Dedicated Microwave Reactors for Organic Synthesis

The growing interest in MAOS during the mid-1990s led to increased demand for more sophisticated microwave instrumentation, offering, for example, stirring of the reaction mixture, temperature measurement, and power control features. For scientifically valuable, safe, and reproducible work, the microwave instruments utilized should offer the following features  

A particularly difficult problem in microwave processing is the correct measurement of the reaction temperature during the irradiation phase. Classical temperature sensors (thermometers, thermocouples) will fail since they will couple with the electromagnetic field. Temperature measurement can be achieved either by means of an immersed temperature probe (fiber-optic or gas-balloon thermometer) or on the outer surface of the reaction vessels by means of a remote IR sensor. Due to the volumetric character of microwave heating, the surface temperature of the reaction vessel will not always reflect the actual temperature inside the vessel [7]. 

Since the early applications of microwave-assisted synthesis were based on the use of domestic multimode microwave ovens, the primary focus in the development of dedicated microwave instruments was inevitably the improvement of multimode 

Due to the cavity design, it is suitable for vessels of different types and sizes, such as sealed 10-80 mL vials or 125 mL round-bottomed flasks. With this system, pressurized reactions in sealed vessels as well as traditional refluxes at atmospheric pressure can be performed. 

Recent advances and further improvements have led to a broad variety of applications for single-mode microwave instruments, offering flow-through systems as well as special features such as solid-phase peptide synthesis or in situ on-line analy- 

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High equipment costs. While prices for dedicated microwave reactors for organic synthesis may have come down considerably in the past decade, the current price range is still many times higher than that of conventional heating equipment,... 

Today, a large body of work on microwave-assisted synthesis exists in the published and patent literature. Many review articles [8-20], several books [21-23], and information on the world-wide-web [24] already provide extensive coverage of the subject. The goal of the present book is to present carefully scrutinized, useful, and practical information for both beginners and advanced practitioners of microwave-assisted organic synthesis. Special emphasis is placed on concepts and chemical transformations that are of importance to medicinal chemists, and that have been reported in the most recent literature (2002-2004). The extensive literature survey is limited to reactions that have been performed using controlled microwave heating conditions, i.e., where dedicated microwave reactors for synthetic applications with adequate... 

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. 

In contrast to single-mode reactors, dedicated multimode instruments allow scale-up to be performed in multivessel rotor systems utilizing various types of sealed vessels. In these systems, reactions can be carried out in batch to synthesize multiple gram quantities (< 250 g) of material in typically up to 1 L processing volume. Most of the multimode instruments available for organic synthesis have been derived from closely related sample preparation equipment [39-41]. The MARS Microwave Synthesis System (Fig. 4) is based... 

This chapter provides a detailed description of the various commercially available microwave reactors that are dedicated for microwave-assisted organic synthesis. A comprehensive coverage of microwave oven design, applicator theory, and a description of waveguides, magnetrons, and microwave cavities lies beyond the scope of this book. Excellent coverage of these topics can be found elsewhere [1—4]. An overview of experimental, non-commercial microwave reactors has recently been presented by Stuerga and Delmotte [4],... 

Other microwave-assisted parallel processes, for example those involving solid-phase organic synthesis, are discussed in Section 7.1. In the majority of the cases described so far, domestic multimode microwave ovens were used as heating devices, without utilizing specialized reactor equipment. Since reactions in household multimode ovens are notoriously difficult to reproduce due to the lack of temperature and pressure control, pulsed irradiation, uneven electromagnetic field distribution, and the unpredictable formation of hotspots (Section 3.2), in most contemporary published methods dedicated commercially available multimode reactor systems for parallel processing are used. These multivessel rotor systems are described in detail in Section 3.4. 

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