A maleimide-reactive reagent used to functionalize peptides or proteins with an azide protecting group. The azide group can then be further modified via a Staudinger reduction.1
Staudinger, H.; Meyer, J. UberneueorganischePhosphorverbindungen III. Phosphinmethylenederivative und Phosphinimine. Helvetica Chim. Acta. 1919 (2) 635-646.