Mechanism and reactivity in the Morita-Baylis-Hillman reaction: the challenge of accurate computations

Mixed Explicit-Implicit Solvation Approach for Modeling of Alkane Complexation in Water-Soluble Self-Assembled Capsules

The host-guest binding properties of a water-soluble resorcinarene-based cavitand are examined using density functional theory methodology. Experimentally, the cavitand has been observed to self-assemble in aqueous solution into both 1:1 and 2:1 host/guest complexes with hydrophobic guests such as n-alkanes. For n-decane, equilibrium was observed between the 1:1 and 2:1 complexes, while 1:1 complexes are formed with shorter n-alkanes and 2:1 complexes are formed with longer ones. These findings are used to assess the standard quantum chemical methodology. It is first shown that a rather advanced computational protocol (B3LYP-D3(BJ)/6-311+G(2d,2p) with COSMO-RS and quasi-rigid-rotor-harmonic-oscillator) gives very large errors. Systematic examination of the various elements of the methodology shows that the error stems from the implicit solvation model. A mixed explicit-implicit solvation protocol is developed that involves a parametrization of the hydration free energy of water such that water cluster formation in water is predicted to be thermoneutral. This new approach is demonstrated to lead to a major improvement in the calculated binding free energies of n-alkanes, reproducing very well the 1:1 versus 2:1 host/guest binding trends.

A paramedic treatment for modeling explicitly solvated chemical reaction mechanisms

We report a static quantum chemistry modeling treatment to study how solvent molecules affect chemical reaction mechanisms without dynamics simulations. This modeling scheme uses a global optimization procedure to identify low energy intermediate states with different numbers of explicit solvent molecules and then the growing string method to locate sequential transition states along a reaction pathway. Testing this approach on the acid-catalyzed Morita-Baylis-Hillman (MBH) reaction in methanol, we found a reaction mechanism that is consistent with both recent experiments and computationally intensive dynamics simulations with explicit solvation. In doing so, we explain unphysical pitfalls that obfuscate computational modeling that uses microsolvated reaction intermediates. This new paramedic approach can promisingly capture essential physical chemistry of the complicated and multistep MBH reaction mechanism, and the energy profiles found with this model appear reasonably insensitive to the level of theory used for energy calculations. Thus, it should be a useful and computationally cost-effective approach for modeling solvent mediated reaction mechanisms when dynamics simulations are not possible.

Intra- versus Intermolecular Carbon-to-Carbon Proton Transfers in the Reactions of Arynes with Nitrogen Nucleophiles: A DFT Study

Carbon-to-carbon proton transfer (PT) is an elementary reaction in organic chemistry. Herein we report a systematic DFT study on the carbon-to-carbon proton transfers in the reactions of arynes with nitrogen nucleophiles, including tertiary amines, aziridines, imines, and N-heteroarenes. DFT calculations indicated that the Gibbs energies of activation for the 1,4- and 1,5-proton transfers involved are lower than 20 kcal/mol in most cases. We also found that the Bell-Evans-Polanyi principle applies to the 1,4-proton transfers, indicating that the more exergonic an intramolecular proton transfer is (the more stable a carbanion is generated), the faster it will be. Previous experimental studies have shown that, in the presence of a carbon nucleophile (NuH; e.g., CHCl₃ and MeCN) as the third component (also as the solvent), intermolecular proton abstractions may compete with the intramolecular proton transfers and lead to different products. Our theoretical rationalization of these competitions is that the introduction of an electron-withdrawing group at the acidic site can accelerate the intramolecular proton transfer dramatically (owing to the generation of a more stable carbanion), making it compete with the intermolecular processes. In addition, we have also discussed the competition between intra- and intermolecular proton transfers when NuH is used as the reactant rather than the solvent.

Computational prediction of chemical reactions: current status and outlook

Over the past few decades, various computational methods have become increasingly important for discovering and developing novel drugs. Computational prediction of chemical reactions is a key part of an efficient drug discovery process. In this review, we discuss important parts of this field, with a focus on utilizing reaction data to build predictive models, the existing programs for synthesis prediction, and usage of quantum mechanics and molecular mechanics (QM/MM) to explore chemical reactions. We also outline potential future developments with an emphasis on pre-competitive collaboration opportunities.