The Sonogashira coupling reaction is pretty unique because it utilizes cocatalysts. Now, we're going to say that this coupling reaction is a palladium catalyzed reaction of a copper(1) alkyne complex with a vinyl or aryl halide. Now, we need to realize what's the function of these 2 catalysts. Well, we're going to say here that the palladium catalyst, all it's going to do is it's going to partake in the oxidative addition step. So it's prepping our R1X group for the coupling reaction. So it's working with the carbon halide. The copper(one) catalyst, what it does is it's prepping our coupling agent. So, our R2C group. And by prepping the R1 and R2 group, that's going to lead them to combining later on to create our coupling product. So we're going to say here that the Sonogashira coupling reaction starts out with a step 0, where we have the preparation of our coupling agent through the use of our copper(one) catalyst. So what's going to happen here is that our alkyne carbon is weakly acidic. Therefore, the triethylamine that we use will deprotonate it. We wind up getting here R2 connected to our triple bonded carbon which now possesses a negative charge.
Here we have copper(1) iodide. Remember, this is an ionic compound. So it's not really together. It's just 2 free-floating ions that are close together because of opposite charges. So as a result of this, this negative carbon will grab onto this positive copper. So what we're going to get at this point is we have our R2 group connected to our alkyne which is now connected to our copper. We're going to say here that our R2 group has been primed or it's been activated so that it can later on react within this coupling reaction. Now, we head into step 1 here which is our oxidative addition step where things are what we are accustomed to seeing in terms of a cross-coupling reaction. We have our palladium with its d orbital electrons with that lone pair. They're going to attach to my X group here and this is going to break and attach right back to the palladium. So we're going to get our palladium still connected to its 2 ligands, connected to the X group and now connected to R1.
This leads us into step 2 transmetalation where our primed R2 group or our copper(1) alkyneal compound is going to react now with the transition metal complex from the oxidative addition step. So, here we're going to have our R2 group being transferred from the copper to the palladium. Remember that's going to cause this bond here to break. It's going to attach to the palladium at the same time this X leaves and attaches to the copper. So what we wind up getting is we're going to get R1 still connected to palladium with its 2 ligands attached. And now my R2 group is attached.
And then finally, we have our final step which is reductive elimination, We're going to have the R1 group attaching to this carbon and then having this bond breaking going to the palladium. So what we're winding up getting is our R1 attaching to the alkyne which is part of R2, and then the regeneration of our transition metal catalyst which is a driving force for this to go through another cycle in terms of the Sonogashira coupling reaction. So remember, this reaction is pretty unique through the use of the 2 catalysts. So they work in conjunction with one another to help prime my R1 group and R2 group so at the end they combine together to form my coupling product. So we have the addition of that, step 0 which we usually don't see in terms of coupling reactions but in this case, we do have it. And then followed by our 3 normal steps of oxidative addition, transmetalation, and reductive elimination to round out the 4 steps involved in the Sonogashira coupling reaction.