Conjugated Transfer Hydrogenations

In disproportionation, rosin is heated over a catalyst to transfer hydrogen, yielding dehydro (5) and dihydro (8) resin acids. The dehydro acids are stabilized by the aromatic ring the dihydro acids contain only an isolated double bond in place of the less stable conjugated double bonds. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

More recently, using the cyclometallated iridium C,(7-benzoate derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP, catalytic carbonyl crotylation employing 1,3-butadiene from the aldehyde, or alcohol oxidation was achieved under transfer hydrogenation conditions [274]. Carbonyl addition occurs with roughly equal facility from the alcohol or aldehyde oxidation level. However, products are obtained as diastereomeric mixtures. Stereoselective variants of these processes are under development. It should be noted that under the conditions of ruthenium-catalyzed transfer hydrogenation, conjugated dienes, including butadiene, couple to alcohols or aldehydes to provide either products of carbonyl crotylation or p,y-enones (Scheme 16) [275, 276]. 

Transfer hydrogenation of dienes to monoenes 1,5-Cyclooctadiene is selectively reduced to cyclooctene by transfer hydrogenation with isopropanol catalyzed by this metal carbonyl cluster. The first step is isomerization to conjugated diene isomers. 1,5-Hexadiene is reduced under these conditions to frms-3-hexene (19%), os-2-hexene (21%), trans-2-, and cw-3-hexene (56%). Ru3(CO)i2, Os3(CO)12, and Ir4(CO)i2 catalyze isomerization of 1,5-cyclooctadiene, but are less active than Rh6(CO)i6 for transfer hydrogenation. 

Transfer hydrogenation. - Mediated by Pd/C, alkenes readily accept hydrogen from ammonium formate. Conjugated carbonyl compounds are more readily reduced. ... 

The transfer hydrogenation of 3-substituted quinolines was also studied, with the particularity that the stereogenic center was generated in this case during the conjugate addition step (Scheme 4.60). ° The reaction needed for a more sterically demanding catalyst such as partially hydrogenated derivative 60f and, in addition, the nature of the Hantzsch ester required further optimization in order to achieve the best enantioselectivities, which still remained in values around 80% ee. In this case, the scope of the reaction was limited to the use of quinolines with aromatic or heteroaromatic substituents at the 3-position. 

Within the context of chiral secondary amine-catalyzed conjugate reduction, the polyethylene glycol grafted polystyrene resin-supported peptide 18 was developed as an effective catalyst for the 1,6-selective transfer hydrogenation of 3-methyl-2,4-dienals with Hantzsch ester, followed by enantioselective 1,4-reduction (Scheme 14) [26]. The combination of the five terminal residues of the catalyst. 

---