Nonlinear Effects in Asymmetric Catalysis by Design: Concept, Synthesis, and Applications

Asymmetric synthesis constitutes a key technology for the preparation of enantiomerically pure compounds as well as for the selective control of individual stereocenters in the synthesis of complex compounds. It is thus of extraordinary importance for the synthesis of chiral drugs, dietary supplements, flavors, and fragrances, as well as novel materials with tunable and reconfigurable chiroptical properties or the assembly of complex natural products. Typically, enantiomerically pure catalysts are used for this purpose. To prepare enantiomerically pure ligands or organocatalysts, one can make use of the natural chiral pool. Ligands and organocatalysts with an atropisomeric biphenyl and binaphthyl system have become popular, as they are configurationally stable and contain a C2-symmetric skeleton, which has been found to be particularly privileged. For catalysts with opposite configurations, both product enantiomers can be obtained. Configurationally flexible biphenyl systems initially appeared to be unsuitable for this purpose, as they racemize after successful enantiomer separation and thus are neither storable nor afford a reproducible enantioselectivity. However, there are strategies that exploit the dynamics of such ligands to stereoconvergently enrich one of the catalyst enantiomers. This can be achieved, for example, by coordinating an enantiomerically pure additive to a ligand-metal complex, which results in deracemization of the configurationally flexible biphenyl system, thereby enriching the thermodynamically preferred diastereomer. In this Account, we present our strategy to design stereochemically flexible catalysts that combine the properties of supramolecular recognition, stereoconvergent alignment, and catalysis. Such systems are capable to recognize the chirality of the target product, leading to an increase in enantioselectivity during asymmetric catalysis. We have systematically developed and investigated these smart catalyst systems and have found ways to specifically design and synthesize them for various applications. In addition to (i) reaction product-induced chiral amplification, we have developed systems with (ii) intermolecular and (iii) intramolecular recognition, and successfully applied them in asymmetric catalysis. Our results pave the way for new applications such as temperature-controlled enantioselectivity, controlled inversion of enantioselectivity with the same chirality of the recognition unit, generation of positive nonlinear effects, and targeted design of autocatalytic systems through dynamic formation of transient catalysts. Understanding such systems is of enormous importance for catalytic processes leading to symmetry breaking and amplification of small imbalances of enantiomers and offer a possible explanation of homochirality of biological systems. In addition, we are learning how to target supramolecular interactions to enhance enantioselectivities in asymmetric catalysis through secondary double stereocontrol. Configurationally flexible catalysts will enable future resource-efficient development of asymmetric syntheses, as enantioselectivities can be fully switched by stereoselective alignment of the stereochemically flexible ligand core on demand.

Controlling the Enantioselectivity in an Adaptable Ligand by Biomimetic Intramolecular Interlocking

For the preparation of chiral drugs, both stereochemically stable and flexible catalysts in combination with chiral auxiliaries can be used. Here, chiral induction plays an important role in generating an enantiomerically pure catalyst. We demonstrate a successful approach to the spontaneous deracemization of tropos ligands for asymmetric catalysis. Three different constitutional isomers of a bisphosphinite ligand decorated with l-valine moieties (interaction units) linked to the flexible biphenyl system by a phenylene bridge for inducing a chiral switch were prepared. The substitution pattern's influence on the attached intermolecular recognition sites was systematically investigated. We can show that biomimetic intramolecular hydrogen bonding leads to a pronounced diastereoselective enrichment of one of the ligand stereoisomers. As a result, in the asymmetric Rh-catalyzed hydrogenation of prochiral olefins using these ligands, enantiomeric ratios of up to 95.8:4.2 (S) were obtained. Of particular note is the inversion of enantioselectivity relative to the previously reported BIBIPHOS-Rh catalyst due to the altered orientation of the biphenyl moiety from (R_ax) to (S_ax). The enantioselectivities achieved by appropriate intramolecular interlocking are remarkable for a tropos ligand/catalyst. The strategy presented here represents a powerful approach for the spontaneous alignment of tropos ligands, yielding high enantioselectivities in asymmetric catalysis.

Diastereoselective synthesis of a cyclic diamide-bridged biphenyl as chiral atropos ligand

Chiral compounds with a 1,2-diamine structure motif and their derivatives are of great interest in organic chemistry and are broadly used in asymmetric transformations, as chiral auxiliaries, (co)ligands, and ligand core structure. Here, we present a straightforward, diastereoselective synthesis for a diamide-bridged biaryl ligand. The ring closing reaction of the racemic atropos biphenyl 6,6'-dimethoxy-[1,1'-biphenyl]-2,2'-dicarboxylic acid with (R,R)-diaminocyclohexane yields the diasteromerically and enantiomerically pure cyclic (S_ax ,R,R)-BIPOL, which can be used as a versatile chiral ligand. By NMR spectroscopy, we observed the formation of intermolecular aggregates of the diamide-bridged BIPOL with anhydrous DMSO-d₆ . DFT calculations at the B3LYP/6-31G* level of theory corroborate the high interconversion barrier for the biaryl axis of ΔG^ǂ  = 148.7 kJ mol^-1 and the favoured formation of (S_ax ,R,R)-BIPOL as single stereoisomer.