Microwave heating high pressure

The first reports of the use of microwave heating to accelerate Heck, Suzuki and Stille reactions on the solid phase [54] and in solution did not appear until 1996 [55], Since then, many metal-catalyzed reactions have been performed within minutes by microwave heating in pressurized systems, sometimes with high regio- and enan-tioselectivity [56, 57]. 

Vessels designed for microwave-assisted SPOS must fulfill several require-menfs because of fhe harsh conditions (i.e., high temperatures and pressures) usually associated with microwave heating. Open vessels are often impractical because of the possible loss of solvent and/or volatile reagents during the heating process. However, in cases where a volatile byproduct inhibits a reaction, their use may be superior over closed systems. A sealed vessel retains the solvents and reagents, but must be sturdily constructed to avoid the obvious safety implications due to the buildup of pressure. 

Decomposition methods are usually classified as melt decompositions, wet decompositions (with liquid decomposing agents) and dry decompositions by combustion. Sample decomposition methods are varied, and involve open and closed systems (at low and high pressure), UV and thermal activation, low or high temperature, and use of conventional convective or microwave heating. Table 8.4 lists the main sample decomposition methods for trace-element determination. 

Matusiewicz [63,68] has reported the development of a high-pressure, high-temperature, focused-microwave-heated acid (HNO3) digestion system. This microwave technique requires only about 3 % of the time necessary for the thermal high-pressure (HPA) technique. The technique of microwave heating samples in sealed containers to speed up acid digestion has been in widespread use for the past few years [69,70]. 

Microwave heating is often applied to already known conventional thermal reactions in order to accelerate the reaction and therefore to reduce the overall process time. When developing completely new reactions, the initial experiments should preferably be performed only on a small scale applying moderately enhanced temperatures to avoid exceeding the operational limits of the instrument (temperature, pressure). Thus, single-mode reactors are highly applicable for method development and reaction optimization. 

Bagley and coworkers have described the preparation of primary thioamides by treatment of nitriles with ammonium sulfide in methanol solution (Scheme 6.139) [276], While the reactions with electron-deficient aromatic nitriles proceeded at room temperature, other aromatic and aliphatic nitriles required microwave heating at 80-130 °C for 15-30 min to furnish the thioamides in moderate to high yields. This protocol avoids the use of hydrogen sulfide gas under high pressure, proceeds in the absence of base, and usually provides thioamides without the need for chromatographic purification. 

High Pressure, and Microwave Heating in Organic Synthesis. JAI Press, Greenwich, CT, 1989, pp. 103-172. 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

An interesting application of this method is the preparation of diamond films which may be obtained from a precursor such as CH4, C2H2 and H2 activated by heating, microwaves, etc. typically at 600-1000°C at a reduced pressure. The direct deposition from the gas to the surface results in the formation of metastable diamond whereas, according to the phase diagram (see Fig. 5.37), the production of stable bulk diamond requires very high pressure and temperature. Kinetically, the... 

Decomposition in closed Teflon vessels at high pressure (up to 85 bar) with microwave heating. (Microwave Acid Digestion Bomb, Parr Instrument Company, USA.)... 

Instrument Company systems apply microwave heating, whereas the High Pressure Asher has a conventional heating system. 

Figure 4, Triplet EPR spectra of ZnTCP ( -10 ) in Cn Cl-CII OH recorded at 10 K before (solid line) and after (dotted line) addition of KCl ( S x 10 ). Microwave power 0.5 mW, field modulation 20 gauss (100 kHz), excitation with square wave modulated (13 Hz) light from an Xe high pressure arc (1000 W) passed through a CuSO heat filter.

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