If I were to run a poll, asking the practitioners of cyclizations about their most commonly dreaded side reactions, I am sure the answer will be clear: oligomerization and polymerization will be close to the top of that list. Entropically, this is to be expected of a bifunctional intermediate, especially when it comes to macrocyclization reactions where entropy is really not our ally. I was interested in seeing a reaction shown below. Not long ago, this process was reported by Kitamura and co-workers in Organic Letters. I find this palladium-catalysed cyclization interesting because it represents a somewhat odd outcome: this is not what you might consider an oligomerization, yet this is also far from a simple annulation. I chose one of the better examples. There are indeed mixtures of products in other entries reported in this work. But still, this “in-between” business when the system is not reaching the oligomeric abyss is interesting and is undoubtedly governed by some good rationale – so you should look at the mechanism.
amphoteros 