A Microwave-Assisted Heck Reaction: Modifying Conventional Heating Protocols

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Nearly any conventionally heated synthetic transformation can be adapted for microwave heating. The power of microwave irradiation has been harnessed in nanomaterial assembly, polymerization reactions, and small molecule synthesis, compatible with air-sensitive reagents and transition-metal catalysts. Benefits of microwave heating include decreased waste generation and increased product purity. Perhaps the largest benefit of microwave-assisted synthesis, however, is the dramatic reduction of reaction time. Because modern microwave reactors like the Discover SP can safely maintain high-pressure atmospheres, reactions can be performed at temperatures exceeding reflux, expediting reaction rates and reducing reaction times.

 Arrhenius Equation

Equation 1.  Arrhenius equation

Standard incremental temperature screening procedures can be employed when adapting a conventional synthesis to a microwave-assisted synthesis. A convenient generalization supported by the Arrhenius equation (Equation 1), is that a reaction rate will double every 10 °C the temperature increases. With this generalization, a microwave reaction time approximation can be derived, presented in Equation 2, where T = microwave heating temperature, T0 = conventional heating temperature, x = the temperature increase coefficient, t = microwave heating time and t0 = conventional heating time.


Equation 2. Microwave reaction time


To demonstrate the ready modification of a conventionally heated transformation to microwave heating, a Heck reaction of iodobenzene with methyl acrylate (Figure 1) was adapted for microwave irradiation. Under conventional heating protocol, the transformation is limited to 80 °C (boiling point of MeCN) and takes 20 h for completion. However, after applying the approximation depicted in Equation 2, simply doubling the temperature to 160 °C (x = 8) via microwave irradiation could lead to a reaction time under 10 min.


Figure 1. Conventionally heated Heck reaction of iodobenzene with methyl acrylate.