Enantioselective Nickel-Catalysed Hydrogenation Reactions
The ninth chapter of the book collects the advances in enantioselective nickel-catalysed hydrogenation reactions reported since 2004. Homogeneous asymmetric hydrogenation of ketones has a 40 year history. In particular, the BINAP–ruthenium/Brønsted acid combined catalyst and the BINAP–ruthenium/diamine ternary catalyst revolutionised the asymmetric hydrogenation of ketones in 1987 and 1995, respectively. Although the complementary use of these two catalysts covers a wide range of ketonic substrates, there is still no universal catalyst for the asymmetric hydrogenation of ketones. Because the production of chiral secondary alcohols through this methodology is so important in asymmetric synthesis, the efficiency of asymmetric hydrogenation of ketones remains challenging. At present, the vast majority of catalysts are based on precious metals, including ruthenium, osmium, rhodium, iridium, and palladium. Replacement of these expensive and toxic elements with more abundant base metals such as nickel should be thoroughly investigated from the viewpoints of cost. Among very good results involving nickel catalysts reported in the last few years is the first use of homogeneous chiral nickel–phosphine complexes in the asymmetric hydrogenation of α-amino-β-keto ester hydrochlorides, achieved in excellent stereoselectivities. In the area of asymmetric hydrogenation of alkenes, there is also a renewed interest in developing cheap, abundant, and less toxic metals. In this context, remarkable results were recently reported with the first highly enantioselective (transfer) hydrogenation of β-acetamidoacrylates using nickel catalysts.