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 pi-acceptor strength of N-heterocyclic carbenes is presented. The Se-77 chemical shifts of the easily available selenium carbene adducts 1.Se-7.Se correlate with the pi-acceptor character of the respective carbenes. The observed delta(Se-77) values cover a range of almost 800 ppm, with increasing pi-acidity leading to a downfield shift of the signal.