On the origin of regio- and stereoselectivity in singlet oxygen addition to enecarbamates

NHC-Catalyzed Transformation Reactions of Imines: Electrophilic versus Nucleophilic Attack

It is generally accepted that N-heterocyclic carbene (NHC)-catalyzed imine transformations are initiated by a nucleophilic attack (NA) by NHC. However, due to significant nucleophilicity of the iminyl nitrogen atom in imines, the electrophilic attack (EA) by electrophiles onto imine would also be a possible mechanism of these kinds of reactions. Therefore, we use the quantum mechanical approach to disclose that both the NA and EA modes could be switchable for a wide range of NHC-catalyzed transformations of imines.

The activation strain model and molecular orbital theory

The activation strain model is a powerful tool for understanding reactivity, or inertness, of molecular species. This is done by relating the relative energy of a molecular complex along the reaction energy profile to the structural rigidity of the reactants and the strength of their mutual interactions: ΔE(ζ) = ΔE_strain(ζ) + ΔE_int(ζ). We provide a detailed discussion of the model, and elaborate on its strong connection with molecular orbital theory. Using these approaches, a causal relationship is revealed between the properties of the reactants and their reactivity, e.g., reaction barriers and plausible reaction mechanisms. This methodology may reveal intriguing parallels between completely different types of chemical transformations. Thus, the activation strain model constitutes a unifying framework that furthers the development of cross-disciplinary concepts throughout various fields of chemistry. We illustrate the activation strain model in action with selected examples from literature. These examples demonstrate how the methodology is applied to different research questions, how results are interpreted, and how insights into one chemical phenomenon can lead to an improved understanding of another, seemingly completely different chemical process. WIREs Comput Mol Sci 2015, 5:324-343. doi: 10.1002/wcms.1221.

Noninnocent role of N-methyl pyrrolidinone in thiazolidinethione-promoted asymmetric aldol reactions

The origin of stereoselectivity in the reaction between α-azido titanium enolate derived from chiral auxiliary N-acyl thiazolidinethione and benzaldehyde is established using the DFT(B3LYP) method. A nonchelated transition state with N-methyl-2-pyrrolidinone (NMP) bound to a TiCl(3) enolate is found to be energetically the most preferred model responsible for the formation of an Evans syn aldol product. The TS model devoid of NMP, although of higher energy, is found to be successful in predicting the right stereochemical outcome.