Transfer-hydrogenation technique

A method has been described for the preparation of N-formyl-a-amino-acid t-butyl esters by formylation of the free base with a mixture of formic acid, DCC, and pyridine, thus overcoming the incompatibility of the t-butyl ester group with the usual acidic formylation conditions. The transfer hydrogenation technique has been recommended for the removal of N-benzyloxycarbonyl and benzyl ester groups from peptides. N-Acetyl-a-amino-acids are converted into the corresponding A-ethyl derivatives by reaction with Meerwein s reagent followed by reduction with sodium borohydride. ... 

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McBride and Hall (37,38) reported the first observation of a controlled catalytic reaction on alumina using IETS. They studied the catalytically induced transfer hydrogenation from water vapor to unsaturated hydrocarbon chains chemisorbed on alumina at both ends of the chain. They absorbed muconic acid ( trans-trans-1,3 butadiene 1,4 dicarboxylic acid, HOOC-CH=CH-CH=CH-COOH ) onto oxidized aluminum strips using the liquid doping technique. The samples were returned to the vacuum system, and in the presence of 0.3 torr of D2O vapor, heated to up to 400° C by passing current through a heater strip evaporated on the back of the glass slide. The films were then allowed to cool and the junctions completed by evaporation of the Pb counter electrode. 

The technique can also be used for the catalytic transfer hydrogenation of carbonyl groups using formate [34, 35] or isopropanol [36] as the hydrogen donor. It is also worth noting, in this context, that Ru/tppts can be used for the selective hydrogenation [37] or transfer hydrogenation [38] of carbohydrates in a monophasic aqueous system (Fig. 7.8). 

An interesting debenzylation technique under essentially chemical conditions has been described having aspects of transfer hydrogenation. A mixture of... 

Polborn and Severin [23] recently reported ruthenium- and rhodium-based TSAs for the transfer hydrogenation reaction. These complexes were used as catalyst precursors in combination with molecular imprinting techniques. Phosphinato complexes were prepared as analogs for the ketone-associated complex. They demonstrated that the results obtained in catalysis were better in terms of selectivity and activity when these TSAs were imprinted in the polymer. This shows that organometallic complexes can indeed serve as stable TSAs (Figure 4.9). 

Brown, J. M. Chaloner, P. A. The catalytic resting state of asymmetric homogeneous hydrogenation. Exchange processes delineated by nuclear magnetic resonance saturation-transfer (DANTE) techniques. /. Chem. Soc., Perkin Trans. II1987,1583-1588. 

Hydrogenation can be performed not only by molecular hydrogen, but also by a multitude of reagents capable of forming hydride complexes in situ. This technique is usually called transfer hydrogenation. The hydride complexes of many late transition metals are formed by water-gas shift reaction from water and CO or its precursors, and thus WGSR conditions... 

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