                 Palladium-Catalyzed Oxidation of Terminal
               Olefins to Methyl Ketones by Hydrogen Peroxide
                   	     JOC 45, 5387 (1980)
                     Michel Roussel and Hubert Miinoun*
       
The use of the Wacker PdCl2-CuCl2 system for the oxidation of higher
terminal olefins to methyl ketones presents major drawbacks, i.e.,
formation of chlorinated, aldehydic, and internal ketones as byproducts,
precipitation of metallic palladium, and corrosion. Some improvements have
been achieved by using basic or alcoholic solvents and phase-transfer
catalysts, but the disadvantages have not been completely eliminated. We
have previously described a highly selective procedure using rhodium
catalysts and involving molecular oxygen activation, but a deactivation of
the catalyst system was observed. We have also recently synthesized a new
class of stable palladium alkyl peroxidic complexes with the formula
[RCO2PdOO-t-Bu]4; they undergo an oxygen transfer to terminal olefins
through a pseudocyclic peroxypalladation mechanism (eqn 1).

                                  [eqn 1]

This paper describes a very effecient catalytic procedure for the oxidation
of terminal olefins to methyl ketones by hydrogen peroxide using a
palladium catalyst and operating in the absence of halogens and co-metals.
Although H2O2 has been used as a reoxidant in the Wacker system for the
transformation of ethylene to acetaldehyde, no systematic investigation of
this type of reaction has been previously reported.


                                  Results

Addition of 30% hydrogen peroxide to a solution of palladium(II) acetate in
acetic acid or tert-butyl alcohol at room temperature resulted in an
immediate decomposition with evolution of molecular oxygen. When this
addition was carried out in the presence of 1-octene (1-octene-Pd = 20), no
apparent decomposition occurred and a change in color from yellow-orange to
deep orange resulted. GLC analysis showed the formation of 2-octanone as
the major product, according to the reaction (eq. 3).

                                  [eqn 3]

This reaction may be carried out either in biphasic medium, using solvents
such as ethyl acetate or dichloroethane, or in homogeneous solution, using
tert-butyl alcohol or acetic acid. Basic solvents, e.g., DMF, DMA, HMPA,
were found to be strong inhibitors for the reaction. Figure 1 shows a
typical plot of 1-octene comsumption, 2-octanone formation, and H2O2
decomposition vs. time at 80 'C, using Pd(OAC)2 catalyst (1-octene-Pd =
1500), AcOH solution, and 30% H2O2 as oxidant (H2O2-1-octene = 5). A high
selectivity in 2-octanone formation was observed throughout the reaction,
and a quite complete conversion of 1-octene (90-95%) was obtained after ca.
3 h of reaction time. 3- and 4-octanone were the major byproducts detected
in the reaction, while no octanal was identified. Under these conditions, I
mol of palladium was found to transform ca. 400 mol of 1-octene into
2-octanone per hour.

Decomposition of hydrogen peroxide occurred, as shown from oxygen evolution
during the reaction. This decomposition was in part due to a thermal effect
(ca. 15%), but the major part was attributed to a palladium-catalyzed
reaction. Taking into account this H2O2 decomposition, ca. 2-3 mol of H2O2
were consumed per 1 mol of 1-octene transformed.

Table I shows that the use of a large excess of H2O2 (H2O2-1-octene > 2:1)
is necessary to achieve a complete conversion of 1-octene. At lower ratios,
a precipitation of metallic palladium was observed and the decomposition of
H2O2 occurred at the expense of 2-octanone formation. If the temperature
was lowered to 60 'C, no significant reduction of H2O2 decomposition on
behalf of 2-octanone formation was observed.

At H2O2-1-octene = 5 and under the conditions of Figure 1, a linear
dependence of initial rates on the concentration of Pd(OAC)2 was observed.

The influence of the nature of the palladium catalysts on the oxidation of
1-octene by H2O2 is shown in Table II. In acetic acid solvent, no major
differences were observed between the different catalysts, probably due to
the exchange of the anion bonded to palladium by AcOH. However, a larger
influence of the anion resulted when the reaction was carried out in
tert-butyl alcohol. Palladium complexes bearing strongly coordinating
anions such as acetylacetonate or hexafluoroacetylacetonate were found to
be inactive for the transformation of 1-octene to 2-octanone, suggesting
the occurrence of a necessary exchange of at least one anion on palladium
by H2O2 (vide infra). The use of Na2PdCl4 as catalyst resulted in a lower
selectivity in the formation of 2-octanone, with extensive double bond
migration and production of internal ketones.

Several modifications of the reaction medium have been tried, using
tert-butyl alcohol as a solvent in order to improve the understanding of
the general features of this catalytic system. The Pd(OAc)2-catalyzed
oxidation of 1-octene by H2O2 (H2O2-olefin = 5; olefin-Pd = 1500) was not
influenced by the addition of a radical inhibitor such as di-tert-butyl-
para-cresol (DTPC-Pd = 10) either in the 2-octanone formation or in H2O2
decomposition.The presence of water in the medium also had no effect on the
reaction and H2O2 Solutions can be used at any dilution. The oxidation is
best carried out in neutral (t-BuOH) or slightly acidic (AcOH) solutions.
The addition of a base such as potassium tert-butoxide (t-BuOK-Pd = 1)
inhibited the oxidation and resulted in a complete decomposition of H2O2.
The addition of a noncoordinating strong acid such as MeS03H (MeS03H-Pd =
1) resulted in lower conversion and selectivity.

The reactivity of several different olefins towards H2O2 in the presence of
Pd(OAc)2 is illustrated in Table III. Only terminal olefins are transformed
to methyl ketones by this catalytic system in both AcOH and t-BuOH
solution. However, AcOH was found to be more efficient in the oxidation of
allyl acetate to 3-acetoxy-2-propanone. Allyl alcohol was transformed into
a mixture of formic, acetic, and propionic acid, presumably via the
formation of pyruvic compounds. Internal and cycloolefins such as
cyclohexene were unreactive, as previously observed with the peroxidic
[RCO2PdOO-t-BU]4 complexes.


                                 Discussion

These results are consistent with the suggested mechanism depicted in
Scheme I. Palladium hydroperoxidic species (1) probably obtained by the
addition of H2O2 to palladium compounds appear to be the most likely active
intermediates in this catalytic oxidation. Because of extensive H2O2
decomposition, our attempts to isolate such species failed. However,
several pieces of evidence favor the involvement of peroxidic palladium
compounds in this reaction.

PdOOH species, generated from a noncoordinating acid hydrolysis of
(Ph3P)2PdO2 have been recently shown by us to be active in the selective
transformation of terminal olefins to methyl ketones. PdOOH species undergo
an oxygen transfer to olefin through a pseudocyclic hydroperoxypalladation
mechanism(such as 2-3) of the coordinated olefin." The involvement of such
pseudocyclic peroxidic intermediates (3) is further supported by the
exchange reaction between Na2PdCl4 and CF3CO2HgCH2CH(Ph)OOH affording
acetophenone.

The reactivity of olefins toward H2O2 in the presence of palladium is very
close to that previously observed with [CF3CO2PdOO-t-BU]4 (PPT). Only
terminal olefins, can be oxidized to methyl ketones. The coordination of
the olefin to the metal prior to its peroxypalladation is strongly
inhibited by competing ligands or o-donor solvents.

PdOOH species appear to be less stable than PdOO-t-Bu species previously
prepared and are probably responsible for the H2O2 decomposition. The
presence of a large excess of H2O2 is necessary not only to compensate its
unavoidable Pd-catalyzed decomposition but also to regenerate PdOOH species
from PdOH (4). At low H2O2 concentrations, PdOH species may undergo a
hydroxypalladation of the olefin, affording the ketone and metallic
palladium precipitation. This metallic palladium does not dissolve again in
the solution when excess H2O2 is added. Furthermore, the presence of excess
H2O2 is necessary to avoid the formation of allylic complexes (5) and
regenerate the initial PdOOH species.

This palladium-catalyzed synthesis of methyl ketones from terminal olefins
and H2O2 appears to be most useful in synthetic applications. It provides
very high selectivity in methyl ketones at quite complete conversions of
the olefins and only needs very low amounts of palladium (20-40 spin) in
the medium.


                            Experimental Section

Material. Reagent-grade tert-butyl alcohol (Prolabo) and glacial acetic
acid (Merck) were used without further purification. Olefins such as
1-octene (Merck), 1-decene, 1-dodecene (Ethyl Corp.), and cyclohexene
(Prolabo) were passed through a column containing active alumina to remove
peroxidic impurities and distilled before use. Other olefinic compounds,
i.e., allyl acetate, allyl alcohol, and ethyl acrylate (Aldrich), were used
without purification. 30% H2O2 (Merck) or 70% (Air Liquide) was used as
such. Palladium catalysts were purchased from Ventron (Pd(OAc)2), Cie des
Metaux Precieux (Pd(acac)2, Pd(Hfacac)2), and Prolabo (Pd(NO3)2, Na2PdCl4).
Palladium trifluoroacetate was synthesized according to Wilkinson. 14
[RCO2PdOOtBu] complexes were prepared according to ref  8.

Apparatus. The olefins were oxidized in a three-necked 1-L thermostated
glass reactor equipped with a magnetic stirrer, a condenser, and a 100-mL
glass funnel (for introduction of H2O2) and connected through a gas counter
(for the evaluation Of O2 evolution) to the atmosphere. H2O2 solution was
introduced dropwise into the mixture of olefin, solvent, and catalyst
during 30 min at the reaction temperature. When the reaction was complete,
the mixture was cooled and water was added. The yellow upper layer
containing the catalyst was separated and passed through a column of
alumina in order to eliminate the catalyst and distilled under reduced
pressure.

Note Added in Proof. In order to determine whether H2O2 or H2O represents
the oxygen source for the methyl ketone formation, we have followed a
pertinent suggestion of a referee and carried out an H2 16 O2-H2O labeling
experiment. Thus, 1-octene (0.18 M) was oxidized by 70% H2O2 (0.54 M) in
the presence of H2180 (99% O, 0.18 M) and Pd(OAc)2 (0.037 M) in tert-butyl
alcohol at 20 'C. The reaction was monitored by GLC-MS coupling (AEI Model
MS 80) and the ratio of 2-octanone '10:110 was accurately determined vs.
time by measuring the molecular 128:130 peak ratio. Figure 2 shows that no
O coming from water was incorporated into the 2-octanone for ca. 20 min,
after which time an isotopic exchange between the obtained 2-octanone and
H2O occurred.' 1,12 Therefore, these results provide strong additional
evidence that, contrary to the Wacker process, water is not involved as the
oxygen source in this reaction. Under our conditions, the internal
pseudocyclic hydroperoxypalladation, i.e., 2 --> 3 of the coordinated
olefin prevails over external nucleophilic attack by water forming a
trans-hydroxy-palladation adduct.

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Posted on The Hive 08-10-98 by SCx

Dream-WoRkS

The Oxidation Dream of our favorite terminal Olefin via Palladium Acetate,
H2O2 & tert Butyl Alcohol...

Someone had a dream, they wrote it down... then it was posted, but I don't
know shit about this stuff. Might as well be Greek or assembly language
(actually, I'd be more comfortable with assembly language). The following
dream occurred in a land where this research is sanctioned by the
administration that runs the country and happened within the last 48 hours!
Obviously this is for educational purposes only and should not be performed
in any country where the oppressive government prohibits aspiring young
scientists from simple research!

200 g safrol + 1.0 g Pd(AcO)2 (Note 1) + 1000 mls tert butyl alcohol + 700
mls 35% H2O2. (Palladium Acetate was purchased) Added everything but H2O2
to a 5 liter two neck flask equipped with a reflux condenser, thermometer,
and dropping funnel, mounted in an oil bath on a good magnetic stirrer.
Started stirring while bringing the temp to 80C. At 78-80C started to add
(dropwise) the H2O2 via a 1 liter addition funnel. It was added over a 30
minute time frame. As this was added, they noticed an evolution of bubbles
coming from the rxn (O2 from the H2O2).

Monitored stirring and temp with no unusual occurrences until the 4 hour
mark... Initially solution was a light orange - brown in color, after 4
hours it started to turn a very deep orange, almost reddish color. Also the
smell of safrol was completely gone and was replaced with a unique aroma of
ketone. One that is attributable to this rxn as it differs from the smell
of the wacker oxidation ketone. Continued rxn for a total of 6 hours,
maintained temp at 79 to 82C. Color at the end of rxn was deep orange-reddish,
pH of solution was 2.

To separate the oil, they added an equal volume of fresh cool water (note:
waited until solution cooled before adding the water). The oil started to
drop out perfectly, used DCM to extract all traces of the oil. This work up
is by far the cleanest, easiest and simplest to date. (This dreamer was
tried all method of ketone synthesis). Once the oil was extracted, the
extracts were pooled washed with sodium bicarbonate 1x, saturated solution
of NaCl 1x, and two washes with fresh dH2O. Some time was required for the
work up as there was a little emulsion from the use of the base wash and
then with the first water wash. The JOC ref suggested using an alumina
column to remove the catalyst (could be a better way to go).

So now we have this solvent containing ketone, it was dried with MgSO4.
Not being able to vac-distill during this dream, they took about 50 mls
of solvent/ketone and placed in beaker on stir plate and boiled off the
solvent. Vac distilling will be performed in a couple of days (I guess
they woke up too early).

The resulting oil was a very nice reddish-orange color. Had a very unique
smell too. Took about 2 grams worth of this oil, added to a test tube
containing a saturated solution of sodium bisulfite. In less than 60
seconds the oil precipitated into a whitish yellow crystalline mass (very
similar to what acetone would do if added to a bisulfite solution). Never
had this quick of a crystallization using the bisulfite procedure. Not
having access to HPLC or other more accurate methods of analysis, they
would say the purity of this ketone is quite high without distillation.
The yield will be minimum 78% and probably as high as 86%.
No isosafrol was formed.

Note 1: In the JOC ref much less catalyst was used.
        At this amount, it was estimated that the
        rxn was finished within 4.5 hours.
        The dreamers decided to run the full 6 hours.

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Posted on The Hive 08-13-98 by Sunlight

Pd(AcO)2 can be purchased, or is really easy to make it: 

1 gram of PdCl2 was dissolved in 100 ml of water and 5 cc of HCl at 80°C, 
then was added 3 ml of formaline and dilute NaOH till ph 8-9. Pd metal 
(black) precipitates, the solution was decanted and the precipitate washed 
three times with water (decanted washes). Finally about 40 ml of acetic 
acid and 0.8 ml of 70% HNO3 was added to the Pd metal, and the solution 
was boiled. The Pd dissolved and the acetic acid was evaporated at 120°C 
to afford 1.2 grams of Pd(AcO)2, 94% yield.

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