Probing the mechanisms of enantioselective hydrogenation of simple olefins with chiral rhodium catalysts in the presence of anions

Article abstract of DOI:10.1002/(SICI)1521-3765(20000103)6:1<139::AID-CHEM139>3.0.CO;2-U

The strong influence of various anions upon the hydrogenation of 2-phenyl-1-butene, catalyzed by chiral rhodium catalysts was investigated. Both sulfonates and halides exert large increases in the enantioselectivity when [Rh{(-)-bdpp}(NBD)]ClO₄ (bdpp = 2,4-bis(diphenylphosphino)pentane, NBD=2,5-norbornadiene) is used as the catalyst precursor at high pressures (70 atm) of dihydrogen in nonpolar solvents. A dihydride mechanism similar to that for Wilkinson's catalyst [RhCl(PPh₃)₃] was shown to be operating at both high- and low-pressure conditions through a combination of catalytic studies, ³¹P, ¹H and parahydrogen-induced polarization (PHIP) NMR experiments. With sulfonate and in neat methanol, however, a mechanistic switch takes place from a dihydride route (dihydrogen addition before olefin binding) at high pressure to an unsaturate route (olefin binding before dihydrogen addition) at low pressures (<30 atm). Olefin isomerization is inhibited by halide addition, but occurs with sulfonate and in neat methanol through what is most likely a π-allyl mechanism. A detailed understanding of the effects of addition of these anions is crucial for development of new classes of catalysts capable of efficient enantioselective reduction of prochiral olefins lacking a secondary polar binding group.

Full text of DOI:10.1002/(SICI)1521-3765(20000103)6:1<139::AID-CHEM139>3.0.CO;2-U

FULL PAPER
Probing the Mechanisms of Enantioselective Hydrogenation of Simple Olefins
with Chiral Rhodium Catalysts in the Presence of Anions
[a]
[a]
[a]
[b]
Jillian M. Buriak,* Jason C. Klein, Deborah G. Herrington, and John A. Osborn*
Abstract: The strong influence of vari-
ous anions upon the hydrogenation of
ing at both high- and low-pressure con-
ditions through a combination of cata-
lytic studies, 31P, H and parahydrogen-
induced polarization (PHIP) NMR ex-
periments. With sulfonate and in neat
rate route (olefin binding before dihy-
drogen addition) at low pressures
(<30 atm). Olefin isomerization is in-
hibited by halide addition, but occurs
with sulfonate and in neat methanol
through what is most likely a p-allyl
mechanism. A detailed understanding of
the effects of addition of these anions is
crucial for development of new classes
of catalysts capable of efficient enantio-
selective reduction of prochiral olefins
lacking a secondary polar binding group.
1
2-phenyl-1-butene, catalyzed by chiral
rhodium catalysts was investigated.
Both sulfonates and halides exert large
increases in the enantioselectivity when
methanol, however,
a
mechanistic
[
2
Rh{(� )-bdpp}(NBD)]ClO
(bdpp ˆ
switch takes place from a dihydride
route (dihydrogen addition before olefin
binding) at high pressure to an unsatu-
4
,4-bis(diphenylphosphino)pentane,
NBD ˆ 2,5-norbornadiene) is used as
the catalyst precursor at high pressures
(70 atm) of dihydrogen in nonpolar
Keywords: alkenes
´
´
asymmetric
rhodium
solvents. A dihydride mechanism similar
to that for Wilkinsonꢀs catalyst
catalysis
´
halides
´
sulfonates
[
RhCl(PPh ) ] was shown to be operat-
3 3
Introduction
stituted olefins capable of only monodentate coordination to
[6]
[7]
the catalyst metal center with chiral titanium, samarium
and late transition metal catalysts[
8, 9]
and have achieved
Asymmetric hydrogenation of prochiral olefins has for over
0 years been intensively investigated because of the utility of
3
moderate to excellent enantioselectivities. These results are
encouraging, and suggest that new approaches towards the
design of catalysts may allow access to a wide range of
reaction conditions and efficient metal complexes, extending
the generality of enantioselective olefin hydrogenation reac-
tions to broad classes of substrates.
the chiral products for pharmaceutical and materials applica-
tions, to name a few.[1] The majority of catalysts tested have
involved rhodium and ruthenium complexes rendered chiral
through incorporation of optically active phosphane ligands.[
Enantioselective olefin hydrogenation with late transition
metal catalysts has been, however, essentially limited to
specific classes of substrates capable of binding in a bidentate
2]
Because catalysts based upon rhodium are extremely active
[
10]
for olefin hydrogenation,
we have been exploring the
[
3]
I
fashion to the metal center, like enamides, unsaturated a,b-
potential of chiral Rh catalyst precursors for hydrogenation
of olefinic substrates lacking a secondary metal binding site.
Recently we demonstrated that the binding of sulfonate
groups to the metal center of the rhodium catalyst [Rh{(� )-
carboxylic acids, and allylic and homoallylic alcohols.[5]
Much more difficult to reduce with good enantiomeric
excesses are olefinic substrates lacking a polar secondary
Lewis basic binding group in the correct position. Several
groups have tackled the challenging class of di- and trisub-
[4]
‡
bdpp}] , formed via hydrogenation of the precursor [Rh{(� )-
[11]
bdpp}(NBD)]ClO (1; Scheme 1), resulted in large increas-
4
es in enantioselectivity for asymmetric imine hydrogena-
tion.[ In this work, we show that binding of anions such as
sulfonates and halides in nonpolar solvents also induces
strong enhancements for the enantioselective hydrogenation
of the prochiral hydrocarbon substrate, 2-phenyl-1-butene (4)
12]
[
a] Prof. Dr. J. M. Buriak, J. C. Klein, D. G. Herrington
Department of Chemistry
Purdue University
West Lafayette, IN 47907 ± 1393 (USA)
Fax : (‡ 1)765-494-0239
(
also known as a-ethylstyrene). We investigated 4 as a
E-Mail: buriak@purdue.edu
substrate because of its inherent difficulty to reduce with
high enantioselectivity as compared to enamide substrates.[
With rhodium catalysts, it is hydrogenated with very poor to
moderate enantioselectivities and thus any increases in ee due
[
b] Prof. Dr. J. A. Osborn
Universit e Louis Pasteur
13]
4
6
rue Blaise Pascal
7000 Strasbourg (France)
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139

J. M. Buriak, J. A. Osborn et al.
Hydrogenation in benzene with
FULL PAPER
0.1m [15]crown-5 gave similar
chemical and optical yields as in
neat benzene (runs 4 and 8),
demonstrating that the crown
ether is not responsible for the
improved
enantioselectivity.
The rates for the latter two runs
in C H could not be accurately
6
6
determined because of the het-
erogeneous nature of the cata-
lytic mixture. Complex 1 is only
slightly soluble in neat benzene,
with or without addition of
[15]crown-5, so only final yields
could be determined with any
accuracy. This lack of solubility
explains the low chemical yields
for these two cases.
Scheme 1. Catalysts, olefins and additives used in this study.
to anion binding would be easily observed and then further
[
14]
analyzed. Takaya and co-workers recently investigated the
influence of anions on the rate and enantioselectivity of
hydrogenation of 4 and related cyclic hydrocarbon olefins
I
with Rh -binap complexes (binap ˆ [1,1'-binaphthalene]-2,2'-
diylbis(diphenylphosphane)) and observed strong effects.[
9]
A
possible mechanistic explanation was proposed which we
3
1
1
substantiate through detailed P, H and parahydrogen-
induced polarization (PHIP) NMR studies. Additional insight
into the problem of concomitant catalytic olefin isomerization
is also provided. The seemingly disparate effects of non-
dissociative and dissociative anions (halides and sulfonates,
respectively) on hydrogenation can be rationalized through
the two mechanistic schemes described here.
Scheme 2. Schematic representation of the role of [15]crown-5 with AOT.
Not only is there a difference in the absolute enantiose-
lectivity for hydrogenation in methanol and AOT/[15]crown-5
in benzene, but there is also a discrepancy in the effect of H2
pressure on enantioselectivity in the two systems as shown in
Figure 1. Catalysis in methanol and in the AOT/[15]crown-5/
Results and Discussion
C H system show remarkably different dependencies on the
6 6
hydrogen pressure. An increase in pressure for hydrogenation
in MeOH results in a lowering of the enantioselectivity, while
for the AOT/[15]crown-5 case the enantioselectivity increases
and reaches a plateau at 30 atm of pressure. The enhancement
of the ee value seen upon the increase of pressure may be due
to a change in mechanism from an unsaturate route to a
dihydride route at high pressures (vide infra).[ Additionally,
catalytic isomerization of 4 to the internal olefin (E)-2-
phenyl-2-butene is observed at 1 atm pressure in both neat
methanol and in the AOT/[15]crown-5/C H system as shown
Catalytic results: Applying the optimum conditions found for
imine hydrogenation, we examined the reactivity of 1 in the
presence of 0.1m of the hydrocarbon-soluble surfactant AOT
[
12]
(see Scheme 1) and 0.1m [15]crown-5 in benzene. AOT is a
commercially available surfactant with a sodium sulfonate
head group and two branched aliphatic tails that forms
16]
[
15]
reverse micelles in nonpolar solvents. As demonstrated in
earlier work, the role of [15]crown-5 is to complex the sodium
cation of AOT, break up reverse micellar aggregates, and
promote binding of the bis(2-ethylhexyl)sulfosuccinate anion
of AOT to the cationic rhodium catalyst (Scheme 2).[
6
6
1
[17]
by GC and H NMR spectroscopy. Isomerization occurs
only at atmospheric pressure and exclusively in the presence
of 1.
12]
Control experiments were carried out in ªregularº solvent
systemsÐneat methanol, benzene, and 1:1 methanol/ben-
zeneÐat varying temperatures and pressures without addi-
tives. The results are summarized in Table 1. Under no
conditions were induction periods observed. Hydrogenation
of 4 in benzene in the presence of AOT and [15]crown-5
results in a dramatic increase in enantioselectivity: with AOT
and [15]crown-5, an ee of 63% (S) can be achieved (run 6),
while in ªregularº solvents, the highest enantioselectivity
observed is 29% (S) in neat methanol (see runs 1 ± 4).
A comprehensive study of different salts was undertaken to
determine the effect of other anions on the hydrogenation of 4
with 1, including sulfates, carboxylates, and halides. The only
other anions that resulted in strong increases in enantiose-
lectivity were chloride and iodide (added in the form of the
tetrabutylammonium salt) as shown in runs 9 ± 16 of Table 1.
Both of these halides induced large increases in enantiose-
lectivityÐup to 71% (S). The rate observed for catalysis in
the presence of iodide in benzene was very rapid and was
140
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Chem. Eur. J. 2000, 6, No. 1

Enantioselective Hydrogenation of Simple Olefins
139±150
Table 1. Results for hydrogenation using 1 as catalyst precursor.[a]
Run
Substrate
Solvent
Additives
pH
2
[atm]
Rate[b]
ee[%]
2
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
5
6
7
7
7
7
MeOH
MeOH
-
-
-
-
70
1
3.0 Â 10
7 (S)
29 (S)
12 (S)
29 (S)
54 (S)
63 (S)
42 (S)
33 (S)
66 (S)
65 (S)
68 (S)
68 (S)
71 (S)
70 (S)
69 (S)
66 (S)
17 (‡)
12 (‡)
40 (‡)
[
c]
81
33
1:1 C
6
H
6
/MeOH
70
70
70
70
1
70
70
70
70
70
70
70
70
1
70
70
70
70
70
1
[
d]
C
C
C
C
C
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
H
H
H
H
6
6
6
6
6
6
6
6
6
6
6
6
6
0.1m AOT
33
0.1m AOT, 0.1m [15]crown-5
0.1m AOT, 0.1m [15]crown-5
0.1m [15]crown-5
10mm N(nBu)
5mM N(nBu)
3.8 Â 10
[
c]
14
[
e]
4
Cl
Cl
8.9 Â 10
1.7 Â 10
1.6 Â 10
4.8 Â 10
1.0 Â 10
1.2 Â 10
1.2 Â 10
1.4 Â 10
1.0 Â 10
2
2
2
3
3
3
2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
4
�
[f]
5mm Cl (in situ)
10mm N(nBu)
5mm N(nBu)
2.5mm N(nBu)
1mm N(nBu)
2.5mm N(nBu)
-
-
0.1m AOT, 0.1m [15]crown-5
5mm N(nBu)
-
-
-
4
I
I
4
4
I
4
I
4
I
1:1 C
6
6
H
H
6
6
/MeOH
/MeOH
[
g]
C
C
C
6
6
6
H
H
H
6
6
6
80
[
h]
4
I
7 (‡)
[
i]
[i]
1:1 C
3
2
2
MeOH
4:6 CH
4:6 CH
4:6 CH
1.4 Â 10
4.8 Â 10
4.8 Â 10
74 (R)
72 (R)
48 (R)
8 (S)
2
2
2
Cl
Cl
Cl
2
/C
2
/C
2
/C
6
6
6
H
H
H
6
6
6
1
1
1
0.1m AOT, 0.1m [15]crown-5
10mm N(nBu)
[
j]
4
I
1
[
a] Conditions: [1] ˆ 5mm; [substrate]/[1] ˆ 100; Tˆ 258C; solvent volume ˆ 10 mL; yields determined by GC or H NMR spectroscopy; all chemical yields
31
[21]
are 100% unless stated otherwise; ee values for hydrogenation product of 4 and 5 determined by P NMR spectroscopy; ee values for hydrogenation
1
product of 7 determined by using optical rotation or by H NMR spectroscopy with [Eu(hfc)
camphorate) in CDCl
3
] (hfc ˆ 3-(heptafluoropropylhydroxymethylene)-d-
as the chiral shift reagent. [b] Units of rate are turnovers h 1 and correspond to the rate recorded for the first hour. Rates are
�
3
accurate to within Æ10%. No induction periods were observed. [c] 10% isomerization to (E)-2-phenyl-2-butene. [d] Conversion after 21 h ˆ 47%.
[
e] Conversion after 21 h ˆ 46%. [f] Prepared in situ from 2.5mm [{Rh(NBD)}
2
(m-Cl)
2
]
and 5mm (� )-bdpp. [g] Conversion after 20 h ˆ 26%.
[
h] Conversion after 14 h ˆ 92%. [i] Conversion after 19 h at 558C ˆ 6%. ee value not determined because of the low yield. [j] Conversion after 24 h ˆ 33%.
Figure 1. Effect of pressure on the enantioselectivity of hydrogenation of 4
with 1 in MeOH (~), in C
with 0.1m AOT/0.1m [15]crown-5 (*), and
with 2.5mm N(nBu) I in C ). The conditions are as in Table 1.
Figure 2. Enantiomeric excess [ee %] (*) and rate (~
concentration of N(nBu) I in C . The hydrogen pressure is 70 atm. All
other conditions as in Table 1. The rate with 0mm N(nBu) I has been
)
versus the
6
H
6
6 6
H
4
4
6
H
6
(
&
4
omitted because of the low solubility of 4 in neat benzene which made an
accurate determination of initial rate impossible.
substantially higher than in neat methanol or in the presence
of AOT/[15]crown-5. Investigation of the effect of iodide
concentration on the enantioselectivity and rate revealed that
only one equivalent or less of iodide with respect to catalyst is
required to bring about an increase in ee to 70% (S) (Figure 2
and Table 1, runs 13 and 14. Iodide is also responsible for large
augmentations in rate; the fastest rates are four times higher
than in neat methanol under the same conditions. Greater
than one equivalent of either iodide or chloride per rhodium
has an inhibiting effect on the rate of catalysis but is not
detrimental to the enantioselectivity (Figure 2). The high
I
Rh ], and keeping the concentration of 4 identical. The rate at
I
0.25 mm of Rh is over an order of magnitude faster than at
I
5 mm Rh . The rate of the hydrogenation reaction increased
with decreasing catalyst concentration, and the enantioselec-
tivity was essentially unchanged (Figure 3).[ In order to slow
the rate of hydrogenation, the pressure was lowered to 1 atm.
Two pertinent observations were made at this pressure:
i) no significant isomerization of 4 was detected (<1% by
GC)
18]
I
reactivity of the iodide/Rh catalyst can be demonstrated upon
lowering the concentration of catalyst precursor [1:1 iodide/
ii) the enantioselectivity did not depend on hydrogen pres-
sure (compare runs 14 and 16 in Table 1).
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141

J. M. Buriak, J. A. Osborn et al.
FULL PAPER
presence of halides, 31P{ H} NMR studies were carried out.
1
Using concentrations identical or similar to those of catalysis,
we detected spectroscopically a total of three species after
‡
hydrogenation of 1 in [D ]benzene to form Rh[(� )-bdpp] ,
6
upon addition of N(nBu) X (X ˆ Cl, I). In absence of halide,
4
the hydrogenated product of 1 (1 atm, 298 K) is only sparingly
31
1
soluble but appears in the P{ H} NMR spectrum as a doublet
1
at d ˆ 41.64 ( J
ˆ 194 Hz). This species has a chemical shift
Rh,P
and coupling constant indicative of the 18-electron complex
6
‡
[
Rh{(� )-bdpp}(h -C D )] which forms readily in the pres-
[21]
6
6
ence of aromatic compounds.
When one equivalent of
6
‡
N(nBu) I is added to [Rh{(� )-bdpp}(h -C D )] in [D ]ben-
4
6
6
6
Figure 3. Enantiomeric excess [ee %] (*) and rate (~) versus the
concentration of 1 (with one equivalent of N(nBu) I)in C H for the
4 6 6
hydrogenation of 4. The [4] is held constant at 0.5m and the hydrogen
pressure at 70 atm. All other conditions as in Table 1.
zene, three distinct species are observed as doublets initially,
two of which decrease in intensity after 1 h of equilibration.
6
‡
One of the species corresponds to [Rh{(� )-bdpp}(h -C
D )] ,
6 6
and is still present in small concentrations after equilibration.
1
The second and most intense doublet at d ˆ 43.08 ( J
ˆ
Rh,P
These observations are in stark contrast to hydrogenation in
1
86 Hz) (Figure 4a) is most likely due to the iodide-bridged
either neat methanol or with 1:1 AOT/[15]crown-5 in C H6
6
dimer [{Rh[(� )-bdpp]} (m-I) ] which would be expected to
2
2
which did have a strong relationship between H pressure and
2
[22]
have a bent structure (aRh-(m-I)-Rh < 1808).
Similar
ee, and was accompanied by 10% isomerization to the internal
E isomer at atmospheric pressure (Table 1, runs 6 and 7).
Other phosphanes and substrates were tested to further
generalize the effect of anion binding on enantioselectivity.
The addition of 0.1m AOT and 0.1m [15]crown-5 for hydro-
genation of 4 with the catalyst precursors [Rh{(‡)-pro-
phos}(NBD)]ClO4 (2) (prophos ˆ 1-methyl-1,2-ethanediyl-
iodide-bridged dimers have been observed with the diphos-
phanes chiraphos and 1,2-bis(diphenylphosphino)ethane
(
(
dppe).[ Upon addition of a second equivalent of N(nBu)4I
23]
with respect to rhodium) to the dimeric[{Rh[(� )-bdpp]} (m-
2
I) ], a new complex forms which corresponds to the third
2
species seen before equilibration with a chemical shift of d ˆ
1
4
2.35 ( J
ˆ 179 Hz) (Figure 4b). Because of the clear
Rh,P
bis(diphenylphosphane)) and [Rh{(‡)-binap}(NBD)]ClO
4
(
3) brought about only small effects on the enantioselectivity.
The effect of iodide on 2, however, is dramatic as the ee inverts
from 5% (R) in MeOH to 51% (S) in benzene with 10mm
N(nBu) I. This result parallels that of Takaya and co-workers
4
who observed an increase in enantioselectivity for the hydro-
[9]
genation of 4 with 3 in CH Cl in the presence of iodide.
2
2
Three other olefins, 5, 6, and 7 (see Scheme 1) were tested for
hydrogenation with 1 as shown in Table 1, runs 17 ± 25. The
highest ee observed for hydrogenation of 5, [40% (‡)], was
attained in the presence of 0.1m AOTand 0.1m [15]crown-5 in
C H . This ee is more than double that observed in 1:1 MeOH/
6
6
C H or neat C H . Surprisingly, iodide, however, causes a
6
6
6
6
decrease in enantioselectivity for the reduction of 5. Hydro-
genation of trans-a-methylstilbene (6) was unsuccessful
probably due to the steric hindrance of the internal, trisub-
stituted olefin. Reduction of the enamide, methyl-(Z)-a-
acetamidocinnamate (7), was not improved in the presence of
either AOT/[15]crown-5 or iodide (runs 22 ± 25). The reasons
for the observed decrease in ee with sulfonate are not clear
since 31P NMR studies revealed that the presence of 0.1m
AOT and 0.1m [15]crown-5 does not prevent formation of
‡
the [Rh{(� )-bdpp}(enamide)] complex (majority isomer) in
[
19]
4
:6 CH Cl /[D ]benzene. Halides, on the other hand, are
2 2 6
known to lower the enantioselectivity of enamide hydro-
[
20]
genation with chiral rhodium catalysts.
3
1
1
P and H NMR studies with halides: It has been demon-
Figure 4. 121 MHz 31P{ H} NMR spectra of 1 (5mm) in [D
1
6
]benzene in the
strated previously that two rhodium complexes form in the
presence of iodide under 1 atm of H
2
at 298 K after 2 h of equilibration
presence of a sulfonate anion in [D ]benzene, [Rh{(� )-
6
6
‡
under these conditions. [Rh{(� )-bdpp}(h -C
6
D
6
)] appears at d ˆ 41.64
6
‡
bdpp}(h -C D )] and [Rh{(� )-bdpp}](RSO ). In order to
1
1
6
6
3
( J
ˆ 194 Hz), [{Rh[(� )-bdpp](m-I)} ] at d ˆ 43.08 ( J
ˆ 186 Hz), and
Rh,P
2
1
Rh,P
elucidate the species present under catalytic conditions in the
[{Rh[(� )-bdpp](m-I)}
2
] at d ˆ 42.35 ( JRh,P ˆ 179 Hz).
142
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Chem. Eur. J. 2000, 6, No. 1

Enantioselective Hydrogenation of Simple Olefins
139±150
dependence on iodide concentration, the smaller 1JRh,P
coupling constant, and similarity of chemical shift, this
complex is most likely the anionic diiodide complex
d ˆ � 11.62 (Figure 5c). These hydride species are able to
withstand stripping of the solvent in vacuo, followed by
redissolution in CD Cl with no decrease in their relative
concentrations, and thus probably represent stable species
outside of the catalytic cycle. The proton spectra have been
simulated and indicate that the features in Figures 5a and 5b
2
2
�
‡
[
Rh{(� )-bdpp}(I) ] with [N(nBu)4] as the counterion.
2
I
Similar monomeric anionic, square-planar Rh complexes
previously characterized include cis-[Rh(PR ) (NHPh) ]Li
3
2
2
[
24]
�
[25]
(
(
R ˆ Ph, Et),
di-tert-butylphosphino)ethane}(C H ) ]Li.
cis-[Rh(CO) (I) ] ,
and [Rh{1,2-bis-
are due coupling with two slightly chemically inequivalent
2
2
[
26]
2
The
cis-
phosphorus atoms and the rhodium center ( J
ˆ 12.5 Hz
6
5
2
P(cis),H
2
1
[
Rh(PR )(NHPh) ]Li complex also involves an equilibrium
(observed), J
ˆ 14.4 Hz (calculated), J
ˆ9.0 (ob-
3
2
P(cis),H
Rh,H
between the amido-bridged dimer [{Rh(PR ) } (m-NHPh) ]
served)). Attempts to simulate the observed spectra with two
equivalent cis phosphane ligands cis to the hydride ligand
failed. The two phosphane ligands are chemically inequivalent
3
2
2
2
and the monomeric anion, the relative ratios depending upon
the equivalents of added lithium anilide. By analogy, a similar
equilibrium between [{Rh[(� )-bdpp]} (m-I) ], [Rh{(� )-
due to loss of the C symmetry of the chiral phosphane.
2
2
2
�
6
‡
bdpp}(I) ] , and [Rh{(� )-bdpp}(h -C D )] is proposed as
The same hydrides observed in the presence of iodide at
2
6
6
shown in box I of Scheme 3. Similar equilibria could not be
atmospheric pressure, form at high pressure as well. [Rh{(� )-
31
1
observed by P{ H} NMR spectroscopy for the related
complexes [Rh(NBD)(dppe)]ClO4 and
[Rh{(‡)-bi-
nap}(NBD)]ClO (5mm concentrations) in [D ]benzene with
bdpp}(NBD)]ClO4 and one equivalent of N(nBu) I were
4
hydrogenated at 70 atm in an autoclave for 3 h at room
temperature in C H , the pressure released, the benzene
4
6
6
6
5
mm N(nBu) I and 1 atm of H at 298 K. Presumably the
removed in vacuo, and the complexes dissolved in CD Cl ,
2 2
4
2
neutral dimeric iodide-bridged complex formed so rapidly
that the anion and benzene solvate were not spectroscopically
yielding a solution with a total [Rh] of 15 mm. The main
species present is the [{Rh[(� )-bdpp]} (m-I) ] dimer as
2
2
[
27]
31
1
observable under these conditions.
observed by P{ H} NMR spectroscopy and only the two
hydrides shown in Figures 5b and 5c are observed. Assuming
that the hydrides originate from dihydrogen, it appears that
1
With H NMR spectroscopy, stable hydride species can be
detected within minutes after hydrogenation of 5mm 1 in
[
D ]benzene at 1 atm pressure in the presence of two
H is activated in a similar fashion at both low and high
2
6
equivalents of N(nBu) Cl, or one or two equivalents of
pressures since the same hydride products are observed in
each case.
4
N(nBu) I (Figure 5). With chloride, an apparent doublet of
4
triplets centered around d ˆ � 15.5 is rapidly produced (Fig-
ure 5a), and with iodide, a similar feature also resembling a
doublet of triplets appears at d ˆ � 14.84 (Figure 5b) but is, in
addition, accompanied by a triplet of triplets of triplets at
Possible structures for the species yielding the patterns
observed in Figures 5a and 5b include a monomeric five-
III
coordinate Rh
trigonal-bipyramidal complex [Rh{(� )-
bdpp}(H)X ] (X ˆ Cl, I) with H occupying an apical position,
2
as represented by
I
in
Scheme 3, but could be six-
coordinate if a molecule of
solvent is bound, as has
been suggested in the case
of
the
related
complex
[
Rh(PPh ) (H)Cl ] in CH Cl .
3
2
2
2
2
A square-pyramidal structure
with the hydride in the apical
position is discounted since the
hydride would be expected to
resonate much farther up-
field.[ The chemical shift of
the hydride (trans to Cl) in
28]
[
Rh(PPh ) (H)Cl ] is d ˆ � 16.1
3
2
2
which compares very favorably
with the d ˆ � 15.5 feature ob-
served for the (� )-bdpp com-
1
[29]
Figure 5. 300 MHz H NMR spectra of the hydride resonances observed after hydrogenation of 1 (5mm) under
plex with added chloride. It
1
atm of H
2
at 298 K. The top three spectra are observed experimentally and the bottom three simulated. a) 10mm
was suggested that [Rh(PPh ) -
3
2
1
of N(nBu)
4
Cl (equivalents of chloride anion) present. Observed and calculated coupling constants: JRh,H ˆ 9.0 Hz
(
H)Cl ] may dimerize in solu-
2
2
2
(
observed), JP(cis),H ˆ 12.5 Hz (observed), JP(cis),H ˆ 14.4 Hz (calculated). Under the assumption of a monohydride
tion through through bridging
chlorides to give a formally
octahedral coordination sphere
around each rhodium. The spe-
cies yielding the spectra of
Figures 5a and 5b may thus be
best interpreted as the dimeric
species, this resonance represents about 0.05 of a proton as determined by integration. b) 5mm (or 10mm) of
1
N(nBu)
4
I (1 or 2 equivalents of iodide anion). Observed and calculated coupling constants:
J
Rh,H ˆ 5.9 Hz
observed), 2JP(cis),H ˆ 15.9 Hz (observed), JP(cis),H ˆ 13.8 Hz (calculated). Under the assumption of a monohydride
2
(
species, this resonance represents about 0.13 of a proton as determined by integration. c) 5mm (or 10mm) of
1
N(nBu)
4
I (1 or 2 equivalents of iodide anion) present. Observed and calculated coupling constants:
J
Rh,H
ˆ
1.0 Hz (observed), 2
J
P(trans),H ˆ 69.0 Hz (observed),
2
J
P(cis),H ˆ 6.0 Hz (observed). The assignment of
1
JRh,H and
2
2
2
1
2
[23b, 33]
J
P(cis),H is ambiguous but in related compounds, JP(trans),H > JRh,H > JP(cis),H. Under the assumption of a
monohydride species, this resonance represents about 0.10 of a proton as determined by integration.
Chem. Eur. J. 2000, 6, No. 1
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143

J. M. Buriak, J. A. Osborn et al.
FULL PAPER
Scheme 3. Catalytic cycle, pre-equilibria and proposed monohydride species. Box I, the pre-equilibria, involves three rhodium species labeled A, B, and C
whose relative concentrations depend upon the quantity of added iodide. Splitting of the dimer [{Rh[(� )-bdpp](m-I)} ] results in formation of two identical
2
1
4-electron monomeric units, [Rh{(� )-bdpp}(I)], which are intermediates in the catalytic cycle for olefin hydrogenation (box II, the catalytic cycle). The
possibility for the existence of equilibria between B and D, and B ‡ C and D are postulated but not determined. Dihydrogen addition to form E (geometry
unknown) is followed by olefin coordination, insertion, and then reductive elimination to yield the reduced, chiral product. Box III includes the proposed
structures for the inactive, observed hydride complexes. The exact stereochemistry of the intermediates shown here is merely tentative.
complex [{Rh[(� )-bdpp](H)(X)} (m-X) ] (X ˆ Cl, I) as shown
be considered and contrasted with a dihydride Wilkinsonꢀs
catalyst type mechanism [Eq. (2)].
2
2
in Scheme 3 (complex J) since it would lead to octahedral
coordination about each rhodium center in the poorly
coordinating benzene solvent. An identical compound based
‰
H2Š
L M�H ‡
!L M(CH ) R ! L M�H‡CH CH R
(1)
(2)
n
n
2
2
n
3
2
[
30]
upon iridium, [{Ir[(� )-bdpp](H)(I)} (m-I) ],
is known
2
2
‰
H2Š
whose hydride is coupled (cis) to two chemically inequivalent
L
n
M!L
n
M(H)
2
! L
n
M‡CH
3
CH
2
R
phosphorus atoms as is the case with the rhodium complex.
The C symmetry of the ligand broken by the apical H and I
substituents oriented trans to each other. Related dimeric
2
The observed monohydride species do not appear to be
involved in catalysis due to their lack of observable reactivity
[
31]
species based upon rhodium have also been characterized,
with an excess of 4 in an argon-purged [D ]benzene solution.
6
[
32]
including [{Rh(PEtPPh ) (Cl) } (m-Cl) ].
An exact deter-
2
2
2
2
2
In addition, no isomerization of the olefin is observed under
mination of these hydride species remains to be seen but
based upon literature precedent, a monomeric five-coordi-
nate Rh trigonal-bipyramidal complex is possible which
catalytic conditions with the [Rh{(� )-bdpp}] iodide or chlor-
ide complexes. While rhodium monohydrides are known to be
III
[10b, 34]
excellent isomerization catalysts,
these hydrides appear
probably dimerizes in the nonpolar benzene solvent to yield
inert and unreactive with respect to alkenes. Because mono-
hydride species may be formed through heterolytic dihydro-
gen activation, the addition of a strong, noncoordinating base
would be expected to increase rates and effect enantioselec-
tivity. Hydrogenation of 4 with 1 and one equivalent of
N(nBu) I in C H , in the presence of a fivefold excess of
[
{Rh[(� )-bdpp](H)(X)} (m-X) ] (X ˆ Cl, I). The compound
2
2
2
producing the triplet of triplets of triplets pattern ( J
6
is almost certainly the bridging monohydride dirhodium
ˆ
P(trans),H
2
9.0 Hz (observed), J
ˆ 6.0 Hz (observed)) of Figure 5c
P(cis),H
complex [{Rh[(� )-bdpp]} (m-H)(m-I)], represented by H in
2
4
6
6
1
Scheme 3, based upon the similarity of the H NMR spectrum
the proton sponge N,N,N',N'-tetramethyl-1,8-naphthalenedi-
amine, however, brought about little change in rate or
enantioselectivity. Thus a mechanism based upon monohy-
dride intermediates appears unlikely.
with several closely related compounds that have been
[
23b]
previously characterized by NMR
spectroscopy and crys-
[
33]
tallographic analysis. The routes leading to these hydride
species remain elusive and are the subject of continuing
investigation in our laboratory.
In order to ascertain definitively whether the hydrogena-
tion mechanism proceeds through a mono or dihydride
1
[35]
mechanism, PHIP H NMR spectroscopy was utilized.
Towards a mechanism in the presence of halide anions:
Because monohydride species were observed with halides, the
possibility for a monohydride mechanism [Eq. (1)] needed to
Since PHIP can only be observed when both H atoms from
a molecule of dihydrogen are transferred in a pairwise fashion
to the same molecule of substrate, enhancement of peaks in a
144
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Enantioselective Hydrogenation of Simple Olefins
139±150
hydrogenated product 1H NMR spectrum would clearly
indicate a dihydride mechanism [Eq. (2)]. In a monohydride
mechanism, the two hydrogen atoms transferred to the
substrate would arise from different molecules of dihydrogen
and thus polarization would not be observed [Eq. (1)].
sonꢀs catalyst [RhCl(PPh ) ].
[41]
Excess iodide inhibits the
3
3
�
reaction rate through formation of [Rh{(� )-bdpp}I ] which is
2
probably not catalytically active. While the 14-electron
monomeric intermediate [Rh{(� )-bdpp}I] has been implicat-
ed in this mechanism, Landis and Chan previously suggested
that dihydrogen addition to one of the rhodium centers of the
chloride-bridged dimer [{Rh[(‡)-diop]} (m-Cl) ], similar to A
[
D ]Styrene, a perdeuterated model substrate studied pre-
8
viously with PHIP, was used in this study since it does not
overlap with any of the hydrogenated product peaks.[
2
2
36, 37]
in Scheme 3, could occur which would result in [Rh{(‡)-
[42]
Upon addition of para-enriched hydrogen to 15 mm [Rh{(� )-
diop}(Cl)H ] after cleavage of the dimer. In the case of the
2
‡
bdpp}] (formed from hydrogenation of 1) in [D ]benzene
[{Rh[(� )-bdpp]} (m-I) ] dimer, dihydrogen addition would
6
2
2
with two equivalents of N(nBu) I and 44 equivalents of
result in the same dihydride intermediate, E, [Rh{(� )-
4
[
D ]styrene, a net polarized spectrum can be observed (Fig-
bdpp}(I)H ]. Although this step could occur in parallel, it
8
2
ure 6a), which is enhanced by ꢀ100 fold as determined by
does not explain the increase in rate with decreasing [Rh] and
is thus improbable in this particular system. Like the
[Rh(PPh ) Cl] complex in the Wilkinson mechanism, the
integration. The polarized peaks correspond to the methylene
and methyl hydrogens arising from incorporation of H in
2
3 2
styrene to yield the expected ethyl benzene product. The
polarization fades within 5 min (Figure 6b). That net polar-
ization is observed reveals that the hydrogenation occurs
before placement of the sample in the magnetic field and thus
hydrogenation commences rapidly under these conditions.[
[Rh{(� )-bdpp]}I complex then undergoes oxidative addition
of dihydrogen to form E which then coordinates to the olefinic
substrate. The geometry of the [Rh{(� )-bdpp}(I)H ] species
2
could be square-pyramidal or trigonal-bipyramidal, as shown.
A recent study of olefin-catalyzed hydrogenation with
[RhCl(PPh ) ] using parahydrogen-induced polarization in-
38]
Pairwise geminal C�H exchange in styrene catalyzed by
3
3
‡
[
Rh{bis(diphenylphosphinobutane)}] solvates in deuterated
dicates formation of the [Rh(PPh ) H Cl(olefin)] species with
3 2 2
1
[43]
acetone has been observed in H PHIP NMR experiments,
cis phosphane ligands; one of the hydrides is forcibly trans
which leads to enhancement of those signals between d ˆ 6
to a phosphane ligand, a geometry previously believed to be
highly unfavorable.
[
39]
[41]
and 5. As can be seen from Figure 6a, no geminal C�H
signal, corresponding to C D CDˆC(para � H) , is observed
6
5
2
which suggests that this exchange process is slow under these
conditions with added iodide.
All available evidence strongly points towards a dihydride
Wilkinsonꢀs catalyst type mechanism. It is well known that
rhodium ± halide complexes form stable dimers in solu-
[
23b, 40]
I
tion.
The faster rates at low Rh concentrations suggest
a monomeric catalytically active species as represented by D
in Scheme 3 whose formation is favored at the low concen-
trations, preventing self-inhibition through dimerization. The
catalytically active monomeric complex is proposed to be the
With the diphosphane (� )-bdpp, the phosphane ligands
must coordinate in a cis fashion to the rhodium center because
of the small bite angle of the ligand. Therefore, the complex
[Rh{(� )-bdpp}(I)H ], having a similar coordination sphere, is
2
1
4-electron [Rh{(� )-bdpp}(I)] which is spectroscopically
reasonable and could be the active species in our case.
unobservable. This coordinatively unsaturated [Rh{(� )-
Because high pressures of H favor formation of dihydride
2
bdpp}I] species is similar to the monomeric intermediate
species through oxidative addition of dihydrogen, the reaction
mechanism at 70 atm is almost certainly a dihydride pathway
[
Rh(PPh ) Cl] inferred for olefin hydrogenation with Wilkin-
3 2
(
dihydrogen addition prior to
olefin coordination). The cata-
lytic results indicate that the
enantioselectivity with halides
is independent of pressure
which suggests that the same
mechanism is in operation at
both 1 and 70 atm, explaining
the lack of dependence of enan-
tioselectivity on hydrogen pres-
sure. Takaya and co-workers
also noted that pressure had
no bearing upon the enantiose-
lectivity of hydrogenation of 4
with [Rh{(‡)-binap}] com-
plexes with halides, thus dem-
onstrating the generality of this
anion effect with monodentate
olefinic substrates.[
Figure 6. 1H NMR spectra of hydrogenation of [D
two equivalents of N(nBu) I in [D ]benzene with one atmosphere of para-enriched hydrogen at 298 K. a) Two
scans of the sample reveal a 100-fold increase in intensity of methylene and methyl peaks of the hydrogenated-
ethylbenzene product due to net polarization. b) Two scans after allowing the sample to relax for 5 min, revealing
complete disappearance of the polarization.
]styrene carried out under PHIP conditions: 15mm of 1 with
8
4
6
9]
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145

J. M. Buriak, J. A. Osborn et al.
FULL PAPER
Towards a mechanism in the presence of sulfonate anion: The
enantioselectivity does show pressure-dependent behavior
with the AOT sulfonate anion in benzene and in neat
methanol without additives (see Figure 1). With 0.1m AOT/
through a p-allyl mechanism (vide infra). At atmospheric
pressure, the rate in neat MeOH is six times higher than in
0.1m AOT/0.1m [15]crown-5 in benzene, while at high
pressure the latter is faster. The reason for the sluggishness
of the AOT/[15]crown-5 system at 1 atm is due to the
competition at the rhodium center for binding by the
sulfonate, the benzene, and the olefin. The olefin must
coordinate to enable dihydrogen addition so olefin binding
becomes rate limiting. At higher pressures, the hydride
mechanism becomes dominant and thus dihydrogen can add
directly to the sulfonate complex (the major species present)
to give the dihydride complex [Rh{(� )-bdpp}(RSO )H ]. The
0
.1m [15]crown-5 in C H , the ee increases from 42% to 66%
6 6
at 70 atm. The enhancement of enantioselectivity seen upon
increased pressure can be explained by a change in mecha-
nism from an unsaturate route (olefin binding prior to
dihydrogen addition) at atmospheric pressure, to a dihydride
route (dihydrogen addition before olefin binding) which is
favored at higher pressures as outlined in Scheme 4. At high
pressures of dihydrogen (>30 atm), the previously postulated
3
2
I
neutral, four-coordinate Rh complex, [Rh{(� )-bdpp}(RSO )]
neutral sulfonate complex [Rh{(� )-bdpp}(RSO )] (K), being
3
3
(
K of Scheme 4) with a bidentate sulfonate ligand is hydro-
more electron rich than the cationic methanol solvate
‡
genated, producing the six-coordinate dihydride species. Due
to the strong trans effect of the hydride ligand, dissociation of
one arm of the sulfonate ligand occurs, opening a binding site
for the olefin (O). The olefin then inserts (P), and the product
reductively eliminates. At low pressure, olefin binding occurs
before dihydrogen addition, which explains the substantial
[Rh{(� )-bdpp}(MeOH) ] , would oxidatively add H faster,
x
2
also explaining at least partially the increase in rate. As a
result, hydrogenation of 4 is not inhibited at high pressures by
the large excess of bis(2-ethylhexyl)sulfosuccinate anion. The
complex [Rh(cod)(dppe)(p-O SC H CH )], which also con-
3
6
4
3
tains a sulfonate counteranion, has been shown to be active
for olefin isomerization at low pressures of hydrogen although
details concerning hydrogen
(10%) isomerization of 4 to (E)-2-phenyl-2-butene, possibly
pressure effects were not stat-
ed.[
44]
Effect of anion on isomeriza-
tion:
A
10% isomerization
turnovers/
(
10 isomerization
molecule of catalyst) is ob-
served in both neat methanol
and in [D ]benzene with AOT/
6
[15]crown-5. At high pressures,
rhodium dihydride species are
formed before olefin binding
and thus a dihydride mecha-
nism is taking place. Because
rhodium dihydride complexes
are generally not good isomer-
ization catalysts,[
10a, 34]
isomeri-
zation is effectively blocked at
high pressures. This explains
the lack of isomerization in the
all cases described here at high
pressure, and with halides at
low pressure as well since a
dihydride mechanism domi-
nates. At atmospheric pressure
in neat MeOH and [D ]benzene
6
with AOT/[15]crown-5, howev-
er, the olefin coordinates to the
complex before dihydrogen ad-
dition, and is isomerized, possi-
bly through a p-allyl mecha-
nism as shown in Scheme 5.
Scheme 4. Olefin hydrogenation in the presence of the sulfonate anion formed from 0.1m AOT and one
)] (K) serves
as the initial, common intermediate in the catalytic cycle. At low pressures (<30 atm of dihydrogen), an
unsaturate route is followed whereby olefin binding (M) precedes dihydrogen addition (O). At high pressures
equivalent of [15]crown-5. The neutral four-coordinate square-planar species [Rh{(� )-bdpp}(RSO
3
When a CD OD solution of
3
(
>30 atm), a dihydride route is operational and dihydrogen first oxidatively adds to K, producing the dihydride
intermediate N which then coordinates a substrate molecule, forming O. Species O is common to both cycles and
H bond to form the alkyl complex P which then reductively
the methanol solvate, [Rh{(� )-
‡
bdpp}(MeOH) ] , is sparged
is followed by insertion of the olefin into the Rh
eliminates, producing the product alkane and the initial catalytic intermediate K. The exact stereochemistry of the
�
x
extensively with Ar to remove
intermediates shown here is merely tentative.
residual H , this complex rap-
2
146
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Enantioselective Hydrogenation of Simple Olefins
139±150
Effect of anion on enantioselectivity: Effects on enantiose-
lectivity due to anion coordination have been observed
previously upon addition of salts to rhodium-, iridium-, and
ruthenium-catalyzed reductions.[ The improvements in ee
observed in this work for hydrogenation of 4 may be due to
one or both of the following factors:
47]
ii) The steric influence of the anionic ligand bound to the
metal center could play an important role. It has been
suggested repeatedly that the chirality of the phosphane
backbone is transmitted to the substrate through the face ±
edge[ or quasi-equatorial array
48]
[49]
of phenyl groups.
Certain quadrants around the metal center become more
sterically hindered than others, forcing the prochiral
substrate into a certain conformation leading to chiral
induction. Therefore, an additional ligand would increase
the steric bulk around the metal center and thus enhance
one substrate binding mode over the other.
Scheme 5. Isomerization of 4 via a p-allyl. The solvated, cationic species
‡
Rh(P�P) forms in neat methanolic solvent, and when the less strongly
binding sulfonate anion (as opposed to halide) dissociates.
ii) Electronic effects could have large effects on catalyst
reactivity. Because electronic effects have been shown to
be important in asymmetric hydrogenation of enamides,[
anion coordination could adjust the electronics of the
metal center and favor certain diastereomeric intermedi-
ates or steps in the catalytic cycle.
As suggested earlier,[ the formation of intermediate E in
Scheme 3 and N in Scheme 4 could be the enantioselective
step. If the anion is trans to the hydride, the trans effect of the
anion could affect the stability of the hydride, modifying the
difference of the free energy of activation of between the
transition states of the diastereomeric complexes for olefin
insertion, augmenting the enantioselectivity of the reaction.
Because several possible isomers are possible, this interpre-
tation remains plausible but speculative.
50]
idly isomerizes 4 in an NMR tube to give exclusively (E)-2-
phenyl-2-butene. While trace, spectroscopically unobservable
monohydrides may catalyze the isomerization, little evidence
supporting active monohydrides has been obtained in this
study. p-Allyl mechanisms have been invoked to explain the
9]
[45]
isomerization activity of rhodium hydrogenation catalysts
which operate through dihydride mechanisms.[46] The weakly
bound anions like sulfonate and perchlorate (the counterion
in 4) dissociate readily, opening up coordination sites which
enable the p-allyl mechanism to operate rapidly. The complex
is also more Lewis acidic due to the positive charge of the
complex which also encourages binding of the olefin to the
metal center. Halide binding, renders the complex neutral and
less Lewis acidic, and therefore olefin binding may occur less
readily, leading to substantially less isomerization through a
p-allyl mechanism under these conditions.
Conclusions
1
Because recent PHIP H NMR experiments by Bargon and
co-workers revealed exchange of the geminal C�H hydrogens
of styrene with parahydrogen mediated by cationic rhodium
complexes under hydrogenation conditions, isomerization of
The presence of anionic achiral additives has very strong
effects on the enantioselectivity, hydrogen pressure depend-
ence, and rate for the asymmetric hydrogenation of mono-
dentate hydrocarbon olefins such as 4. Strongly coordinating
halides such as iodide bind to the cationic rhodium complex in
nonpolar benzene to form neutral and anionic complexes,
depending upon the relative ratio of anion/rhodium. Excess
halide brings about a decrease in rate due to inhibition, but
does not lower the enantioselectivity of the reaction. The
NMR studies, H, P and PHIP, indicate that a dihydride
mechanism is in operation at both low and high pressures
where dihydrogen addition occurs before olefin binding.
Monohydride species observed by NMR spectroscopy with
chloride and iodide are inactive, stable species not involved in
the catalytic cycle. With the more dissociative sulfonate anion,
however, catalytic data suggest that a dihydride mechanism
occurs only at high pressure; an ªunsaturateº route, that is
olefin binding before H2 addition, occurs at atmospheric
pressure. Olefin isomerization does not occur when halides
are present at any pressure, but is seen with sulfonate at low
pressure through what is probably mechanism involving a p-
allyl. The effect of anion binding is clearly general and is the
focus of continuing work. It is obvious that simple achiral
4
to the internal isomer (E)-2-phenyl-2-butene could occur
[
39]
through a similar route. Insertion of the olefin into one of
the Rh�H bonds of the dihydride complex can then undergo
b-hydride elimination, reforming either 4 or (E)-2-phenyl-2-
butene. The key to the mechanism is secondary coordination
[
39]
by the phenyl group of styrene. Secondary phenyl coordi-
nation is much more likely in the case of cationic rhodium
complexes because of their higher Lewis acidity and more
open coordination sphere as compared to the neutral halide
adducts. Takaya and co-workers proposed that reductive
elimination of the alkane product from neutral [Rh(binap)]
complexes in the presence of halide was too rapid to allow for
b-hydride elimination, explaining the lack of isomerization.[
We observed, however, that hydrogenated 1, [Rh{(� )-
1
31
9]
‡
bdpp}] , in the absence of dihydrogen (in an argon-sparged
solution of CD OD), is capable of isomerization of 4 which
3
supports a p-allyl type mechanism. Trace hydrogen may,
however, remain in the solution and be responsible for the
observed isomerization through the discussed mechanism
involving dihydrides.
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147

J. M. Buriak, J. A. Osborn et al.
FULL PAPER
additives have a dramatic influence on catalytic mechanisms
in late transition metal catalyzed reactions and a detailed
understanding of their role is extremely useful, if not essential
for the development of highly enantioselective catalysts
capable of reducing new and broad classes of olefinic
substrates.
was extracted with pentane. Previously, the ee of the 2-phenylbutane
product had been determined in the literature through optical rotation, but
31
in this work we utilize exclusively a quick and effective P NMR method
‡
involving the catalyst itself, [Rh{(� )-bdpp} ], as an inorganic chiral shift
reagent since it requires only small quantities of material and is easily
[
21]
Final yields for 6 and 7 were determined by 1H NMR
performed.
spectroscopy. Compound 7 was separated from its hydrogenation product
[
55]
by chromatography as previously reported. The ee can be determined by
optical rotation {[a]2
CDCl
0
[56]
D
ˆ ‡16.4 (c ˆ 2.0, MeOH)} or using [Eu(hfc)
3
] in
3
.
Experimental Section
For the ee determination of the 2-phenylbutane hydrogenation products of
and 6, the 31P NMR method previously described was utilized. For an
4
The reagents and solutions were handled under nitrogen or argon
accurate measurement of the hydrogenation product of 4, the 2-phenyl-
butane had to be free of 4 and the isomerization product (E)-2-phenyl-2-
butene; the following two methods could be used to purify the desired
atmosphere through standard Schlenk techniques or in
Atmospheres glovebox. NMR spectra were obtained using either
00 MHz or 500 MHz Bruker spectrometers, or a 200 MHz 200 XLA
Varian instrument. NMR spectra were simulated with NMR'' Version 1.0,
8881(2) Specific Code (developed by A. K. Rapp e and C. J. Casewit). Gas
a Vacuum
3
2
product. Reverse-phase HPLC using 85:15 H O/MeOH with a Zarbox
ODS semi-preparative reverse phase column (25 cm  4.6 mm i.d.) cleanly
separated the three compounds. Alternatively, ªsilveredº 2-mm thick
preparative TLC silica plates could be used. Commercial TLC plates
6
chromatography (GC) was carried out with a Hewlett Packard Serie-
s II 5890 equipped with a semicapillary column (HP-1, methylsilicone gum,
(
Merck) were silvered by soaking in 80:20 EtOH/H
2 3
O with 10% AgNO
1
0 m  0.53 mm  2.65 mm) and a flame ionization detector (FID). Optical
(
by mass) for 2 min and then dried overnight in a dessicator in vacuo in
rotations were performed by using a Perkin Elmer 241 polarimeter.
Solution samples were taken in a 10 cm glass cell and neat samples in a
absence of light. The plates are then heated in an oven in air for 1 h at
608C or until they become pale red/brown. Small quantities (25 ± 50 mg)
1
1
5
cm glass cell. High-pressure hydrogenation reactions were carried out in a
0 mL stainless steel autoclave (Autoclave Engineers). The temperature is
31
of pure 2-phenylbutane (sufficient for P NMR ee determination) could be
eluted with neat pentane. If, however, the total percentage of 4 and (E)-2-
phenyl-2-butene in the sample was greater than 55%, obtaining a pure
sample of 2-phenylbutane through this procedure was virtually impossible
and HPLC had to be used. Compound 6 did not have to be separated from
held constant by a thermostated oil bath and the catalytic solution agitated
by magnetic stirring. Hydrogenations carried out at 1 atm were performed
on a dual manifold Schlenk line (Ace Glass) with H fed directly into the
2
manifold. Solvents were cyclically distilled under an inert atmosphere and
dried over the usual drying agents or were purified by using a homemade
Dow Chemical/Grubbs solvent purification system.[ 2-phenyl-1-butene
‡
its hydrogenated product since it binds only weakly to [Rh{(� )-bdpp}] and
is well separated in the 31P NMR spectrum from the diastereomers formed.
51]
(
4) and a-cyclohexylstyrene (5) were synthesized by a Wittig reaction
Hydrogenations in NMR tube: The catalyst and additives were loaded into
a 5 mm NMR tube and taken into the glovebox. The deuterated solvent was
added and the tube sealed with a rubber septum and parafilm. The tube was
according to known procedures and their purities verified by NMR
spectroscopy and GC.[52] trans-a-Methylstilbene and N,N,N',N'-tetrameth-
yl-1,8-naphthalenediamine were purchased from Aldrich and used as
received. [15]crown-5 was purchased from Aldrich and dried 12 h in vacuo
2
removed from the box and then purged with H through a stainless steel
needle piercing the rubber septum. In the case of the halides, if the halide
and catalyst were thoroughly mixed before hydrogen addition, the lemon-
over molecular sieves. N(nBu)
and used as received while N(nBu)
4
I was purchased from Fluka as white crystals
Cl (Fluka) was dried 12 h in vacuo.
yellow five-coordinate complex [Rh(NBD)(P
formed. Since this complex reacts very slowly with dihydrogen, the
following precautions were taken to avoid its formation:
�
P)X] (X ˆ Cl, I) was
4
AOT (Aldrich, 99% stated purity) was purified by stirring 12 h with
activated charcoal in dry methanol, filtered repeatedly over celite to
eliminate charcoal and insoluble solids, and dried in vacuo 48 h according
[Rh(NBD)(P
�
P)]ClO4 and the halide salt were placed in the bottom of
to a literature procedure.[53] [D
the NMR tube, and the [D ]benzene were added carefully without
6
]benzene was purchased from Cambridge
6
Isotope Laboratories and used as received. Methyl-(Z)-a-acetamidocinna-
dissolution of the solids. The sealed tube was removed from the glovebox
mate was a generous gift from Dr. Claire Newton. [{Rh(NBD)}
2
(m-Cl)
2
]
and purged with hydrogen through the rubber septum. Only at this point
the tube was shaken to induce hydrogenation of [Rh(NBD)(P�P)]ClO4
was prepared according to published procedures.[
54]
[Rh{(� )-
bdpp}(NBD)]ClO
nap}(NBD)]ClO
acac ˆ acetylacetonate), 70% HClO
phosphane purchased from Strem Chemicals, as previously described.
4
,
[Rh{(‡)-prophos}(NBD)]ClO
were prepared halide-free from [Rh(acac)(NBD)]
, and the enantiomerically pure
4
and [Rh{(‡)-bi-
and then complexation of the resulting complex with halide.
[Rh(NBD)(P
formation of [Rh(NBD)(P
studies.
�
4
P)]ClO reacts with dihydrogen almost immediately before
4
�
P)]X can take place, as demonstrated by NMR
(
4
[
12]
High-pressure hydrogenation procedure: For a series of experiments in the
same solvent and same surfactant concentration, a standard solution was
prepared and kept in the glovebox. A typical hydrogenation experiment
was carried out as follows. The entire autoclave was taken into the
Parahydrogen-induced polarization NMR procedure: Parahydrogen was
[
58]
prepared according to the method of Bargon and co-workers. [Rh{(� )-
�
6
� 5
bdpp}(NBD)]ClO
4
(6.7 mg, 9 Â 10 n) and N(nBu)
4
I (6.6 mg, 1.8 Â 10 n,
2 equivalents) were weighed and placed into an NMR tube and brought
�
5
nitrogen-filled glovebox and filled with 1 (37 mg, 5 Â 10 mol), the olefinic
6
into the dry box. [D ]benzene (0.6 mL) was then added to make the final
�
3
substrate (5 Â 10 mol), the surfactant/solvent mixture (10 mL) and any
co-additives (salts, [15]crown-5, etc.). The autoclave was sealed and
removed from the glovebox and attached to a high-pressure stainless steel
hydrogen line. The line was purged of air and the autoclave pressurized to
[Rh] 15 mm, taking care not to disturb the solid on the bottom. The tube
was then sealed and removed from the dry box. It was immersed in liquid
nitrogen and purged with parahydrogen for 1 min. The tube was stored in
liquid nitrogen, then thawed, shaken, and inserted into the NMR
spectrometer. Two scans were immediately recorded; the process from
70 atm hydrogen pressure. The pressure was released and then refilled with
1
hydrogen, repeating this procedure three times in total. The autoclave was
refilled again to the desired pressure and the timing started with stirring of
the solution. Periodic samples of the reaction solution were taken utilizing
the dip/sample tube and analyzed. Hydrogenation of 4 and 5 could be
thawing to accumulation taking no more than 30 s. The H NMR spectrum
failed to show any polarization due to metal hydride species. The tube was
�
4
then brought back into the dry box and [D
8
]styrene (0.05 mL, 4 Â 10 n,
44 equivalents) added and the tube again sealed, removed from the
glovebox, frozen in liquid nitrogen, and purged with parahydrogen. After
the sample had been thawed, shaken, and immediately inserted into the
NMR, two scans revealed a large enhancement of the alkyl signals at d ˆ
2.45 and 1.05 of greater than 100 times. The two signals correspond to the H
of the methylene and the methyl, respectively, of the ethylbenzene
hydrogenation product. Repeated removal of the tube, shaking, and
reinsertion in the instrument (8 times) renewed the observed polarization.
1
monitored by H NMR spectroscopy, or more conveniently, by GC; yields
by these two methods correlate within 2%. Isomerized 4, (E)-2-phenyl-2-
1
butene, was identified by its H NMR spectrum and was well separated
from 4 and the hydrogenation product 2-phenylbutane by GC. Samples
1
taken from hydrogenation reactions of 6 and 7 were analyzed by H NMR
spectroscopy. Following completion of the reaction, the pressure was
released and the products separated from the catalyst and other additives
by passing through a short (3 cm) plug of silica gel with a small excess of
pentane. For hydrogenation of 4 in MeOH, the 2-phenylbutane product
1
A H NMR spectrum taken 5 min after shaking with parahydrogen showed
loss of all polarization.
148
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Chem. Eur. J. 2000, 6, No. 1

Enantioselective Hydrogenation of Simple Olefins
139±150
iodide-bridged dimers which catalyze imine hydrogenation,[30, 31]
proving that the monomeric complexes are active. In this case, we
do not observe a linear relationship between rate and [Rh ] ,
2 o
probably due to other competing equilibria as shown in Scheme 3
which render this system more complicated.
Acknowledgements
1/2
J. M. B. is grateful for an NSF Career Award (1999 ± 2003) and a Camille
and Henry Dreyfus New Faculty Award (1997 ± 2002). Purdue University,
the Government of France (Minist eÁ re des Affaires Etrang eÁ res), and
DuPont are thanked for their generous support.
[
19] C. R. Landis, J. Halpern, J. Am. Chem. Soc. 1987, 109, 1746. Other
species observed under these conditions include [Rh{(� )-
‡
bdpp}(C
6
D
6
)] (16% of total rhodium ± phosphane complexes as
31
determined by P NMR integration) and [Rh{(� )-bdpp}{bis(2-ethyl-
hexyl)sulfosuccinate}] (13% of total rhodium ± phosphane com-
plexes). Only the major diastereomeric complex of [Rh{(� )-
[
[
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(
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1
[
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of N(nBu)
4
I in C
6
D
6
(5mm) are as follows: dppe complex, d ˆ 76.23
1
1
5
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[
[
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[
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a catalytically active monomer should be directly proportional to
[
1/2
[
2 o
Rh ] . This relationship has been observed in the case of iridium
Chem. Eur. J. 2000, 6, No. 1
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149

J. M. Buriak, J. A. Osborn et al.
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[
Received: March 10, 1999 [F F1666]
150
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