Determination of Absolute Configuration of Acyclic 1,2-Diols with [Mo2(OAc)4], Part 2: New Structural Evidence toward a Rationale of the Method: What Remains of [Mo2(OAc)4] in DMSO Solution?

The nature of the CD-active species obtained by mixing dimolybdenum tetraacetate and a chiral 1,2-diol in DMSO has been studied by different techniques (1 D and 2D H-1 NMR, CD, UV/Vis) with two substrates, (R)phenyl-1,2-ethanediol (1) and (RR)butane-2,3-diol (2). The diol/dimolybdenum adducts have diagnostic CID spectra whose sign correlates with the absolute configuration of the organic substrate. It is demonstrated that, in DMSO solution, the acetate ligands of [Mo-2(OAc)(4)] dissociate to a large extent under the action of the dissolved water, yielding acetic acid and a poly-hydrated dimolybdenum species. Addition of a 1,2-diol leads to chelation with formation of one main active species for 2 and two for 1, all with 1:1 stoichiometries at diol/dimolybdenum molar ratios less than 1.5. Only a small fraction (less than 20%) of the 1,2-diol is bound. The structures of the active complexes are estimated on the basis of NMR spectra, by correlating the observed chemical shifts with the quadruple bond diamagnetic anisotropy. In the predominating complexes for 1 and 2, the 1,2-diol moiety bridges the Mo-2, core forming a six-membered ring.