Selective Reduction of Carbonyl Compounds via (Asymmetric) Transfer Hydrogenation on Heterogeneous Catalysts

Abstract

Based on the ever-increasing demand for optically pure compounds, the development of efficient methods to produce such products is very important. Homogeneous asymmetric catalysis occupies a prominent position in the ranking of chemical transformations, with transition metals coordinated to chiral ligands being applied extensively for this purpose. However, heterogeneous catalysts have the ability to further extend the field of asymmetric transformations, because of their beneficial properties such as high stability, ease of separation and regeneration, and the possibility to apply them in continuous processes. The main challenge is to find potential synthetic routes that can provide a chemically and thermally stable heterogeneous catalyst having the necessary chiral information, whilst keeping the catalytic activity and enantioselectivity equally high (or even higher) than the corresponding homogeneous counterpart. Within this short review, the most relevant immobilization modes and preparative strategies depending on the support material used are summarized. From the reaction scope viewpoint, metal catalysts supported on the various solid materials studied in (asymmetric) transfer hydrogenation of carbonyl compounds are selected and represent the main focus of the second part of this overview.1 Introduction2 Synthesis of Chiral Heterogeneous Catalysts2.1 Immobilization of Homogeneous Asymmetric Catalysts2.1.1 Immobilization on Inorganic Supports2.1.2 Immobilization on Organic Polymers as Supports2.1.3 Immobilization on Dendrimer-Type Materials as Supports2.1.4 Self-Supported Chiral Catalysts: Coordination Polymers2.1.5 Immobilization Using Non-Conventional Media2.2 Chirally Modified Metal Surfaces for Heterogeneous Asymmetric Catalysis3 Examples of Transfer Hydrogenation on Heterogeneous Catalysts3.1 Silicon-Immobilized Catalysts3.2 Carbon-Material-Immobilized Catalysts3.3 Polymer-Immobilized Catalysts3.4 Magnetic-Nanoparticle-Immobilized Catalysts4 Conclusions

Synthesis and Reactivity of Poly(propyleneimine) Dendrimers Functionalized with Cyclopentadienone N-Heterocyclic-Carbene Ruthenium(0) Complexes

“…In particular, the downfield-shifted resonance of the coordinated carbon atom of the carbene (δ = 174.80 ppm) appeared in the 13 C NMR spectrum. In addition, the imidazole moiety displays signals at chemical shifts ( 1 H NMR: δ = 8.03, 7.33, 7.08 ppm; 13 C NMR: δ = 136.95, 130.88, 116.97 ppm) close to those of the starting material CDI, while the C=O group resonates at 147.89 ppm, typical for a carbamate moiety (-OC(O)N-). In the infrared spectrum in CH2Cl2, CO stretching bands (νCO = 2007 and 1948 cm -1 ) are consistent with those of 1c, and the stretching at 1768 cm -1 is ascribable to the carbamate group formed during the reaction.…”

“…They were characterized by 1 H and 13 C NMR, IR spectroscopy and mass spectrometry, when feasible ( Figures S5-S21 in the SM). Considering the NMR spectra registered in CDCl 3 , they show the signals due to the diaminobutane-based PPIs skeleton ( 1 H: four broad multiplets in the range 3.8-1.3 ppm; 13 C: four resonances in the range 63-25 ppm) and those given by the anchored peripheral NHC ruthenium moieties (see Materials and Methods section). Even if all the proton signals appear as broad peaks typical of a polymer-like structure, it is clearly detectable that the resonances of the two methylene groups belonging to the -NHCCH 2 CH 2 OC(O)NHunit of the organometallic moieties are at 3.77 and 3.59 ppm, which are more shielded than those of 2 (4.05 and 3.99 ppm in -NHCCH 2 CH 2 OC(O)Im).…”

“…Dendritic catalysts are functional macromolecules with precise and unambiguous structures, which, thanks to their monodisperse nature, maintain the advantages of homogeneous catalysts showing fast kinetic responses and easy tunability [1][2][3][4][5][6][7][8][9][10][11][12]. Furthermore, they can be removed from the reaction mixture by membrane or nanofiltration techniques and precipitation, exploiting their bigger sizes compared to products, and this confers the advantages of heterogeneous catalysts [13]. Catalytic sites grafted on the periphery of dendrimers can give rise to active, multivalent species that might result in high reaction rates.…”

“…Both negative [24] and positive [25][26][27] dendritic effects have been observed when hydrogenation is carried out with increasing sizes of dendrimers. On the contrary, less examples regarding transfer hydrogenation are reported [13,28,29]. Worthy of note is the tetrabranched phosphoranyl-terminated carbosilane ruthenium(II) derivative able to catalyze the transfer hydrogenation of cyclohexanone, where cyclohexadiene or formic acid acts as stoichiometric hydrogen donor species.…”

“…3g3, prepared from g3 (0.021 g, 0.01 mmol) and 3 (0.17 g, 0.21 mmol), yield of 32% 1. H NMR (399.9 MHz, 298 K, CDCl 3 ): δ 7.76-6.61 (CH aryl ), 6.96 (m, 16H, CH NHC ), 6.78 (m, 16H, CH NHC ), 3.77 (m, 32H, -CH 2 -),3.71 (s, 96H, -OCH 3 ), 3.67 (m, 32H, -CH 2 -), 3.10 (s, 48H, -NCH 3 ), 3.20-1.50 (br, 176H, CH 2 , DAB-dendr) ppm 13. C NMR (100.6 MHz, 298 K, CDCl 3 ): δ 202.17 (C=O), 173.94 (C carbene ), 169.46 (C=O, Cp), 158.60 (-COCH 3 ), 153.57 (-OC(O)N-), 135.00 (Cq aryl ), 133.57 (CH aryl ), 129.29 (CH aryl ), 127.59 (CH aryl ), 125.52 (Cq aryl ), 124.54 (CH aryl ), 124.29 (CH NHC ), 122.38 (CH NHC ), 112.95 (CH aryl ), 103.98 (C2,5, Cp), 78.67 (C3,4, Cp), 63.69 (-CH 2 -), 55.01 (-OCH 3 ), 50.01 (-CH 2 -), 38.30 (CH 3 , NHC), 53.39, 51.39, 38.41, 26.78, 24.02 (CH 2 , DAB-dendr) ppm.…”