Regioselective cyclopolymerization of 1,7-octadiynes

Article abstract of DOI:10.1021/ma201749n

The regioselective cyclopolymerization of two structurally different 1,7-octadiynes, i.e. of 1,4-dihexyloxy-2,3-dipropargylbenzene (M1) and (R,R)/(S,S)-4,5-bis(trimethylsilyloxy)-1,7-octadiyne (M2) has been achieved with the modified Grubbs-Hoveyda-type metathesis initiator Ru(NCO) ₂(IMesH₂)(=CH-(2-(2-PrO)C₆H₄)) (I1, IMesH₂ = 1,3-dimesitylimidazolidin-2-ylidene) and with a series of Schrock initiators in the presence of quinuclidine, yielding conjugated polymers consisting virtually exclusively of 1,2-cyclohex-1-enylenvinylene units. In contrast to I1, Mo(N-2,6-(2-Pr)₂C₆H₃) (CHCMe₂Ph)(OCHMe₂)₂ (I3) allows for establishing a living polymerization with M2 in the presence of quinuclidine. The structure of the polymers, which represent highly soluble and stable poly(acetylene) analogues, was confirmed by comparing the ¹³C NMR shifts of model compounds with those of the corresponding polymer. Poly(ene)s that are virtually solely based on six-membered repeat units show a bathochromic shift in UV-absorption compared to poly(ene)s based on six- and seven-membered rings.

Full text of DOI:10.1021/ma201749n

ARTICLE
pubs.acs.org/Macromolecules
Regioselective Cyclopolymerization of 1,7-Octadiynes
Stefan Naumann,†,§ J€org Unold,†,§ Wolfgang Frey,‡ and Michael R. Buchmeiser*,†,§
†Institute of Polymer Chemistry and ‡Institute of Organic Chemistry, University of Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany
§Institute of Textile Chemistry and Chemical Fibers, K€orschtalstrasse 26, D-73770 Denkendorf
S Supporting Information
b
ABSTRACT: The regioselective cyclopolymerization of two
structurally different 1,7-octadiynes, i.e. of 1,4-dihexyloxy-2,3-
dipropargylbenzene (M1) and (R,R)/(S,S)-4,5-bis(trimethy-
lsilyloxy)-1,7-octadiyne (M2) has been achieved with the
modified Grubbs-Hoveyda-type metathesis initiator Ru(NCO)2
(IMesH2)(dCH-(2-(2-PrO)C6H4)) (I1, IMesH2 = 1,3-dimesi-
tylimidazolidin-2-ylidene) and with a series of Schrock initiators in
the presence of quinuclidine, yielding conjugated polymers con-
sisting virtually exclusively of 1,2-cyclohex-1-enylenvinylene units. In contrast to I1, Mo(N-2,6-(2-Pr)2C6H3)(CHCMe2Ph)(OCHMe2)2
(I3) allows for establishing a living polymerization with M2 in the presence of quinuclidine. The structure of the
polymers, which represent highly soluble and stable poly(acetylene) analogues, was confirmed by comparing the 13C
NMR shifts of model compounds with those of the corresponding polymer. Poly(ene)s that are virtually solely based on
six-membered repeat units show a bathochromic shift in UV-absorption compared to poly(ene)s based on six- and seven-
membered rings.
’ INTRODUCTION
Scheme 1 illustrates the changes when moving from 1,6-
heptadiynes to 1,7-octadiynes. Thus, with 1,7-octadiynes, α- or
β-addition will deliver either six- or seven-membered repeat units
instead of five- or six-membered ones. Clearly, the six-membered
repeat units resulting from the α-addition of 1,7-octadiynes are
thermodynamically favored. We were interested, whether the
concept of regioselective cyclopolymerization with modified
Grubbs-Hoveyda and with Schrock-type initiators could be
extended to 1,7-octadiynes, the more since α-insertion-derived
poly(1,7-octadiynes) could serve as soluble progenitors to poly-
(o-phenylenevinylene)s, which are accessible from the corre-
sponding poly(1,2-cyclohex-1-enylenevinylene)s via oxidation
or thermally induced elimination of suitable substituents at the
4,5-position.
Centered in the fields of conjugated materials and metathesis,
cyclopolymerization has found considerable interest during the
last years.1À3 This is a direct consequence of the shortcomings
of other conjugated polymers like poly(acetylene)s, which are
either nonprocessable, unstable or display poor effective con-
jugation lengths.4À8 The cyclopolymerization of 1,6-hepta-
diynes has first been accomplished with ZieglerÀNatta
catalysts, Pd- and Ni-initiators and binary or ternary Mo- and
W-catalysts to yield soluble, conjugated poly(acetylene)-type
polymers.9À17 More recently, carefully designed Mo-based
Schrock initiators and appropriate reaction conditions allowed
for the regioselective and stereoselective cyclopolymerization
of various 1,6-heptadiynes to yield polymers consisting exclu-
sively of either cyclopent-1-enylene-1-vinylene- or cyclohex-1-
ene-3-methylidenes based repeat units.18À20 Complementary,
modified Grubbs- and Grubbs-Hoveyda catalysts allowed for
the regioselective cyclopolymerization of 1,6-heptadiynes to
yield poly(ene)s that are exclusively based on five-membered
repeat units, i.e., cyclopent-1-enylene-1-vinylenes.21,22 With
these Ru-based initiators, the polarization and thus activation
of the ruthenium-carbene by strongly electron withdrawing
ligands and electron-rich N-heterocyclic carbenes (NHCs) is a
prerequisite to render this type of initiator suitable for cyclo-
polymerization while maintaining high selectivity. Generally,
the key to any selective formation of a certain repeat unit is
selectivity in α- or β-addition, which in turn depends on the
steric demand and electronic situation in both the initiator and
the monomer.
’ MATERIALS AND CHARACTERIZATION
All manipulations where conducted in an N2-filled glovebox (MBraun
Lab Master 130) or standard Schlenk techniques were used. CH2Cl2,
diethyl ether, toluene, pentane and THF were dried by a solvent
purification system (SPS, MBraun). 1,2-Dichloroethane and CCl4 were
stirred over CaH2 for several hours and then distilled under nitrogen.
Starting materials were purchased from Aldrich (Munich, Germany),
TCI Europe (Zwijndrecht, Belgium) and ABCR (Karlsruhe, Germany)
and used without further purification. All initiators were prepared as
described in the literature.23À26 NMR measurements were recorded on a
Received: July 29, 2011
Revised:
September 20, 2011
Published: October 05, 2011
r
2011 American Chemical Society
8380
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Scheme 1. Two Possible Modes of Insertion Leading to Different Repeat Unitsa
a Left: 1,7-octadiynes yielding six- (top) and seven-membered (bottom) ring structures (dashed: aromatic backbone as present in M1). Right:
Corresponding structures derived from 1,6-heptadiynes.
Bruker Avance 250 or Bruker Avance III 400. IR spectra were measured
on an IFS 28 (Bruker) using NaCl cuvettes. UV/vis measurements were
carried out in CHCl3 on a Perkin-Elmer Lambda 2. GCÀMS data were
recorded on an Agilent Technologies device consisting of a 7693
autosampler, a 7890A GC and a 5975C quadrupole MS. Dodecane
was used as internal standard. A SPB-5 fused silica column (34.13 m Â
0.25 mm  0.25 μm film thickness) was used. The injection temperature
was set to 150 °C. The column temperature ramped from 45 to 250 °C
within 8 min, and was then held for further 5 min. The column flow was
1.05 mL per minute. GPC measurements were carried out on a system
constituted by a Waters 515 HPLC pump, a Waters 2707 autosampler,
PolyPore columns (300 Â 7.5 mm, Agilent technologies, B€oblingen,
Germany), a Waters 2489 UV/vis- and a Waters 2414 refractive index
detector.
Synthesis of M1. 1,4-Dihexyloxy-2,3-dimethylbenzene (2). 2,3-
Dimethylhydroquinone (1, 7.50 g, 54.3 mmol) and 1-hexyl bromide
(20.5 g, 130.3 mmol, 2.4 equiv) were dissolved in 80 mL of acetonitrile,
K2CO3 (48.25 g, 0.35 mol) was added, and the mixture was refluxed for
25 h. The reaction was monitored by GCÀMS. The salts were removed
by filtration and carefully washed with acetonitrile until colorless. Then,
the solvent was removed and the dark brown residue was dissolved in
petrol ether. After washing with 1 M NaOH solution, the organic layer
was washed with water and saturated NaCl solution and dried over
Na2SO4. The excess of hexyl bromide was removed in vacuo. The
resulting brown liquid was further purified by column chromatography
(silica) using petrol ether to yield 7.61 g (24.8 mmol, 46%) of a colorless
liquid. 1H NMR (CDCl3, δ): 6.65 (s, 2H); 3.89 (t, 3J = 6.5 Hz, 4H); 2.18
(s, 6H); 1.79 (quint, 3J = 6.4 Hz, 4H); 1.41À1.55 (m, 4H); 1.28À1.41
(m, 8H); 0.92 (t, 3J = 6.5 Hz, 3H). 13C NMR (CDCl3, δ): 151.6, 127.4,
109.6, 69.4, 32.0, 29.9, 26.3, 23.0, 14.5, 12.5. GCÀMS (relative
intensity): m/z calcd for C20H34O2, 306.26; found, 306.1 (19%),
222.1 (7%), 138.1 (100%), 123.0 (4%), 107.0 (4%), 55.1 (13%). IR
(film, cmÀ1): 2933 (s), 2860 (s), 1599 (w), 1468 (s), 1381 (m), 1254
(s), 1211 (s), 1107 (s), 1014 (w), 787 (m).
of a pale yellow crystalline solid. 1H NMR (CDCl3, δ): 6.80 (s, 2H), 4.76
3
3
(s, 4H), 3.97 (t, J = 6.45 Hz, 4H), 1.82 (quint., J = 6.45 Hz, 4H),
1.43À1.59 (br s, 4H), 1.27À1.43 (br s, 8H), 0.92 (t, 3J = 6.65 Hz, 6H).
13C NMR (CDCl3, δ): 151.6, 126.8, 113.5, 69.4, 31.9, 29.7, 26.2, 24.6, 23.0,
14.5. GCÀMS (relative intensity): m/z calcd for C20H32Br2O2, 464.08;
found, 464.1 (22%), 383.1 (24%), 304.1 (13%), 216.9 (39%), 135.0
(100%), 83.1 (89%), 55.1 (93%). IR (film, cmÀ1): 2929 (s), 2870 (m),
1714 (w), 1593 (w), 1487 (m), 1466 (s), 1267 (s), 1068 (w).
1,4-Dihexyloxy-2,3-bis(3-trimethylsilyl-2-propinyl)benzene (4)27. At
À78 °C, n-BuLi (8.1 mL, 1.6 M in hexane; 12.9 mmol) was added to a
solution of trimethylsilylacetylene (1.47 g, 15 mmol) in 20 mL of dry
THF. The cooling bath was removed and the mixture was stirred for a
further 50 min. 3 (1.00 g, 2.15 mmol) was dissolved in 5 mL of THF and
added to this solution at À78 °C. The mixture was warmed to room
temperature and stirred overnight. After heating to 50 °C for 8 h, the
reaction was stopped with saturated NaCl solution. The phases were
separated and the aqueous phase was extracted with diethyl ether. The
combined organic phases were washed with water and saturated NaCl
solution and dried over Na2SO4. After removal of the solvent, a dark
reddish oil was isolated. Purification by column chromatography (silica,
petrol ether: ethyl acetate = 50:1) yielded an orange colored solid (660
mg, 1.32 mmol, 61%). 1H NMR (CDCl3, δ): 6.73 (s, 2H), 3.93 (t, 3J =
6.45 Hz, 4H), 3.79 (s, 4H), 1.69À1.87 (m, 4H), 1.20À1.58 (m, 12H),
0.92 (br s, 6H), 0.11 (s, 18H). 13C NMR (CDCl3, δ): 152.2, 126.6,
111.9, 105.2, 83.9, 69.9, 32.0, 29.8, 27.3, 23.0, 17.1, 14.5, 0.5. GCÀMS
(relative intensity): m/z calcd for C30H50Si2O2, 498.33; found, 498.3
(4%), 413.2 (3%), 329.1 (22%), 308.9 (31%), 278.8 (9%), 241.0 (10%),
216.9 (5%), 84.0 (44%), 55.0 (100%). IR (film, cmÀ1): 2929 (s),
2858 (m), 2173 (s), 1466 (s), 1250 (s), 1074 (m), 1020 (m), 843 (s),
760 (m).
1,4-Dihexyloxy-2,3-dipropargylbenzene (M1)28. 4 (300 mg, 0.60
mmol) was dissolved in a mixture of CH2Cl2, acetone and water
(7:4:1). Under exclusion of light, AgNO3 (450 mg, 2.60 mmol) was
added. After stirring for 45 min at room temperature, 5 mL of water and
4 mL of 12.5 N hydrochloric acid were added. After stirring for another
hour, the phases were separated and the aqueous part was extracted
twice with CH2Cl2. The combined organic phases were washed with
water and saturated NaCl solution and dried over Na2SO4. The solvent
was removed in vacuo to yield a red-brown oil. It was purified by column
chromatography (silica, petrol ether:ethyl acetate=200:3). 185 mg
(0.52 mmol, 87%) of a yellow oil were obtained. 1HNMR(CDCl3, δ): 6.74
2,3-Bis(bromomethyl)-1,4-dihexyloxybenzene (3). 2 (560 mg, 1.83
mmol) was dissolved in 15 mL of dry CCl4 and the solution was heated
to reflux. Then, N-bromosuccinimide (650 mg, 3.66 mmol, 2 equiv) and
AIBN (8 mg, 0.05 mmol) were added simultaneously. After 30 min, the
reaction was finished as evidenced by discoloration of the solution and
formation of succinimde at the surface. The latter was removed by
filtration and the solvent was evaporated to yield 715 mg (1.54 mmol, 84%)
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(s, 2H), 3.93 (t, 3J = 6.45 Hz, 4H), 3.74 (d, 3J = 2.70 Hz, 4H), 1.94 (t, 3J =
2.70 Hz, 2H), 1.79 (quint, 3J = 6.45 Hz, 4H), 1.19À1.58 (m, 12H), 0.91 (br
s, 6H). 13C NMR (CDCl3, δ): 151.1, 125.9, 111.8, 82.7, 69.6, 68.1, 32.0,
29.8, 26.2, 23.0, 15.7, 14.4. HRMS (ESI): m/z calcd for C24H34O2,
354.2559; found, 354.2560. IR (film, cmÀ1): 3311 (s), 2931 (s), 2870
(m), 2117 (w), 1630 (m), 1597 (w), 1466 (s), 1261 (s), 1080 (s), 796 (m),
633 (m).
(DMSO-d6, δ): 8.53 (s, 2H), 6.45 (s, 2H), 3.07 (s, 4H), 1.75 (s, 6H). 13
C
NMR (DMSO-d6, δ): 147.5, 123.2, 123.0, 112.3, 32.2, 19.5. IR
(film, cmÀ1): 3230 (br), 2856 (w), 1600 (w), 1423 (m), 1335 (s),
1244 (s), 1119 (m), 968 (m), 804 (s), 741 (m).
5,8-Dihexyloxy-2,3-dimethyl-1,4-dihydronaphthaline (A). 6 (270 mg,
1.42 mmol), 1-hexyl bromide (700 mg, 4.26 mmol, 3 equiv), and K2CO3
(1.17g, 8.46mmol) wereaddedto20mLofacetoneandrefluxedfor2days.
The reaction was monitored by GCÀMS. The suspension was filtered to
remove all salts and vacuum distilled to remove any excess of hexyl bromide.
Column purification (silica, petrol ether:ethyl acetate = 100:0 to 50:1)
yielded 100 mg (0.28 mmol, 20%) of a greenish solid. 1H NMR (CDCl3,
Synthesis of M2. (R,R),(S,S)-4,5-Dihydroxy-1,7-octadiyne (7)31.
Lithium acetylide (2.9 g, 32.1 mmol) was dissolved in 80 mL of THF/
DMSO (3:1), diastereomeric diepoxybutane (1.3 g, 16 mmol) was
added dropwise at 0 °C and the mixture was stirred for 3 h at 0 °C. After
stirring overnight at room temperature, the reaction was stopped by
adding 5 mL of a saturated NH4Cl solution. The THF was removed
in vacuo and the mixture was extracted with CH2Cl2. The combined
organic phases were washed with saturated NaCl solution and dried
over Na2SO4. After removal of the solvent, a white solid remained.
Purification by column chromatography (silica, ethyl acetate:pentane = 3:1)
yielded 7 (718 mg, 5.20 mmol, 38%, ee = 4%). The enantiomers were
separated by chiral GC-chromatography on a Carlo Erba Strumenta-
zione HRGC-5300 Mega Series instrument using an Ambidex B column
(20 m x 0.3 mm x 0.25 μm; based on 21 mol % valinbornylamid, 5 mol %
permethyl-β-cyclodextrin). The injection temperature was 40 °C, the
temperature was then increased to 200 °C with a heating rate of 2.5 °C/
min. 1H NMR (CD3OD, δ): 4.78 (s, 2H, OH), 3.67 (m, 2H, CH), 2.31
(m, 4H, CH2), 2.17 (t, 2H, CtH). 13C NMR (CD3OD, δ): 82.32,
72.31, 71.48, 24.61. GCÀMS (EI, 70 eV): m/z calcd for C8H10O2,
138.07; found, 138 (M•+). Elem. Anal. calcd for C8H10O2: C, 69.54; H,
7.30. Found: C, 69.53, H, 7.37. IR (cmÀ1): 3286 (s), 2928 (s), 2891 (s),
2114 (m), 2170 (w), 2170 (w), 1469 (w), 1430 (w), 1331 (w), 1258
(w), 1219 (w), 1155 (w), 1124 (w), 1012 (s), 862 (w), 893 (w), 831
3
δ): 6.61 (s, 2H), 3.92 (t, J = 6.5 Hz, 4H), 3.23 (s, 4H), 1.81 (m, 4H),
1.29À1.59 (m, 12H), 0.94 (t, 3J = 7.0 Hz, 6H). 13C NMR (CDCl3, δ):
150.4, 125.7, 122.9, 108.2, 68.7, 32.0, 31.6, 29.8, 26.3, 23.1, 19.1, 14.5;
GCÀMS(relativeintensity):m/zcalcd for C24H38O2, 358.29; found, 358.3
(51%), 343.1 (1%), 274.1 (7%), 201.0 (4%), 187.0 (100%), 173.0 (71%),
159.1 (12%), 145.0 (15%), 115.0 (12%), 55.0 (46%). IR (film, cmÀ1):
2937 (s), 2922 (s), 2854 (s), 1610 (s), 1464 (s), 1255 (s), 1128 (s), 1066
(s), 779 (s).
trans-4,5-Bis(trimethylsilyloxy)-1-cyclohexene (B). trans-1-Cyclo-
hexene-4,5-diol (8) was prepared as described in the literature.30 (8)
(175 mg, 1.53 mmol) and triethylamine (0.60 mL, 3.98 mmol) were
dissolved in 40 mL of CH2Cl2 and trimethylchlorosilane (0.50 mL, 3.52
mmol) was added at 0 °C. After stirring for 5 h at room temperature, the
reaction was stopped by addition of 2 mL of saturated aqueous NH4Cl-
solution. The phases were separated and the aqueous phase was
extracted with CH2Cl2. The combined organic phases were washed
with water and saturated NaCl solution and dried over Na2SO4. Column
purification (silica, CH2Cl2) yielded 127 mg (0.49 mmol, 33%) of a
colorless oil. 1HNMR(CDCl3, δ): 5.44À5.33 (m, 2H), 3.60À3.46 (m, 2H),
2.31À2.13 (m, 2H), 1.99À1.80 (m, 2H), 0.00 (s, 18H); 13C NMR
(CDCl3, δ): 124.14, 71.32, 33.96, 0.00; GCÀMS (EI, 70 eV) m/z calcd
for C12H26O2Si2 = 258.1; found: 258.1 (M•+).
(w) cmÀ1
.
(R,R)/(S,S)-4,5-Bis(trimethylsilyloxy)-1,7-octadiyne (M2). Triethyla-
mine (0.80 mL, 5.6 mmol) and trimethylchlorosilane (0.7 mL, 0.5
mmol) were added to a solution of 7 (309 mg, 2.2 mmol) in 60 mL of
CH2Cl2. After stirring for 5 h at room temperature, the reaction was
stopped by addition of 5 mL of saturated NH4Cl solution. The phases
were separated and the aqueous phase was extracted with CH2Cl2. The
combined organic phases were washed with water and saturated NaCl
solution and dried over Na2SO4. After removal of the solvent, a
crystalline, white solid (512 mg, 1.8 mmol, 80%) was isolated. 1H
NMR (CDCl3, δ): 3.92À2.83 (m, 2H, CHÀOTMS), 2.47 (ddd,2J =
16.6 Hz, 3J = 6.1 Hz,4J = 2.7 Hz, 2H, CH2), 0.2.24 (ddd,2J = 16.6 Hz, 3J =
6.5 Hz,4J = 2.7 Hz, 2H, CH2) 1.95 (q, 4J = 2.7 Hz, 2H, CtCH), 0.19 (s,
18H, OSi(CH3)3. 13C NMR (CDCl3, δ): 82.23, 72.12, 70.46, 23.29,
0.81. Elem. Anal. Calcd for C14H26O2Si2: C, 59.52; H, 9.82. Found: C,
59.60, H 9.85. IR (cmÀ1): 3277 (m), 3254 (m), 2959 (m), 2921 (w),
2908 (w), 2888 (vw), 1429 (w), 1371 (m), 1253 (s), 1204 (w), 1127
(m), 1038 (s), 1002 (m), 968 (s), 947 (m), 843 (s), 751 (s), 685 (s).
Synthesis of Model Compounds A and B. 6,7-Dimethylte-
trahydro-4a,5,8,8a-1,4-naphtoquinone (5). Benzoquinone (2.00 g,
18.5 mmol) was dissolved in 18 mL of acetic acid. Two portions of
2,3-dimethyl-1,3-butadiene (each portion 675 mg; 9.25 mmol) were
added slowly to this solution. After stirring at room temperature
overnight, the crystalline needles that had formed were removed by
filtration and washed with water to yield 5 (630 mg, 3.3 mmol, 22%). 1H
NMR (CDCl3, δ): 6.63 (s, 2H), 3.17 (m, 2H), 2.37 (m, 2H), 2.04 (m,
2H), 1.60 (s, 6H). 13C NMR (CDCl3, δ): 200.7, 139.7, 47.4, 30.8, 19.2.
IR (film, cmÀ1): 2925 (m), 1687 (s), 1267 (m), 1090 (m), 847 (w).
6,7-Dimethyl-5,8-dihydronaphthaline-1,4-diol (6)29. 5 (325 mg,
1.71 mmol) was dissolved in 4 mL of acetic acid and the mixture was
heated to 80 °C under stirring. The oil bath was removed and one drop
of hydrobromic acid (32% in acetic acid) was added. The reaction
solution immediately solidified. The resulting white solid was then
Cyclopolymerization with Grubbs (ÀHoveyda)-Type Initiators. The
monomer was dissolved in 1,2-dichloroethane (ca. 0.12 mmol/mL), the
solution was filtered through neutral Al2O3 and heated to 45 °C. Under
nitrogen, a solution of I1 in 1,2-dichloroethane was added via syringe.
After 150 min, the polymerization was stopped by addition of ethyl vinyl
ether. After stirring for another 20 min, the solution was concentrated
and precipitated in methanol. The resulting red solid was washed twice
with methanol and acetone and dried in vacuo.
1
Poly-M1. H NMR (CDCl3, δ): 7.37À7.75 (br s, 1H), 6.39À6.80
(br s, 2H), 3.36À4.23 (br s, 8H), 0.50À1.98 (m, 25 H). 13C NMR
(CDCl3, δ): 150.2, 130.8, 125.7, 123.7, 107.3, 68.3, 31.7, 29.5, 26.6, 26.2,
22.8, 14.1. IR (film, cmÀ1): 2929 (s), 2868 (s), 1603 (m), 1468 (s), 1379
(m), 1257 (s), 1084 (s), 951 (m), 787 (m). UV/vis (CHCl3): λmax
=
470 nm.
1
Poly-M2. H NMR (CDCl3, δ): 6.95À6.63 (br,s, 1H), 3.75À3.45
(br s, 3H), 2.77À2.07 (m, 4H), 0.19À0.13 (m, 24). 13C NMR (CDCl3,
δ): 131.7, 125.4, 71.5, 34.6, 1.3. IR (film, cmÀ1): 2960 (m), 2907 (m),
1580 (w), 1443 (w), 1411 (w), 1319 (vw), 1251 (vs), 1097 (vs), 1012
(vs), 917 (m), 885 (m), 837 (s), 795 (vs), 742 (s), 683 (w). UV/vis
(CHCl3): λmax = 483 nm.
Cyclopolymerization with Schrock-Type Initiators. The monomer
was dissolved in CH2Cl2 or THF (c = 0.12 mol/L) and the solution
was filtered through neutral Al2O3. The initiator was dissolved in the
same solvent (1 mL) and added to the one of the monomer. For
polymerizations carried out at low temperature, both solutions were
first placed into the freezer and cooled to À36 °C. After 90 min, the
reaction was stopped by adding ferrocenylaldehyde (10 molequiv.
with respect to initiator), and after another 15 min the solution was
concentrated and precipitated in acetone. The resulting red solid was
washed twice with methanol and acetone or methanol/pentane (3:1)
and dried in vacuo.
1
washed with water to yield 280 mg (1.47 mmol, 86%) of 6. H NMR
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Figure 1. Initiators employed for the cyclopolymerizations of M1 and M2 and structure of the model compounds A and B.
Scheme 2. Synthesis of M1 and M2 and of the Model Compounds A and B
’ RESULTS AND DISCUSSION
The synthesis of M2 was accomplished starting from commer-
cially available diastereomeric butadiene diepoxide via a two-step
route (Scheme 2). Nucleophilic ring-opening of the diepoxide
with lithium acetylide was followed by standard trimethylsilyl-
protection of the resulting diol 7. Spectroscopic results and the
X-ray structure of M2 provide an argument for the stereoselec-
tive ring-opening of only one of the diastereomers, i.e., of the
(R,R)/(S,S) but not of the (R,S)/(S,R) couple, thereby selectively
forming diastereomeric (R,R)/(S,S)-4,5-bis(trimethylsilyloxy)-
1,7-octadiyne (M2). Model compound B was accessible via
epoxidation of 1,4-cyclohexadiene, followed by acid-catalyzed
ring-opening of the epoxide to yield the diol 8. Its etherification
with trimethylsilyl chloride in the presence of NEt3 finally yielded
model compound B (Scheme 2).
Synthesis ofInitiators, Monomers, and ModelCompounds.
The modified Grubbs-Hoveyda catalysts I1 and I2 (Figure 1) as
well as the Schrock-type initiators I3-I5 were prepared according
to the literature.23À26
1,4-Dihexyloxy-2,3-dipropargylbenzene (M1) was accessible
starting from commercially available 2,3-dimethylhydroquinone
via a four-step synthesis (Scheme 2). A simple sequence of
etherification, benzylic bromination and nucleophilic substitu-
tion yielded trimethylsilyl-protected M1. Deprotection proved
critical because standard procedures32,33 resulted in the partial
formation of allenes. It was thus necessary to deprotect M1 with
the aid of AgNO3 under acidic conditions.28
Model compound A was synthesized via the DielsÀAlder
reaction of benzoquinone with 2,3-dimethylbutadiene, followed
by acid-catalyzed aromatization and etherification (Scheme 2).
Cyclopolymerizations. The cyclopolymerization of both M1
and M2 by the action of I1 and I3ÀI5 quinuclidine (Figure 1)
3
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Table 1. Cyclopolymerization of M1 and M2 by Initiators I1ÀI5
no.
initiator
monomer
Mn (theor.) (g/mol)
Mna (g/mol)
PDI
λmaxb (nm)
%c α-insertion
yield (%)
1d
2e
3e
4f
I1
I3
M1
M1
M1
M1
M1
M2
M2
M2
M2
M2
17 700
26 600
26 600
26 600
26 600
22000
28000
28000
20000
28000
12 000
16 000
25 000
13 000
15 000
11000
57000
32000
7000
2.0
1.6
1.9
1.5
1.6
1.7
1.3
1.2
1.2
1.3
470
458
461
454
461
483
485
485
484
487
>98
90
24
64
48
47
54
17
80
96
89
95
I3 quinuclidine
80
3
I4 quinuclidine
79
3
5f
I5 quinuclidine
83
3
6d
7h
8g
9g
10g
I1
>95
>94
>98
>98
>94
I3 quinuclidine
3
I4 quinuclidine
3
I5 quinuclidine
3
I5 quinuclidine
67000
3
a Vs PS. b In CHCl3. c By 13C NMR. d ClCH2CH2Cl, 45 °C. e CH2Cl2, À36 °C f room temperature. f THF, À36 °C f room temperature. g CH2Cl2,
room temperature. h THF, room temperature.
resulted in red, film-forming polymers. The materials showed no
signs of decomposition even after prolonged storage under air or
in solution. Thus, no significant changes in the corresponding
NMR and UV spectra or in GPC were observed. Solubility was
very good in THF and halogenated solvents such as CHCl3.
Acetone could be used for washing poly-M1, while poly-M2
proved to be soluble in acetone at least up to number-average
molecular weights of 65 000 g/mol. Some significant differences,
however, between I1/I2 and I3ÀI5 quinuclidine have to be
3
stated (Table 1). Thus, the cyclopolymerization of M1 by the
ruthenium-based initiator I1 resulted in yields e24% when used
in 1,2-dichloroethane at 45 °C. Lowering the reaction tempera-
ture to room temperature resulted in yields <5%. Interestingly,
initiator I2 was not able to induce the cyclopolymerization of M1
at all. This contrasts former investigations regarding various 1,6-
heptadiynes, where the use of I2 allowed for controlled, some-
Figure 2. Graph of ln(C0/Ct) vs time for the cyclopolymerization of M2
times living, regioselective cyclopolymerizations1,21,22,34 and
by the action of I3 quinuclidine. C0 = 44.3 mmol/L, T = 25 °C, THF.
3
clearly illustrates the low cyclopolymerization propensity of this
monomer. Consequently, a more drastic activation of the
determined for kp/ki35 using a noncoordinating solvent, i.e.
ruthenium alkylidene as is given in I1, where the highly activating
C6D6. This value also accounts for the positive intercept in
isocyanates coordinate to the metal center23 appears to be
Figure 2 (vide supra). On the other hand, the rigid nature of the
necessary for this particular 1,7-octadiyne. Even so, the latter
poly(1,7-octadiynes) plays a key role. As found for poly(1,6-
only delivers low yields. Similar accounts for the cyclopolymer-
heptadiynes),18 where the rod-like structure leads to apparent
ization of M2 by the action of I1. GPC traces of both poly-M1
number-average molecular weights up to 50% higher than the
and poly-M2 prepared by the action of I1 were monomodal with
theoretical ones in case molecular weights are determined vs PS,
1.7 < PDI < 2.0 and an Mn around 12 000 g/mol (vs polystyrene);
higher apparent number-average molecular weights must be
however, yields were poor (17%). In no case could a controlled
expected for poly-M1 and poly-M2, too. Changing the solvent
or even living polymerization setup be accomplished.
from CH2Cl2 to THF did not only result in lower PDIs, but also
In contrast to these findings, the more reactive molybdenum-
allowed for the polymerization of M2 by the action of I3
quinculidine in a living manner. As can be deduced from
Figure 2, a linear plot of ln(M0/M) vs time, which supports
first-order polymerization kinetics, was obtained.
3
based Schrock initiators I3ÀI5 quinuclidine allowed for the
3
complete consumption of both M1 and M2. The quinuclidine
adducts of the Schrock initiators were chosen since they some-
times allow for more controlled polymerizations, i.e. lower values
of kp/ki. As can be seen in Table 1, the Schrock initiators
produced the corresponding cyclopolymers in up to 96% isolated
yield. In no case, any cross-linking, leading to insoluble polymers
or polymers with residual alkyne groups was observed. Values for
Microstructure and Optical Spectra. For reasons of sym-
metry, the 13C NMR spectrum of poly-M1 based on six-
membered repeat units prepared via selective α-addition should
show five signals in the aromatic region. This is what was
observed in case I1 was used for polymerization (Figure 3).
Mn were between 12 000 and 67 000 g/mol, poldispersity indices
ranged from 1.2 to 2.0. It is worth emphasizing that despite the
fact that isolated yields were <100%, in most cases the experi-
mentally determined number-average molecular weights were
significantly higher than the theoretical ones. This is, on the one
hand, attributable to initiation kinetics. Thus, for the polymer-
Three signals that can be assigned to the carbon atoms incorpo-
rated in the aromatic ring (A, B, and C) were observed at
δ = 150.2, 125.7, and 107.3 ppm. These chemical shifts perfectly
fit the ones of model compound A, where A*, B*, and C* appear
at δ = 150.1, 125.5, and 107.9 ppm. The additional peaks at
δ = 130.8 and 123.7 ppm were assigned to the conjugated
chain. Interestingly, these signals are comparably sharp, pointing
ization of M2 by the action of I3 quinuclidine, a value of 60 was
3
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Figure 3. Aromatic region of the 13C NMR spectrum (CDCl3) of model compound A (top), poly-M1 prepared by the action of I1 (middle) and of
poly-M1 prepared by the action of I3 quinuclidine (bottom).
3
toward a more flexible and most probably predominant all-trans-
configuration of the polymer, which is also supported by the IR
spectrum in which a strong signal at 960 cmÀ1 can be observed
for CdCtrans. The chemical shift of the methylene carbon inside
the newly formed 6-membered ring of the repeat units overlaps
with signals of the alkoxy side chains; tentatively, one peak at
δ = 26.6 ppm can be assigned to this atom. These findings clearly
indicate that the cyclopolymerization of this 1,7-octadiyne by the
modified Grubbs-Hoveyda initiator I1 proceeds, as observed for
1,6-heptadiynes, via regioselective α-insertion. Most probably,
the same arguments for the regioselective α-insertion, i.e. a
strong steric interaction between the N-heterocyclic carbene
and the second metallacyclobutene ring in β-insertion derived
intermediates, apply.36
integration can help to calculate the ratio of αÀ over β-addition
(Table 1). The highest content of 6-membered repeat units was
obtained with I3 and I5 quinuclidine. As has been found for
3
4-aza-1,6-heptadiynes,37 I4 containing the sterically most de-
manding ligands displays the lowest regioselectivity in insertion,
resulting in only 79% of α-insertion-derived repeat units
(Table 1).
In the cyclopolymerization of M2 by the ruthenium-based
initiator I1, again a regioselective α-addition occurs as evidenced
by the 13C NMR spectrum of poly-M2, which displays 5 signals.
Two signals (D, E) that can be assigned to the conjugated chain
appear at δ = 131.7 and 125.4 ppm. The chemical shift of the
methylene carbon of the newly formed 6-membered ring of the
repeat unit appears at δ = 34.6 ppm (F), the low field shifted
signal for the tertiary carbon (G) and finally the signal of the
TMS group (H) can be found at δ = 71.5 and 1.27 ppm,
respectively. These values again perfectly fit the chemical shifts
of model compound B, where E*, F*, G*, and H* appear at
δ = 124.1, 71.3, 33.9, and 0.0 ppm. Again, an all-trans structure is
Using the Schrock-type initiators I3ÀI5, the 13C NMR of
poly-M1 showed additional signals, irrespective of the catalyst
used. The spectra are consistent with a polymer structure
containing both α- and β-addition derived repeat units. The
arylic CB next to the oxygen serves as a sensitive probe to
calculate the ratio of six-membered units as given in Table 1.37
Thus, two signals at δ = 150.2 and 149.5 ppm are observed
in polymers containing both six- and seven-membered repeat
units (Figure 3). Since the carbon atoms are of the same kind,
proposed for poly-M2 as indicated by the IR-signal at 958 cmÀ1
.
In contrast to the 13C NMR spectrum of poly-M1, the ones of
poly-M2 prepared by any of the Schrock-type initiators I3ÀI5
3
quinuclidine showed two signals at δ = 71.0 and 70.4 ppm,
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Figure 4. 13C NMR (CDCl3) of model compound B (top) and poly-M2 prepared by the action of I1 (middle) and I4 quinuclidine (bottom).
3
however, no additional signals in the olefinic region that would
suggest a mixed polymer structure resulting from both α- and
β-addition (Figure 4).
(IMesH2)(dCHÀ(2-(2-PrO)C6H4)) (I1) is able to cyclopoly-
merize 1,4-dihexyloxy-2,3-dipropargylbenzene (M1) and (R,R)/
(S,S)-4,5-bis(trimethylsilyloxy)-1,7-octadiyne (M2) exclusively
via α-addition to yield unprecedented poly(1,2-cyclohex-1-en-
ylenevinylene)s, albeit in low yields and in a nonliving manner.
With M2, selected Schrock-type initiators also deliver poly-
(ene)es based on 6-membered repeat units via regioselective
α-insertion. Furthermore, a living polymerization setup can
be created with Mo(N-2,6-(2-Pr)-C6H3)(CHCMe2Ph)(OCH-
(CH3)2)2 in the presence of quinuclidine. Both poly-M1 and
poly-M2 that are virtually solely based on 6-membered repeat
units possess higher absorption maxima than the corresponding
polymers containing both 6- and 7-membered repeat units.
Apparently, in the presence of quinuclidine, all Mo-based
initiators, I3ÀI5, produce cyclopolymerization-derived poly-M2
via regioselective α-insertion. From the corresponding 13C NMR
spectra, an α-insertion selectivity upto >98% was determined. The
signal splitting around δ = 71 and 34.0 ppm is attributed to a
poly(ene)s in which an (R,R)-configured monomer is followed by
an (R,R) or an (S,S)-configured one. The different regioselectiv-
ities in insertion are also reflected by the UVÀvis spectra of the
corresponding polymers. Thus, UV/vis measurements revealed a
maximum in absorption at λmax = 470 nm for poly-M1 prepared
by the action of I1. In contrast, lower absorption maxima were
’ ASSOCIATED CONTENT
found for poly-M1 prepared by the action of I3ÀI5 quinucli-
3
dine, which all contain a lower fraction of 6-membered repeat
units. Vice versa, poly-M2 produced by the action of I1 as well as
Supporting Information.13C NMR spectra of M1, A, poly-
S
b
M1,andM2,1HNMRspectrumofpoly-M1,M2,andB, IR spectra of
poly-M1 and poly-M2, UVÀvis spectrum of poly-M2, GC chroma-
togram of 7, and X-ray structure and crystal data of M2.Thismaterialis
available free of charge via the Internet at http://pubs.acs.org/.
by I3ÀI5 quinuclidine showed virtually identical absorption
3
maxima in the range of 483À487 nm, which further supports
the high regioselectivity in insertion.
’ CONCLUSIONS
’ ACKNOWLEDGMENT
A “pseudo-halide”-modified GrubbsÀHoveyda type initiator
This work was supported by the Deutsche Forschungsge-
meinschaft (DFG, Project Number BU 2174/4-1).
bearing strongly electron withdrawing groups, i.e., Ru(NCO)2-
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