Subscriber access provided by Fudan University
Article
Rational and Then Serendipitous Formation of AzaAnalogues of Hoveyda-Type Catalysts Containing a Chelating Ester Group Leading to a Polymerisation Catalyst Family Stefan J Czarnocki, Izabela Czelusniak, Tomasz K OLSZEWSKI, Maura Malinska, Krzysztof Wozniak, and Karol Grela ACS Catal., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Rational and Then Serendipitous Formation of Aza-Analogues of Hoveyda-Type Catalysts Containing a Chelating Ester Group Leading to a Polymerisation Catalyst Family Stefan J. Czarnocki a,†, Izabela Czeluśniak,b Tomasz K. Olszewski,c Maura Malinska,a Krzysztof Woźniaka and Karol Grela a* a.
b. c.
Faculty of Chemistry, Biological and Chemical Research Centre University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland. Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland. Faculty of Chemistry, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 54-054 Wrocław, Poland.
Supporting Information Placeholder
[
]
ABSTRACT: Analogues of the well-known Hoveyda-Grubbs catalyst bearing both a chelating ester function and a chelating nitrogen atom were obtained. These complexes behave differently depending on the character of the chelating amine. Complexes containing a secondary amine underwent unexpected spontaneous oxidation of the amine group leading to the Schiff base analogues. In contrast, complexes containing a tertiary amine were prone to intramolecular cyclization in the presence of a base (Et3N). Probing the activity of such (pre)catalysts in ring-closing metathesis reaction (RCM) revealed their dormant character, excellent thermoswitchability. Especially the complexes bearing an iminium nitrogen fragment were found to be very useful in a delayed ringopening metathesis polymerization (ROMP) and were therefore commercialized. Keywords: Olefin metathesis; Latent catalysts; Polymerisation; Imines; Ruthenium; Catalysis
mentally friendly solvents without the protective atmosphere of an inert gas.7 The success of olefin metathesis is associated mostly with the development of effective and stable catalysts displaying excellent functional group tolerance.8 The most popular, modern, air and moisture stable, very reactive Ru-based catalyst such as 1 or 2 (Figure 1) however, do not always meet the specific requirements of industrial processes. This is especially true for ring-opening metathesis polymerization (ROMP) of reactive monomers like dicyclopentadiene (DCPD).6a Since this process, on industrial scale, is conducted using reaction-injection molding (RIM) technique, instant polymerization after mixing the catalyst with the monomer is undesired.6g Time is required to handle the formulation thus; the ideal catalyst should initiate polymerACS Paragon Plus Environment
Introduction Olefin metathesis represents a powerful set of chemical transformations based on reacting olefins in the presence of transition metal-containing catalyst and leading to formation of new carbon-carbon double bonds.1 This reaction has found many applications not only in modern organic synthesis carried out in academia, but also most importantly it is successfully applied by many branches of chemical industry as effective tool for preparation of pharmaceuticals,2 agrochemicals,3 flavour and fragrances components,4 transformation of biomass into valuable chemicals,5 and preparation of polymers.6 Importantly, in-line with the current sustainability trends in industrial-scale synthesis, olefin metathesis can be efficiently conducted in environ-
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ization in the appropriate time after being activated by chemical or physical methods. The same properties of catalysts are desired by the novel applications of olefin metathesis in the field of three-dimensional (3D) printing by stereolithography and in photolithography.9 To meet this requirement Ru-catalysts with reduced initiation speed, due to chelation caused by the presence of oxygen, sulphur or nitrogen in the benzylidene fragment of the catalyst, were developed. These, so-called “latent” catalysts can be conveniently activated by elevating reaction temperature or by introducing of a proper agent, usually Brønsted or Lewis acid.10 As for polymerization especially the nitrogenchelated catalysts such as 4a,b bearing a Schiff-base ligands, are well suited (Figure 1).11 Interestingly, while a number of structures were synthesised and tested, only a very few were commercialised and used in industrial context. Therefore, the search for new catalysts for applications in that area is still highly desired. As part of our continued interest in design and synthesis of new latent catalysts for use in olefin metathesis,12 herein, we present our latest results on the synthesis and catalytical activity of new N-chelating ruthenium-based complexes, playing a special emphasis on their application in ringopening metathesis polymerization (ROMP) .
Page 2 of 15
was used as a starting material. Wittig olefination followed by reduction of the nitro group led to the desired propenyl aniline (7), the key synthetic intermediate bearing free amino group for further functionalization. Alkylation of 7 with methyl bromoacetate afforded the desired ligand precursor 9. Subsequent methylation of the secondary nitrogen in 9 with CH3I under basic conditions produced ligand precursor 10. Alternatively, 10 could be obtained in a reverse order protocol, via methylation of the secondary nitrogen in 7 and then introduction of the ester moiety, by means of simple alkylation with methyl bromoacetate (Scheme 1).
Scheme 1. Preparation of ligands 9 and 10.
Figure 1. Structures of 2nd generation Grubbs (1) and HoveydaGrubbs (2) catalyst, a representative “scorpio-type” catalyst 3 and representative Ru-based catalysts containing Schiff base motive 4a,b suitable for application in ROMP. Cy = cyclohexyl; Mes = 2,4,6-trimethylphenyl.
Results and discussion In a quest for new N-chelated catalysts we decided to introduce an ester moiety into the nitrogen-chelated analogue13a of Hoveyda-Grubbs catalyst 2 (where the oxygen is formally replaced with nitrogen atom). The concept of creating a nitrogen analogue of the very reactive and stable so-called “scorpio-type” catalysts 313,7d was not explored before,13e and we were interested in testing the application profiles of such analogues. We started the investigation from the preparation of the precursors of the appropriate N-chelating benzylidene ligands 9 and 10 (Scheme 1), bearing secondary or tertiary chelating nitrogen atom, respectively. In our synthetic strategy easy available, affordable 2-nitrobenzaldehyde (5)
Having both ligand precursors 9 and 10 in hand we began the synthesis of the corresponding Ru complexes. Reaction under argon atmosphere of ligand precursor 9 with [(SIMes)Ru(PCy3)(Ind)Cl2] (M2) or with 1 in the presence of CuCl used as phosphane scavenger yielded, after chromatographic purification, the desired complexes 11a and 11b in 60% and 55% yield, respectively as green microcrystalline solids (Scheme 2). To our great surprise, unpredicted reactivity of both complexes 11a and 11b leading to formation of new structures was then observed. Unexpectedly, we noted that the removal of an argon atmosphere after the in situ formation of product 11a and continuation of heating for an additional time resulted in isolation of a new red-coloured product, which was later characterised as a Schiff base type complex (12, Scheme 2). Apparently the action of oxygen from air caused oxidation of the N-chelating amine to imine and as a result, formation of new Schiff-base complex 12. After optimization of the reaction conditions14 we isolated complex 12 with 64 % yield as a stable, red, microcrystalline solid.
ACS Paragon Plus Environment
Page 3 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
The structure of 12 was confirmed by spectroscopic techniques. Additionally, single crystals of complex 12 (Figure 2) suitable for and X-ray diffraction analysis were grown from CH2Cl2-MeOH solution, which also helped us to
monitored by TLC) and continuing the heating for additional 30 min in air resulted in isolation of the desired complex 12.
Figure 2. Molecular structure of complex 12.
Scheme 2. Synthesis of complexes 11a,b and their unexpected reactivity leading to formation of new catalysts 12 and 13. Cy = cyclohexyl; Mes = 2,4,6-trimethylphenyl; a) solution of complex 12 in CH2Cl2; b) solution of complex 13 in CH2Cl2.
confirm the molecular assignment of the new structure (for crystallographic details see Supporting Information). The ruthenium atom in 12 is six coordinated (Figure 2), with the Ru(1)-O(1) bond length of 2.419(2)Å close to the length of Ru-Cl bonds confirming the molecular structure presented on Scheme 2. The Ru(1)-N(3) bond length is 2.067(2)Å, which is the shortest distance reported to date for this type of precatalyst.15 The Ru=C(22) and Ru-C(1) bonds length are 1.846(2)Å and 2.075(2)Å, comparable to the previously reported Hoveyda precatalyst.16 Also the SIMes ligand in the catalyst is symmetrically placed above the Ru-C(1) direction, with the difference between the Ru-C(1)-N(1) and Ru-C(1)-N(2) angles equal to 5.5(1)˚. To the best of our knowledge, spontaneous oxidation of the chelating secondary amine to imine “on” the preformed ruthenium benzylidene metathesis catalyst was not reported in the literature thus far. From the practical point of view the simplicity of this methodology is worth mentioning, as the observed oxidation reaction requires neither additional reagents nor equipment. Simple removal of protecting atmosphere of argon after the in situ formation of intermediate 11a (toluene 60oC, 30 min, progress of the reaction
Another interesting aspect of the synthesis of new nitrogen analogue of the “scorpio-type” catalysts was the serendipitous isolation of catalyst 13 during the work-up of the reaction mixture after synthesis of 11b (Scheme 2). In the course of chromatographic purification of 11b with an eluent containing 1% of Et3N (used to improve the elution of the product), we observed a partial formation of another product according to a TLC plate (a new blue spot). After extensive investigations, we discovered that the formation of the new product is due to the presence of the base (Et3N). After optimization of the reaction conditions we have isolated this product in 58 % yield as microcrystalline dark blue solid (13, Scheme 2). The structure of 13 was elucidated by means of spectroscopic characterisation and was unambiguously confirmed by X-ray diffraction analysis (Figure 3). Here, the ruthenium atom is five coordinated with the Ru-O(2) bond much shorter than in the 12 catalyst (2.043(3)Å). Complex 13 has a longer Ru-N bond (2.205(3)Å) and a shorter Ru=C(22) and Ru-C(1) bonds than previously reported analogues.17 Similarly, there is an asymmetrical distortion of the SIMes ligand with Ru-C(1)N(1) and Ru-C(1)-N(2) angles equal to 118.0(3)˚ and 135.2(5)˚, respectively, that cannot be explain by π-π stacking.17b We suggest this is a simple steric effects that allow the mesityl group to move closer to the ruthenium center.
Figure 3. Molecular structure of complex 13.
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 15
Subsequently, we began testing the activity of the new complexes in olefin metathesis transformations. Complexes 11b, 12 and 13 (1 mol%) were first evaluated in the benchmark ring-closing metathesis reaction (RCM) of diethyl diallylmalonate 14, as the model substrate (Scheme 3). Complex 11a was found to be unstable and spontaneously underwent oxidation therefore it was excluded from catalytic tests.
Scheme 3. Model RCM reaction of diethyl diallylmalonate 14.
The preliminary tests carried out in CH2Cl2 at 40 oC (reflux) with 1 mol% of the catalysts 11b, 12 and 13, revealed relatively low catalytic activity of all the complexes under the tested conditions (conversions below 20% after 5 h reaction time, even with the use of dioxane solution of HCl [50 mol%] as an activator). Extending the reaction time to 72 h resulted in improving the conversion to 85 % however, only in the case of 11b (improvement in the case of 12 and 13 was not noticeable) (see Supporting Information). Changing the reaction solvent to toluene and elevating the temperature of the process to 80 oC enhanced the reactivity of the tested catalysts (Figure 4 and 5). In the case of complex 11b conversion reached 96 % after 6 h however, in the case of 12 and 13 even after 72 h the conversions were still moderate, 82 % and 50 %, respectively. Interestingly, while examining the influence of additives such as HCl (50 mol%) and trimethylsilyl chloride (TMSCl) (10 mol%) on the reactivity of the tested catalysts, we observed that in the case of catalysts 11b and 12 those additives had an inverse influence than expected and observed several times before for related latent catalysts,11 namely their presence diminished the reactivity of the complexes (Figure 5). Whereas, in the case of catalyst 13 the presence of those activators resulted in improving the conversion (eg. up to 95% after 72 h in the presence of TMSCl, Figure 5), which is the expected behaviour of those additives.11
Figure 4. Reactivity of catalysts 11b, 12, and 13 in model RCM reaction of 14 carried out with 1 mol% of catalyst (C = 0.1 M) in toluene at 80 oC.
Figure 5. Reactivity of catalysts 11b, 12, and 13 in model RCM reaction of 14 carried out with 1 mol% of catalyst (C = 0.1 M) in toluene at 80 oC and in the presence of HCl and TMSCl as activators.
Figure 6. Thermo-switchability of the catalyst 12 in the RCM reaction of diethyl diallylmalonate 14.
Finally, to test further the effect of temperature we performed the model RCM reaction of diethyl diallylmalonate 14 catalysed by the N-chelating Schiff-base containing complex 12 in mesitylene at 120 oC (Figure 6). In this case an acceptable 87 % conversion was observed after 6 h reaction time. Additionally, this experiment is not only a proof for thermo-switchable properties of the new complex 12 but also its excellent stability. Thermo-switchability of catalyst 12 (Fig.4 and 6) needs some comments: transition from 40°C to 80°C indicates the formation of active species from dormant catalyst as expected. However, these active species seem not to be stable and probably are transformed to the less active ones (nearly linear part of conversion curve at prolonged times). Further temperature increase to 120 °C need not indicate the rising amounts of active species (initial reaction rate did not increase), but the rising propagation rate may outcompete the decrease in the amount of active species. The results presented in figures 4-6 clearly show that the new N-chelating complexes 11b, 12 and 13 belong to the “latent” catalysts family, despite the presence of an ester group and thus their analogy to very active so called “scorpio-type” catalysts.13 However, the new catalysts showed good thermo-switchability (the case of complexes 11b and 12) and chemo-switchability
ACS Paragon Plus Environment
Page 5 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Scheme 4. Model ROMP reaction of norbornene (NBE) and dicyclopentadiene (DCPD).
Figure 7. Structures of complex 12 (HeatMetTM) and its analogues 12a-e with altered NHC ligand and also ester moiety. Mes = 2,4,6-trimethylphenyl; DIPP = 2,6-diisopropylphenyl.
(the case of complex 13) therefore we decided to investigate their reactivity in ring-opening metathesis polymerization (ROMP). Additionally, to acquire more information about the influence of the structure of the catalysts on their reactivity in ROMP we prepared analogues of catalyst 12 with altered NHC ligand and also ester moiety (Figure 7) (see Supporting Information). To gain information on the latency of the studied complexes in ring opening metathesis polymerization (ROMP) we conducted model polymerizations utilizing highly strained monomers, such as norbornene (NBE) and dicyclopentadiene (DCPD) (Scheme 4).18 First ROMP of NBE under several conditions were carried out, as detailed in Table 1. In all cases the monomer-to-initiator ratio was kept constant at 300/1 and 1000/1 and the monomer concentration was always 0.1M. Reactions were conducted at various temperature and for this reason toluene was used as the solvent due to its considerable high boiling point. In addition, a reference initiator, Hoveyda-Grubbs 2nd generation catalyst (2), was used for comparison as an example for a rather slowly initiator.19 Not surprisingly, in the presence of all of the investigated complexes the monomer was polymerized even at 25°C leading poly(NBE)s. While reducing the catalysts’ loadings to 0.1 % mol the ROMP reactions required longer time to complete. However, it should be noted that the reactions still proceeded much more slowly than that mediated by Hoveyda-Grubbs 2nd generation initiator (2), and the monomer was consumed over 5 hour period. As expected, increasing reaction temperature up to 100ºC accelerated the polymerization rate while reducing reaction times. Close examination of Table 1 reveals that these ruthenium catalysts do not form well-defined polymeric structures. The differences between the theoretical and experimentally determined values for Mn of
poly(NBE)s suggest competing chain-transfer reactions occurred during polymerization.19 Furthermore, the relatively high polydispersity indexes (in the range of 1.13 and 3.14) of the resulting polymers could be the result of high propagation rates and slow initiation rates.19c In fact, monitoring the ROMP of NBE (50 equivalents) using catalyst 12a by 1H NMR spectroscopy (C6D6 25ºC) indicated that less than 5% of the catalyst initiated before ROMP was complete. Even after 2 days the proton signal of unreacted initiator (δ = 17.16) was still observed and no alkylidene protons of propagating species were detected during this reaction. This could be qualitatively observed, as a solution of the initiator and NBE retained the colour of the initiator, even after complete conversion of monomer.” The cis/trans ratio of the obtained poly(NBE)s by using the investigated initiators varied from 38/62 to 61/39 and was temperature dependent; higher cis polymers being formed at lower temperature (Table 1). Interestingly, the observed cis-content was higher for polymers prepared with Ru alkylidenes bearing SIMes ligand (12a, 12c and 12) while with those containing SIPr ones (12b, 12d and 12e). Regarding to the type of chelating alkylidene ligand no differences in the cis contents were observed. It should be noted that Hoveyda-Grubbs 2nd generation initiator (2) giving polymer with 65% of cis content ranked among our SiMesbased catalysts. This phenomenon could be explained by steric feature of substituents of the N-heterocyclic carbene ligand since the actually formed catalytically active species are the same.20b,21 After benchmark reactions with norbornene and bearing in mind potential industrial applications, the initiators were tested in the bulk polymerisation of dicyclopentadiene (DCPD). From the industrial point of view there is a strong need of reducing the catalyst loading while a bulk production is assumed.22 Moreover, the RIM process of DCPD requires the use of catalysts which activity can’t be so fast in order to ensure enough time to fill the mould. On the other hand, some of the poly(DCPD) produced using the conventional catalyst systems are blackened or darkened in appearance and are opaque which prevents its applications that require transparent materials. For these reasons there is a need of a method for producing transparent polymeric materials while still retaining excellent physical and chemical properties. Considering all the above mentioned,
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
polymerisation tests with our catalysts were carried out in order to assess the lowest possible initiator-to-monomer ratio on the one hand, and to obtain a transparent and good Table 1. ROMP of NBE with Ru initiatorsa. Initiator
12a
12b
12c
12d
12
12e
2
Page 6 of 15
quality solid poly(DCPD) on the other. In order to minimize an impact of catalyst’s solubility in the monomer
T(°C)
t (min)b
Mnc
PDIc
cis/trans ratiod
1/300 1/1 000
25 25
120 300
16 770 20 427
1.32 1.20
59/41 59/41
1/1 000
40
120
28 947
2.78
58/42
1/1 000
100
20
109 002
3.82
53/46
1/300 1/1 000
25 25
60 300
20 659 13 517
1.22 1.63
47/53 45/54
1/1 000
40
210
65 333
3.61
44/56
1/1 000
100
20
88 513
2.36
40/60
1/300 1/1 000
25 25
60 300
22 903 15 350
1.24 1.34
60/40 61/39
1/1 000
40
240
69 486
3.14
58/42
1/1 000
100
20
121 222
2.31
55/44
1/300 1/1 000
25 25
60 300
18 823 77 895
1.28 2.89
44/57 43/57
1/1 000
40
240
76 673
3.11
41/59
1/1 000
100
20
87 419
2.74
38/62
1/300 1/1 000
25 25
60 300
19 880 64 623
1.27 2.18
59/41 58/42
1/1 000
40
180
82 819
2.33
55/45
1/1 000
100
30
69 126
2.44
51/49
1/300 1/1 000
25 25
60 300
21 191 18 908
1.20 1.30
43/57 46/54
1/1 000
40
180
15 829
1.33
44/56
1/1 000
100
30
34 916
1.13
45/55
1/1 000
25
20
115 597
4.08
66/34
[Ru]/[NBE]
a)
Solvent: toluene; b) Time until conversion of NBE was complete based on 1H NMR measurements; c) Mn and PDI were determined by GPC in THF relative to polystyrene standard; d) Cis/trans double bonds of poly(NBE) calculated from 1H NMR.
ACS Paragon Plus Environment
Page 7 of 15
ACS Catalysis
Table 2. Mechanical characterization of selected poly(DCPD)s.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Sample
[Ru-I] % mol
Temp./time
Rm (MPa)a
E (GPa)a
P1
0.002
120ºC/4 h
46 ±1
1.66 ±0.07
P2
0.01
60ºC/2 h
47 ±1
1.48 ±0.06
P3
0.01
60ºC/1 h and 120ºC/1 h
50 ±1
1.59 ±0.06
a
Rm = ultimate strength; E = Young’s modulus
on the herein described results, the complexes were always dissolved in small amount of CH2Cl2 before mixing with DCPD. Investigated complexes 12, 12a-e polymerized DCPD with extremely low catalyst loadings; up to a monomer/catalyst ratio of 1/100 000, in excellent yields, giving brownish to light amber coloured solid poly(DCPD)s (see Supporting Information). To our satisfaction the DCPD is completely inert when mixed even with 100 ppm of 12a at 30ºC for 20 min. Extending the polymerisation’s time to 120 min. resulted in jelly-like specimens. Moreover, in order to obtain a soft rubbery form of polymer the process required a reaction time of 18 hours. In comparison, a solid poly(DCPD) was formed after 120 and 30 min when the reaction was performed at 60 and 120ºC, respectively. These high gel times (determined as the time needed for the DCPD solution to flow like a honey) are needed for good handling of the DCPD/catalyst formulation. However, placing the reaction vessel containing the DCPD/catalyst mixture into an oil bath heated to 120ºC resulted in fast polymerisation, leading to a hard polymer with cracks, which are formed deep within the structure. Further decrease of the catalyst loading resulted in the isolation of good quality, solid poly(DCPD) without any cracks within the structure (see Supporting Information). However, while reducing the catalyst loading the process requires higher temperatures and longer time. For qualitative observations, at the lowest catalyst loadings we were not able to obtain the hard poly(DCPD) even if the reaction was performed at 120ºC. This can be explained by the fact that with a lower amount of catalyst, relative to monomer, there are less propagating species at any time and a given degree of cure can not be achieved under these conditions.23 It should be noted, that the relatively clear, transparent and light amber polymeric materials were obtained in the polymerisations at 20 and 10 ppm initiator’s loadings. This practical advantage is highly desirable in applications where the products of a variety of colours are manufactured. The most important test for the applicability of the initiators is the evaluation of the mechanical properties of resulting poly(DCPD) work pieces. We chose the initiator 12a, since all studied catalysts exhibited similar activity in ROMP of DCPD. Accordingly, shoulder test bars were prepared with two initiator’s loadings; 0.01 and 0.002% mol and at various temperatures to assess the lowest pos-
sible initiator amount in order to achieve typical mechanical properties reported for commercialized poly(DCPD).24 Test bars were prepared from a formulation of liquid DCPD (at 30°C) and the initiator 12a dissolved in dichloromethane, and that was filled into glass moulds. The moulds were put into an oil-bath at appropriate temperatures. After cooling the polymerised specimens were removed from the mould. The evaluation of the mechanical properties of such obtained poly(DCPD) specimens were done by tensile tests and the results are within the specifications of industrially produced poly(DCPD), i.e. Young’s modulus of 1.6 – 1.9 GPa and Rm of 40-50 MPa.22 Polymerisations initiated with 0.002 and 0.01% mol of initiator resulted in hard test pieces (P1 and P2) characterized by an maximal stress (Rm) of 46 and 47 MPa and Young’s modulus (E) of 1.66 and 1.48 GPa, respectively, showing that higher catalyst’s loadings do not significantly improve the mechanical properties of polymer (Table 2). Conducting the reaction firstly at lower temperature (60ºC) and then post-cured at 120°C for 1h yielded the specimen (P3) with slightly better mechanical properties (Rm = 50 MPa and E = 1.59 GPa) than this prepared at 60ºC (P2). In general, more densely cross-linked polymer networks gives larger rigidity as well as E value.23 Thus poly(DCPD) obtained with the lowest amount of catalyst but at the highest temperature seems to be the most crosslinked material among investigated polymers. Conclusions In conclusion, we synthesised aza-analogues 11a and 11b of the well-known Hoveyda-Grubbs catalyst, containing additionally an ester group in the Ru coordination sphere. These complexes behave differently depending of the character of the chelating amine. In the case of 11a bearing a secondary amine we observed an unexpected spontaneous oxidation of the amine group, by oxygen from air, leading to isolation of its iminium analogue 12. In turn, in the case of 11b an unpredicted intramolecular cyclization, in the presence of Et3N, producing new cyclic Ru-carboxylate complex 13 was discovered. Catalytic activity of complexes 11b, 12 and 13 in model RCM of diethyl diallylmalonate 14 revealed their latent character, good thermo-switchability and intriguing chemoswitchability. Accordingly, after our preliminary studies concerning catalytic activity of the new complexes in olefin metathesis transformations, we think that the main
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
application of those catalysts can be found in ROMP of reactive monomers like DCPD. Good mechanical properties of the poly(DCPD) specimens together with their high transparency make investigated catalysts promising as potential candidates for industrial applications. Importantly, due to the huge potential for application in polymerisation on industrial scale, the catalyst 12a, under the name HeatMetTM is now commercially available.25
AUTHOR INFORMATION Corresponding Author * Tel: (+48) 22 55 26 513; Fax: (+48) 22 632 66 81; E-mail:
[email protected]; Homepage: http://www.karolgrela.eu/
Present Addresses † Current affiliation: Apeiron Synthesis S.A., Duńska 9, 54-427 Wrocław, Poland.
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interest.
Supporting Information For full details on synthetic procedures, characterization of all new compounds and for details on X-ray measurements of complexes 12 (CCDC151837) and 13 (CCDC1518372) see the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.
ACKNOWLEDGMENT K.G. is grateful to the National Science Centre (Poland) for the NCN MAESTRO Grant No. DEC-2012/04A/ST5/00594. This work was supported in part by the Support Programme of the Partnership between Higher Education and Science and Business Activity Sector financed by City of Wroclaw (No. BWU8/2014/M3). The authors thank Dr Piotr Kotowski for mechanical analyses.
REFERENCES (1) (a) Handbook of Metathesis, 2nd ed.; Grubbs, R. H., Wenzel, A. G., O’Leary, D. J., Khosravi, E., Eds.; John Wiley & Sons, Weinheim, 2015. (b) Olefin Metathesis: Theory and Practice, 1st ed.; Grela, K., Ed.; John Wiley & Sons, Hoboken, 2014. (c) Metathesis in natural product synthesis: Strategies, substrates and catalysts; Cossy, J., Arseniyadis, A.S., Meyer, C., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA Weinheim 2010. (2) For recent review articles see: (a) Olszewski, T.K.; Figlus, M.; Bieniek, M. Chim. Oggi / Chemistry Today 2014, 32, 22-29. (b) Magano, J.; Dunetz, J. R. Chem. Rev., 2011, 111, 2177-2250. (c) Busacca, C. A.; Fandrick, D. R.; Song, J. J., Senanayake C. H. Adv. Synth. Catal., 2011, 353, 1825-1864. (3) (a) Schaetzer, J.; Edmunds, A. J. F.; Gaus, K.; Rendine, S.; De Mesmaeker, A.; Rueegg, W. Bioorg. Med. Chem. Lett., 2014, 24, 4643-4649. (b) Daeuble, J.; Sparks, T. C.; Johnson, P.; Graupner P. R. Bioorg. Med. Chem. 2009, 17, 4197−4205. (c) Loiseleur, O.; Schneider, H.; Huang, G.; Machaalani, R.; Sellès, P.; Crowley, S.; Hanessian, S. Org. Process Res. Dev., 2006, 10, 518–524. (4) (a) Davey, P. N.; Lovchik, M. A.; Goeke, A.; Lorincz, K.; Toth,F.; Ondi L. PCT Int. Appl. WO 2015136093 A1 (13.03.2015.). (b) Mathys, M.; Kraft P. Chem. Biodiversity 2014, 11, 1597-160. (c) Kraft, P.; Berthold C. Synthesis 2008, 4, 543-550.
Page 8 of 15
(5) (a) Vignon, P.; Vancompernolle, T.; Couturier, J.-L.; Dubois, J.-L.; Mortreux, A.; Gauvin R. M. ChemSusChem 2015, 8, 1143-1146. (b) Bidange, J.; Dubois, J.-L.; Couturier, J.-L.; Fischmeister, C.; Bruneau, C. Eur. J. Lipid Sci. Technol. 2014, 116, 1583-1589. (c) Firth, B.; Pease, B. M.; Ilseman, A. D.; Zopp, G.; Murphy, T. A.; Weitkamp, R.; Morie-Bebel M. PCT Int. Appl. WO 2014058872 A1 (08.10.2013). For review articles see: (d) Behr, A.; Vorholt, A. J.; Ostrowski, K. A.; Seidensticker T. Green Chem., 2014, 16, 982–1006. (e) Chikkali, S.; Mecking S. Angew. Chem., Int. Ed., 2012, 51, 5802-5808. (6) For a recent review on possible application of olefin metathesis in rubber technology see: (a) Leimgruber, S.; Trimmel G. Monatsh Chem. 2015, 146, 1081-1097. Other examples include: (b) Slugovc C. In Olefin Metathesis: Theory and Practice, 1st ed.; Grela, K., Ed.; John Wiley & Sons, Hoboken, 2014; pp. 329-333. (c) Martinez, H.; Ren, N.; Matta, M. E.; Hillmyer M. A. Polym. Chem., 2014, 5, 3507-3532. (d) Flores, J.-C.; Martin, E.; Bujard, P. PCT Int. Appl. (2014), WO 2014206916 A1 (23.06.2014). (e) Smit, T.; Mueller, K.; Dahmen, S.; Negrete, H.; Norma, L.; Sturm B. PCT Int. Appl. WO 2014026865 A1 (02.08.2013). (f) Obrecht, W.; Mueller, J. M.; Nuyken, O.; Schneider M. PCT Int. Appl. WO 2012104183 (25.01.2012). (g) Rule, O. D.; Moore, J. S. Macromolecules 2002, 35, 7878-7882. (7) (a) Piola, L.; Nahra, F.; Nolan S. P. Beilstein J. Org. Chem. 2015, 11, 2038–2056 and references cited therein. (b) Guidone, S.; Songis O.; Nahra, F.; Cazin C. S. J. ACS Catal., 2015, 5, 2697-2701. (c) Skowerski, K.; Białecki, J.; Tracz, A.; Olszewski, T. K. Green Chem., 2014, 16, 1125-1130. (d) Skowerski, K.; Kasprzycki, P.; Bieniek, M.; Olszewski T. K. Tetrahedron 2013, 69, 7408-7415. (8) For selected recent reviews see: (a) Herbert, M. B.; Grubbs R. H. Angew. Chem. Int. Ed. 2015, 54, 5018 – 5024. (b) Fustero, S.; Simón-Fuentes, A.; Barrio, P.; Haufe G. Chem. Rev. 2015, 115, 871−930. (c) Hoveyda A. H. J. Org. Chem., 2014, 79, 4763–4792. (d) Szczepaniak, G.; Kosiński, K.; Grela K. Green Chem., 2014, 16, 4474-4492. (e) Olszewski, T. K.; Bieniek, M.; Skowerski, K.; Grela, K. Synlett 2013, 24, 903-919. (f) Fürstner A. Science 2013, 341, 1357-1364. (g) Tomasek, J.; Schatz J. Green Chem., 2013, 15, 2317-2338. (9) (a) Weitekamp, R. A.; Wtwater, H. A.; Grubbs, R. H. J. Am. Chem. Soc., 2013, 135, 16817-16820. (b) Sutar, R. L.; Levin, E.; Butilkov, D.; Goldberg, I.; Reany, O.; Lemcoff, N. G. Angew. Chem. Int. Ed., 2016, 55, 764-767 (and the references cited therein). (10) For comprehensive reviews on latent metathesis catalysts see: (a) Monsaert, S.; Lozano A.; Drozdzak, V., R., Van Der Voort, P.; Verpoort F. Chem. Soc. Rev., 2009, 38, 3360–3372. (b) Szadkowska, A.; Grela K. Curr. Org. Chem. 2008, 12, 1631-1647. (c) Vidavsky, Y.; Anaby, A.; Lemcoff, G. N. Dalton Trans., 2012, 41, 32-43. (d) Naumann, S.; Buchmeiser, M. R. Macromol. Rapid Commun. 2014, 35, 682-701. (11) (a) Morin, Y., Gauvin R. M. In Olefin Metathesis: Theory and Practice, 1st ed.; Grela, K., Ed.; John Wiley & Sons, Hoboken, 2014; pp. 453-471. (b) Drozdzak, R.; Allaert, B.; Ledoux, N.; Dragutan, I.; Dragutan, V.; Verpoort F. Coord. Chem. Rev. 2005, 249, 3055–3074. (12) (a) Kozłowska, A.; Dranka, M.; Zachara, J.; Pump, E.; Slugovc, C.; Skowerski, K.; Grela, K. Chem. – Eur. J. 2014, 20, 14120-14125. (b) Pietraszuk. C.; Rogalski, S.; Powała, P.; Miętkiewicz, M.; Kubicki, M.; Spólnik, G.; Danikiewicz, W.; Woźniak, K.; Pazio, A.; Szadkowska, A.; Kozłowska, A.; Grela, K. Chem. – Eur. J. 2012, 18, 6465-6469. (c) Żukowska, K.; Szadkowska, A.; Pazio, A. E.; Woźniak, K.; Grela, K. Organometallics, 2012, 31, 462-469. (d) Tzur, E.; Szadkowska, A.; Ben-Asuly, A.; Makal, A.; Goldberg, I.; Woźniak, K.; Grela, K.; Lemcoff, N. G. Chem. – Eur. J. 2010, 16, 8726-8737. (13) (a) Bieniek, M.; Bujok, R.; Cabaj, M.; Lugan, N.; Lavigne, G.; Arlt, D.; Grela K. J. Am. Chem. Soc. 2006, 128, 13652-13653. (b) Gawin, R.; Makal, A.; Woźniak, K.; Mauduit, M.; Grela K. Angew. Chem. Int. Ed. 2007, 46, 7206-7209. (c) Bieniek, M.; Samojlowicz, C.; Sashuk, V;. Bujok, R.; Sledz, P.; Lugan, N.; Lavigne, G.; Arlt, D.; Grela, K. Organometallics 2011, 30, 4144–
ACS Paragon Plus Environment
Page 9 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
4158. (d) Guidone, S.; Blondiaux, E.; Samojłowicz, C.; Gułajski, Ł.; Kędziorek, M.; Malinska, M.; Pazio, A.; Woźniak, K.; Grela, K.; Doppiu, A.; Cazin, C. S. J. Adv. Synth. Catal. 2012, 354, 2734– 2742. (e) In parallel a related work was published recently: Duan, Y.; Wang, T.; Xie, Q.; Yu, X.; Guo, W.; Wang, J.; Liu, G. Dalton Trans., 2016, 45, 19441-19448. (14) For details see Supporting Information. (15) Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 171-179. (16) Barbasiewicz, M.; Bieniek, M.; Michrowska, A.; Szadkowska, A.; Makal, A.; Woźniak, K.; Grela, K. Adv. Synth. Catal., 2007, 349, 193–203. (17) (a) Chang, S.; Jones, L.; Wang, C.; Henling, L.M.; Grubbs, R.H. Organometallics. 1998, 17, 3460–3465. (b) Allaert, B.; Dieltiens, N.; Ledoux, N.; Vercaemst, C.; Van Der Voort, P.; Stevens, C.V.; Linden, A.; Verpoort, F., J. Mol. Catal. A: Chem. 2006, 260, 221–226. (18) (a) Pump, E.; Leitgeb, A.; Kozlowska, A.; Torvisco, A.; Falivene, L.; Cavallo, L.; Grela, K.; Slugovc, Ch. Organometallics, 2015, 34, 5383-5392. (b) Leitgeb, A.; Wappel, J.; Urbino-Blanco, C. A.; Strasser, S.; Wappl, Ch.; Cazin, C. S. J.; Slugovc, Ch. Monatsh Chem. 2014, 145, 1513-1517.
(19) (a) Love, J. A.; Sanford. M. S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc., 2003, 125, 10103-10109. (b) Buchmeiser, M. R.; Ahmad, I.; Gurram, V.; Kumar, P. S. Macromolecules, 2011, 44, 4098-4106. (c) Sanford, M. S.; Ulman, R. H.; Grubbs, R. H. J. Am. Chem. Soc., 2001, 123, 749-750. (20) (a) Choi, T.-L.;. Grubbs, R. H. Angew. Chem. Int. Ed., 2003, 42, 1743-1746. (b) Vehlow, K.; Wang, D.; Buchmeiser, M. R.; Blechert S. Angew. Chem. Int. Ed., 2008, 47, 2615-2618. (21) (a) Keitz, B. K.; Fedorov, A.; Grubbs, R. H. J. Am Chem. Soc., 2012, 2040-2043. (b) Peeck, L. H.; Leuthäusser, S.; Plenio H. Organometallics, 2010, 29, 4339-4345. (22) Hubner, S.; De Vries, J. G.; Farina, V. Adv. Synth. Catal. 2016, 358, 3-25. (23) Mauldin, T. C.; Kessler, M. R. J. Therm Anal. Calorim. 2009, 96, 705-713. (24) Polymer material property database EFUNDA. http://www.efunda.com/materials/polymers/properties/polymer_dat asheet.cfm?MajorID=PDCP&MinorID=1 (25) https://secure.strem.com/catalog/v/44-0760/ruthenium
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 15
TOC
10
ACS Paragon Plus Environment
Page 11 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Figure 2. Molecular structure of complex 12. 49x31mm (300 x 300 DPI)
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 3. Molecular structure of complex 13. 62x49mm (300 x 300 DPI)
ACS Paragon Plus Environment
Page 12 of 15
Page 13 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
TOC 156x61mm (300 x 300 DPI)
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Preparation of ligands 9 and 10. 83x112mm (300 x 300 DPI)
ACS Paragon Plus Environment
Page 14 of 15
Page 15 of 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Synthesis of complexes 11a,b and their unexpected reactivity leading to formation of new catalysts 12 and 13 87x140mm (300 x 300 DPI)
ACS Paragon Plus Environment
