Uranium = C-H Activation

Uranium is the 92nd element, and the last naturally occurring element, in the periodic table.  I've always held an intense disdain for it since my final project in junior Inorganic Chemistry was to elucidate the energy levels and their symmetry of UO₂. F AND d orbitals?  Seriously?  No one else had to deal with f orbitals. I know it was a test because I was a straight A student and he wanted to see if I was as brilliant as I seemed... nope.  It was an awful presentation, and a classmate was nice enough to bring me home on her red, brand new Vespa to cheer me up. (Bitch.)  Anyway, what I didn't know at the time was that organouranium compounds, and organoactinides on the whole, are actually freaking awesome.  Uranium complexes can catalyze oligomerization, dimerization, hydrosilation and hydroamination of terminal alkynes, hydrogenation of arenes, polymerization of olefins, and coupling of isonitriles with terminal alkynes, all extensively reviewed in Coordination Chemistry Reviews in 2006 (doi:10.1016/j.ccr.2005.12.007).  It's a really interesting review, even if you just check out the schemes.  Another published last year on organouranium and organothorium specifically is also worth checking out (doi 10.1039/b614969n).

What caught my eye was a paper that just came out in Early View in ACIE by the Diaconescu group at UCLA, in which examined the reaction of a dibenzyl uranium complex with multiple equivalents of methylimidazole.  They found that instead of merely coordinating, as one would expect such a heterocycle to do with a transition metal, the metal center inserted itself into the C2 position's C-H bond on two separate methylimidazole molecules; a third equivalent coordinates via the lone pair.  By using deuterated benzene as the solvent, they were able to observe that two equivalents of toluene are also formed, confirming the C-H insertion steps. Mechanistically, uranium doesn't proceed through oxidative addition or reductive elimination steps like the transition metals; rather, it goes through a sigma-bond metathesis type 4-center transition state. The C-H insertion positions are highlighted in red.

That's not all - after the imidazoles are bonded to uranium, crazy sh#t happens.  Two of the imidazoles couple, which prepares one for ring-opening, then migratory insertion to create a crazy structure that  I wouldn't believe if they weren't supported by crystal structures. It's not surprising that it requires a lot of heat and a lot of time to accomplish.

 The intermediates in brackets were not able to be isolated in this case, but when 1-methylbenzimidazole was used instead of 1-methylimidazole, the reaction stopped at the first intermediate directly after the imidazole coupling, and they were able to isolate and confirm the structure of the complex.  Here are the crystal structures of the reaction with 1-methylimidazole to compare with the products (with 50% and 35% ellipsoids, respectively).