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In Situ Generation of Ru-Based Metathesis Catalyst. a Systematic 2 Study 3 4 Daniel S

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In Situ Generation of Ru-Based Metathesis Catalyst. a Systematic 2 Study 3 4 Daniel S View metadata, citation and similar papers at core.ac.uk brought to you by CORE Page 1 of 9 ACS Catalysis provided by Archive Ouverte en Sciences de l'Information et de la Communication 1 In Situ Generation of Ru-Based Metathesis Catalyst. A Systematic 2 Study 3 4 Daniel S. Müller,a Yann Raoul,b Jérôme Le Nôtre,c Olivier Basléa,† and Marc Mauduit a* 5 a 6 Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR UMR 6226, F-35000 Rennes, France 7 b OLEON SAS, Venette BP 20609, 60206 Compiègne Cedex, France 8 c PIVERT SAS, Rue les Rives de l’Oise CS50149, 60201 Compiègne Cedex, France 9 10 KEYWORDS: Olefin metathesis, Ruthenium, Arene complexes, in situ, NHC ligand 11 12 ABSTRACT: A practical and cost-effective protocol for the in situ generation of Ru-based metathesis catalysts was developed. 13 Assembly of commercially available and inexpensive reagents [Ru(p-cymene)Cl2]2, SIPr.HCl and n-BuLi led to the formation of 18 14 electron arene-ruthenium complexes that, in the presence of additives such as alkynes, cyclopropenes and diazoesters, generated 15 highly selective and efficient catalytic systems applicable to a variety of olefin metathesis transformations. Notably, we were able 16 to achieve a productive TON of 4500 for the self-metathesis of methyl oleate, a reaction which could be easily upscaled to 2 kg. 17 18 19 20 Since its discovery in the mid 1950's, the metathesis reaction containing the saturated NHC ancillary ligand (SIMes) is quite gained significant importance for organic syntheses and unstable and remains difficult to isolate.10,18 Inspired by the 21 polymer chemistry.1 In its early days, the metathesis reaction pioneering work of Grubbs,14a Noels14c,16 and Dixneuf14b we 22 relied on ill-defined in situ generated catalysts. Only upon the opted for a systematic study to generate 14 electron active 23 emergence of well-defined air stable Ru-complexes, such as species by simply mixing a Ru(II) salt, an ancillary NHC 24 the Grubbs 1st generation Ru-1a, and the more performant precursors and a carbene activator, thus avoiding any isolation 25 Grubbs 2nd generation Ru-1b containing an N-heterocyclic steps of unstable complexes (Figure 1). 26 carbene (NHC) ligand, olefin metathesis started to 27 significantly impact organic synthesis.2 Since then the field 28 has grown steadily, resulting in numerous applications for 29 both academia and industry.3 An early mechanistic study from 30 Grubbs demonstrated that Ru-1a,b are in fact precatalysts 31 which, only after dissociation of the PCy3 ligand, provide the corresponding catalytically active 14-electron benzylidene- 32 ruthenium species (Figure 1-A).4 Obviously the dissociated 33 Manuscript ligand (e.g. PCy3) is no longer needed in the catalytic cycle 34 and has been shown to even be involved in catalyst 35 decomposition.5 In the light of this, several research groups 36 were interested to develop the in situ formation of metathesis 37 active species from alkylidene-free Ru-precatalysts. In that 38 respect, the 18-electron phosphine-based p-cymene-ruthenium 39 complex Ru-2,6 and its NHC ligated congeners Ru-3,7 Ru-48 9,10 40 and Ru-5 showed metathesis activity in the presence of 11 41 carbene activator (Figure 1-B). This in situ protocol was successfully illustrated in ring-opening metathesis 42 polymerization (ROMP),6a,12 self-metathesis13,14d and ring- 43 closing metathesis (RCM) reactions.14 Of note, Thieuleux and 44 Basset demonstrated by 1H-NMR studies that the self- 45 metathesis of ethyl oleate with PCy3-based complex Ru-2 and 46 (trimethylsilyl) diazomethane (TMSD) as carbene activator 47 generated the same observable resting states as the Grubbs-1 48 catalyst (Ru-1).13 Moreover, independent studies by Fürstner15 49 and Noels12d,16 showed that light could also initiate metathesis 50 reactions with 18-electron arene-ruthenium complexes (e.g. 51 Ru-2). More recently, Lewis acid assisted activation of 18- electron arene-ruthenium complexes containing various 52 unsaturated NHCs was reported by Beller and coworkers as 53 wellAccepted as the influence of trace impurities upon metathesis Figure 1. Previous strategies used for the formation of 14-electron 54 activity.17 Despite all these cost-effective methodologies to Ru-active species toward olefin metathesis (A and B). The proposed 55 furnish active metathesis species from simple 18-electron Ru- synthetic route involving inexpensive commercially available reagents (this work). SIMes = N,N'-bis-mesityl-imidazolin-2-ylidene 56 complexes, few drawbacks merit to be solved to render this 57 protocol more attractive. For instance, the most efficient Ru-5 58 59 60 ACS Paragon Plus Environment ACS Catalysis Page 2 of 9 IMes = N,N'-bis-mesityl-imidazol-2-ylidene, IPr = N,N'-bis-2,6-iPr- Figure 2. Ratio of RCM-product 2 (blue) and cycloisomerization 1 C6H3-imidazol-2-ylidene. product 3 (red) in the absence (a) and in the presence (b) of phenyl 2 We report herein that transient alkylidene Ru-species may be acetylene A1. SIPr = N,N'-bis-2,6-iPr-C6H3-imidazolin-2-ylidene. 3 readily formed by mixing inexpensive commercially available This reaction was carried out in the presence (Figure 2-a) and 19 absence (Figure 2-b) of phenyl acetylene A1,20 which has been 4 [(p-cymene)RuCl2]2, an azolium salt, a base and a carbene activator (Figure 1-this work). The choice of the base and the previously described as a convenient and efficient 5 activator appeared to be pivotal factors in the quest for highly activator.14c,d In the absence of phenylacetylene, the use of 6 active catalytic species towards the targeted metathesis SIPr.HCl salt as NHC precursor afforded the highest 7 transformation. Furthermore, the results of this systematic selectivity towards the metathesis product 2 (Figure 2-a). In 8 approach led to the development of in situ made ruthenium- fact, the noticeable activity observed using SIMes ligand 9 catalysts exhibiting catalytic performances that can compete precursor led mostly to the cycloisomerization product 3. 10 with some well-defined 1st generation catalysts (e.g. Ru-1a). Interestingly, in all examined cases a strong enhancement of 11 We began our study by in situ generation of ruthenium arene the metathesis reactivity and selectivity could be observed 14c,d 12 complex Ru-NHC from four commercially available azolium upon the addition of 20 mol% of phenyl acetylene, with the 13 salts which were deprotonated with NaHMDS and then SIPr-based complex demonstrating the highest catalytic 20 activity (Figure 2-b). It should be noted that a reduction of the 14 exposed to [(p-cymene)RuCl2] (Figure 2). There is literature 4:1 phenyl acetylene:ruthenium ratio significantly lowered 15 precedent on the exchange of p-cymene to deuterated benzene 9 yields and selectivities towards the metathesis product 2 (see 16 and based on this observation, it is reasonable to assume that 21 Supporting Information; SI). Encouraged by these initial 17 p-cymene is rapidly replaced by toluene at 80 °C. The solution of the in situ generated 18-electron complex Ru-NHC results, we decided to further investigate the influence of the 18 was then immediately tested for the ring-closing metathesis base on the outcome of the metathesis reaction. Therefore, the 19 (RCM) of diethyl diallylmalonate 1. catalytic activity obtained with 3 mol% of SIPr-based complex 20 generated using NaHMDS was compared in terms of 21 efficiency to catalytic systems generated from the use of other 22 bases such as ZnEt2, KHMDS, n-BuLi and KOtBu (Figure 3). used in situ The catalytic systems generated from the various bases 23 NaHMDS (1.0 equiv) + toluene Me evidenced tremendous metathesis activity differences after 15 80 °C, 15 min 24 Azolium salt EtO2C CO2Et Ru min of reaction. Whereas Et2Zn gave a very modest 7% yield (1.0 equiv) - p-cymene Cl 25 + NHC Cl of the desired RCM-product 2, NaHMDS and KOtBu gave [(p-cymene)RuCl2]2 26 (0.5 equiv) Ru-NHC (5 mol %) 3 significantly improved results (37% and 63% yield). The best 27 performances were obtained with KHMDS and n-BuLi (~90 EtO2C CO2Et 28 EtO2C CO2Et % GC-yield), which compare favorably to the previous + Ph 29 toluene, 80 °C, 2 h procedure described by Grubbs requiring prolonged reaction A1 times and higher catalyst loading for similar results (5 mol%, 30 1 (0 or 20 mol%) 2 31 10 h).14a With the objective to provide catalytic systems of 32 Azolium salt broad application, we therefore decided to continue our study with n-BuLi, which has the advantage of only producing 33 N N N N Manuscript butane and LiCl side products with low potential for 34 Cl Cl interfering with the metathesis catalytic cycle. 35 IMes.HCl IPr.HCl used in situ 36 base (1.0 equiv) + toluene Me 37 SIPr.HCl (1.0 equiv) 80 °C, 15 min NN NN Ru - p-cymene Cl 38 + SIPr Cl 39 Cl Cl [(p-cymene)RuCl2]2 (0.5 equiv) Ru-6 (3 mol %) 40 SIMes.HCl SIPr.HCl EtO2C CO2Et 41 EtO2C CO2Et + Ph 42 toluene, 80 °C, 15 min. A1 43 1 (12 mol%) 2 44 45 46 47 48 49 (%) 50 2 51 yield of 52 53 Accepted 54 Et2Zn NaHMDS KOtBu KHMDS n-BuLi 55 (0.5 equiv) 56 57 58 59 60 ACS Paragon Plus Environment Page 3 of 9 ACS Catalysis Figure 3. Influence of the base on the catalytic efficiency after 15 Concerning the activation of 18 electron arene-ruthenium- 1 min in the RCM of diethyl diallylmalonate 1 using 3 mol% of in complex Ru-6 with alkyne, cyclopropene and diazo activators 2 situ generated Ru-6.
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