Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2011 Jun 1;133(21):8317-25.
doi: 10.1021/ja2017043. Epub 2011 May 9.

On the mechanism of the palladium-catalyzed tert-butylhydroperoxide-mediated Wacker-type oxidation of alkenes using quinoline-2-oxazoline ligands

Brian W Michel  1 , Laura D Steffens, Matthew S Sigman
Affiliations

On the mechanism of the palladium-catalyzed tert-butylhydroperoxide-mediated Wacker-type oxidation of alkenes using quinoline-2-oxazoline ligands

Brian W Michel et al. J Am Chem Soc. .

Abstract

The mechanism of the tert-butylhydroperoxide-mediated, Pd(Quinox)-catalyzed Wacker-type oxidation was investigated to evaluate the hypothesis that a selective catalyst-controlled oxidation could be achieved by rendering the palladium coordinatively saturated using a bidentate amine ligand. The unique role of the Quinox ligand framework was probed via systematic ligand modifications. The modified ligands were evaluated through quantitative Hammett analysis, which supports a "push-pull" relationship between the electronically asymmetric quinoline and oxazoline ligand modules.

PubMed Disclaimer

Figures

Figure 1
Figure 1
a) Proximal Lewis-basic groups proposed to chelate to palladium resulting in mixtures of products. b) Oxypalladation may occur through a hydrogen bonding network of three water molecules. c) Hypothesized coordinatively saturated palladium complex with anionic tert-butylperoxide ligand.
Figure 2
Figure 2
a) Previously reported oxidation of styrenes using TBHP as the terminal oxidant. b) Reaction performed with anhydrous TBHP and 18OH2 shows the rate of 18O incorporation in the product under reaction conditions is the same as the rate of washing 18O into the product via hydrate formation. Use of α-deuterostyrene shows that the hydrogen atoms in the substrate are maintained in the product.
Figure 3
Figure 3
a) Evaluation of bidentate amine ligands for the TBHP-mediated Wacker-type oxidation. b) Summary of substrate scope.
Figure 4
Figure 4
Initial rates to determine dependencies a) Conditions: [Pd(Quinox)Cl2] = 0 to 5 × 10-3 M; [AgSbF6] = 2.5 × [Pd(Quinox)Cl2]; aqueous [TBHP] = 1.2 M; [alkene] = 0.05 M; CH2Cl2; rt. b) Conditions: [Pd(Quinox)Cl2] = 2 × 10-3 M; [AgSbF6] = 5 × 10-3 M; aqueous [TBHP] = 3.0 M; [alkene] = 0 to 0.2 M; CH2Cl2; rt. c) Conditions: [Pd(Quinox)Cl2] = 2 × 10-3 M; [AgSbF6] = 5 × 10-3 M; anhydrous [TBHP] = 0 to 1.80 M; [H2O] = 2.5 M; [alkene] = 0.045 M; CH2Cl2; rt. d) Conditions: [Pd(Quinox)Cl2] = 2 × 10-3 M; [AgSbF6] = 5 × 10-3 M; anhydrous [TBHP] = 1.2 M; [H2O] = 0.3 – 4.0 M; [alkene] = 0.05 M; CH2Cl2; rt.
Figure 5
Figure 5
a) Plot of the log of the rate as a function of counterion conjugate acid pKa. b) Evaluation of inhibitor concentration on initial rate of reaction.
Figure 6
Figure 6
a) Proposed coordination model of the penultimate intermediate prior to oxypalladation. b) Alternative possible coordination model of the penultimate intermediate prior to oxypalladation.
Figure 7
Figure 7
Hammett analysis of electronically disparate styrenes. For 4-methoxystyrene the σρ+ value provides a good linear fit, which is a nearly identical fit to a plot with that substrate omitted. The σρ value for 4-methoxystryrene is also plotted to highlight its deviation from linearity.
Figure 8
Figure 8
a) Linear free energy relationship observed for plotting the log(rate) for a series of 4-substituted quinox ligands 8a-d as a function of Hammett σp values. b) Linear free energy relationship observed for plotting the log(rate) for a series of ligands 16a-d as a function of Hammett σp values
Scheme 1
Scheme 1
Mechanism proposed by Mimoun.
Scheme 2
Scheme 2
Possible mechanisms.
Scheme 3
Scheme 3
Synthesis of Quinoline-2-oxazole 2 and Quinoline-2-(5,5-dimethyloxazoline) 3.
Scheme 4
Scheme 4
Synthesis of 4-substituted quinox ligands.
Scheme 5
Scheme 5
Stille cross-coupling preparation of 4-quinoline-2-(4-pyridyl) ligands.
See this image and copyright information in PMC

References

    1. Tsuji J, Nagashima H, Nemoto H. Org. Synth. 1984;62:9–13.
    1. Tsuji J. Synthesis. 1984:369–384.
    1. Takacs JM, Jiang X.-t. Curr. Org. Chem. 2003;7:369–396.
    1. Cornell CN, Sigman MS. Inorg. Chem. (Washington, DC, U. S.) 2007;46:1903–1909. - PubMed
    1. Kang S-K, Jung K-Y, Chung J-U, Namkoong E-Y, Kim T-H. J. Org. Chem. 1995;60:4678–4679.

Publication types