text: The first reported decarboxylative cross coupling reaction was an Ullmann reaction, in 1966 by Nilsson et al. Thermal decarboxylation of copper benzoates, in the presence of an aryl halide, was found to produce (both symmetric and unsymmetric) biaryls through aryl-Cu intermediates.
  This monometallic copper system required drastic conditions for complete cross-coupling, and had various intrinsic limitations, both of which prevented development of a catalytic, preparatory version of this reaction.
  It was not until 2009 that Liu and Shang et al. found that decarboxylative cross-coupling of aryl bromides and iodides with potassium polyfluorobenzoates could be achieved using monometallic copper iodide as a catalyst. The oxidative addition step was determined to be the rate-limiting step in the copper-only catalyst cycle (a contrast with Pd-catalyzed decarboxylative cross-coupling).
  Cu(I)-only systems have also been found to promote coupling of alkynyl carboxylic acids with aryl halides (see aryl alkynes below), as well as decarboxylative dehydrogenative cross-coupling of amino acids with alkynes (or similar nucleophiles).
  Catalysts for decarboxylative cross-coupling are of the general form ML2, with a wide variety of ligand types optimized for different substrates. Copper (and silver) centers are often complexed with phenanthrolines, and activity is reported to increase with electron-rich substituents on the ligands.