Microwave heterogeneous 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 development of microwave heterogeneous catalysis is, however, impeded because most synthetic chemists are not well acquainted with factors affecting both heterogeneous catalysis and microwaves. Although attempts have been made to accelerate reaction rate or to improve yield and selectivity, they have been based on the rule let s try microwaves and see what happens . 

Since the revised Biginelli mechanism was reported in 1997, numerous papers have appeared addressing improvements and variations of this reaction. The improvements include Lewis acid catalysis, protic acid catalysis, non-catalytic conditions, and heterogeneous catalysis. In addition, microwave irradiation (MWI) has been exploited to increase the reaction rates and yields. 

Microwave technology has now matured into an established technique in laboratory-scale organic synthesis. In addition, the application of microwave heating in microreactors is currently being investigated in organic synthesis reactions [9-11] and heterogeneous catalysis [12, 13]. However, most examples of microwave-assisted chemistry published until now have been performed on a... 

Zhang, X., Hayward, D.O. and Mingos, D.M.P., Effects of microwave dielectric heating on heterogeneous catalysis, Catal. Lett., 2003, 88, 33. 

Chemat, F., Esveld, D.C., Poux, M. and DiMartino, J.L., The role of selective heating in the microwave activation of heterogeneous catalysis reactions using a continuous microwave reactor, /. Microwave Power Electromag. Energy, 1998, 2, 88. 

Silica-supported Lewis acids are useful catalysts with microwave irradiation for conjugate additions. The silica-supported catalysts are obtained by treatment of silica with ZnCh [Si(Zn)], Et2AlCl [Si(Al)] or TiCl4 [Si(Ti)] [ 150-152], The Michael addition of methyl a-acetamidoacrylate (196) with indole (2) under Si(M) heterogeneous catalysis assisted by microwave irradiation afforded the alanine derivative 197 within 15 min and/or bis-indolyl 198, depending on the reaction conditions (Scheme 45) [153]. While the bis-indolyl product 198 is only formed when Si(Zn) was used as catalyst, the alanine derivative 197, as a single product is formed under thermal heating in a yield of 12%. The best yields were observed with Si(Al) (Table 5). The product 198 was obtained by elimination of acetamide followed by a-Michael addition between intermediate 199 with a second mole of indole. 

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. 

Simultaneous cooling whilst microwave heating is a new method in synthetic organic chemistry. It is most recently discovered microwave effect in heterogeneous catalysis and can substantially improve yields and selectivity of catalytic reactions [77]. 

Porous Carbon Materials from Sustainable Precursors 33 Heterogeneous Catalysis for Today s Challenges Synthesis, Characterization and Applications 34 Chemical Biotechnology and Bioengineering 35 Microwave-Assisted Polymerization... 

Lubinu MC, De Luca L, Porcheddu A et al (2011) Microwave-promoted selective mono-Al-afky-lation of anifines with tertiary amines by heterogeneous catalysis. Chem-Eur J 17(l) 82-85... 

Moyes, R.B. and Bond, G., Microwave heating in catalysis, in Handbook of Heterogeneous Catalysis, VCH VerlagsgeseUschft, Weinheim, Germany, 1997. 

The most studied and perhaps promising catalytic systems in direct amidation so far are boric and boronic acid-based systems. However, other catal3Aic systems, such as triazines and other metal-based catalysts, have also been found to be impressive. In addition, heterogeneous catalysis is also possible in this area, with the application of microwave conditions. ... 

In these cases, reduction of the copper precursors was carried out by borohy-dride [263, 264], by microwave irradiation [262], or even by spontaneous dissolution of macroscopic copper powders (copper bronze) reacting with the counterions of the IL through an oxidation/reduction multistep process that occurs while using NPs in heterogeneous catalysis (as a part of the overall catalytic cycle) [261],... 

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