Microwaves irradiation with

Oxidative Heck arylation of enamides with arylboronic acids, using oxygen gas as a reoxidant for Pd(0) and 2,9-dimethyl-1,10-phenanthroline as a chelating regiocontrolling ligand, yielded a (= internally) arylated reaction product as the major compound with a very good a//3 selectivity [92]. Microwave irradiation with prepressurized sealed vials proved useful in reducing the reaction time (Scheme 82). 

Tricyclic pyrazolo-quinolines 239 were prepared from /1-chloro arylalde-hydes and hydrazine derivatives under microwave irradiation with an acid support [156]. The method, appHed to a series of tricyclic compounds (Scheme 89), can be used, in principle, also for the synthesis of bicycHc and even monocycHc pyrazoles. 

The feasibility of synthesizing oxovanadium phthalocyanine (VOPc) from vanadium oxide, dicyanobenzene, and ethylene ycol using the microwave synthesis was investigated by comparing reaction temperatures under the microwave irradiations with the same factors of conventional synthesis. The efficiency of microwave synthesis over the conventional synthesis was illustrated by the yield of crude VOPc. Polymorph of VOPc was obtained ttough the acid-treatment and recrystallization step. The VOPos synthesized in various conditions were characterized hy the means of an X-ray dif actometry (XRD), a scanning electron microscopy (SEM), and a transmission electron Microscopy (TEM). 

Freitag and John [96] studied rapid separation of stabilisers from plastics. Fairly quantitative extraction (>90% of the expected content) of stabilisers from a powdered polymer was achieved by MAE within 3 to 6 min, as compared to 16 h of Soxhlet extraction for the same recovery. MAE and Soxhlet extraction have also been compared in the analysis of cyclic trimer in PET [113]. On the other hand, Ganzler et al. [128] compared the extraction yields for various types of compounds from nonpolymeric matrices for microwave irradiation with those obtained by the traditional Soxhlet or shake-flask extraction methods. Microwave extraction was more effective than the conventional methods, in particular in the case of polar compounds. As expected, the efficiency of the former is high especially when the extraction solvents contain water. With the high dipole moment of water, microwave heating is more... 

If it is considered that at 150 °C water can be removed equally under both types of activation, the noticeable difference in yields is clearly indicative of an improvement in the nucleophilic addition of the amine to the carbonyl group when performed under the action of microwave irradiation, with important specific effects. 

This type of reaction is usually best performed in DMSO solution. A simpler procedure has been proposed which uses anionic activation and microwave irradiation, with a metallic salt as the reagent and a PTC in the absence of solvent [75], This procedure was applied to the striking example of cyclic /J-ketoesters with considerable improvements (Eq. 53 and Tab. 5.25) which are readily apparent when the maximum yields obtained under classical Krapcho conditions (<20% when R-H) are considered [76],... 

Noticeable improvements were achieved for steps (i) and (iv) when the experimental conditions were realized under the action of microwave irradiation with the same concentration of starting material and volume of dimethylformamide. Product 35a (R = Me, X = H) was obtained in 91% yield after 5 min under the action of micro-wave irradiation at 150 W conventional heating in oil bath resulted in only 65% yield of 35a after 180 min (DMF, reflux). For step (iv), transposition of the conventional heating conditions for reaction of 38a (40% yield, 60 min, reflux) to micro-wave irradiation of solutions gave the desired compound 38a in better yield (60%) and reduced reaction time (5 min, 80 °C, 150 W). 

It should be noted that the Hantzsch-1,4-DHP synthesis can be conducted with a soluble polymer. Successful derivatization of poly(styrene-co-allyl alcohol) under the action of microwave irradiation with a variety of ethyl oxopropanoates and ethyl 3-aminobut-2-enoates were reported by Vanden Eynde s group [27]. 

Among the various derivatives of biomass, furanic compounds obtained from furfural are important (200,0001 year-1). A new family of furanic diethers has been obtained by alkylation of 2,5-furandimethanol or furfuryl alcohol under microwave irradiation with PTC solvent-free conditions [83] (Scheme 8.59). 

Under the action of microwave irradiation, with toluene as a solvent, cycloadduct 76 was formed in 35% yield after 15 min at 800 W [72] this is an improvement on the yields obtained by conventional heating and probably occurs because of a decrease in the reversion of the cycloaddition in the shorter period of time needed for the irradiated reaction. It is remarkable that under microwave conditions the formation of bis adducts was not observed in these reactions. 

A perhaps more exotic substrate for the Heck reaction is 1,2-cyclohexanedione [25], The reactivity of this molecule under Heck coupling conditions can probably be attributed to its resonance enol form. This reaction is attractive, because the literature contains relatively few examples of the preparation of 3-aryl-l,2-cyclohexane-diones. Yields varied from good to modest when classic heating and electron-rich aryl bromides were used, and reaction times typically ranged from 16 to 48 h. Similar yields were obtained under continuous microwave irradiation with a single-mode microwave reactor for 10 min at 40-50 W (Eq. 11.10) [25],... 

Hemandez-Chavama, F., and Vargas-Montero, M. 2001. Rapid contrasting of ultrathin sections using microwave irradiation with heat dissipation. J. Microsc. 203 227-230. 

Microwave-assisted organic synthesis may also be used for carrying out the multicomponent reactions of ketones and 1,2-diamines [20, 21, 92, 100]. For example, the three-component reaction of o-PDA 1 with acetoacetic acid ethyl ester 83 and a series of aromatic and heteroaromatic aldehydes 84 proceeds under microwave irradiation with very high yields of diazepines 85 (up to 95%) [100]. Reaction of 2 equiv of cyclohexanone 86 with o-PDA 1 was also realized in a microwave field on a basic alumina surface in 4 min [92] (Scheme 4.27). 

The palladium-catalyzed intramolecular coupling between an aryl iodide and an allyl moiety 460 proceeds under thermal heating or microwave irradiation, with improved yields of the l-vinyl-benzo[/]chroman obtained with the latter (Equation 189) <2005JA72>. 

Hydroxylamine supported on magnesia reacted, under microwave irradiation, with nitriles and benzoyl chlorides to afford in a one-pot reaction 1,2,4-oxadiazoles <07TL2829>. In a stepwise procedure, amino acids and dicarboxylic acids were transformed into the corresponding 3,5-disubstituted 1,2,4-oxadiazoles, useful as peptidomimetic building blocks <07TL1465>. 

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. 

Carbon-carbon bond formation via the Michael addition of a,P-unsaturated ketone and 1,3-diketone is achieved in high yields and short times to give (61) by employing catalytic amounts of EUCI3 in dry media under microwave irradiation (Soriente et al, 1997). Ranu et al. (1997) reported the Michael addition of ethyl acetoacetate, acetyl acetone, and ethyl cyanoacetate to cycloalkenones, P-substituted enones and enal. The reaction accomplished efficiently on the surface of alumina under microwave irradiation in dry media. Baruah et al. (1997a,b) also demonstrated the BiClj and Cdl2 catalyzed solvent-free Michael addition of 1,3-dicarbonyl compounds under microwave irradiations with good yields. 

Fig. 10. (a) Time-integrated emission spectrum and (b) PMDR (phosphorescence microwave double resonance) spectrum of Pd(2-thpy)2 at T = 1.4 K dissolved in an n-octane Shpol skii matrix. Concentration = 10 mol/1. Aexc = 330 nm. The PMDR spectrum is induced by a microwave irradiation with a frequency of 2886 MHz, which is in resonance with the energy difference between the triplet substates I and III. An intensity increase (+) signifies vibrational satellites that belong to an emission from the short-lived substate III, while a decrease (-) characterizes satellites of the emission spectrum from the long-lived substate I (Compare Ref. [61])... 

Several interesting reactions of pyrazole or pyrazolines have been noted in the literature. Efficient aromatization of l,3,5-trisubstituted-2-pyrazolines 49 to pyrazoles 50 was achieved under microwave irradiation with silica-gel supported jV-bromosuccinimide and solvent-free conditions or with bismuth(iii) nitrate pentahydrate in acetic acid <0513(65)865, 05SC2581>. 

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