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Green Processing and Synthesis

Editor-in-Chief: Hessel, Volker / Tran, Nam Nghiep

Editorial Board: Akay, Galip / Arends, Isabel W.C.E. / Cann, Michael C. / Cheng, Yi / Cravotto, Giancarlo / Gruber-Wölfler, Heidrun / Kralisch, Dana / D. P. Nigam, Krishna / Saha, Basudeb / Serra, Christophe A. / Zhang, Wei


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Volume 2, Issue 4

Issues

Microwave heating and conventionally-heated continuous-flow processing as tools for performing cleaner palladium-catalyzed decarboxylative couplings using oxygen as the oxidant – a proof of principle study

DiAndra M. Rudzinski
  • Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269, USA
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Nicholas E. Leadbeater
  • Corresponding author
  • Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269, USA
  • Email
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Published Online: 2013-07-27 | DOI: https://doi.org/10.1515/gps-2013-0043

Abstract

A microwave unit interfaced with a gas-loading accessory is used as a tool for facilitating the palladium-catalyzed decarboxylative Heck reaction of 2,6-dimethoxybenzoic acid and methyl acrylate, using molecular oxygen as the oxidant. The reaction is complete in less time and at a lower catalyst loading than when using conventional approaches. The reaction is scaled up using continuous-flow processing, employing a reactor in which both gas input and heating can be performed simultaneously. An 86% isolated product yield is obtained. This proof-of-principle study paves the way for the technology to be used in other cases of these increasingly popular decarboxylative coupling reactions.

Keywords: decarboxylative coupling; flow chemistry; microwave heating

1 Introduction

Palladium-catalyzed coupling and cross-coupling reactions are the subjects of significant attention, most probably due to their wide application in academic and industrial laboratories [1–3]. While powerful synthetic methods, many coupling reactions involve the use of an organometallic reagent, with the added synthetic steps for its preparation, and generate significant waste. As a result, they are often neither atom- nor step-economic [4]. An area of recent research effort is the use of aryl carboxylic acids as alternatives to organometallic reagents in coupling reactions, with an electrophilic coupling partner such as an aryl halide [5, 6]. This has met with considerable success; examples are added to the synthetic chemists’ repertoire on a regular basis [7–10]. As an extension to this work, benzoic acids have also been coupled with boronic esters, alkenes, and arenes (via C–H bond activation) by means of decarboxylative oxidative couplings [11–13].

Many decarboxylative coupling protocols involve the use of a stoichiometric or super-stoichiometric quantity of an oxidant, such as a silver salt [14–17]. A case in point, is the coupling of carboxylic acids with alkenes, first reported by Myers et al. in 2002 [18]. The reaction was performed using palladium(II) trifluoroacetate as the catalyst and three equivalents of silver carbonate (3.0 equiv.) as an oxidant. A combination of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) in a 95:5 ratio as solvent was key to the success of the reaction [19]. The silver salt can be replaced by benzoquinone as the oxidant, when highly electron-rich benzoic acids are used as substrates [20]. Also, when using electron-rich substrates, molecular oxygen can be used as the oxidant [21]. This represents a significant step forward in developing a clean, green methodology. The reactions are performed using 10 mol% palladium(II) acetate as the catalyst and 1–3 atm of oxygen. Heating at 120°C for 10 h is used for the reaction with styrene and acrylate coupling partners.

One aspect of the research in our laboratory is a focus on the use of new technology for synthetic chemistry, in particular, the application of microwave heating and continuous-flow processing. These techniques offer a way to perform reactions safely and effectively at elevated temperatures and pressures [22–26]. Since decarboxylative couplings often require heating, it is not surprising that microwave technology has proven valuable in this endeavor [27–30]. As well as using these tools for performing reactions at a high temperature, we have also had interest in using a microwave unit interfaced with a gas-loading accessory as a “modern autoclave” [31, 32]. For example, we have performed palladium-catalyzed carbonylation reactions using a near-stoichiometric quantity of carbon monoxide [33]. We recently had the need to prepare the methyl ester of 2,6-dimethoxycinnamic acid (3) and we decided to leverage our microwave apparatus in developing a palladium-catalyzed decarboxylative Heck coupling approach using 2,6-dimethoxybenzoic acid (1) and methyl acrylate (2) as substrates, and molecular oxygen as the oxidant (Scheme 1). We report our results here, together with our scale up approach using continuous-flow processing.

2 Experimental section

2.1 General experimental

2.1.1 Materials

All chemical transformations requiring inert atmospheric conditions or vacuum distillation utilized Schlenk line techniques, with a 3, 4, or 5-port dual-manifold (ChemGlass, Vineland, NJ, USA). Nitrogen was used to provide an inert atmosphere. Nuclear magnetic resonance (NMR) spectra (1H, 13C) were performed at 25˚C on a Brüker DRX-400 400 MHz spectrometer (Billerica, MA, USA). 1H-NMR spectra obtained in CDCl3 were referenced to residual non-deuterated chloroform (7.26 ppm) in the deuterated solvent. Deuterated NMR solvent was purchased from Cambridge Isotope Laboratories (Andover, MA, USA) and stored over 4 Å molecular sieves and K2CO3. Starting materials and palladium(II) acetate were purchased from Sigma-Aldrich Corporation (St. Louis, MO, USA). Oxygen gas (high purity) was obtained from Airgas Inc.

2.1.2 Apparatus

Microwave reactions were performed in a CEM Discover microwave equipped with a gas-loading accessory (CEM Corp, Matthews, NC, USA). Continuous-flow reactions were performed in a Vapourtec R-Series flow unit (Vapourtec Ltd, Bury St Edmunds, Suffolk, UK). The system was equipped with two gas loading reactor coils. The “reagent out” port on the first reactor coil was connected to the “reagent in” port on the second reactor coil, using a 32 mm length of tubing. The system was initially primed using the equipment manufacturer’s suggested start-up sequence. After priming the unit, the reactor coils were each heated to the requisite temperature, whereupon the system was ready for loading the reagent solution.

2.2 Preparation of 2,6-dimethylcinnamic acid methyl ester (3) using microwave heating

2,6-Dimethoxybenzoic acid (0.0364 g, 0.2 mmol, 1.0 equiv.) was added to a 10 ml capacity glass microwave tube equipped with a Teflon-coated magnetic stir bar. Methyl acrylate (36 μl, 0.4 mmol, 2.0 equiv.) was added to the vessel, which was then purged with nitrogen. Dry DMF (2 ml) was added via a syringe followed by 0.1 ml of a stock solution of 5% Pd(OAc)2 in DMSO (0.01 mmol Pd added). The reaction vessel was placed into the microwave cavity, the gas-loading accessory connected. The tube was loaded with a pressure of 13 bar of oxygen gas. Using an initial microwave power of 100 W, the contents of the reaction mixture were heated to 140°C. The microwave power was then automatically attenuated, so as to hold the reaction mixture at this target temperature for 30 min. After this period of time, microwave irradiation was stopped and the reaction vessel and its contents were cooled to below 50°C, whereupon the residual pressure was released. The contents of the tube were placed in an Erlenmeyer flask and diethyl ether (5 ml) added. Another aliquot of diethyl ether was added to the reaction tube to wash it, these washings being placed into the Erlenmeyer flask. The contents of the flask were filtered through a small layer of silica gel, and the flask was washed with more diethyl ether (4×10 ml), each washing being passed through the silica gel pad. The filtrates were transferred into a separatory funnel, and washed with brine (2×20 ml). The organic layer was dried over sodium sulfate, filtered and the solvent removed in vacuo yielding pure product 3 (0.029 g, 67%). 1H-NMR (CDCl3, 400 MHz) δ ppm 3.79 (s, 3H), 3.87 (s, 6H), 6.55 (d, J=8.37 Hz, 2 H), 6.88 (d, J=16.26 Hz), 7.26 (t, J=8.37 Hz), 8.13 (d, J=16.26 Hz).

2.3 Preparation of 2,6-dimethylcinnamic acid methyl ester (3) using continuous-flow processing

2,6-Dimethoxybenzoic acid (0.9109 g, 5.0 mmol, 1.0 equiv.) was added to a 100 ml capacity reagent bottle. Dry DMF (50 ml) and methyl acrylate (0.900 ml, 10.0 mmol, 2.0 equiv.) were added to the bottle, followed by 2.5 ml of a stock solution of 5% Pd(OAc)2 in DMSO (0.25 mmol Pd added). With this solution prepared, the flow reactor was readied using the equipment manufacturer’s suggested start-up sequence, including starting a feed of oxygen (17 bar), heating the reactor coils to 140°C and passing DMF through. When at the required temperature, the reaction mixture was then loaded into the reactor. Product collection was commenced immediately after this switch. After the reaction mixture had been completely loaded into the coils, the reactor pump was set back to pumping solvent. The product mixture was diluted with diethyl ether (100 ml), filtered through a small layer of silica gel, and extracted with diethyl ether (3×50 ml). The organics were transferred into a separatory funnel, and washed with brine (3×50 ml). The organic layer was dried over sodium sulfate and the solvent removed in vacuo, yielding the pure product 3 (0.957 g, 86%).

3 Results and discussion

We embarked on our study by employing one of our monomode microwave units equipped with a gas-loading accessory, which we could interface with a 10 ml reaction vessel [34]. It is possible both to flush the reaction vessels with inert gas, as well as load up to 17 bar of gas with this apparatus. To start our optimization of reaction conditions (Table 1), we elected to use a 1:2 stoichiometric ratio of 12, 10 mol% Pd(OAc)2 as the catalyst, and a 95:5 ratio of DMF:DMSO as the solvent; we loaded our reaction vessel with 14 bar of oxygen. Heating at 150°C for 1 h led to a 74% conversion to the desired product, 1 (Table 1, entry 1). Lowering the catalyst loading from 10 mol% to 5 mol% Pd(OAc)2 resulted in an increase of conversion to 90% (Table 1, entry 2). Neither decreasing the temperature to 140°C nor reducing the reaction time from 1 h to 30 min had an effect on the outcome of the reaction (Table 1, entries 3 and 4). Reducing the reaction time to 15 min led to a concomitant reduction in product conversion, as did performing the reaction for 30 min at a catalyst loading of 1 mol% (Table 1, entries 5 and 6). To confirm that the reaction did indeed have to be performed under a positive pressure of oxygen, we performed a trial in a sealed tube that was not pre-pressurized with the gas. No product was obtained (Table 1, entry 7).

Table 1

Optimization of reaction conditions using microwave heating.a

While scale up of reactions performed under an atmosphere of reactive gas using microwave heating is theoretically possible [35], to proceed to larger scales in a single sealed-vessel approach was not feasible using the microwave apparatus at our disposal. We were also concerned about safety; working under an oxygen atmosphere with palladium catalysts that can, under certain circumstances, arc in a microwave field [36]. We therefore decided to turn to continuous-flow processing, where a smaller amount of gas would be in the reactor at any point in time [37]. Our interest (and the novelty of this portion of our work) therefore lay in performing in flow a metal-catalyzed, cross-coupling reaction involving a gaseous component.

Traditional cross-coupling reactions have been performed successfully in flow [38]. In addition, the use of reactive gases as reagents in flow processing has been widely studied. Hydrogenation reactions can be performed using the now ubiquitous H-cube system [39]. Gases such as oxygen and ozone [40–44], carbon monoxide [45–47], and fluorine [48, 49] have also been used in flow. An effective method for performing reactions involving gaseous reagents was reported by O’Brien and co-workers in 2010 [50]. It employs a “tube-in-tube” design, which is comprised of an outer polytetrafluoroethylene (PTFE) tube and a gas-permeable Teflon inner tube (Figure 1A). The reagent stream flows within the inner membrane tubing, while the gas fills the PTFE outer tubing. Diffusion through the inner tube membrane allows for the transfer of gas into the reagent stream. This approach has been successful for numerous reactions [51–56]. While excellent for ambient reactions, this tube-in-tube design is not amenable to operation at elevated temperatures. This is because, firstly, the liquid runs through the inner tube and so is difficult to heat externally. Secondly, the gas-permeable Teflon tube is not particularly robust. The reaction is thereby limited by the quantity of gas that can be loaded into the solution in the tube-in-tube unit prior to entering the heated zone. This necessitates running reactions at more dilute concentrations, to match the concentration of reagent and gas, or performing multiple passes through the entire system to ensure complete consumption of starting materials. We have access to an alternative design in which gas input and heating can occur simultaneously (Figure 1B) [57]. We have used this “on-demand” gas delivery device to perform Pd-catalyzed alkoxycarbonylation reactions [58] and rhodium-catalyzed hydrogenation [59]. The reactor is comprised of a coil of stainless steel tubing, through which liquid flows on the outside of a gas-permeable inner membrane. The reactor has a working liquid volume of approximately 15 ml when the inner gas tube is inflated. We decided to use this apparatus to scale up our decarboxylative Heck reaction (Table 2).

(A) “Tube-in-tube” and (B) “on-demand” approaches to performing reactions involving gaseous reagents. [Modified from an original figure from Vapourtec].
Figure 1

(A) “Tube-in-tube” and (B) “on-demand” approaches to performing reactions involving gaseous reagents. [Modified from an original figure from Vapourtec].

Table 2

Scale up of the reaction using continuous-flow processing.a

We began by mimicking the optimized conditions determined for the reaction using microwave heating. To do this, working on the 5 mmol scale and operating at a concentration of approximately 0.1 m in 1, we ran the reaction at 140°C with a residence time of 30 min in one coil, by using a flow rate of 0.5 ml/min. A pressure of 17 bar of oxygen was used. Only a 10% conversion to product 3 was obtained. Increasing the residence time to 1 h by slowing the flow rate to 0.25 ml/min had a significant effect on the outcome of the reaction, a 70% conversion being obtained. By introducing a second coil, and maintaining a flow rate of 0.25 ml/min, resulting in a 2 h residence time, we were able to obtain a near quantitative conversion. The product was collected and passed through a pad of silica gel, resulting in an 86% yield of isolated product. If 2,4,5-trimethoxybenzoic acid was used as a carboxylic acid substrate instead of 1, a 70% conversion to the corresponding product was observed in the reaction with methyl acrylate. We did find that when other alkenes, such as styrene and acrylonitrile, were employed, decomposition and byproducts were observed rather than product formation. This is not unprecedented when working at elevated temperatures for prolonged reaction times.

4 Conclusion

In summary, we show here that microwave heating can be used as a tool for facilitating the palladium-catalyzed decarboxylative Heck reaction of 2,6-dimethoxybenzoic acid and methyl acrylate, using molecular oxygen as the oxidant. The fact that the microwave unit, interfaced with a gas loading accessory, allows us to access elevated temperatures and pressures easily, reproducibly, and safely, means that we were able to perform the reaction in less time and at a lower catalyst loading than conventional approaches, obtaining a 90% conversion to the desired product. To scale up the reaction, we chose continuous-flow processing, employing a reactor in which we were able to input both gas and heat simultaneously. Using this equipment, we were able to obtain an 86% isolated product yield. These results pave the way for this technology to be used in other cases of these increasingly popular decarboxylative coupling reactions.

Funding from the National Science Foundation (CAREER award CHE-0847262) is acknowledged. Vapourtec Ltd and CEM Corp. are thanked for equipment support. Michael Mercadante of the University of Connecticut is thanked for training on the flow equipment.

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About the article

DiAndra M. Rudzinski

DiAndra M. Rudzinski earned a BS in Chemistry in 2010 at Niagara University in New York State, where she contributed to three peer-reviewed articles. In 2013, under the advisement of Dr. Nicholas E. Leadbeater, she received a Research Master’s Degree in synthetic organic chemistry, from the University of Connecticut. Her research was focused on new organofluorine chemistry, including the formation of trifluoromethyl ketones from Weinreb amide precursors. She also explored the use of microwave and continuous-flow technologies as tools for cleaner and greener metal catalyzed-cyanation and decarboxylative Heck reactions. After an internship at Boehringer-Ingelheim (Ridgefield, CT), she accepted a position at CheminPharma (Farmington, CT) where she is currently employed as a medicinal chemist.

Nicholas E. Leadbeater

Dr. Nicholas E. Leadbeater is currently an Associate Professor at the University of Connecticut in the USA. The overarching theme of his research group is the development of new methods for synthetic chemistry and the use of new technology in both research chemistry and in the undergraduate teaching laboratory. The group’s current hot topics are clean, green oxidation methods, the selective incorporation of fluorine into organic molecules, and the application of flow processing in synthetic chemistry.


Corresponding author: Nicholas E. Leadbeater, Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269, USA


Received: 2013-05-30

Accepted: 2013-06-28

Published Online: 2013-07-27

Published in Print: 2013-03-01


Citation Information: Green Processing and Synthesis, Volume 2, Issue 4, Pages 323–328, ISSN (Online) 2191-9550, ISSN (Print) 2191-9542, DOI: https://doi.org/10.1515/gps-2013-0043.

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