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Microwave Synthesis

It has long been known that molecules undergo excitation with electromagnetic radiation. This effect is utilized in household microwave ovens to heat up food. However, chemists have only been using microwaves as a reaction methodology for a few years. Some of the first examples gave amazing results, which led to a flood of interest in microwave-accelerated synthesis.

The water molecule is the target for microwave ovens in the home; like any other molecule with a dipole, it absorbs microwave radiation. Microwave radiation is converted into heat with high efficiency, so that "superheating" (external link) becomes possible at ambient pressure. Enormous accelerations in reaction time can be achieved, if superheating is performed in closed vessels under high pressure; a reaction that takes several hours under conventional conditions can be completed over the course of minutes.

Thermal vs. Nonthermal Effects

Excitation with microwave radiation results in the molecules aligning their dipoles within the external field. Strong agitation, provided by the reorientation of molecules, in phase with the electrical field excitation, causes an intense internal heating. The question of whether a nonthermal process is operating can be answered simply by comparing the reaction rates between the cases where the reaction is carried out under irradiation versus under conventional heating. In fact, no nonthermal effect has been found in the majority of reactions, and the acceleration is attributed to superheating alone. It is clear, though, that nonthermal effects do play a role in some reactions.

Is a Home Microwave Suitable for Organic Synthesis?

The discussion on the use of microwave units specially designed for synthesis use, which are often quite expensive, becomes rather heated at times. Unmodified home microwave units are suitable in some cases. However, simple modifications (for example, a reflux condenser) can heighten the safety factor. High-pressure chemistry should only be carried out in special reactors with a microwave oven specifically designed for this purpose. A further point in favor of using the more expensive apparatus is the question of reproducibility, since only these specialized machines can achieve good field homogeneity, and in some cases can even be directed on the reaction vessel.

Links of Interest

Microwave Chemistry Highlights
Laboratory microwave apparatus manufacturers
www.maos.net - MICROWAVE-ASSISTED ORGANIC SYNTHESIS (MAOS) WEBPAGES
www.cyf-kr.edu.pl/~pcbogdal/ - Darek Bogdal's Page with some literature citations

Reviews on Microwave Synthesis

C. O. Kappe, "Controlled Microwave Heating in Modern Organic Synthesis", Angew. Chem. Int. Ed. 2004, 43, 6250. DOI

Books on Microwave Synthesis

Microwaves in Organic and Medicinal Chemistry
C. Oliver Kappe, Alexander Stadler
Hardcover, 410 Pages
First Edition, 2005
ISBN: 3-527-31210-2 - Wiley-VCH

Microwaves in Organic Synthesis
André Loupy
Hardcover, 499 Pages
First Edition, November 2002
ISBN: 3-527-30514-9 - Wiley-VCH

Recent Literature

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DABCO promotes an efficient, solvent-free, and eco-friendly domino reaction of various β,γ-unsaturated α-ketocarbonyls with 5/6-membered cyclic sulfamidate imines in neat conditions under MW irradiation to provide densely functionalized picolinates in short reaction times.
S. Biswas, D. Majee, S. Guin, S. Samanta, J. Org. Chem., 2017, 82, 10928-10938.

A transition-metal-free synthesis of a series of primary arylamines from potassium aryltrifluoroborates and phenylboronic acids uses hydroxylamine-O-sulfonic acid as a mild, inexpensive source of nitrogen in cooperation with aqueous sodium hydroxide in acetonitrile. Both a sonication and a microwave-assisted method were developed.
D. Kuik, J. A. McCubbin, G. K. Tranmer, Synthesis, 2017, 49, 2555-2561.

Polyphosphoric acid (PPA) esters promote a microwave-assisted procedure for the synthesis of 5- to 7-membered cyclic iminoethers from amido alcohols. 2-Aryl-2-oxazolines and 5,6-dihydro-4H-1,3-oxazines were efficiently prepared using ethyl polyphosphate/CHCl₃ in very good yields and short reaction time. Trimethylsilyl polyphosphate under solvent-free conditions enables the synthesis of 4,5,6,7-tetrahydro-1,3-oxazepines.
M. C. Mollo, L. R. Orelli, Org. Lett., 2016, 18, 6116-6119.

A microwave-assisted flow generation of primary ketenes by thermal decomposition of α-diazoketones at high temperature followed by in situ reaction with amines and imines provides a number of amides and trans β-lactams, respectively, in very good yields.
B. Musio, F. Mariani, E. P. Śliwiński, M. A. Kabeshov, H. Odajima, S. V. Ley, Synthesis, 2016, 48, 3515-3526.

In a palladium catalyzed Negishi-type α-arylation of sulfones and sulfonamides with a broad range of aryl bromides, the substrates are selectively metalated in situ with tmp·ZnCl·LiCl base and cross-coupled in the presence of a catalyst system that is generated from Pd(dba)₂ and XPhos.
T. Knauber, J. Tucker, J. Org. Chem., 2016, 81, 5636-5648.

An acceptorless coupling of o-aminobenzamides with methanol has been accomplished in the presence of the metal-ligand bifunctional catalyst [Cp*Ir(2,2′-bpyO)(H₂O)] to provide quinazolinones in good yields.
F. Li, L. Lu, P. Liu, Org. Lett., 2016, 18, 2580-2583.

Heck isomerization of aryl bromides and allyl alcohols provides 3-arylpropanals, that can readily be transformed into 3-arylmethylindoles by Fischer indole synthesis in a consecutive three-component fashion in good yields. This sequence can be expanded to a four-component Heck isomerization-Fischer indolization-alkylation (HIFIA) synthesis.
J. Panther, T. J. J. Müller, Synthesis, 2016, 48, 974-986.

A Schiff’s base complex nickel catalyst (Ni-C) enables a highly efficient one-pot microwave-assisted synthesis of 2,4,5-trisubstituted imidazoles in excellent yields from aldehydes, benzil, and ammonium acetate. The catalyst could be easily recovered by simple filtration and reused.
T. S. Chundawat, N. Sharma, P. Kumari, S. Bhagat, Synlett, 2016, 27, 404-408.

A New Simplified Protocol for Copper(I) Alkyne-Azide Cycloaddition Reactions Using Low Substoichiometric Amounts of Copper(II) Precatalysts in Methanol
B. R. Buckley, M. M. P. Figueres, A. N. Khan, H. Heaney, Synlett, 2016, 27, 51-56.

A microwave-assisted, metal-free direct decarboxylative elimination of arylacetic acids in the presence of PIFA as oxidant readily provides alkenes in good yields.
S.-W. Wu, J.-L. Liu, F. Liu, Org. Lett., 2016, 18, 1-3.

A microwave-assisted hydrophosphinylation of unactivated alkenes with phosphinic acid and its derivatives under metal- and initiator-free conditions is operationally simple and seems to proceed via a radical mechanism. Good isolated yields were obtained using a reasonable excess of the appropriate reagent.
P. Troupa, G. Katsiouleri, S. Vassiliou, Synlett, 2015, 26, 2714-2719.

A protocol with small amounts of palladium and ligand, without any additive solvent and under microwave heating enables cross-coupling reactions between 2-methyl-2-propen-1-ol and various boronic acids to yield aromatic 2-methylallyl derivatives.
C. Tabélé, C. Curti, N. Primas, Y. Kabri, V. Remusat, P. Vanelle, Synthesis, 2015, 47, 3339-3346.

A mild gold-catalyzed protodeboronation reaction can be carried out in green solvents without any acid or base additives. Therefore, the reaction is very functional-group-tolerant and enables the use of the boronic acid group as an effective traceless directing or blocking group.
G. Barker, S. Webster, D. G. Johnson, R. Curley, M. Andrews, P. C. Young, S. A. Macgregor, A.-L. Lee, J. Org. Chem., 2015, 80, 9807-9816.

Enyne cross metathesis of propargylamines with ethyl vinyl ether provides a series of substituted pyrroles, bearing alkyl, aryl, and heteroaryl substituents under microwave irradiation.
H. Chachignon, N. Scalacci, E. Petricci, D. Castagnolo J. Org. Chem., 2015, 80, 5287-5295.

A microwave-assisted method for the palladium-catalyzed direct arylation of quinazolin-4-one has been developed under copper-assistance. This method is applicable to a wide range of aryl iodides and substituted (2H)-quinazolin-4-ones. This protocol provides a simple and efficient way to synthesize biologically relevant 2-arylquinazolin-4-one backbones.
S. Laclef, M. Harari, J. Godeau, I. Schmitz-Afonso, L. Bischoff, C. Hoarau, V. Levacher, C. Fruit, T. Besson, Org. Lett., 2015, 17, 1700-1703.

A gold-catalyzed, microwave protocol activates alcohols through an intermolecular, S_N1-type reaction to directly form unsymmetrical ethers and Cbz-protected amines in good yields. This reaction is highly reproducible and tolerates moisture.
A. R. S. Vinson, V. K. Davis, A. Arunasalam, K. A. Jesse, R. E. Hamilton, M. A. Shattuck, A. C. Hu, R. G. Iafe, A. G. Wenzel, Synlett, 2015, 26, 765-770.

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