March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Chapter 7. Irradiation Processes in Organic Chemistry

7.C. Microwave Chemistry

In 1986, independent work by Gedye et al.,¹⁰⁵ as well as Majetich and co-workers¹⁰⁶ reported the use of microwave irradiation for organic reactions. Gedye described four different types of reactions, including the hydrolysis of benzamide to benzoic acid under acidic conditions, and all reactions showed significant rate enhancements when compared to the same reactions done at reflux conditions.¹⁰⁷ Majetich, Giguere and co-workers¹⁰⁶ reported rate enhancements for microwave-promoted Diels–Alder, Claisen, and ene reactions. Many publications¹⁰⁸ have appeared that describe chemical synthesis promoted by microwave irradiation, including many review articles¹⁰⁹ and books.¹¹⁰

Microwaves are electromagnetic waves (see Sec. 7.A.i) and there are electric and magnetic field components. Charged particles start to migrate or rotate as the electric field is applied,¹¹¹ which leads to further polarization of polar particles. Because the concerted forces applied by the electric and magnetic components of microwaves are rapidly changing in direction (2.4 × 10⁹ s⁻¹), warming occurs.¹¹¹ In general, the most common frequencies used for microwave dielectric heating¹¹² are 918 MHz and 2.45 GHz¹¹³ (wavelengths of 33.3 and 12.2 cm, respectively), which are in the region between the IR and radiowave wavelengths in the electromagnetic spectrum. For chemical reactions done with microwave irradiation, rapid heating is usually observed,¹¹⁴ and if a solvent is used superheating of that solvent was always observed.¹¹² Agitation is usually important.¹¹⁵ In the early days of microwave chemistry, reactions were often done in open vessels, but also in sealed Teflon or glass vessels using unmodified domestic household ovens.¹¹⁶ Dielectric heating is direct so if the reaction matrix has a sufficiently large dielectric loss tangent, and contains molecules possessing a dipole moment, a solvent is not required. The use of dry-reaction microwave chemistry is increasingly popular.¹¹⁷

Microwave dielectric heating was initially categorized by thermal effects and nonthermal effects.¹¹⁸ “Thermal effects are those which are caused by the different temperature regime which can be created due to microwave dielectric heating. Nonthermal effects are effects,¹¹⁹ which are caused by effects specifically inherent to the microwaves and are not caused by different temperature regimes.”¹¹¹ Some claimed special effects¹²⁰ in microwave chemistry, such as lowering of Gibbs energy of activation, but later study under careful temperature control indicated no special rate effects.¹²¹ When conventional microwave ovens were used, temperature control was difficult, particularly when reactions are carried out in closed reaction vessels. The main contributing factor to any rate acceleration caused by microwave dielectric heating seems to be due to a thermal effect. The thermal effect may be due to a faster initial heating rate or to the occurrence of local regions with higher temperatures.¹¹¹

Conventional microwave ovens are used less often for microwave chemistry today. Microwave reactors for chemical synthesis are commercially available and widely used in academia and in industry. These instruments have built-in magnetic stirring, direct temperature control of the reaction mixture, shielded thermocouples or IR sensors, and the ability to control temperature and pressure by regulating microwave output power.

The applications of microwave chemistry to organic chemistry are literally too numerous to mention. A few representative examples will be given to illustrate the scope and utility. The combined use of microwaves and ultrasound is important in process chemistry and organic synthesis.¹²² Microwave chemistry is widely used in synthesis,¹²³ including organocatalyzed asymmetric reactions.¹²⁴ Examples include the Heck reaction (Reaction 13-10),¹²⁵ the Suzuki reaction (Reaction 13-12),¹²⁶ the Sonogashira reaction (Reaction 13-13),¹²⁷Ullman-type couplings (Reaction 13-3),¹²⁸ cycloaddition reactions (Reactions 15-58–15-66),¹²⁹ dihydroxylation (Reaction 15-48),¹³⁰ and the Mitsunobu reaction (Reaction 10-23).¹³¹ There are a multitude of other reactions types from earlier literature that can be found in the cited review articles. When microwave chemistry is an important feature of a chemical reaction, this fact will be noted in the reactions presented in Chapters 10–19.

Notes

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2. Wessig, P. Angew. Chem. Int. Ed. 2006, 45, 2168.

3. Zimmerman, H.E. Pure Appl. Chem. 2006, 78, 2193.

4. Formerly, millimicrons (mμ) were frequently used; numerically they are the same as nanometers.

5. For monographs, see Zollinger, H. Color Chemistry, VCH, NY, 1987; Gordon, P.F.; Gregory, P. Organic Chemistry in Colour, Springer, NY, 1983; Griffiths, J. Colour and Constitution of Organic Molecules, Academic Press, NY, 1976. See also, Fabian, J.; Zahradník, R. Angew. Chem. Int. Ed. 1989, 28, 677.

6. See Kurreck, H. Angew. Chem. Int. Ed. 1993, 32, 1409.

7. See Koziar, J.C.; Cowan, D.O. Acc. Chem. Res. 1978, 11, 334.

8. An n electron is one in an unshared pair.

9. See Malkin, Yu.N.; Kuz'min, V.A. Russ. Chem. Rev. 1985, 54, 1041.

10. Bohlmann, F.; Mannhardt, H. Chem. Ber. 1956, 89, 1307.

11. For a review of the azo group as a chromophore, see Rau, H. Angew. Chem. Int. Ed.1973, 12, 224.

12. These values are from Silverstein, R.M.; Bassler, G.C. Spectrometric Identification of Organic Compounds, 2nd ed., John Wiley, NY, 1967, pp. 164–165. Also see Jaffé, H.H.; Orchin, M. Theory and Applications of Ultraviolet Spectroscopy, Wiley, NY, 1962, p. 257.

13. See Pitts, Jr., J.N.; Wilkinson, F.; Hammond, G.S. Adv. Photochem. 1963, 1, 1; Porter, G.B.; Balzani, V.; Moggi, L. Adv. Photochem. 1974, 9, 147; Braslavsky, S.E.; Houk, K.N. Pure Appl. Chem. 1988, 60, 1055.

14. For reviews of the structures of excited states, see Zink, J.I.; Shin, K.K. Adv. Photochem. 1991, 16, 119; Innes, K.K. Excited States 1975, 2, 1; Hirakawa, A.Y.; Masamichi, T. Vib. Spectra Struct. 1983, 12, 145.

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16. Merer, A.J.; Mulliken, R.S. Chem. Rev. 1969, 69, 639.

17. Garrison, B.J.; Schaefer III, H.F.; Lester Jr., W.A. J. Chem. Phys. 1974, 61, 3039; Streitwieser Jr., A.; Kohler, B. J. Am. Chem. Soc. 1988, 110, 3769. For reviews of excited states of formaldehyde, see Buck, H.M. Recl. Trav. Chim. Pays-Bas 1982, 101, 193, 225; Moule, D.C.; Walsh, A.D. Chem. Rev. 1975, 75, 67.

18. See Ireland, J.F.; Wyatt, P.A.H. Adv. Phys. Org. Chem. 1976, 12, 131.

19. Weller, A. Z. Phys. Chem. (Frankfurt am Main) 1955, 3, 238, Discuss. Faraday Soc. 1959, 27, 28.

20. Lubitz, W.; Lendzian, F.; Bittl, R. Acc. Chem. Res. 2002, 35, 313.

21. See Turro, N.J.; Ramamurthy, V.; Cherry, W.; Farneth, W. Chem. Rev. 1978, 78, 125.

22. See Lin, S.H. Radiationless Transitions, Academic Press, NY, 1980. For reviews, see Kommandeur, J. Recl. Trav. Chim. Pays-Bas 1983, 102, 421; Freed, K.F. Acc. Chem. Res. 1978, 11, 74.

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26. See Lower, S.K.; El-Sayed, M.A. Chem. Rev. 1966, 66, 199. For a review of physical and chemical processes of triplet states see Wagner, P.J.; Hammond, G.S. Adv. Photochem. 1968, 5, 21.

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28. In certain cases excited states can be produced directly in ordinary reactions. See White, E.H.; Miano, J.D.; Watkins, C.J.; Breaux, E.J. Angew. Chem. Int. Ed. 1974, 13, 229.

29. For another table of this kind, see Calvert, J.G.; Pitts, Jr., J.N. Photochemistry, Wiley, NY, 1966, p. 89.

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42. Adapted from Calvert, J.G.; Pitts, Jr., J.N. Photochemistry, Wiley, NY, 1966, p. 367.

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