Diazo compounds and palladium-aryl complexes: trapping the elusive carbene migratory insertion organometallic products.

The reactions of Pd-aryl complexes with diazo compounds NCH-CHCHPh and NCHPh allowed us to isolate the organometallic products formed right after the migratory insertion of a non-stabilized CHR carbene into the Pd-aryl bond. η-Allylic and η-benzylic palladium complexes were formed respectively. This is compelling experimental evidence for the key step in the palladium-catalyzed cascade transformations of diazo derivatives leading to multiple C-C or C-X bond formation.