Ion-pairing strategies for electrostatic control of selectivity in homogeneous catalysis
Abstract/Contents
- Abstract
- Controlling the selectivity of catalytic reactions remains an enduring challenge in synthetic chemistry, limiting the utility of many known chemical transformations. Reaction selectivity is often achieved through steric and/or electronic modification of catalysts. An alternative mode of selectivity uses electrostatic interactions to differentiate between competing transition states based on their charge distributions via (de)stabilizing interactions with proximal charge. Ion pairing can be used to position a charge very close to an ionic product-determining transition state, enforcing strong electrostatic (charge--dipole) interactions. This is a mode of ion pairing-- mediated selectivity that has not previously been addressed in homogeneous catalysis. In this work, efforts to understand and exploit electrostatic interactions to achieve regioselectivity in Au(I)-catalyzed reactions are described. The Au(I)-catalyzed rearrangement of 3-substituted aryl alkynyl sulfoxides to form dihydrobenzothiepinones is examined as a model reaction. The placement of an anion near a cationic Au(I)-bound transition state is achieved through ion pairing, which is modulated by solvent dielectric. Pulsed-field gradient diffusion NMR experiments show that cationic Au(I) catalysts form strong ion pairs with their weakly coordinating counterions in low-dielectric solvents. The regioselectivity of the rearrangement increases monotonically as solvent dielectric decreases, and independent of molecular properties of the solvent. DFT calculations of putative product-determining transition states show that the magnitude of the ion pairing effect on selectivity correlates with the magnitude of the differences in dipole moments between these transition states. The strongest electrostatic effect is observed in low-dielectric solvents, and stabilizes the more polar of two competing transition states. The understanding of ion pairing and electrostatic effects garnered from studies of the model sulfoxide rearrangement reaction is used to enable the regioselective synthesis of 2H-chromenes via Au(I)-catalyzed hydroarylation of phenyl propargyl ethers. The strength of electrostatic stabilization is tuned by the choice of catalyst, counterion, substrate, and reaction solvent. Where competing product-determining transition states differ significantly in charge distribution, up to 7-fold changes in regioselectivity are achieved by electrostatic stabilization. Ion pairing acts in concert with steric effects to achieve up to 14:1 regioselectivity for 7- substituted 2H-chromenes. In sum, the results show that selectivity in homogeneous catalysis can be enabled by the control of ion pairing-induced electrostatic interactions.
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2016 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Lau, Vivian Mei-Yan |
|---|---|
| Associated with | Stanford University, Department of Chemistry. |
| Primary advisor | Kanan, Matthew William, 1978- |
| Thesis advisor | Kanan, Matthew William, 1978- |
| Thesis advisor | Burns, Noah |
| Thesis advisor | Du Bois, Justin |
| Advisor | Burns, Noah |
| Advisor | Du Bois, Justin |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Vivian Mei-Yan Lau. |
|---|---|
| Note | Submitted to the Department of Chemistry. |
| Thesis | Thesis (Ph.D.)--Stanford University, 2016. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2016 by Vivian Mei-Yan Lau
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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