Microwave-assisted organic synthesis vessels

In modern microwave synthesis, a variety of different processing techniques can be utilized, aided by the availability of diverse types of dedicated microwave reactors. While in the past much interest was focused on, for example, solvent-free reactions under open-vessel conditions [1], it appears that nowadays most of the published examples in the area of controlled microwave-assisted organic synthesis (MAOS) involve the use of organic solvents under sealed-vessel conditions [2] (see Chapters 6 and 7). Despite this fact, a brief summary of alternative processing techniques is presented in the following sections. 

As was mentioned previously, pressurized conditions have been reported for microwave-assisted organic synthesis [50], The reactions were carried out at a domestic microwave oven and commercially available screw-up pressure vessels made of Teflon, PET and PEEK, which are both microwave transparent (Fig. 5.1). It has to be pointed out that reactions under such conditions have to be strictly controlled with an efficient power feed-... 

By working with pressure resistant quartz vessels, Kappe and Kremsner recently extended the temperature of microwave-assisted organic synthesis... 

The reaction vessels used in microwave-assisted organic synthesis should preferably be made up of teflon, polystyrene or glass (Bose et al., 1994 Strauss and Train-or, 1995). These vessels should be tall and loosely covered with a capacity much greater than total volume of the reactants. Metallic containers are not advised for use as these are heated soon due to preferential absorption and reflection of rays. 

Alternatively, microwave-assisted synthesis has been carried out using standard organic solvents under open-vessel conditions. If solvents are heated by microwave irradiation at atmospheric pressure in an open vessel, the boiling point of the solvent typically limits the reaction temperature that can be achieved. In order to none-... 

Whereas in the last decade microwave irradiation was mainly applied to accelerate and optimize well-known and established reactions, current trends are indicative of the future use of microwave technology for the development of completely new reaction pathways in organic synthesis. Limited by vessel and cavity size, microwave-assisted synthesis has hitherto been focused predominantly on reaction optimiza-... 

Most examples of microwave-assisted chemistry published to date and presented in this book (see Chapters 6 and 7) were performed on a scale of less than 1 g (typically 1-5 mL reaction volume). This is in part a consequence of the recent availability of single-mode microwave reactors that allow the safe processing of small reaction volumes under sealed-vessel conditions by microwave irradiation (see Chapter 3). While these instruments have been very successful for small-scale organic synthesis, it is clear that for microwave-assisted synthesis to become a fully accepted technology in the future there is a need to develop larger scale MAOS techniques that can ultimately routinely provide products on a multi kg (or even higher) scale. 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

A related support frequently used for liquid phase synthesis is methoxypoly(ethylene glycol) (MeO-PEG) and a general procedure for microwave-assisted synthesis of organic molecules on MeO-PEG has been reported [77]. The use of MeO-PEG under microwave conditions in open vessels simplified the process of polymer-supported synthesis (Scheme 16.53), because the polymer-bound... 

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