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Microwave heating in chemical reactions was first reported in 1986. There have since been many reports employing microwave heating in organic chemistry, where microwave heating has afforded higher yields of products in shorter time periods. However, such reactions are challenging to scale in batch due to the limited penetration depth of microwaves as well as the wave propagation dependence on cavity size. Continuous flow has addressed both these issues, enabling scalability of microwave processes. As such, a host of reports employing microwave flow chemistry have emerged, employing various microwave heating and reactor configurations in the context of either custom‐built or commercial apparatus. The focus of this review is to present the benefits of microwave heating in the context of continuous flow and to characterize the different types of microwave flow apparatus by their design (oscillator, cavity type and reactor vessel). We advocate the adoption of tunable, solid‐state oscillator single‐mode microwave flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications. We present the benefits of microwave (MW) heating in the context of continuous flow and characterize different types of MW flow apparatus by their design. Various MW flow reactors and reaction examples (including our own) are presented on g/h to 100's g/h scale. We advocate for tunable, solid‐state oscillator single‐mode MW flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications. [ABSTRACT FROM AUTHOR]

We have developed a single-mode microwave reactor for continuous-flow synthesis with various methods of operation. This device measures the resonant frequency and modulates the oscillation frequency accordingly to maintain the maximum electric field intensity in the cavity. It can be operated either using constant applied irradiation power or using new programs which change the electric power (E-GRA) and change the flow rate (Fl-GRA), in order to rapidly screen various reaction conditions. As a model reaction, the Claisen rearrangement reaction of allyl 1-naphthyl ether 1 was rapidly optimized in this device, affording the desired product 2-allylnapthalen-1-ol 2 in high (91%) yield and up to 20 g/h productivity. [ABSTRACT FROM AUTHOR]