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Halides Inhibit Olefin Isomerized By-Products from Phosphine-Based Grubbs' Metathesis Catalysts in Polar Protic Solv

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Halides Inhibit Olefin Isomerized By-Products from Phosphine-Based Grubbs' Metathesis Catalysts in Polar Protic Solv Copper(I) halides inhibit olefin isomerized by-products from phosphine-based Grubbs’ metathesis catalysts in polar protic solvents Michael D. Schulz, Manza B. J. Atkinson, Rachel J. Elsey & Martin M. Thuo Transition Metal Chemistry ISSN 0340-4285 Volume 39 Number 7 Transition Met Chem (2014) 39:763-767 DOI 10.1007/s11243-014-9858-1 1 23 Your article is protected by copyright and all rights are held exclusively by Springer International Publishing Switzerland. This e- offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy Transition Met Chem (2014) 39:763–767 DOI 10.1007/s11243-014-9858-1 Copper(I) halides inhibit olefin isomerized by-products from phosphine-based Grubbs’ metathesis catalysts in polar protic solvents Michael D. Schulz • Manza B. J. Atkinson • Rachel J. Elsey • Martin M. Thuo Received: 25 June 2014 / Accepted: 7 July 2014 / Published online: 31 July 2014 Ó Springer International Publishing Switzerland 2014 Abstract Copper(I) halides are employed as ‘phosphine degradation to the undesired olefin isomerization catalysts sponges’ to sequester phosphor-ylides when using phos- [24–28]. Scheme 1 shows two examples of olefin isomer- phine-based Grubb’s metathesis catalysis in polar protic ization reactions from literature when Grubbs’ catalyst was solvents and under heat. These cuprous halides are used in polar protic solvents [20]. Attempts to improve the hypothesized to significantly slow the formation of the Grubbs’ metathesis catalysts for use in polar protic media ruthenium hydride olefin isomerization catalyst. We dem- include: use of solid supports [16], ligand modification onstrate their use in both cross metathesis and ring-closing occlusion/encapsulation in polymeric materials [13–15, metathesis. 17–21], and use of quinone additives [22, 23]. Olefin isomerization during metathesis has been attrib- uted to degradation of the Grubbs’ catalysts to ruthenium Introduction hydrides (Scheme 2). This degradation is induced by heat or polar protic solvents [20, 24–28]. The mechanism of Olefin metathesis is one of the most important reactions in degradation depends on the structure of the catalyst, with C–C bond formation, and its importance has been high- the carbene-based second-generation catalyst, 2, being lighted by the number of catalysts developed [1–12]. more resistant to degradation than catalyst 1 (Fig. 1)[25, Applications of olefin metathesis range from small mole- 26]. Grubbs and co-workers isolated the dinuclear hydrido cule synthesis to living polymerization [13–23]. One of the complex, 10, when catalyst 2 was thermally degraded in more versatile metathesis catalysts is the ruthenium-based benzene (Scheme 2)[26, 28]. Grubbs and others also Grubbs’ catalysts [1–12]. The use of these catalysts in polar showed that rutheno hydrides like, 10, caused olefin media or elevated temperatures, however, is limited by isomerization [26, 28]. Labeling experiments by Wagener and co-workers also showed that olefin isomerization was catalyzed by a metal hydride via an addition elimination M. D. Schulz mechanism rather than via a p-allyl, g3-ruthenium medi- Department of Chemistry, University of Florida, Gainesville, ated rearrangement as previously proposed [29]. A key FL 32611, USA intermediate in the formation of the rutheno hydride, M. B. J. Atkinson Á M. M. Thuo however, is the dissociated phosphine ligand, without Department of Chemistry, University of Massachusetts, 100 which degradation would only lead to catalyst depreciation Morrissey Blvd, Boston, MA 02125, USA without the undesired olefin isomerization (Scheme 2)[26, 27]. We therefore hypothesized that removal of the disso- R. J. Elsey College of Pharmacy, South Dakota State University, 10000E ciated phosphine would exclusively lead to olefin metath- 23rd St., Brookings, SD 57105, USA esis without any isomerization. Copper(I) has been regarded as a ‘phosphine sponge’ & M. M. Thuo ( ) [30, 31]. In principle, the addition of Cu(I) halides during Center for Green Chemistry, University of Massachusetts, 100 Morrissey Blvd, Boston, MA 02125, USA metathesis reactions is appealing because: (1) it would e-mail: [email protected] inhibit formation of unwanted metal hydrides and therefore 123 Author's personal copy 764 Transition Met Chem (2014) 39:763–767 Fig. 1 Examples of the ruthenium-based Grubbs’ metathesis pre- catalysts (1 and 2) with the corresponding metathesis-active methyl- Scheme 1 Examples of two products reported in literature generated idene catalysts (3 and 4) through isomerization of olefins by Grubbs’ metathesis catalysts when used in polar protic media. The first reaction leads to deprotection of alcohols while the second reaction gives an isomerized olefin pathway with the ruthenium species, or a reagent that prevents degradation of the Grubbs’ catalyst to the unwanted hydrides. This latter approach would seek to minimize olefin isomerization, (2) the inorganic salts are inhibit, degrade, or re-route one or several of various readily removed, alongside the catalyst by-products, from intermediates (Scheme 2, complexes 5–9) or fragments the final product(s), (3) Cu(I) salts are cheap and readily thereof. We desired to apply the second approach by available, and (4) only small quantities of Cu(I) are needed sequestering the dissociated phosphine ligand using Cu(I). (*1:1 Cu(I):catalyst). In this paper, we discuss our find- ings on the effectiveness of Cu(I) halides as inhibitors of olefin isomerization while performing olefin metathesis Experimental under conditions in which isomerization is favored. We chose to perform our study under heat (50 °C) and in polar To ensure that the catalyst was well dissolved, we initially protic media (MeOH/ethylene glycol), conditions that are dissolved it in 1 mL of CH2Cl2. The MeOH/ethylene gly- known to readily decompose Grubbs’ catalysts to give high col was then added followed by an additive of choice and or exclusively isomerized olefins [20]. We show that the finally the olefin. The reaction mixture was heated to addition of Cu(I) halides inhibits olefin isomerization. 50 °C. The highly metathesis-active diethyl diallyl mal- Efforts to prevent olefin isomerization include two main onate, allyl phosphine oxide, and an allyl ether were used approaches: (1) Hydride traps can be introduced to elimi- to study the effects of inorganic additives on the metathesis nate any isomerization catalysts formed [29]. This reaction in polar protic solvent. All solvents were degassed, approach was, for example, reported by Grubbs and co- and experiments performed under inert atmosphere. For all workers when they employed benzoquinones [32]. (2) The the reactions, Grubbs’ first-generation metathesis catalyst application of hydride formation inhibitor(s) that may was used unless otherwise stated. We monitored the reac- entail an additive that competes for the hydride formation tions by periodically taking aliquots and running 1HNMR Scheme 2 The proposed degradation pathway for Grubbs’-type generation catalyst to the isomerizing dinuclear ruthenium hydride olefin metathesis catalysts. The dissociated phosphine ligand (PCy3) (10). Sequestering this phosphine ligand would, subsequently, inhibit is important in the degradation of the activated Grubbs’ second- formation of the hydride isomerization catalyst 123 Author's personal copy Transition Met Chem (2014) 39:763–767 765 Table 1 A comparison of adding copper halide versus alkali metals to the metathesis reactions of substrates that readily isomerizes in presence of MeOH Entry Substrate Additivea Time (h) Metathesis product (%)b Isomerization product (%)b 1 O CuBr 39 0 0e 2 O NaCl 39 0 100 3 O KI 72 66d 25 P Ph Ph 4 O NaCl 39 63 37 P Ph Ph 5 O CuBr 72 81c 0 P Ph Ph All reactions were performed at 50 °C a 0.08 Equivalents relative to the olefin b Determined by 1H NMR c 19 % Starting olefin d 9 % Starting olefin e Only allyl phenyl ether was recovered and no phenols were observed in CDCl3 (with TMS as an internal standard). The Table 2 The effect of adding copper halides to the metathesis metathesis reaction products were differentiated from reaction in MeOH/ethylene glycol mixture 1 isomerization products by their unique H NMR signals Entry Catalyst (%) Additive Time (h) Isomerizationa (%)b (for general procedure1). 14 – 1022 2 4 KI 3 33 Results and discussion 3 4 KI 16 51 4 4 NaI 22 50 Cross metathesis reactions 5 4 HCl 12 0 6 4 CuBr 2 0 To demonstrate the potential for Cu(I) halides to inhibit 7 4 CuBr 14 0 isomerization, we employed substrates that have been 8 4 CuBr 24 0 9 4 CuCl 22 0 10 4 CuI 22 0 1 General experimental procedure: To a clean flask was added Grubbs’ first-generation catalyst (17 mg, 21 lmol, 1 mol %) dis- All reactions were performed at 50 °C solved in 1 mL dichloromethane under inert atmosphere. To the flask a Obtained as a mixture of the isomerized starting material and the was then added 5 mL of 1:1 MeOH:Ethylene glycol followed by NaI isomerized product (3.2 mg, 21 lmol, 1 mol %). To the flask was added 1-propenyldi- b phenylphosphine oxide (509 mg, 2.1 mmol), and the reaction mixture Relative to the metathesis product was maintained at 50 °C using a temperature controlled oil-bath for 22 h. For analysis, an aliquot was drawn, dissolved in CDCl3 (containing 1 % TMS internal standard), and sample analyzed by 1H reported to preferentially isomerize instead of undergoing NMR.
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