Dedicated microwave reactors

The Bischler-Napieralski reaction has been described to proceed imder microwave irradiation to give very good yields of dihydroisoquinolines [140] and other polycyclic compounds (see below) in the presence of POCI3 and P2O5 (classical conditions) in toluene (10 cycles of 60 s each using a dedicated microwave reactor). 

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. 

The major advantages in using dedicated microwave reactors in organic synthesis can be summarized as follows ... 

Dedicated microwave reactors have been developed that are capable of reliable and safe operation with volatile organic solvents at elevated temperatures andpressures13,65,91-95. Independent investigations into optimal parameters for microwave chemistry support this approach90,96. 

Even though in some cases the original works given in the references have been conducted in household microwave ovens, all the experiments presented in the book have been carried out in the dedicated microwave reactors (see Section 3.4), which can result in differences in reaction temperatures and times while compared to the original references. 

The delivery and introduction of energy has been closely connected with the discovery and investigation of new chemistry. It is with pleasure that we have seen an increased use of microwave irradiation over the years and we hope that this volume will reflect the current interest in expanding the scope of microwave applications in both organic and medicinal chemistry. One important explanation behind the growth of microwave-enhanced chemistry has been the introduction of dedicated microwave reactors. 

Investigation of these propositions required dedicated microwave reactors. It was essential that such systems operated reliably, safely, and routinely with volatile compounds, including organic solvents at high temperatures and at moderate pressures. To attract the attention and maintain the interest of chemists, microwave technology would need to have advantages that were unavailable with the conventional chemical process equipment of the day. 

By mid-1995, only 220 papers had been published on microwave-assisted organic chemistry [15]. Until then the vast majority of the papers had covered dry media reactions and for approximately the next five years this trend continued. By the turn of the 21st Century, however, the applicability of dedicated microwave reactors and associated chemistry had stimulated other researchers and encouraged commercial manufacturers to construct systems for laboratory and pilot-scale studies [13, 14]. 

Industrial dedicated microwave reactors have an edge over domestic units as the grafting efficiency... 

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,... 

The reactions described previously are a typical example of MAOS performed in a sealed reaction vessel at a temperature above the boiling point of the solvent. Dedicated microwave reactors provide simple and safe operating conditions to perform such pressurized reactions. 

---