Mechanistic Insight toward Understanding the Role of Charge in Thiourea Organocatalysis.

Abstract

Pyranylation and glycosylation are pivotal for accessing a myriad of natural products, pharmaceuticals, and drug candidates. Catalytic approaches for enabling these transformations are of utmost importance and integral to advancing this area of synthesis. In exploring this chemical space, a combined experimental and computational mechanistic study of pyranylation and 2-deoxygalactosylation catalyzed by a cationic thiourea organocatalyst is reported. To this end, a thiourea-cyclopropenium organocatalyst was employed as a model system in combination with an arsenal of mechanistic techniques, including 13C kinetic isotope effect experiments, deuterated labeling studies, variable-temperature 1H NMR spectroscopy, and density functional theory calculations. From these studies, two distinct reaction pathways were identified for this transformation corresponding to either dual hydrogen bond (H-bond) activation or Brønsted acid catalysis. The former involving thiourea orchestrated bifurcated hydrogen bonding proceeded in an asynchronous concerted fashion. In contrast, the latter stepwise mechanism involving Brønsted acid catalysis hinged upon the formation of an oxocarbenium intermediate accompanied by subsequent alcohol addition.