Microwave-assisted hydrogen

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. 

Wada, Y., Yin, H.B., Kitamura, T. and Yanagida, S., Microwave-assisted hydrogenation of chlorinated phenols for complete dechlorination, Chem. Lett., 2000, 632-633. 

Desai, B. and Danks, T.N., Thermal- and microwave-assisted hydrogenation of electron-deficient alkenes using a polymer-supported hydrogen donor, Tetrahedron Lett., 2001,42, 5963. 

In the microwave-assisted or hot-filament-assisted CVD of diamond, methane and hydrogen gases (CH ca 1—5% and 95—99%) are used. In... 

Apart from the preparation of radiotracers, microwave-assisted transformations have also been utilized to carry out simple hydrogen-deuterium exchange reactions. In the case of acetophenone, for example, simple treatment with deuterium oxide as solvent in the presence of molecular sieves at 180 °C for 30 min led to complete... 

Microwave-assisted catalytic hydrogenation of steroid compounds, e. g. cholesterol, campesterol, sitosterol, etc., in the presence of Pd/C catalyst and ammonium formate in glycol solvent was fast and afforded the corresponding products in high yield (80-95%) and purity [42]. 

Hydrogenation is widely used for deproteetion of benzyl and benzyloxycarbonyl groups. Kappe s group [67,68] deprotected benzyl esters using continuous flow and then performed comparative experiments using conventional heating and microwave assisted transfer hydrogenation, whieh resulted in lower yields (53-65%) than when eompared to flow (80-85%). 

Dominguez, A. and Fidalgo, B. and Fernandez, Y.and Pis, J.J.and Menendez, J.A. (2007) Microwave-assisted catalytic decomposition of methane over activated carbon for C02-lree hydrogen production. International Journal of Hydrogen Energy, 32, 4792-4799. 

Menendez et al. reported the synthesis of murrayafoline A (7) by palladium(II)-mediated oxidative double C-H activation of a diarylamine assisted by microwave irradiation (585). The aniline derivative 598 was obtained by O-methylation of 5-methyl-2-nitrophenol (625) followed by catalytic hydrogenation. The required diarylamine 654 was obtained by N-arylation of the aniline derivative 598 with phenyllead triacetate (653) in the presence of copper(II) acetate. Under microwave-assisted conditions, in the presence of more than the stoichiometric amount of palladium(II) acetate and a trace of dimethylformamide, the diarylamine 654 was cyclodehydrogenated to murrayafoline A (7) (585) (Scheme 5.47). 

Carbon-carbon multiple bonds are most commonly reduced to saturated hydro carbons by hydrogenation or by other methods, such as the use of hydrides. While a large number of alkenes have been reduced under microwave irradiation, surprisingly little work has been published on microwave-assisted reduction of alkynes. 

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. 

Until now, hydrogen sources other than formates have been rarely reported in microwave-assisted transfer hydrogenations of carbon-carbon multiple bonds. An exception is a transfer hydrogenation of electron-deficient alkenes where a series of 1,4-dihydropyridines supported on silica gel were used as the hydrogen source (Scheme 4.6). The influences of electronic effects of the alkene, steric effects of the dihydropyridine and type and power of the microwave irradiation were studied24. 

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. 

Banik, B.K., Barakat, K.J., Wagle, D.R., Manhas, M.S. and Bose, A.K., Microwave-induced organic reaction enhancement (MORE) chemistry, Part 13. Microwave-assisted rapid and simplified hydrogenation, /. Org. Chem., 1999, 64, 5746-5753. 

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. 

Scheme 3.11 Microwave-assisted transfer-hydrogenation of cinnamaldehyde with palladium on activated charcoal...

Fulop and co-workers [124] have reported the synthesis of some new (ami-noalkyl) naphthols and (aminoalkyl) quinolinols 85 in good yields through a one-pot, microwave-assisted three-component modified Mannich reaction of naphthol or quinolinol with two equivalents of an aldehyde using ammonium carbamate or ammonium hydrogen carbonate as solid ammonia sources. It was observed that aliphatic aldehydes did not lead to the formation of the desired (aminoalkyl) quinolinols 85 (Scheme 64). 

Szatmari I, Fulop F (2009) Microwave-assisted one-pot synthesis of (aminoalkyl) naphthols and aminoalkyl) quinolinols by using ammonium carbamate or ammonium hydrogen carbonate as solid ammonia source. Synthesis 5 775-778... 

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