Microwave irradiations, monomode

Free radical copolymerization of methyl methacrylate and styrene as well as butyl methacrylate with styrene or isoprene in toluene under microwave irradiation (monomode microwave reactor) has also been carried out (Fellows, 2005). However, no changes in reactivity ratios were observed although more detailed studies were required for the copolymerization of butyl methacrylate and isoprene. The microwave-assisted polymerization procedure accelerated the polymerizations by a factor of 1.7, may be due to an increase in radical flux. It was proposed that the increased radical flux under microwave irradiation is due to rapid orientation of the radicals that are formed from decomposition of the azoisobutyronitrile. This orientation would reduce the number of direct terminations by recombination of the two radical fragments under microwave irradiation and thus, cause a higher radical flux. 

For the synthesis of coumarins, the Pechmann reaction [145] is one of the most popular synthetic routes. As the reaction is conventionally carried out at high temperature, two microwave-assisted versions have been recently described. Besson and co-workers described the cyclocondensation of different m-amino phenols 226 with /1-ketoesters 227 on graphite/montmorillonite KIO support (Scheme 83). The use of graphite was crucial in the development of the reaction conditions. In fact, microwave irradiation of the reagents using different conditions gave poor results in terms of yields and purity. The optimized conditions, using a monomode microwave system, employed... 

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. 

Fig. 5.1 Thermal behavior induced by microwave irradiation of PhC02l< under different conditions (monomode reactor, 180W).

A recent report [6] has discussed the effect of monomode microwave irradiation in the palladium-catalyzed phenylation of 5-iodouracil 4 with the nontoxic sodium tet-raphenylborate 5 as phenyl reagent (Scheme 8.3). The authors showed that the use of monomethylformamide (MMF) as solvent increases the yield of 6 (70%), because MMF has a high boiling point (180 °C) and is more polar (s = 182.5) than other amides used in microwave-activated reactions. 

Deshayes described the cydoaddition of 1-amino-1,3-butadienes 63 with ethyl acrylate (34) under the action of microwave irradiation in a monomode reactor with temperature control [59]. Irradiation for 30 min at 70 °C in the absence of solvent afforded a 60 40 ratio of an inseparable mixture of the endo and exo isomers in 90% yield. Classical heating under the same conditions did not affect the selectivity but the yield was lower (Scheme 9.17). 

Garrigues described the reaction of the 1-azadiene 84 with dimethyl acetylenedi-carboxylate (26b) on a graphite support [35], After sequential microwave irradiation for 10 min in a monomode reactor pyridine 85 was obtained in 60% yield. This azadiene does not react when conventional heating is used (Scheme 9.25). 

Other microwave-assisted SPOS processes reported in the literature are summarized in Scheme 12.8. The addition of isocyanates to amines bound to Wang resin, for example, was studied both under conventional conditions at room temperature and under the action of microwave irradiation in open vessels by use of a monomode instrument. By monitoring the progress of the addition by on-bead FTIR it was demonstrated that the microwave procedure proceeded significantly faster than the reaction at room temperature (12 compared with 210 min) [38], The temperature during the microwave irradiation experiment was not determined, however, so it is unclear if any nonthermal microwave effects were responsible for the observed rate-enhancements (Scheme 12.8a) [38]. 

Another example of microwave-assisted PSR chemistry involves the rapid conversion of amides to thioamides by use of a polystyrene-supported Lawesson-type thio-nating reagent. By use of microwave irradiation at 200 °C in sealed vessels (monomode reactor), a range of secondary and tertiary amides was converted within... 

The first use of room temperature ionic liquids as potential novel soluble phases for combinatorial synthesis has recently been described. As model reaction the Knoevenagel condensation of salicyl aldehyde grafted on to an imidazolium-derived ionic liquid was studied under the action of microwave irradiation (Scheme 12.19) [66]. Reactions were performed without additional solvent in the presence of a basic catalyst, utilizing microwave irradiation in a designated monomode microwave reac-... 

Microwave irradiation, in contrast to thermal heating, produces very efficient heat transfer resulting in even heating throughout the sample. The process can be optimized by giving careful thought to the dimensions of the reaction vessel and volume of reactants [9] it is fortunate that radiochemical syntheses are usually performed on a very small scale (< 5 cm3) where a high and stable E-field intensity is easier to maintain, especially if a monomodal cavity, rather than a multimodal mode, is adopted. 

Aldehyde 1 (0.2 mmol), alkene 2 (1.0 mmol), 2-amino-3-picoline (0.08 mmol), benzoic acid (0.02 mmol) and RhCl(PPh3)3 (0.01 mmol) were mixed in the absence of any organic solvent and then submitted for 10 min to microwave irradiation inside a domestic microwave oven (Samsung, RE-431H, 700 W) or in a monomode Syntliewave 402 Prolabo where power is enslaved to temperature at 140 °C. After the reaction, the product 3 was purified by column chromatography 38-85%. 

A mixture of. sy/i-benzaldoxime 1 (10 mmol, 1.21 g) or 2-hydroxyacetophenone oxime 4 (10 mmol, 1.52 g) and one equivalent of 98% anhydrous zinc chloride (10 mmol, 1.36 g) was introduced in a Pyrex tube and submitted to microwave irradiation for 20 min in the Synthewave S402 monomode reactor at 140 °C. The mixture was cooled to room temperature and dissolved in methanol and then filtered through silica gel. Products were analyzed by capillary gas chromatography using methyl benzoate as an internal standard and compared by NMR with authentic samples. 

The electrocyclic cyclization was investigated by heating the sample in o-dichlorobenzene (o-DCB), both under conventional heating conditions and under microwave irradiation in a dedicated monomode apparatus. It was found that the microwave-assisted cyclization provided slightly elevated yields at the lower reaction temperature of 150 °C, in comparison with the conventional heating experiment conducted at 180 °C (Scheme 14). The intermediate isoquinoline was then converted to the final alkaloid using known synthetic manipulations to complete the synthesis. 

Bougrin et al. (1995) reported the first practical utilization of microwave irradiation with nitrile imine as 1,3-dipole using solvent-free conditions. In this work, a comparative study of the reactivity of diphenylnitrilimine (DPNI) (1) with some dipola-rophiles was made in dry media using microwave irradiation. The good yields were obtained on mineral support in a monomode reactor. 

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