Determining the π‑Acceptor Properties of N‑Heterocyclic Carbenes by Measuring the ⁷⁷Se NMR Chemical Shifts of Their Selenium Adducts

A new method for the assessment of the π-acceptor strength of N-heterocyclic carbenes is presented. The ⁷⁷Se chemical shifts of the easily available selenium carbene adducts <b>1</b>·Se–<b>7</b>·Se correlate with the π-acceptor character of the respective carbenes. The observed δ­(⁷⁷Se) values cover a range of almost 800 ppm, with increasing π-acidity leading to a downfield shift of the signal