Monomode microwave heating

One of the first published microwave-assisted synthesis of benzothiazoles is the condensation of a dinucleophile such as 2-aminothiophenol, with an ortho-ester (neat) in the presence of KSF clay in a mono-mode microwave reactor operating at 60 W under a nitrogene atmosphere [ 12] (Scheme 12). Traditional heating (oil bath, toluene as solvent and KSF clay) gave the expected products in similar yields compared to the microwave experiments but more than 12 h were required for completion. Solvent-free microwave-assisted syntheses of benzothiazoles was also described by attack of the dinucleophiles cited above on benzaldehydes and benzaldoximines [13] (Scheme 12). This methodology was performed in a dedicated monomode microwave reactor... 

The Friedlander annulation is one of the most straightforward approaches towards poly-substituted quinolines. Thus, a 22-membered library of quinolines was synthesized in a TsOH-catalyzed cyclocondensation-dehydration of 2-aminoaryl ketones and 2-aminoarylaldehydes with ketones in a household microwave oven (with power control) under solvent-free conditions [112]. It was observed that the Friedlander reaction occurred readily also in an oil-bath (at 100 °C). To compare the conventional and dielectric heating conditions precisely, a purpose-built monomode microwave system with temperature control was employed instead of the household oven. The experiments at 100 °C under otherwise identical conditions demonstrated that the dielectric heating protocol was only slightly faster. Products were isolated by a simple precipitation-neutralization sequence (in the case of solid products) or neutralization-extraction for oily or low melting point products (Scheme 43). 

Purification of carbon nanotubes has been performed in multimode [31] and monomode [32] systems. Prato purified HIPCO carbon nanotubes in a multimode oven. Common impurities are amorphous carbon and iron particles. The raw material was soaked in diethyl ether to obtain a more compact material. After evaporation of the solvent the flask was subjected to microwave heating (80 W) and an immediate weight loss occurred (5 s). This process was then repeated. Results from iron analysis were 16% (iv/iv) for the first run and 9% wjw) after the second heating cyde. It is clear in this instance that the microwave heating is selectively directed to the iron particles. It can be seen from Fig. 5.12 that, although the quality of the tubes remained similar to the original material, most of the iron spots had disappeared. 

It should, however, be emphasized that solvent-free methods possibly suffer from technical difficulties relating to nonuniform heating, mixing, and precise determination of the reaction temperature. Nevertheless, these drawbacks are circumvented when operating with efficient mechanical stirring in open vessels in a monomode microwave reactor [23]. 

When performing a reaction on a small scale using a monomode microwave unit, a temptation is to heat the reaction mixture using the full 300-400 W available. However, this can sometimes lead to uncontrollable temperature and pressure rises and, as a result, vessel failure. When scaling up reactions using larger multimode microwave units, the ability to heat the reaction mixture as intensively is much reduced since the power density is significantly lower. 

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. 

Experimental protocols using microwave heating have been added to Experiments 7, 8,15,22, and 30. The experiments in this book can be performed on a range of these commercially available microwave units. The procedures are split into two classes the first is generally for use with monomode microwave apparatus (Biotage Initiator and CEM Discover) and the second for use wifh multimode microwave units (Anton Fhar Synthos 3000, CEM MARS, and Milestone MicroSYNTH). A modified version of the monomode procedure for use wifh fhe Anton Fhar Synthos 3000 in conjunction with the silicon carbide plate format for microscale chemistry is added as a footnote in the monomode protocol. 

Leukart reductive amination of carbonyl compounds was considerably enhanced (in comparison to conventional heating) by a specific microwave effect under solvent-free conditions using monomode microwave reactor (Scheme 25). 

Stange et al. (2006) investigated the free radical copolymerization of methyl methaciy late and styrene with different initiators in different solvents nsing both microwave (monomode microwave reactor) and thermal heating. Agarwal et al. (2005) studied the copolymerization of 2,3,4,5,6-pentafluorostyrene and phei lmaleimide. They revealed that a higher initial polymerization rate and a lower final monomer conversion for the microwave-assisted procedure were there as compared to thermal heating. 

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