Outlook and Conclusions

The diverse examples provided in Chapters 6 and 7 of this book should make it obvious that perhaps the overwhelming majority of chemical transformations can be successfully carried out under microwave conditions. This does not necessarily imply that dramatic rate enhancements compared to a classical, thermal process will be observed in all cases, but the simple convenience of using microwave technology will make this non-classical heating method a standard tool in the laboratory within a few...

Thermal and Mechanical Stability of Polymer Supports

As far as polymer supports for microwave-assisted SPOS are concerned, the use of cross-linked macroporous or microporous polystyrene (PS) resins has been most prevalent. In contrast to common belief, which states that the use of polystyrene resins limits reaction conditions to temperatures below 130 C 14 , it has been shown that these resins can withstand microwave irradiation for short periods of time, such as 20-30 min, even at 200 C in solvents such as 1-methyl-2-pyrrolidone or...

Preparation of Isotopically Labeled Compounds

The rapid synthesis of short-lived radiolabeled (for example 11C, 18F) substances used in positron emission tomography (PET) was one of the first applications of single-mode microwave-assisted synthesis, and this area has been extensively reviewed 6 . A typical application of this technique is shown in Scheme 6.170. In recent years, considerable effort has been devoted to the design, synthesis, and pharmacological characterization of radiofluorinated derivatives of the 5-HT1A receptor...

O

After the reaction, the solution was again suspended with the aldehyde resin and subjected to the microwave-assisted scavenging (domestic microwave oven). After five irradiation cycles for 4 min each, the resin was filtered off and the filtrate was concentrated to obtain the corresponding glycinamide. Further modification and cyclization steps furnished the desired heterocyclic scaffolds. Finally, Porco and Lei have described the use of polymer-bound anthracene as...

Info

Microwave-enhanced chemistry is based on the efficient heating of materials by microwave dielectric heating effects 4, 5 . Microwave dielectric heating is dependent on the ability of a specific material (for example, a solvent or reagent) to absorb microwave energy and convert it into heat. Microwaves are electromagnetic waves which consist of an electric and a magnetic field component (Fig. 2.2). For most practical purposes related to microwave synthesis it is the electric component of the...

Nh2

Scheme 6.187 Fused prolines from azomethine ylide-maleimide 3+2 cycloadditions. has reported the generation of 2-heteroaryl-5-methoxyindoles using a solvent-free, clay-catalyzed protocol, starting from the corresponding in situ generated arylhydra-zones (Scheme 6.188) 350 . The microwave-induced Fischer indole synthesis was performed on montmorillonite K 10 clay modified with zinc chloride, providing the desired indoles within a relatively short timeframe. The 2-substituted indole products were...

M

Scheme 4.32 Suzuki reactions in a micro reactor environment. MW, ca 100 C contact time 44 s flow rate 3 L min Critically evaluating the currently available instrumentation for microwave scale-up in batch and continuous-flow Chapter 3 , one may argue that for processing volumes of lt 1000 mL a batch process may be preferable. By carrying out sequential runs in batch mode, kg quantities of product can easily be obtained. When larger quantities of a specific product need to be prepared on a...

References

Abramovitch, Org. Prep. Proced. Int. 1991, 23, 685-711 S. Caddick, Tetrahedron 1995, 51, 1040310432 P. Lidstr m, J. Tierney, B. Wathey, J. Westman, Tetrahedron 2001, 57, 92259283 B. L. Hayes, Aldrichimica Acta 2004, 37, 66-77 C. O. Kappe, Angew. Chem. Int. Ed. 2004, 43, 6250-6284. 2 C. R. Strauss, R. W. Trainor, Aust. J. Chem. 1995, 48, 1665-1692 C. R. Strauss, Aust. J. Chem. 1999, 52, 83-96. 3 Open-vessel technology MORE A. K. Bose, B. K. Banik, N....

B

Scheme 6.60 Synthesis of aryl aminobenzophenones using palladium-catalyzed aminations. cacy in the palladium-catalyzed amination, the X-Phos ligand system 2 mol palladium acetate, 4 mol ligand provided the highest product yields and cleanest reaction profiles. As a base, both sodium tert-butoxide and cesium carbonate worked equally well in a 5 1 toluene tert-butyl alcohol solvent mixture. Amination of an electronically diverse array of aryl halides with a variety of anilines was realized in...

Mitsunobu Reactions

The Mitsunobu reaction offers a powerful stereochemical transformation. This reaction is very efficient for inverting the configuration of chiral secondary alcohols since a clean SN2 process is generally observed Mitsunobu inversion . Considering the fact that Mitsunobu chemistry is typically carried out at or below room temperature, high-temperature Mitsunobu reactions performed under microwave con ditions would appear to have little chance of success. In 2001, however, it was established by...