Formation of Uranium Pinacolate from the Reductive Coupling of Acetone

1,2‐Diols are elemental building blocks in organic synthesis. The standard preparation methods involve the reductive coupling of ketones such as acetone, often using in situ reduced electropositive metal species. Uranium tetrachloride in combination with Na(Hg) as reductant was shown previously to allow for mechanistic studies of the pinacol coupling reaction, but not for direct isolation of intermediates. In this study, using three well‐defined uranium(III) complexes with different cyclopentadienyl ligands, pinacol coupling of acetone was observed resulting in a pinacolate moiety between two uranium(IV) compounds. The uranium pinacolate complexes were characterized by IR, NMR, mass spectrometry, and in one case by single crystal x‐ray diffraction. This first direct structural characterization of the reaction product confirms the previously suggested role of metallapinacolates in the pinacol coupling mechanism. Mass spectrometry additionally confirmed the proposed formation of uranium hydroxide complexes in the subsequent hydrolysis step releasing the pinacol.