An important aspect of the assay is the effect of carrying out the reaction in deuterated solvent (Scheme 2). If the xylyl radical attacks benzene (kC-C) to form 11-d6 in a rate-determining step (i.e. if the subsequent conversion to 7 including the cleavage of the C-D bond is not rate limiting, then carrying out the reaction with benzene-d6 would not incur a primary KIE and thus no fall in the yield of 7-d5 (relative to the yield of 7 when C6H6 was the solvent) will be seen. (relative to 8 when the reaction is carried out in C6H6). If 7 is instead formed via an organometallic pathway such as CMD, a large KIE effect would be expected for formation of 7. For all palladium/ligand combinations tested (Table ), replacing benzene with benzene-d6 led to no change, or indeed a slight increase, in the yield of 7. This was accompanied by a significant fall in the yield of 8, and 9 to a lesser extent. This is consistent with the formation of a xylyl radical from 6. The lesser fall in the yield of 9, and the fact that it remains predominantly unlabelled (deuterium incorporation between 2.5% and 7.2%), has been explored in our previous ground state studies with this assay and is due to the large concentration of reactive benzylic C-H bonds providing a ready source of hydrogen atoms. In entries 2, 4 and 6, while the major coupled products in all cases are 7-d5 and 8-d10, there are traces of 7, 8-d5 and 8 arising from incorporation of the phenyl groups on the ligands (see S.I. for details). In the case of dippf, however, 7 and 8 were entirely 7-d5 and 8-d10 (entry 8).