Structures of Zinc Ketone Enolates

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• 作者：Mark L. Hlavinka and John R. Hagadorn
• 刊名：Organometallics
• 出版年：2006
• 出版时间：July 3, 2006
• 年：2006
• 卷：25
• 期：14
• 页码：3501 - 3507
• 全文大小：125K
• 年卷期：v.25,no.14(July 3, 2006)
• ISSN：1520-6041

文摘

Solution- and solid-phase structural characterizations of a series of simple zinc ketone enolates arepresented. The new enolate derivatives were prepared (isolated yields 38-69%) by the deprotonation ofketones by the bis(arylzinc) species ^RLZn₂Ph₂ (1) (where ^RL^2- are bis(amidoamine) ligands). Ketonesused 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 varyingdegrees of substitution at the acidic carbon (i.e., to the carbonyl group). Thus, the effect of sterics onthe structural preferences of zinc enolates was examined. In all cases, both solid- and solution-phasestructural data indicate that the anionic enolate ligands have isolated (and uncoordinated) carbon-carbondouble bonds. The enolate anions bind to the Zn centers with O-bridged structures. Single-crystal X-raydiffraction 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-O_enolate = 1.975(2)-1.982(2) Å). In contrast, the structures of 2b and 4b,which are supported by the more sterically hindered ligand ^iPrL^2-, display parallelogram-shaped Zn₂O₂cores and longer intermetal distances. These structural distortions are accompanied with changes in thedegree of polarization of the carbon-carbon double bonds of the enolate ligands. This is observed byconsidering the shift difference between the two ¹³C resonances arising from the carbons of the enolateC-C double bond. The general trend is that the more hindered supporting ligand, ^iPrL^2-, forms enolatederivatives with less polarized carbon-carbon double bonds. As expected, the Zn enolates also displaycarbon-carbon double-bond polarizations that are intermediate to those of closely related Li and Sienolates.