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The coordinated, cooperative use of microwave heating with conventional heating can provide advantages in chemical synthesis. Here, heterogeneous mixtures comprising ionic, highly microwave‐absorbing organic reagents and nearly microwave‐transparent arene solvents are heated conventionally and/or with microwaves, resulting in faster and, in some cases, higher yielding reactions when the two heating methods are applied cooperatively as compared to either method independently. Control experiments in more polar arene solvents show no advantage of cooperative heating, consistent with selective microwave heating phenomena. The experiments are facilitated by reactor technology that regulates internal reaction temperature and coordinates the application of conventional and microwave heating. The positive outcomes in this initial exploratory system suggest that cooperative heating can offer benefits in other systems designed for selective microwave heating.

 

Microwave (MW) heating is more effective than conventional (CONV) heating for promoting a high‐temperature oxidative cycloisomerization reaction that was previously reported as a key step in a total synthesis of the natural product illudinine. The thermal reaction pathway as envisioned is an inverse electron‐demand dehydro‐Diels–Alder reaction with in situ oxidation to generate a substituted isoquinoline, which itself is unstable to the reaction conditions. Observed reaction yields were higher at a measured bulk temperature of 200 °C than at 180 °C or 220 °C; at 24 hours than at earlier or later time points; and when the reaction solution was heated using MW energy as opposed to CONV heating with a metal heat block. Selective MW heating of polar solute aggregates is postulated to explain these observations.

 

Macroscopically homogeneous mixtures of p -nitroanisole ( p NA) and mesitylene (MES) can be selectively heated using microwave (MW) energy. The p NA solutes agglomerate into distinct phase domains on the attoliter-scale (1 aL = 10 −18 L), and these agglomerates can be MW-heated selectively to temperatures that far exceed the boiling point of the surrounding MES solvent. Here, a 1 : 20 mixture of p NA : MES is used as a mixed solvent for aryl Claisen rearrangement of allyl naphthyl ether (ANE). ANE itself does not heat effectively in the MW, but selective MW heating of p NA allows for transfer of thermal energy to ANE to accelerate rearrangement kinetics above what would be expected based on Arrhenius kinetics and the measured bulk solution temperature. This focused study builds on prior work and highlights 1 : 20 p NA : MES as a mixed solvent system to consider for strategically exploiting MW-specific thermal effects. 

A high-temperature retro-Diels–Alder reaction is accelerated by microwave (MW) heating to rates higher than expected based on Arrhenius kinetics and the measured temperature of the reaction mixture. Observations are consistent with selective MW heating of the polar reactant relative to other, less polar components of the reaction mixture. 

After some initial false-starts, the international synthetic organic community is slowing warming to the possibility of certain strategic advantages of microwave heating in chemical synthesis.