I was running a copper-catalyzed coupling reaction the other day when my summer intern asked me how it worked. I showed her the mechanism that the authors of the paper had proposed, but pointed out that it was mostly hand-waving. The general features are probably more or less right: the copper iodide presumably does form some kind of soluble complex with the amino acid that’s needed in the reaction mix, and that may well form some sort of complex with the aryl halide, which opens up the ring to nucleophilic substitution, etc. If this were an exam, I’d give full points for that one.
But a lot of these couplings are, as I pointed out to her, very hazily worked out. The Ullman reaction, in various forms, has been with us for many decades, and there are more variations on it than you can count. If it always worked reasonably well, or if people had any strong ideas about how it did so, the literature on it wouldn’t be in the shaggy shape it is. Copper chemistry in particular has been (simultaneously) a very useful area for people to discover new reactions, and a horrible trackless swamp for people trying to explain how they work.
All you have to do is look at the vicious exchanges between Bruce Lipschutz and Steve Bertz during the 1990s about whether such as thing as a “higher-order cuprate” exists. I have absolutely no intention of reconstructing this argument; I would have to be paid at a spectacular hourly rate to even attempt it. It's enough to say that the arguments raged, in an increasingly personal manner, about what state the copper metal was in, what ligands coordinated to it, and what the active form of these reagents might be (as opposed to what the bulk of the mixture was at any given time). It culminated in what must be one of the most direct titles for a scientific paper I've ever seen: It's on lithium! An answer to the recent communication which asked the question: 'if the cyano ligand is not on copper, then where is it?'. That's in Chemical Communications 7, 815 (1996), if you're interested (here's the PDF for subscribers). Bertz continued to shell Lipshutz's position past the time when any fire was being returned, as far as I can tell, and continues to work in the area. Lipshutz, for his part, hasn't published on the higher-order cuprates in some time (being no doubt heartily sick of the whole topic), but has kept up a steady stream of work on new reactions involving copper, nickel, and other metals.
So if well-qualified researchers, brimming with grad students, postdocs, and grant money, can argue for years about copper mechanisms, I'm going to stay out of it. As time goes on, I'm increasingly indifferent to reaction mechanisms, anyway. I want to get product out the other end of the reaction. And while there are times when knowing the mechanism can help reach that goal, those times do not occur as frequently as you might hope.