So, the first step of our Heck reaction mechanism begins with oxidative addition. This involves the addition of the carbon halide to the transition metal complex. Now, here we have our palladium catalyst. It possesses a lone pair which comes from the d orbital electrons of palladium. By utilizing this lone pair, we're going to connect to our X group of the carbon halide. When we connect to that X group at the same time, we have the breaking of this bond here, where it connects to palladium. So what we end up making is our transition metal complex where R and X have attached to the palladium.
Now, normally with a lot of these coupling reactions, we have 3 major steps: oxidative addition, transmetalation, and reductive elimination. When it comes to the Heck reaction, there's a small wrinkle. Instead of doing a traditional transmetalation step, instead we do a syn addition step. Now with syn addition, because of the presence of the alkene, we're going to say the resulting complex reacts with the alkene and adds R1 and palladium across the pi bond on the same side. Because they're adding on the same side that's why it's called syn addition.
So, we have our X group that got added to the Palladium, and we have our R group. We're going to say here that my R group and then my Palladium. I'm going to say our Palladium complex. Two separate things are getting attached on the same side onto the pi bond. So that means that our new structure R group and our palladium complex are being added to the same side.
Next, we have our reductive eliminations which can be broken down into 3a and 3b. The first one, step 3a, has a new complex undergo a carbon-carbon bond rotation followed by a syn elimination to give an alkene. So from the complex we created in step 2 under syn addition, we're going to bring that down. Then we have our R here and the palladium complex. We have to undergo a 60 degree bond rotation because we need both the hydrogen and this palladium complex to be on the same side so that they can be eliminated by syn elimination. So we're going to rotate around this single bond, we're going to keep the palladium complex in the same position. All that's rotating is the Hydrogen and the R group. So Hydrogen will rotate when we rotate this way, H moves over here. So now, the H is here and then the R gets moved over here. Now, we're going to have our reductive elimination that happens here.
So, under this reductive elimination, these bonds break because hydrogen escaping as hydride connects to the palladium. And then this bond here breaks, carbon being more electronegative than palladium, takes the bond to help make a pi bond. So, we're going to make a double bond here, and then the R group is right here. So under the first step within step 3a, we have the formation of our alkene. What we wanted to do.
Step 3b is all about regenerating the palladium catalyst. So here the bases that we usually use are our acetate ion or hydrogen carbonate or bicarbonate. So what happens here is that they represent a base, they're going to remove the hydrogen which causes this bond to break in palladium to hold on to the electrons and at the same time for the X group to leave because it represents a good leaving group. So we wind up getting the regeneration of our palladium catalyst.
Remember there are 2 major driving forces that come with these coupling reactions. We're trying to make a more conjugated product, and also we're trying to get the transition metal to follow either the 18 or 16 electron rule. At this point, by regenerating the catalyst in its original form, the palladium no longer follows the 18 or 16 electron rule. So this puts pressure on it to go through the cycle again to get closer to the 18 or 16 electron rule. So this is the driving force that's going to want us to redo the whole reaction, get palladium back to its optimal 18 or 16 electron count, and also to generate more alkene product. Now that we've seen this mechanism, we're going to attempt to answer the example question by following this whole mechanism of the Heck reaction.
