Access to P- and Axially Chiral Biaryl Phosphine Oxides by Enantioselective (CpIrIII)-Ir-x-Catalyzed C-H Arylations

An enantioselective C-H arylation of phosphine oxides with o-quinone diazides catalyzed by an iridium(III) complex bearing an atropchiral cyclopentadienyl (Cp-x) ligand and phthaloyl tert-leucine as co-catalyst is reported. The method allows access to a) P-chiral biaryl phosphine oxides, b) atropo-enantioselective construction of sterically demanding biaryl backbones, and also c) selective assembly of axial and P-chiral compounds in excellent yields and diastereo- and enantioselectivities. Enantiospecific reductions provide monodentate chiral phosphorus(III) compounds having structures and biaryl backbones with proven importance as ligands in asymmetric catalysis.

Development of Catalytic Asymmetric C-H Functionalizations with Chiral Cyclopentadienyl Iridium(III) Complexes

Yun-Suk Jang

Cp*Ir(III) catalyzed C-H functionalization is a growing field, since it provides novel complementary reactivities, but enantioselective C-H functionalization reactions with this metal remain a niche. This thesis investigates CpXIr(III) catalyzed asymmetric, ortho-directed C-H functionalization reactions. While exploring an efficient enantioselective C-H amidation process, several chiral carboxylic acids were evaluated. A pronounced matched/ mismatched phenomenon between a chiral CpXIr(III) catalyst and (S)- and (R)-phthaloyl-tert leucine was found. This phenomenon was exploited to access highly enantioenriched P-chiral compounds. The corresponding phosphines are highly promising candidates as ligands for asymmetric catalysis. The enantioselective C(sp2)-H amidation of prochiral biaryl-phosphorus(V) compounds with various sulfonyl azides furnished the corresponding P-chiral products with up to 95% yield and up to 99:1 er. The cooperative system of CpXIr(III) and (S)-phthaloyl-tert leucine catalyzes a highly enantioselective C-H arylation reaction of phosphine oxides with ortho-quinone diazides. The catalytic reaction furnished the corresponding P-chiral phosphine oxides in up to 98% yield with 98.5:1.5 er. By using 1,2 diazonaphthol derivatives for the asymmetric C-H arylation reaction of phosphine oxides, a stable chiral axis was successfully implemented, giving access to P-chiral and axially chiral diastereomeric phosphorus(V) compounds with high selectivities and yields. Purely axially chiral phosphine oxides were also provided via an atropo-enantioselective C-H functionalization with up to 96:4 er and up to 95% yield. Additionally, chiral phosphorus(V) compounds were alkylated and reduced to their corresponding chiral phosphorus(III) compounds, without erosion of optical purity.

EPFL2019

Neutral chiral cyclopentadienyl Ru(II)Cl catalysts enable enantioselective [2+2]-cycloadditions

Nicolai Cramer, David Kossler

Cyclopentadienyl ruthenium(II) complexes with a large number of available coordination sites are frequently used catalysts for a broad range of transformations. To be able to render these transformations enantioselective, we have designed a chiral neutral (CpRu)-Ru-x(II) Cl complex basing on an atropchiral cyclopentadienyl (Cp-x) ligand which is accessed in a streamlined C-H functionalisation approach. The catalyst displays excellent levels of reactivity and enantioselectivity for enantioselective [2+2]-cycloadditions leading to strained chiral cyclobutenes, allowing for catalyst loadings as low as 1 mol%. A very strong counterion effect of a bound chloride anion transforms the corresponding unselective cationic complex into a highly enantioselective neutral version. Moreover, by adding norbornadiene at the end of the reaction the catalyst can be recovered and subsequently reused.

2017

Exploiting the chirality of intermetallic PdGa

Samuel Thomas Stolz

The intermetallic compound PdGa has recently attracted considerable interest for combining high catalytic activity with high reaction selectivity in symmetric heterogeneous catalysis, in particular semi-hydrogenation of acetylene and methanol steam reforming. However, its potential for asymmetric heterogeneous catalysis, for which its chiral bulk crystal structure predestines it, remained largely unexplored. The possibility to prepare clean, atomically well-defined, and bulk-truncated low Miller-index surfaces of chiral PdGa (Pd1Ga1) under ultra-high vacuum conditions paves the way to tackle several fundamental research questions in chemistry and physics under idealized conditions. In particular, the PdGa{111} surfaces and their unique properties are ideally suited to serve as model system to study ensemble (geometry) and ligand (electronic) effects and show the enormous potential of PdGa towards asymmetric heterogeneous catalysis. In this work, the focus is on obtaining fundamental insights into chirality transfer of intrinsically chiral surfaces on the electronic structure and on each of the principal steps of a catalytic reaction pathway by experimental surface analytical tools. With regard to the relation of the chiral crystal structure of PdGa and its electronic structure, crystals of the space group P213, such as PdGa, are predicted to exhibit peculiar symmetry-protected fermionic quasiparticles. Using ARPES, we were able to evidence such symmetry-protected fermionic quasiparticles in PdGa, to exhibit multifold band-crossings at the high-symmetry points G and R in the Brillouin zone are fermionic quasiparticles with topological charge of magnitude 4 The non-centrosymmetry of PdGa is not only manifested in the emergence of chiral Fermi-arcs, but its chirality governs also the interaction with molecules and its surfaces, which is expressed, for instance in unidirectional motion or enantioselective on-surface reactions. In particular, we have evidenced and induced directed rotation with a directionality of 98% of isolated small, achiral acetylene (C2H2) molecules on Pd3-terminated PdGa{111} surfaces by using LT-STM. The rotor of this exceptionally small motor consists of 4 atoms only and thus operates not only in the classical, electrically driven regime, but also in an unexpected quantum tunneling regime with persisting high directionality. This strong chirality transfer from the substrate to the molecule is also encountered in enantiospecific interactions with prochiral molecules which is a prerequisite for enantioselective control in on-surface reactions, i.e., asymmetric heterogeneous catalysis. With this focus, we have studied three different reactions on the two structurally different Pd1- and Pd3-terminated PdGa{111} surfaces with regard to enantiomeric excess and the relevance of ensemble effect (geometry) over the ligand effect (electronic structure): Enantioselective debromination of 5-Bromo-7Methylbenz(a)Anthracene (BMA) Nearly enantiopure homocoupling of 9-Ethynylphenanthrene (9-EP) Regio- and enantioselective azide-alkyne Huisgen cycloaddition between 3-(4-Azidophenyl)propionic azid (APA) and 9-EP Our results highlight the outstanding potential of intermetallic PdGa in numerous fields of research, specifically as a chiral topo-logical material with non-trivial bandstructure, as stator for molecular motors, or as template for asymmetric heterogeneous catalysis.

EPFL2020

Exploiting the chirality of intermetallic PdGa

Samuel Thomas Stolz

EPFL2020