Palladium nanocatalyst

Dynamic Kinetic Resolution of Primary Amines with a Recyclable Palladium Nanocatalyst (Pd/A10(0H)) for Racemization... 

Kim et al. have developed a practical procedure for the DKR of primary amines illustrated by substrate 56c (Scheme 2.28). They employed a supported palladium nanocatalyst as the racemization catalyst and commercially available CALB as the enantioselective catalyst for acylation of the amine using ethyl methoxyacetate as the acyl donor. High yields and enantiomeric excesses were achieved [30]. 

Fig. 25. (a) SEM-BSE image and (b) STEM-HAADF image of palladium nanocatalysts on a carbon support (119). 

Changing Catalytic Selectivity Atom by Atom Polymerization of Acetylene on Palladium Nanocatalysts... 

Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this astonishing catalytic selectivity is still lacking. In this example, we illustrate such a size-dependent selectivity with the polymerization of acetylene on palladium nanocatalysts [46]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [66]. In ultra-high vacuum (UHV) [67] as well as under high pressure, large palladium particles of typically thousands of atoms show an increased selectivity for the formation of benzene with increasing particle size [66]. In contrast, small palladium particles of typically hundreds of atoms are less selective for the cyclotrimerization, and catalyze butadiene and butene as additional products [66]. 

Similarly impressive results were obtained in the DKR of P-amino esters using a combination of GALA and a palladium nanocatalyst the product amides were obtained in high yield (up to 97%) and high ee (up to 99%) (Scheme 4.33) [84]. 

Scheme 4.33 DKR of p-amino esters using palladium nanocatalyst.

Engstroem, K., Shakeri, M., and Baeckvall, J.-E. (2011) Dynamic kinetic resolution of p-amino esters by a heterogeneous system of a palladium nanocatalyst and Candida antarctica lipase A. Eur. J. Org. Chem., 2011 (10), 1827-1830, S1827/1-S1827/5. 

In 2007, Kim et al. reported the direct DKR of primary amines using a recyclable palladium nanocatalyst combined with a lipase in the presence of ethyl acetate or ethyl methoxyacetate as the acyl donor. As shown in Scheme 4.52, a series of primary amines and one amino acid amide were efficiently resolved with good yields and high enantioselectivities. The catalyst, Pd/A10(OH), was prepared as palladium nanoparticles entrapped in aluminium hydroxide. Because this catalyst was highly thermostable, the DKR reactions could be operated at 100°C with multiple recycling of the catalyst. 

Scheme 4.52 DKRs of primary amines with a palladium nanocatalyst.

Palladium nanocatalyst [Pd°/AlO(OH)] was shown by Kim et al. to be able to catalyse, in combination with lipase Novozym 435, the DKR of primary benzyl amines. As shown in Scheme 8.74, the corresponding chiral amides were produced in remarkable yields and enantioselectivities ranging from 90 to >99% ee. 

In the same area, the DKR of p-amino esters, including aliphatic, aromatic as well as heteroaromatic ones, was performed hy Backvall et al, using a palladium nanocatalyst [Pd°/AlO(OH)] in combination with immobilised Candida antarctica lipase A (CALA/GAmP-MCF), providing the corresponding chiral amides in both excellent yields and enantioselectivities, as shown in Scheme 8.75. ... 

In another area, the asymmetric reductive acylation of ketoxime of m-methoxyacetophenone was developed by Kim and co-workers, in 2010. This process was catalysed by a combination of lipase CALB with a palladium nanocatalyst in the presence of ethyl methoig acetate as an acyl donor, molecular sieves in toluene at 70 °C under 0.1 bar of hydrogen pressure. It allowed the formation of the corresponding almost enantiopure amide in high yields of up to 91% and with enantioselectivity of 98% ee, as shown in Scheme 8.77. The utility of this novel methodology was applied to the total synthesis of the calcimimetic (-l-)-NPS R-568. 

Different N-donor compounds have already proved their efficiency as protective agents in the case of colloidal suspensions in non-aqueous ionic liquids. As an example, the synthesis of phenanthroline stabilized palladium nanocatalysts and their use for olefin hydrogenation in l-butyl-3-methylimidazolium hexafluorophosphate media was reported (Huang et al., 2003). 

Engstrom K, Shaken M, Backvall J-E. Dynamic kinetic resolution of -amino esters by a heterogeneous system of a palladium nanocatalyst and Candida antarctica lipase A. Eur. 

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