Microwave-induced organic reaction enhancement (more) chemistry: Techniques for rapid, safe and inexpensive synthesis

Research on Chemical Intermediates (Impact Factor: 1.54). 06/2010; 20(1):1-11. DOI: 10.1163/156856794X00027

ABSTRACT Synthetic organic reactions have been conducted under microwave irradiation in open vessels in unaltered domestic microwave
ovens. Reaction times vary from a few seconds for sub-milligram reactions to about 15 minutes for reactions carried out on
a scale of hundreds of grams. Promising results have been obtained for several condensations, as well as the Bischler-Napieralski
reaction, the Wolff-Kishner reduction, free radical dehalogenation reactions, and other standard synthetic operations. Rapid
catalytic transfer hydrogenation using ammonium formate as the source of hydrogen has been conducted at about 100-130 °C under
microwave irradiation.

Meaningful, safe and inexpensive synthetic experiments for undergraduate and pre-college students have been developed and
tested. The MORE chemistry techniques make it possible to use simple apparatus and very short reaction times.

Commercial microwave ovens are now essential equipment in our research and teaching laboratories [1-3]. These ovens are relatively
inexpensive, easy to move from one laboratory and set up in another, and safe to operate. Glass, plastics, and ceramics are
essentially transparent to microwaves whereas many organic compounds are dipolar in nature and absorb microwave energy readily.
We have found that untraditional experimental arrangements are possible for conducting a wide variety of organic reactions
in open vessels inside domestic microwave ovens. Depending on the quantity of reactants, most reactions (on a scale of milligrams
to several grams) can be completed in minutes instead of hours. One important element of our “Microwave-induced Organic Reaction
Enhancement” (MORE) chemistry is the proper choice of a microwave energy transfer agent as the reaction medium.

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    ABSTRACT: 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 degrees 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 degrees C. Conversely, under constant temperature microwave conditions (200 degrees C), a negligible (similar to 1.5-fold) microwave-specific rate enhancement was observed. The largest microwave-specific rate enhancement was observed when a series of 300 W pulses, programmed for 145-175 degrees C and 85-155 degrees 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 microwave-absorbing 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.
    The Journal of Organic Chemistry 05/2014; 79(16). DOI:10.1021/jo5011526 · 4.64 Impact Factor
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    ABSTRACT: The use of microwave energy in chemical reactions has revolutionized the field of heterocyclic chemistry in the past two decades. Synergy of microwave methodology with reactions performed on support media and/or in the absence of solvent constitutes an environmentally clean technique, that offers tremendous advantages such as clean chemistry, reduction in reaction times, improved yields, and applicability to wide range of reactions, safety and tremendous scope for automation over the traditional heating. The benzoannulated azaheterocycles display an impressive repertoire of biological activities. The present review will provide an in-depth view of microwave-assisted synthetic methodologies of benzo-fused seven-membered azaheterocycles such as benzodiazepines, benzothiazepines and benzoxazepines.
    Mini-Reviews in Organic Chemistry 09/2014; 11(1). DOI:10.1002/chin.201440271 · 0.83 Impact Factor
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    ABSTRACT: The recrystallization of CL-20 and HNFX to form different polymorphs was investigated by employing various experimental analyses, including micro-crystallization under myriad conditions, novel means of crystallization, analysis of polymorphs using powder diffraction, and Rietveld analysis. In the investigation all three groups of solvents: polar protic, non-polar aprotic, and dipolar aprotic solvents were employed. These results are reported in two parts with the experimental results reported here and the mathematical modeling results reported in Part I of this paper. The conventional crystallization techniques included combinations of solvent and anti-solvent system (including high molecular weight polymer melts), combination of solvents, the use of habit modifiers, and seeding. Other recrystallization techniques, which were also investigated included recrystallization under UV, under microwaves, upon freeze drying, upon annealing, in the presence of high molecular weight polymers, and under different hydrodynamic and hence different micro-mixing conditions.As will be seen in Part I of this paper (this issue) our computations suggest (in agreement with earlier studies) that multiple polymorphs with various densities for both CL-20 and HNFX are possible. However, the recrystallization studies reported here revealed only one type of polymorph for HNFX, i.e., the Ci R-3 polymorph. On the other hand, CL-20 gave rise to indeed multiple polymorphs which changed with the crystallization conditions, especially with the solvent/anti-solvent systems utilized. An X-ray based method was used to determine the types of polymorphs and also the percentages of the polymorphs generated upon crystallization. The presence of multiple polymorphs or polymorph impurities in CL-20 may have significant ramifications in the sensitivity and other ultimate properties of energetic grains containing CL-20. On the other hand the existence of only one low-density polymorph for HNFX significantly limits the application areas for HNFX.
    Journal of Energetic Materials 07/2006; 24(2):103-139. DOI:10.1080/07370650600672090 · 1.36 Impact Factor