Alternative microwave reactors

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. 

If increased reaction yields and/or cleaner reactions are demonstrable, to add to large reductions in reaction time, one has the best of all possible opportunities to profitably partner the process development chemist and chemical engineer in the development of a pilot plant scale microwave reactor as an alternative to conventionally heated plant equipment. As with any relatively new technology, sober-minded and realistic evaluation is needed in order to gain credibility with one s supporters and to overcome the inertia, often born of skepticism, which is frequently associated with excursions into the unknown. 

Thus, in the presence of a dirhodium(II) tetraoctanoate catalyst, 1-sulfonyl triazoles react with nitriles, forming imidazoles [123]. The reaction proceeds at 60-80 °C with conventional heating or can be performed in a microwave reactor, and generally provides imidazoles in good to excellent yields (Scheme 7.11 A). The sufonyl group can be readily removed, revealing the parent NH-imidazole. Alternatively, sulfonyl imidazoles can be converted to 1,2,5-trisubstituted imidazoles by simple alkylation (Scheme 7.11B). 

On the other hand, RPBs suffer from poor heat transfer possibilities. Heat input could theoretically be achieved by use of eddy currents, microwaves, or sonic energy, and thus endothermic reactions are, in principle, possible. The heat removal is more problematic and exothermic reactions must be conducted adiabatically within the rotor. Alternating packing and heat transfer plates could perhaps be an option, although it would greatly increase the complexity and the price of the reactor. 

The implementation of in-line ultrasound for sample preparation in flow analysis is analogous to microwave or UV irradiation. The coiled reactor (or a mini-chamber) is immersed in a water bath to which ultrasound is applied. As mechanical waves are involved, the inner walls of the tubing should be thin and flexible to avoid wave damping, which would decrease the efficiency of the ultrasound. Alternatively, an ultrasound probe can be placed near the ultrasound reactor [143]. The use of an... 

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