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Volumn 73, Issue 1, 2008, Pages 36-47

Nonthermal microwave effects revisited: On the importance of internal temperature monitoring and agitation in microwave chemistry

Author keywords

[No Author keywords available]

Indexed keywords

CYCLOADDITION; DIELECTRIC HEATING; TEMPERATURE MEASUREMENT; THERMAL EFFECTS; THERMAL GRADIENTS;

EID: 37549015931     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo7022697     Document Type: Article
Times cited : (478)

References (102)
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    • For a more detailed definition and examples for thermal, specific, and nonthermal microwave effects, see: Kappe, C. O.; Stadler, A. Microwaves in Organic and Medicinal Chemistry; Wiley-VCH, Weinheim, 2005; Chapter 2, pp 9-28. See also refs 3a and 12.
    • For a more detailed definition and examples for thermal, specific, and nonthermal microwave effects, see: Kappe, C. O.; Stadler, A. Microwaves in Organic and Medicinal Chemistry; Wiley-VCH, Weinheim, 2005; Chapter 2, pp 9-28. See also refs 3a and 12.
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    • (c) Perreux, L.; Loupy, A. In Microwaves in Organic Synthesis, 2nd ed.; Loupy, A., Ed.; Wiley-VCH: Weinheim, Germany, 2006; Chapter 4, pp 134-218.
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    • It should be emphasized that specific microwave effects are essentially still the result of a thermal phenomenon that is, a change in temperature compared to heating by standard convection methods, although it may be difficult to experimentally determine the exact reaction temperature in these cases, for example, on the surface of a strongly microwave-absorbing catalyst that is selectively superheated by microwave irradiation. For examples, see refs 11 and 12
    • It should be emphasized that specific microwave effects are essentially still the result of a thermal phenomenon (that is, a change in temperature compared to heating by standard convection methods), although it may be difficult to experimentally determine the exact reaction temperature in these cases, for example, on the surface of a strongly microwave-absorbing catalyst that is selectively superheated by microwave irradiation. For examples, see refs 11 and 12.
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    • For very rapid microwave-assisted reactions in the order of seconds, see: (a) Enquist, P.-A.; Nilsson, P.; Larhed, M. Org. Lett. 2003, 5, 4875.
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    • For a theoretical prediction of this effect and modeling of the field distribution in a single-mode cavity, see: (a) Berlan, J. Rad. Phys. Chem. 1995, 45, 581
    • For a theoretical prediction of this effect and modeling of the field distribution in a single-mode cavity, see: (a) Berlan, J. Rad. Phys. Chem. 1995, 45, 581.
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    • For a previous observation of this phenomenon using a prototype single-mode cavity, see: (c) Kaiser, N.-F. K. Ph.D. thesis, Uppsala University, 2001, p 37.
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    • For temperature gradients in the heating of solids under microwave conditions in multimode cavities, see: (a) Stuerga, D.; Gaillard, P. Tetrahedron 1996, 52, 5505.
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    • Related to the issue of nonthermal microwave effects is the concept that simultaneous external cooling of the reaction mixture (or maintaining subambient reaction temperatures) while heating by microwaves can in some cases lead to an enhancement of the overall process. Here, the reaction vessel is cooled from the outside by compressed air or with the aid of a cooling fluid while being irradiated by microwaves. This allows a higher level of microwave power to be directly administered to the reaction mixture thereby potentially enhancing nonthermal microwave effects that rely on the electric field strength. At the same time, overheating will be prevented by continuously removing heat. For recent examples using this technique. see ref 17 and references cited therein
    • Related to the issue of nonthermal microwave effects is the concept that simultaneous external cooling of the reaction mixture (or maintaining subambient reaction temperatures) while heating by microwaves can in some cases lead to an enhancement of the overall process. Here, the reaction vessel is cooled from the outside by compressed air or with the aid of a cooling fluid while being irradiated by microwaves. This allows a higher level of microwave power to be directly administered to the reaction mixture thereby potentially enhancing nonthermal microwave effects that rely on the electric field strength. At the same time, overheating will be prevented by continuously removing heat. For recent examples using this technique. see ref 17 and references cited therein.
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    • For a recent qualitative description of problems resulting from agitation issues in microwave chemistry involving single-mode cavities, see: Moseley, J. D.; Lenden, P.; Thomson, A. D.; Gilday, J. P. Tetrahedron Lett. 2007, 48, 6084.
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    • Solvents used for microwave synthesis can be classified as high (tan δ > 0.5; for example, ethanol, DMSO, methanol, formic acid), medium (tan δ 0.1-0.5; for example, acetic acid, 1,2-dichlorobenzene, NMP, DMF, water), and low microwave absorbing (tan δ < 0.1; for example, chloroform, ethyl acetate, THF, dichloromethane, toluene, o-xylene, hexane). Other common solvents without a permanent dipole moment such as carbon tetrachloride, benzene, p-xylene and dioxane can be considered as microwave transparent.
    • Solvents used for microwave synthesis can be classified as high (tan δ > 0.5; for example, ethanol, DMSO, methanol, formic acid), medium (tan δ 0.1-0.5; for example, acetic acid, 1,2-dichlorobenzene, NMP, DMF, water), and low microwave absorbing (tan δ < 0.1; for example, chloroform, ethyl acetate, THF, dichloromethane, toluene, o-xylene, hexane). Other common solvents without a permanent dipole moment such as carbon tetrachloride, benzene, p-xylene and dioxane can be considered as microwave transparent.
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    • For examples of microwave-assisted transformations that require open vessel processing, see: Razzaq, T.; Kappe, C. O. Tetrahedron Lett. 2007, 48, 2513 and references cited therein.
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    • For related examples, see: (a) Kiddle, J. J. Synth. Commun. 2001, 31, 3377.
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    • By microwave irradiating a biphasic system consisting of immiscible solvents/reagents with vastly different loss tangents, differential heating of the two phases will occur. Depending on where and how the reaction temperature is measured, different values will be obtained. A case in point are biphasic mixtures of strongly microwave abosorbing ionic liquids and nearly microwave transparent organic solvents. See ref 16 for more details
    • By microwave irradiating a biphasic system consisting of immiscible solvents/reagents with vastly different loss tangents, differential heating of the two phases will occur. Depending on where and how the "reaction temperature" is measured, different values will be obtained. A case in point are biphasic mixtures of strongly microwave abosorbing ionic liquids and nearly microwave transparent organic solvents. See ref 16 for more details.
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    • Ab initio calculations at the B3LYP/6-31G* level confirmed that the N1 isomer (11, Ar = phenyl) is more stable than the N4 isomer (10, Ar = phenyl) by ca. 8 kcal/mol.
    • Ab initio calculations at the B3LYP/6-31G* level confirmed that the N1 isomer (11, Ar = phenyl) is more stable than the N4 isomer (10, Ar = phenyl) by ca. 8 kcal/mol.
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    • Difficulties reproducing this reaction in a solvent-free regime under microwave conditions have been recently reported. For more information, see: Lebouvier, N, Giraud, F, Corbin, T, Na, Y.-M, Le Baut, G, Marchand, P, Le Borgne, M. Tetrahedron Lett. 2006, 47, 6479
    • Difficulties reproducing this reaction in a solvent-free regime under microwave conditions have been recently reported. For more information, see: Lebouvier, N.; Giraud, F.; Corbin, T.; Na, Y.-M.; Le Baut, G.; Marchand, P.; Le Borgne, M. Tetrahedron Lett. 2006, 47, 6479.
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    • In order to mimic the rapid heating profiles observed under microwave conditions, the conventionally heated experiments were conducted using a differential calorimetric scanning (DSC) apparatus
    • In order to mimic the rapid heating profiles observed under microwave conditions, the conventionally heated experiments were conducted using a differential calorimetric scanning (DSC) apparatus.
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    • For further discussion on this reaction, see also the follwowing references: (a) Koopmans, C.; Ianelli, M.; Kerep, P.; Klink, M.; Schmitz, S.; Sinwell, S.; Ritter, H. Tetrahedron 2006, 62, 4709.
    • For further discussion on this reaction, see also the follwowing references: (a) Koopmans, C.; Ianelli, M.; Kerep, P.; Klink, M.; Schmitz, S.; Sinwell, S.; Ritter, H. Tetrahedron 2006, 62, 4709.
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    • Reference 12c, pp 162-164
    • (b) Reference 12c, pp 162-164.
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    • Reference 12d, pp 254-256
    • (c) Reference 12d, pp 254-256.


* 이 정보는 Elsevier사의 SCOPUS DB에서 KISTI가 분석하여 추출한 것입니다.