Catalytic hydrogen transfer

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

An unusual sensitivity of this reaction to structure was reported by Ram and Neumeyer (51). When R = H (1), hydrogenolysis could not be effected either directly or by catalytic hydrogen transfer (13), but etherification to give 2 (R = CH3) permitted slow formation of 3, The mild conditions of hydrogenation were required to avoid racemization at the 6a-position. Hydrogenolysis is usually much more facile than is indicated by this example. 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can be generated in situ, finds specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano are unaffected. The mechanism of the reaction is pictured as a concerted transfer of hydrogen via a nonpolar cyclic TS. 

Hydrogenolysis of the benzyl group in a dioxane-containing compound under catalytic hydrogen-transfer conditions (10% Pd/C in MeOH and HC02NH4 for 10 minutes at room temperature) proceeded without affecting the dioxane moiety (Scheme 4.57).169,260... 

Neither tritium or deuterium gas, with zero dipole moments, can be expected to interact positively with microwave radiation. Their low solubilities are seen as a further disadvantage. Our thoughts therefore turned towards an alternative procedure, of using solid tritium donors and the one that has found most favor with us is formate, usually as the potassium, sodium or ammonium salt. Catalytic hydrogen transfer of this kind is remarkably efficient as the results for a-methylcinnamic acid show [50]. The thermal reaction, when performed at a temperature of 50 °C, takes over 2 h to come to equilibrium whereas the microwave-enhanced reaction is complete within 5 min. A further advantage is that more sterically hindered al-kenes such as a-phenylcinnamic acid which are reduced with extreme difficulty when using H2 gas and Wilkinson s catalyst are easily reduced under microwave-enhanced conditions. 

Other amino alcohols have also been used as chiral ligands in asymmetric catalytic hydrogen transfer. Scheme 6-54 depicts another example. Ruthenium complex bearing 2-azanorbornyl methanol was used as the chiral ligand, and the corresponding secondary alcohols were obtained in excellent ee.116... 

The aim of this chapter is to review the catalytic hydrogen transfer chemistry wherein Cp Ir complexes play an important role as catalysts. To date, a number... 

Catalytic hydrogen transfer results usually in cis (jyn) addition of hydrogen and is sometimes more selective than catalytic hydrogenation with hydrogen gas (p. 44). 

Formic acid, anhydrous (M.W. 46.03, m.p. 8.5°, b.p. 100.8°, density 1.22), or a 90% aqueous solution, is an excellent hydrogen donor in catalytic hydrogen transfer carried out by heating in the presence of copper [77] or nickel [77]. Also its salt with triethylamine is used for the same purpose in the presence of palladium [72, 73], Conjugated double bonds, triple bonds, aromatic rings and nitro compounds are hydrogenated in this way. 

Recently, the influence of the preparation method of various MgO samples on their catalytic activity in the MPV reaction of cyclohexanone with 2-propanol has been reported 202). The oxides were prepared by various synthetic procedures including calcination of commercially available magnesium hydroxide and magnesium carbonate calcination of magnesium hydroxides obtained from magnesium nitrate and magnesium sulfate sol-gel synthesis and precipitation by decomposition of urea. It was concluded that the efficiency of the catalytic hydrogen transfer process was directly related to the number of basic sites in the solid. Thus, the MgO (MgO-2 sample in Table IV) prepared by hydration and subsequent calcination of a MgO sample that had been obtained from commercially available Mg(OH)2 was the most basic and the most active for the MPV process, and the MgO samples with similar populations of basic sites exhibited similar activities (Table IV). 

S. Ram u. R. E. Ehrenkaufer, Synthesis 1988,91 - -95 . .Ammonium Formate in Organic Synthesis, a Versatile Agent in Catalytic Hydrogen-Transfer Reductions". 

Choukroun, R.. Iraqi, A.. Gervais, D., Daran, f.C. and Jeannin, Y. (1987) A semibridging hydrido zirconium-rhodium complex A possible way to catalytic hydrogen transfer on do-de systems. Organometallics. 6. 1197-1201. 

Catalytic hydrogenation of l,2,3,4-tetrahydro-ll//-pyrido[2,l-6]quina-zolin-ll-one and its 6-, 7-, 8-, and 9-methyl derivatives over Pd/C catalyst in ethanol gave 1,2,3,4,6,7,8,9-octahydro-ll //- derivatives (87JMC1543). A solution of a 1 1 mixture of 9-methyl-l,2,3,4-tetrahydro-ll/f-pyrido-[2,l-6]quinazolin-11 -one (73) and 2-benzyl-9-methyl-l,2,3,4-tetrahydro-ll//-dipyrido[l,2-a 4,3-catalyst yielded a complex reaction mixture containing 15% of 9-methyl-l,2,3,4,6,7,8,9-octahydro-ll//-pyrido[2,l-/)]quinanolizin-Tl-one and other products (87T1157). The double bonds of the pyridine of compound 73 was also saturated by intermolecular catalytic hydrogen transfer from 2-(4-cyclohexenyl)-l,2,3,4,5,6,7,8-octahydroquinazolin-4-one in the presence of Pd/C catalyst [85H(23)3095]. 

The possibility of using of aliphatic alcohols as hydrogen donors for the catalytic transfer reduction of nitro group over MgO was examined. Catalytic hydrogen transfer was found to be effective and selective method for reduction of nitrobenzene, A-nitrotoluene, A-chloronitrobenzene, 4-nitro-m-xylene, 3-nitro-styrene, 3-nitrobenzaldehyde, 1-nitropropane, and 1-nitrobutane. Conversion of starting nitro compound into desired product depended on the alcohol used as a donor. Adsorption of reactant and catalyst deactivation were studied by esr. New aspects of a role of one-electron donor sites in hydrogen transfer over MgD were demonstrated. 

The Kp values for the reactions of hydrogen transfer between primary and secondary alcohols and nitro compounds were calculated using Van Krevelen and Chermin procedure (ref. 4). Similarly as the simple reduction with hydrogen the catalytic hydrogen transfer from alcohols to nitro derivatives is strongly favored thermodynamically. For all studied reactions with primary alcohols the K... 

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