Stereochemistry of the Wacker Reaction: Modes of Addition of Hydroxide, Methoxide, and Phenyl at High and Low [Cl-]. A Study Using Chirality Transfer

Full text of DOI:10.1021/jo971051q

7
082
J . Org. Chem. 1997, 62, 7082-7083
Ster eoch em istr y of th e Wa ck er Rea ction :
Mod es of Ad d ition of Hyd r oxid e,
Sch em e 1
Meth oxid e, a n d P h en yl a t High a n d Low
-
[
Cl ]. A Stu d y Usin g Ch ir a lity Tr a n sfer
,
†
Othman Hamed, Charles Thompson,* and
Patrick M. Henry*
Department of Chemistry, Loyola University of Chicago,
Chicago, Illinois 60626
Received J une 11, 1997
The Pd(II)-catalyzed oxidation of ethene in aqueous
solution (Wacker reaction) gives exclusively acetaldehyde
under the usual Wacker conditions of low [Cl ] (>1.0 M).
Sch em e 2
-
1
Under the same conditions, Pd(II) oxidizes R-olefins to
methyl ketones and aldehydes. Thus, propene gives a
mixture of acetone and propanal. At high chloride (>2.5
M) and high [CuCl ] (>3 M) formation of ethylene
2
chlorohydrin becomes a serious side reaction. The Wack-
er controversy concerns the stereochemistry of addition
of the elements of Pd(II) and OH to the ethene double
bond (hydroxypalladation) under the usual Wacker con-
-
ditions of low [Cl ].
The rate expression for the oxidation of ethene to
acetaldehyde, given by eq 1, is consistent with (a) syn
2
-
+
- 2
rate ) k[PdCl4 ][olefin]/[H ][Cl ]
(1)
addition by coordinated hydroxyl on a Pd(II)-olefin
π-complex to give 1 in the slow step (eq 2), or (b) anti
attack by external water on the corresponding aquo-
complex in an equilibrium step to give 1 (eq 3). In both
-
chloride and cupric chloride concentrations ([Cl ] > 3 M;
CuCl ] > 2.5 M), conditions where the main product is
-chloroethanol.5 It was assumed that the same inter-
mediate, 1, which decomposes to acetaldehyde, is inter-
cepted by CuCl to give chlorohydrin. The present report
[
2
2
2
provides evidence that the modes of addition at high and
low chloride concentrations are different so the stereo-
chemical studies at high chloride are not a valid indicator
of the stereochemistry under the usual Wacker conditions
of low chloride.
mechanisms 1 decomposes to acetaldehyde and Pd(0).
Oxidation of allyl alcohols occurs by the rate expression
-
given by eq 1 at low [Cl ] (<1 M). The products are a
mixture of hydroxyacetone and 3-hydroxypropanal, the
-
expected Wacker oxidation products. At [Cl ] > 3 M the
oxidation of allyl alcohol effectively ceases and nonoxi-
Studies under conditions far removed from those of the
aqueous acyclic olefin oxidation suggest an anti mode of
dative isomerization of allyl alcohol-1,1-d
2
into its allylic
predominates.6 The rate
isomer, allyl alcohol-3,3-d
2
2
-4
hydroxypalladation.
The study that uses conditions
expression for the nonoxidative isomerization, given by
eq 4, is most consistent with trans attack of water on an
closest to those of the Wacker oxidation employs high
†
Present address: Department of Chemistry, University of Mon-
rate ) k[PdCl42-][allyl alcohol]/[Cl-]
(4)
tana, Missoula, Montana 59812.
(
1) For general discussion and references, see: Henry, P. M.
Palladium Catalyzed Oxidation of Hydrocarbons; D. Reidel: Dordrecht,
Holland, 1980; pp 41-84.
allyl alcohol-Pd(II) π-complex as shown in Scheme 1.
Since the isomerization shown in Scheme 1 is the
(
2) Stille, J . K.; J ames, D. E. J . Organomet. Chem. 1976, 108, 401-
-
4
2
6
08.
predominant pathway at high [Cl ], it seems reasonable
(3) Stille, J . K.; Divakarumi, R. J . Organomet. Chem. 1979, 169,
that a trichloro-hydroxypalladation adduct analogous to
39-248.
2
2
is the species intercepted by CuCl rather than 1. Thus
(4) Åkermark, B.; S o¨ derberg, B. C.; Hall, S. S. Organometallics 1987,
, 2608-2610.
a demonstration that the stereochemistry of hydroxy-
(5) B a¨ ckvall, J . E.; Åkermark, B.; Ljunggren, S. O. J . Am. Chem.
-
palladation is different at high and low [Cl ] would
Soc. 1979, 101, 2411-2416.
6) Gregor N.; Zaw, K.; Henry, P. M. Organometallics 1984, 3, 1251-
256.
support different mechanistic pathways for these two
(
-
- 5
1
regions of [Cl ]. Since addition is anti at high [Cl ], such
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Communications
J . Org. Chem., Vol. 62, No. 21, 1997 7083
Ta ble 1. Ster eoch em istr y of Ad d ition of P h en yl, Hyd r oxid e, a n d Meth oxid e to Ch ir a l Allylic Alcoh ols
-
run
[Cl ] (M)
X
solvent
allyl alcohol (%ee)
product (%ee)
1
2
3
4
5
6
0.1
2.5
0.1
2.0
0.1
2.5
Ph
Ph
OH
OH
OCH3
OCH3
CH3OH
CH3OH
H2O
H2O
CH3OH
CH3OH
(R)-(Z)-3a (53)
(S)-(Z)-3a (74)
(R)-(Z)-3b (66)
(R)-(Z)-3c (76)
(R)-(Z)-3a (53)
(S)-(Z)-3a (74)
(R)-CH3C(dO)CH2CH(Ph)CH3, (R)-5a ′′ (30%)
(S)-(Z)-CH3CHdCHCH(Ph)CH3, (S)-Z-3a ′′ (68%)
(R)-CH3C(dO)CH2CH(OH)C2H5, (R)-5b (42%)
(S)-(Z)-C2H5CHdCHCH(OH)CH3, (S)-Z-3b (76%)
(R)-CH3C(dO)CH2CH(OCH3)CH3, (R)-5a ′ (42%)
(R)-(Z)-CH3CHdCHCH(OCH3)CH3, (R)-Z-3a ′ (68%)
a demonstration would be evidence for syn addition at
low chloride concentration.
Studies in methanol at low and high [Cl ] showed that
PdCl) formed by exchange of the PhHgCl with PdCl
2
.13
This nucleophile was chosen because, being a carbanoid
species, it can only exist in hydroxylic solvents bonded
to Pd(II). For that reason it must add syn under all
experimental conditions, and syn addition has been
-
the kinetics and mechanism of the Pd(II)-catalyzed
reactions in this solvent are exactly analogous to those
7
,8
14
found in aqueous solution. Oxidation to acetals obeying
demonstrated for this nucleophile. For solubility rea-
-
eq 1 occurs at low [Cl ], and isomerization obeying eq 4
sons, methanol was the solvent. As discussed above,
methanol behaves in the same fashion as water in
Wacker chemistry.
-
is found at high [Cl ].
Determination of the relative modes of addition under
the two regimes of chloride concentration is possible by
chirality transfer. This technique employs the directing
influence of the hydroxyl group in chiral allylic alcohols.
This directing ability of the hydroxyl function has been
Table 1 summarizes the results. Runs 1 and 2 provide
evidence for the reaction sequence outlined in Scheme
2
. Thus, as predicted by Scheme 2, the products from syn
phenylation had the same absolute configuration as the
9
shown for several reactions of allylic alcohols. Previ-
-
starting allylic alcohol at both high and low [Cl ]. This
ously the exchange and isomerization of chiral tetrasub-
situted allylic alcohols, which cannot undergo Wacker-
type oxidation, was examined. The results were consistent
result is strong evidence for addition to the most stable
-
π-complex at both high and low [Cl ]. The remainder of
the data in Table 1 can now be interpreted in light of
this analysis. The most significant result is the relative
configurations of the oxidation and exchange products in
runs 3 and 4. The product of oxidation of (R)-(Z)-3b is
-
10
with different modes of addition at low and high [Cl ].
In the present study we examine the reactions of disub-
stituted allylic alcohols, (S)-(Z)-3-penten-2-ol (3a ), (R)-
(Z)-3-hexen-2-ol (3b), and (R)-(Z)-4-hexen-3-ol (3c), which
(
(
R)-5 while the product of isomerization of (R)-(Z)-3c is
S)-(Z)-3b. Thus, according to Scheme 2, the stereochem-
can undergo both exchange and oxidation and thus are
more closely related to the allylic alcohols previously
studied.
The most stable π-complex intermediate in these re-
1 2
actions is the one in which the R and R groups are
istry of hydroxypalladation must be opposite at high and
-
low [Cl ]! The results are best accommodated by syn
-
-
addition at low [Cl ] and anti addition at high [Cl ]. This
interpretation is consistent with kinetic studies discussed
furthest apart. Scheme 2 outlines one possible reaction
sequence involving syn addition to the most stable
π-complex. As shown in Scheme 2, the face to which the
Pd(II) is directed will depend on the absolute configura-
tion of the starting alcohol. If, as shown, (R)-(Z)-3 is the
allylic alcohol, the intermediate 4 will have the (R,R)
configuration. The products that retain this chiral center
will have the (R)-absolute configuration. Conversely, if
above and with the stereochemistry studies in the pres-
ence of CuCl .
2
5
Runs 5 and 6 confirm the mechanistic similarity of
water and methanol solvents. The results parallel those
in water with different modes of addition at high and low
-
[
Cl ]. Thus the phenylation results in runs 1 and 2 are
valid models for the results in aqueous solution.
(
S)-(Z)-3 is the allylic alcohol, syn addition will result in
Acknowledgment is made to the donors of the Petro-
leum Research Fund, administered by the American
Chemical Society, for their support. C.M.T. thanks the
National Institutes of Health (Grant no. ES04434) for
their support. The authors also thank Loyola Univer-
sity for the purchase of the VXR-300 NMR used in this
work.
products with the (S)-absolute configuration. Anti ad-
dition to the most stable π-complex with either (R)- or
(
configuration from that of the starting material. On the
other hand, anti addition to the least stable π-complex
will give the same absolute configuration as the starting
material. The important result is that one type of
addition to a given π-complex will give both isomeriza-
tion and oxidation products with the same absolute
configuration.
S)-(Z)-3 will give products with the opposite absolute
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedure and characterization data (10 pages).
The approach assumes only that the same π-complex
J O971051Q
-
11
is attacked at high and low [Cl ].
It would be very
unlikely that one mode of addition occurs to one π-com-
(
(
7) Henry, P. M.; Lee, H. B. Can. J . Chem. 1976, 54, 1726-1738.
8) Dumlao, C. M.; Francis, J . W.; Henry, P. M. Organometallics
-
plex at low [Cl ] and another mode of addition to the
-
other π-complex at high [Cl ]. To test this assumption,
1991, 10, 1400-1405.
(
9) See reference 10 for a discussion of chirality transfer.
a nucleophile, whose mode of addition is known and is
the same at both low and high chloride, is required. Such
a nucleophile is the phenyl group. As shown in eq 5, the
(10) (a) Francis, J . W.; Henry, P. M. Organometallics 1991, 10,
3
498-3503. (b) Francis, J . W.; Henry, P. M. Organometallics 1992,
11, 2832-2836.
(11) A reviewer pointed out that it is also assumed that the same
π-complex is attacked regardless of nucleophile. Certainly methanol
and water are so similar that this assumption must be valid. Also
there appears to be no reason coordinated hydroxide or methoxide
would add to a different π-complex than coordinated phenyl.
-
phenylation (Heck reaction) of allyl alcohol at low [Cl ]
(
(
(
12) Heck, R. F. J . Am. Chem. Soc. 1968, 90, 5526-5531.
13) Heck, R. F. J . Am. Chem. Soc. 1968, 90, 5518-5526.
14) (a) Heck, R. F. J . Am. Chem. Soc. 1969, 91, 6707-6714. (b)
gives a â-phenyl ketone analogous to 5′.12 The actual
reagent in the Heck reaction is “phenylpalladium” (Ph-
Henry P. M.; Ward, G. A. Ibid. 1972, 94, 673-674.