Structures of Zinc Ketone Enolates

Solution- and solid-phase structural characterizations of a series of simple zinc ketone enolates are
presented. The new enolate derivatives were prepared (isolated yields 38−69%) by the deprotonation of
ketones by the bis(arylzinc) species RLZn2Ph2 (1) (where RL2- are bis(amidoamine) ligands). Ketones
used include 2‘,3‘,4‘,5‘,6‘-pentamethylacetophenone, cyclohexanone, 2,2-dimethylcyclopentanone, 2,4-dimethyl-3-pentanone, and isobutyrophenone. These ketones were chosen because they have varying
degrees of substitution at the acidic carbon (i.e., α to the carbonyl group). Thus, the effect of sterics on
the structural preferences of zinc enolates was examined. In all cases, both solid- and solution-phase
structural data indicate that the anionic enolate ligands have isolated (and uncoordinated) carbon−carbon
double bonds. The enolate anions bind to the Zn centers with O-bridged structures. Single-crystal X-ray
diffraction data are reported for MeLZn2[OC(CMe2)iPr]2 (2a), iPrLZn2[OC(CMe2)iPr]2 (2b), and
iPrLZn2[OC(CH2)C6Me5]2 (4b). The Zn2O2 core of 2a is symmetrical with the four Zn−O bond lengths,
varying only slightly (Zn−Oenolate = 1.975(2)−1.982(2) Å). In contrast, the structures of 2b and 4b,
which are supported by the more sterically hindered ligand iPrL2-, display parallelogram-shaped Zn2O2
cores and longer intermetal distances. These structural distortions are accompanied with changes in the
degree of polarization of the carbon−carbon double bonds of the enolate ligands. This is observed by
considering the shift difference between the two 13C resonances arising from the carbons of the enolate
C−C double bond. The general trend is that the more hindered supporting ligand, iPrL2-, forms enolate
derivatives with less polarized carbon−carbon double bonds. As expected, the Zn enolates also display
carbon−carbon double-bond polarizations that are intermediate to those of closely related Li and Si
enolates.