Why Are BINOL-Based Monophosphites Such Efficient Ligands in Rh-Catalyzed Asymmetric Olefin Hydrogenation?

Probing machine learning models based on high throughput experimentation data for the discovery of asymmetric hydrogenation catalysts

Enantioselective hydrogenation of olefins by Rh-based chiral catalysts has been extensively studied for more than 50 years. Naively, one would expect that everything about this transformation is known and that selecting a catalyst that induces the desired reactivity or selectivity is a trivial task. Nonetheless, ligand engineering or selection for any new prochiral olefin remains an empirical trial-error exercise. In this study, we investigated whether machine learning techniques could be used to accelerate the identification of the most efficient chiral ligand. For this purpose, we used high throughput experimentation to build a large dataset consisting of results for Rh-catalyzed asymmetric olefin hydrogenation, specially designed for applications in machine learning. We showcased its alignment with existing literature while addressing observed discrepancies. Additionally, a computational framework for the automated and reproducible quantum-chemistry based featurization of catalyst structures was created. Together with less computationally demanding representations, these descriptors were fed into our machine learning pipeline for both out-of-domain and in-domain prediction tasks of selectivity and reactivity. For out-of-domain purposes, our models provided limited efficacy. It was found that even the most expensive descriptors do not impart significant meaning to the model predictions. The in-domain application, while partly successful for predictions of conversion, emphasizes the need for evaluating the cost-benefit ratio of computationally intensive descriptors and for tailored descriptor design. Challenges persist in predicting enantioselectivity, calling for caution in interpreting results from small datasets. Our insights underscore the importance of dataset diversity with broad substrate inclusion and suggest that mechanistic considerations could improve the accuracy of statistical models.

Paving the road towards automated homogeneous catalyst design

In the past decade, computational tools have become integral to catalyst design. They continue to offer significant support to experimental organic synthesis and catalysis researchers aiming for optimal reaction outcomes. More recently, data-driven approaches utilizing machine learning have garnered considerable attention for their expansive capabilities. This Perspective provides an overview of diverse initiatives in the realm of computational catalyst design and introduces our automated tools tailored for high-throughput in silico exploration of the chemical space. While valuable insights are gained through methods for high-throughput in silico exploration and analysis of chemical space, their degree of automation and modularity are key. We argue that the integration of data-driven, automated and modular workflows is key to enhancing homogeneous catalyst design on an unprecedented scale, contributing to the advancement of catalysis research.

Dynamic Thermodynamic Resolution of Racemic 1,1'-Binaphthyl-2,2'-diol (BINOL)

A dynamic thermodynamic resolution method for converting (R/S)-BINOL (1,1'-binaphthyl-2,2'-diol) into (R)-BINOL in 100% theoretical yield is reported. This technique involves mixing (R/S)-BINOL with N-benzyl cinchonidinium bromide (1 equiv) and a [Cu2(tmeda)2(μ-OH)2]Br2 (2.5 mol %) redox catalyst in acetonitrile. In the background of this process is the observation that the energy for atropoisomerization decreases significantly when an electron is removed from BINOL. Therefore, it is possible to convert both enantiomers into the thermodynamically favorable [N-benzyl cinchonidinium bromide·(R)-BINOL] adduct.

Metal-Free Heterogeneous Asymmetric Hydrogenation of Olefins Promoted by Chiral Frustrated Lewis Pair Framework

The development of metal-free and recyclable catalysts for significant yet challenging transformations of naturally abundant feedstocks has long been sought after. In this work, we contribute a general strategy of combining the rationally designed crystalline covalent organic framework (COF) with a newly developed chiral frustrated Lewis pair (CFLP) to afford chiral frustrated Lewis pair framework (CFLPF), which can efficiently promote the asymmetric olefin hydrogenation in a heterogeneous manner, outperforming the homogeneous CFLP counterpart. Notably, the metal-free CFLPF exhibits superior activity/enantioselectivity in addition to excellent stability/recyclability. A series of in situ spectroscopic studies, kinetic isotope effect measurements, and density-functional theory computational calculations were also performed to gain an insightful understanding of the superior asymmetric hydrogenation catalysis performances of CFLPF. Our work not only increases the versatility of catalysts for asymmetric catalysis but also broadens the reactivity of porous organic materials with the addition of frustrated Lewis pair (FLP) chemistry, thereby suggesting a new approach for practical and substantial transformations through the advancement of novel catalysts from both concept and design perspectives.

Leveraging Chiral Zr(IV)-Based Metal-Organic Frameworks To Elucidate Catalytically Active Rh Species in Asymmetric Hydrogenation Reactions

One of the most widely employed strategies to produce chiral molecules involves the asymmetric hydrogenation of functionalized olefins using rhodium catalysts. Despite their excellent performance, the exact identity of the active Rh species is still ambiguous as each site may plausibly feature one or two phosphorus ligands. In this work, we used a sequential postsynthetic modification approach to successfully incorporate single-site Rh species into a zirconium-based metal-organic framework comprised of chiral spinol-based ligands. These Rh species feature one phosphorus ligand per Rh, which contrasts with the molecular analogue that contains two phosphorus ligands per Rh site. Following extensive characterization of the Rh-monophosphorus material using techniques including solid-state NMR and extended X-ray absorption fine-structure (EXAFS) spectroscopy, we studied their catalytic performance in the asymmetric hydrogenations of enamides and α-dehydroamino acid esters and observed excellent yields and enantioselectivities (up to 99.9% ee). Notably, the Rh-monophosphorus catalyst is 5 times more active than the homogeneous Rh-biphosphorus control, which we attributed to the higher activity of the single-site Rh-monophosphorus species and the confined MOF cavities that can enrich reactants. In addition, we observed a unique topology-dependent behavior in which linker expansion leads to the formation of a novel Zr-MOF with a distinct 4,8-connected net that cannot be phosphorylated, presumably due to intense tensile strain and steric repulsion present within this framework. Finally, we demonstrate the utility of this single-site Rh-monophosphorus catalyst in the gram-scale synthesis of (R)-cinacalcet hydrochloride, a first-in-class drug in the therapy of secondary hyperparathyroidism and parathyroid carcinoma, with 99.1% ee.

Au(I)/(R)-BINOL-Ti(IV) Concerted Catalyzed Asymmetric Cascade Cycloaddition Reaction of Arylalkynols

An efficient catalytic asymmetric cascade cycloaddition reaction of arylalkynols with dioxopyrrolidines was developed. This reaction was achieved using Au(I) and (R)-BINOL-Ti(IV) bimetallic catalysts and exclusively delivered a series of chiral oxo-bridged bicyclic benzooxacine compounds in up to 86% yield with 96% ee as well as >33:1 dr. Meanwhile, three new σ bonds and three new stereogenic centers were formed in a one-pot process.

How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5

Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C₄–C₈ alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy barriers for various octene β-scission reactions, determined via umbrella sampling simulations at operating temperature (773 K). Cracking modes A (3° → 3°) and B₂ (3° → 2°) are expected to be dominant at operating conditions, whereas modes B₁ (2° → 3°), C (2° → 2°), D₂ (2° → 1°), and E₂ (3° → 1°) are expected to be less important. All β-scission modes in which a transition state with primary carbocation character is involved have high intrinsic free energy barriers. Reactions starting from secondary carbenium ions will contribute less as these intermediates are short living at the high cracking temperature. Our results show the importance of simulations at operating conditions to properly evaluate the carbenium ion stability for β-scission reactions and to assess the mobility of all species in the pores of the zeolite.

Application of Q2MM to predictions in stereoselective synthesis

Quantum-Guided Molecular Mechanics (Q2MM) can be used to derive transition state force fields (TSFFs) that allow the fast and accurate predictions of stereoselectivity for a wide range of catalytic enantioselective reactions. The basic ideas behind the derivation of TSFFs using Q2MM are discussed and the steps involved in obtaining a TSFF using the Q2MM code, publically available at github.com/q2mm, are shown. The applicability for a range of reactions, including several non-standard applications of Q2MM, is demonstrated. Future developments of the method are also discussed.

Highly efficient solid catalysts for asymmetric hydrogenation fabricated via facile adsorption of Rh–MonoPhos on porous silicas

Via a facile adsorption strategy, highly efficient solid catalysts for asymmetric hydrogenation of itaconic acid dimethyl ester were fabricated.

Tunable asymmetric catalysis through ligand stacking in chiral rigid rods

Chiral benzene-1,3,5-tricarboxamide (BTA) ligands, comprising one diphenylphosphino group and one or two remote chiral 1-methylheptyl side chains, were evaluated in the rhodium-catalyzed asymmetric hydrogenation of dimethyl itaconate. Despite the fact that the rhodium atom and the chiral center(s) are separated by more than 12 covalent bonds, up to 82% ee was observed. A series of control and spectroscopic experiments confirmed that the selectivity arises from the formation of chiral helical polymers by self-association of the BTA monomers through noncovalent interactions. The addition of a phosphine-free chiral BTA, acting as a comonomer for the chiral BTA ligands, increases the level of enantioselectivity (up to 88% ee). It illustrates how the selectivity of the reaction can be increased in a simple fashion by mixing two different BTA monomers. The concept was further probed by performing the same experiment with an achiral BTA ligand, i.e. a phosphine-functionalized BTA that contains two remote octyl side chains. It afforded an encouraging 31% ee, thus demonstrating the catalytically relevant transfer of chirality between the self-assembled units. It constitutes a unique example of the sergeants-and-soldiers principle applied to catalysis.

Rhodium-catalyzed asymmetric hydrogenation of olefins with PhthalaPhos, a new class of chiral supramolecular ligands

A library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (PhthalaPhos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2-acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1-yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3-phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect.

Hybrid bidentate phosphorus ligands in asymmetric catalysis: privileged ligand approach vs. combinatorial strategies

In this perspective the development of chiral phosphorus ligands for asymmetric catalysis is discussed, with a special focus on hybrid bidentate phosphorus ligands, in particular phosphine-phosphoramidites. An attempt is made to compare privileged ligand and combinatorial approaches to ligand development--for which the class of phosphine-phosphoramidite ligands is well suited--highlighting differences, similarities and their complementary use.