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Highly Selective Olefin Metathesis with CAAC Containing Ruthenium Benzylidenes Danielle Butilkov, Alexander Frenklah, Illya Rozenberg, Sebastian Kozuch, and Norberto Gabriel Lemcoff ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02409 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017
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Highly Selective Olefin Metathesis with CAAC Containing Ruthenium Benzylidenes Danielle Butilkov, Alexander Frenklah, Illya Rozenberg, Sebastian Kozuch and N. Gabriel Lemcoff* Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, 84105, Israel. ABSTRACT: Several olefin metathesis reactions are studied, namely: jojoba oil oligomerization, methyl oleate selfmetathesis, ring-closing metathesis (RCM) to form a nitrogen heterocycle, and 1,5-hexadiene acyclic diene metathesis polymerization (ADMET). The catalyst containing the Bertrand-Grubbs cyclic alkyl amino carbene (CAAC) ligand showed high selectivity by diminishing isomerization reactions; this was especially clear at high temperatures where the more widely used nitrogen heterocyclic carbene (NHC) based catalysts show side reactions. Experimental and computational studies determined that it is much more difficult to produce ruthenium hydrides with CAAC, a property that can explain the improved observed activity. This finding opens a pathway for the development of even more selective olefin metathesis catalysts for reactions that require harsh conditions.
KEYWORDS: Olefin metathesis, homogeneous catalysis, double bond isomerization, CAAC ligand, ruthenium. The ruthenium olefin metathesis story is certainly one of the most salient examples of transition metal catalyst development by judicious design of their ligand shell. For example, the design of novel alkylidenes have led to more stable and more active catalysts with improved properties;1 special anionic ligands have been used to promote cis-selective olefin metathesis2 and some forms of ligand chelation gave rise to the property of latency.3 Nevertheless, it was probably the introduction of the Nheterocyclic carbene (NHC) ligands that brought about the most significant breakthrough in the family of the Grubbs’ catalysts.4 Recently, Bertrand, Grubbs and others have studied the properties of yet another remarkable stabilized carbene ligand on ruthenium olefin metathesis, i.e. the cyclic alkyl-amino carbene (CAAC) ligand.5 Indeed, ruthenium catalysts with CAACs have shown impressive results, such as 340,000 turnover numbers in ethenolysis reactions6 and great efficiency in other olefin metathesis reactions.7 These advances may have great implications in the developing field of advanced biofuels and bio-based energy alternatives.6b However, ruthenium based olefin metathesis complexes are known to catalyze also non-metathetic reactions;8 at times beneficial,9 but largely in detriment of the desired olefin metathesis main products.10 Thus, considerable effort has been spent on diminishing these side reactions, either by the effect of solvents or additives11 or by the design of novel ligands for the ruthenium catalysts.12 In our investigation of the self-metathesis of jojoba oil to produce bio-based jet fuel and degradable polyesters,13 we noticed a significant double bond migration that led to a broad distribution of the distilled hydrocarbons. Our continuing efforts on this area led us to study this reac-
tion with the efficient Bertrand-Grubbs catalysts. In an unexpected turn of events, we noticed that the isomerization under harsh conditions was almost completely suppressed. In light of this, herein we report the effect of using CAAC ligands (Figure 1) on olefin isomerization and concomitant side-reactions, especially under harsh conditions (i.e. high temperatures). We also discuss the reasons why CAAC ligands on ruthenium catalysts diminish the occurrence of the pernicious non-metathetic side reactions. Et
R1 N
N
Cl Ru Cl O
HG2
N Cl (iPrO)3P
N
N Ph
R2
Ru
Cl Ru Cl
Cl
O
cis caz -1
N
1 R1 = R2 = iPr 2 R1 = R2 = Et 3 R1 = iPr, R2 = Me
Et
Ph
Cl Ru Cl Et N Et
4
Figure 1. Ruthenium precatalysts used in this work.
In order to remove the hydrocarbon chains in the acyclic diene metathesis polymerization (ADMET) reaction of jojoba oil (Scheme 1), high temperatures (about 200°C) are needed. These harsh conditions brought about extensive isomerization of the double bond and a broad distillate distribution. Accordingly, as measured by GC-MS, only 28% of the distillate consisted of the expected octadec-9-ene molecules (C-18) when the reaction was catalyzed by the commercially available Hoveyda-Grubbs 2nd generation catalyst (HG2). Thus, we set out to determine the effect of CAAC containing catalysts on this important reaction. The first experiments using both diisopropyl- (1) and diethyl-CAAC (2) Hoveyda type complexes did not afford an improvement in conversion (see supporting information); yet, we were surprised to
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O 7
n O
m
[Ru]-precatalyst (0.1% mol) 200oC, 0.1-0.3 torr
O
7
n O
7
jojoba oil n = 7, 9, 11 or 13 m = 8, 10, 12 or 14
7
m x
7
in deuterated 1,1,2,2-tetrachloroethane (0.5M) and heated to 150oC for 1 hour under nitrogen. In both cases quantitative conversions were observed. 1H-NMR analysis (See SI) clearly revealed that RCM catalyzed by HG2 was accompanied by significant double bond migration to produce a mixture of symmetric 7 (58%) and asymmetric 8 (8%), in addition to the expected 6 as a minor product (34%). Notably, 3 catalyzed the desired RCM reaction to give 6 with high selectivity (87%) and only 13% of 7 as a minor byproduct. No asymmetric isomerization product was observed.
a)
7
[Ru]-precatalyst (0.1% mol) 100 oC, 1h
O
Scheme 1. General scheme for ADMET of jojoba oil.
7
7
O O
MO
+
7
DE
7 O 7
C-18
86 75
80
75 66
51
60 40
O 7
O
b) 100 C-18 (%)
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20 0
precatalyst Figure 2. C-18 composition of the distillate of jojoba oil ADMET with several precatalysts.
Following the observation that CAAC based ruthenium complexes showed a substantial reduction in double bond migration in the jojoba oil metathesis reaction, an extended study on other olefin metathesis reactions was conducted. Self-metathesis of methyl oleate. Methyl oleate was mixed with several ruthenium precatalysts at different temperatures for 1 hour. At 200°C, the complexes HG2 and cis caz-13e were efficient cross-metathesis catalysts; but, as observed in the jojoba reaction, there was substantial isomerization of the double bond, with C-18 accounting for just 20-30% of the hydrocarbon chain. Satisfactorily, complex 3 was again the most efficient precatalyst with 73% selectivity for the C-18 chain under the testing conditions. However, at 100°C the advantages of using the CAAC ligand in the ruthenium complexes become even more evident. While isomerization using 3 was suppressed almost completely, the popular HG2 precatalyst still afforded large amounts of isomerized products (Figure 3). Ring-closing metathesis (RCM) of N-allyl-N(pent-4-en-1-yl)tosylsulfonamide. The problem of double bond isomerization is not exclusive for crossmetathesis reactions; many RCM reactions are also hindered by this side reaction.14 Thus, a typical benchmark RCM reaction of N-allyl-4-methyl-N-(pent-4-en-1yl)benzenesulfonamide was studied (Scheme 2). The substrate was reacted in the presence of both HG2 and 3
Figure 3. a) Reaction scheme and b) GC-MS analyses of self-metathesis of methyl oleate with HG2 (top, blue) and 3 (bottom, red). Ts N
[Ru]-precatalyst (1.0% mol) 1h, 150oC, N2 1,1,2,2-tetrachloroethane-d2 5
Ts N
Ts N +
Ts N +
6 7 8 RCM product isomerization products
Scheme 2. RCM of yl)tosylsulfonamide.
N-allyl-N-(pent-4-en-1-
ADMET polymerization of 1,5-hexadiene. One of the most significant reactions where double bond migration must be avoided is the ADMET reaction to produce well defined polymers. 1,5-hexadiene is a typical ADMET substrate that affords a polybutadiene like polymer. The need to remove volatiles to achieve high conversions (and consequently high molecular weights) in this important reaction encourages polymer chemists to work at high temperatures, where most ruthenium catalysts can decompose and promote double bond migrations. In this polymer, a migration of the double bond may be easily detected by 13C-NMR analysis, because it will produce a methylene flanked by two sp2 carbons, notably different than the expected signal for the vicinal methylene pairs. Separate mixtures of HG2 and 3 in neat monomer under inert atmosphere at 100 oC were stirred for 4 hours. 13C-NMR analyses clearly show the significant difference
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in the microstructure of the ADMET derived polymers (Figure 4). While, CAAC containing complex 3 produced the desired well-defined polymer (polybutadiene like), extensive isomerization and concomitant defects could be seen in the HG2 derived polymer. Naturally, hydrogenation of the double bonds can hide these defects, but the use of 3 will give a precision polymer without the imperfections that originate from carbon-carbon double bond migrations.
dene complexes are more than 3 kcal/mol greater for the CAAC containing ruthenium alkylidene than for the NHC ruthenium alkylidene (Scheme 3); hence supporting the hypothesis that the greater difficulty to form the ruthenium hydride (π-allyl complex) is the reason behind the beneficial effects seen for the Bertrand-Grubbs family of catalysts. Furthermore, according to the performed calculations, the distance between the propene double bond and the metal at the starting reactant is shorter by an average of 0.2Å when NHC is present (stabilizing the starting material and favoring the isomerization pathway). Also, at the transition state, the distance between the hydride and the metal center is shorter by 0.04Å, for the NHC bearing catalyst; probably influencing the energy difference in the direction noted.
a)
b)
Scheme 3. Influence of L ligand on ruthenium mediated hydride transfer.
39
37
35
33
31
29
27 ppm
25
23
21
19
17
15
Figure 4. a) General 1,5-hexadiene ADMET reaction. b) 13C-NMR of ADMET of 1,5-hexadiene catalyzed by HG2 (top, blue) or 3 (bottom, red).
Having unequivocally shown the advantage of using the complex developed by Bertrand and Grubbs in situations where heating of the reaction is beneficial, or even required, we turned our attention to understanding this unusual outcome. Ruthenium-hydrides. One of the leading hypotheses for double bond migration in olefins is through the assistance of metal hydride complexes and bimetallic species.15 Formation of the binuclear species (and other decomposition products) requires the presence of a phosphine nucleophile;16 absent in our study. The propensity towards hydride formation in the relevant complexes was thus probed. For this, HG2 and 3 were simply mixed with sodium cyanoborohydride in deuterated THF at room temperature.17 Indeed, 1H-NMR immediately revealed the clear formation of a ruthenium hydride with HG2. However, under the same conditions complex 3 did not show any change (see SI). Moreover, DFT calculations of the rate determining transition state according to Nolan-Prunet18 isomerization mechanism revealed that, in relative terms, the differences in activation energies between the ruthenium mediated hydride transfer transition states and their corresponding alkyli-
13
Conclusions. We have investigated the influence of CAAC bearing ruthenium precatalysts on double bond isomerization in different types of olefin metathesis reaction (RCM, CM, and ADMET) under harsh conditions. In all cases, these precatalysts showed significant reduction in double bond migration (especially complex 3) in comparison to the ubiquitously used HG2 precatalyst. When introduced to sodium cyanoborohydride, complex 3 remained unscathed, while HG2 immediately reacted to give a ruthenium-hydride complex. The experimental results are supported by the DFT calculations. While other types of novel ruthenium precatalysts may show improved selectivities in olefin metathesis reactions where double bond isomerization may be detrimental;7,12 the CAAC family, due to its versatility and ease of synthesis, gives a proper solution to this predicament. We propose that to further deter formation of hydrides, tuning CAAC ligands may produce even more efficient catalysts for olefin metathesis reactions carried out at severe conditions where non-metathetic side reactions become prevalent.
ASSOCIATED CONTENT Supporting Information. Experimental data and general procedures, GC-MS analyses, NMR spectra of products of catalysis, NMR spectra of hydride study, DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
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*E-mail:
[email protected]; Fax: (+972) 8647-1740; Tel: (+972) 8646-1196.
Funding Sources The Israel Science Foundation is gratefully acknowledged for funding this research.
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ACS Catalysis (17) Schmidt has previously shown that adding borohydride to ruthenium benzylidenes promotes alkene isomerization: Schmidt, B. Eur. J. Org. Chem. 2003, 2003, 816−819.
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