skip to main content


Title: Microwave Heating Outperforms Conventional Heating for a Thermal Reaction that Produces a Thermally Labile Product: Observations Consistent with Selective Microwave Heating
Abstract

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.

 
more » « less
Award ID(s):
1665029
NSF-PAR ID:
10118034
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Chemistry – An Asian Journal
Volume:
14
Issue:
15
ISSN:
1861-4728
Page Range / eLocation ID:
p. 2594-2597
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. 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. 
    more » « less
  2. Abstract

    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.

     
    more » « less
  3. 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. 
    more » « less
  4. Abstract

    Graphene‐supported single atomic metals (G‐SAMs) have recently attracted considerable research interest for their intriguing catalytic, electronic, and magnetic properties. The development of effective synthetic methodologies toward G‐SAMs with monodispersed metal atoms is vital for exploring their fundamental properties and potential applications. A convenient, rapid, and general strategy to synthesize a series of monodispersed atomic transition metals (for example, Co, Ni, Cu) embedded in nitrogen‐doped graphene by two‐second microwave (MW) heating the mixture of amine‐functionalized graphene oxide and metal salts is reported here. The MW heating is able to simultaneously induce the reduction of graphene oxide, the doping of nitrogen, and the incorporation of metal atoms into the graphene lattices in one simple step. The rapid MW process minimizes metal diffusion and aggregation to ensure exclusive single metal atom dispersion in graphene lattices. Electrochemical studies demonstrate that graphene‐supported Co atoms can function as highly active electrocatalysts toward the hydrogen evolution reaction. This MW‐assisted method provides a rapid and efficient avenue to supported metal atoms for wide ranges of applications.

     
    more » « less
  5. Abstract

    Reaction‐bonded boron carbide composites were fabricated by both microwave (under Ar/10% H2) and conventional heating (under vacuum or Ar/10% H2). Silicon carbide (SiC) formation occurred in all cases and was slightly favored in the case of microwave heating under Ar/H2. The resulting microstructures were influenced by the heating process and atmosphere; the SiC existed in the form of needles with conventional heating under vacuum. SiC small polygonal grains were present after microwave heating under Ar/H2. Both the atmosphere and the electromagnetic field influence the SiC morphology. Despite this difference, the hardness and toughness of composites obtained by both heating techniques were similar.

     
    more » « less