Solvent-Free Rearrangement under Microwave Irradiation

A suspension of the compound (0.5-1 g) in distilled water (150 mL) was irradiated with stilling using a Hanovia medium-pressure 450-W lamp and a Pyrex filter for ca. 35 h. The suspension was then extracted with CH2CI2 and the residue obtained after removal of solvent was chromatographed on a silica gel column (60-120 mesh). The ketene was eluted with a mixture of ethyl acetate (5-10%) in petroleum ether (bp 60-80 °C). 

References M. Chanda, D. Maji, S. Lahiri, Chem. Commun., 543 (2001). 

3 Solvent-Free Rearrangement under Microwave Irradiation 

Type of reaction rearrangement Reaction condition solvent-free 

Keywords acetylenic alcohol, KSF clay, microwave irradiation, a,/ -unsaturated ketone 

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Solvent-Free Rearrangement under Microwave Irradiation 375 Experimental procedures ... 

Phosphoric acid is an efficient and mild reagent for conversion of ketoximes to amide or lactam by Beckmann rearrangement (Baneijee and Mitra, 2005). The reactions are carried out in solvent-free condition under microwave irradiation. It gives good to excellent yields (73-82%). 

Bagheri and Karimkoshteh (2013) used nano silica-H SO as an efficient and mild catalyst for the Beckmann rearrangement of ketoximes to amides. The reactions were carried out in solvent-free conditions under microwave irradiation (600 W) within 50-120 sec in good yields. 

Moghaddam et reported that the Beckmann rearrangement of oximes 2 (R = alkyl or aryl and = aryl) in the presence of AlCls-ZnCb mixture supported on silica gel under microwave irradiation in solvent-free conditions furnished the corresponding amides 3 in excellent yields. 

Potassium dihydrogen phosphate (KH2PO4) was found to be a new and efficient medium for the Beckmann rearrangement in solvent-free conditions under micro-wave irradiation (Niralwad et al, 2013). It was observed that the yield of the products increases with reduction in time as the power of microwaves was increased. The yield was 93% in 25 min for power 600 watts. 

An ecofriendly solvent-free Fries rearrangement with acyl/alkyl migration has been observed by Selvakumar et al. (2007). Rearrangement of N-acylaniline in presence of AlClj adsorbed on neutral alumina under microwave irradiation afforded exclusively the p-rearranged product. Reactions were completed within 3-5 min and the yield varies between 86-96%. 

A few solvent-free examples of Claisen rearrangement reactions under the action of microwave irradiation have been described. One involves the double Claisen rearrangement of bis(4-allyloxyphenyl)sulfone into bis(3-allyl-4-hydroxy-phenyl)sulfone (Scheme 8.43) [111]. Similarly, 3 -allyl-2 -hydroxyacetophenone has been obtained in quantitative yield from 2 -allyloxyacetophenone by Bennett et al. [112]. 

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