol, PDI: 1.26] as a gray solid.
(6-oxohexyl)carbamate
liquid.
5 (512.8 mg, 52%) as colorless
1H NMR (400 MHz, CDCl3) d 9.66 (s, 1H), 7.11–7.34 (m, 5H),
5.00 (s, 2H), 3.11 (dd, J 5 13.2, 6.6 Hz, 2H), 2.34 (t, J 5 7.0
Hz, 2H), 1.13–1.61 (m, 6H). 13C NMR (101 MHz, CDCl3) d
202.38, 156.41, 136.61, 128.53, 128.12, 66.65, 43.72, 40.79,
29.80, 26.21, 21.63. HR-MS (ESI) calcd. For C14H19NO3Na1
[M 1 Na1]: 272.1257; Found: 272.1257.
1H NMR (300 MHz, CDCl3) d 7.16–7.33 (m), 5.40–5.71 (m),
2.64–3.21 (m), 1.99–2.11 (m), 1.47–1.75 (m), 1.17–1.23 (m),
0.76–0.81 (m). 13C NMR (75 MHz, CDCl3) d 178.37, 133.52,
132.07, 131.86, 131.84, 128.56, 126.31, 52.67, 51.11, 51.02,
45.82, 45.63, 42.14, 41.98, 40.87, 31.77, 29.88, 29.70, 27.98,
26.66, 24.83, 24.77. MALDI-ToF MS calcd. For
C158H173N15O30Na1 [M 1 Na1]: 2783.24; Found: 2783.24.
6-(((Benzyloxy)carbonyl)amino)hex-1-en-1-yl Acetate (6)
Benzyl-(6-oxohexyl)carbamate 5 (1 eq, 510 mg, 2 mmol),
acetic anhydride (2 eq, 0.38 mL, 4 mmol), potassium carbon-
ate (0.12 eq, 33.2 mg, 0.24 mmol) and sodium acerate (0.3
eq, 49.2 mg, 0.6 mmol) were heated under reflux for 4 h.
The reaction was quenched with water and the organic layer
was extracted with DCM. The organic phase was washed
with aqueous solution of sodium bicarbonate until the pH of
the aqueous phase was higher than 8. The organic layer was
dried over magnesium sulfate and the product was concen-
trated under reduced pressure. The crude product was puri-
fied by chromatography (Hexane:EtOAc 5 10:1) to afford 6-
Polymer 3 Terminated with But-1-en-1-yl Acetate (BAc)
But-1-en-1-yl acetate (50 eq, 247 mg, 2.17 mmol) was added
to a propagating solution of polymer 1 (1.5 mL, see above)
and the reaction stirred for 12 h. The solution was concen-
trated by a slight flow of nitrogen. The polymer was precipi-
tated by slow addition into a 10-fold excess of methanol. The
mixture was filtered to afford the respective polymer
[142 mg, 94% yield, Mn(GPC, THF) 5 5800 g/mol, PDI: 1.21].
(((benzyloxy)carbonyl)amino)hex-1-en-1-yl
acetate
6
(135.5 mg, 23%, cis:trans 5 50:50) as a colorless liquid.
1H NMR (300 MHz, CDCl3) d 7.16–7.33 (m), 5.36–5.71 (m),
2.64–3.23(m), 1.47–2.07 (m), 1.19 (s), 0.90–0.97 (m). 13C
NMR (75 MHz, CDCl3) d 178.37, 133.46, 132.06, 131.88,
1H NMR (400 MHz, CDCl3) d 7.13-7.33 (m, 5H), 6.96
(d, J 5 12.4 Hz, 0.5H), 6.92 (d, J 5 6.4 Hz, 0.5H), 5.25–5.32
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131.84, 131.71, 128.56, 126.31, 52.67, 51.11, 51.02, 45.82,
45.63, 42.15, 41.98, 40.87, 29.88, 27.98, 26.66, 24.83, 24.77.
MALDI-ToF MS calcd. For C150H166N14O28Na1 [M 1 Na1]:
2634.19; Found: 2634.19.
in a Schlenk flask, which was evacuated and charged with
argon three times. The polymerization was initiated by quick
addition of G1(1 eq, 41.1 mg, 0.05 mmol) in degassed DCM
(1 mL). After 1 h, 3-(4-(tert-butoxy)phenyl)propenyl acetate
(5 eq, 62.1 mg, 0.25 mmol) was added to the polymer solu-
tion and the mixture stirred for 12 h. The solution was con-
centrated by a slight flow of nitrogen. The polymer was
precipitated by slow addition into a ten-fold excess of metha-
nol. The mixture was filtered to afford the respective poly-
mer [305.7 mg, 97% yield, Mn(GPC, THF) 5 6800 g/mol, PDI:
1.18] as a light yellow solid.
Polymer 4 Terminated with 3-(4-(Tert-
butoxy)phenyl)propenyl Acetate (4)
Exo-N-methyl-5-norbornene-2,3-dicarboximide
(10
eq,
88.6 mg, 0.5 mmol) was dissolved in degassed DCM (1 mL) in
a Schlenk flask, which was evacuated and charged with argon
three times. The polymerization was initiated by quick addi-
tion of G1(1 eq, 41.1 mg, 0.05 mmol) in degassed DCM (1 mL).
After 1 h, 3-(4-(tert-butoxy)phenyl)propenyl acetate (5 eq,
62.1 mg, 0.25 mmol) was added to the polymer solution and
the mixture stirred for 12h. The solution was concentrated by
a slight flow of nitrogen. The polymer was precipitated by slow
addition into a ten-fold excess of methanol. The mixture was
filtered to afford the respective polymer [111 mg, 81% yield,
Mn(GPC, THF) 5 3900 g/mol, PDI: 1.12] as a light yellow solid.
1H NMR (400 MHz, CDCl3) d 7.10–7.36 (m), 6.97 (dd,
J 5 66.0, 8.3 Hz), 5.50–5.74 (m), 2.69–3.33 (m), 1.53–2.15
(m), 1.19–1.32 (m). 13C NMR (101 MHz, CDCl3) d 178.37,
136.19, 133.48, 132.05, 131.87, 131.84, 128.78, 128.67,
128.56, 128.55, 126.30, 124.25, 124.18, 113.97, 112.76,
52.65, 51.10, 51.01, 45.80, 45.62, 42.14, 41.98, 40.86, 31.76
(t, J 5 10.1 Hz), 29.96, 29.87, 28.84, 28.82, 27.97 (t, J 5 5.4 Hz),
24.82, 24.76. MALDI-ToF MS calcd. For C249H278N23O47Na1
[M1 Na1]: 4362.0; Found: 4362.1.
1H NMR (400 MHz, CDCl3) d 7.21–7.37 (m), 6.97 (dd,
J 5 65.6, 8.3 Hz), 5.52–5.75 (m), 2.57–3.32 (m), 1.53–2.16
(m), 1.22–1.31 (m). 13C NMR (101 MHz, CDCl3) d 178.36,
153.51, 136.85, 136.20, 135.07, 133.49, 132.06, 131.87,
131.84, 131.71, 130.88, 130.58, 130.36, 128.79, 128.67,
128.64, 128.55, 128.32, 127.50, 126.30, 124.25, 124.18,
52.66, 51.11, 51.01, 45.81, 45.62, 42.13, 41.96, 40.86, 31.77
(t, J 5 9.9 Hz), 29.87, 28.85, 27.97 (t, J 5 5.3 Hz), 26.66,
24.82, 24.76. MALDI-ToF MS calcd. For C189H209N17O35Na1
[M 1 Na1]: 3299.5; Found: 3299.6
Polymer 7 Terminated with 6-
(((Benzyloxy)carbonyl)amino)hex-1-en-1-yl Acetate (6)
MNI (20 eq, 177.2 mg, 1.0 mmol) was dissolved in degassed
DCM (1 mL) in a Schlenk flask, which was evacuated and
charged with argon three times. The polymerization was ini-
tiated by quick addition of G1(1 eq, 41.1 mg, 0.05 mmol) in
degassed DCM (1 mL). After 1 h, 6-(((benzyloxy)carbonyl)a-
mino)hex-1-en-1-yl acetate (5 eq, 73 mg, 0.25 mmol) was
added to the polymer solution and the mixture stirred for
12 h. The solution was concentrated by a slight flow of nitro-
gen. The polymer was precipitated by slow addition into a
10-fold excess of methanol. The mixture was filtered to
afford the respective polymer [190.7 mg, 82% yield, Mn(GPC,
THF) 5 5800 g/mol, PDI: 1.18] as a brown solid.
Polymer 5 Terminated with 3-(4-(Tert-
butoxy)phenyl)propenyl Acetate (4)
Exo-N-methyl-5-norbornene-2,3-dicarboximide (20 eq, 177.2 mg,
1.0 mmol) was dissolved in degassed DCM (1 mL) in a Schlenk
flask, which was evacuated and charged with argon three times.
The polymerization was initiated by quick addition of G1 (1 eq,
41.1 mg, 0.05 mmol) in degassed DCM (1 mL). After 1 h, 3-(4-
(tert-butoxy)phenyl)propenyl acetate (5 eq, 62.1 mg, 0.25 mmol)
was added to the polymer solution and the mixture stirred for
12 h. The solution was concentrated by a slight flow of nitrogen.
The polymer was precipitated by slow addition into a 10-fold
excess of methanol. The mixture was filtered to afford the respec-
tive polymer [201 mg, 89% yield, Mn(GPC, THF) 5 5400 g/mol,
PDI: 1.18] as a light yellow solid.
1H NMR (400 MHz, CDCl3) d 7.12–7.30 (m), 5.43–5.68 (m),
2.84–3.18 (m), 2.63 (s), 1.16–2.09 (m). 13C NMR (101 MHz,
CDCl3) d 178.36, 133.48, 132.06, 131.88, 131.83, 128.55,
128.52, 128.11, 126.31, 52.67, 51.11, 51.02, 45.81, 45.62,
42.14, 41.97, 40.87, 24.83, 24.76. MALDI-ToF MS calcd. For
C120H133N11O22Na1 [M 1 Na1]: 2103.0; Found: 2103.0.
Polymer 8 Terminated with 3-(4-(Tert-
butoxy)phenyl)propenyl Acetate (4)
1H NMR (400 MHz, CDCl3) d 7.13–7.37 (m), 6.97 (dd, J 5 65.7,
8.3 Hz), 5.51–5.75 (m), 2.56–3.34 (m), 1.53–2.15 (m), 1.16-
1.31 (m). 13C NMR (101 MHz, CDCl3) d 178.36, 136.20, 133.49,
132.06, 131.88, 131.84, 128.79, 128.67, 128.55, 126.30,
124.25, 124.18, 113.97, 52.66, 51.11, 51.01, 45.81, 45.62,
42.14, 41.97, 40.86, 31.77 (t, J 5 10.0 Hz), 29.87, 28.85, 28.82,
27.97 (t, J 5 5.4 Hz), 26.66, 24.83, 24.76. MALDI-ToF MS calcd.
For C259H286N24O49Na1 [M 1 Na1]: 4539.1; Found: 4539.1.
Exo-N-hexyl-5-norbornene-2,3-dicarboximide
(20
eq,
247.3 mg, 1.0 mmol) was dissolved in degassed DCM (1 mL)
in a Schlenk flask, which was evacuated and charged with
argon three times. The polymerization was initiated by quick
addition of G1 (1 eq, 41.1 mg, 0.05 mmol) in degassed DCM
(1 mL). After 1 h, 3-(4-(tert-butoxy)phenyl)propenyl acetate
(5 eq, 62.1 mg, 0.25 mmol) was added to the polymer solu-
tion and the mixture stirred for 12 h. After addition of EVE
(50 eq) the solution was stirred for 15 min and then concen-
trated by a slight flow of nitrogen. The polymer was precipi-
tated by slow addition into a 10-fold excess of methanol. The
mixture was filtered to afford the respective polymer
Polymer 6 Terminated with 3-(4-(Tert-
butoxy)phenyl)propenyl Acetate (4)
Exo-N-methyl-5-norbornene-2,3-dicarboximide
(30
eq,
265.8 mg, 1.5 mmol) was dissolved in degassed DCM (1 mL)
4
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towards the ruthenium center27 [see Fig. 2(a)] this simplified
model can be used to understand the regioselectivity of the
reaction with electron rich olefins such as EVE.
The lone pair of the oxygen in VA, however, is less
p-donating than that of EVE [Figs. 1(b) and 2(b)] and we
were interested in investigating whether this would result in
a reduced regioselectivity in the reaction with Grubbs-type
ruthenium carbene complexes.
FIGURE 1 Resonance structures of EVE (a) and VA (b).
In an NMR experiment Grubbs first generation complex
(benzylidene-bis(tricyclohexylphosphine)-dichlororuthenium,
G1) was exposed to VA in benzene-d6 (Fig. 3). An immediate
shift of the carbene proton signal from 20.7 to 15.8 ppm
was observed. Furthermore, this acyl-Fischer-carbene28–30
reacted with EVE to form the vinyl ether Fischer carbene
with a carbene signal at 14.7 ppm in a Fischer-carbene to
Fischer-carbene transformation. While the observed upfield
shift of the carbene signal was in agreement with the respec-
tive p-donation strength of oxygen lone pairs on EVE and
VA, the NMR experiment gave no indication about a loss of
regioselectivity.
[280 mg, 95% yield, Mn(GPC, THF) 5 5200 g/mol, PDI: 1.16] as
a brown solid.
1H NMR (300 MHz, CDCl3) d 7.07–7.35 (m), 7.00 (d, J 5 8.4
Hz), 6.84 (d, J 5 8.4 H), 5.69 (d, J 5 6.2 Hz), 5.38–5.50 (m),
3.27–3.44 (m), 2.92 (s), 2.61 (s), 2.00–2.16 (m), 1.59 (s),
1.41–1.52 (m), 1.26 (s), 1.21 (s), 0.81 (t, J 5 6.5 Hz). 13C
NMR (75 MHz, CDCl3) d 178.32, 136.90, 132.05, 131.86,
131.81, 128.55, 124.20, 52.63, 50.99, 50.89, 45.85, 42.21,
42.04, 40.90, 38.57, 31.38, 31.30, 28.85, 27.64, 26.55, 26.45,
22.48, 14.00. MALDI-ToF MS calcd. For C274H379N17O35Na1
[M 1 Na1]: 4490.83; Found: 4490.98.
A model polymer (poly(MNI)) was therefore prepared using
G1 and exo-N-methyl-5-norbornene-2,3-dicarboximide (MNI).
The solution of the propagating ruthenium carbene complex
was divided into three reaction vessels and the first was
reacted with excess VA (polymer 1) and the second with
excess EVE (polymer 2). MALDI-ToF mass spectrometric
analysis of the isotopically resolved signals revealed that in
both cases all polymer chains were terminated with methy-
lene groups therefore indicating that the reaction with VA
and EVE occurred regioselectively.
Polymer 9 Terminated with 3-(4-(Tert-
butoxy)phenyl)propenyl Acetate (4)
Exo-N-methyl-7-oxanorbornene-2,3-dicarboximide (20 eq,
179.2 mg, 1.0 mmol) was dissolved in degassed DCM (1 mL)
in a Schlenk flask, which was evacuated and charged with
argon three times. The polymerization was initiated by quick
addition of G1(1 eq, 41.1 mg, 0.05 mmol) in degassed DCM
(1 mL). After 1 h, 3-(4-(tert-butoxy)phenyl)propenyl acetate
(5 eq, 62.1 mg, 0.25 mmol) was added to the polymer solu-
tion and the mixture stirred for 12 h. The solution was con-
centrated by a slight flow of nitrogen. The polymer was
precipitated by slow addition into a 10-fold excess of metha-
nol. The mixture was filtered to afford the respective poly-
mer [198.5 mg, 88% yield, Mn(GPC, THF) 5 3200 g/mol, PDI:
1.11] as a brown solid.
We next prepared a cis/trans mixture of but-1-en-1-yl acetate
BAc and used it to terminate the propagating ruthenium car-
bene complex of poly(MNI) (third reaction vessel, polymer 3).
An NMR-scale reaction of G1 benzylidene and BAc carried out
in parallel showed that the cis-BAc was consumed much faster
than the trans-BAc. Reaction of the cis/trans mixture of BAc
1H NMR (400 MHz, CDCl3) d 7.30–7.41 (m), 6.98 (dd,
J 5 66.0, 8.2 Hz), 5.91 (d, J 5 106.9 Hz), 4.99 (s), 4.49 (s),
3.34 (s), 2.96 (s), 2.14 (d, J 5 21.8 Hz), 1.74 (s), 1.28 (d,
J 5 27.7 Hz). 13C NMR (101 MHz, CDCl3) d 175.82, 130.98,
128.68, 126.81, 124.25, 80.83, 53.46, 52.36, 28.82, 25.11.
MALDI-ToF MS calcd. For C100H103N9O28Na1 [M 1 Na1]:
1900.7; Found: 1900.7.
RESULTS AND DISCUSSION
The reaction of Grubbs-type ruthenium carbene complexes
with EVE always occurs in a regioselective manner resulting
in the formation of a Fischer-carbene. An explanation for this
regioselectivity can be found in one of the resonance struc-
tures of EVE in which the b-carbon of the vinyl group carries
a negative charge [Fig. 1(a)]. This very simple resonance the-
ory model illustrates the polarization of the HOMO of EVE,
which undergoes the reaction with the LUMO of the rutheni-
um carbene. As the ruthenium carbene p-bond is polarized
FIGURE 2 Reaction of substituted vinyl ethers (a) and substitut-
ed enolesters (b) with ruthenium carbene complexes (A) form-
ing a metallacyclobutane (B) and the corresponding Fischer
carbene complexes (C).
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SCHEME 1 Synthesis of the functional enol esters 4 and 6.
chains carried the protected amine end group at one chain end
[Mn(GPC, THF) 5 5900, PDI 5 1.18, vs. PS standards, polymer 7, see
Supporting Information]. A second mass distribution was attribut-
ed to the aldehyde terminated polymer chains, a known side reac-
tion between the propagating ruthenium carbene complex and
oxygen (Supporting Information).32
FIGURE 3 1H NMR spectra (400 MHz, benzene-d6). Bottom: Car-
bene proton signal of Grubbs first generation (G1) initiator
(20.7 ppm) Middle: Acylcarbene of the G1 initiator (15.8 ppm)
Top: Fischer carbene from EVE and G1 (14.7 ppm). The signal
for the Fischer-carbene shifts from d 5 15.8 ppm to d 5 14.7
ppm indicating metathesis activity of the acyl carbene.
The observed difference between the molecular weight mea-
sured by GPC (THF) vs. PS standards and the observed mass
distribution in the MALDI-ToF mass spectrum is due to the
difference in hydrodynamic radius of polynorbornenes ver-
sus PS and has been described before in a similar context.3
with the propagating chain end of poly(MNI) and subsequent
MALDI-ToF mass spectrometric analysis (polymer 3) showed
that all polymer chains carried the corresponding propenyl
(CH3CH2CH@) end group as expected from a regioselective
reaction between the propagating G1 alkylidene and BAc.
Poly(MNI)s of different molecular weight (Mn 5 3900, 5400,
6800, see Supporting Information, polymers 4–6) were pre-
pared by varying the ratio of G1 to MNI and all were functional-
ly terminated with 4 to give tert-butyl protected phenol end
groups (Supporting Information). Low PDI values and a linear
relationship between the ratio of monomer to initiator and the
molecular weight was found. This strongly indicates that the
polymerization reactions were living (Supporting Information).
Enolacetates can be prepared readily from the corresponding
aldehydes. Although both, enol ethers and enol acetates are readi-
ly synthetically accessible in few reaction steps the cis:trans ratio
for enolacetates was slightly more favorable than those reported
for enol ethers.23,31 We therefore investigated the synthesis of
functional enolacetates carrying either a protected phenolic
alcohol or an amine group. As shown in Scheme 1, methyl 3-(4-
hydroxyphenyl)propanoate was first protected as the tert-butyl
ether 2 (81% yield) followed by DIBAL reduction to give alde-
hyde 3 (70% yield). Enolisation and acetylation under basic con-
ditions gave the target enolacetate 4 (53% yield). The protected
amine carrying enolacetate 6 (23% yield) could be prepared
readily via oxidation of benzyl (6-hydroxyhexyl)carbamate using
PCC to give the corresponding aldehyde 5 (52% yield), followed
by reaction with acetic anhydride under basic condition.
Exo-N-methyl-7-oxanorbornene-2,3-dicarboximide and exo-N-
hexyl-5-norbornene-2,3-dicarboximide were synthesized and
polymerizations initiated with G1. Both, poly(exo-N-hexyl-5-nor-
bornene-2,3-dicarboximide) [Mn(GPC, THF) 5 5200 g/mol, PDI:
1.16, polymer 8, Supporting Information] and poly(exo-N-methyl-
7-oxanorbornene-2,3-dicarboximide)
[Mn(GPC,
THF) 5 3200
g/mol, PDI: 1.11, polymer 9, Supporting Information] were
An excess of 4 was subsequently employed in a functional termi-
nation experiment using poly(MNI) prepared from G1 and MNI
(Scheme 2, polymer 4). As can be seen from the MALDI-ToF
(matrix: DCTB, sodium trifluoroacetate 5 NaTFA) mass spectro-
metric analysis of the obtained polymer (Fig. 4), every polymer
chain carries the tert-butyl protected phenol end group. 1H-NMR
spectroscopy of the polymer showed both required end groups,
one resulting from the reaction with 4 and the other resulting
from initiation with G1 benzylidene (Supporting Information).
GPC analysis in tetrahydrofuran (THF) showed a monomodal
distribution (Mn5 3900, PDI 5 1.12, vs. PS standards, see Sup-
porting Information). Similarly, if an excess of 6 was added to ter-
minate the reaction with poly(MNI), the vast majority of polymer
SCHEME 2 Functional end-capping of a propagating poly(MNI)
ruthenium carbene complex with enol acetate end-capping
reagent 4.
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complex yielding acyl Fischer-carbenes. Grubbs’ third generation
complex, however, does not react regioselectively with enolester
substrates. Reactions with the first generation complex were
employed to end-functionalize several poly(norbornene) deriva-
tives of different molecular weights with protected phenols and
protected amines. This new method describes a synthetically
straight-forward route to functional terminating reagents for
ROMP which can be prepared readily from functional enolisable
aldehydes.
ACKNOWLEDGMENTS
€
A.F.M. Kilbinger, P. Liu, M. Yasir, N. Hanik, and M. Schafer thank
the Swiss National Science Foundation (SNF) for funding of this
research.
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