Pressurized microwave reactors

Roles of Pressurized Microwave Reactors in the Development of Microwave-assisted Organic Chemistry... 

The Claisen rearrangement was also investigated in aqueous medium at high temperature by using a pressurized microwave reactor. The example in Scheme 5.42 is the first Claisen rearrangement of an allyl aryl ether in water. Under neat condition, the transposition of allyl phenyl ether (128) occurred at 290°C, while in water a temperature of 240°C was enough. The control of temperature was important at 245°C a mixture of 2-allylphenol (46%), 2-(2-hydroxyprop-l-yl)-phenol (31%), and 2-methyl-2,3-dihydrobenzofuran (23%) was obtained. 

Figure 5.2. Two of the more common types of low pressure CVD reactor, (a) Hot Filament Reactor - these utilise a continually pumped vacuum chamber, while process gases are metered in at carefully controlled rates (typically a total flow rate of a few hundred cubic centimetres per minute). Throttle valves maintain the pressure in the chamber at typically 20-30 torr, while a heater is used to bring the substrate up to a temperature of 700-900°C. The substrate to be coated - e.g. a piece of silicon or molybdenum - sits on the heater, a few millimetres beneath a tungsten filament, which is electrically heated to temperatures in excess of 2200 °C. (b) Microwave Plasma Reactor - in these systems, microwave power is coupled into the process gases via an antenna pointing into the chamber. The size of the chamber is altered by a sliding barrier to achieve maximum microwave power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on top of the heated substrate, onto which the diamond film is deposited.

The Anton Paar Synthos 3000 (Fig. 3.16 and Table 3.5) is the most recent multi-mode instrument to come onto the market. It is a microwave reactor dedicated for scaled-up synthesis in quantities of up to approximately 250 g per run and designed for chemistry under high-pressure and high-temperature conditions. The instrument enables direct scaling-up of already elaborated and optimized reaction protocols from single-mode cavities without changing the reaction parameters. 

The second example involves the synthesis of ortho-dipropynylated arenes (Scheme 4.12b), which serve as precursors to tribenzocyclyne by way of an alkyne metathesis reaction (see also Scheme 6.31). Here, a Sonogashira reaction was carried out in a pre-pressurized (propyne at ca. 2.5 bar) sealed microwave vessel in a standard single-mode microwave reactor. Double-Sonogashira coupling of the dibromodiiodo-benzene was completed within 20 min at 110 °C [30]. 

Another example involves dioxygen-promoted regioselective oxidative Heck aryla-tions of electron-rich alkenes with arylboronic acids (Scheme 4.12c). For this, two types of microwave reactors have been used. In a single-mode instrument (1 mmol run 25 mL vessel), the Heck arylation was performed by first pre-pressurizing the... 

There are many other examples in the literature where sealed-vessel microwave conditions have been employed to heat water as a reaction solvent well above its boiling point. Examples include transition metal catalyzed transformations such as Suzuki [43], Heck [44], Sonogashira [45], and Stille [46] cross-coupling reactions, in addition to cyanation reactions [47], phenylations [48], heterocycle formation [49], and even solid-phase organic syntheses [50] (see Chapters 6 and 7 for details). In many of these studies, reaction temperatures lower than those normally considered near-critical (Table 4.2) have been employed (100-150 °C). This is due in part to the fact that with single-mode microwave reactors (see Section 3.5) 200-220 °C is the current limit to which water can be safely heated under pressure since these instruments generally have a 20 bar pressure limit. For generating truly near-critical conditions around 280 °C, special microwave reactors able to withstand pressures of up to 80 bar have to be utilized (see Section 3.4.4). 

Operating with chemicals and pressurized containers always carries a certain risk, but the safety features and the precise reaction control of the commercially available microwave reactors protect the users from accidents, perhaps more so than with any classical heating source. The use of domestic microwave ovens in conjunction with flammable organic solvents is hazardous and must be strictly avoided as these instruments are not designed to withstand the resulting conditions when performing chemical transformations. 

The temperature/pressure monitoring mechanisms of modem microwave reactors allow for an excellent control of reaction parameters, which generally leads to more reproducible reaction conditions. 

First, laboratory and experimental reactors will be described. The vessel containing reactants or their supports are made of convenient dielectric materials (cylindrical or egg-shaped reactor). Original microwave reactors will be described. The first one is a metallic cylindrical reactor which is also the microwave applicator. It allow to reaches high pressures. The other one is a egg-shaped microwave reactor leading to high focusing level of microwave power. 

For several years M. Delmotte et al. have designed a microwave reactor for high pressure chemistry [63]. The microwave applicator and reactor are identical in order to accommodate the mechanical constraints induced by high pressure within liquids. This is the main interest of this device. The metallic cylindrical pipe is simultaneously a waveguide and the reactor. The device is described by Fig. 1.15. 

More recently [29] the microwave-mediated Biginelli dihydropyrimidine synthesis (Eq. 2) was reinvestigated using a purpose-built commercial microwave reactor with on-line temperature, pressure, and microwave power control. Transformations performed with microwave heating at atmospheric pressure in ethanol solution resulted in neither a rate increase nor an increase in yield when the temperature was identical to that used for conventional thermal heating. The only significant rate and yield enhancements were found when the reaction was performed under solvent-free conditions in an open system. 

Khadilkar and Rebeiro have investigated a new method [107] that overcomes all these problems and is far safer. The authors used closed pressure reactor [108], with no apparent loss of yield. The microwave reactor used for these reactions has a possibility for recording temperature and pressure during irradiation. For example, 1-bu-tyl-3-methylimidazolium chloride was isolated in 91% yield in 24 min [109] at 150 °C and 57 psig was the maximum pressure reached. 

---