Multimode reactor

Recently, Murray and Gellman demonstrated that parallel synthesis in inexpensive 96-well polypropylene filter plates with microwave irradiation in a multimode reactor is a simple and effective method for the rapid preparation of j8-peptide hbraries on sohd support in acceptable purities [156]. 

The synthesis of imidazoles is another reaction where the assistance of microwaves has been intensely investigated. Apart from the first synthesis described since 1995 [40-42], recently a combinatorial synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles has been described on inorganic solid support imder solvent-free conditions [43]. Different aldehydes and 1,2 dicarbonyl compounds 42 (mainly benzil and analogues) were reacted in the presence of ammonium acetate to give the trisubstituted ring 43. When a primary amine was added to the mixture, the tetrasubstituted imidazoles were obtained (Scheme 13). The reaction was done by adsorption of the reagent on a solid support, such as silica gel, alumina, montmorillonite KIO, bentonite or alumina followed by microwave irradiation for 20 min in an open vial (multimode reactor). The authors observed that when a non-acid support was used, addition of acetic acid was necessary to obtain good yields of the products. 

The development of multimode reactors for organic synthesis occurred mainly from already available microwave acid-digestion/solvent-extraction systems. Instruments for this purpose were first designed in the 1980s and with the growing demand for... 

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. 

Consequently, which strategy is utilized in reaction optimization experiments is highly dependent on the type of instrument used. Whilst multimode reactors employ powerful magnetrons with up to 1500 W microwave output power, monomode reactors apply a maximum of only 300 W. This is due to the high density microwave field in a single-mode set-up and the smaller sample volumes that need to be heated. In principle, it is possible to translate optimized protocols from monomode to multimode instruments and to increase the scale by a factor of 100 without a loss of efficiency (see Section 4.5). 

Similar parallel reactors as described above for the ETHOS multimode microwave reactor (Milestone, Inc.) are also available for the MARS-S multimode reactor from CEM Corp. [81]. Recently the construction of a parallel reactor with expandable reaction vessels that accommodate the pressure build-up during a microwave irradiation experiment has been reported [87]. The system was used for the parallel synthesis of a 24-membered library of substituted 4-sulfanyl-lff-imidazoles [87]. 

The most widely nsed eqnipment for organic synthesis on a laboratory scale is the domestic oven, which is a multimode reactor. The distribution of electric field inside the cavity results from multiple reflections on the walls and on the products and hence the heating is totally nonuniform. 

Tab. 6.35. Oxidation of secondary alcohols by means of hydrogen peroxide urea adduct (UHP) under MW- - PTC conditions (multimode reactor, 600 W).

Recently, a wide range of organic reactions have been promoted by microwave irradiation," but in the field of Heck chemistry only a limited number of papers have appeared. " " "" Two types of microwave heating equipment have been used, a multimode reactor or a monomode reactor.The latter is more expensive but allows the placement of the reaction mixture at a fixed position of much higher continuous electric field strength than can be obtained in a multimode reactor." This is particularly important with Pd-catalyzed reactions since the reaction mixture must be heated to a high temperature in a reproducible and homogeneous fashion. 

The polymer particles can pass through these two sections of the reactor several times before exiting the reactor. If these two (or more) zones are kept in difference polymerization conditions, a multimodal reactor blend polymer can be produced. It is claimed that because the polymer particles can be made to circulate between the different reactor zones several times before exiting the reactor, a more uniform distribution of blend components will result than in an equivalent resin made on two reactors in series. Of course, this is what would be expected from a reactor blend made in several reactors in series. 

This is the main issue in the industries. Two main approaches have evolved for scaling up microwave reactors. The first approach scales up single-mode reactors through a flow-through reactor, and the second scales up multimode reactors to a batch reactor. 

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