Palladium t-Butyl Peroxide Trifluoroacetate

[75123-99-8]  · C6H9F3O4Pd  · Palladium t-Butyl Peroxide Trifluoroacetate  · (MW 308.57)

(oxidizing agent for the selective transformation of terminal alkenes to methyl ketones;1,2 oxidative ring cleavage of cyclic acetals5)

Alternate Name: PPT.

Physical Data: yellow crystals, mp 130-150 °C (dec).

Solubility: sol most common organic solvents, e.g. CH2Cl2, toluene, THF; destroyed by alcohols through anion exchange.

Analysis of Reagent Purity: IR: n(O-O) = 855-860 cm-1; n(C=O)asym = 1615-1630 cm-1; 1H NMR: d 1.36-1.41 (t-Bu).

Preparative Methods: 1 g Palladium(II) Acetate is dissolved in 2 mL of Trifluoroacetic Acid, and 10 mL of anhydrous 80% t-Butyl Hydroperoxide is added to the brown slurry at room temperature. After stirring for ca. 6 h, fine yellow crystals precipitate from the reaction mixture. The solid is filtered, washed repeatedly with pentane, and dried in vacuo; yield 85%.

Handling, Storage, and Precautions: not explosive or shock sensitive, but must be stored in a refrigerator.

Oxidation of Terminal Alkenes to Methyl Ketones.

PPT is a powerful and selective reagent for the oxidation of terminal alkenes to methyl ketones according to eq 1. The reaction is carried out at rt in solvents such as benzene, toluene, or dichloromethane, using a large excess of alkene (alkene/Pd > 5). Alkenes such as 1-hexene, 1-octene, or styrene are selectively converted to 2-hexanone, 2-octanone, and acetophenone, respectively.

Internal alkenes are not oxidized, but form stable h3-allyl Pd complexes (eq 2).

The mechanism of the ketonization of terminal alkenes (eq 3) has been shown to proceed via coordination of the alkene to palladium followed by intramolecular Markovnikov nucleophilic attack of the t-BuOO moiety on the coordinated alkene, forming the five-membered peroxometallocycle (pseudocyclic peroxymetalation); this decomposes with a b-hydride migration to form the methyl ketone and the palladium t-butoxy complex. The latter can react with excess alkene to give the h3-allyl complex, or with excess t-BuOOH in a catalytic ketonization of terminal alkenes which is less effective than the oxidation by Hydrogen Peroxide (see below).

Palladium complexes are powerful catalysts for the selective oxidation of terminal alkenes to methyl ketones. In contrast to the Wacker Palladium(II) Chloride-Copper(II) Chloride-H2O-O2 system, this reagent system (eq 4) can be successfully applied to most terminal higher alkenes, does not give chlorinated byproducts, and is not corrosive.

This reaction can be carried out at room to moderate (70 °C) temperature in a two-phase (benzene, toluene, AcOEt, MTBE) or single-phase (t-BuOH, AcOH) mixture. Several palladium complexes (particularly Pd(OAc)2) can be used as catalysts for this transformation in concentrations as low as 10-4 molar in solution, with turnover numbers higher than 400 mol ketone produced per mol Pd and per hour. The reaction can be applied to most terminal alkenes, e.g. 1-hexene, 1-octene, 1-dodecene, or allyl acetate, and gives the corresponding methyl ketones with conversions and selectivities higher than 80%. Byproducts include mainly the internal isomers of the starting alkenes, and some internal ketones. During the reaction, a Pd-catalyzed parallel decomposition of H2O2 occurs, and two equiv or more are often necessary to reach a complete conversion of the substrate.

The mechanism of this reaction (eq 5) has been shown to proceed through a hydroperoxypalladation of the coordinated alkene, similar to that described in eq 3. Labeling studies have shown that the oxygen atom incorporated into the methyl ketone exclusively derives from H2O2, not H2O.2

Almost quantitative yields of methyl ketones were obtained in biphasic conditions by adding a terminal alkene (from 1-hexene to 1-octadecene) to a suspension of Tetrakis(triphenylphosphine)palladium(0) in a 3.2 M aqueous solution of H2O2 at 40-80 °C (alkene:Pd = 2000; H2O2:alkene = 10). The ketonization is thought to proceed via the formation of an active anionic intermediate [(Ph3P)3Pd(OOH]-H+ which transfers oxygen to the alkene as in eq 5.3

A heterogenized reusable Pd complex on layered montmorillonite silylpropylethylenediamine (Figure 1) was shown to exhibit a much higher activity for the selective ketonization of terminal alkene by H2O2 (substrate:Pd = 4500; AcOH, 80 °C, with alkenes ranging from 1-hexene to 1-dodecene, styrene, allyl alcohol, and allyl acetate; yields higher than 90%). Consistent activity was found for 4 catalyst recycles.4

Oxidative Cleavage of Cyclic Acetals.

The trifluoroacetate catalyzes the oxidative cleavage of cyclic acetals by t-butyl hydroperoxide to give diol monoesters. In a typical reaction, the cyclic acetal and two equivalents of t-butyl hydroperoxide were mixed with 5 mol % of the catalyst in dry benzene at 50 °C (eqs 6, 7).5

1. Mimoun, H.; Charpentier, R.; Mitschler, A.; Fischer, J.; Weiss, R. JACS 1980, 102, 1047.
2. Roussel, M.; Mimoun, H. JOC 1980, 45, 5387.
3. Ioele, M.; Ortaggi, G.; Scarsella, M.; Sleiter, G. G 1992, 122, 531.
4. Rao, Y. V. S.; Rani, S. S.; Choudary, B. M. J. Mol. Catal. 1992, 75, 141.
5. Hosokawa, T.; Imada, Y.; Murahashi, S. CC 1983, 1245.

Hubert Mimoun

Firmenich, La Plaine-Geneva, Switzerland

Joseph A. Picard

Parke-Davis Pharmaceutical Research, Ann Arbor, MI, USA

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