NMR Quantification of Halogen-Bonding Ability To Evaluate Catalyst Activity.

Quantification of halogen-bonding abilities is described for a series of benzimidazolium-, imidazolium- and bis(imidazolium) halogen-bond donors (XBDs) using 31P NMR spectroscopy. The measured Δδ(31P) values correlate with calculated activation free energy ΔG‡ and catalytic activity for a Friedel-Crafts indole addition. This rapid method also serves as a sensitive indicator for Brønsted acid impurities.