Elucidation of Microwave Effects: Methods, Theories, and Predictive Models

Abstract

IntroductionThe impact of microwave (MW) irradiation in organic synthesis has increased over the years and especially since the development of new dedicated and reliable MW instruments [1].MW-assisted organic synthesis has been characterized by the spectacular accelerations produced in many reactions as a consequence of a heating rate which, in most cases, cannot be reproduced by classical heating. This effect is particularly important in (i) the preparation of isotopically labeled drugs for positron emission tomography (PET) that have a short half-life ( 11 C, t1 / 2 = 20 min; 122 I, t1 / 2 = 3.6 min; 18 F, t1 / 2 = 100 min) [2], (ii) high-throughput chemistry (combinatorial and parallel chemistry) where a rapid synthesis may increase the efficiency [3], (iii) catalysis where the short reaction time protects the catalyst from decomposition and increases the catalyst efficiency [4], and (iv) the synthesis of unstable or sensitive compounds, for example, natural products, where the short reaction time prevent the decomposition that occurs with long reaction times [5].The occurrence of results that cannot be explained exclusively by rapid heating led several authors to postulate the existence of a so-called ''microwave effect.'' Hence acceleration or modifications of the selectivity and reactivity could be explained by an effect of the electromagnetic radiation and not merely by the heating effect. In this way, in combination with the rapid heating, specific thermal effects and non-thermal effect could be the responsible for the improvement of many chemical processes.The occurrence of MW effects has been the subject of much controversy. In this chapter, we describe the state-of-the-art of this subject and we present many examples in favor of and against the presence of MW effects with a critical appraisal.Microwaves in Organic Synthesis, Third Edition.

Microwave Irradiation as a Powerful Tool for the Preparation of n-Type Benzotriazole Semiconductors with Applications in Organic Field-Effect Transistors

“…), and in most cases, remarkable improvements have been observed [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. Our research group has been using microwave radiation since 1992 in numerous and varied organic chemical processes in the absence of solvents, such that it is now the most common and sustainable source of energy used in our laboratory [ 25 , 26 , 27 , 28 , 29 , 30 , 31 ].…”

Microwave-assisted organic synthesis of nucleoside ProTide analogues

“…A number of articles report the optimisation of a variety of transformations exploiting the thermal/kinetic effect of the microwave irradiation (MWI) to enhance the reaction rate. 1,2 A few examples include Mitsunobu reaction, 3 Suzuki coupling, 4 Buchwald-Hartwig amination. 5 Prodrug nucleotide (ProTide) technology was originally designed by Professor Chris McGuigan and co-workers at Cardiff University in the early 1990s.…”

Iron-catalyzed cross-coupling of arylboronic acids with unactivated N-heterocycles and quinones under microwave heating

“…There is therefore an opportunity for further exploration of iron catalyzed reactions under microwave heating.Microwave heating in liquid-liquid biphasic systems are uncommon. Reactions in such mixtures often exhibit differential heating of the two phases, especially if the two have significantly different tanδ values 88,137,138. This can lead to altered reactivity but can also makes it difficult to accurately measure the temperature of the reaction mixture; since most microwave systems use an external IR sensor, the location of the sensor determines whether temperature is being measured in the upper phase, lower phase, or somewhere in between.…”