Enantiodivergent Pd-catalyzed C-C bond formation enabled through ligand parameterization

Identifying high-performance catalytic conditions for carbon dioxide reduction to dimethoxymethane by multivariate modelling

In times of a warming climate due to excessive carbon dioxide production, catalytic conversion of carbon dioxide to formaldehyde is not only a process of great industrial interest, but it could also serve as a means for meeting our climate goals. Currently, formaldehyde is produced in an energetically unfavourable and atom-inefficient process. A much needed solution remains academically challenging. Here we present an algorithmic workflow to improve the ruthenium-catalysed transformation of carbon dioxide to the formaldehyde derivative dimethoxymethane. Catalytic processes are typically optimised by comprehensive screening of catalysts, substrates, reaction parameters and additives to enhance activity and selectivity. The common problem of the multidimensionality of the parameter space, leading to only incremental improvement in laborious physical investigations, was overcome by combining elements from machine learning, optimisation and experimental design, tripling the turnover number of 786 to 2761. The optimised conditions were then used in a new reaction setup tailored to the process parameters leading to a turnover number of 3874, exceeding by far those of known processes.

Predictive Multivariate Linear Regression Analysis Guides Successful Catalytic Enantioselective Minisci Reactions of Diazines

The Minisci reaction is one of the most direct and versatile methods for forging new carbon-carbon bonds onto basic heteroarenes: a broad subset of compounds ubiquitous in medicinal chemistry. While many Minisci-type reactions result in new stereocenters, control of the absolute stereochemistry has proved challenging. An asymmetric variant was recently realized using chiral phosphoric acid catalysis, although in that study the substrates were limited to quinolines and pyridines. Mechanistic uncertainties and nonobvious enantioselectivity trends made the task of extending the reaction to important new substrate classes challenging and time-intensive. Herein, we describe an approach to address this problem through rigorous analysis of the reaction landscape guided by a carefully designed reaction data set and facilitated through multivariate linear regression (MLR) analysis. These techniques permitted the development of mechanistically informative correlations providing the basis to transfer enantioselectivity outcomes to new reaction components, ultimately predicting pyrimidines to be particularly amenable to the protocol. The predictions of enantioselectivity outcomes for these valuable, pharmaceutically relevant motifs were remarkably accurate in most cases and resulted in a comprehensive exploration of scope, significantly expanding the utility and versatility of this methodology. This successful outcome is a powerful demonstration of the benefits of utilizing MLR analysis as a predictive platform for effective and efficient reaction scope exploration across substrate classes.

Driving factors in amiloride recognition of HIV RNA targets

Noncoding RNAs are increasingly promising drug targets yet ligand design is hindered by a paucity of methods that reveal driving factors in selective small molecule : RNA interactions, particularly given the difficulties of high-resolution structural characterization. HIV RNAs are excellent model systems for method development given their targeting history, known structure-function relationships, and the unmet need for more effective treatments. Herein we report a strategy combining synthetic diversification, profiling against multiple RNA targets, and predictive cheminformatic analysis to identify driving factors for selectivity and affinity of small molecules for distinct HIV RNA targets. Using this strategy, we discovered improved ligands for multiple targets and the first ligands for ESSV, an exonic splicing silencer critical to replication. Computational analysis revealed guiding principles for future designs and a predictive cheminformatics model of small molecule : RNA binding. These methods are expected to facilitate progress toward selective targeting of disease-causing RNAs.

Preparation of Enantioenriched Alkylcarbastannatranes via Nucleophilic Inversion of Alkyl Mesylates for Use in Stereospecific Cross-Coupling Reactions

We report the preparation of enantioenriched secondary alkylcarbastannatranes via a stereoinvertive S_N2 reaction of enantioenriched alkyl mesylates and carbastannatranyl anion equivalents. Using this process, enantioenriched secondary alcohols may be converted into highly enantioenriched alkylcarbastannatranes, which are useful in stereospecific cross-coupling reactions.

Synthesis of Biaryls via Decarbonylative Palladium-Catalyzed Suzuki-Miyaura Cross-Coupling of Carboxylic Acids

The biaryl motif is a building block in many drugs, agrochemicals, and materials, and as such it is highly desirable as a synthesis target. The state-of-the-art process for biaryl synthesis from ubiquitous carboxylic acids is decarboxylative cross-coupling involving loss of carbon dioxide (CO₂). However, the scope of these methods is severely limited, mainly due to specific substitution required to promote decarboxylation. The present report implements a decarbonylative version with loss of carbon monoxide (CO) that enables to directly engage carboxylic acids in a Suzuki-Miyaura cross-coupling to produce biaryls as a general method with high cross-coupling selectivity using a well-defined Pd(0)/(II) catalytic cycle. This protocol shows a remarkably broad scope (>80 examples) and is performed in the absence of exogenous inorganic bases. In a broader context, the approach shows promise for routine applications in the synthesis of biaryls by carefully controlled decarbonylation of prevalent carboxylic acids.

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First decarbonylative Suzuki cross-coupling of carboxylic acids via Pd catalysis

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Rapid synthesis of functionalized biaryls from ubiquitous carboxylic acids

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Mechanistic insights from DFT studies point at the origin of high selectivity

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CO loss as a strategy for expanding access to aryl metals (cf. CO₂ loss)

Sustainable Knowledge-Driven Approaches in Transition-Metal-Catalyzed Transformations

The sustainable synthesis of relevant scaffolds for their use in the pharmaceutical, agrochemical, and materials sectors constitutes one of the most urgent challenges that the chemical community needs to overcome. In this context, the development of innovative and more efficient catalytic processes based on a fundamental understanding of the underlying reaction mechanisms remains a largely unresolved challenge for academic and industrial chemists. Herein, selected examples of computational and experimental knowledge-driven approaches for the rational design of transition-metal-catalyzed transformations are discussed.

Facile access to functionalized chiral secondary benzylic boronic esters via catalytic asymmetric hydroboration

Allylic and homoallylic phosphonates bearing an aryl or heteroaryl substituent at the γ- or δ-position undergo rhodium-catalyzed asymmetric hydroboration by pinacolborane to give functionalized chiral secondary benzylic boronic esters in yields up to 86% and enantiomer ratios up to 99 : 1. Compared to minimally-functionalized terminal and 1,1-disubstituted vinyl arenes, there are relatively few reports of efficient catalytic asymmetric hydroboration (CAHB) of more highly functionalized internal alkenes. Phosphonate substrates bearing a variety of common heterocyclic ring systems, including furan, indole, pyrrole and thiophene derivatives, as well as those bearing basic nitrogen substituents (e.g., morpholine and pyrazine) are tolerated, although donor substituents positioned in close proximity of the alkene can influence the course of the reaction. Stereoisomeric (E)- and (Z)-substrates afford the same major enantiomer of the borated product. Deuterium-labelling studies reveal that rapid (Z)- to (E)-alkene isomerization accounts for the observed (E/Z)-stereoconvergence during CAHB. The synthetic utility of the chiral boronic ester products is illustrated by stereospecific C-B bond transformations including stereoretentive electrophile promoted 1,2-B-to-C migrations, stereoinvertive S_E2 reactions of boron-ate complexes with electrophiles, and stereoretentive palladium- and rhodium-catalyzed cross-coupling protocols.

Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP)

Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst ( EHOMO), percent buried volume ( Vbur%), and distortion energy of the catalyst (Δ Edist), to describe the electronic, steric, and flexibility effects on reactivity, respectively. A robust and simple predictive model for ligand effect on reactivity is thereby established by correlating these three descriptors with experimental activation rate constants using multivariate linear regression. Validation using a structurally diverse set of ligands revealed the average error is less than ±2 kcal/mol compared to the experimentally derived activation energies. The same approach was also applied to develop a predictive model for reactivity of different alkyl halide initiators using R-X bond dissociation energy (BDE) and Cu-X halogenophilicity as descriptors.

Alkyl Carbagermatranes Enable Practical Palladium-Catalyzed sp2-sp3 Cross-Coupling

Pd-catalyzed cross-coupling reactions have achieved tremendous accomplishments in the past decades. However, C(sp³)-hybridized nucleophiles generally remain as challenging coupling partners due to their sluggish transmetalation compared to the C(sp²)-hybridized counterparts. While a single-electron-transfer-based strategy using C(sp³)-hybridized nucleophiles had made significant progress recently, fewer breakthroughs have been made concerning the traditional two-electron mechanism involving C(sp³)-hybridized nucleophiles. In this report, we present a series of unique alkyl carbagermatranes that were proven to be highly reactive in cross-coupling reactions with our newly developed electron-deficient phosphine ligands. Generally, secondary alkyl carbagermatranes show slightly lower, yet comparable activity to its Sn analogue. Meanwhile, primary alkyl carbagermatranes exhibit high activity, and they were also proved stable enough to be compatible with various reactions. Chiral secondary benzyl carbagermatrane gave the coupling product under base/additive-free conditions with its configuration fully inversed, suggesting that transmetalation was carried out in an "S_E2(open) Inv" pathway, which is consistent with Hiyama's previous observation. Notably, the cross-coupling of primary alkyl carbagermatranes could be performed under base/additive-free conditions with excellent functional group tolerance and therefore may have potentially important applications such as stapled peptide synthesis.

Access to P-chiral phosphine oxides by enantioselective allylic alkylation of bisphenols

A novel biscinchona alkaloid-catalyzed highly enantioselective desymmetrization reaction of bisphenol compounds with achiral Morita-Baylis-Hillman carbonate agents was developed. Through the asymmetric allylic alkylation strategy, a broad range of optically active P-stereogenic phosphine oxides were generated with excellent to good yields (up to 99%) and high enantioselectivities (up to 98.5 : 1.5 e.r.). The reaction was further investigated by the linear free energy relationship (LFER) analysis. A possible transition state was proposed and furthered verified by theoretical calculations.

Axial shielding of Pd(II) complexes enables perfect stereoretention in Suzuki-Miyaura cross-coupling of Csp3 boronic acids

Stereocontrolled Csp³ cross-coupling can fundamentally change the types of chemical structures that can be mined for molecular functions. Although considerable progress in achieving the targeted chemical reactivity has been made, controlling stereochemistry in Csp³ cross-coupling remains challenging. Here we report that ligand-based axial shielding of Pd(II) complexes enables Suzuki-Miyaura cross-coupling of unactivated Csp³ boronic acids with perfect stereoretention. This approach leverages key differences in spatial orientation between competing pathways for stereoretentive and stereoinvertive transmetalation of Csp³ boronic acids to Pd(II). We show that axial shielding enables perfectly stereoretentive cross-coupling with a range of unactivated secondary Csp³ boronic acids, as well as the stereocontrolled synthesis of xylarinic acid B and all of its Csp³ stereoisomers. We expect these ligand design principles will broadly enable the continued search for practical and effective methods for stereospecific Csp³ cross-coupling.

Despite the progress in C(sp³) cross-coupling reactions, full control over the stereochemistry remains a challenge. Here, the authors show that phosphine-containing axially shielded Pd(II) complexes enable Suzuki-Miyaura cross-couplings of unactivated C(sp³) boronic acids with perfect stereoretention.

Enantiospecific Synthesis of ortho-Substituted 1,1-Diarylalkanes by a 1,2-Metalate Rearrangement/anti-SN 2' Elimination/Rearomatizing Allylic Suzuki-Miyaura Reaction Sequence

The one-pot sequential coupling of benzylamines, boronic esters, and aryl iodides has been investigated. In the presence of an N-activator, the boronate complex formed from an ortho-lithiated benzylamine and a boronic ester undergoes stereospecific 1,2-metalate rearrangement/anti-SN 2' elimination to form a dearomatized tertiary boronic ester. Treatment with an aryl iodide under palladium catalysis leads to rearomatizing γ-selective allylic Suzuki-Miyaura cross-coupling to generate 1,1-diarylalkanes. When enantioenriched α-substituted benzylamines are employed, the corresponding 1,1-diarylalkanes are formed with high stereospecificity.