Mechanism of the Regio- and Stereoselective Cyclopolymerization of 1,6-Hepta- and 1,7-Octadiynes by High Oxidation State Molybdenum-Imidoalkylidene N -Heterocyclic Carbene Initiators

Article abstract of DOI:10.1021/acs.macromol.5b01185

The regio- and stereoselective cyclopolymerization of a series of 1,6-heptadiynes and 1,7-octadiynes, i.e., of 4,4-bis(ethoxycarbonyl)-1,7-heptadiyne (M1), 4-ethoxycarbonyl-4-(1R,2S,5R)-(-)-menthyloxycarbonyl-1,6-heptadiyne (M2), (R,S)-4,5-bis((1R,2S,5R)-

Full text of DOI:10.1021/acs.macromol.5b01185

Article
pubs.acs.org/Macromolecules
Mechanism of the Regio- and Stereoselective Cyclopolymerization of
1,6-Hepta- and 1,7-Octadiynes by High Oxidation State
Molybdenum−Imidoalkylidene N‑Heterocyclic Carbene Initiators
Katharina Herz,† Jorg Unold,§ Johannes Hanle,† Roman Schowner,† Suman Sen,† 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, Korschtalstr. 26, D-73770 Denkendorf, Germany
̈
S
* Supporting Information
ABSTRACT: The regio- and stereoselective cyclopolymerization
of a series of 1,6-heptadiynes and 1,7-octadiynes, i.e., of 4,4-
bis(ethoxycarbonyl)-1,7-heptadiyne (M1), 4-ethoxycarbonyl-4-
(1R,2S,5R)-(−)-menthyloxycarbonyl-1,6-heptadiyne (M2), (R,S)-
4,5-bis((1R,2S,5R)-(−)-menthyloxymethylcarboxymethyl)-1,7-oc-
tadiyne (M3), and (S,S/R,R)-4,5-bis((1R,2S,5R)-(−)-menthylox-
ymethylcarboxymethyl)-1,7-octadiyne (M4), by the action of the
molybdenum−imidoalkylidene N-heterocyclic carbene (NHC)
complexes (Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OTf)2(IMesH2)
(I1), Mo(N-2,6-Me2C6H3)(CHCMe3)(OTf)2(IMesH2) (I2), (Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(OTf)(OCH(CF3)2)-
(IMesH2) (I3), Mo(N-2,6-Cl2C6H3)(CHCMe3)(OTf)2(IMes) (I4, IMesH2 = 1,3-dimesitylimidazolidin-2-ylidene, IMes = 1,3-
dimesitylimidazolin-2-ylidene) and by the molybdenum−imidoalkylidene monoaryloxide monopyrrolide (MAP) complexes
Mo(N-2,6-iPr2-C6H3)(CHCMe2Ph)(2,5-Me2-pyrrolide)(O-2,6-Me2-C6H3) (MAP1), Mo(N-2,6-iPr2-C6H3)(CHCMe2Ph)(2,5-
Me2-pyrrolide)(O-2,6-Mes2-C6H3) (MAP2), and Mo(N-2-tBu-C6H4)(CHCMe2Ph)(2,5-Me2-pyrrolide)(O-2,6-Mes2-C6H3)
(MAP3) is described. With initiators I1−I4, high regioselectivity >96% was observed in the polymerization of M1−M4,
allowing for virtually selective α-insertion. These initiators displayed also high stereoselectivity, allowing for all-trans polyenes
with up to 96% syndioselectivity. By contrast, the MAP initiators MAP1−MAP3 showed poor regioselectivity (26−62% α-
selectivity). A polymerization mechanism is proposed that explains for the high stereo- and regioselectivity of the Mo−
imidoalkylidene NHC complexes.
a predominant role in the control of the regioselectivity.32,33
INTRODUCTION
■
Recently, molybdenum−imidoalkylidene NHC bis(triflate)34
The cyclopolymerization of 1,6-hepta- and 1,7-octadiynes has
been widely used for the syntheses of soluble, conjugated
and cationic tungsten−oxoalkylidene NHC complexes35 have
polymers possessing a rigid backbone.1−29 Equally important, it
can also be used as a probe to determine both the regio- and
stereoselectivity of insertion and to come up with a
polymerization mechanism for a specific initiator system. Two
different pathways for the cyclopolymerization of α,ω-diynes
with Schrock-type molybdenum carbenes exist (Scheme 1).3,4
Thus, the monomer can undergo either α- or β-addition. The
first addition step of the monomer to the metal alkylidene is
decisive for the structure of the resulting repeat unit. In the
absence of any backbiting, the polymer structure is a concise
record of all previous insertion steps. For first-generation
Schrock initiators, the concept of large and small alkoxides was
developed by the Schrock group and allowed for the selective
β-addition of monomer.1,2 Our group was capable of extending
this concept to selective α-addition by using first-generation
Schrock initiators bearing small alkoxides, an additional
coordinating base, and low temperatures.18,20−24,30,31 In that
context not only the alkoxide but also the imido ligands played
been developed in our group. Particularly molybdenum−
imidoalkylidene NHC bistriflates display high reactivity for
various norborn-2-enes, 7-oxanorborn-2-enes, and diynes and
are also tolerant toward nitrile, amino, hydroxyl, and carboxylic
acid groups.34 We were interested in the polymerization
mechanism that applies in the polymerization of α,ω-diynes
with these novel initiators. Equally important, we wished to find
out about the stereo- and regioselectivity of these novel
complexes and therefore addressed the issues of tacticity and
cis/trans selectivity in cyclopolymerization. Since the cationic
species derived from Mo−imidoalkylidene NHC bis(triflate)
complexes resemble to some extent molybdenum−imidoalky-
lidene monoalkoxide monopyrrolide (MAP) catalysts devel-
oped by the Schrock group,36,37 and since these systems have
Received: June 2, 2015
Revised: June 30, 2015
© XXXX American Chemical Society
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Scheme 1. Pathways for the Cyclopolymerization of α,ω-Diynes with a Metal Alkylidene3,9
Figure 1. Structure of model compound A and of initiators I1−I4 and MAP1−MAP3 used for the cyclopolymerization of monomers M1−M4.
also been reported to display a high degree of regio- and
stereospecifity both in olefin metathesis38−42 and in ring-
opening metathesis polymerization,43−45 we also extended our
investigations onto this class of initiators. Here we report our
results.
as the monoalkoxide monopyrrolide (MAP) initiators MAP1
and MAP236,43,47,48 (Figure 1) were prepared according to the
literature. Starting from Mo(N-2-t-BuC6H4)(CHCMe2Ph)-
(OTf)2(DME) (1), the bispyrrolide (2) could be prepared by
adding 2 equiv of lithium 2,5-dimethylpyrrolide. Conversion of
2 with 1 equiv of 2,2″,4,4″,6,6″-hexamethyl-m-terphenol led to
the formation of MAP3 (Scheme 2).
MAP1, MAP2, and 2 were characterized by single-crystal X-
ray structure. MAP1 (Figure 2) crystallizes in the triclinic space
group P1, a = 936.88(4) pm, b = 960.41(4) pm, c = 1894.22(9)
pm, α = 86.863(3)°, β = 85.260(2)°, γ = 70.338(2)°, and Z = 2.
MAP2 (Figure 3) also crystallizes in the triclinic space group,
P1, a = 1039.30(7) pm, b = 1191.83(7) pm, c = 3756.2(2) pm,
α = 92.528(3)°, β = 93.652(4)°, γ = 107.923(3)°, and Z = 4.
RESULTS AND DISCUSSION
■
Synthesis of Initiators, Monomers, and Model
Compound A. Mo(N-2,6-Me2C6H3)(CHCMe2Ph)-
(OTf)2(IMesH2) (I1),34 Mo(N-2,6-Me2C6H3)(CHCMe3)-
(OTf)2 (IMesH2 ) (I2),34 (Mo(N-2,6-Me2 -C6 H3 )-
(CHCMe2Ph)(OTf)(OCH(CF3)2)(IMesH2) (I3),46 and
Mo(N-2,6-Cl2C6H3)(CHCMe3)(OTf)2(IMes) (I4)46 as well
B
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Scheme 2. Synthesis of MAP3 and of Model Compound A
Figure 2. Single-crystal X-ray structure of MAP1. Selected bond
lengths (pm) and angles (deg): Mo(1)−N(1) 173.53(16), Mo(1)−
C(1) 187.4(2), Mo(1)−O(1) 192.75(14), Mo(1)−N(2) 203.07(17);
N(1)−Mo(1)−C(1) 103.70(8, N(1)−Mo(1)−O(1) 115.19(7),
C(1)−Mo(1)−O(1) 106.70(7), N(1)−Mo(1)−N(2) 108.93(7),
C(1)−Mo(1)−N(2) 106.63(8), O(1)−Mo(1)−N(2) 114.72(7).
Figure 3. Single-crystal X-ray structure of MAP2. Selected bond
lengths (pm) and angles (deg): Mo(1A)−N(1A) 173.2(3), Mo(1A)−
C(1A) 188.8(4), Mo(1A)−O(1A) 192.8(3), Mo(1A)−N(2A)
202.6(3), O(1A)−C(29A) 135.4(5); N(1A)−Mo(1A)−C(1A)
101.88(16), N(1A)−Mo(1A)−O(1A) 117.37(14), C(1A)−Mo-
(1A)−O(1A) 112.92(15), N(1A)−Mo(1A)−N(2A) 103.39(15), C-
(1A)−Mo(1A)−N(2A) 103.16(16), O(1A)−Mo(1A)−N(2A)
116.17(13).
Compound 2 (Figure 4) again crystallizes in the triclinic space
group, P1, a = 867.15(7) pm, b = 1132.46(10) pm, c =
1519.45(13) pm, α = 92.600(5)°, β = 99.484(4)°, γ =
104.196(4)°, and Z = 2. In all three complexes, the ligands
adopt a tetrahedral configuration. Relevant bond lengths and
angles for MAP1, MAP2, and 2 are summarized in Figures
2−4. Both in MAP1/MAP2 and in 2 the length of the metal
alkylidene as well as the metal−nitrogen distances are
inconspicuous and comparable to related compounds.36,37
4-(Ethoxycarbonyl)-4-(1R,2S,5R)-(−)-menthoxycarbonyl-
1,6-heptadiyne (M2)49 and 4,4-bis(ethoxycarbonyl)-1,6-hepta-
diyne (M1)23,24 were synthesized according to the literature.
Model compound A was synthesized by alkylation of ethyl-
(1R,2S,5R)-(−)-menthylmalonate with NaH/allyl bromide
followed by ring-closing metathesis (RCM) using the second-
generation Grubbs initiator (Scheme 2).23
Regio- and Stereoselective Cyclopolymerization with
Mo−Imidoalkylidene−Bistriflate NHC Initiators. Cyclo-
polymerization of M1−M4 (Figure 1) was accomplished by the
action of initiators I1−I4, respectively. Cyclopolymerization of
M1 by the action of I1 yielded poly-M1 with ≥99% α-insertion
selectivity (Figure S26, Supporting Information). This high α-
insertion selectivity is in line with previous reports on the
cyclopolymerization of other 1,6-heptadiynes such as 4,4-
bis[(3,5-diethoxybenzoyloxy)methyl]-1,6-heptadiyne by the
action of I2.34
Generally, cyclopolymerization-derived polyenes can exist in
their cis or trans form and be either isotactic (it), syndiotactic
(st), or atactic (at). If initiation and propagation proceed in a
C
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spectroscopy, the tacticity of poly(α,ω-diyne)s cannot be
determined unless the monomer contains at least one chiral
center, usually in the side chain. In this case, and only in this
case, one can together with IR data and with the aid of
1
correlated H NMR spectroscopy distinguish between cis-/
trans-st/it structures.3−5,9,20−24,31,50,51 The cyclopolymerization
of 4-ethoxycarbonyl-4-(1R,2S,5R)-(−)-menthyloxycarbonyl-
1,6-heptadiyne (M2) was accomplished by the action of I2,
I3, and I4. The 13C NMR spectra of the corresponding
polymers were virtually identical, which suggests that neither
the nature of the NHC (IMes vs IMesH2) nor of the imido-
ligand (2,6-Me2-C6H4 vs 2,6-Cl2-C6H4) has a strong influence
on regio- and stereoselectivity. The 13C NMR spectrum of
poly-M2 prepared by the action of I2, representative for the
polymers obtained with all other initiators, is shown in Figure 5.
Signals at δ = 172.0 and 171.4 ppm can be assigned to the
two carbonyls. Their chemical shifts are in full agreement with
those in model compound A (δ = 172.4 and 171.8 ppm). The
ipso-carbon at δ = 57.5 ppm is typical for α-insertion-derived
polyenes, and its chemical shift fits again to the one in model
compound A (δ = 59.1 ppm). No additional signals for β-
insertion-derived structures are visible. The IR spectrum of this
polymer shows only trans-configured double bonds at 955
cm−1. Consequently, either trans-it or trans-st diads must be
present. As outlined in Figure 6, a trans-st structure would have
uncoupled olefinic protons, while a trans-it structure would
show coupled protons.
Figure 4. Single-crystal X-ray structure of 2. Selected bond lengths
(pm) and angles (deg): Mo(1)−N(1) 173.3(2), Mo(1)−C(1)
192.5(2), Mo(1)−N(2) 209.5(2), Mo(1)−C(28) 236.0(2), Mo(1)−
C(27) 240.0(2), Mo(1)−C(29) 243.4(2), Mo(1)−N(3) 245.5(2),
Mo(1)−C(30) 247.9(3); N(1)−Mo(1)−C(1) 101.25(10), N(1)−
Mo(1)−N(2) 100.27(9), C(1)−Mo(1)−N(2) 101.95(9), N(1)−
Mo(1)−N(3) 159.00(8), C(1)−Mo(1)−N(3) 86.94(9).
controlled and defined manner, then the resulting polymers
should either display a cis- or trans-syndiotactic (st) or isotactic
(it) structure. While any predominant cis- or trans-configuration
in the polymer chain can easily be determined by IR
1
Based on the H,1H−COSY spectrum of poly-M2 prepared
by the action of I2 (Figure S29, Supporting Information),
which shows no coupling of the main signal at δ = 6.68 ppm, a
Figure 5. 13C NMR spectrum (CDCl3) of poly-M2 prepared by the action of I2.
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should show two coupled olefinic protons, while the olefinic
protons in a trans-st structure should be uncoupled. The
1H,1H−COSY spectrum of poly-M3 prepared by the action of
I1 (Figure S41, Supporting Information) shows one large
olefinic peak at δ = 6.96 ppm and no cross-coupling signals.
Olefinic signals in the range of δ = 5.4−6.6 ppm are assigned to
the end groups. Therefore, even when taking the end groups
into account, poly-M3 prepared by the action of I1 is formed
via >99% α-addition and exist as >99% trans, >72% st polymer.
The polymerization of (racemic) (S,S/R,R)-4,5-bis-
((1R,2S,5R)-(−)-menthyloxymethylcarboxymethyl)-1,7-octa-
diyne (M4) by the action of I1 yielded poly-M4. Its 13C NMR
spectrum is shown in Figure 9.
Figure 6. Structure of trans-it and trans-st diads of poly-M2 (R = chiral
group).
>96% α-insertion derived, all-trans, >96% st structure is
Signals at δ = 170.9 (CO), 131.8 (CH), 125.2 (CC),
and 28.4 ppm (CH2) can again be clearly assigned to an α-
insertion-derived, i.e., 6-membered, repeat unit.5 As for poly-
M3, no signals for β-insertion-derived repeat units are observed.
The IR spectrum of poly-M4 shows both trans- and cis-
configured double bonds (950 and 700 cm−1, Figure S46,
Supporting Information). The 1H,1H−COSY spectrum of poly-
M4 prepared by the action of I1 (Figure S45, Supporting
Information) shows one large uncoupled signal at δ = 6.97
ppm. Again, the coupled signals at 5.8 < δ < 6.8 ppm are
assigned to end groups. According to the possible regular
structures shown in Figure 8b, either cis-it or trans-it structures
must be expected. Accordingly, poly-M4 forms by the action of
I1 via selective α-addition and exists as cis/trans-it polymer
(>66% it).
assigned to this polymer. Notably, because of the rigid
1
character of the backbone, the H NMR spectra of poly(ene)s
1
show a by far lower resolution than the H NMR spectra of
tactic poly(cyclopropene)s, poly(norbornene)s, or poly-
(norbornadiene)s.52−56 Nonetheless, the 1H,1H−COSY spectra
still allow for concluding on tacticity.5
The polymerization of M3 by the action of I1 yielded poly-
M3. The 13C NMR spectrum of poly-M3 prepared by the
action of I1 is shown in Figure 7.
Signals at δ = 132.0 (CH), 125.1 (CC), and 28.5 ppm
(CH2) can be clearly assigned to an α-insertion-derived, i.e., 6-
membered repeat unit.5 The absence of any additional signals
for β-insertion-derived repeat units and the number of carbons
visible in the 13C NMR clearly point toward selective α-
insertion (>99%). The IR spectrum of poly-M3 (Figure S42,
Supporting Information) shows trans-configured double bonds
(νCC = 951 cm−1). According to Figure 8a, a trans-it structure
Polymerization Mechanism. Polymerization of M2 and
M3 by the action of I1−I4 yields predominantly trans-st-
Figure 7. 13C NMR spectrum (CDCl3) of poly-M3 prepared by the action of I1.
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Figure 8. Possible microstructures of cyclopolymerization-derived chiral poly(1,7-octadiyne)s. R = chiral group; (a) poly-M3; (b) poly-M4.
Figure 9. 13C NMR spectrum (CDCl3) of poly-M4 prepared by the action of I1.
polymers. According to calculations carried out on other group
6 and 7 metathesis catalysts,57−59 as well as in view of results
published by the Schrock group on 2,5-dimethylpyrrolide
complexes,60 which propose attack of monomer trans to the
strongest σ-donor, i.e., trans to the NHC, we suggest the
mechanism shown in Scheme 3.
weakly bound OTf− groups are found at 77.2 < δ < 79 ppm.
Consequently, both the initiators and propagating species are
drawn in their cationic form. Starting from a cationic syn-
isomer, M3 approaches the metal α-selectively and trans to the
NHC. The two chiral R-groups point away from the R-groups
at the growing polymer chain (chain end control). An α-
addition-derived transition state forms, resulting in the
formation of a disubstituted alkylidene. Formation of a trans-
double bond must be a result of the relatively large substituents
at the molybdacyclobutene. Notably, the configuration at
19F NMR measurements (Figures S47−S49) suggest that one
OTf− group in I1 and in I3 is only weakly bound to Mo (δ =
−76.6 and −78.1 ppm), forming a (weak) contact ion pair
(Figures S47−S49). Upon addition of monomer, signals for
F
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Scheme 3. Formation of α-Insertion-Derived trans-st-Poly-M3 (R = (-)-Menthyl-O-CH2-COOCH2−)
Scheme 4. Formation of α-Insertion-Derived trans-it-Poly-M4 (R = (-)-Menthyl-O-CH2-COOCH2−)
molybdenum has changed at that point from S to R (or vice
versa). Now, the second alkyne group present in the monomer
again approaches the metal intramolecularly trans to the NHC.
Metallacyclobutene formation followed by cycloreversion
results in an α-addition-derived, trans-st-polymer. Provided
the fact that the bulky menthyl groups in the monomer point
away from those in the growing polymer chain (anti-approach),
the consecutive insertion of two monomers again leads to the
propagating species I. The same sequence is also possible for
M2 (not shown), and consequently poly-M2 has a >96% α-
insertion derived, all-trans, >96% st structure. The situation
with M4 seems to be different. There, one might speculate
whether the trans-configuration of the two chiral groups always
leads to some steric interaction between the growing polymer
chain and the monomer, irrespective of the initial approach of
the monomer to the initiator. However, as outlined in Scheme
4, in case M4 follows the same reaction pathway as M2 and
M3, an α-insertion-derived, it polymer is formed. The situation
where an S-configured initiator reacts with the S,S-enantiomer
has been chosen arbitrarily; however, the arguments apply for
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a
Table 1. Results of the Cyclopolymerization of M1 by the Action of MAP1−MAP3
b
c
d
initiator
additive
solvent, T [°C]
Mn (found) [g mol−1
]
PDI
λmax [nm]
yield [%]
χ5 [%]
χ6 [%]
MAP1
MAP2
MAP2
MAP3
−
−
−
−
CH2Cl2, rt
CH2Cl2, rt
toluene, rt
toluene, rt
20500
27000
52700
53500
1.4
8
−
542
93
91
84
95
57
29
26
62
43
71
74
38
10
11
542
548, 585
a
b
c
Poly-M1: Mn(theor) = 12126 g/mol; monomer:initiator = 50:1, Q = quinuclidine. Isolated yields. Fraction of α-insertion-derived repeat units.
Fraction of β-insertion-derived repeat units; rt = room temperature.
d
Figure 10. Integration of carbonyl signals (left) and aliphatic, ipso-carbons (right) of poly-M1 prepared by the action of MAP2.
any combination provided the fact that the corresponding
initiator−monomer couples form selectively, i.e., S/S,S and R/
R,R or S/R,R and R/S,S.
In line with Schrock’s concept of small and large alkoxides,
the use of the sterically demanding terphenoxide in MAP2
allowed for a β-insertion selectivity of 74%, which is however
still lower than the ones reported earlier for the bis-
(triphenylacetate) complexes (100% β-insertion).1,2 However,
we obviously cannot rule out that a careful ligand tuning in
MAP-type complexes would also provide improved regiose-
lectivity in the cyclopolymerization of diynes. Obviously, the
novel Mo−imidoalkylidene NHC complexes closely follow the
polymerization mechanism proposed for MAP-type cata-
lysts.51,61 The reason for their substantially better regioselec-
tivity might be related to a lower propensity to undergo Berry
rotation; however, this is purely speculative at this stage.
We finally wish to address the issue of whether the proposed
tacticities could be confirmed by an alternative, independent
method. The Schrock group very recently proposed low degree
epoxidation or bromination of poly(norbornene)s as a viable
method.62 Unfortunately, despite its beauty, this method
cannot be applied to polymers with conjugated double bonds
for obvious reasons.
Finally, a few additional issues have to be kept in mind. First,
initiators I1−I4 exist in their racemic forms. Despite the fact
that release of triflate can well be expected to occur selectively
for the triflate that experiences the strongest trans-effect,34 the
resulting cationic species must therefore exist in form of their
racemates, too. Now, what is the consequence? For both M2
and M3, enantiomeric polymer chains can form, no matter
which enantiomer of the initiator forms the propagating
species. With M4, which exists in form of a racemate, the
situation must be somewhat different. There, two enantiomeric
polymer chains can only form if the (S)-enantiomer of the
initiator selectively reacts with the (S,S) enantiomer and the
(R)-enantiomer of the initiator selectively reacts with the (R,R)-
enantiomer of the monomer. This implies that consecutive
insertion of two enantiomeric monomers into the metal
alkylidene, i.e., (S,S) followed by (R,R), does not occur. In
fact, this makes sense. A situation in which one enantiomer
could either be followed by the same or by the other
enantiomer would imply negligible differences between the
corresponding diastereotopic transitions states (ΔΔG† < 2
kcal) and would therefore probably lead to mostly atactic
structures.
Cyclopolymerization with Mo-MAP Initiators. In terms
of structure, I1−I4 are at least to some extent comparable to
MAP initiators. Since no data on the cyclopolymerization
propensity of MAP initiators existed, we prepared MAP1−
MAP3 for these purposes. MAP1 is based on a small alkoxide;
MAP2 and MAP3 contain a large alkoxide. An additional
feature of MAP3 is the “small” arylimido group. The
cyclopolymerization of M1 by the action of MAP1−MAP3 in
toluene or CH2Cl2 led to purple polymers with complete
monomer conversion. Polymerization results are summarized in
Table 1. Analysis of the 13C NMR spectra revealed poor α-
insertion selectivity in the range of 26−62%. A typical 13C
NMR spectrum is shown in Figure 10.
SUMMARY
■
The regio- and stereoselective cyclopolymerization of 1,6-
hepta- and 1,7-octadiynes by Mo−imidoalkylidene NHC-based
initiators has been carried out and a viable mechanism has been
developed. According to the proposed mechanism, initiators
I1−I4 behave quite similarly to the known MAP initiators;
however, at least based on the results obtained with MAP1−
MAP3, they display higher regioselectivity. Current work
focuses on tuning the activity and selectivity of these initiators,
e.g., by replacing one triflate by fluorinated alkoxides or by
using NHCs with a second, chelating group. Results will be
reported in due course.
EXPERIMENTAL SECTION
General. All experiments were carried out in a N2-filled glovebox
(Lab Master 130, MBraun, Garching, Germany) or by standard
Schlenk techniques. All chemicals used were purchased from Sigma-
■
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Aldrich (Munich, Germany), Alfa Aesar (Karlsruhe, Germany), ABCR
(Karlsruhe, Germany), or Merck (Darmstadt, Germany) and were
dried and distilled prior to use. All other chemicals were dried and
degassed by well-established techniques. Dichloromethane, diethyl
ether, toluene, pentane, and tetrahydrofuran (THF) were dried using a
solvent purification system (SPS, MBraun) and stored over molecular
sieves. Polymer solutions were filtered through a 0.2 μm filter
(Sartorius, Germany) prior to GPC measurements to remove any
insoluble particles. GPC measurements were carried out on a system
consisting of a Waters 515 HPLC system with a Waters autosampler,
was dissolved in 10 mL of dry THF and added slowly to a stirred
suspension of NaH (3.55 g, 60 wt %, 88.8 mmol) in 100 mL of THF.
Once the formation of gas had ceased, allyl bromide (7.6 mL, 88.8
mmol) was added, and the mixture was stirred for another 6 h. Then,
the mixture was chilled to 0 °C, and a saturated aqueous NH4Cl
solution (40 mL) was added. A white solid precipitated. The organic
layer was separated, washed with water (2 × 20 mL) and brine (2 × 20
mL), and dried over MgSO4. Finally, the solvent was purified by bulb
distillation. A colorless oil (6.23 g, 81%) was isolated after freeze-
drying. 1H NMR (CDCl3): δ = 5.69−5.56 (m, 2H, CHCH2), 5.16−
5.06 (m, 4H, CHCH2), 4.69 (t × d, 3J1 = 10.9 Hz, 3J2 = 4.4 Hz, 1H,
PolyPore columns (300 × 7.5 mm, Agilent Technologies, Boblingen,
̈
2
3
Germany), and a Waters 2489 UV/vis and a Waters 2414 refractive
index detector. For thin layer chromatography a coated aluminum foil
with silica gel endowed with a fluorescence indicator (Merck, silica gel
60 F254) was used. UV-active substances were detected at 254 and
366 nm; for coloring KMnO4 was used. For column chromatography,
silica gel (Fluka, 60M: 0.040−0.063 mm grain size, 230−400 mesh
ASTM) was used as stationary phase. For the different mobile phases
used vide infra. NMR spectra were recorded on a Bruker Avance III
400 at 400 MHz for proton and 101 MHz for carbon. All spectra were
recorded at room temperature and calibrated to characteristic solvent
signals. Analysis of the NMR data was accomplished with the aid of the
Topspin 3.0 program package. Chemical shifts (δ) are reported in ppm
relative to the solvent signal, coupling constants (J) in hertz (Hz). The
following abbreviations were used: s = singlet, d = doublet, t = triplet,
q = quartet, m = multiplet, and b = broad. The pulse sequences
cosyqf90 (Bruker) and zgpg30 were used for 2D COSY and 13C
NMR measurements. IR spectra were carried out on an IFS 28
(Bruker) using NaCl cuvettes. UV/vis values were measured in CHCl3
using a PerkinElmer Lambda 2 or a UV-1800 Shimadzu UV
spectrometer. GC-MS data were recorded on an Agilent Technologies
device consisting of a 7693 autosampler, a 7890A GC, and a 5975C
quadrupole MS equipped with an SPB-5 fused silica column (34.13 m
× 0.25 mm × 0.25 μm film thickness). Dodecane was used as internal
standard. 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/min. IR spectra
were measured on an ATR/FT-IR spectrometer Bruker IFS 128 in the
area 4000 to 400 cm−1 and analyzed by the OPUS software (Version
7.2). Wavenumbers are reported in cm−1. Mass spectra were measured
at the Institute of Organic Chemistry of the University of Stuttgart on
a Bruker Daltonics Microtof Q mass spectrometer. Initiators I1,34 I2,34
I3,42 and I446 and the MAP initiators MAP1−MAP236,43,47,48 were
prepared as described in the literature. 4,4-Bis(ethoxycarbonyl)-1,7-
heptadiyne (M1),4 4-(ethoxycarbonyl)-4-(1R,2S,5R)-(−)-menthoxy-
carbonyl-1,6-heptadiyne (M2),23,24 (R,S)-1,7-octadiyne-4,5-dimethyl
dimenthylate (M3),5 and (R,R/S,S)-1,7-octadiyne-4,5-dimethyl di-
menthylate (M4)5 were prepared according to the literature.
CO2CH), 4.20 (d × q, J1 = 10.8 Hz, J2 = 7.1 Hz, 1H, CO2CHH),
2 3
4.07 (d × q, J1 = 10.8 Hz, J2 = 7.1 Hz, 1H, CO2CHH), 2.71−2.57
(m, 4H, CH2CHCH2), 2.01−1.94 (m, 1H, Hmenthyl), 1.86 (d × sept,
3J1 = 7.0 Hz, 3J2 = 2.8 Hz, 1H, CH(Me)2), 1.71−1.62 (m, 2H,
3
CO2CHCH2), 1.54−1.31 (m, 2H, Hmenthyl), 1.24 (t, J1 = 7.1 Hz, 3H,
CO2CH2CH3), 1.09−0.70 (m, 12H, Hmenthyl).13C{1H}NMR (CDCl3):
δ = 171.0 (s, CO2Et), 170.4 (s, CO2menthyl), 132.4 (s, CHCH2),
119.3 (s, CHCH2), 75.6 (s, CO2CHmenthyl), 61.4 (s, CO2CH2), 57.1
(s, Cipso), 47.0, 40.7, 36.6, 34.3, 31.5, 25.7, 22.9, 22.1, 21.0, 15.9, 14.2.
IR (cm−1): 3078 (w), 2954 (m), 2924 (m), 2869 (m), 1729 (s), 1641
(w), 1455 (m), 1417 (w), 1387 (m), 1366 (m), 1323 (w), 1285 (s),
1215 (s), 1194 (s), 1141 (m), 1096 (m), 1038 (m), 991 (m), 961 (m),
917 (m), 845 (w), 637 (w).
Ethyl ((1R,2S,5S)-2-Isopropyl-5-methylcyclohexyl)cyclopent-3-
ene-1,1-dicarboxylate (A). 4-(Ethoxycarbonyl)-4-(1S,2R,5S)-
(−)-menthoxycarbonyl-1,6-heptadiene (110 mg, 323.8 μmol) was
dissolved in 5 mL of CH2Cl2. Then a solution of RuCl2(PCy3)-
(IMesH2)(CHPh) (14,4 mg, 17.5 μmol) in 2 mL of CH2Cl2 was
added slowly, and the mixture was stirred for 4 h. After adding ethyl
vinyl ether and 10 mL of water, the organic layer was separated, the
aqueous fraction was extracted with CH2Cl2, and the solvent was
removed in vacuo to yield a colorless oil, which was further purified by
column chromatography. A colorless oil (97.8 mg, 97%) was obtained.
1H NMR (CDCl3): δ = 5.64−5.57 (m, 2H, CHCH), 4.69 (t × d, 3J1
= 10.9 Hz, 3J2 = 4.4 Hz, 1H, CO2CH), 4.22 (d × q, 2J1 = 10.8 Hz, 3J2 =
2
7.1 Hz, 1H, CO2CHH), 4.10 (d × q, J1 = 10.8 Hz, 3J2 = 7.1 Hz, 1H,
CO2CHH), 3.07−2.91 (m, 4H (O2C)2C(CH2)2), 2.02−1.94 (m, 1H,
3
3
Hmenthyl), 1.84 (d × sept, J1 = 7.0 Hz, J2 = 2.8 Hz, 1H, CH(Me)2),
1.73−1.60 (m, 2H, CO2CHCH2), 1.55−1.32 (m, 2H, Hmenthyl), 1.25
3
(t, J1 = 7.1 Hz, 3H, CO2CH2CH3), 1.10−0.71 (m, 12H, Hmenthyl).
13C{1H} NMR (CDCl3): δ = 172.4 (s, CO2Et), 171.8 (s, CO2menthyl),
127.7 (s, CHCH), 127.9 (s, CHCH), 75.5 (s, CO2CHmenthyl),
61.7 (s, CO2CH2), 59.1 (s, Cipso), 47.0, 41.0, 40.9, 40.5, 34.3, 31.5,
26.0, 23.2, 22.1, 20.9, 16.1, 14.1. IR (cm−1): 3063 (w), 2953 (m), 2924
(m), 2869 (m), 1728 (s), 1455 (m), 1387 (m), 1367 (m), 1339 (w),
1252 (s), 1178 (s), 1097 (m), 1067 (m), 1038 (m), 1010 (w), 961
(m), 915 (m), 697 (w).
Ethyl ((1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl)malonate. The
compound was prepared according to ref 23. Ethyl chloromalonate
(5.00 g, 33.2 mmol, 1 equiv) was dissolved in 100 mL of dry diethyl
ether. (−)-Menthol (5.19 g, 33.2 mmol, 1 equiv) was dried in vacuo
and dissolved in 10 mL of Et2O and added at 0 °C to the reaction
mixture over a period of 45 min. The solution was stirred for 20 h at
room temperature. Then a saturated solution of aqueous NaHCO3 was
added until the formation of gas had ceased. The aqueous fraction was
extracted three times with water and once with brine. The combined
organic fractions were dried over Na2SO4 and filtered, and the solvent
was removed in vacuo. The resulting oil was precipitated from n-
pentane at −80 °C. A colorless oil could be obtained (7.58 g, 28.1
mmol, 84%). H NMR (CDCl3): δ = 4.73 (t × d, 1H, J1 = 10.8 Hz,
3J2 = 4.4 Hz, OCH), 4.26−4.12 (m, 2H, CH2), 3.34 (s, 2H, CH2),
2.08−1.98 (m, 1H, CH), 1.89 (d × sept, 1H, 3J1 = 7.0 Hz, 3J2 = 2.7 Hz,
CH), 1.74−1.62 (m, 2H, CH), 1.56−1.32 (m, 2H, CH2), 1.27 (t, 3H,
3J1 = 7.2 Hz, CH3), 1.15−0.71 (m, 12H, CH, CH3). IR (cm−1): 2954
Mo(N-2-t-BuC6H4)(CHCMe2Ph)(2,5-dimethylpyrrolide)2 (2). A sol-
ution of Mo(N-2-t-BuC6H4)(CHCMe2Ph)(OTf)2(DME) (280 mg,
0.389 mmol) in 50 mL of diethyl ether was chilled to −30 °C, and
lithium-2,5-dimethylpyrrolide (80.6 mg, 0.798 mmol) was added
slowly. The mixture was stirred for 3 h, yielding a red solution. The
solvent was removed in vacuo; then the residue was dissolved in
CH2Cl2 and filtered through Celite. The solution was concentrated
and pentane was added. An orange solid was obtained by fractionated
crystallization. 1H NMR (C6D6): δ = 13.41 (s, 1H, MoCH), 7.55 (d ×
d, 3J1 = 7.8 Hz, 2J2 = 1.5 Hz, 1H), 7.45−7.39 (m, 2H), 7.24−7.18 (m,
2H), 7.15 (d × d, 3J1 = 7.9 Hz, 2J2 = 1.4 Hz, 1H), 7.13−7.09 (m, 1H),
6.90 (t × d, 3J1 = 7.6 Hz, 4J2 = 1.6 Hz, 1H), 6.85−6.80 (m, 1H), 6.59
(br s, 2H, 2xNC(Me)CH), 5.89 (br s, 2H, 2 × NC(Me)CH), 2.49−
1.87 (br s, 12H, 4 × NCCH3), 1.77 (s, 6H, MoCHC(CH3)2), 1.42
(s, 9H, C(CH3)3). 13C{1H} NMR (C6D6): δ = 153.5 (s, NCCtBu),
147.6 (CH3)2CC(C5H5), 140.6 (C(CH3)3), 133.6, 127.1, 125.8, 125.6,
125.2, 124.8, 124.4, 109.0 (br s, NCMeC), 101.5 (br s, NCMeC), 57.9,
34.4, 30.3, 28.5, 15.87 (br s, NCCH3).
1
3
(m), 2928 (m), 2869 (m), 1729 (s), 1456 (m), 1411 (w), 1387 (m),
1368 (m), 1265 (s), 1180 (s), 1145 (w), 1096 (m), 1080 (w), 1035
(m), 1011 (w), 989 (m), 904 (w), 842 (m), 685 (w), 598 (w).
4-(Ethoxycarbonyl)-4-(1R,2S,5R)-(−)-menthoxycarbonyl-1,6-hep-
tadiene. Ethyl-(1R,2S,5R)-(−)-menthylmalonate (6.00 g, 22.2 mmol)
Mo(N-2-t-BuC6H4)(CHCMe2Ph)(2,2″,4,4″,6,6″-hexamethyl-m-
terphenyl)(2,5-dimethylpyrrolide) (MAP3). A chilled solution of
2,2″,4,4″,6,6″-hexamethyl-m-terphenol (148.5 mg, 449.4 μmol) in 5
mL of diethyl ether was added over a period of 10 min to a chilled
I
DOI: 10.1021/acs.macromol.5b01185
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
solution of 2 (252 mg, 447.5 μmol) in 50 mL of diethyl ether and
stirred for 5 h. All volatiles were removed, and the residue was washed
once with diethyl ether and twice with pentane to obtain a yellow
solid. 1H NMR (C6D6): δ = 11.28 (s, 1H), 7.26−7.22 (m, 2H), 7.14−
7.07 (m, 3H), 7.03−6.88 (m, 6H), 6.84 (s, 2H), 6.78 (s, 2H), 6.68 (d
× d, 3J1 = 7.7 Hz, 4J2 = 1.6 Hz, 1H), 6.07 (s, 5H), 2.37 (s, 3H), 2.20 (s,
6H), 2.14 (s, 6H), 2.04 (s, 6H), 1.78 (s, 3H), 1.62 (s, 3H), 1.48 (s,
3H). Anal. Calcd for C50H58MoN2O: C, 75.16; H, 7.32; N, 3.51.
Found: C, 75.09; H, 7.37; N, 3.49.
Cyclopolymerizations. Cyclopolymerization of α,ω-diynes was
carried out in dry dichloromethane or chloroform at the indicated
temperature inside a nitrogen-filled glovebox using the indicated
initiators. All monomers and solvents were dried and filtered through
neutral Al2O3 prior to use. Polymerizations were started by the
addition of a solution of the initiator in 1 mL of the indicated solvent
to the monomer dissolved in the same solvent. In case quinuclidine
was used, it was added to the solution of the initiator. Polymerizations
with MAP catalysts were terminated by addition of ferrocenylaldehyde
(10 mol equiv with respect to the initiator). All other polymers were
collected by precipitation from methanol or n-pentane containing 1 vol
% of CF3COOH, followed by centrifugation and drying in vacuo. All
polymers were stored under a N2 atmosphere.
Representative Polymer Data. Poly-M1. The polymer was
prepared from I1 (0.004 g, 0.0042 mmol) and the monomer M1
(0.05 g, 0.213 mmol) in CH2Cl2 in 84% isolated yield. Polymerization
was started at −30 °C and allowed to run for 1 h. 1H NMR (CDCl3):
δ = 6.68 (br, m, 2H), 4.27 (br, m, 4H), 3.43 (br, m, 4H), 1.30 (br, m,
6H). 13C NMR (CDCl3): δ = 172.1, 137.1, 128.4, 126.3, 123.4, 62.1,
57.0, 41.6, 14.2. IR (cm−1): 2977 (m), 1720 (s), 1444 (w), 1367 (s),
1245 (s), 1157 (w), 1065 (s), 946 (s), 629 (m). UV/vis (CHCl3): λmax
= 592, 547 nm; Mn = 8500 g/mol, PDI = 2.1; α-insertion: ≥99%.
Poly-M2-(I2). 1H NMR (CDCl3): δ = 6.68 (bs, 2H, CH), 4.74 (bs,
1H, OCH), 4.50−2.59 (m, 6H, CH2), 2.58−0.42 (m, 22H, CH, CH2,
CH3). 13C NMR (CDCl3): δ = 172.0, 171.4 137.1, 123.2, 76.1, 62.0,
57.5, 46.9, 41.5, 40.4, 34.3, 34.2, 31.5, 26.1, 23.4, 22.1, 21.0, 16.3, 14.2.
IR (cm−1): 2953 (m), 2925 (m), 2868 (m), 1727 (s), 1455 (m), 1387
(m), 1367 (m), 1247 (s), 1176 (s), 1097 (m), 1048 (m), 1008 (m),
981 (m), 913 (m), 862 (m), 845 (m), 802 (w) 634 (m), 598 (w), 576
(w). UV/vis (CHCl3): λmax = 550 nm, 591 nm; Mn = 32 300 g/mol,
PDI = 2.1; α-insertion: ≥96%; isolated yield: 83%.
Poly-M4-(I1). 1H NMR (CDCl3): δ = 7.0 (bs, 1H, CH), 6.75−5.71
(m, 1H, CH), 4.60−3.77 (m, 8H, OCH2), 3.32−3.03 (m, 2H, CH),
2.83−1.89 (m, 10H, CH, CH2), 1.87−1.13 (m, 8H, CH2), 1.1−0.55
(m, 24H, CH, CH3). 13C NMR (CDCl3): δ = 170.9, 131.8, 128.3,
126.5, 125.2, 80.4, 66.1, 66.0, 65.2, 48.3, 40.1, 34.5, 33.7, 31.6, 29.0,
28.4, 25.6, 23.4, 22.5, 21.1, 16.5. IR (cm−1): 2951 (m), 2918 (m), 2867
(m), 1756 (s), 1732 (s), 1454 (m), 1368 (m), 1276 (m), 1236 (w),
1179 (m), 1118 (s), 1027 (w), 950 (m), 909 (m), 873 (m), 843 (m),
700 (m), 575 (w). UV/vis (CHCl3): λmax = 463 nm; Mn = 24 200 g/
mol, PDI = 1.5; α-insertion: ≥99%; isolated yield: 24%.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Selected H, 13C, 19F, COSY NMR, and GC-MS spectra of
monomers, intermediates, and polymers; crystallographic
details on MAP1, MAP2, and 2. The Supporting Information
is available free of charge on the ACS Publications website at
DOI: 10.1021/acs.macromol.5b01185.
AUTHOR INFORMATION
Corresponding Author
*E-mail michael.buchmeiser@ipoc.uni-stuttgart.de (M.R.B.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG, BU 2174/19-1).
■
REFERENCES
■
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Poly-M2-(I3). 1H NMR (CDCl3): δ = 6.69 (bs, 2H, CH), 4.74 (bs,
1H, OCH), 4.35−3.0 (m, 6H, CH2), 2.20−0.42 (m, 22H, CH, CH2,
CH3). 13C NMR (CDCl3): δ = 172.0, 171.4 137.1, 123.1, 76.1, 62.0,
57.5, 46.9, 41.6, 40.4, 34.2, 31.7, 31.5, 26.1, 23.4, 22.5, 22.3, 22.1, 21.0,
16.3, 14.2. IR (cm−1): 2953 (m), 2926 (m), 2868 (m), 1725 (s), 1454
(m), 1387 (m), 1366 (m), 1245 (s), 1160 (s), 1096 (m), 1066 (w),
1048 (m), 1009 (m), 981 (m), 947 (s), 912 (m), 862 (m), 845 (m),
700 (w) 630 (m), 597 (w). UV/vis (CHCl3): λmax = 552 nm, 588.5
nm; Mn = 27 500 g/mol, PDI = 1.8; α-insertion: ≥90%; isolated yield:
77%.
Poly-M2-(I4). 1H NMR (CDCl3): δ = 6.63 (bs, 2H, CH), 4.67 (bs,
1H, OCH), 4.41−2.59 (m, 6H, CH2), 2.19−0.38 (m, 22H, CH, CH2,
CH3). 13C NMR (CDCl3): δ = 172.0, 171.5, 137.4, 123.4, 76.1, 62.0,
57.7, 46.9, 41.6, 40.5, 36.1, 34.3, 34.3, 31.6, 26.2, 23.4, 22.5, 22.1, 20.9,
16.3, 14.2. IR (cm−1): 2953 (m), 2925 (m), 2868 (m), 1724 (s), 1450
(m), 1386 (m), 1367 (m), 1245 (s), 1167 (s), 1096 (m), 1027 (s), 980
(m), 952 (m), 911 (m), 861 (m), 844 (m), 797 (w), 720 (w), 635
(m), 576 (m), 515 (m). UV/vis (CHCl3): λmax = 547 nm, 585 nm; Mn
= 47 500 g/mol, PDI = 4.0; α-insertion: ≥93%; isolated yield: 60%.
Poly-M3-(I1). 1H NMR (CDCl3): δ = 7.0 (bs, 1H, CH), 6.72−5.66
(m, 1H, CH), 4.60−3.84 (m, 8H, OCH2), 3.25−2.99 (m, 2H, CH),
2.88−1.85 (m, 10H, CH, CH2), 1.75−1.13 (m, 8H, CH2), 1.0−0.55
(m, 24H, CH, CH3). 13C NMR (CDCl3): δ = 170.9, 132.0, 128.3,
126.5, 126.1, 125.1, 80.4, 66.4, 66.1, 48.2, 40.1, 34.5, 34.2, 31.6, 25.6,
23.4, 22.4, 22.3, 21.1, 16.5. IR (cm−1): 2951 (m), 2918 (m), 2867 (m),
1757 (s), 1732 (m), 1454 (m), 1384 (m), 1367 (m), 1277 (m), 1187
(m), 1118 (s), 951 (m), 909 (m), 843 (m), 700 (w), 573 (w). UV/vis
(CHCl3): λmax = 469 nm; Mn = 25 300 g/mol, PDI = 1.3; α-insertion:
≥99%; isolated yield: 47%.
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DOI: 10.1021/acs.macromol.5b01185
Macromolecules XXXX, XXX, XXX−XXX