Explaining the Disparate Stereoselectivities of N-Oxide Catalyzed Allylations and Propargylations of Aldehydes

Mechanism and Enantioselectivity in QUINOX-Catalyzed Asymmetric Allylations of Aromatic Aldehydes: Solvent and Substituent Effects

Computational investigations were conducted on the QUINOX-catalyzed asymmetric allylation of aromatic aldehydes with allyltrichlorosilanes. Our calculations provide evidence that the catalytic allylation can follow distinct mechanisms, depending on the solvent employed. In toluene and CH2Cl2, the QUINOX-catalyzed allylation predominantly follows an associative pathway, while in CH3CN, a dissociative pathway becomes more favorable. Noncovalent interactions, such as π-stacking effects for the associative mechanism and CH/π interactions for the dissociative mechanism, play a pivotal role in enantiostereodifferentiation in the asymmetric QUINOX-catalyzed reactions of benzaldehyde. Furthermore, the study unveils how different aldehyde substituents exert differing influences on the catalytic allylation reaction. Specifically, the QUINOX-catalyzed allylation of 4-(trifloromethyl)benzaldehyde displays a strong preference for the associative pathway, yielding excellent results in both yield and enantioselectivity. Conversely, 4-methoxybenzaldehyde tends to favor a dissociative mechanism with reduced yields and enantioselectivity. The mechanistic basis for these remarkable substituent effects on the catalytic allylation reaction was also elucidated. In summary, this research enhances our understanding of the QUINOX-catalyzed asymmetric allylation, shedding light on the role of solvents and substituents in the reaction mechanism and enantioselectivity.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Portable Models for Entropy Effects on Kinetic Selectivity

Differences in entropies of competing transition states can direct kinetic selectivity. Understanding and modeling such entropy differences at the molecular level is complicated by the fact that entropy is statistical in nature; i.e., it depends on multiple vibrational states of transition structures, the existence of multiple dynamically accessible pathways past these transition structures, and contributions from multiple transition structures differing in conformation/configuration. The difficulties associated with modeling each of these contributors are discussed here, along with possible solutions, all with an eye toward the development of portable qualitative models of use to experimentalists aiming to design reactions that make use of entropy to control kinetic selectivity.

Mechanism-Dependent Selectivity: Fluorocyclization of Unsaturated Carboxylic Acids or Alcohols by Hypervalent Iodine

To understand the unprecedented difference between 6-endo and 5-exo selectivity in hypervalent iodine (III) promoted fluorocyclization of unsaturated carboxylic acids or alcohols by difluoroiodotoluene, density functional theory (DFT) studies have been performed to systematically compare both the previous proposed “fluorination first and cyclization later” mechanism and the alternative “cyclization first and fluorination later” mechanism. Our results revealed that the selectivity is mechanism-dependent. The unsaturated alcohol prefers the fluorination first and the 6-endo-tet cyclization later pathway, leading to the experimentally observed 6-endo ether product. In contrast, the unsaturated carboxylic acid plausibly undergoes the 5-exo-trig cyclization first and the fluorination later to the experimentally observed 5-exo lactone product. The pKa property of the functional group of the substrate is found to play a key role in determining the reaction mechanism. The provided insights into the mechanism-dependent selectivity should help advance the development of fluorocyclization reactions with hypervalent iodine reagents.

Steps toward Rationalization of the Enantiomeric Excess of the Sakurai–Hosomi–Denmark Allylation Catalyzed by Biisoquinoline N,N’-Dioxides Using Computations

Allylation reactions of aldehydes are chemical transformations of fundamental interest, as they give direct access to chiral homoallylic alcohols. In this work, we focus on the full computational characterization of the catalytic activity of substituted biisoquinoline-N,N′-dioxides for the allylation of 2-naphthaldehyde. We characterized the structure of all transition states as well as identified the π stacking interactions that are responsible for their relative energies. Motivated by disagreement with the experimental results, we also performed an assessment of 34 different density functional methods, with the goal of assessing DFT as a general tool for understanding this chemistry. We found that the DFT results are generally consistent as long as functionals that correctly account for dispersion interactions are used. However, agreement with the experimental results is not always guaranteed. We suggest the need for a careful synergy between computations and experiments to correctly interpret the data and use them as a design tool for new and improved asymmetric catalysts.

Design and synthesis of 3,3'-triazolyl biisoquinoline N,N'-dioxides via Hiyama cross-coupling of 4-trimethylsilyl-1,2,3-triazoles

A new strategy to effectively lock the conformation of substituents at the 3,3'-positions of axial-chiral biisoquinoline N,N'-dioxides was developed based on the strong dipole-dipole interaction between 1,2,3-triazole and pyridine N-oxide rings. The crystal structure and the DFT calculations of 3,3'-bis(1-benzyl-1H-1,2,3-triazole-4-yl)-1,1'-biisoquinoline N,N'-dioxide (3a) provided strong support for this strategy. Furthermore, we successfully demonstrated that readily available 4-trimethylsilyl-1,2,3-triazoles are viable nucleophiles for Hiyama cross-coupling.

Evaluation of 3,3′-Triazolyl Biisoquinoline N,N′-Dioxide Catalysts for Asymmetric Hydrosilylation of Hydrazones with Trichlorosilane

A new class of axial-chiral biisoquinoline N,N′-dioxides was evaluated as catalysts for the enantioselective hydrosilylation of acyl hydrazones with trichlorosilane. While these catalysts provided poor to moderate reactivity and enantioselectivity, this study represents the first example of the organocatalytic asymmetric reduction of acyl hydrazones. In addition, the structures and energies of two possible diastereomeric catalyst–trichlorosilane complexes (2a–HSiCl₃) were analyzed using density functional theory calculations.

Reaction-based machine learning representations for predicting the enantioselectivity of organocatalysts

Hundreds of catalytic methods are developed each year to meet the demand for high-purity chiral compounds. The computational design of enantioselective organocatalysts remains a significant challenge, as catalysts are typically discovered through experimental screening. Recent advances in combining quantum chemical computations and machine learning (ML) hold great potential to propel the next leap forward in asymmetric catalysis. Within the context of quantum chemical machine learning (QML, or atomistic ML), the ML representations used to encode the three-dimensional structure of molecules and evaluate their similarity cannot easily capture the subtle energy differences that govern enantioselectivity. Here, we present a general strategy for improving molecular representations within an atomistic machine learning model to predict the DFT-computed enantiomeric excess of asymmetric propargylation organocatalysts solely from the structure of catalytic cycle intermediates. Mean absolute errors as low as 0.25 kcal mol-1 were achieved in predictions of the activation energy with respect to DFT computations. By virtue of its design, this strategy is generalisable to other ML models, to experimental data and to any catalytic asymmetric reaction, enabling the rapid screening of structurally diverse organocatalysts from available structural information.

Characterization of Highly Coordinated Allylgermanes: Pivotal Players for Enhanced Nucleophilicity and Stereoselectivity

Allylgermanes with a 4-, 5-, and 6-coordinated germanium center were characterized by X-ray crystallography. Cationic 6-coordinated group 14 allylmetals, which were hitherto assumed to be a transition-state structure of allylations, were successfully isolated. Forming high coordination states significantly enhanced the reactivity of the allylgermanes. In contrast to the 4-coordinated allylgermanes with low reactivity, the highly coordinated species readily reacted with several aldehydes. Furthermore, the high coordination states exerted a significant effect on the E/Z selectivity of allylation depending on external additives. The coordination structure had a dramatic influence on the electronic and steric environments around the Ge center, enabling the geometrically controlled allylation of aldehydes.

Axial-Chiral Biisoquinoline N, N'-Dioxides Bearing Polar Aromatic C-H Bonds as Catalysts in Sakurai-Hosomi-Denmark Allylation

The design, synthesis, and evaluation of axial-chiral biisoquinolines bearing polar aromatic C-H bonds as Lewis base catalysts are reported. Lewis bases containing the 3,5-bis(trifluoromethyl)phenyl group were found to be significantly more enantioselective for a wider range of substrates than those bearing aromatic residues that are not strongly electron-deficient in the allylation of aldehydes with allyltrichlorosilane. Also, optically pure 3,3'-dibromo-1,1'-biisoquinoline N, N'-dioxide that has not been previously reported was synthesized as a common catalyst precursor to facilitate the study.

Mechanism and Origin of the Unexpected Chemoselectivity in Fluorocyclization of o-Styryl Benzamides with a Hypervalent Fluoroiodane Reagent

The mechanism and origin of the unexpected chemoselectivity in fluorocyclization of o-styryl benzamide with a cyclic hypervalent fluoroiodane reagent were explored with DFT calculations. The calculations suggested an alternative mechanism that is broadly similar to, but also critically different from, the previously proposed mechanism for the formation of an unexpected structurally novel seven-membered 4-fluoro-1,3-benzoxazepine. The amide group of o-styryl benzamide was revealed to be crucial for activating the fluoroiodane reagent and facilitating C-F bond formation. In contrast to the popular electrophilic N-F reagent Selectfluor, the F atom in the fluoroiodane reagent is nucleophilic, and the I(III) atom is the most electrophilic site, thus inducing a completely different reactivity pattern. The insights reported here will be valuable for the further development of new reactions based on the hypervalent fluoroiodane reagent.

Noncovalent Interactions in Organocatalysis and the Prospect of Computational Catalyst Design

Noncovalent interactions are ubiquitous in organic systems, and can play decisive roles in the outcome of asymmetric organocatalytic reactions. Their prevalence, combined with the often subtle line separating favorable dispersion interactions from unfavorable steric interactions, often complicates the identification of the particular noncovalent interactions responsible for stereoselectivity. Ultimately, the stereoselectivity of most organocatalytic reactions hinges on the balance of both favorable and unfavorable noncovalent interactions in the stereocontrolling transition state (TS). In this Account, we provide an overview of our attempts to understand the role of noncovalent interactions in organocatalyzed reactions and to develop new computational tools for organocatalyst design. Following a brief discussion of noncovalent interactions involving aromatic rings and the associated challenges capturing these effects computationally, we summarize two examples of chiral phosphoric acid catalyzed reactions in which noncovalent interactions play pivotal, although somewhat unexpected, roles. In the first, List's catalytic asymmetric Fischer indole reaction, we show that both π-stacking and CH/π interactions of the substrate with the 3,3'-aryl groups of the catalyst impact the stability of the stereocontrolling TS. However, these noncovalent interactions oppose each other, with π-stacking interactions stabilizing the TS leading to one enantiomer and CH/π interactions preferentially stabilizing the competing TS. Ultimately, the CH/π interactions dominate and, when combined with hydrogen bonding interactions, lead to preferential formation of the observed product. In the second example, a series of phosphoric acid catalyzed asymmetric ring openings of meso-epoxides, we show that noncovalent interactions of the substrates with the 3,3'-aryl groups of the catalyst play only an indirect role in stereoselectivity. Instead, the stereoselectivity of these reactions are driven by the electrostatic stabilization of a fleeting partial positive charge in the SN2-like transition state by the chiral electrostatic environment of the phosphoric acid catalyst. Next, we describe our studies of bipyridine N-oxide and N,N'-dioxide catalyzed alkylation reactions. Based on several examples, we demonstrate that there are many potential arrangements of ligands around a hexacoordinate silicon in the stereocontrolling TS, and one must consider all of these in order to identify the lowest-lying TS structures. We also present a model in which electrostatic interactions between a formyl CH group and a chlorine in these TSs underlie the enantioselectivity of these reactions. Finally, we discuss our efforts to develop computational tools for the screening of potential organocatalyst designs, starting in the context of bipyridine N,N'-dioxide catalyzed alkylation reactions. Our new computational tool kit (AARON) has been used to design highly effective catalysts for the asymmetric propargylation of benzaldehyde, and is currently being used to screen catalysts for other reactions. We conclude with our views on the potential roles of computational chemistry in the future of organocatalyst design.

A Highly Efficient Chirality Switchable Synthesis of Dihydropyran-Fused Benzofurans by Fine-Tuning the Phenolic Proton of β-Isocupreidine (β-ICD) Catalyst with Methyl

A highly enantioselective β-isocupreidine (β-ICD) catalyzed synthesis of dihydropyran-fused benzofurans through [4+2] cycloaddition of allenoates and benzofuranone alkenes was developed. Switchable chirality inversion of cycloaddition products was achieved by replacing the phenolic proton of the catalyst with a methyl, demonstrating an amazing effect of minimal structural variation on inverting enantioselectivity. DFT calculations were utilized to elucidate the origin of the observed phenomena. Computation also provided a clue for a rational design in which the multi-hydrogen bond with the alcohol additive was found to improve the enantioselectivity of the cycloaddition. Finally, the substrate scope was examined, in which a number of functionalized dihydropyran-fused benzofurans could be obtained in high yields (up to 97 %) with very good regio- (>20:1) and enantioselectivities (up to 98:2 e.r.).

Toward a more complete understanding of noncovalent interactions involving aromatic rings

Noncovalent interactions involving aromatic rings, which include π-stacking interactions, anion-π interactions, and XH-π interactions, among others, are ubiquitous in chemical and biochemical systems. Despite dramatic advances in our understanding of these interactions over the past decade, many aspects of these noncovalent interactions have only recently been uncovered, with many questions remaining. We summarize our computational studies aimed at understanding the impact of substituents and heteroatoms on these noncovalent interactions. In particular, we discuss our local, direct interaction model of substituent effects in π-stacking interactions. In this model, substituent effects are dominated by electrostatic interactions of the local dipoles associated with the substituents and the electric field of the other ring. The implications of the local nature of substituent effects on π-stacking interactions in larger systems are discussed, with examples given for complexes with carbon nanotubes and a small graphene model, as well as model stacked discotic systems. We also discuss related issues involving the interpretation of electrostatic potential (ESP) maps. Although ESP maps are widely used in discussions of noncovalent interactions, they are often misinterpreted. Next, we provide an alternative explanation for the origin of anion-π interactions involving substituted benzenes and N-heterocycles, and show that these interactions are well-described by simple models based solely on charge-dipole interactions. Finally, we summarize our recent work on the physical nature of substituent effects in XH-π interactions. Together, these results paint a more complete picture of noncovalent interactions involving aromatic rings and provide a firm conceptual foundation for the rational exploitation of these interactions in a myriad of chemical contexts.

Performance of DFT methods and origin of stereoselectivity in bipyridine N,N′-dioxide catalyzed allylation and propargylation reactions

It is shown that many DFT methods correctly predict the stereoselectivity of bipyridine N,N′-dioxide catalyzed alkylation reactions despite predicting the incorrect low-lying transition state structures. A novel explanation of the origin of stereoselectivity in these reactions is also provided.