Nickel Catalysis in a High Speed Ball Mill: A Recyclable

Mar 31, 2016 - A solvent-free, nickel-catalyzed [2 + 2+2 + 2] cycloaddition of alkynes to synthesize substituted cyclooctatetraene (COT) derivatives h...
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Nickel Catalysis in a High Speed Ball Mill: A Recyclable Mechanochemical Method for Producing Substituted Cyclooctatetraene Compounds Rebecca A. Haley, Aaron R. Zellner, Jeanette A. Krause, Hairong Guan,* and James Mack* Department of Chemistry, University of Cincinnati, 301 Clifton Court, Cincinnati, Ohio 45221-0172, United States

ACS Sustainable Chem. Eng. 2016.4:2464-2469. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 09/04/18. For personal use only.

S Supporting Information *

ABSTRACT: A solvent-free, nickel-catalyzed [2 + 2+2 + 2] cycloaddition of alkynes to synthesize substituted cyclooctatetraene (COT) derivatives has been developed. This mechanochemical approach takes advantage of the frictional energy created by reusable nickel pellets, which also act as the catalyst. In contrast to solution chemistry, the major products are cyclooctatetraene isomers rather than substituted benzenes.

KEYWORDS: Green chemistry, Mechanochemistry, High speed ball mill, Nickel catalysis, Cyclooctatetraene, Cyclotrimerization, Cyclotetramerization

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INTRODUCTION The field of mechanochemistry was first introduced in the late 1890s with the studies of M. Carey Lea on the effect of mechanical energy on silver halides.1−4 More recently, mechanochemical methods have been employed in organic syntheses5−7 and transition metal catalysis reactions8 because of the reduction in the amount of solvent used for carrying out the reaction.8 The area of transition metal catalysis has become a more integrated part of mechanochemistry. For example, classic metal-catalyzed reactions such as Sonogashira,9,10 Suzuki,11−14 and Heck15,16 reactions have proven to be successful under solvent-free mechanochemical conditions. While these reactions take place under solvent-free conditions, they still employ catalysts that are typically used in homogeneous solutions. Our group and others17 have previously shown that some of these traditional catalysts can be replaced with elementary metals as catalysts under mechanochemical conditions. For instance, the copper cocatalyst needed for the Sonogashira reaction can be replaced by using a copper ball and copper vial.9 This concept has been further tested in copper-catalyzed azide−alkyne click reactions (CuAAC).18 The success of using copper vials and balls as catalysts for various chemical transformations has led us to investigate the suitability of other metals. The most recent example involves a silver-catalyzed cyclopropanation reaction, where silver foil acts as a relatively inexpensive and recyclable catalyst.19 One metal that has not been previously studied but has great potential in mechanochemical research is nickel. Reactions that use low-valent nickel catalysts such as Ni(COD) 2 (COD = 1,5-cyclooctadiene) could be more © 2016 American Chemical Society

conveniently catalyzed under ball milling conditions. For example, Ni(COD)2 is highly oxygen-sensitive and thermally unstable thus requiring high catalyst loadings for most catalytic reactions; whereas, the nickel vial, ball, or foil used in ball milling is in the zero oxidation state and insensitive to the ambient environment. Though there is most likely a protective coating of nickel oxide on the surface of these nickel materials, we theorize that under the milling conditions this layer is broken down to reveal fresh Ni(0) metal. Additionally, these fine nickel particles are relatively unreactive in the presence of air since any oxygen or vapor present in the reaction vial are constantly displaced by the ball bearing(s).20 This work uses a nickel-mediated cycloaddition reaction to illustrate that nickel metal under high speed ball milling conditions is a suitable catalyst for these types of Ni(0)-catalyzed reactions.

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EXPERIMENTAL (MATERIALS AND METHODS) General Remarks. Cycloaddition reactions were carried out by mechanochemical milling in a SPEX8000 M Mixer Mill at a frequency of 18 Hz using a stainless steel vial with 1/8 in. nickel pellets. 1H and 13C {1H}NMR spectra were obtained on a Bruker Avance 400 MHz spectrometer, and all chemical shift values are reported in ppm on the δ scale. GC-MS data were obtained with a Hewlett-Packard 6890 series GC-MS with a Zebron ZB-5, 15 mm × 0.25 mm × 0.25 mm column. Mass Received: February 20, 2016 Revised: March 23, 2016 Published: March 31, 2016 2464

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ACS Sustainable Chemistry & Engineering spectral determinations were carried out using ESI as the ionization method. Normal-phase TLC was done using silica gel plates purchased from Silicycle, and reverse-phase TLC was done with C18-W impregnated silica gel plates from Sorbtech, both analyzed by UV light. Flash column chromatography was performed using a Combiflash automated flash column chromatography system with RediSep Rf gold high performance flash columns (fine spherical silica gel 20−40 μm). Deuterated chloroform was obtained from Cambridge Isotope Laboratories, Inc., Andover, MA, and used without further purification. All alkynes were purchased from Sigma-Aldrich and used without further purification. All reaction vials were custom-made at the University of Cincinnati machine shop, and the metal rods were purchased from McMaster Carr. The nickel pellets and nickel foil were purchased from ESPI Corp, Inc. Simiriz 486 Perfluoroelastomer O-rings (6/16 in. ID × 7/16 in. OD × 3/32 in. width) were purchased from Small Parts, Inc. The neodymium magnet (Ni plated, 2 in. × 1/2 in.) was purchased from K&J Magnetics, Inc. Typical Procedure for Preparing Cyclooctatetraene (COT) Compounds. To a custom-made 2.0 in. × 0.5 in. stainless steel screw-capped vial was added 19 g of 1/8 in. Ni pellets. The alkyne substrate (500 μL) was added via syringe. To the cap, a perfluoroelastomer O-ring was added. The vial and cap were screwed together using a vice and wrench and placed in a Spex Certiprep 8000 M mixer mill. These reagents were milled for 16 h. After milling, the reaction mixture was dissolved in dichloromethane and transferred to an Erlenmeyer flask. The mixture was separated from the nickel pellets by placing a neodymium magnet on the outside of the Erlenmeyer flask and decanting into a recovery flask. The solvent was removed under reduced pressure to afford a mixture of products. The cyclooctatetraene compounds were separated from the benzene derivatives and oligomer byproducts by flash column chromatography on silica gel using petroleum ether/ ethyl acetate or 100% petroleum ether as the eluent. 1H NMR and GC-MS were used to assess the purity of these target molecules.

Scheme 2. Mechanochemical [2 + 2+2 + 2] Cycloaddition

traene (COT) molecules were identified as the major products; the ratio of COTs to aromatic products was calculated to be 90:10. Of the four COT products, the major isomers were found to be 1,2,4,6- and 1,2,4,7-substituted isomers in a ratio of 51:42 (Scheme 2). The substitution pattern of the 1,2,4,7substituted isomer 5a was further confirmed by X-ray crystallography (Figure 1).

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RESULTS AND DISCUSSION To begin our study of nickel-catalyzed cycloadditions under mechanochemical conditions, ethyl propiolate (1a) was chosen as the substrate. In our previous solution studies, 1a was shown to oligomerize to 1,2,4- and 1,3,5-trisubstituted benzenes in a 97:3 ratio when a catalytic amount of Ni(PPh3)4 or Ni(COD)2PPh3 was employed (Scheme 1).21 No ligand additives were necessary for mechanochemically catalyzed reactions using copper and silver,18,19 so the initial reaction was performed with just a nickel vial, nickel pellet, and 1a (Scheme 2). After 16 h of ball milling, approximately 10% conversion was observed by 1H NMR spectroscopy. In addition to the expected aromatic products, substituted cyclooctate-

Figure 1. ORTEP drawing of 5a at the 50% probability level. H atoms omitted for clarity.

Synthesis of unsubstituted COT via tetramerization of acetylene can be traced back to a multistep transformation described by Willstätter in 1911.22 In 1948, Reppe introduced a nickel-catalyzed synthesis of cyclooctatetraene using Ni(CN)2/ CaC2,23−25 which opened the door for the synthesis of COT and related compounds with other metal systems.26 Although the synthesis of substituted COTs from substituted acetylene is limited due to difficulty in designing a regioselective process, their use in several applications, including building blocks for pharmaceutical drugs and ligands for lanthanide metals, is well known.27,28 Since existing synthetic methods for COTs use relatively high loadings of nonrecyclable catalysts, it would be particularly attractive if the reaction in Scheme 2 could be optimized to give synthetically useful yields. To increase the conversion of alkyne to COT products, a number of different reaction conditions were screened, as summarized in Table 1. The initial low conversion led to the hypothesis that perhaps the impact between the nickel vial and nickel ball was too soft to produce sufficient energy to complete the reaction. To

Scheme 1. Previous Solution Study of [2 + 2 + 2] Cycloaddition

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ACS Sustainable Chemistry & Engineering Table 1. Optimization for Synthesis of COTsa

entry 1 2 3 4 5 6 7 8 9 10 11 12

vial nickel stainless stainless nickel nickel nickel stainless stainless stainless stainless stainless Teflon

steel steel

steel steel steel steel steel

ball

Ni source

time (h)

% conversionb

nickel stainless steel tungsten carbide stainless steel tungsten carbide none none none stainless steelc stainless steel tungsten carbide Teflon

vial/ball Ni foil Ni foil vial vial Ni pellets Ni pellets Ni pellets Ni powder − − −

16 16 16 16 16 1 1 16 16 16 16 16

10 0 37 28 85 39 54 94 57 0 0 0

Reactions were performed on a 2.48 mmol scale (250 μL). bDetermined by 1H NMR. cMultiple stainless steel balls were used to emulate nickel pellet conditions.

a

increase the energy while maintaining the recyclability of the nickel source, Ni foil was used to cover the interior of a stainless steel vial. In this way, a harder impact of stainless steel against stainless steel could be achieved. Although this method is effective in mechanochemical silver-catalyzed cyclopropanation reactions,19 for the current study, it was unsuccessful (entry 2). Replacing the stainless steel ball with a tungsten carbide ball showed some improvement (37% conversion, entry 3), suggesting that increasing the hardness of the ball results in more energy on impact. While encouraging, the nickel foil was significantly damaged so that it could not be reused. Subsequent optimization was focused on the nickel vial and using a ball made of harder materials, e.g., stainless steel and tungsten carbide. This led to substantially higher conversions as compared to entry 1, 28% conversion (entry 4) with the stainless steel ball and 85% conversion (entry 5) with the tungsten carbide ball and a nickel vial. Variation in grinding mode may offer an alternative approach to achieve increased energy for reactivity. Takacs29 showed that oblique collisions occurring in planetary ball mills induce a much higher “effective temperature” than direct collisions between one ball and a vial in a mixer mill. This has also been supported by computational30 and experimental results from our group and others.31−35 On the basis of this hypothesis, a vial made of either nickel or stainless steel was filled with 1/8 in. nickel pellets to simulate oblique collisions (entries 6−8). When operating in a nickel vial, 39% conversion was achieved within 1 h (entry 6). With a stainless vial, the conversion of 1a was higher, reaching 54% within 1 h (entry 7) and 94% within 16 h (entry 8). This method also proved to be even more attractive for product isolation. Since nickel is ferromagnetic,

the pellets could be separated from the product mixture by using a strong neodymium magnet. Subsequent purification of the product mixture is achieved by column chromatography with petroleum ether and ethyl acetate (Figure 2). A series of control reactions were also performed. The first control reaction was done to determine whether or not nickel powder generated in situ is responsible for the catalytic activity. Using similar planetary mill conditions as entries 6−8, several stainless steel balls were added to a stainless steel vial along with excess nickel powder compared to 1a. A product conversion of 57% indicates that a combination of micronsized particles and fresh nickel surface provides the catalytic activity (entry 9). The other three control reactions were done to be confident that the nickel was responsible for the catalysis. Experiments using stainless steel, tungsten carbide, and Teflon media were performed, all resulting in 0% conversion to any product (entries 10−12). To investigate the scope of the reaction, the optimized conditions were applied to several terminal alkynes. These results are summarized in Table 2. In addition to ethyl propiolate, other commercially available propiolates that are amenable to the methodology included methyl and tert-butyl propiolates. While there is no difference in the regioselectivity for the individual COTs, there does appear to be a lower selectivity for the COT isomers obtained from tert-butyl propiolate, as indicated by a lower tetramer to trimer ratio (entry 3). On the basis of the commonly proposed cycloaddition mechanisms,24,26,36 it is possible that the bulkiness of the tert-butyl group promotes reductive elimination to give more trimeric isomers rather than further incorporation of an alkyne molecule to give the tetrameric isomers. To determine if 2466

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ACS Sustainable Chemistry & Engineering Table 2. Substrate Scope for Cycloaddition Reaction

a

Combined isolated yield for 2−7. isolation. cDetermined by 1H NMR. Figure 2. Visualized process of catalyst and product separation. (A) Nickel pellets before use. (B) Reaction mixture and pellets after removed from the reaction vial. (C) Using the neodymium magnet to separate nickel pellets from crude reaction mixture. (D) Separated reaction mixture. (E). Isolated products in ethyl acetate after column chromatography.

b

Determined after product

In conclusion, we have developed a unique ligand-free method of synthesizing various substituted cyclooctatetraene (COT) compounds that is simple, inexpensive, and uses a recyclable Ni(0) source. The method also shows that in a mixer mill, the impact energy can be increased by increasing the number of balls (in this case Ni pellets). This increases the amount of shear friction in the system, thus simulating a planetary mill-like motion. In addition, this proof-of-principle study shows that nickel in its metallic state can be an effective catalyst for nickel-catalyzed reactions. This method has broader implications in synthesis as it potentially allows researchers to perform these types of reactions without using Schlenk lines and gloveboxes for air-sensitive catalysts. Future investigation will focus on the effects of ligand additives to this unique mechanochemical system.

electronic effects influence the reaction outcome, a variety of substituted phenylacetylene derivatives were used as the starting materials. Phenylacetylene itself affords a fairly high ratio of COTs to cyclotrimerization products with 94% of the products being COTs (entry 4). It appears that introducing any group to the para position of the phenylacetylene decreases selectivity and yields more trimer products (entries 5−9). In the case of 4-ethynyltoluene vs 4-tert-butylphenylactylene (entries 5 and 6), the difference in tetramer to trimer ratio is not so pronounced. However, the selectivity observed for 4ethynylanisole (entry 7) was reversed with the trimer products being the major products. These data seem to suggest that electron-donating groups increase the amount of trimer products. However, phenylactylenes with electron-withdrawing substituents such as fluorine and ester groups (entries 8 and 9) are also less selective for the COT products, when compared to the unsubstituted phenylacetylene (entry 4).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00363. Characterization data of COT compounds and details of the crystallographic study for 5a. (PDF) 2467

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ACS Sustainable Chemistry & Engineering

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.G.). *E-mail: [email protected] (J.M.). 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.

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ACKNOWLEDGMENTS We are thankful for financial support for this research from the National Science Foundation, CHE-1465110 (J.M.) and CHE1464734 (H.G.). We also express gratitude for the support from the Undergraduate Student Research Fellowship, University of Cincinnati, University Research Council. Funding for the diffractometer was through NSF-MRI Grant CHE0215950. Lastly, we are very appreciative of Ronald Hudepohl’s (University of Cincinnati machine shop) assistance in making the vials used in this work.

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DEDICATION We dedicate this article to Rajender S, Varma, a pioneer in Green Chemistry, on the occasion of his 65th birthday. REFERENCES

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