The Gilded Edge in Acetylenic Scaffolding II: A Computational Study of the Transmetalation Processes Involved in Palladium-Catalyzed Cross-Couplings of Gold(I) Acetylides

The
palladium-catalyzed cross-coupling reaction between phosphine-gold­(I)
acetylides and aryl iodides has recently proven as a convenient alternative
to the standard Sonogashira reaction, which instead employs terminal
alkynes as substrates. This alternative reaction does not require
the presence of an amine base, but still, however, requires a copper
cocatalyst (CuI). In this theoretical work we have investigated the
possible roles that this copper catalyst may play. Three transmetalation
pathways can be imagined, proceeding by either (i) transferring the
acetylide from gold to copper and thereafter to palladium, (ii) directly
transferring the acetylide from gold to palladium, or (iii) directly
transferring the acetylide from gold to palladium but aided by a copper
coordination to the triple bond. Calculations reveal that the first
of these is the most viable reaction pathway, as it involves the initial
formation of a very favorable copper/gold acetylide complex. The transmetalations
along this pathway run via several equilibria.