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.

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.

Hydrogen-Bond-Dependent Conformational Switching: A Computational Challenge from Experimental Thermochemistry

We have compiled an experimental data set (SWITCH10) of equilibrium constants for a series of hydrogen-bond-dependent conformational switches. These organic molecules possess common functionalities and are representative in terms of size and composition of systems routinely studied computationally. They exist as two well-defined conformations which serve as a useful tool to benchmark computational estimates of experimental Gibbs energy differences. We examine the performance of HF theory and a variety of density functionals (B3LYP, B3LYP-D3, CAM-B3LYP, ωB97X-D, M06-2X) against these experimental benchmarks. Surprisingly, despite a strong similarity between the two switch conformations, the average errors (0.4-1.7 kcal·mol^-1) obtained across the data set for all methods are larger than obtained with HF calculations. B3LYP was found to outperform implicitly and explicitly dispersion-corrected functionals, with an average error smaller by 1 kcal·mol^-1. Unsystematic errors in the optimized structures were found to contribute to the relatively poor performance obtained, while quasi-rigid rotor harmonic oscillator thermal contributions are important in improving the accuracy of computed Gibbs energy differences. These results emphasize the challenge of quantitative accuracy in computing solution-phase thermochemistry for flexible systems and caution against the often used (but unstated) assumption of favorable error cancellation in comparing conformers or stereoisomers.

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.

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.

Enantioselectivity in CPA-catalyzed Friedel–Crafts reaction of indole and N-tosylimines: a challenge for guiding models

Reaction models derived from theoretical investigations for predicting the enantioselectivity of organocatalytic reactions are very useful, but difficult to formulate for the Friedel–Crafts reaction of indole and N-tosylimines.

Selecting Chiral BINOL-Derived Phosphoric Acid Catalysts: General Model To Identify Steric Features Essential for Enantioselectivity

Choosing the optimal catalyst for a new transformation is challenging because the ideal molecular requirements of the catalyst for one reaction do not always simply translate to another. Large groups at the 3,3' positions of the binaphthol rings are important for efficient stereoinduction but if they are too large this can lead to unusual or poor results. By applying a quantitative steric assessment of the substituents at the 3,3' positions of the binaphthol ring, we have systematically studied the effect of modulating this group on enantioselectivity for a wide range of reactions involving imines, and verified this analysis using ONIOM calculations. We have shown that in most reactions, the stereochemical outcome depends on both proximal and remote sterics. Summarising detailed calculations into a simple qualitative model identifies and explains the steric features required for high selectivity. This model is consistent with seventy seven papers reporting reactions (over 1000 transformations in total), and provides a straightforward decision tree for selecting the best catalyst.

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.