Sie sind echt: Mikrowelleneffekte in der organischen Synthese bleiben umstritten, dennoch kann es keinen Zweifel mehr an ihrer Existenz geben. Im Fokus dieser Korrespondenz steht die Frage, ob Synthesechemiker versuchen sollten, Mikrowelleneffekte strategisch zu nutzen.
Thermally promoted Friedel-Crafts benzylation of arene solvents has been examined under both conventional convective heating with an oil bath and heating using microwave (MW) energy. Bulk solution temperatures-as measured by internal and external temperature probes and as defined by solvent reflux-were comparable in both sets of experiments. MW-specific rate enhancements were documented under certain conditions and not others. The observed rate enhancements at a given temperature are proposed to arise from selective MW heating of polar solutes, perturbing thermal equilibrium between the solute and bulk solution. Central to MW-specific thermal phenomena is the difference between heat and temperature. Temperature is a measure of the ensemble average kinetic molecular energy of all solution components, but temperature does not provide information about solute-specific energy differences that may arise as a consequence of selective MW heating. Enhanced chemical reactivity of the MW-absorbing solute can be described as a MW-specific "extra-temperature thermal effect", because the measurable solution temperature only captures a portion of the solute kinetic molecular energy. Experimental factors that favor MW-specific rate enhancements are discussed with an eye toward future development of MW-actuated organic reactions, in which the observed thermal reactivity exceeds what is predicted from temperature-based Arrhenius calculations.
Under appropriate conditions, significant microwave-specific enhancement of the reaction rate of an organic chemical reaction can be observed. Specifically, the unimolecular Claisen rearrangement of allyl p-nitrophenyl ether (ApNE) dissolved in naphthalene was studied under microwave heating and conventional convective (thermal) heating. Under constant microwave power, reaching a temperature of 185°C, a 4-fold rate enhancement was observed in the microwave over that using convective heating; this means that the microwave reaction was proceeding at an effective temperature of 202°C. Conversely, under constant temperature microwave conditions (200°C), a negligible (∼1.5-fold) microwavespecific rate enhancement was observed. The largest microwave-specific rate enhancement was observed when a series of 300 W pulses, programmed for 145− 175°C and 85−155°C cycles, where 2-and 9-fold rate enhancements, over what would be predicted by conventional thermal heating, was observed, respectively. The postulated origins of the microwave-specific effect are purely thermal and arise from selective heating of ApNE, a microwaveabsorbing reactant in a nonabsorbing solvent. Under these conditions, excess heat is accumulated in the domains around the ApNE solute so that it experiences a higher effective temperature than the measured temperature of the bulk medium, resulting in an accelerated unimolecular rearrangement.
Sie sind echt: Mikrowelleneffekte in der organischen Synthese bleiben umstritten, dennoch kann es keinen Zweifel mehr an ihrer Existenz geben. Im Fokus dieser Korrespondenz steht die Frage, ob Synthesechemiker versuchen sollten, Mikrowelleneffekte strategisch zu nutzen.
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