Understanding the Mechanism of the Asymmetric Propargylation of Aldehydes Promoted by 1,1′-Bi-2-naphthol-Derived Catalysts

Highly Regioselective Propargylation/Allenylation of Organolanthanum Reagents with Aldehydes

The metal-mediated propargylation or allenylation of carbonyl compounds is well-adapted to the preparation of homopropargylic or allenylic alcohols, which are multifunctional intermediates in synthetic chemistry. However, the regioselectivity of reactions using propargyl or allenyl metal reagents is difficult to control, owing to the equilibrium between the two species. In our study, propargyl or allenyl organolanthanum reagents were prepared using trimethylsilylpropyne or prop-1-yn-1-ylbenzene substrates. The treatment of the organolanthanum reagents with aldehydes yielded the regioselective products, respectively. This study provides a better understanding of structural specificity and the special chemoselectivity of rare earth metal reagents.

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.

Noncovalent Stabilization of Radical Intermediates in the Enantioselective Hydroamination of Alkenes with Sulfonamides

Noncovalent interactions (NCIs) are critical elements of molecular recognition in a wide variety of chemical contexts. While NCIs have been studied extensively for closed-shell molecules and ions, very little is understood about the structures and properties of NCIs involving free radical intermediates. In this report, we describe a detailed mechanistic study of the enantioselective radical hydroamination of alkenes with sulfonamides and present evidence suggesting that the basis for asymmetric induction in this process arises from attractive NCIs between a neutral sulfonamidyl radical intermediate and a chiral phosphoric acid (CPA). We describe experimental, computational, and data science-based evidence that identifies the specific radical NCIs that form the basis for the enantioselectivity. Kinetic studies support that C-N bond formation determines the enantioselectivity. Density functional theory investigations revealed the importance of both strong H-bonding between the CPA and the N-centered radical and a network of aryl-based NCIs that serve to stabilize the favored diastereomeric transition state. The contributions of these specific aryl-based NCIs to the selectivity were further confirmed through multivariate linear regression analysis by comparing the measured enantioselectivity to computed descriptors. These results highlight the power of NCIs to enable high levels of enantioselectivity in reactions involving uncharged open-shell intermediates and expand our understanding of radical-molecule interactions.

Unusual Enantiodivergence in Chiral Brønsted Acid‐Catalyzed Asymmetric Allylation with β‐Alkenyl Allylic Boronates

We report herein a rare example of enantiodivergent aldehyde addition with β-alkenyl allylic boronates via chiral Brønsted acid catalysis. 2,6-Di-9-anthracenyl-substituted chiral phosphoric acid-catalyzed asymmetric allylation using β-vinyl substituted allylic boronate gave alcohols with R absolute configuration. The sense of asymmetric induction of the catalyst in these reactions is opposite to those in prior reports. Moreover, in the presence of the same acid catalyst, the reactions with β-2-propenyl substituted allylic boronate generated homoallylic alcohol products with S absolute configuration. Unusual substrate-catalyst C-H⋅⋅⋅π interactions in the favoured reaction transition state were identified as the origins of observed enantiodivergence through DFT computational studies.

Enantioselective Friedel-Crafts Reaction of 2-Alkynyphenols with Aromatic Ethers by Chiral Brønsted Acid Catalysis

Herein, we report chiral strong Brønsted acid-catalyzed enantioselective Friedel-Crafts reaction of 2-alkynyphenols with aromatic ethers. The reaction affords the corresponding axially chiral styrenes in up to 91% yield and 97% ee.

Chiral Phosphoric Acid-Catalyzed Enantio- and Diastereoselective Allylboration of Aldehydes with β,γ-Substituted Allylboronates

The catalytic asymmetric addition of β,γ-substituted allylboronates to aldehydes has been described. Promoted by 5 mol % chiral phosphoric acid, the reactions were broadly applicable, scalable, and efficient, allowing for the formation of 3,4-anti/syn-homoallylic alcohols bearing adjacent tertiary or quaternary stereogenic centers in a highly enantio- and diastereoselective manner (≤99% ee and dr >20:1). The rigid chairlike transition state involving the chiral phosphoric acid contributed to the highly controlled reaction.

An overview of the applications of chiral phosphoric acid organocatalysts in enantioselective additions to CO and CN bonds

Since 2004, chiral phosphoric acids (CPAs) have emerged as highyl efficient organocatalysts, providing excellent results in a wide reaction scope. In this review, the applications of CPA for enantioselective additions to CO and CN bonds are covered.

Interrogating the Thionium Hydrogen Bond as a Noncovalent Stereocontrolling Interaction in Chiral Phosphate Catalysis

CH⋯O bonds are a privileged noncovalent interaction determining the energies and geometries of a large number of structures. In catalytic settings, these are invoked as a decisive feature controlling many asymmetric transformations involving aldehydes. However, little is known about their stereochemical role when the interaction involves other substrate types. We report the results of computations that show for the first time thionium hydrogen bonds to be an important noncovalent interaction in asymmetric catalysis. As a validating case study, we explored an asymmetric Pummerer rearrangement involving thionium intermediates to yield enantioenriched N,S-acetals under BINOL-derived chiral phosphate catalysis. DFT and QM/MM hybrid calculations showed that the lowest energy pathway corresponded to a transition state involving two hydrogen bonding interactions from the thionium intermediate to the catalyst. However, the enantiomer resulting from this process differed from the originally published absolute configuration. Experimental determination of the absolute configuration resolved this conflict in favor of our calculations. The reaction features required for enantioselectivity were further interrogated by statistical modeling analysis that utilized bespoke featurization techniques to enable the translation of enantioselectivity trends from intermolecular reactions to those proceeding intramolecularly. Through this suite of computational modeling techniques, a new model is revealed that provides a different explanation for the product outcome and enabled reassignment of the absolute product configuration.

Transferable selectivity profiles allow data from intermolecular reactions using iminium substrates to be applied to predict intramolecular reactions involving thioniums.

Identifying the true origins of selectivity in chiral phosphoric acid catalyzed N-acyl-azetidine desymmetrizations

The first catalytic intermolecular desymmetrization of azetidines was reported by Sun and coworkers in 2015 using a BINOL-derived phosphoric acid catalyst (J. Am. Chem. Soc. 2015, 137, 5895-5898). To uncover the mechanism of the reaction and the origins of the high enantioselectivity, Density Functional Theory (DFT) calculations were performed at the B97D3/6-311+G(2d,2p)/SMD(toluene)//B97D3/6-31G(d,p)/CPCM(toluene) level of theory. Comparison of four possible activation modes confirms that this reaction proceeds through the bifunctional activation of the azetidine nitrogen and the thione tautomer of the 2-mercaptobenzothiazole nucleophile. Upon thorough conformational sampling of the enantiodetermining transition structures (TSs), a free energy difference of 2.0 kcal mol-1 is obtained, accurately reproducing the experimentally measured 88% e.e. at 80 °C. This energy difference is due to both decreased distortion and increased non-covalent interactions in the pro-(S) TS. To uncover the true origins of selectivity, the TSs optimized with the full catalyst were compared to those optimized with a model catalyst through steric maps. It is found that the arrangements displayed by the substrates are controlled by strict primary orbital interaction requirements at the transition complex, and their ability to fit into the catalyst pocket drives the selectivity. A general model of selectivity for phosphoric acid-catalyzed azetidine desymmetrizations is proposed, which is based on the preference of the nucleophile and benzoyl group to occupy empty quadrants of the chiral catalyst pocket.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

Identification of Bi-2-naphthol and Its Phosphate Derivatives Complexed with Cyclodextrin and Metal Ions Using Trapped Ion Mobility Spectrometry

The separation of chiral enantiomers has gained increasing importance in many research fields, becoming a major research hotspot. 1,1'-Bi (2-naphthol) (BINOL) and 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNP) are referred to as atropisomer chiral molecules, which are essential chiral catalysts and intermediates in several reactions. In this work, BINOL and BNP atropisomers are separated and identified using trapped ion mobility spectrometry (TIMS) to monitor the different mobilities of their derivative complexes. The latter are obtained by the simple mixing of BINOL/BNP, cyclodextrin (CD), and the metal ions through noncovalent interactions. The results indicate that the enantiomer complexes of BINOL/BNP can be separated with a certain specificity, showing that R-, S-BINOL can be separated by the ternary complexes of [BINOL+γ-CD + Rb]⁺, [BINOL+γ-CD + Cu-H]⁺, and [BINOL+β-CD + Cu-H]⁺ based on the difference in their mobility; similarly, the R-, S-BNP enantiomer can be isolated by the formed ternary complexes of [BNP+α-CD + Ba-H]⁺, [BNP+β-CD + Co-H]⁺, [BNP+β-CD + Ca-H]⁺, [BNP+β-CD + Cu-H]⁺, [BNP+β-CD + Fe-H]⁺, [BNP+β-CD + Li]⁺, and [BNP+β-CD + Sr-H]⁺. Furthermore, the peak separation rate (R_p-p) of the complexes was calculated, with the R_p-p of the three enantiomers of BINOL being 1.130 and the R_p-p of the seven complexes of BNP reaching 2.089. At last, the different survival yields for the collision energies were found for the enantiomer complexes, revealing the rigid structural differences in the stereospecificity of the enantiomer complexes that result in the separation by the TIMS. Additionally, due to the advantages of simple operation, fast speed, and high sensitivity and because chemical derivatization and chromatographic separation are not required, the developed method can provide a promising and powerful strategy for the separation and identification of binaphthyl derivatives or even other enantiomers of the reaction intermediates.

Stereochemical Control via Chirality Pairing: Stereodivergent Syntheses of Enantioenriched Homoallylic Alcohols

We report herein the development of stereodivergent syntheses of enantioenriched homoallylic alcohols using chiral nonracemic α-CH₂ Bpin-substituted crotylboronate. Chiral phosphoric acid (S)-A-catalyzed asymmetric allyl addition with the reagent gave Z-anti-homoallylic alcohols with excellent enantioselectivities and Z-selectivities. When the enantiomeric acid catalyst (R)-A was utilized, the stereoselectivity was completely reversed and E-anti-homoallylic alcohols were obtained with high E-selectivities and excellent enantioselectivities. By pairing the chirality of the boron reagent with the catalyst, two complementary stereoisomers of chiral homoallylic alcohols can be obtained selectively from the same boron reagent. DFT computational studies were conducted to probe the origins of the observed stereoselectivity. These reactions generate highly enantioenriched homoallylic alcohol products that are valuable for rapid construction of polyketide structural frameworks.

Nazarov Cyclizations Catalyzed by BINOL Phosphoric Acid Derivatives: Quantum Chemistry Struggles To Predict the Enantioselectivity

Quantum chemical calculations have successfully predicted the stereoselectivities of many BINOL phosphoric acid catalyzed reactions over the past 10-15 years. Herein we report a contrasting example: a reaction for which standard quantum chemistry techniques have proven unexpectedly ineffective at explaining the stereoselectivity. The Nazarov cyclizations of a divinyl ketone catalyzed by a BINOL phosphoric acid or H₈-BINOL dithiophosphoric acid were studied with a conventional contemporary quantum chemical approach, consisting of transition state optimizations with B3LYP-D3(BJ) and single-point calculations with several functionals in implicit solvent. Unexpectedly, different functionals gave widely different predictions of the level of enantioselectivity and were unable even to agree on which enantiomer of the product would predominate. Molecular dynamics simulations with the OPLS-AA force field provided evidence that the transition state geometries optimized with DFT in the gas phase or in implicit solvent are not good representations of the true transition states of these reactions in solution. One possible reason for this, which may also explain the failure of quantum chemical techniques to reliably predict the enantioselectivity, is the fact that the transition states contain ion pairs which are not highly organized and do not contain any strongly directional noncovalent interactions between the substrate and the catalyst.

Chiral Phosphoric Acid Catalysis: The Terada Model Revisited

Since Akiyama and Terada independently reported the introduction of chiral phosphoric acids (CPAs) as effective catalysts for Mannich-type reactions in 2004, the field of CPA catalysis has grown immensely. Terada reported in 2008 the first example of the activation of aldehydes by a CPA. Based on density functional theory (DFT) calculations, Terada proposed a dual activation mode for this enantioselective aza-ene-type reaction between an aldehyde and an enecarbamate. In this model, hydrogen bonds between the catalyst's hydroxyl group and the carbonyl oxygen and the catalyst's P═O and the formyl proton were observed; the nucleophile then attacks without coordination to the catalyst. This reaction model provided the mechanistic basis for understanding Terada's reaction and many other asymmetric transformations. In the present study, DFT calculations are reported that identify a lower-energy mechanism for this landmark reaction. In this new model, hydrogen bonds between the catalyst's hydroxyl group and the aldehyde oxygen and the catalyst's P═O and the NH group of the enecarbamate are seen. The new model rationalizes the stereoselective outcome of Terada's reaction and offers insight into why a more sterically demanding catalyst gives lower levels of enantioselectivity.

Selective Synthesis of C1-Symmetric BINOL-phosphates and P-chiral Phosphoramides Using Directed ortho-Lithiation

Directed ortho-lithiation (DoL) has been developed as an effective method for the ortho-substitution of BINOL-phosphoric acid and BINOL-N-triflylphosphoramide (BINOL-P-acids). It can be employed in the rapid assembly of either mono- or disubstituted BINOL-P-acids, including unsymmetrical disubstitution through iterative DoL. Most significantly, DoL has proven to be highly effective in the diastereoselective desymmetrization of pseudo-C₂-symmetric BINOL-N-triflylphosphoramide, affording a chiral P-group.

Trends in computational molecular catalyst design

Computational methods have emerged as a powerful tool to augment traditional experimental molecular catalyst design by providing useful predictions of catalyst performance and decreasing the time needed for catalyst screening. In this perspective, we discuss three approaches for computational molecular catalyst design: (i) the reaction mechanism-based approach that calculates all relevant elementary steps, finds the rate and selectivity determining steps, and ultimately makes predictions on catalyst performance based on kinetic analysis, (ii) the descriptor-based approach where physical/chemical considerations are used to find molecular properties as predictors of catalyst performance, and (iii) the data-driven approach where statistical analysis as well as machine learning (ML) methods are used to obtain relationships between available data/features and catalyst performance. Following an introduction to these approaches, we cover their strengths and weaknesses and highlight some recent key applications. Furthermore, we present an outlook on how the currently applied approaches may evolve in the near future by addressing how recent developments in building automated computational workflows and implementing advanced ML models hold promise for reducing human workload, eliminating human bias, and speeding up computational catalyst design at the same time. Finally, we provide our viewpoint on how some of the challenges associated with the up-and-coming approaches driven by automation and ML may be resolved.

Dynamic parallel kinetic resolution of α-ferrocenyl cation initiated by chiral Brønsted acid catalyst

The dynamic parallel kinetic resolution (DPKR) of an α-ferrocenyl cation intermediate under the influence of a chiral conjugate base of a chiral phosphoric acid catalyst has been demonstrated in an S_N1 type substitution reaction of a racemic ferrocenyl derivative with a nitrogen nucleophile. The present method provides efficient access to a ferrocenylethylamine derivative in a highly enantioselective manner, which is potentially useful as a key precursor of chiral ligands for metal catalysis. The mechanism of the present intriguing resolution system was elucidated by control experiments using the enantio-pure precursor of relevant α-ferrocenyl cation intermediates and the hydroamination of vinylferrocene. Further theoretical studies enabled the elucidation of the origin of the stereochemical outcome as well as the efficient DPKR. The present DPKR, which opens a new frontier for kinetic resolution, involves the racemization process through the formation of vinylferrocene and the chemo-divergent parallel kinetic resolution of the enantiomeric α-ferrocenyl cations generated by the protonation/deprotonation sequence of vinylferrocene.

The dynamic parallel kinetic resolution (DPKR) of an enantiomeric α-ferrocenyl cation using a chiral phosphate anion of an acid catalyst was accomplished by the combination of the PKR and the racemization through the formation of vinylferrocene.

Computational Insights into Privileged Stereocontrolling Interactions Involving Chiral Phosphates and Iminium Intermediates

The precise design of a catalyst for a given reaction is extremely difficult, often requiring a significant empirical screening campaign to afford products in high yields and enantiomeric excess. Design becomes even more challenging if one requires a catalyst that performs well for a diverse range of substrates. Such "privileged" catalysts exist, but little is known why they operate so generally. We report the results of computations which show that when substrate and catalyst features are conserved between significantly different mechanistic regimes, similar modes of activation can be invoked. As a validating case study, we explored a Hantzsch ester hydrogenation of α,β-unsaturated iminiums involving BINOL-derived chiral phosphates and find they impart asymmetric induction in an analogous fashion to their acid counterpart. Specifically, DFT calculations at the IEFPCM(1,4-dioxane)-B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level predicted enantioselectivity to be close to the experimental value (82% ee calculated, 96% ee experimental) and showed that the reaction proceeds via a transition state involving two hydrogen-bonding interactions from the iminium intermediate and nucleophile to the catalyst. These interactions lower the energy of the transition structure and provide extra rigidity to the system. This new model invokes "privileged" noncovalent interactions and leads to a new explanation for the enantioselectivity outcome, ultimately providing the basis for the development of general catalyst design principles and the translation of mechanistically disparate reaction profiles for the prediction of enantioselectivity outcomes using statistical models.

Understanding the mechanism of the chiral phosphoric acid-catalyzed aza-Cope rearrangement

The first catalytic enantioselective aza-Cope rearrangement was reported in 2008 by Rueping et al. The reaction is catalyzed by a 1,1'-bi-2-naphthol-derived (BINOL-derived) phosphoric acid and achieved high yields and enantioselectivities (up to 97 : 3 er with 75% yield). This work utilizes Density Functional Theory to understand the mechanism of the reaction and explain the origins of the enantioselectivity. An extensive conformational search was carried out to explore the different activation modes by the catalyst and, the Transition State (TS) leading to the major product was found to be 1.3 kcal mol-1 lower in energy than the TS leading to the minor product. The origin of this stabilization was rationalized with NBO and NCI analysis: it was found that the major TS has a greater number of non-bonding interactions between the substrate and the catalyst, and shows stronger H-bond interactions between H atoms in the substrate and the O atoms in the phosphate group of the catalyst.

Insights into the Chiral Phosphoric Acid-Catalyzed Dynamic Kinetic Asymmetric Hydroamination of Racemic Allenes: An Allyl Carbocation/Phosphate Pair Mechanism

Computational studies of chiral phosphoric acid (CPA)-catalyzed dynamic kinetic asymmetric hydroamination (DyKAH) of racemic allenes show that the reaction proceeds through a catalytic asymmetric model involving a highly reactive π-allylic carbocationic intermediate, generated from a racemic allene through an intermolecular proton transfer mediated by CPA, which also results in a high E/Z selectivity. Moreover, the distortion-interaction, atom in molecule, and electrostatic interaction analyses and space-filling models are employed on the basis of the DyKAH catalyzed by (S)-A5 (reaction 1) or (R)-A2 (reaction 2) to explain the high enantioselectivity and the controlling effects of SPINOL scaffolds on the signs of enantioselectivity. Our calculations indicate that the enantioselectivity of reactions 1 and 2 can be mainly ascribed to the favorable noncovalent interactions within the stronger chiral electrostatic environment created by the phosphoric acid in the preferential transition states. Finally, the effect of (S/R)-SPINOL-based CPAs on the signs of enantioselectivity can be explained by the different combination modes of substrates into the chiral binding pocket of the catalyst controlled by the chirality of SPINOL backbones. Overall, the new insights into the reaction rationalize the outcome and these key factors that affect the product enantioselectivity are important to guide the DyKAHs.

Computational Studies of Chiral Hydroxyl Carboxylic Acids: The Allylboration of Aldehydes

The mechanism of the asymmetric BINOL-derived hydroxyl carboxylic acid catalyzed allylboration of benzaldehyde was investigated using density functional theory calculations. A new reaction model is proposed, and the roles of the two Brønsted acidic sites of the catalyst elucidated. Catalyst distortion was found to be a key factor in determining stereoselectivity. The flexibility of the hydroxyl carboxylic acid catalyst leads to significant differences in the mechanism and origins of selectivity compared to the equivalent phosphoric acid catalyzed reaction.

Bi(cyclopentyl)diol-Derived Boronates in Highly Enantioselective Chiral Phosphoric Acid-Catalyzed Allylation, Propargylation, and Crotylation of Aldehydes

In this study, we disclose the catalytic addition of bi(cyclopentyl)diol-derived boronates to aldehydes promoted by chiral phosphoric acids, allowing for the formation of enantioenriched homoallylic, propargylic, and crotylic alcohols (up to >99% enantiomeric excess (ee), diastereomeric ratio (dr) >20:1). These boronate substrates provided superior enantioselectivities, allowing for the reactions to proceed with low catalyst loading (0.5-5 mol %) and reduced reaction time (15 min at room temperature for aldehyde allylboration). A wide substrate scope was exhibited, and the novel boronates provided high enantiocontrol. Reactions with substituted allylboronates and aldehydes yielded vicinal stereogenic alcohols bearing β-tertiary or quaternary carbon centers. High enantio- and diastereoselectivities were found due to the closed six-membered chair-like transition state, with backbone modifications of the boronate and its interactions with the chiral phosphoric acid being the most likely contributing factor.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Enantioselective Syntheses of (Z)-6'-Boryl-anti-1,2-oxaborinan-3-enes via a Dienylboronate Protoboration and Asymmetric Allylation Reaction Sequence

The enantioselective synthesis of 6'-boryl-anti-1,2-oxaborinan-3-enes is reported. A Cu-catalyzed highly stereoselective 1,4-protoboration of 1,1-bisboryl-1,3-butadiene is developed to generate (E)-α,δ-bisboryl-crotylboronate. The chiral phosphoric-acid-catalyzed asymmetric allylboration of aldehydes with the boron reagent produces 6'-boryl-anti-1,2-oxaborinan-3-enes with excellent Z-selectivities and enantioselectivities. The product contains a vinyl and alkyl boronate unit that can directly participate in a variety of subsequent transformations.

Theoretical Characterization of Catalytically Active Species in Reductive Hydroxymethylation of Styrene with CO2 over a Bisphosphine-Ligated Copper Complex

In this work, a density functional theory (DFT) study was performed to identify the catalytically active species in the copper-catalyzed three-component reductive hydroxymethylation of styrene with CO₂ and hydrosilane. The calculations reveal that the dimeric copper(I) hydride species, formed in a mixture of the bisphosphine ligand, Cu(OAc)₂, and hydrosilane, probably acts as the catalyst precursor. In the beginning, this species is catalytically competent to trigger the hydrocupration of styrene, along with the formation of the dimeric copper(I) alkyl intermediate. Subsequently, CO₂ insertion into the dimeric copper(I) alkyl intermediate occurs, which is accompanied by the cleavage of the Cu-Cu bond and the generation of the monomeric copper(I) carboxylate intermediate. In the end, the sequential reduction of the monomeric copper(I) carboxylate intermediate with the hydrosilane produces the monomeric copper(I) hydride species as the actual catalyst and turns on the catalytic cycle. On the other hand, the monomeric copper(II) hydride species, yielded as the kinetic product in the initial reaction of the bisphosphine ligand, Cu(OAc)₂, and hydrosilane, is also reactive for the hydrocupration of styrene. However, the resulting monomeric copper(II) alkyl intermediate is found to be the catalyst resting state, because of the much higher energy barrier demanded for the subsequent nucleophilic attack toward CO₂. On the basis of the results of an activation-strain model (ASM) analysis and charge decomposition analysis (CDA), the low activity of the monomeric copper(II) alkyl intermediate can be ascribed to the more crowded environment around the central copper(II) ion and the weaker nucleophilicity of the alkyl moiety. Furthermore, all of the possible CuH species generated in the system are competent to promote the two-component hydrosilylation of CO₂ with hydrosilane, which is an inevitable side reaction along with the reductive hydroxymethylation of styrene with CO₂ and hydrosilane.

Theoretical Study of the Borono-Mannich Reaction with Pinacol Allenylboronate

A density functional theory study of the mechanism of the Borono-Mannich reaction using benzylamine and piperidine as representative examples of primary and secondary amines with pinacol allenylboronate is presented. The study shows that both reactions progress through coordination between the boron and the phenolic oxygen. Ring size strain and hydrogen bond activation appear to determine the observed divergent regioselectivity. In the case of benzylamine, the eight-membered ring transition structure that leads to the propargylamine exhibits a hydrogen bond between the hydrogen attached to the nitrogen and the phenolic oxygen (γ-attack), whereas for piperidine a hydrogen bond between the hydrogen on the imine carbon and one of the oxygens of the pinacol group was observed in the six-membered ring transition structure toward the allenylamine (α-attack).

Mechanism and Origin of Stereoselectivity in Chiral Phosphoric Acid-Catalyzed Aldol-Type Reactions of Azlactones with Vinyl Ethers

The precise mechanism of the chiral phosphoric acid-catalyzed aldol-type reaction of azlactones with vinyl ethers was investigated. DFT calculations suggested that the reaction proceeds through a Conia-ene-type transition state consisting of the vinyl ether and the enol tautomer of the azlactone, in which the catalyst protonates the nitrogen atom of the azlactone to promote enol tautomerization. In addition, the phosphoryl oxygen of the catalyst interacts with the vinyl proton of the vinyl ether. The favorable transition structure features dicoordinating hydrogen bonds. However, these hydrogen bonds are not involved in the bond recombination sequence and hence the catalyst functions as a template for binding substrates. From the results of theoretical studies and experimental supports, the high enantioselectivity is induced by the steric repulsion between the azlactone substituent and the binaphthyl backbone of the catalyst under the catalyst template effect.

Efficient method for propargylation of aldehydes promoted by allenylboron compounds under microwave irradiation

The propargylation of aldehydes promoted by microwave irradiation using allenylboron compounds in a chemo- and regioselective way is described. The corresponding products were obtained in short reaction time, high yield and purity without the need of any solvent when allenylboronic acid pinacol ester was used, or using a minimal amount of acetone when potassium allenyltrifluoroborate was used.

F10BINOL-derived chiral phosphoric acid-catalyzed enantioselective carbonyl-ene reaction: theoretical elucidation of stereochemical outcomes

An enantioselective carbonyl-ene reaction of 1,1-disubstituted olefins with ethyl glyoxylate was accomplished using an F₁₀BINOL-derived chiral phosphoric acid of which the perfluoro-binaphthyl skeleton is beneficial not only for adopting high catalytic activity but also for creating an effective chiral environment.

An F₁₀BINOL-derived chiral phosphoric acid was shown to be an effective catalyst for an enantioselective carbonyl-ene reaction of 1,1-disubstituted olefins with ethyl glyoxylate as the common enophile. The perfluoro-binaphthyl skeleton is beneficial not only for adopting high catalytic activity but also for creating an effective chiral environment for enantioselective transformations. Indeed, the reaction afforded enantio-enriched homoallylic alcohols in high yields with high enantioselectivities. Theoretical studies identified that the multi-point C–H···O hydrogen bonds and the π interactions between the substrates and the 6-methoxy-2-naphthyl substituents at the 3,3′-positions of the F₁₀BINOL skeleton play a crucial role in determining the stereochemical outcomes. The significance of the perfluoro-binaphthyl skeleton in achieving the high enantioselectivity was also evaluated through a structural analysis of the catalysts.

The control effects of different scaffolds in chiral phosphoric acids: a case study of enantioselective asymmetric arylation

DFT calculations disclosed that the sign of enantioselectivity in chiral-phosphoric-acid catalyzed reactions can be tuned by BINOL- or SPINOL-derived backbones.

A Versatile Bis-Allylboron Reagent for the Stereoselective Synthesis of Chiral Diols

Allylboron reagents are popular in synthesis owing to their versatility and the predictable stereochemical outcomes of their reactions with carbonyl compounds. Herein, we describe the synthesis of (Z,Z)-hexadienyl bis-boronate 1, a configurationally stable, crystalline, and easy to handle compound, which represents a class of bis-allylic boron reagents with heretofore untapped synthetic potential. In combination with a chiral phosphoric acid catalyst, the reagent can be employed for the enantioselective allyl transfer reaction to a variety of one-pot transformations, enabling swift access to functionalized 1,n-diols. The in situ conversion of the reagent into the corresponding bis-borinic ester allows for the direct and diastereoselective two-fold allyl transfer to aldehydes. This affords C₂ - or C_i -symmetric stereotetrads containing a 1,4-diol moiety for natural product synthesis. The usefulness of our method was demonstrated with a short synthesis of the lignan (±)-neo-olivil.

Chiral Brønsted acid-catalyzed intramolecular SN2' reaction for enantioselective construction of a quaternary stereogenic center

Construction of a quaternary stereogenic center was accomplished through the enantioselective intramolecular allylic substitution reaction of bis-trichloroacetimidate catalyzed by a chiral phosphoramide derivative.

An enantioselective intramolecular anti-S_N2′ cyclization reaction for the construction of a quaternary stereogenic center was accomplished through the activation of the leaving group using a binaphthol-derived phosphoramide as the chiral Brønsted acid catalyst. The present allylic substitution reaction is beneficial not only for the regioselective nucleophilic substitution at the multi-substituted site of the double bond but also for controlling the stereochemical outcome because of using a geometrically defined double bond. Indeed, the reaction afforded synthetically useful amino alcohol derivatives having a tetra-substituted carbon center in a highly enantioselective manner in most cases, in which the modification of the sulfonamide unit of the phosphoramide catalyst was demonstrated to improve the enantioselectivity. Experimental and theoretical elucidation of the reaction mechanism suggested that the reaction proceeds through a synchronous anti-S_N2′ pathway, although NMR monitoring of the reaction indicated the formation of the phosphorimidate ester via the S_N2 reaction of the catalyst with the substrate, which results in catalyst deactivation. Further theoretical studies of the origin of the stereochemical outcome at the generated quaternary stereogenic center were performed. Structural analysis of the transition states at the enantio-determining step revealed that the distinct discrimination of the substituents attached to the geometrically defined double bond is achieved by the anthryl and sulfonamide substituents of the catalyst through the three-point hydrogen bonding interactions and the T-shaped C–H···π interactions.

Chiral Phosphoric Acid Catalyzed Enantioselective Ring Expansion Reaction of 1,3-Dithiane Derivatives: Case Study of the Nature of Ion-Pairing Interaction

Chiral counterion controlled asymmetric catalysis via an ion-pairing interaction has attracted immense attention in recent years. Despite a number of successful studies, the mechanistic elucidation of the stereocontrolling element in the ion-pairing interaction is rarely conducted and hence its nature is still far from being well understood. Herein we report an in-depth mechanistic case study of a newly developed enantioselective ring expansion reaction of 1,3-dithiane derivatives catalyzed by chiral phosphoric acid (CPA). An unprecedented enantioselective 1,2-sulfur rearrangement/stereospecific nucleophilic addition sequence was proven to be the stereoselective pathway. More importantly, by thorough investigation of the intrinsic nature of the stereospecific nucleophilic addition to the cationic thionium intermediate, we discovered that the key interaction in this process is the nonclassical C-H···O hydrogen bonds formed between the conjugate base of the CPA catalyst and the cationic intermediate. These C-H···O hydrogen bonds not only bind the catalyst to the substrates to form energetically favored states throughout the overall processes but also firmly maintain the relative positions of these fragments as the "fixed" contact ion pair to sustain the chiral information generated at the initial sulfur rearrangement step. This mechanistic case study provides a very clear understanding of the nature of the ion-pairing interaction in organocatalysis. The conclusion encourages the further development of the research field with the focus to design new organocatalysts and cultivate novel organocatalytic transformations.

Computational insight into the cooperative role of non-covalent interactions in the aza-Henry reaction catalyzed by quinine derivatives: mechanism and enantioselectivity

Density functional theory (DFT) calculations were performed to elucidate the mechanism and the origin of the high enantioselectivity of the aza-Henry reaction of isatin-derived N-Boc ketimine catalyzed by a quinine-derived catalyst (QN). The C-C bond formation step is found to be both the rate-determining and the stereo-controlled step. The results revealed the important role of the phenolic OH group in pre-organizing the complex of nitromethane and QN and stabilizing the in situ-generated nitronate and protonated QN. Three possible activation modes for C-C bond formation involving different coordination patterns of catalyst and substrates were studied, and it was found that both the ion pair-hydrogen bonding mode and the Brønsted acid-hydrogen bonding mode are viable, with the latter slightly preferred for the real catalytic system. The calculated enantiomeric excess (ee) favouring the S enantiomer is in good agreement with the experimental result. The high reactivity and enantioselectivity can be ascribed to the cooperative role of the multiple non-covalent interactions, including classical and non-classical H bonding as well as anionπ interactions. These results also highlight the importance of the inclusion of dispersion correction for achieving a reasonable agreement between theory and experiment for the current reaction.