**6. Practical industrial applications of asymmetric transfer hydrogenation**

Even though asymmetric transfer hydrogenation was originally merely a subject of academic interest, nowadays this reaction finds its use also in several sectors of chemical industry. There are a large number of optically active amines and alcohols used as active substances and a suitable method for ATH preparation. As any method or technology, ATH inclusively has its pros and cons. The indisputable advantages, compared to asymmetric hydrogenation by gaseous hydrogen, are primarily the catalyst stability under air conditions (AH catalyst typically contains phosphine ligands, which easily undergo oxidation by atmospheric oxygen) and second, avoiding the use of gaseous hydrogen. These two facts actually permit testing many structurally different catalysts in a relatively short period of time. Eliminating the use of pressure hydrogen and thus the demanding apparatus for reactions at a high pressure noticeably simplifies the production facilities, which is an important aspect from the economical point of view.

Relatively small values of turnover frequencies (TOF) present one of the major drawbacks compared to AH catalysts. Extending the reaction time or using a higher amount of the catalyst represent potential solutions (since the catalysts for ATH are inexpensive compared to AH, this solution is economically acceptable). Increasing the reaction temperature is, however, not the optimal solution, since enantioselectivity is decreasing with a higher temperature. Eliminating metal residues from the product is not an issue in these days as applying commercially available methods allows reducing the number of residues to units of *ppm*.

In 1997, Avencia Company patented rhodium catalysts bearing diamine, aminoalcohol ligands, respectively, for asymmetric transfer hydrogenation of imines and ketones. These Rh complexes are analogues to original Noyori's ruthenium catalysts. However, except for the central atom, these catalysts also differ in their aromatic ligand, which is in the most cases η5 pentamethylcyclopentadienyl. These catalysts are used for the production of several different types of chiral alcohols and amines. To name some examples, one of the running processes is ATH of tetralone to (*R*)-1-tetralol with the capacity of 200 dm3 and the corresponding yield of 95 and 97% *ee*, than the process of preparation of (*S*)-1-(4-flurophenyl)ethanol with its yield of 85 and 98.4% *ee* or the production of (*R*)-N-diphenylphosphinyl-1-methylamine with 95% yield and 99% *ee* [26].

Asymmetric transfer hydrogenation has the potential to find use also in the production of fine chemicals such as drugs, where a high optical purity of the final products is demanded. To provide an example, the preparation of the precursor for the synthesis of muscle relaxant, mivacurium-chloride, can be mentioned [27]. In this case, the application of ATH for the preparation of mivacurium-chloride seems to be a more favorable, since the cleavage used in the classical preparation, capitalizing on using *L*-dibenzoyltartaric acid to separate both enantiomers, is rather ineffective and produce a lot of nonrecyclable waste.
