Ligand Self-Sorting and Nonlinear Effects in Dinuclear Asymmetric Hydrogenation: Complexity in Catalysis

Abstract

Nature has been a source of inspiration for scientists as billion years of evolution have resulted in magnificent examples of how processes can be controlled efficiently. In the field of supramolecular catalysis, enzymes have been the major source of inspiration. As such, many synthetic systems have been prepared to mimic certain aspects of enzymes, with a strong focus on connecting catalytically active sites to cavities or binding sites having affinity for the substrate.[1, 2] Although such approaches have resulted in interesting new tools to control selectivity,[3–5] we are not nearly close to the abilities of Nature to control chemical transformations. One of the major differences between Nature and synthetic approaches is that in biological systems, chemical processes take place in a complex out-of-equilibrium environment.[6] In a cell, many chemical transformations occur simultaneously or in controlled sequence, involving an impressive number of different components. An important challenge for biologists is to understand how this organized complexity leads to emerging properties. As a result, new fields such as systems biology[7] including non-equilibrium thermodynamics[ 8] have been developed. Synthetic chemists traditionally aim for systems that are as clean and pure as possible,[9] and it is only recently that complexity in chemical systems received attention.[10–12] Despite the interesting perspectives, complexity in homogeneous catalysis has not been a research focus in itself, although many aspects related to complexity have been reported. Product inhibition and catalyst poisoning are relevant to complex chemistry as they imply processes with feedback loops. Dynamic catalyst libraries and combinatorial catalysis require selection procedures,[6, 13] and nonlinear effect (NLE) in asymmetric catalysis[14–16] can lead to emerging properties. With this in mind we decided to study in detail NLE using dinuclear hydrogenation catalysts C that have four chiral ligands. Application of nonenantiopure ligands can lead to formation of heterochiral complexes (with more than two chiral ligands) with different properties and these complexes can theoretically be more active and selective than their homochiral analogues.[16] Catalytically active complexes with four chiral ligands are rare, and for hydrogenation complex C is, to the best of our knowledge, the only example. The use of a racemic ligand could lead to the formation of ten stereoisomers of C (Figure 1).