Microwave accelerated reactions

In a faster, selective and cleaner applications of the microwave-accelerated reactions, Stone-Elander et al. have synthesized a variety of radiolabeled (with 3H, 11C, and 19F) organic compounds via the nucleophilic aromatic and aliphatic substitution reactions, esterifications, condensations, hydrolysis and complexation reactions using monomodal MW cavities on microscale [121]. A substantially reduced level of radioactive waste is generated in these procedures that are discussed, at length, in Chapt. 13 [122]. 

Microwave-accelerated reactions, miscellaneous, 76 580-584 Microwave-accelerated solvent-free organic reactions, 76 551, 555—584 Microwave-assisted synthesis, of... 

The most common synthetic method towards quinazolin-4-ones is the Niementowski reaction, a cyclocondensation of anthranilic acid with for-mamide which requires high temperatures (130-150 °C) and long reaction times (6 hours). It is noteworthy that a remarkable reduction of the reaction time (20 min) was achieved under microwave heating conditions (150 °C) [123]. Moreover, microwave-accelerated reactions were cleaner and afforded higher yields than those under conventional thermal conditions (Scheme 48). 

Wisen S, Androsavich J, Evans CG et al (2008) Chemical modulators of heat shock protein 70 (Hsp70) by sequential, microwave-accelerated reactions on solid phase. Bioorg Med Chem Lett 18 60-65... 

Again, the conventional heating approach delivered only poor yields with other more fluorous-tagged organotins (F-21), whereas the microwave-accelerated reaction delivered 75% yield after 6 min [Equation (5.3.24)].72... 

Trace levels of Cr(VI) were determined by FIA method employing microwave-accelerated reactions of Cr207 with dibromocarboxyarsenazo or dibromo-o-carborylchlorophonazo and spectrophotometric detection at 535 and 556 nm, respectively [8]. The detection limits were 0.087 and 0.1 g ml, respectively. The methods were applied to determine Cr in steels and electroplating solutions. 

A common commercial closed system is the Microwave Accelerated Reaction System (MARS ) 5, as supplied by the CEM Corporation, USA (Figure 5.7). This system allows up to 14 extraction vessels (XP-1500 Plus ) to be irradiated simultaneously. In addition, other features include a function for monitoring both pressure and temperature, and most notably, the system is equipped with... 

Functionalization of pyridines from the corresponding bromo substrates has been achieved by use of Reformatsky reagents and a microwave-accelerated reaction procedure [65]. This approach utilizes Pd(0)-catalyzed a-arylation of esters and amides (Scheme 10.29). The Reformatsky reagent was prepared by microwave irradiation of tert-butyl bromoacetate or dibenzyl bromoacetamide with Zn in THF for 5 min at 100 °C. 

Kidwai and Kumar ° reported a microwave-accelerated reaction of urea 232 with phenacylbromide to yield the iminooxazoline 233 (Scheme 1.64). In this case, the total reaction time required was only 1.0-1.5 min. Patel and Fernandes prepared the 2-amino-4-substituted oxazoles 235 as precursors to novel oxa-zolo[5,4-c]pyrazoles and oxazolo[5,4-c]pyridazines, which were evaluated as antibacterial agents. Cyclization of a-bromoketones 234 with urea in refluxing DMF afforded 235 (Scheme 1.65). 

Experiments were carried out under microwave in a microwave-accelerated reaction system model Mars X which operates at 2.45 GHz with a maximum power supply of 1200W. Reagents La(N03)3.9H20 (Fluka, 99%), Mn(N03)2.5H20 (Alfa Aesar, 99%) and citric acid (Prolabo, 97%) were used as received. The resulting aqueous solution was transferred in a pyrex flask coimected to a reflux set-up to collect the evaporated solvent. The step duration (min) of the microwave irradiation sequences were given in Table 1. For comparison a reference sample LaMnOs+g was synthesized by the citrate method as in ref [6] (calcination temperature 700°C kept for 5h). 

Shie and Fang (2003) carried out oxidation of some aromatic aldehydes with in ammonia water and in situ cycloadditions with NaNj/ZnBr under microwave irradiation at 80 °C for 10 min to afford 5-aryl-l,2,3,4-tetrazoles in 70-83% yields. This microwave-accelerated reaction in aqueous media is quite safer and more efficient than conventional heating using prolonged reflux (17-48 h) at a high temperature (>100 °C)... 

Bu3Sn)20 BzCl. The use of microwaves accelerates this reaction. Bu2Sn(OMe)2 is reported to work better than Bu2SnO in the monoprotection of diols. The monoprotection of diols at the more hindered position can be accomplished through the stannylene if the reaction is quenched with PhMe2SiCl (45-77% yield).Microwave heating has been found to be effective for this transformation in some cases. ... 

Microwave chemistry has been found to be a useful method for accelerating reactions or catalyzing reactions that are difficult to carry out by other methods. A modification of the Hantzsch method to directly obtain pyridines has been communicated. A dry medium using ammonium nitrate bentonitic clay system with microwave irradiation affords pyridines 96 in a single pot within 5 minutes. When the pyridine is not the major product (> 75% yield), the dealkylated pyridine 97 becomes an... 

There is sfill some dispufe about how microwave irradiation accelerates reactions. Besides the generally accepted thermal effects, one beheves that there are some specific (but also thermal) microwave effects, such as the formation of hot spots . There is still some controversy about the existence of non-thermal (athermal) microwave effects. At the present time, new techniques such as coohng while heating are being investigated and the problem of upscahng... 

Inhibitors for proteases plasmepsin I and II of the malaria parasite Plasmodium falciparum, with a good plasmepsin/human protease cathepsin D selectivity, have been identified via library construction involving rapid microwave-accelerated Suzuki reactions [57]. The phenyl ring of the biphenyl unit in the lead compound M-((lS)-l- [((lS,2S)-3- [(lS)-2-amino-l-(4-phenyl-benzyl)-2-oxoethyl]amino -2-hydroxy-l-phenoxypropyl)amino]carbonyl -2-methylpropyl)pyridine-2-carboxamide has been altered by performing Suzuki reactions on N-((lS)-l- [((lS,2S)-3- [(lS)-2-amino-l-(4-bromobenzyl)-2-oxoethyl]amino -2-hydroxy-l-phenoxypropyl)amino]carbonyl -2-methyl-propyl)pyridine-2-carboxamide (Scheme 37). In particular, a 2-benzofuryl moiety proved to be interesting since a Ki value of 13 nM for plasmepsin I and... 

Organ et al. from York University demonstrated that a diarylated IH-pyrazole-based library, based on the structure of the potent COX II inhibitor Celecoxib [4-(3-trifluoromethyl-5-(4-methylphenyl)-lH-pyrazol-l-yl)benzenesulfonamide], could be rapidly prepared using MAOS [59]. Microwave-accelerated Suzuki reaction on 4-(5-iodo-3-methyl-lH-pyrazol-l-yl)-benzenesulfonamide using heterogeneous Pd/C was the principal diversification step investigated (Scheme 41). The interest of the team in microwave... 

Different approaches towards pyrazoles have been described using microwave-accelerated cyclizations. A classical approach to pyrazoles is the cyclization of a j6-diketone with hydrazines. A series of 5-trichloromethyl-pyrazoles 58 and pyrazolium chlorides 59 were synthesized by reaction of a 4-methoxy-trihalo-3-alken-one 57 with differently substituted hydrazines (Scheme 19). The use of microwave and conventional heating for making pyrazoles gave... 

The common microwave oven has been brought into the laboratory. Using special Teflon reaction vessels, components are mixed together, the vessel sealed and put into the microwave oven. Reaction times are greatly accelerated in many reactions, and reactions that took hours to be complete in refluxing solvents are done in minutes. Benzyl alcohol was converted to benzyl bromide, for example, using microwaves (650 W) in only 9 min on a doped Montmorillonite K-10 clay. This is a growing and very useful technique. 

Moberg et al. [146] modified further the bis(pyridylamide) ligand described by Trost for the preparation of a polymer-supported pyridylamide (113 in Scheme 60) for the microwave-accelerated molybdenum-catalyzed al-lylic alkylation. TentaGel resin was tested in the presence of high concentrations of reactants and gave, after a 30 min reaction, total conversion in the... 

If tight ion pairs (between two hard ions) are involved in the reaction the microwave-accelerating effect then becomes more important, because of enhancement of ionic dissociation during the course of the reaction as tight ion pairs (GS) are transformed into more polar loose ion pairs (TS). 

The microwave-specific effect is more apparent in the case of demethylation (Sn2). The microwave acceleration clearly is more pronounced with the difficulty of the reaction, thus constituting a clear example of an increased microwave effect with a more difficult reaction, indicative of a later TS position along the reaction coordinate. The microwave effect may also be connected to the more localized charge in the Sn2 transition state (three centers) when compared to that of p-E2 (charge developed over five centers). 

Microwave-accelerated Solvent-free Organic Reactions... 

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