Microwave irradiation, use

Abstract Controlled microwave heating has foimd many important applications in the synthesis of heterocycles. Almost all kinds of heterocycles have been prepared (or their preparation attempted) with the aid of microwaves. Many examples of cyclocondensations, reactions where two or more fimctional groups combine with the loss of another small molecule (usually water), have been described. Moreover, microwave irradiation successfully induces cycloaddition reactions, especially in the cases where high temperatures are required. This review collects the most representative examples of the application of microwaves to these two kinds of transformations. Except for a few examples, all the reactions selected have been carried out imder controlled microwave irradiation using dedicated instruments. 

This cyclization reaction can be accelerated by the use of microwave irradiation, using DMF as the solvent and Ba(OH)2 as the base C1998JOC4854, 1998TL3379>. 

The application of deuterated ammonium formate as a deuterium source in transfer deuteration reactions of aromatic heterocycles has been reported by Derdau (Scheme 6.173 c) [329]. It was found that the reaction time could be reduced from 12-18 h (50 °C, oil bath) to 20 min at 80 °C by employing microwave irradiation, using deuteriomethanol as solvent and 10% palladium-on-charcoal as a catalyst. 

Nitrofurfurylidines have been prepared by condensation of 5-nitrofurfuralde-hyde with active methylene compounds under microwave irradiation using ZnCl2 and K 10 as catalysts [67]. 

Cycloaddition of the nitrone 161 to the lactone 160 in boiling benzene for 6 h gave a 53 37 10 mixture of the three optically active adducts 162-164 in 66% combined yield (Scheme 9.50). Formation of the diastereoisomers 162-164 can be rationalized in terms of a highly preferred anti approach of the nitrone to the hydroxymethyl group in the transition state. The isomer ratio in the adducts was found to be dependent upon the solvent used in the reaction. Optimization of the reaction or the dia-stereoselectivity by Lewis acid catalysis failed. However, attempts to accelerate the cycloadditions by microwave irradiation, using 1,4-dioxane as the solvent, were successful and the reaction time decreased from hours to less than 10 min with only a... 

V-Benzoyl-TV, TV -dialkylthioureas have been prepared under microwave irradiation using potassium fluoride impregnated on alumina as support. Reaction... 

Thiopyrimidines can be prepared by thiolysis of halopyrimidines or by thiation of pyrimidinones with phosphorus pentasulfide or Lawesson s reagent (2,4-bis-/i-methoxyphenyl-l,3,2,4-dithiadiphosphetane-2,4-disulfide) <2003JOC9971>. The dithiation of uracil and thiamine has also been performed with the assistance of microwave irradiation using phosphorus pentasulfide absorbed on silica gel <2001SC2231>. 

The Pechmann and Knoevenagel reactions have been widely used to synthesise coumarins and developments in both have been reported. Activated phenols react rapidly with ethyl acetoacetate, propenoic acid and propynoic acid under microwave irradiation using cation-exchange resins as catalyst <99SL608>. Similarly, salicylaldehydes are converted into coumarin-3-carboxylic acids when the reaction with malonic acid is catalysed by the montmorillonite KSF <99JOC1033>. In both cases the use of a solid catalyst has environmentally friendly benefits. Methyl 3-(3-coumarinyl)propenoate 44, prepared from dimethyl glutaconate and salicylaldehyde, is a stable electron deficient diene which reacts with enamines to form benzo[c]coumarins. An inverse electron demand Diels-Alder reaction is followed by elimination of a secondary amine and aromatisation (Scheme 26) <99SL477>. 

SH Chiou, Wang KT. A rapid and novel means of protein hydrolysis by microwave irradiation using Teflon-Pyrex tubes. In Current Research in Protein Chemistry. San Diego Academic Press, 1990, pp 3-10. 

Unsaturated sterols such as cholesterol, campesterol, sitosterol and bile alcohols with unsaturated side chains can be transfer hydrogenated efficiently and with high yields under microwave irradiation, using ammonium formate and a Pd/C catalyst in methylene chloride/propylene glycol solvents (Scheme 4.3)17. 

Recently, Deetlefs and Seddon83 reported the solvent-free synthesis and scale-up of ionic liquids under microwave irradiation. Using a commercial microwave reactor they prepared ionic liquids based upon the 1-alkylpyridinium, l-alkyl-3-methylimida-zolium, l-alkyl-2-methylpyrazolium and 3-alkyl-4-methylthiazolium cations, on scales from 50 mmol to 2 mol (Scheme 9.2). Under microwave irradiation, because of efficient... 

Solvent-free three-component Biginelli reactions of 2-phenyl-l,3-oxazol-5-one, D-xylose/D-glucose and (thio)ureas 136 in the presence of Ce2(S04)3 salt (10%) under microwave irradiation produced perhydropyrido[l,2-c] pyrimidines 137 and 138 with 94% trans diastereoselectivity (Scheme 3). In the absence of Ce3+ salt no reaction occurred, and without microwave irradiation using conventional heating at 90 °C yields were significantly lower (39-51%) even for longer reaction times (3-5 h). If shorter reaction periods (4-7 min) was applied addition products 139 could be isolated, which then were converted to pyrido[l,2-c]pyrimidines 137 (X = O, R = H) and 138 (X = S, R = Ph) (07SL1905). 

Dharman MM, Choi H-J, Kim D-W et al (2011) Synthesis of cyclic carbonate through microwave irradiation using silica-supported ionic liquids Effect of variation in the silica support. Catal Today 164(1) 544—547... 

Alkylations of nitrophenylacetates, indole, and methyl benzylidene glycinate under dry conditions induced by microwaves have been studied, leading to the synthesis of arenes (66) linked by a long hydrocarbon chain (Abramovitch et al., 1995). Kumar et al. (1999) reported the enviromnental-friendly process for the synthesis of 2-alkylated hydroquinones (67) under microwave irradiation using 1,4-cyclohexanedione and aldehydes catalyzed by KF-AI2O3. An attractive methodology using microwave irradiation is described by Vanelle et al. (1999) for reactions of... 

Khan A, Alam M et al (2008) The synthesis of 20-amino-5a-cholest-6-eno [6,7-d] thiazole derivatives under microwave irradiation using dry-media conditions. Chin Chem Lett 19 1027-1030... 

A single pot synthetic protocol for the synthesis of 2-sulphonamide-l,3,4-oxadiazoles from 1,2-diacylhydrazine (xxix) under microwave irradiation using PS-BEMP (xxxi) and corresponding sulfonyl chloride (xxx) is reported by Baxendale et al. [33]. 

Substituted-2-(2-methyl-4-nitro-1 -imidazomethyl)-1,3,4-oxadiazoles (xxxviii) have been prepared under microwave irradiation using 2-methyl-4-nitro-1-imidazo-acethydrazide (xxxvii), aromatic acid and phosphorus oxychloride as the cyclizing agent [36]. 

CuO nanoparticles with an average size of ca. 4 nm have been successfully prepared by microwave irradiation, using copper(II) acetate and sodium hydroxide as the starting materials and ethanol as the solvent [202]. The as-prepared CuO nanoparticles have regular shape, narrow size distribution and high purity. The band gap is estimated to be 2.43 eV according to the results of the optical measurements of the CuO nanoparticles. 

Hydantoins. Cyclization of ureas containing an ester group occurs on microwave irradiation using barium hydroxide as a base. 

Loupy, A., A. Petit, M. Ramdani, C. Yvanaeff, M. Majdoub, B. Labiad, and D. Villemin, The Synthesis of Esters under Microwave Irradiation Using Dry-Media Conditions, Can. J. Chem., 71, 90 (1993). 

A variety of fused 3-aminoimidazoles has been synthesized by a microwave-assisted Ugi three-component coupling reaction catalyzed by scandium triflate in methanol as solvent. The reactions of heterocyclic amidines with aldehydes and isocyanides were performed in 33 to 93% yields within only 10 min of microwave irradiation using a simple one-stage procedure [119]. 

Formation of several pyrazolyl-pyrazolino[60]fullerene adducts (45a-c) from nitrile imines by use of this procedure has been described (Scheme 21.17) [53, 54]. The nitrile imines are generated in situ from the corresponding hydrazone 42a-c and NBS in the presence of EtsN and then reacted with Ceo (1) under the action of microwave irradiation, using a focused microwave oven. The route is simpler than the previously described method, which involved cycloaddition of Qo to nitrile imines prepared from the corresponding N-chlorobenzylidene derivatives [55]. 

The fullerodendrimers 52a,b were prepared in 31-34% yield by the procedure outlined in Scheme 21.19, with microwave irradiation used as the energy source. 

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