Research Article Cite This: ACS Catal. 2019, 9, 951−959
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The Initiation Reaction of Hoveyda−Grubbs Complexes with Ethene Natalie Peschek, Klaus-Jürgen Wannowius, and Herbert Plenio* Organometallic Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
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S Supporting Information *
ABSTRACT: The initiation reactions of the three Hoveyda−Grubbs complexes (GH(H), GH(4-NO2), and GH(5-NO2)) with ethene were studied via UV−vis spectroscopy. The unsubstituted complex GH(H) initiates approximately 5 times slower with ethene than with 1-hexene, and the electron-deficient GH(4-NO2) and GH(5-NO2) initiates 1.2 and 1.7 times slower with ethene than with 1-hexene. A detailed multiwavelength analysis of the absorbance−time traces reveals the multistep nature of the reaction of the Hoveyda−Grubbs complexes with ethene. In addition to the established rate constant k1 (being first order in ethene), two additional, c(olefin) independent reactions k2 and k3 were identified: k2 depends on the nature of the GH precatalyst, while k3 is nearly independent of the precatalyst. The reaction associated with k1 precedes the faster k2 leading to low stationary concentration of an intermediate formed from the respective Hoveyda−Grubbs complex and ethene, corresponding to a postinitiation species. KEYWORDS: olefin metathesis, UV−vis spectroscopy, mechanism, ruthenium, ethene
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INTRODUCTION The simplest olefin ethene is the most common molecule in olefin metathesis reactions,1 and it is formed in numerous variants of this reaction2,3 (a prominent exception is ROMP).4 Apart from being an easily overlooked reaction product, ethene is also employed as a reactant in cross metathesis reactions with internal olefins to yield new terminal olefins.5 Ethenolysis reactions are very useful for the synthesis of α-olefins from unsaturated fatty acids contained in seed oils.1e,6 Ethenolysis has also been applied in the depolymerization of synthetic polyolefins7 or terpenes such as squalene or natural rubber.8 Ethene plays a critical role in olefin metathesis, since in the course of olefin metathesis reactions with ethene9 the respective ruthenium methylidenes are formed.10 Such methylidenes are considered to be key species in the decomposition of olefin metathesis catalysts.11 The thermolysis of a Grubbs-II-complex-derived ruthenium methylidene generates dinuclear decomposition products.12 Consequently, a number of studies report on the detrimental effect of ethene in substrate conversion.13 In ring-closing metathesis (RCM)14 and in ADMET (acyclic diene metathesis),15 the removal of ethene from the reaction mixture represents a very significant driving force.13d,16 The reaction of Grubbs I complex with ethene was studied experimentally and via DFT and found to result in the decomposition of the ruthenium complex and the formation of different olefins, primarily propene and 1butene.17 The reactions of Hoveyda−Grubbs complexes with ethene were investigated by NMR spectroscopy, but an intermediate ruthenium methylidene could not be observed. On the basis of 1H NMR spectra, it is believed that ruthenium hydrides are finally formed in this reaction.12a Ethene is not necessarily detrimental to olefin metathesis and such reactions can also benefit from the presence of this © XXXX American Chemical Society
simple olefin. Once a stable metal carbene is formed in enyne metathesis, the catalytically active RuCH2 species can be regenerated by an ethene-induced conversion,18 which is known as the Mori effect.19 Diver et al. showed that the presence of ethene in enyne metathesis leads to a ruthenacyclobutane resting state and lower catalytic efficiency.20 Detailed NMR spectroscopic studies with ethene employing rapidly initiating precatalysts of the Piers type were done at low temperatures to establish ruthenacyclobutanes (RCB)21 as key intermediates in olefin metathesis reactions.22 Piers et al. also showed that precatalyst (Pcat) initiation can be accelerated dramatically by the presence of ethene.23 Schrock, Hoveyda et al. reported an ethene-induced degenerate olefin metathesis, in which ethene helps to establish Curtin−Hammett kinetics leading to excellent stereoselectivities.24 Little is known about the initiation of Grubbs-type precatalysts with ethene. DFT calculations on this reaction provide evidence for a smaller activation barrier of ethenebased initiation, compared with the analogous ethyl vinyl ether reaction.25 Based on DFT calculations, an interchange pathway for the ethene-induced initiation of Hoveyda−Grubbs complexes was considered likely.26 Preliminary attempts were made to explore the ethene initiation of the Hoveyda−Grubbs complex with ethene using 1H NMR spectroscopy. However, incomplete precatalyst activation was observed after several hours of reaction time, and the topic was not pursued further.27 Received: August 28, 2018 Revised: December 19, 2018
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DOI: 10.1021/acscatal.8b03445 ACS Catal. 2019, 9, 951−959
Research Article
ACS Catalysis
1,2-dibromoethane (formed in virtually quantitative yield with ethene) was determined by quantitative gas chromatography. The maximum concentration of ethene in toluene under an ethene atmosphere was found to be c(ethene) = 0.093 mol·L−1 (at 303 K). This value is very close to that determined by 1H NMR for a saturated solution of ethene in benzene-d6 c(ethene) = 0.089 mol·L−1 (at 296 K)10b and in toluene.36 The initiation of three different Hoveyda−Grubbs complexes (Schemes 1) with ethene was studied in the concentration range c(ethene)= 0.009−0.093 mol·L−1. Since ethene is a gas under standard conditions with limited solubility in organic solvents, the available concentration range for initiation experiments is also limited. The initial concentration of the Hoveyda−Grubbs complex is set to 1.0 × 10−4 mol·L−1, thus establishing pseudo-first order conditions (at least 90-fold excess of ethene vs ruthenium complex) in all initiation experiments. Spectral Changes during the Initiation Reaction. The most prominent spectral change in the course of the initiation reaction is the disappearance of the strong absorbance at around 380 nm (a typical series of UV−vis spectra is shown in Figure 1), which is believed to be a charge-transfer band.37
The role of ethene in olefin metathesis is ambiguous and remains insufficiently understood.28 Since the initiation of Hoveyda−Grubbs type complexes with olefins was shown to also follow an interchange pathway,26,29 the nature of the olefin can be expected to influence the initiation kinetics. The rate of precatalyst initiation is essential for the performance of the catalyst in olefin metathesis reactions.28 The rate at which the active species is generated should be matched to the rate at which the active species is consumed in a catalytic transformation. On this basis, the fast olefin metathesis reactions give best results for rapidly initiating precatalysts,30 while slow olefin metathesis reaction perform best with slowly initiating precatalyst. In the latter case, slowly activating precatalysts act as a long time reservoir of the active species.31 In both cases, low stationary concentrations of the catalytically active species diminish the rate of bimolecular deactivation, leading to enhanced catalyst performance.28 Diver et al. recently showed that subtle conformational effects influence initiation rates and based on this the yields of metathesis reactions.32 Studies by Grubbs et al. are aimed at identifying physical parameters of Hoveyda−Grubbs complexes correlated with the initiation rate, to enable the easy prediction of this important property. It was found that the Ru−O bond strength correlates with the initiation rate.33 More recently a computational model was developed by Liu, Houk, Grubbs et al., enabling the prediction of initiation rates relying on easily computed thermodynamic metrics and without making any assumption on a mechanistic model for the initiation reaction.34 On this basis and with a view to ethenolysis reactions, we have decided to study in detail the role of ethene in the activation of Hoveyda−Grubbs type precatalysts.
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RESULTS AND DISCUSSION Experimental Setup. The reactions of three different Hoveyda−Grubbs type complexes (Scheme 1) with ethene Scheme 1. Hoveyda−Grubbs Complexes Used for the UV/ vis Initiation Experiments with Ethene Figure 1. UV−vis spectra during the initiation reaction of GH(5NO2) with ethene at 303 K in toluene (c(Ru) = 1.0 · 10−4 mol·L−1, c(ethene)= 0.081 mol·L−1 (arrows indicate changes of absorbances during the reaction, two arrows denote absorbance changes due to two consecutive processes).
dissolved in toluene were monitored by UV−vis spectroscopy at 303 K. Solutions of ethene in toluene with different gas content were prepared, by exposing toluene to pure ethene or nitrogen/ethene mixtures with different partial pressure of ethene. All mixtures were allowed to equilibrate under the respective atmosphere for at least 12 h, prior to the initiation experiments, to establish the thermodynamic equilibrium concentration of ethene in solution. This ethene atmosphere was maintained throughout the entire initiation reaction. To the stirred solutions were added solutions of Hoveyda−Grubbs complex in the same solvent to result in c(Ru) = 1.0 × 10−4 mol·L−1.35 Changes in the UV−vis spectrum were observed via a quartz glass immersion probe connected to a UV−vis spectrometer. Immediately following each completed initiation measurement, the concentration of ethene in toluene was determined. Aliquots of the reaction mixture were withdrawn and treated with bromine in CHCl3 solution. The amount of
The initiation of Hoveyda−Grubbs complexes by olefins is a multistep reaction.38 There is the chance that intermediates formed during this reaction will accumulate in sufficient concentrations to enable their spectroscopic detection and to determine the respective kinetics. Previously, we and others primarily utilized the changes in the absorbance maximum of Hoveyda−Grubbs complexes at around 380 nm.26,29,33,39 Early studies had already provided evidence for additional reaction steps.29 This is based on the formation of an intermediate with absorbance at around 500 nm, which is observed for rapidly initiating complexes at high olefin concentrations. For the full analysis of ethene-induced initiation, a detailed multiwavelength analysis of the UV−vis spectra of the Hoveyda−Grubbs complexes was done. In this way, additional characteristic changes in the absorbances were identified (Figure 2). Analytically most useful are spectral regions whose timedependent changes in absorbance rely on a single chemical 952
DOI: 10.1021/acscatal.8b03445 ACS Catal. 2019, 9, 951−959
Research Article
ACS Catalysis
broad and weak absorbances in the 420−620 nm range are observed. We had realized earlier that the 515 nm absorbance may be the key to the understanding of the intermediate formed in the reaction of Hoveyda−Grubbs complexes with olefins.29 Very important evidence for the formation of such an intermediate comes from the work of Diver et al.: The reaction of GH(H) with 1-hexene leads to the disappearance of the precatalyst, but the formation of an active carbene without the isopropoxybenzylidene ligand appears to be retarded. Following the initiation of GH(H) with 1-hexene, the isocyanide-based Buchner reaction41 exclusively trapped species containing the isopropoxybenzylidene ligand. It was concluded that the precatalyst initiates to an intermediate retaining the styrene.39 Diver et al. suggested this intermediate to be either the metallacyclobutane derived from the precatalyst or the ruthenium carbene bearing the η2- bound 2-isopropoxystyrene ligand.39 The detailed analysis of the complete set of time-dependent UV−vis−spectra reveals spectral regions (typically close to the pseudoisosbestic points), in which the two processes k1 and k2 are easily identified (Figure 3), especially when they contribute to the spectral changes with opposite signs of the amplitude (Figure 2). Each of the spectral changes can be fitted with an exponential function resulting in excellent fit parameters and a statistical distribution of the residuals (Supporting Information). The time-dependent UV−vis traces collected over a long period of time (Figure 1) do not show well-defined isosbestic point. However, since the half times of the three steps observed in the reaction with ethene are sufficiently different (t1/2 = 22, 132, and 425 s, Table 1), the spectral changes can be viewed as if they were separate events (in a reasonable approximation). Plotting the first series of spectra in the time interval of t = 10− 50 s provides a clean isosbestic point at 340 nm (Supporting Information), meaning that the specified time interval is dominated by a single chemical process. The next time interval chosen from t = 90−270 s spectra also shows clean isosbestic points at 347 and 434 nm (Supporting Information). Once more there is evidence for a dominant step in the designated period of time. Concerning the third step, the spectral changes associated are less distinct and the spectral analysis does not provide clear results. Determination of UV−vis-Derived Rate Constants. The quantitative analysis of the time-dependent UV−vis spectra of the reaction of several Hoveyda−Grubbs complexes with ethene reveals three distinguishable chemical processes characterized by the rates kobs,1, kobs,2, and kobs,3 for each of the precatalysts studied (Figures 4−7). The rates kobs,1 were obtained from the fitting of the absorbance−time traces at different wavelengths according to the kinetic model reported previously (Table 1).29,40a Excellent fit parameters for the absorbance−time traces were obtained (Supporting Information). The rate constant k1 depends on c(ethene) according to kobs,1 = b + k1 · c(ethene) (first order in ethene, Figure 5) using the kinetic treatment reported previously.29,40a The detailed analysis reveals additional processes kobs,2 and kobs,3 in the fitting of the absorbance− time traces with exponential functions. The obtained rates are independent of c(ethene), and the respective rate constants k2 and k3 are obtained from linear fits of the respective kobs vs c(ethene) (Figures 4, 5). The linear fit of kobs,1 vs c(ethene) (Figure 5) shows a nonzero intercept b. This intercept can be
Figure 2. Representative absorbance- time trace (including red fit curve based on two exponential functions, R2 = 0.993 and chi2 = 8.1 × 10−7) and fit residuals-time trace (bottom, red) for the initiation reaction of GH(5-NO2) (c = 1.0 × 10−4 mol·L−1) with ethene (c = 0.081 mol·L−1).
process only. However, the more common scenario is that several chemical reactions simultaneously lead to changes in the absorbance at certain wavelengths. In such a case, it is helpful to select the wavelength for data analysis such that the amplitudes associated with the different processes have similar amplitudes (= change in the absorbance in the course of the chemical reaction) but different signs. Analytically useful regions of the spectra for GH(5-NO2) tend to be close to the pseudoisosbestic points at around 350 and 425 nm. This detailed analysis of the time-dependent UV−vis traces at selected wavelengths finally reveals three different kobs based on characteristic changes in the absorption spectra (Figure 2 and Supporting Information). The multistep nature of precatalyst reaction with olefins can be illustrated in the difference spectra of complex GH(5-NO2) since the three respective reactions are characterized by sufficiently different rates (Figure 3). A fast, c(ethene)-
Figure 3. UV−vis−difference spectrum (ΔA vs wavelength) during the ethene initiation in the interval t = 10−50 s for c(GH(5-NO2) = 1.0 × 10−4 mol·L−1 with c(ethene) = 0.081 mol·L−1). The increase of an absorbance in the spectra leads to a positive sign of the amplitude change ΔA.
dependent reaction with kobs,1 leads to the loss of intensity in the 376 nm absorbance and the generation of a weak absorbance at 515 nm. The amplitude of this absorbance is small, but this absorbance is nonetheless highly characteristic, since it is observed in many initiation reactions of Hoveyda− Grubbs complexes with olefins.29,40 Another process with kobs,2 leads to a decrease in the 515 nm absorbance, accompanied by the growth of weak absorbances at 470 and 590 nm. During the third period (primarily associated with kobs,3), the spectroscopic changes are less clear, but the evolution of 953
DOI: 10.1021/acscatal.8b03445 ACS Catal. 2019, 9, 951−959
Research Article
ACS Catalysis
Table 1. Rates kobs,1 and Rate Constants k1, k2, and k3 for the Initiation of Hoveyda−Grubbs Complexes (c(Pcat) = 1.0 × 10−4 mol·L−1) with Ethene (c(Ethene) = 0.009−0.088 mol·L−1) at T = 303 K in Toluene Solvent; kobs,1 Data for the Initiation of Hoveyda−Grubbs Complexes with Ethene and 1-Hexene (c(Olefin) = 0.10 mol·L−1 (the kobs,1 for Ethene in the Table Are Obtained from a Linear Extrapolation from the Data in the Range of 0.078−0.088 mol·L−1) to c(Ethene) = 0.1 mol·L−1)a ethene, k1 [L·mol−1·s−1]
ethene, k2 [s−1]
ethene, k3 [s−1]
ethene, kobs,1 [s−1] at 0.1 L·mol−1
1-hexene, kobs,1 [s−1] at 0.1
GH(H)
0.016
0.0054
0.0031
0.0018
0.0095
GH(4-NO2) GH(5-NO2)
0.11 0.050
0.026 0.027
0.0030 0.0016
0.0120 0.0060
0.0143 0.0106
BuVE kobs,1 [s−1] at 0.1 L·mol−1 0.0062b 0.0069c 0.0133b 0.0130b
a The errors for the individual rates and for the respective linear fits obtained from the statistics of the data fitting procedure are given in the supporting information (Tables S1−S3) and are always smaller than 15%. The experimental error of the rates is very different since the amplitude of spectral changes associated with the different processes varies. The final error is estimated to be smaller than 20% for k1, but up to a factor of 0.5−2 for k2 and k3. bMeasured in 2012,40a and cremeasured in 2018.
Figure 4. Log−log plot of kobs vs c(ethene) for complex GH(H) and k1(black), k2 (red), and k3 (green). Wavelengths used for data analysis are 376 nm for kobs,1 and 650 nm for kobs,2 and kobs,3.
Figure 7. Log−log plot of kobs vs c(ethene) for complex GH(5-NO2) and k1 (black), k2 (red) and k3 (green). Wavelengths used for data analysis are 378 nm for kobs,1 and 343 nm for kobs,2 and 416−430 nm for kobs,3.31
corresponding to the intercept, or (b) a second reaction (independent of ethene) takes place. The reaction of a Hoveyda−Grubbs complex with ethene is generally considered to be reversible, but it is likely that the equilibrium is very much in favor of the initial Hoveyda−Grubbs complex, which should lead to an intercept smaller than the one observed. Considering the dual initiation pathway (D, I) of such complexes with olefins, scenario (b) appears to be more likely. The obtained k1 corresponds to the olefin dependent initiation reactions reported previously for other olefins.40a However, the comparison of the k1 rate constants with those of other olefins reported previously requires some consideration. It was shown before that the initiation mechanism of Hoveyda−Grubbs complexes with other olefins follows both a dissociative and an interchange pathway.40a This interpretation is supported by various DFT calculations, which show similar activation barriers for a dissociative and an interchange pathway.27,38,43 For the Hoveyda−Grubbs complexes studied, the relative population of D and I pathways depends on the concentration of the respective olefin. At low c(olefin) the D pathway is more populated than the I pathway, while at high c(olefin), the I pathway is more important. Because of the limited solubility of ethene in toluene, the variation of c(ethene) for the experiments reported here tends to be relatively small and is always
