## 2 Applications of Chiral Nanoparticles
### Asymmetric Catalysis  
Catalysis is the process by which a chemical compound, the catalyst, is able to facilitate the conversion of the substrate into the product by lowering the energy of activation of the reaction. Eventually, the reaction can also be stereoselective if the catalyst is chiral. Chemical ways to obtain enantio-enriched, and eventually enantiopure, products have attracted great attention over the past decades, inspiring extraordinary development in enantioselective catalysis, for example for the synthesis of enantiomerically pure pharmaceuticals or chemical products. This research led in 2001 to the award of the Nobel Prize for Chemistry to [PERSON] and [PERSON], and [PERSON], for their work on asymmetric hydrogenation and oxidation reactions, respectively [43; 44]. Numerous approaches have been investigated for obtaining heterogeneous enantioselective catalysts that could associate high catalytic activity to appropriate stereochemical mechanism of reaction, the most encouraging comprising alteration of the catalytic metal surface by a chiral compound with strong adsorbing capacity [45; 46; 47]. The high interest in heterogeneous enantioselective catalysis is also revealed by the ever-increasing number of research articles and by the amount of reviews on this topic [48; 49; 50; 51]. More recently, nanotechnology has been also exploited for catalyzing chiral reactions as this modern approach can also help to override major hurdles of traditional catalyst,  
Figure 3: Mechanism of formation of helical structures for (L, L) and (D, D) threonine. Reprinted with permission from [34], Copyright Elsevier, 2016.  
for example purification [52]. The main finding about application of chiral NPs to asymmetric catalysis are summarized in Table 2.  
One of the most extensively investigated reactions is the carbonyl hydrogenation of \(\beta\)-ketoesters and \(\beta\)-diketoesters over Ni-based catalysts in the presence of a chiral converter. [PERSON] and co-workers adsorbed (\(R\),\(R\))-tartaric acid onto the surface of Ni NPs and exploited photoabsorption spectra to investigate in situ the chiral modification occurring during the catalysis reaction [53]. By means of scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) they proposed that the tartaric acid placed onto the surfaces of the NPs from aqueous solutions experiences a ketoenol tautomerization. In addition, no \"particle size effect\" of the adsorption type of the tartaric acid was observed among NPs with a diameter between 90 and 300 nm. Thermoregulated phase-separable Pt nanoparticle catalyst, functionalized with a cinchonidine derivative, also demonstrated excellent e.e. (>99%) in the enantioselective hydrogenation of activated ketones for synthesizing chiral \(\alpha\)-hydroxy acetals and chiral 1,2-diols [54]. The chiral catalyst could be easily removed by phase separation and straightly recycled in the following process without losing its catalytic activity or enantioselectivity, even in a gram-scale reaction. Hydroamination is another important synthetic method widely used for the preparation of chiral amines, very important functional groups in many APIs. [PERSON] and colleagues attached ZnCl\({}_{2}\) on chiral mesoporous silica (Z-n-CMS, \(n=1\), 3, 5 mmol) obtaining strong Lewis acid catalysts on solid support for the hydroamination of cyclohexene [55]. The homogeneously dispersed ZnCl\({}_{2}\) nanoparticles (\(\approx\)10 nm) presented catalytic synergistic outcome (>61% conversion) in the hydroamination of cyclohexene under mild conditions (60 \({}^{\circ}\)C) and the catalyst showed superior activity compared to other homogeneous and heterogeneous braced mesoporous catalysts seemingly because of the helical mesostructure that enhanced dispersion of the substrates and the availability of the Lewis acid sites. [PERSON] et al.