Solvent-Free Elimination under Microwave Irradiation

2 Solvent-Free Elimination under Microwave Irradiation 

Type of reaction elimination Reaction condition solvent-free 

Keywords benzylamino alcohol, microwave irradiation, retro-Diels-Alder reaction, unsaturated benzylamino alcohol 

Neat amino alcohols la-e (10 mmol) were placed in a Pyrex open flask allowing the removal of furan. After microwave irradiation or conventional heating, the products were removed with methylene dichloride and analyzed by ll NMR spectroscopy. Unsaturated amino alcohols 2 obtained under microwaves were highly pure and did not need further purification. 

Type of reaction eUmination Reaction condition solvent-free 

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Gomez et al. (2009) reported that the cycloaddition reactions of nitropyrroles in solvent-free conditions under microwave irradiation give 27-71% yields of the aromatic indoles followed by elimination of the nitro group and subsequent aroma-tization. 

The use of sodium hydrogen carbonate under solvent-free conditions and microwave irradiation is best method for N-alkylating pyrazoles. The yields are good and the method is devoid of side reactions like quatemization, isomerization and hydrogen halide elimination. Solvent-free conditions are the only ones that allow the preparation of 1-substituted pyrazoles from secondary halides (Almena et al., 2009). 

A set of Diels-Alder reactions of fused pyran-2-ones with ethyl vinyl ether (an appropriate synthetic equivalent of acetylene) gave fused carbocyclic systems. DABCO was used as a catalyst for the elimination of ethanol under microwave irradiation (Juranovic et al., 2012). The Diels-Alder cycloaddition reaction in 3-nitro-l-(p-toluenesulfonyl)indole with dienes under microwave irradiation in solvent-free conditions gave carbazole derivatives after elimination of the nitro group and in situ aromatization (Victoria et al., 2009). 

Cycloaddition reactions of electron rich 6-[(dimethylamino)methylene] aminouracil with various electron deficient substrates gives pyrimido[4,5-d]pyrimi-dines and pyrido[2,3-d]pyrimidines. This reaction takes place in two steps, firstly formation of cycloadducts and then elimination of dimethylamine from the cycloadducts and oxidative aromatization (Gohain et al, 2004). The reaction gives excellent yields, when carried out under microwave irradiation and solvent-free conditions. 

Anionic -elimination Ketene acetal synthesis by j8-elimination of haloacids from halogenated acetals under solvent-free PTC conditions under well controlled conditions using thermal activation (A), ultrasound (US), or microwave irradiation [152] (MW) has been described. Mechanistically, as the TS is more charge delocalized than the GS and the polarity is enhanced during the course of the reaction, a favorable microwave effect is expected, and is actually observed (Eqs. (65) and (66), Scheme 4.16). 

Solvent selection. One of the most important considerations when designing amicrowave-assisted reaction is whether or not a solvent is actually needed for the reaction. Some reactions will not be successful under solvent-free conditions however, since the solvents are typically disposed of at the end of reactions, the elimination of solvents from chemical reactions is a step forward when designing sustainable chemical reactions. The vast majority of molecules containing functional groups will have a dipole moment and absorb microwave irradiation without the addition of a solvent. If a solvent is required for the success of the reaction, a minimal amount of solvent should be used. These near-solvent-free reactions still significantly reduce the amount of solvent used by the synthesis. 

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