Variable Temperature LED-NMR: Rapid Insights into a Photocatalytic Mechanism from Reaction Progress Kinetic Analysis

J Org Chem. 2022 Sep 2;87(17):11776-11782. doi: 10.1021/acs.joc.2c01479. Epub 2022 Aug 15.

Abstract

A multitude of techniques are available to obtain a useful understanding of photocatalytic mechanisms. The combination of LED illumination with nuclear magnetic resonance spectroscopy (LED-NMR) provides a rapid, convenient means to directly monitor a photocatalytic reaction in situ. Herein, we describe a study of the mechanism of an enantioselective intermolecular [2 + 2] photocycloaddition catalyzed by a chiral Ir photocatalyst using LED-NMR. The data-rich output of this experiment is suitable for same-excess and variable time normalization analyses (VTNA). Together, these identified an unexpected change in mechanism between reactions conducted at ambient and cryogenic temperatures. At -78 °C, the kinetic data are consistent with the triplet rebound mechanism we previously proposed for this reaction, involving sensitization of maleimide and rapid reaction with a hydrogen-bound quinoline within the solvent cage. At room temperature, the cycloaddition instead proceeds through intracomplex energy transfer to the hydrogen-bound quinolone. These results highlight the potential sensitivity of photocatalytic reaction mechanisms to the precise reaction conditions and the further utility of LED-NMR as a fast, data-rich tool for their interrogation that compares favorably to conventional ex situ kinetic analyses.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Catalysis
  • Hydrogen*
  • Kinetics
  • Magnetic Resonance Spectroscopy / methods
  • Temperature

Substances

  • Hydrogen