Microwave domestic

A related example involves the N-alkylation of azaheterocycles such as imidazole (Scheme 4.8). These reactions are typically performed using 1.5 equivalents of the alkyl halide and a catalytic amount of tetrabutylammonium bromide (TBAB). The reactants are adsorbed either on a mixture of potassium carbonate and potassium hydroxide or on potassium carbonate alone and then irradiated with microwaves (domestic oven) for 0.5 to 1 min. With benzyl chloride, a yield of 89% was achieved after 40 s [18]. 

Benzoyl-3,3-diethylthiourea 1 (2 mmol) was dissolved in acetone. An equimolar amount of the indicated amines were then added and the mixture then smoothly mixed with 1 g of KF-alumina. The solvent was removed under reduced pressure. The resulting mixture was placed into a Pyrex-glass open vessel and irradiated in a microwave domestic oven at 560 W for the times and final temperature as indicated. The products were extracted from the support with acetone and precipitated with ice water. 

The synthesis of a series of pyrimido[l,2-a]benzimidazoles 98 has been reported by Dandia and co-workers [136] through a MCR of 2-aminobenzimidazole, mal-ononitrile or ethylcyanoacetate and a carbonyl compound in water. Cyclic as well as acyclic carbonyl compounds could be used. The reaction highly benefits from microwave (domestic oven) irradiation in terms of rate and the compounds 98 were obtained in good yields (Scheme 75). 

Polycarbonates have proved attractive in domestic appliances. Examples include food processor bowls, coffeemaker cold water reservoirs, vacuum cleaner housings, food mixer housings, power tool housings, hair drier and electric razor housings, and microwave cookware. 

A solution of a benzene-1,2-diaminc (15mmol) and an cnol ether 4 (15mmol) in o-xylene (lOmL) was heated in a domestic microwave oven for a specified time. The reaction mixture was cooled and evaporated under reduced pressure, and the residue was purified by chromatography (silica gel. hexane/ CHCl3 1 4). 

The domestic microwave oven is one of the magnificent inventions used in the kitchen that contributes to simplify ing the lives of many people, as the time for cooking an acceptable meal can be reduced to the time it takes to defrost and heat vacuum-packed food, altogether consuming less than half an hour. 

It all started almost 60 years ago when P. Spencer, studying high-power microwave sources for radar applications, observed the melting of a chocolate bar in his pocket at least that is the story told. The first patent in this field was filed by him in 1946 and one year later the first commercial microwave oven appeared on the market. We had to wait until 1955 for domestic models, but by 1976 almost 60% of US households already had a microwave oven. 

The rapid synthesis of heteroaromatic Hantzsch pyridines can be achieved by aromatization of the corresponding 1,4-DHP derivative under microwave-assisted conditions [51]. However, the domino synthesis of these derivatives has been reported in a domestic microwave oven [58,59] using bentonite clay and ammoniiun nitrate, the latter serving as both the source of ammonia and the oxidant, hi spite of some contradictory findings [51,58,59], this approach has been employed in the automated high-throughput parallel synthesis of pyridine libraries in a 96-well plate [59]. In each well, a mixture of an aldehyde, ethyl acetoacetate and a second 1,3-dicarbonyl compound was irradiated for 5 min in the presence of bentonite/ammonium nitrate. For some reactions, depending upon the specific 1,3-dicarbonyl compound used. 

This transformation can also be carried out under solvent-free conditions in a domestic oven using acidic alumina and ammoniiun acetate, with or without a primary amine, to give 2,4,5-trisubstituted or 1,2,4,5-tetrasubstituted imidazoles, respectively (Scheme 15A) [69]. The automated microwave-assisted synthesis of a library of 2,4,5-triarylimidazoles from the corresponding keto-oxime has been carried out by irradiation at 200 ° C in acetic acid in the presence of ammonium acetate (Scheme 15B) [70]. Under these conditions, thermally induced in situ N - O reduction occurs upon microwave irradiation, to give a diverse set of trisubstituted imidazoles in moderate yield. Parallel synthesis of a 24-membered library of substituted 4(5)-sulfanyl-lff-imidazoles 40 has been achieved by adding an alkyl bromide and base to the reaction of a 2-oxo-thioacetamide, aldehyde and ammonium acetate (Scheme 15C) [71]. Under microwave-assisted conditions, library generation time was dramatically re-... 

Condensation of 2-aminothiophenol with the /3-chlorocinnamaldehyde in the presence ofp-toluene sulfonic acid (PISA) gave good yield of benzothia-zole (Scheme 14). The mechanism suggested in this work is beUeved to proceed via a nucleophilic attack of the sulfur atom in an addition-ehmination sequence followed by a spontaneous cyclization and ejection of acetaldehyde [15]. These investigations were performed in a domestic microwave reactor and need 1.5 min for completion (65% yield). Here again, oil bath heating seems to be inferior, providing a maximum conversion of 53% after... 

Manganese(III)-promoted radical cyclization of arylthioformanilides and a-benzoylthio-formanilides is a recently described microwave-assisted example for the synthesis of 2-arylbenzothiazoles and 2-benzoylbenzothiazoles. In this study, manganese triacetate is introduced as a new reagent to replace potassium ferricyanide or bromide. The 2-substituted benzothiazoles are generated in 6 min at 110°C imder microwave irradiation (300 W) in a domestic oven with no real control of the temperature (reflux of acetic acid) (Scheme 15). Conventional heating (oil bath) of the reaction at 110 °C for 6 h gave similar yields [16]. 

Li and co-workers introduced a rapid and efficient microwave-assisted method to prepare new disubstituted 1,3,4-thiazoles from 1,4-disubtituted thiosemicarbazides with the objective to obtain biologically active molecules. The intermediate l-aryloxyacetyl-4-(4-methoxybenzoyl)thiosemicarbazide was irradiated in an excess of glacial acetic acid in a domestic microwave oven and led to the formation of 2-(methoxybenzoyl-5-aryloxymethyl)-l,3,4-dithiazoles in good yields [30] (Scheme 20). 

In addition, thionation-cyclisation of 1,2-diacylhydrazidines to 1,3,4-thiadiazoles has been achieved by the action of Lawesson s reagent under solvent-free microwave irradiation in a domestic microwave oven (Scheme 21). This ring-closure methodology was extended for the synthesis of various liquid crystals [1]. 

Since 1986, when the very first reports on the use of microwave heating to chemical transformations appeared [147,148], microwave-assisted synthesis has been shown to accelerate most solution-phase chemical reactions [24-27,32,35]. The first application of microwave irradiation for the acceleration of reaction rate of a substrate attached to a solid support (SPPS) was performed in 1992 [36]. Despite the promising results, microwave-assisted soHd-phase synthesis was not pursued following its initial appearance, most probably as a result of the lack of suitable instriunentation. Reproducing reaction conditions was nearly impossible because of the differences between domestic microwave ovens and the difficulties associated with temperature measurement. The technique became a Sleeping Beauty interest awoke almost a decade later with the publication of several microwave-assisted SPOS protocols [37,38,73,139,144]. There has been an extensive... 

Independently, Caddick et al. reported microwave-assisted amination of aryl chlorides using a palladium-N-heterocyclic carbene complex as the catalyst (Scheme 99) [lOlj. Initial experiments in a domestic microwave oven (reflux conditions) revealed that the solvent is crucial for the reaction. The Pd source also proved very important, since Pd(OAc)2 at high power in DMF gave extensive catalyst decomposition and using it at medium and low power gave no reaction at all. Pd(dba)2/imidazohum salt (1 mol% catalyst loading) in DME with the addition of some DMF was found to be suitable. Oil bath experiments indicated that only thermal effects are governing the amination reactions. 

Oxetanes are present in several biologically active natural compounds as, for example, the taxol ring skeleton. An interesting method used to obtain this particular ring is the thermal [2 -i- 2] cycloaddition reaction. Longchar and co-workers reported a novel [2-1-2] cycloaddition of /1-formil enamides 5, often used in other cycloaddition and condensation processes, with acetylenic dienophiles 6 under microwave irradiation (in a domestic oven) to afford ox-etenes 7 in 80% yields [29]. This reaction was directed towards the synthesis of D-ring annelated heterosteroids (Scheme 2). 

The first report appeared in 1999 when Banks described the basic reaction of 2,5-hexanedione and aniline, which was successfully carried out mixing the neat reagents and irradiating for 30 s in a domestic microwave oven. Several 2,5 dimethypyrroles 9 were prepared starting from the same 1,4 diketone [30] (Scheme 3). 

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