Microwave-homogeneous catalysis

This chapter focuses exclusively on microwave heterogeneous catalysis. Microwave homogeneous catalysis by transition metal complexes is treated in Chapt. 11, phase transfer catalysis in Chapt. 5, catalytic reactions on graphite in Chapt. 7, photocataly-tic reactions in Chapt. 14, and catalytic synthesis oflabeled compounds in Chapt. 13. 

The experimental configuration in Figure 4.1 does not provide in situ capability for sono-, photo- and microwave-induced homogeneous catalysis. The chemistry occurring in the fluid elements of an irradiated or sonicated CSTR are not similar to the fluid elements in the recycle stream and sample cells. However, Figure 4.1 maintains the potential for on-line measurement capability, particularly for product, and for some side-products. The issue concerning intermediates is more complex. 

Larhed, M., Moberg, C. andHallberg, A., Microwave-accelerated homogeneous catalysis in organic chemistry, Acc. Chem. Res., 2002, 35, 717-727. 

Keywords Homogeneous catalysis Cross-coupling Microwave Heck Sonogashira Buchwald-Hartwig... 

The characteristics of microwave heating have been exploited efficiently in catalysis. In homogeneous catalysis [13] several advantages have been observed ... 

When using radiation for promoting rates or selectivities, the entire liquid phase is usually irradiated with the intent to achieve vmiform radiant power input. Although UV radiation is the most common photon source, microwave and even sono sources are also used. Numerous examples of the use of light (26), microwaves (27), and ultrasound (28) in homogeneous catalysis exist. 

We review herein progress in microwave-assisted homogeneous metal catalysis in the liquid phase. Most of the work published in this field is, as the reader will see, associated with palladium chemistry, and this will, by necessity, also be reflected in the text. 

Phase-transfer catalysis (PTC) is the most widely used method for solving the problem of the mutual insolubility of nonpolar and ionic compounds. Basic principles, synthetic uses, industrial applications of PTC, and its advantages over conventional methods are well documented [1-3]. PTC has become a powerful and widely accepted tool for organic chemists due to its efficiency, simplicity, and cost effectiveness. The main merit of the method is its universality. It may be applied to many types of reactions involving diverse classes of compounds. An important feature of PTC is its computability with other methods for the intensification of biphasic reactions (sonolysis, photolysis, microwaving, etc.) as well as with other types of catalysis, in particular, with transition-metal-complex catalysis. Homogeneous metal-complex catalysis under PTC conditions involves the simul-... 

The appropriate catalyst to choose for heterogeneous (or immobilized homogeneous) gas-phase catalysis under the action of microwaves unclear. The choice is difficult because catalysts must have appropriate catalytic properties and suitable... 

Microwave irradiation has been successfully applied in chemistry since 1975 and many examples in organic synthesis have been described [3, 4], Several reviews have been published on the application of this technique to solvent-free reactions [5], cycloaddition reactions [6], synthesis of radioisotopes [7], fullerene chemistry [8] and advanced materials [9], polymers [10], heterocyclic chemistry [11], carbohydrates [12], homogeneous [13] and heterogeneous catalysis [14], medicinal and combinatorial chemistry [15], and green chemistry [16]. All these applications are described elsewhere in this book. 

Kappe and Stadler have developed an MW procedure for rapid production of triaryl phosphines by coupling diphenylphospine with aryl halides and triflates [134]. Taking into account the importance of phosphine ligands in a variety of transition metal-catalyzed reactions, convenient procedures for their production is valuable. Both homogeneous Pd-Ni and heterogeneous Pd catalysts were explored and the more unusual substrate phenyl triflate could also be coupled swiftly by use of nickel catalysis (Scheme 15.68). Couplings with other aryl halides proceeded in 26-85% yield after 3-30 min microwave irradiation at 180-200 °C. 

---