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The choice is yours: using liquid assisted grinding to choose between products in the palladium catalyzed dimerization of terminal alkynes Longrui Chen, Mark Regan, and James Mack ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02001 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 17, 2015
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The choice is yours: using liquid assisted grinding to choose between products in the palladium catalyzed dimerization of terminal alkynes Longrui Chen, Mark Regan, and James Mack University of Cincinnati, 301 Clifton Court, Cincinnati, OH 45221-0172, United States KEYWORDS: catalysis, mechanochemistry, polymer support, homocoupling, solvent-free, green chemistry, ball mill
ABSTRACT: Herein, we report on the dimerization of terminal alkynes using various palladium catalyst under solvent-free mechanochemical conditions. When tetrakis(triphenylphosphine)palladium(0) was employed as the catalyst we observed the 1,3 butadiyne as the major product. However, when we employed bis(triphenylphosphine)palladium(II) dichloride as the catalyst, we observed the trans enyne as the major product. When we used a polymer supported bis(triphenylphosphine)palladium(II) dichloride catalyst under liquid assisted grinding conditions, we discovered the ability to tune the catalyst to generate either the diyne or trans enyne as the major product depending on the grinding medium. The area of mechanochemistry is a burgeoning field that has been the subject of various books and reviews over the past five years.1-7 Although performing chemical reactions under these novel conditions has been known for more than a century, the push towards developing more sustainable reaction conditions has caused a renewed interest in this area.8 Furthermore, the ability to run these reactions on multikilogram scale and beyond is becoming more of a realization.9-20 Metal catalyzed reactions have shown to be very effective under mechanochemical conditions.21-31 We looked to extend these conditions to the palladium catalyzed dimerization of terminal alkynes.
The homocoupling of alkynes have been traditionally used with copper catalysts,32-35 however it usually needs stoichiometric amounts of copper salts. More recently the palladium catalyzed dimerization of terminal alkynes has been of significant interest, since only catalytic amounts of palladium are needed. Although the palladium catalyzed alkyne dimerization is a well-known reaction in solution, it suffers from poor selectivity, cumbersome reaction conditions and the use of exotic catalysts. Typically, when conducting this reaction in solution, in addition to the desired 1,3-diyne dimer, the reaction is often accompanied by three enyne by-products (Scheme 1). Recently, we
reported on the palladium catalyzed homocoupling of terminal alkynes under solvent-free mechanochemical conditions.27 Using phenylacetylene as the substrate and tetrakis(triphenylphosphine)palladium(0) as the catalyst, we observed the expected 1,4-diphenylbuta-1,3-diyne in 74% yield, with no observable amount of any of the enyne by-products, even under an inert atmosphere. We have since extended these conditions to a variety of alkyne substrates to demonstrate the versatility of these reaction conditions (Table 1). Although most of the catalysts we tested gave the 1,3 butadiyne as the major product, bis(triphenylphosphine)palladium(II) dichloride gave the trans enyne as the major product and was the sole enyne isomer produced. The trans enyne scaffold has been shown to be present in biologically active compounds.36 Various reaction conditions have been developed to provide the trans enyne selectively in solution.37-45 Many of these reports use exotic ligand combinations, inert reaction conditions and often both. We have demonstrated this desired selectivity can be achieved by simply using commercially available bis(triphenylphosphine)palladium(II) dichloride under mechanochemical conditions. By comparison, when bis(triphenylphosphine)palladium(II) dichloride is used as the catalyst for the dimerization of phenylacetylene in solution, the 1,3 butadiyne is seen as the major product (Table 2).46-49 We extended these conditions to aliphatic alkynes and observed the diyne dimer as the major isomer and very little of the trans enyne. The low yield of trans enyne with long chained aliphatic substrates are consistent with the previously reported conditions of using [π-allyl)PdCl2, tris(2,6-dimethoxyphenyl)phosphine as catalyst. This suggests that the mechanism of our conditions may be similar to what was previously reported by Gevorgyan.42, 45
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Table 1. Dimerization of various alkynes under mechanochemical conditions.
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product with no observable amount of enyne. By comparison when we milled phenylacetylene in the presence of bis(triphenylphosphine)palladium(II) dichloride and excess triphenylphosphine, we also observed the 1,3 butadiyne product exclusively with no trace of enyne present (Table 3, entry 3). Table 3. Comparison polymer supported versus non supported catalyst under mechanochemical conditions
Table 2. Comparison of phenylacetylene dimerization using bis(triphenylphosphine)palladium(II) dichloride as catalyst in solution versus mechanochemical conditions.
It has be demonstrated in solution that bis(triphenylphosphine)palladium(II) dichloride in the presence of excess triphenyl phosphine gives exclusively the diyne
One area that has been a recent focus of our group is the study of polymer supported reagents under mechanochemical conditions, more specifically the study of catalysts on solid support. Catalysts on solid support have many advantages over their non-supported analogues.50-54 Polymer supported catalysts can be recovered and reused, making for more environmentally benign reactions conditions. However, polymer supported reagents can give slower reaction rates and poor stereoselectivity which has limited their use in chemistry. This is especially true with polymer supported enantioselective catalysts, where the polymer support restricts the movement of the catalytic active site.55-58 Many of these problems can be traced to the swelling of the polymer support in a solvent.59 Blum recently demonstrated mechanochemical activation of the polymer support gives higher catalytic efficiency than just swelling alone.60-61 We have observed similar enhancements with polymer supported reagents under our own mechanochemical conditions.62 We wanted to determine
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how polymer supported catalyst would affect the homocoupling of terminal alkynes under these unique conditions. To our surprise, when using polymer supported tetrakis(triphenylphosphine)palladium(0) (PS-Pd(PPh3)4) as catalyst in the dimerization of phenylacetylene, we observed the trans enyne product not the expected 1,4-diphenylbuta-1,3-diyne (Table 3, entry 4).
Table 4. Alkyne dimerization using polymer supported catalyst under mechanochemical conditions
In order to further understand the difference between polymer bound catalyst and non-supported catalyst under mechanochemical conditions, we compared bis(triphenylphosphine)palladium(II) dichloride to its polymer supported analogue (PS-PdCl2(PPh3)2) ( Table 3, entry 2 and 6). The polymer supported bis(triphenylphosphine)palladium(II) dichloride reacts similar to the non-supported version, both gave the enyne as the major product in similar ratios and similar yields. Furthermore, the PS-Pd(PPh3)4 gave similar yields and product ratios as the PdCl2(PPh3)2 (Table 3 entry 2 and 4) suggesting similarities in catalytic activity. In order to determine if the polymer supported catalyst were susceptible to triphenylphosphine addition, we added excess triphenylphosphine to the PS-Pd(PPh3)4 reaction and observed a 92:8 butadiyne : enyne product ratio, suggesting again the PS-Pd(PPh3)4 reacts similar to the PdCl2(PPh3)2 catalyst not the non-supported Pd(PPh3)4 catalyst. This suggests that the same rationale for the selectivity difference between the two catalysts in solution may be applied to mechanochemical conditions.42, 45 When we milled our alkyne substrate with polymer supported tetrakis palladium triphenylphosphine in the presence of polymer supported triphenyl phosphine, we observed the 1,3 butadiyne product as the major product (Table 2, entry 5). This demonstrates the two polymers combined to form a cooperatively interactive polymer support. We extended this to various other substrates to explore the tolerance of different functional groups to these unique conditions (Table 4.) Liquid assisted grinding (LAG) has been demonstrated to facilitate various mechanochemical reactions.6369 Previously, we observed the LAG medium can have a strong influence on the product ratio of polymer supported reactions. As an example, conducting the Wittig reaction using polymer supported triphenylphosphine, benzaldehyde and benzyl bromide under non-polar LAG conditions gave mostly the trans stilbene isomer whereas when we used polar LAG conditions we observed the cis stilbene isomer as the major product.70 Therefore, we conducted LAG experiments to determine if we would observe similar effects on the product distribution. From the results in table 5 and 6, we observed that when non-polar LAG conditions are employed there is a preference for the diyne product, however, when polar LAG conditions are used there is a preference for the trans enyne product. When LAG studies were performed with the non-supported catalysts, we observed no change in catalytic activity or product distribution whether using polar or non-polar LAG conditions.
Table 5. The effect of liquid assisted grinding on diyne/enyne selectivity
Table 6. Dimerization of various alkynes under liquid assisted grinding conditions.
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trans enyne. The combination of mechanochemistry and catalysis has already shown to be very powerful. These reactions have shown to be safe, easy to set-up and give high yields and selectivity. As we previously showed27 the inclusion of polymer support catalysts under these unique conditions gives the added benefit of recovery and recyclability of the catalyst. Furthermore, conducting polymer supported catalyst under LAG conditions provides a new avenue for catalysts to switch between different products. Although the field of mechanochemistry is still at its infancy, it has been demonstrated it could have a large impact in various areas of chemistry.
ASSOCIATED CONTENT Supporting information The following file is available free of charge on the ACS Publications website at DOI: Synthetic methods, experimental protocols, 1H and C NMR data and time intervals experiments (PDF). 13
AUTHOR INFORMATION Corresponding Author * James Mack, Department of Chemistry, University of Cincinnati, 301 Clifton court, Cincinnati, OH 45221-0172
Email:
[email protected] In conclusion we demonstrated that mechanochemical conditions are well suited for the palladium catalyzed dimerization of terminal alkynes. These conditions can give unique product ratios and greater selectivity than observed in solution. Furthermore with the use of liquid assisted grinding in the presence of polymer supported palladium catalyst, we showed the ability to develop reaction conditions to easily switch selectivity from the diyne to the
ACKNOWLEDGMENT This research is supported by the National Science Foundation grant number CHE-1058627, National Science Foundationgrant number CHE-1465110 and the University of Cincinnati Research Council (URC).
The authors declare no competing financial interest.
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