Microwave irradiation study using

Microwave irradiation is used to create hot spots on metal clusters in solution, facilitating catalytic cycles in which these clusters participate. An 8x 12 parallel screening system is built based on this concept, and tested using the Heck reaction as a case study. The spatial reproducibility of this system is examined and the pros and cons of monomode and multimode m/w setups are discussed. 

In a more recent study, microwave irradiation was used in the accelerated solid-phase synthesis of aryl triflates [67] (Scheme 16.45). Reducing the reaction times from 3-8 h under conventional heating conditions to only 6 min under the action of microwaves has made this procedure more amenable to high-throughput synthesis. The use of the commercially available chlorotrityl linker as a solid support... 

Regioselective Heck vinylations of enamides have been studied under controlled microwave irradiation conditions using a palladium catalyst and fluorous bidentate... 

Despite that the thiophene ring is considered as a bioisoster of the benzene ring, the synthesis and chemistry of thiophene analogs of heterocycles with therapeutic interest remain poorly studied. One of the most recent examples concerns the synthesis of new substituted thioisatoic anhydrides (6 and 7-arylthieno[3,2-d] [1,3]oxazine-2,4-diones), which were prepared on a large scale under microwave irradiation conditions. A small library of thiophene ureidoacids was easily performed using a Normatron microwave reactor (500 W) with high yields and good purity [4,5] (Scheme 4). 

Besides sulfonamides, chiral sulfoximines have also been used in C - N bond formation under microwave irradiation [103]. The only heteroaryl chloride used in the study—namely, 2-chloropyridine—gave the desired M-(pyridin-2-yl)sulfoximine at a yield of 43% (Scheme 101). Interestingly,... 

However, most of the reactions are reported to be slow, taking up to 12 h for complete conversion of the starting materials. A Diels-Alder reaction of the pyrazinone scaffold with dimethyl acetylenedicarboxylate (DMAD) [57] has been studied in view of investigating the swiftness of this cycloaddition-fragmentation protocol (Scheme 20). The authors investigated the reaction with DMAD (lOequiv) under microwave irradiation at an elevated temperature of 190 °C, using small amounts of ionic liquid (bmimPFe) in... 

In order to study any chemoselectivity influences of microwave irradiation on the domino Knoevenagel/hetero-Diels-Alder process (the so-called Tietze reaction), Raghunathan and coworkers [31a] investigated the transformation of 4-hydroxy coumarins (10-85) with benzaldehydes 10-86 in EtOH to afford pyrano[2,3-c]cou-marin 10-87 and pyrano[2,3-b]chromone derivatives 10-88. Normal heating of 10-85a and 10-86a at reflux for 4h gave a 68 32 mixture of 10-87a and 10-88 in 57% yield, whereas under microwave irradiation a 97 7 mixture in 82% yield was obtained. Similar results were found using the benzo-annulated substrates 10-85b and 10-86b. 

The diazines pyridazine, pyrimidine, pyrazine, and their benzo derivatives cinnoline, phthalazine, quinazoline, quinoxaline, and phenazine once again played a central role in many investigations. Progress was made on the syntheses and reactions of these heterocycles, and their use as intermediates toward broader goals. Some studies relied on solid-phase, microwave irradiation, or metal-assisted synthetic approaches, while others focused attention more on the X-ray, computational, spectroscopic, and natural product and other biological aspects of these heterocycles. Reports with a common flavor have been grouped together whenever possible. 

Scheme 6.93) [192]. Using either of the two solvent systems, all studied cycloaddition reactions were completed in less than 1 min upon microwave irradiation at 50 °C employing 3 mol% of the catalyst. An additional advantage of using the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) as solvent is that it facilitates catalyst recycling. 

In 2002, Leadbeater and Torenius reported the base-catalyzed Michael addition of methyl acrylate to imidazole using ionic liquid-doped toluene as a reaction medium (Scheme 6.133 a) [190], A 75% product yield was obtained after 5 min of microwave irradiation at 200 °C employing equimolar amounts of Michael acceptor/donor and triethylamine base. As for the Diels-Alder reaction studied by the same group (see Scheme 6.91), l-(2-propyl)-3-methylimidazolium hexafluorophosphate (pmimPF6) was the ionic liquid utilized (see Table 4.3). Related microwave-promoted Michael additions studied by Jennings and coworkers involving indoles as heterocyclic amines are shown in Schemes 6.133 b [230] and 6.133 c [268], Here, either lithium bis(trimethylsilyl)amide (LiHMDS) or potassium tert-butoxide (KOtBu) was em-... 

Raghavan and coworkers have reported on the preparation of 4-hydroxybenzoic add esters (parabans) possessing antimicrobial activity by esterification of 4-hydroxybenzoic acid (Scheme 6.153) [299]. Optimum results were obtained using the alcohol (1-butanol) as solvent in the presence of catalytic amounts of zinc(II) chloride or p-toluenesulfonic acid (pTsOH) under atmospheric conditions. After 5 min of microwave irradiation at 120 °C, ca. 40% conversion to the ester was observed. Related studies on the synthesis of long-chain aliphatic esters have been described by Mariani and coworkers [300]. 

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