Highly Regio- and Enantioselective Hydroformylation of Vinyl Esters Using Bidentate Phosphine,P-Chiral Phosphorodiamidite Ligands

Article abstract of DOI:10.1021/acscatal.5b02846

Hybrid bidentate phosphine-phosphorodiamidite ligands based on a chiral Betti base backbone and diphenylphosphinoaniline derivatives have been prepared (BettiPhos). The ligands possess a stereogenic P atom at the phosphorodiamidite moiety, whose configuration can be largely controlled by the synthetic route and the choice of base and solvent. The new ligands were applied in the rhodium-catalyzed asymmetric hydroformylation (AHF) of vinyl esters and vinyl amides. Very high enantioselectivities of up to 97% ee accompanied by excellent regioselectivities (up to b/l > 1000) were obtained using the BettiPhos ligand (S_C,S_C,R_P,S_C)-4b bearing an additional chiral group at the aniline nitrogen. The catalyst resting state [RhH(CO)₂{(S_C,S_C,R_P,S_C)-4b}] was investigated by high pressure-NMR studies, revealing an equatorial-apical coordination of the bidentate ligand where the two phosphorus donors rapidly exchange their positions through an intermediate with the ligand bound via the phosphine group only.

Full text of DOI:10.1021/acscatal.5b02846

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Research Article
pubs.acs.org/acscatalysis
Highly Regio- and Enantioselective Hydroformylation of Vinyl Esters
Using Bidentate Phosphine,P-Chiral Phosphorodiamidite Ligands
Christian Schmitz, Katharina Holthusen, Walter Leitner,* and Giancarlo Francio*
̀
̈
Institut fur Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany
*
S Supporting Information
ABSTRACT: Hybrid bidentate phosphine-phosphorodiamidite ligands based on
a chiral Betti base backbone and diphenylphosphinoaniline derivatives have been
prepared (BettiPhos). The ligands possess a stereogenic P atom at the
phosphorodiamidite moiety, whose configuration can be largely controlled by
the synthetic route and the choice of base and solvent. The new ligands were
applied in the rhodium-catalyzed asymmetric hydroformylation (AHF) of vinyl
esters and vinyl amides. Very high enantioselectivities of up to 97% ee
accompanied by excellent regioselectivities (up to b/l > 1000) were obtained
using the BettiPhos ligand (S ,S ,R ,S )-4b bearing an additional chiral group at
C
C
P C
the aniline nitrogen. The catalyst resting state [RhH(CO) {(S ,S ,R ,S )-4b}]
2
C
C
P C
was investigated by high pressure-NMR studies, revealing an equatorial−apical coordination of the bidentate ligand where the
two phosphorus donors rapidly exchange their positions through an intermediate with the ligand bound via the phosphine group
only.
KEYWORDS: asymmetric catalysis, hybrid phosphorus ligands, hydroformylation, rhodium, P-stereogenic ligands
INTRODUCTION
ligands with two different phosphorus donors have been
1,3f,12
■
intensively investigated for AHF.
Very high enantio-
Hydroformylation allows atom-efficient and direct formation of
aldehydes from olefins and synthesis gas and has become a
powerful synthetic tool for the preparation of these key organic
selectivities were achieved in the AHF of various substrate using
1
1d,e
a Binaphos derivative (up to 98% ee)
or the related
13
1
phosphine-phosphoramidite ligand Yanphos (up to 98% ee).
intermediates. Nowadays, hydroformylation is the largest
Both ligands, however, do not excel in regioselectivity (89−
application of homogeneous catalysis on an industrial scale
with a capacity of more than 10 million tons of oxo products
per year and is almost exclusively targeted to the production of
9
5%). In contrast, the phospholane-phosphite Bobphos led to
higher regioselectivities of 98−99% but lower enantioselectiv-
ities of 83−92% ee.
14
2
the linear isomer. In contrast, the regio- and stereoselective
1
Herein we report the synthesis of a new family of hybrid
phosphine-phosphorodiamidite bidentate ligands bearing a
stereogenic P atom (BettiPhos) and their application in the
asymmetric hydroformylation of vinyl ester derivatives with
exceptionally high regioselectivities (up to >99.9% correspond-
ing to a b/l ratio >1000) accompanied by very high
enantioselectivities (ee up to 97%). NMR investigations
synthesis of chiral branched aldehydes is far less established,
although these compounds are of special interest for the
preparation of a variety of biologically active compounds and
fine chemicals, such as chiral alcohols, acids, amines, diols, and
3
,4
amino alcohols.
Among the reported ligands for asymmetric hydroformyla-
tion (AHF), only very few are capable of giving high chemo-,
regio-, and enantioselectivity at the same time. For example,
showed that in the resting state [HRh(CO) (P-P′)] the
2
phosphine and the phosphorodiamidite moieties occupy the
equatorial and apical positions (major isomer 60% at −70 °C)
or vice versa (minor isomer 40% at −70 °C). The two
structures interconvert rapidly through an intermediate where
the ligand is bound via the phosphine group only.
5
diphosphite ligands such as (S,S)-Chiraphite and (S,S)-
6
Kelliphite and diphosphine ligands such as (S,S,S)-Bisdiaza-
7
,4c,d
8
phos
and (R,R)-Ph-BPE provided good to high
enantioselectivities of 82−96% ee for styrene or vinyl acetate
but only in a few cases have high regioselectivities (92−99%)
9
been obtained as well. Recently, the AHF of Z-enamides and
RESULTS AND DISCUSSION
■
various enol esters with a very high level of stereocontrol using
S,S,S)-Bisdiazaphos was demonstrated.
4
c,d
Synthesis of Phosphine-Phosphorodiamidites. The
ligand synthesis was accomplished from (R,R)-, (R,S)-, or
(
Very recently,
Vidal-Ferran reported that very high regio- and enantioselec-
tivity can be achieved using bidentate phosphite ligands with a
polyether backbone tunable through the addition of “regulating
15,16
(
S,S)-Betti base 1
and 2-(diphenylphosphino)-N-phenyl-
1
0
agents”.
Since the seminal report of Takaya and Nozaki in 1993 on
Received: December 14, 2015
Revised: January 19, 2016
1
1
the phosphine-phosphite ligand Binaphos, hybrid bidentate
©
XXXX American Chemical Society
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Research Article
1
7
aniline 5a or (S)-2-(diphenylphosphino)-N-(1-phenylethyl)-
unambiguously determined via X-ray crystal structure anal-
1
7
16
aniline 5b, using a previously explored synthetic method-
ysis.
1
6
In the 31P NMR, ligands (R
ology. The methods depicted in Scheme 1 via different
routes, solvents, and bases provide good control of the
configuration of the newly generated P-stereogenic center at
the phosphorodiamidite moiety.
,R ,S )-4a and (R ,S ,R )-4a
C C P C C P
gave two sharp doublets for the phosphine and phosphor-
amidite moieties with through-space coupling constants JP,P′ of
4
8.5 and 42.6 Hz, respectively. In contrast, broad signals were
obtained for the phosphorodiamidite groups in (R ,R ,R )-4a,
C
C
P
(
S ,S ,R ,S )-4b, and (R ,S ,R ,S )-4b suggesting the presence
Scheme 1. Ligand Synthesis
C C P C C C P C
3
1
of rotamers. Indeed, variable-temperature (VT) P NMR
showed for (S ,S ,R ,S )-4b (see Figure S1 in the Supporting
C
C
P C
Information) two distinct sets of doublets of doublets at −40
°
3
C with a ratio of 94:6 (major, δ −21.1 ppm, 140.6 ppm, J
=
P,P′
P
0.3 Hz; minor, δ −19.4 ppm, 130.7 ppm, J = 33.1 Hz).
P
P,P′
The conformers may originate from hindered rotation at the
C−N bonds of the 1-phenylethyl groups both characterized by
1
broad signals in the H NMR even at −40 °C (see Figure S2 in
the Supporting Information).
Rh-Catalyzed Asymmetric Hydroformylation. The
BettiPhos ligands were applied in the asymmetric hydro-
formylation of vinyl acetate. The catalytic systems were
prepared in situ by addition of the bidentate ligand to a
solution of [Rh(CO) (acac)] in a Rh:L ratio of 1:4, and the
2
reaction was carried out under standard conditions (p(CO/H2)
3
0 bar (ratio 1:1), T = 60 °C, t = 14 h). The results are
summarized in Table 1.
Table 1. Rh-Catalyzed Asymmetric Hydroformylation of
a
Vinyl Acetate Using Phosphine-Phosphorodiamidites 4
b
c
c
entry
ligand
conversn (%)
iso (%)
ee (%)
Method A involves first the treatment of Betti base (R,R)-1
or (R,S)-1 with n-butyllithium in THF followed by the addition
1
2
3
4
5
(R
(R
(R
C
C
C
,R
,R
C
C
,R
,S
P
)-4a
13
28
30
27
57
98
99
32.6 (S)
29.6 (S)
34.5 (R)
83.4 (R)
93.8 (R)
)-4a
)-4a
P
of PCl and allows the stereoselective formation of phosphor-
,S
C
,R
P
99
3
(R ,S ,R ,S )-4b
C
>99.9
>99.9
amidochloridites (R ,R ,S )-2 and (R ,S ,R )-2, respectively.
C
P C
C
C
P
C
C
P
(S ,S ,R ,S )-4b
C
The epimerically pure phosphoramidochloridites were then
reacted with the lithiated 2-(diphenylphosphino)-N-phenylani-
line 3a or (S)-2-(diphenylphosphino)-N-(1-phenylethyl)aniline
C
P C
a
Conditions: substrate, 2 mmol; sub/Rh/L, 1000/1/4; toluene, 1 mL;
p(CO/H
NMR. Determined by chiral GC.
b
1
) = 30 bar (1:1); T = 60 °C, t = 14 h. Determined by H
2
c
3
b to give the desired phosphine-phosphorodiamidite ligands
(
R ,R ,S )-4a and (R ,S ,R ,S )-4b in 31% and 24% yields,
C
C
P
C
C
P C
respectively, after recrystallization from toluene/ethanol
All ligands afforded moderately active hydroformylation
catalysts exhibiting excellent regioselectivities. The relative
configuration of the stereogenic phosphorus has little impact on
the catalyst performance, and low enantioselectivities in favor of
the S-aldehyde were obtained with both (R ,R ,R )-4a and
(Scheme 1).
Method B follows a similar reaction sequence using,
however, triethylamine as base and CH Cl as solvent. This
method was applied for the preparation of ligand (R ,R ,S )-4a
2
2
C
C
P
C
C
P
from (R,S)-1 and 2-(diphenylphosphino)-N-phenylaniline (5a).
After recrystallization from toluene/ethanol, (R ,R ,S )-4a was
obtained in 23% yield (Scheme 1).
(R ,R ,S )-4a (Table 1, entries 1 and 2). In contrast, the
change of the stereochemistry of the Betti-base backbone in
C C P
C
C P
(R ,S ,R )-4a had a very significant effect and led to the
C
C
P
Method C, which comprises the in situ formation of
dichloraminophosphines 6a,b, was used for the synthesis of
preferential formation of the opposite enantiomer with a similar
ee value (entry 3 vs entry 1). The enantioselectivity could be
substantially increased using the ligands 4b bearing an
additional chiral moiety at the aniline nitrogen. In the presence
of (R ,S ,R ,S )-4b, 83.4% (R) ee and very high regioselectivity
(
R ,R ,R )-4a and (S ,S ,R ,S )-4b, respectively. After purifi-
C C P C C P C
cation by recrystallization from toluene/ethanol, the bidentate
ligands were obtained in epimerically pure form and 25% and
C
C
P C
6
1% yields, respectively (Scheme 1).
The absolute configuration of the phosphorodiamidite P
were achieved, albeit at low conversion (entry 4). Finally, the
diastereomer (S ,S ,R ,S )-4b based on Betti base (S,S)-1
C
C
P C
atom of all BettiPhos ligands was assigned by comparison of the
provided higher conversion (57%), a regioselectivity exceeding
99%, and 93.8% ee (entry 5).
1
13
31
very characteristic chemical shifts in the H, C, and P NMR
1
H,
31
P
13 31
spectra and coupling constants J
and J C, P to the related
Next, the reaction conditions for the AHF of vinyl acetate
using BettiPhos (S ,S ,R ,S )-4b were optimized (see Table S1
monodentate phosphorodiamidite ligands based on the same
C
C
P C
Betti base backbone, where the P configuration was
in the Supporting Information for full screening). For better
1
585
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a
Table 2. AHF of Vinyl Acetate Using (S ,S ,R ,S )-4b
C
C
P
C
b
c
c
entry
Sub/Rh
T (°C)
CO/H2
L/Rh
conversn (%)
b/l
ee (%)
1
2
3
4
5
6
7
8
1000
500
60
60
80
45
60
60
60
40
25
40
1
4
4
57
> 99
> 99
54
>1000
>1000
>1000
497
93.8
94.0
66.6
95.1
94.2
64.4
94.2
95.7
96.7
1
1000
500
1
4
1
4
1000
1000
1000
300
0.2
1
4
86
>1000
76
2
67
1
10
4
29
>1000
859
0.2
0.2
0.2
91
9
100
4
49
257
d
1
0
4000
4
22
495
94.7
a
b
1
c
Conditions unless specified otherwise: substrate, 2 mmol; toluene, 1 mL; p(CO/H ) = 30 bar; t = 14 h. Determined by H NMR. Determined by
2
d
chiral GC (R enantiomer always preferentially formed). Reaction performed in 0.75 mL of substrate; p = 40 bar.
a
comparison the regioselectivities will be expressed as b/l ratios.
The most important results are summarized in Table 2.
Increasing reaction temperature led to enhanced reaction
rate and lower ee (Table 2, entry 3), whereas the
regioselectivity remained excellent. An enantiomeric excess of
Table 3. AHF of Vinylic Substrates using (S ,S ,R ,S )-4b
C C P C
95.1% was achieved when the reaction was performed at 45 °C
with b/l = 497 (Table 2, entry 4). No significant effect of the
variation of the overall pressure was observed (see Table S1 in
the Supporting Information), whereas an increased partial
pressure of hydrogen (CO/H = 1/5) accelerated the reaction
2
according to the hydroformylation kinetic law (Table 2, entry 5
2
a
vs 1). The variation of the ligand/Rh ratio strongly affected
the outcome of the reaction. When only 2 equiv of ligand was
used, ee and b/l values dropped dramatically at a slightly
increased reaction rate (Table 2, entry 6). The use of 10 equiv
of ligand provided excellent regio- and enantioselectivity, albeit
at the expense of activity (Table 2, entry 7). When the AHF
was carried out at lower temperature, substrate/Rh ratio, and
CO partial pressure, the enantioselectivity could be further
increased. For instance, 95.7% ee was obtained at 40 °C with
91% conversion and b/l = 859 (Table 2, entry 8). This result
represents the best compromise in terms of catalyst activity and
regio- and enantioselectivity. Furthermore, the catalyst was also
active at 25 °C, providing 96.7% ee, albeit at slightly reduced
regioselectivity (b/l = 257) (Table 2, entry 9). When the
reaction was performed in neat substrate at 40 °C, 22%
conversion was achieved within 14 h with 94.7% ee and b/l =
−1
4
95 (Table 2, entry 10). This corresponds to TOF = 63 h .
av
Attempts to preform the catalyst and then conduct AHF led to
a racemic product mixture, because irreversible catalyst
decomposition occurs upon releasing syngas before introducing
1
8
the substrate.
A series of vinyl esters and vinyl amides were hydro-
formylated using BettiPhos (S ,S ,R ,S )-4b under the
C
C
P
C
optimized conditions identified above. The results are
summarized in Table 3. In the AHF of vinyl propionate ee
values of up to 95.5% were obtained, although a somewhat
reduced branched/linear ratio of 121 was found in comparison
to the vinyl acetate (Table 3, entries 1 and 2). The use of vinyl
pivalate as substrate resulted again in excellent regioselectivities
with b/l values up to 451 and 94.6% ee. Interestingly, the
catalyst activity was not negatively affected by the increased
steric bulk of the tert-butyl group in the substrate (Table 3,
entries 3 and 4). A comparable enantiomeric excess of 94.7%
was achieved using vinyl benzoate as substrate, albeit at reduced
yet high regioselectivity (b/l = 100) (Table 3, entries 5 and 6).
For vinyl 4-methoxybenzoate again excellent regioselectivity
a
Conditions: substrate, 2 mmol; Rh/L, 1/4; toluene, 1 mL; p(CO/
2
b
1
c
H ) = 30 bar (1/5); t = 14 h. Determined by H NMR. Determined
by chiral GC.
(b/l > 1000) was observed, leading practically to a single
regioisomer, accompanied by a slightly reduced enantioselec-
tivity of 91.3% ee (Table 3, entries 7 and 8). Comparable
results were obtained for vinyl 4-fluorobenzoate with 73%
conversion, b/l > 1000, and 92.1% ee (Table 3, entry 10). The
AHF of vinyl 2-naphthoate resulted in up to 88% ee and b/l =
1
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1 at 80% conversion (Table 3, entries 11 and 12). The long-
Research Article
8
BettiPhos ligand) which is not resolved on the NMR time
1
1k
chain vinyl laurate underwent hydroformylation with up to
5.1% ee and excellent regioselectivity (b/l = 477) (Table 3,
scale at room temperature, as already observed for Binaphos.
9
This hypothesis was verified by conducting VT HP NMR
entries 13 and 14). When vinyl methacrylate was used as
substrate, the exclusive hydroformylation of the vinyl group was
experiments. Indeed, when the temperature was decreased, the
hydride signal in the H NMR broadened and then sharpened
again to give two resolved but still slightly broad doublets at
1
observed, whereas the conjugated double bond was untouched
1
(
>99% chemoselectivity in H NMR). Enantioselectivities of up
−70 °C (δ −9.0 and −9.1 ppm) exhibiting coupling constants
H
2
2
to 95.8% ee and a b/l ratio of 381 were obtained in this
transformation (Table 3, entries 15 and 16). In contrast to the
smooth hydroformylation of vinyl esters under mild conditions,
the AHF of allyl acetate required elevated temperatures and
afforded the desired aldehyde in only a poor enantiomeric
excess of 5% (Table 3, entries 17 and 18).
of JH,P = 121 Hz (phosphine group) and JH,P = 194 Hz
phosphorodiamidite group), respectively (see Figures S5 and
S6 in the Supporting Information). Additionally, a new broad
(
2
doublet signal ( J = 118 Hz, phosphine group) appeared at
H,P
21
−
7.9 ppm at T ≤ −50 °C. The large coupling constants
observed are characteristic for H,P-trans coupling, whereas the
small H,P-cis and the H−Rh couplings were not resolved. The
ratio of the hydride signals accounts for 37% (−7.9 ppm), 26%
Finally, vinylamides were hydroformylated with BettiPhos
(
S ,S ,R ,S )-4b. Vinylacetamide and N-methyl vinylacetamide
C C P C
31
(
−9.0 ppm), and 37% (−9.1 ppm). The P NMR spectra
underwent hydroformylation with 82% and 83% ee, respec-
tively (Table 3, entries 19−22). The conversions obtained were
somewhat lower in comparison to the previously investigated
vinyl ester substrates, and also the regioselectivities were
significantly lower (b/l = 25 and 6, respectively), but still
varied accordingly upon a decrease in temperature. Three
distinct broad peaks for the phosphine at δ 14.7, 21.7, and 29.2
P
ppm as well as for the phosphoramidite group at δ 170.9,
P
1
61.0, and 129.0 ppm were visible at −70 °C (see Figures S7−
19
S9 in the Supporting Information). When the sample was
heated again, these signals disappeared and signals were
observed at room temperature that were identical with those
found before the sample was cooled.
represent one of the best reported examples so far.
Investigations on Catalyst Resting State [RhH(CO) (P-
2
P′)]. The coordination mode of the ligand in the catalytic
resting state is believed to play an important role for
3d,f
These observations strongly support the presence in solution
of the two stereoisomeric Rh species 7-ea and 7-ae (Scheme 2),
stereocontrol in the AHF.
For instance, the high level of
enantioselectivity obtained with Binaphos was correlated with
the equatorial−apical coordination mode adopted by the ligand
11b,g
Scheme 2. Equilibrium of Diastereomeric
in the resting state [HRh(CO) (P-P′)].
A recent NMR
2
[
HRh(CO) {(S ,S ,R ,S )-4b}] with Equatorial−Apical
2
C
C
P
C
study showed, however, that the major species with the
phosphine at the equatorial position and the phosphite group in
the apical position is in rapid equilibrium with the isomer with
reverse coordination with a ratio of up to 3:1 in CD Cl at
2
1
κ (P,P′) Coordination in 7 and κ (P,P′) in 8
2
2
1
1k
−
90 °C.
The coordination mode of the BettiPhos ligand in [HRh-
(
CO) (P-P′)] was investigated via high pressure (HP) NMR.
2
In a HP NMR tube (sapphire tube with a non-magnetic
titanium head), the complex [Rh{(S ,S ,R ,S )-4b}(acac)] was
C
C
P C
generated in situ from [Rh(CO) (acac)] and the ligand. In the
2
3
1
P NMR, this precursor displays two sets of doublets of
both exhibiting an equatorial−apical coordination of the
bidentate ligand where the phosphine group of (S ,S ,R ,S )-
doublets (δ = 34.4 ppm, dd, JP,Rh = 187.9 Hz, JP,P′ = 86.6 Hz;
δP′ = 153.9 ppm, dd, JP′,Rh = 264.7 Hz, JP,P′ = 86.6 Hz). The HP
P
C
C
P C
4
b occupies the axial position and the phosphorodiamidite the
NMR tube was then pressurized with CO/H (30 bar), leading
2
equatorial position (7-ea, δphosphine(−70 °C, CD Cl ) 21.7 ppm,
2
2
to the quantitative formation of the catalyst resting state
δphosphorodiamidite 161.0 ppm) and vice versa (7-ae, δphosphine 14.7
ppm, δphosphorodiamidite 170.9 ppm) with an approximate ratio of
40:60. The additional NMR signals appearing at lower
temperatures should arise from the square-planar species 8,
[
HRh(CO) {(S ,S ,R ,S )-4b}] with the following resonances
2 C C P C
31
in P NMR: δ 19.3 ppm, dd, J
= 112.3 Hz, JP,P = 59.4 Hz;
P,Rh
P
δP 167.1 ppm, dd, J
P,Rh
= 174.6 Hz, J = 59.4 Hz. A single
P,P
1
hydride species was visible in the H NMR at −9.2 ppm (ddd,
where only the phosphine group at δ 29.2 ppm is coordinated
P
1
2
2
J
= 12.0 Hz, J = 66.7 Hz, J = 87.0 Hz; see Figure S3
H,Rh
H,P H,P
to the Rh trans to the hydride and with a pendant
phosphorodiamidite group characterized by the chemical shift
δP 129.0 ppm upfield with respect to that observed in the free
ligand (δP 140.5 ppm). The species 8 may serve as an
intermediate in the isomerization of 7-ea and 7-ae.
in the Supporting Information). Selective decoupling of each
inequivalent P nucleus allowed us to unequivocally associate
2
the J
coupling constants of 87.0 and 66.7 Hz to the
H,P
phosphorodiamidite and diphenylphosphine groups, respec-
tively (see Figure S4 in the Supporting Information). The
At this stage it is not possible to distinguish which isomer(s)
is/are responsible for the highly enantioselective turnovers in
the AHF. However, following the analogy with Binaphos, the
stereoisomer 7-ae with the phosphorodiamidite trans to the
hydride would be the most plausible candidate.
1
chemical shifts of the phosphorus signals as well as the J
H,Rh
11
and JP,Rh coupling constants are in the expected range for a
trigonal-bipyramidal structure with equatorial−apical coordina-
2
tion of the ligand. The similar J
values, however, are
H,P
indicative of a weighted average of cis and a trans coupling and
20
CONCLUSIONS
do not support the presence of a single isomer. This
observation rather suggests the presence of an equilibrium of
two stereoisomeric Rh complexes [HRh(CO) {(S ,S ,R ,S )-
■
New hybrid phosphine-phosphorodiamidite bidentate Betti-
Phos ligands based on a chiral Betti-base backbone and
diphenylphosphinoaniline derivatives have been prepared. The
2
C
C
P C
4
b}] (each with equatorial−apical coordination of the
1
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ligands possess a stereogenic P atom in the phosphorodiamidite
moiety, whose configuration can be controlled to a large extent
by the synthetic route. The BettiPhos ligands were applied in
the rhodium-catalyzed asymmetric hydroformylation, combin-
ing very high enantioselectivities (up to 97%) with exceptional
regioselectivities (up to b/l > 1000) for a variety of vinyl esters.
Thus, the highly modular BettiPhos ligands define a new lead
structure for the AHF of vinyl esters, providing synthetically
useful selectivities.
216. (e) Allmendinger, S.; Kinuta, H.; Breit, B. Adv. Synth. Catal. 2015,
3
(
2
1
57, 41−45.
5) (a) Dieg
004, 15, 2113−2122. (b) Babin, J. E.; Whiteker, G. T. WO 9303839,
993. (c) Buisman, G. J. H.; Vos, E. J.; Kamer, P. C. J.; van Leeuwen,
́ ̀
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(
In the resting state [HRh(CO) {(S ,S ,R ,S )-4b}], the
2
C
C
P C
ligand exhibits an equatorial−apical coordination with the two
P moieties rapidly exchanging their positions through a square-
planar intermediate where the ligand is bonded to the Rh via
the phosphine donor only. These findings corroborate the
assumption that an equatorial−apical coordination of the
phosphorus ligand is a structural requirement for achieving
highly enantioselective AHF, whereas a rigid environment is
not and a fluxional behavior is tolerated.
6) (a) Cobley, C. J.; Klosin, J.; Qin, C.; Whiteker, G. T. Org. Lett.
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9) van Leeuwen, P. W. N. M. In Science of Synthesis: Stereoselective
5
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02846.
■
2
(
*
S
Synthesis 1; de Vries, J. G., Ed.; Thieme: Stuttgart, Germany, 2010; pp
409−476.
(
́
10) (a) Vidal-Ferran, A.; Mon, I.; Bauza, A.; Frontera, A.; Rovira, L.
Experimental details, NMR spectra, and GC traces
Chem. - Eur. J. 2015, 21, 11417−11426. (b) Rovira, L.; Vaquero, M.;
(
PDF)
Vidal-Ferran, A. J. Org. Chem. 2015, 80, 10397−10403.
(11) (a) Sakai, N.; Mano, S.; Nozaki, K.; Takaya, H. J. Am. Chem. Soc.
AUTHOR INFORMATION
Corresponding Authors
E-mail for W.L.: leitner@itmc.rwth-aachen.de.
E-mail for G.F.: francio@itmc.rwth-aachen.de.
■
1993, 115, 7033−7034. (b) Nozaki, K.; Sakai, N.; Nanno, T.;
Higashijima, T.; Mano, S.; Horiuchi, T.; Takaya, H. J. Am. Chem. Soc.
1997, 119, 4413−4423. (c) Nozaki, K.; Takaya, H.; Hiyama, T. Top.
Catal. 1997, 4, 175−185. (d) Nozaki, K.; Matsuo, T.; Shibahara, F.;
Hiyama, T. Organometallics 2003, 22, 594−600. (e) Nozaki, K.;
Matsuo, T.; Shibahara, F.; Hiyama, T. Adv. Synth. Catal. 2001, 343,
*
*
Notes
The authors declare no competing financial interest.
6
1
1−63. (f) Francio,
̀
G.; Leitner, W. Chem. Commun. 1999, 1663−
664. (g) Francio, G.; Wittmann, K.; Leitner, W. J. Organomet. Chem.
̀
ACKNOWLEDGMENTS
■
2001, 621, 130−142. (h) Horiuchi, T.; Ohta, T.; Shirakawa, E.;
Nozaki, K.; Takaya, H. J. Org. Chem. 1997, 62, 4285−4292.
(i) Horiuchi, T.; Shirakawa, E.; Nozaki, K.; Takaya, H. Organometallics
1997, 16, 2981−2986. (j) Shibahara, F.; Nozaki, K.; Hiyama, T. J. Am.
Chem. Soc. 2003, 125, 8555−8560. (k) Castillo Molina, D. A.; Casey,
Funding was provided by the European Union with the FP-7
integrated project SYNFLOW, http://synflow.eu. We thank
Ines Bachmann-Remy for NMR measurements, Julia Wurlitzer
and Hannelore Eschmann for GC measurements, and Sven
Wiezorkowski for experimental assistance.
C. P.; Mu
367.
12) For selected examples not included in ref 3f see: (a) Francio,
Faraone, F.; Leitner, W. Angew. Chem., Int. Ed. 2000, 39, 1428−1430.
b) Hammerer, T.; Leitner, W.; Francio, G. ChemCatChem 2015, 7,
583−1592. (c) Fernandez-Perez, H.; Benet-Buchholz, J.; Vidal-
̈
ller, I.; Nozaki, K.; Jak
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el, C. Organometallics 2010, 29, 3362−
3
(
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G.;
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repressurization with CO/H a defined catalyst species is no longer
2
formed. Unmodified Rh species might then be responsible for the
catalytic activity without any asymmetric induction.
(
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588
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ACS Catalysis
Research Article
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(
20) In the case of an equatorial−axial coordination of the ligand a
2
small J value in the range of 3−12 Hz would be expected for the cis
H,P
2
coupling, whereas a J value in the range of 120−230 Hz would be
H,P
expected for the trans coupling, depending on the electronic
2
environment of the respective P atom. Here, the J values observed
H,P
are between what is expected.
(
21) Hydride signals in this region are commonly observed for Rh
hydrido carbonyl complexes with ligands exhibiting a bis-equatorial
coordination, as well as for complexes with monodentate ligands: see
ref 5.
1
589
DOI: 10.1021/acscatal.5b02846
ACS Catal. 2016, 6, 1584−1589