Microwave-assisted hydrogen transfer reaction

Bose reported the dehalo genation of bromoanthracene, bromonaphthalene and several bromobenzenes under microwave-assisted hydrogen transfer conditions (ammonium formate, 10% Pd/C, ethylene glycol). An application of this reaction is the synthesis of several (3-lactams and isoquinoline derivatives14. 

The rapid microwave-assisted deprotection of N-benzyl carbamate (Cbz) and AT-benzyl (Bn) derivatives in solution as well as on solid support was reported by Daga et al.26 Within this report, amino groups protected as benzyl carbamates or with simple benzyl groups could be deprotected in a few minutes by microwave-assisted catalytic transfer hydrogenation with palladium charcoal in isopropanol, employing ammonium formate as the hydrogen donor (Scheme 7.6). Both MeO-PEG and PS Wang-resin were used as soluble and solid supports, respectively, in these reactions. 

Similar reaction conditions as those by Bose were used for a range of other applications, for example, the synthesis of heterocycles. A combination of a microwave-assisted Paal-Knorr reaction15 with a transfer hydrogenation takes place in the preparation of 2,5-di- and 1,2,5-trisubstituted pyrroles from -l,4-diaryl-2-butene-l,4-diones in a one-pot operation. Hydrogenation was achieved with ammonium formates and 10% Pd/C as catalyst in PEG-200. Yields of up to 92% were obtained within 0.5-2 min (Scheme 4.2)16. 

Carbonyl)chlorohydridotris(triphenylphosphine)ruthenium(II) was used as a catalyst in the transfer hydrogenation of benzaldehyde with formic acid as a hydrogen source. Under these conditions, the reduction ofbenzaldehyde to benzyl alcohol is accompanied by esterification of the alcohol with the excess of formic acid to provide benzyl formate (Scheme 4.16). In this microwave-assisted reaction, the catalyst displayed improved turnover rates compared to the thermal reaction (280 vs. 6700 turnovers/h), thus leading to shorter reaction times36. 

Besson and co-workers have exploited microwave-assisted Pd/C-catalyzed transfer hydrogenation conditions in the multistep synthesis of some important thiazoloquinazolinone derivatives involving the reduction of a nitro substituent in several intermediate steps [43]. Herein, thermal reactions carried out under microwave conditions have greatly favoured in reducing the overall time... 

Microwave-assisted reduction reactions are among the last to be studied. The kinetics of catalytic transfer hydrogenation of soybean oil under MW and thermal conditions have been examined and reaction rates are found to be eight times greater... 

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