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               The electrochemical Wacker (typed up and pondered)
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(Legion Bob introduces this pretty little monster:   This article was on Rhodium
in the form of some scanned in pictures.  Bob typed it up for the sake of making
it easier to view, and so it can be copied and pasted wildely for the sake of
discussion.   Bob's comments are liberally sprinkled in–hopefully it's still
clear who wrote what.   This article describes the Electrochemical Wacker
Oxydation of olefins (in our case, safrole.)  The process is essentially one of
using two catalysts, with electrical current run through the solution to
continuously regenerate them.   The graphics from the origional articles are
ommited, check them out at Rhodium (E-wacker.))

Chemistry Letters, pp. 121-124, 1994
Copyright 1994 The Chemical Society of Japan

Electrochemical Wacker Type Reaction with a Double Mediatory System Consisting
of Palladium Complex and Tri(4-bromophenyl)amine

Tsutomu INOKUCHI, Liu PING, Fumihiro HAMAUE, Miyoko Izawa, and Sigeru TORII
Department of Applied Chemistry, Faculty of Engineering,
Okayama University, Tsushuma-Naka 3, Okayama 700

The electrochemical Wacker type oxidation of terminal olefins by using palladium
chloride or palladium acetate and tri(4-bromophenyl)amine as a recyclable
mediator in either a devided cell or and undivided cell afforded the
corresponding methyl ketones in good yields.

    (Bob translates:   Safrole is a 'terminal olefin.'  Dump it in a container
with palladium chloride, tri(4-bromophenyl)amine and a solvent (what?) and run
electricity through it.  POOF!  Ketones.  In the case of safrole, the MDP2P
ketone.  It *sounds* so simple...)

      The Wacker type process with palladium(II) chloride and copper(II) as
catalysts in aqueous media under oxygen (Bob:  Which is to say, the famous O2
wacker) is one of the useful methods for conversion of terminal olefins to
methyl ketones (#1).  A variety of modified methods of this reaction by devising
the reoxidation process for palladium(0) to palladium(II) have been reported in
order to avoid chlorination encountered in the process with copper(II) chloride
(#2).

Electrooxidation methods were also employed for the direct oxidation of Pd(0) to
Pd(II)  (#3) or for generation of recyclable oxidents such as quinone (#4) or
ferric chloride (#5) as a co-oxidant for regeneration of Pd(II) catalyst.
Usually divided cell systems have been utilized to avoid the deposition of
palladium metal onto the cathode, which often led to unsatisfactory reaction
conversion. (#6)   Herein, we report an improved procedure for the
electrochemical Wacker type reaction by use of triarylamine as a recyclable
mediator (#7) for regeneration of Pd(II) from Pd(0).   The electrolysis can be
achieved in both a divided or an undivided cells.  In this double mediatory
system, electrooxidation effects formation of triarylamine cation radical from
triarylamine which regenerates palladium(II) species.

[diagram]

      The cation radicals of triarylamines are stable (#8) and hence useful as
redox catalysts for indirect electrooxidations such as deprotonation and
irreversible cleavage of carbon-sulfur bond (#7).  Redox step by the cation
radical of triarylamine is considered to involve the formation of an
intermediate complex, which would lead to a negative shift of the oxidation
potential compared with those of the substrates. (#9)   These features of the
cation radicals of triarylamines are of interest as a recyclable organic redox
for regeneration of Pd(II) from Pd(0) in the double mediatory system.

     (Bob tries to translate:   The electricity flowing through the solution
converts the triarylamine into a radical (positively charged ion).   The
palladium is being converted from it's Pd(II) form to Pd(0) when it converts the
safrole into MDP2P ketone.   The triarylamine radicals then react with the the
Pd(0), turning the palladium back into it's catalytically usefull Pd(II) form,
and in the process the radicals get converted back to plain old triarylamine.
The process repeats itself as long as the safrole and electricity last.  :-)

     The electrolyses were carried out either in a divided or undivided cell
under a constant applied voltage.   Typically, the electrolysis in an undivided
cell was performed by using the substrate (1 mmol), PdCl2 (5 mol%), and
(4-BrC6H4)3N (5 mol%) in an MeCN-H20 (7:1)-(Pt)-(Pt) system at an applied
voltage of 3 volts and a current of 2-3 milliamps for 2-3 Faradays/mol, giving
the desired methyl ketones.  The results of the electrochemical Wacker
oxidations are given in Table 2.

     (Bob:  A Faraday is one mole of electrons (ie. it's a measurement of how
much electricity flows through based on the ammount of current and how long it
flows for.)   To calculate F, you take amps of current * time in seconds /
96,500.  So, 10 mA of power flowing for one hour (3600 seconds) gives us ( 0.010
amps * 3600 sec/96500 conversion factor), which comes out to a wretched .00037
Faradays (moles of electrons.)   Note that the reaction is described in terms of
F/mole of olefin, which is only logical (the more material processed the more
electrons needed.)  What this means is that if you want to process .01 moles
(just under 2 grams) of safrole, you'll need to leave on that 10 mA current for
about 50 to 80 hours.    Now, I'm sure your first impulse is to say 'scew that',
but the methods and cells used by these scientists were tiny experimental ones,
not the sort of industrial-scale visions a bee might have.   For instance, how
much current can the system withstand?  How well does it scale up?  A reasonably
sized aparatus capable of Wacking a whole mole of safrole (~190g) might be
achieved with 30 amps (@3 volts) in about 2-3 hours.   Ahh, but what bored bee
will do the experiment?  :-)   And how should the cell be built?   Bob likes
glass (any old cooking pan may work), but what material should be used for the
electrodes?)   Putting together this unusual power supply would also be a
nuisance, but certainly no great obstacle to the electronics geeks.   Bob is
also semi-concerned about the high palladium:olefin ratio (1:20) but the
palladium appears to recover and recycle well, and the ratio might be lowerable
without adverse effect.   On the bright side, Bob is 99% sure this thing could
be scaled waaaay up.)


       In our attempt to find out the most favorable organic mediator for the
electrochemical process in the double mediatroy system, three kinds of organic
redoxes such as triarylamine (entry 1 in chart below), dephenoqhinone (entry 2
in chart) and hydroquinone (our old friend, entry 3 in chart) were examined for
the oxidation of 1-undencene (1a).  It is known that in general unsubstituted
terminal olefines like 1a are susceptible to the isomerization of the double
bond to internal posititions during the palladium-catalyzed Wacker process.   As
shown in Table 1, the ratio of the desired 2-undecane (2a) vs. other ketones
such as 3-, 4-, and 5- undecanones is affected by the kind of organic mediator.

       (Bob:  Like the man says--your choice of mediator (the chemical that gets
ionized by the electricity so it can restore the palladium to P(II)) will affect
yield.   However, note that the same mediator chemical will give better or worse
results with different starting olefins (In our case, safrole.))

      Among three organic mediators attempted, the highest yield of the Wacker
oxydation product and the highest product selectivity of 2a are observed with
tri(4-bromophenyl)amine. (Bob:   In other words, it not only reacted more of the
olefin, it produced a higher percentage of the desired ketone vs. the undesired
ones.)

      The efficiency of the electrooxidation is also affected by the kind of
palladium complexes.  PdCl2 and Pd(OAc)2 were preferentially used in the
undivided and divided cell systems, respectively.

[Table 1]

(Bob:  In the above table, the reaction in question was as follows:)

a)  Reaction is carried out in a CH3CN/H2O (9:1 V/V, 10 ml)-Et4NOTs-(Pt)-(Pt)
system in the presence of Pd(OAc)2 (5 mol%) and an organic mediator (20 mol%) at
room temperature (20 C) in a divided cell.   Conditions:  3.0 Volts (using
0.4-5.5 mA of current); Electricity charged: 2.5-3.0 Faradays/mol.  b) Yields
are determined by gas chromatagraph analyses based on internal standard
(2-octanone.)  Numbers in parenthesis show selectivity of 2a.  c) Combined yield
of 3-, 4-, and 5-undecanones by gas chromatograph analyses.  d) Combined yield
of 2a and other ketones.

    (Bob bitches:  And that's the bad news.  The best reaction still only gave
39% yield of the desired ketone when using undecane.   But...keep reading, true
believers.   Bob has spotted an old friend in the below table of other olefins
tried.  Yep, right there at the top.   Safrole!   AND IT YIELDS AT 85% in an
undivided cell!)   YEEEEHA!    OK, that's still not as good as the best O2
wackers...but this method doesn't seem to exactly have been pushed to it's
limits yet (recall that the first O2 wackers described at Rhodium yielded about
70% with a lot of work and some unreliability.))

[Big graphic Table 2....]

      The combined use of PdCl2 or Pd(OAc)2 palladium chloride or palladium
acetate with tri(4-bromophenyl)amine (3) was applied to the oxidation of a
variety of terminal olefins and the results are shown in Table 2.  The desired
methyl ketones 2 are obtained in good yields when the starting olefins possess
substituents such as a methoxy group (Entry 2), carbonyl functions (Entries 3-5)
and an amino group (Entry 6) at the neighboring positions.   A small ammount of
by-products such as aldehyde (Entry 2) and dehydrated compound (Entry 9) are
found.   Up to 50 turnovers of the palladium catalyst could be achieved without
noticeable loss of activity of the electrolysis system.

      (Bob Lusts:   Re-using the palladium 50 times and it's still ticking?
It's a Good Thing.)

      The present work was partially supported by The Grant-in-Aid for
Scientific Research No. 05650883 from the Ministry of Education, Science and
Culture of Japan.  We thank Prof. S. Saito for helpful discussions and are
grateful to the SC-NMR laboratory of Okayama University for experiments with the
Varian VXR-500 and -200 instruments.


References.   (Bob hates references.   Too much damn punctuation.)

1)  J. Tsuji, "Organic Synthesis with Palladium Compounds", Springer Verlag,
    Berlin (1980); R. F. Heck, "Palladium Reagents in Organic Synthesis",
    Academic Press, London (1985).
2)  J. Tsuji, Synthesis, 1984, page 369; J. Tsuji, J. Nokami, and T. Mandai,
    Journal of Synthetic Organic Chemistry, Japan,  47, 649 (1989).
3)  D. L. Klass, US Patent # 3147203; Chemical Abstracts 61 13195 (1964); D. L.
    Klass. US Patent #3245890; Chemical Abstracts, 17427 (1966);  A. R. Blake
    and J. G. Sunderland, Journal of the Chem. Society (A), 1969, 3015; F.
    Goodridge and C. J. H. King, Trans. Faraday Society, 66, 2889 (1970);  K.
    Otsuka and A. Kobayashi, Chem. Lett., 1991, 1197.
4)  J. Tsuji and M. Minato, Tetrahedron Lett., 28, 3683 (1987); J.-E. Backvall
    and A. Gogoll, J. Chem. Commun., 1987, 1236;  H. Riering and H. J. Shafer,
    Abstract of 15th Sandbjerg Meeting, Denmark (1990), P60.
5)  H. H. Horrowitz, Journal of Applied Electrochemistry,  14, 779 (1984).
6) D. D. M. Wayner and F. W. Hartstock, Journal of Molecular Catalysis, 48, 15
   (1988).
7) E. Steckhan, "Topics in Current Chemistry", 142, Springer-Verlag (1987)
   pp49-57;  W. Schmidt and E. Steckhan, Angew. Chem. Int. Ed. Engl., 17, 673,
   (1978).
8)  E. T. Seo, R. F. Nelson, J. M. Fritsch, L. S. Marcoux, D. T. Leedy, R. N.
    Adams, Journal of the American Chem. Society, 88, 3498, (1966);  J. F.
    Ambrose, L. L. Carpenter, R. F. Nelson, Journal of the Electrochem. Society,
    122, 876, (1975).
9)  L Eberson, Adv. Phys. Organic Chemistry, 18, 79 (1982).

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