Key Benefits of Open Vessel Microwave Assisted Synthesis

All microwave synthesizers, including the Discover SP (Figure 1) and MARS 6 (Figure 2) Synthesis units, offered by CEM are equipped with the open vessel synthesis option, which facilitates performing microwave chemistry at atmospheric pressure either at or below the boiling point of the solvent. This results in rapid reaction free from the potential problems associated with sealed vessels.

Figure 1. The Discover SP Synthesis Unit

Figure 2. The MARS 6 Synthesis Unit

Advantages of Performing Microwave Synthesis under Open Vessel Conditions

The following are the benefits to carrying out microwave chemistry under open vessel conditions:

• Standard laboratory glassware can be used
• Does not require special vessels
• Yield more product than in sealed vessels
• Easy addition or removal of reagents during the reaction
• Easy access to the reaction mixture
• Larger operating scale
• No concern about pressure
• Overall safer technique compared to sealed vessels

N-Aryl Hydrazones

Rapid synthesis of N-Aryl hydrazones is possible with microwave heating in open vessel mode in conjunction with concurrent cooling to obtain the desired product in high yield (Scheme 1 as shown in Figure 3). The process involves reaction of phenylhydrazine hydrochloride with 2-acetylpyridine in the presence of 96% ethanol solution of sodium acetate to yield 2-(1-(2-phenylhydrazono)ethyl)pyridine. It took 5hr to complete the reaction under conventional heating at reflux.

Figure 3. Scheme 1. Open vessel microwave assisted synthesis of N-aryl hydrazones.

Carrying out the same reaction in the microwave in a sealed vessel for two minutes at 80°C gave rise to low yield of the product to both low yield of the product both with and devoid of simultaneous cooling. Raising the temperature above 80°C also gave rise to low yields, most likely owing to decomposition of both the raw materials and product. Conversely, the performing the same reaction under open vessel microwave heating conditions resulted in 98% yield of the product after 5min at 80°C with simultaneous cooling.

A focused library of N-aryl hydrazones were then synthesized using these optimized conditions from 4- and 2,4-(di)substituted phenylhydrazines, bearing both electron-donating (4-CH₃, 4-OCH₃) and -withdrawing (4-Cl, 4-Br, 4-CF₃, 4-NO₂, 2,4-Cl₂) groups, with 2-, 3-, and 4-acetylpyridine. This microwave procedure enables synthesizing N-aryl hydrazones in unprecedented isolated yields within 5min. Carrying out these reactions in open vessel mode facilitates rapid scaling up of the chemistry to multigram amounts with the requirement to alter the reaction conditions.

Group VI Complexes

The microwave synthesis of Group VI tetracarbonyl phosphine and tertiary amines complexes is performed in a few minutes at moderate temperature, atmospheric pressure, and employing NaBH₄ as a catalyst (Scheme 2 shown in Figure 4). These reactions were carried out in alcohol solvents with borohydride salts to achieve rapid heating of the reaction mixture under microwave irradiation. The microwave synthesis of several Group VI complexes at reflux took a reaction time of 5-20min. Controlling the reaction temperature was easily achieved by changing the alcohol utilized as the solvent.

Figure 4. Scheme 2. Open vessel microwave assisted synthesis of Group VI Complexes.

This technique also led to selective formation of the car-bonyl complex in some cases. The isolation of pure cis-(CO) ₄ (PPh₃)₂Mo in 73% yield took only 20min at 85°C. Conversely, higher temperatures or longer reaction times with traditional heating yielded the trans isomer or a mixture of the cis and trans isomers. The microwave synthesis of these common Group VI complexes involves shorter reaction times, lower temperatures, lower pressures, and benign solvents, compared to the conventional thermal syntheses. This results in a rapid, eco-friendly, and safe technique that avoids the issues associated with high pressure.

Nanoparticles

A one-pot synthetic technique involving nucleation and growth utilizing microwave irradiation can be used to prepare Rh, Pd, and Pt nanoparticles. It is a known fact that being able to control the addition rate of nanoparticle precursors provides optimized experimental reproducibility and control over the kinetics of particle growth. The open vessel feature of the CEM microwave systems enables integrating a syringe pump that is capable of accurately controlling the rate of precursor addition.

The data depicted in Figure 5 shows an analysis involved the addition of RhCl₃ precursor in an initial aliquot influencing nucleation of small isotropic Rh seeds, followed by stirring the seeds isothermally for a specific time. Then a second aliquot of RhCl₃ was added slowly followed by a second isothermal ripening. The slow addition facilitated the controlled growth of the resulting Rh nanoparticles.

Figure 5. Syringe pump precursor addition rate programs employed to achieve controlled nucleation and growth: optimized conditions (blue) and examples of alternatives also studied (dashed lines); (A) nucleation phase; (B) seed ripening; (C) nanoparticle overgrowth; (D) nanoparticle ripening; (E) reaction quenched at 0°C. (Reprinted with permission from reference 3. Copyright 2012 American Chemical Society.)

The microwave synthesis of nanoparticles provides several benefits over traditionally prepared materials, including higher crystallinity, morphological control, and improved monodispersity. The catalytic activity of these Rh nanoparticles is higher than comparably sized conventionally prepared nanoparticles.

Conclusion

The aforementioned chemistries demonstrate the improved synthesis results with the open vessel feature of CEM microwave synthesizers. High yields of N-aryl hydrazones were achieved within shorter reaction times, yielding multigram quantities in a single reaction. The microwave synthesis of Group VI carbonyl complexes is a safer and more environmentally friendly technique requiring below 30min. The synthesis of the Rh nanoparticles was achieved with a high degree of control utilizing syringe pump addition to control the addition rate of precursor.

About CEM

CEM is a company based on innovation that touches many different industries and scientific disciplines. It helped pioneer the field of microwave chemistry and has long been recognized for its expertise on the subject through publications and awards.

For more than 35 years, CEM has been designing and developing laboratory instruments and scientific methods (both microwave-based and non-microwave technologies) that are used by major companies, prestigious research institutes, and universities around the world.

CEM is the largest provider of microwave laboratory systems worldwide and has sold over 35,000 systems. The company also has the largest portfolio of microwave technology patents worldwide with over 300 patents. It has subsidiaries in the UK, France, Germany, Italy, and Japan and distributors in over 50 countries.

This information has been sourced, reviewed and adapted from materials provided by CEM Corporation.

For more information on this source, please visit CEM Corporation.