Ternary complexes of chiral disulfonimides in transfer-hydrogenation of imines: the relevance of late intermediates in ion pair catalysis

An NMR Spectroscopy View on London Dispersion in Catalysis: Detection, Quantification, and Application in Ion Pair and Transition Metal Catalysis

ConspectusThe energetic contribution of London dispersion (LD) can cover a broad range from very few to hundreds of kJ mol-1 for extended interaction interfaces due to its pairwise additivity. However, for a designed and successful application of LD in chemical catalysis, there are still many obstacles and questions that remain. In principle, LD can be regarded as the attractive part of the van der Waals potential. Thus, considering the whole van der Waals potential, including the repulsive part (steric repulsion), the ideal solution to the problem in catalysis would be to design compatible interaction interfaces at exactly the correct distance. In the case of a self-assembled, flexible structure arrangement, entropic contributions and solvent interactions might be detrimental. In the case of a rigid catalyst pocket, steric hindrance might not allow for large substituents that are usually applied as dispersion energy donors (DEDs). For a working catalytic system, the following question arises: how is it possible to dissect the complex interaction interfaces in terms of energetic contributions? Usually, the energetic contribution of LD to catalysis is addressed by using calculations. However, adequately computing the correct energetic contributions can be extremely challenging for a vast conformational space with all kinds of intermolecular interactions. Thus, experimental data are essential for comparison or benchmarking.Therefore, in this Account, we describe our quest for detailed experimental data obtained via NMR spectroscopy to experimentally dissect and quantify LD in catalytic systems. In addition, we address the question of whether bulky substituents used as DEDs can be used in confined catalytic pockets. With the example of Pd phosphoramidite complexes, we show how it is possible to experimentally dissect and quantify the contribution of individual interaction areas in complicated transition metal complexes. Furthermore, a correlation between conformational rigidity and heterodimer preference clearly reveals that LD can only unfold its full potential in cases where entropic contributions are minimized. This finding can also explain the small contribution of LD in flexible and solvent-exposed molecular balances. In the field of Brønsted acid catalysis, we demonstrated that LD has a strong influence on the structures, stability, and populations of confined catalytic intermediates. LD is key for populating higher aggregates such as dimers. In addition, offsets between the experimental and computational results were observed and attributed to solvent-solute dispersion interactions. We studied the delicate interplay of attractive and repulsive interactions by adding bulky DED substituents onto a substrate, which can function as a molecular balance system. Intriguingly, the effect of LD on the free substrate was straightforwardly transferred onto the highly confined intermediates. Furthermore, this effect could even be read out in the enantioselectivities of the underlying reaction. This conceptualized a general approach regarding how LD can be used beneficially in catalysis to convert from moderate/good to excellent stereoselectivities. It showcased that bulky groups such as tert-butyl must not only be regarded as occupied volumes.

Applications of Hantzsch Esters in Organocatalytic Enantioselective Synthesis

Hantzsch esters (1,4-dihydropyridine dicarboxylates) have become, in this century, very versatile reagents for enantioselective organic transformations. They can act as hydride transfer agents to reduce, regioselectively, a variety of multiple bonds, e.g., C=C and C=N, under mild reaction conditions. They are excellent reagents for the dearomatization of heteroaromatic substances, and participate readily in cascade processes. In the last few years, they have also become useful reagents for photoredox reactions. They can participate as sacrificial electron and hydrogen donors and when 4-alkyl or 4-acyl-substituted, they can act as alkyl or acyl radical transfer agents. These last reactions may take place in the presence or absence of a photocatalyst. This review surveys the literature published in this area in the last five years.

Enantioselective Giese Additions of Prochiral α-Amino Radicals

Amines featuring an adjacent stereocenter are important building blocks, and recent years have seen remarkable growth in methods forming these via prochiral α-amino radical intermediates. However, very few can exert control over the newly formed stereocenter. We disclose a strategy to overcome this in the context of one of the most widely used radical carbon-carbon bond forming reactions, the Giese reaction. Incorporation of a removable basic heteroarene into the substrate enables a network of attractive noncovalent interactions between a phosphoric acid catalyst, the subsequently formed α-amino radical, and the Giese acceptor, allowing the catalyst to exert control during the C-C bond forming step. Deprotection of the products leads to analogues of γ-aminobutyric acid. We anticipate that this strategy will be applicable to other asymmetric radical transformations in which catalyst control is presently challenging.

Tilting the Balance: London Dispersion Systematically Enhances Enantioselectivities in Brønsted Acid Catalyzed Transfer Hydrogenation of Imines

London dispersion (LD) is attracting more and more attention in catalysis since LD is ubiquitously present and cumulative. Since dispersion is hard to grasp, recent research has concentrated mainly on the effect of LD in individual catalytic complexes or on the impact of dispersion energy donors (DEDs) on balance systems. The systematic transfer of LD effects onto confined and more complex systems in catalysis is still in its infancy, and no general approach for using DED residues in catalysis has emerged so far. Thus, on the example of asymmetric Brønsted acid catalyzed transfer hydrogenation of imines, we translated the findings of previously isolated balance systems onto confined catalytic intermediates, resulting in a systematic enhancement of stereoselectivity when employing DED-substituted substrates. As the imine substrate is present as Z- and E-isomers, which can, respectively, be converted to R- and S-product enantiomers, implementing tert-butyl groups as DED residues led to an additional stabilization of the Z-imine by up to 4.5 kJ/mol. NMR studies revealed that this effect is transferred onto catalyst/imine and catalyst/imine/nucleophile intermediates and that the underlying reaction mechanism is not affected. A clear correlation between ee and LD stabilization was demonstrated for 3 substrates and 10 catalysts, allowing to convert moderate-good to good-excellent enantioselectivities. Our findings conceptualize a general approach on how to beneficially employ DED residues in catalysis: they clearly showcase that bulky alkyl residues such as tert-butyl groups must be considered regarding not only their repulsive steric bulk but also their attractive properties even in catalytic complexes.

Density Functional Theory Study on the Selective Reductive Amination of Aldehydes and Ketones over Their Reductions to Alcohols Using Sodium Triacetoxyborohydride

Reductive amination is one of the most important methods to synthesize amines, having a wide application in the pharmaceutical, fine chemicals, and materials industries. In general, the reaction begins with dehydration between a carbonyl compound and an amine compound, forming an imine, which is then reduced to an alkylated amine product. Sodium triacetoxyborohydride (STAB) is a popular choice for the reducing agent as it shows selectivity for imines over aldehydes and ketones, which is particularly important in direct reductive amination where the imine and carbonyl compounds are present concurrently. Here, we analyze the reaction pathways of acid-catalyzed direct reductive amination in 1,2-dichloroethane (DCE) with acetaldehyde and methylamine. We find that the transition states for the formation and subsequent reduction of Z-methylethylideneimine (resultant aldimine from acetaldehyde and methylamine) have lower energies than the reduction of acetaldehyde. Transition state structures for the hydride transfers are organized by the Lewis-acidic sodium ion. Additionally, reduction reactions with formaldehyde and acetone and their imine derivatives (with methylamine) are investigated, and again, the hydride transfer to the resultant aldimine or ketimine is lower in energy than that of their parent carbonyl compound.

Discovering and Evaluating the Reducing Abilities of Polar Alkanes and Related Family Members as Organic Reductants Using Thermodynamics

In this work, the pK_a values of 69 polar alkanes (YH₂) in acetonitrile were computed using the method developed by Luo and Zhang in 2020, and representative 69 thermodynamic network cards on 22 elementary steps of YH₂ and related polar alkenes (Y) releasing or accepting H₂ were naturally established. Potential electron reductants (YH^-), hydride reductants (YH^-), antioxidants (YH₂ and YH^-), and hydrogen molecule reductants (YH₂) are unexpectedly discovered according to thermodynamic network cards. It is also found that there are great differences between YH₂ and common hydrogen molecule reductants (XH₂), such as Hantzsch ester (HEH₂), benzothiazoline (BTH₂), and dihydro-phenanthridine (PH₂), releasing two hydrogen ions to unsaturated compounds. During the hydrogenation process, XH₂ release hydrides first, then the oxidation state XH⁺ release protons. However, in the case of YH₂, YH₂ release protons first, then YH^- release hydrides. It is the differences on acidic properties of YH₂ and XH₂ that result in the behavioral and thermodynamic differences on YH₂ and XH₂ releasing two hydrogen ions (H^--H⁺). The redox mechanisms and behaviors of Y, YH^-, and YH₂ as electron, hydrogen atom, hydride, and hydrogen molecule donors or acceptors in the chemical reaction are reasonably investigated and discussed in this paper using thermodynamics.