N-Heterocyclic Carbene Organocatalyzed Redox-Active/Ring Expansion Reactions: Mechanistic Insights Unveiling Base Cooperativity.

N-Heterocyclic carbene (NHC) organocatalyzed transformations of redox-active chemical manifolds is a powerful strategy for interconverting and expanding the chemical space. This approach in the context of ring expansion holds promise for preparing lactones from plentiful redox active aldehydes, despite a lack of rigorous mechanistic insights into the underlying elements governing this reactivity and with-it relevance to other NHC organocatalyzed transformations. Herein, in investigating this reactivity under the lens of modern day quantum mechanical calculations, we explore the mechanism of redox-active/ring expansion reactions of aldehydes furnishing lactone products by means of NHC organocatalysis. Through this comprehensive study, the underpinning mechanism of Breslow intermediate formation and ensuing downstream processes such as intermolecular C-C bond formation providing benzoin products versus intramolecular redox pathways are outlined. Notably, this study of NHC organocatalysis reveals the diverse role of bases as cooperative agents in directing and selectively routing reactivity, as highlighted here toward ring expanded lactone products.