Reductions catalytic transfer hydrogenation

Catalytic transfer hydrogenations for the reduction of carbon-carbon double bonds are illustrated in Scheme 4.18. Reductions of azide functionalities to amines with lipases suspended in organic media under microwave conditions have also been reported [206]. 

In MW-assisted chemistry reduction reactions were the last to appear on the scene the use of ammonium formate and catalytic transfer hydrogenation were initial examples [23 b]. 

R.A.W. Johnstone, A.H. Wilby and I.D. Entwistle, Heterogeneous catalytic transfer hydrogenation and its relation to other methods for reduction of organic compounds, Chem. Rev., 85 (1985) 129-170. 

A further step towards optimised conditions in the catalytic transfer hydrogenation of alkenes was achieved with the introduction of the ionic liquid N-butyl-N -methylimidazolium hexafluorophosphate (BMIMPFg) as a solvent. The reduction of alkenes with formates and Pd/C in BMIMPF6 leads to saturated hydrocarbons in high yields. With an alkyne, a mixture of cis/trans alkenes and the saturated alkane was obtained (Scheme 4.5). Sufficiently pure products were isolated by a simple liquid-liquid... 

The value of the catalytic transfer hydrogenation route is demonstrated by the selective, high yield and rapid reduction of nitro aliphatic compounds to their corresponding amine derivatives using anhydrous ammonium formate (equation 26). A wide variety of nitro compounds are reduced in the presence of other functional groups including acids, esters and nitriles. Furthermore, the method is stereospecific and proceeds with retention of configuration pure racemic syn-nitro alcohols (39a) and (39b) were converted to the 5yn-amino alcohols (40a) and (40b) and the axial nitrosteroid (41a) afforded the 63-amine (41b). 

An interesting variation on conventional catalytic hydrogenation is catalytic transfer hydrogenation, in which formic acid or formate serves as the hydrogen source. Thus, indoles are smoothly reduced (and formylated) to the corresponding A -formylindoles (equations 42 and 43). Since it is difficult to avoid formylation of the indoline, it is best to employ triethylammonium formate to maximize yields of the reduction product (equation 43). 

The improvement results from using a catalytic transfer hydrogenation [113] for the reductive cyclization. The critical step of this method is the demethylation of the methyl acetal. This problem has been a main subject of a first review on the synthesis of benzoxazinoid aglucones [104]. 

Reductive cyclisations of methyl 2-methoxy-2-(2-nitrophenyl-thio)acetate by means of a catalytic transfer hydrogenation or of methyl 2-(2-nitrophenylthio)acetate by electrochemical reduction have been used as key steps in the synthesis of DIBTA and the corresponding lactam [125]. The hemiacetal function results in all pathways by hydrolysis from halogen precursor. 

Aminoarylhydrazones can be prepared by selective reduction of nitroarylhydrazones by catalytic transfer hydrogenation using... 

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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