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

Complexity in Acid-Base Titrations: Multimer Formation Between Phosphoric Acids and Imines

Solutions of Brønsted acids with bases in aprotic solvents are not only common model systems to study the fundamentals of proton transfer pathways but are also highly relevant to Brønsted acid catalysis. Despite their importance the light nature of the proton makes characterization of acid-base aggregates challenging. Here, we track such acid-base interactions over a broad range of relative compositions between diphenyl phosphoric acid and the base quinaldine in dichloromethane, by using a combination of dielectric relaxation and NMR spectroscopy. In contrast to what one would expect for an acid-base titration, we find strong deviations from quantitative proton transfer from the acid to the base. Even for an excess of the base, multimers consisting of one base and at least two acid molecules are formed, in addition to the occurrence of proton transfer from the acid to the base and simultaneous formation of ion pairs. For equimolar mixtures such multimers constitute about one third of all intermolecular aggregates. Quantitative analysis of our results shows that the acid-base association constant is only around six times larger than that for the acid binding to an acid-base dimer, that is, to an already protonated base. Our findings have implications for the interpretation of previous studies of reactive intermediates in organocatalysis and provide a rationale for previously observed nonlinear effects in phosphoric acid catalysis.

Novel efficient hole-transporting materials based on a 1,1′-bi-2-naphthol core for perovskite solar cells

New hole-transporting materials based on 1,1′-bi-2-naphthol and carbazole.

Origins of Selectivity and General Model for Chiral Phosphoric Acid-Catalyzed Oxetane Desymmetrizations

The origins of the high enantioselectivity of chiral phosphoric acid-catalyzed oxetane desymmetrizations were investigated by density functional theory (DFT) calculations. Distortion of the catalyst structure, caused by steric crowding in the catalyst pocket of one enantiomeric transition state, is the main cause for stereochemical preference. A general model was developed to assist in the rational design of new catalysts for related transformations.

Goldilocks Catalysts: Computational Insights into the Role of the 3,3' Substituents on the Selectivity of BINOL-Derived Phosphoric Acid Catalysts

BINOL-derived phosphoric acids provide effective asymmetric catalysis for many organic reactions. Catalysts based on this scaffold show a large structural diversity, especially in the 3,3' substituents, and little is known about the molecular requirements for high selectivity. As a result, selection of the best catalyst for a particular transformation requires a trial and error screening process, as the size of the 3,3' substituents is not simply related to their efficacy: the right choice is neither too large nor too small. We have developed an approach to identify and quantify structural features on the catalyst that determine selectivity. We show that the application of quantitative steric parameters (a new measure, AREA(θ), and rotation barrier) to an imine hydrogenation reaction allows the identification of catalyst features necessary for efficient stereoinduction, validated by QM/MM hybrid calculations.

A Practical Guide for Predicting the Stereochemistry of Bifunctional Phosphoric Acid Catalyzed Reactions of Imines

Chiral phosphoric acids have become powerful catalysts for the stereocontrolled synthesis of a diverse array of organic compounds. Since the initial report, the development of phosphoric acids as catalysts has been rapid, demonstrating the tremendous generality of this catalyst system and advancing the use of phosphoric acids to catalyze a broad range of asymmetric transformations ranging from Mannich reactions to hydrogenations through complementary modes of activation. These powerful applications have been developed without a clear mechanistic understanding of the reasons for the high level of stereocontrol. This Account describes investigations into the mechanism of the phosphoric acid catalyzed addition of nucleophiles to imines, focusing on binaphthol-based systems. In many cases, the hydroxyl phosphoric acid can form a hydrogen bond to the imine while the P═O interacts with the nucleophile. The single catalyst, therefore, activates both the electrophile and the nucleophile, while holding both in the chiral pocket created by the binaphthol and constrained by substituents at the 3 and 3' positions. Detailed geometric and energetic information about the transition states can be gained from calculations using ONIOM methods that combine the advantages of DFT with some of the speed of force fields. These high-level calculations give a quantitative account of the selectivity in many cases, but require substantial computational resources. A simple qualitative model is a useful complement to this complex quantitative model. We summarize our calculations into a working model that can readily be sketched by hand and used to work out the likely sense of selectivity for each reaction. The steric demands of the different parts of the reactants determine how they fit into the chiral cavity and which of the competing pathways is favored. The preferred pathway can be found by considering the size of the substituents on the nitrogen and carbon atoms of the imine electrophile, and the position of the nucleophilic site on the nucleophile in relation to the hydrogen-bond which holds it in the catalyst active site. We present a guide to defining the pathway in operation allowing the fast and easy prediction of the stereochemical outcome and provide an overview of the breadth of reactions that can be explained by these models including the latest examples.

QM/MM study on the enantioselectivity of spiroacetalization catalysed by an imidodiphosphoric acid catalyst: how confinement works

An explanation of why confined imidodiphosphoric acid catalyst succeeds where other chiral phosphoric acid catalysts fail.

Ruthenium-Catalyzed Asymmetric Hydrohydroxyalkylation of Butadiene: The Role of the Formyl Hydrogen Bond in Stereochemical Control

The catalyst generated in situ from RuH2(CO)(PPh3)3, (S)-SEGPHOS, and a chiral phosphoric acid promotes asymmetric hydrohydroxyalkylation of butadiene and affords enantioenriched α-methyl homoallylic alcohols. The observed diastereo- and enantioselectivities are determined by both the chiral phosphine and chiral phosphate ligands. Density functional theory calculations (M06/SDD-6-311G(d,p)-IEFPCM(acetone)//B3LYP/SDD-6-31G(d)) predict that the product distribution is controlled by the kinetics of carbon-carbon bond formation, and this process occurs via a closed-chair Zimmerman-Traxler-type transition structure (TS). Chiral-phosphate-dependent stereoselectivity arising from this TS is enabled through a hydrogen bond between the phosphoryl oxygen and the aldehyde formyl proton present in TADDOL-derived catalysts. This interaction is absent in the corresponding BINOL-derived systems, and the opposite diastereo- and enantioselectivity is observed. Additional factors influencing the stereochemical control are determined.

Asymmetric boronate addition to o-quinone methides: ligand exchange, solvent effects, and Lewis acid catalysis

Density functional theory calculations suggest that asymmetric boronate addition to o-quinone methides proceeds via a Lewis acid catalyzed process through a closed six-membered transition structure. The BINOL-derived catalyst undergoes an exchange process with the original ethoxide boronate ligands. This activation mode successfully accounts for the sense and level of enantioselectivity observed experimentally. A qualitative model which accurately predicts the observed enantioselectivity has been developed and is consistent with results from our study of ketone propargylation demonstrating the reaction model's generality. The effects of replacing the BINOL framework with H8-BINOL have been rationalized.

Base-mediated cascade rearrangements of aryl-substituted diallyl ethers

Two base-mediated cascade rearrangement reactions of diallyl ethers were developed leading to selective [2,3]-Wittig-oxy-Cope and isomerization-Claisen rearrangements. Both diaryl and arylsilyl-substituted 1,3-substituted propenyl substrates were examined, and each exhibits unique reactivity and different reaction pathways. Detailed mechanistic and computational analysis was conducted, which demonstrated that the role of the base and solvent was key to the reactivity and selectivity observed. Crossover experiments also suggest that these reactions proceed with a certain degree of dissociation, and the mechanistic pathway is highly complex with multiple competing routes.

Mechanism and selectivity of N-triflylphosphoramide catalyzed (3(+) + 2) cycloaddition between hydrazones and alkenes

Brønsted acid catalyzed (3(+) + 2) cycloadditions between hydrazones and alkenes provide a general approach to pyrazolidines. The acidity of the Brønsted acid is crucial for the catalytic efficiency: the less acidic phosphoric acids are ineffective, while highly acidic chiral N-triflylphosphoramides are very efficient and can promote highly enantioselective cycloadditions. The mechanism and origins of catalytic efficiencies and selectivities of these reactions have been explored with density functional theory (M06-2X) calculations. Protonation of hydrazones by N-triflylphosphoramide produces hydrazonium-phosphoramide anion complexes. These ion-pair complexes are very reactive in (3(+) + 2) cycloadditions with alkenes, producing pyrazolidine products. Alternative 1,3-dipolar (3 + 2) cycloadditions with the analogous azomethine imines are much less favorable due to the endergonic isomerization of hydrazone to azomethine imine. With N-triflylphosphoramide catalyst, only a small distortion of the ion-pair complex is required to achieve its geometry in the (3(+) + 2) cycloaddition transition state. In contrast, the weak phosphoric acid does not protonate the hydrazone, and only a hydrogen-bonded complex is formed. Larger distortion energy is required for the hydrogen-bonded complex to achieve the "ion-pair" geometry in the cycloaddition transition state, and a significant barrier is found. On the basis of this mechanism, we have explained the origins of enantioselectivities when a chiral N-triflylphosphoramide catalyst is employed. We also report the experimental studies that extend the substrate scope of alkenes to ethyl vinyl ethers and thioethers.

Secondary stereocontrolling interactions in chiral Brønsted acid catalysis: study of a Petasis–Ferrier-type rearrangement catalyzed by chiral phosphoric acids

Guided by computational studies, the involvement of non-classical C–H⋯O hydrogen bonds and π–π stacking interactions were found to be crucial for high stereocontrol in a chiral phosphoric acid-catalyzed reaction.