DiPAMP's big brother "i-Pr-SMS-Phos" exhibits exceptional features enhancing rhodium(I)-catalyzed hydrogenation of olefins

Article abstract of DOI:10.1002/adsc.200900621

Switching Knowles DiPAMP's {DiPAMP = l,2-bis[(o-anisyl)(phenyl)phosphino] ethane} MeO groups with i-PrO ones led to the iPr-SMS-Phos {i-Pr-SMS-Phos = l,2-bis[(o-isopropoxyphenyl)(phenyl)phosphino]ethane} ligand which displayed a boosted catalyst activity coupled with an enhanced enantioselectivity in the rhodium(I)catalyzed hydrogenation of a wide-range of representative olefinic substrates (dehydro-a-amido acids, itaconates, acrylates, enamides, enol acetates, α,α-diarylethylenes, etc). The rhodium(I)-(i-PrSMS-Phos) catalytic profile was investigated revealing its structural attributes and robustness, and in contrast to the usual trend, ³¹P NMR analysis revealed that its methyl (Z)-α-acetamidocinnamate (MAC) adduct consisted of a reversed diastereomeric ratio of 1.4:1 in favour of the most reactive diastereomer.

Full text of DOI:10.1002/adsc.200900621

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DOI: 10.1002/adsc.200900621
DiPAMPꢀs Big Brother “i-Pr-SMS-Phos” Exhibits Exceptional
Features Enhancing Rhodium(I)-Catalyzed Hydrogenation of
Olefins
a,b,
a
a
ˇ
Michel Stephan, * Damjan Sterk, and Barbara Mohar
a
Laboratory of Organic and Medicinal Chemistry, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana,
Slovenia
Fax : (+386)-1-476-0300; e-mail: michel.stephan@ki.si
PhosPhoenix SARL, 115, rue de l’Abbꢀ Groult, 75015 Paris, France
Fax : (+33)-1-4828-3011; e-mail: mstephan@phosphoenix.com
b
Received: September 7, 2009; Revised: October 1, 2009; Published online: November 18, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900621.
Abstract:
{DiPAMP=1,2-bis
ethane} MeO groups with i-PrO ones led to the i-
Pr-SMS-Phos {i-Pr-SMS-Phos=1,2-bis[(o-isoprop-
oxyphenyl)ACHTUNGTRENNUNG(phenyl)phosphino]ethane} ligand which
Switching
Knowles
(phenyl)phosphino]-
DiPAMPꢁs
ing due to the structurally diverse plethora of desira-
ble prochiral substrates to undergo transformation.
In our endeavour to prepare a portfolio of new effi-
cient P-stereogenic ligands for industrial implementa-
tion,[3] we had a strong belief that a golden opportuni-
ty still remains in investigating further the pioneering
ligand design optimization undertaken by Knowles
et al. In particular, during their ligand optimization,
Knowles and co-workers prepared and tested a set of
A
displayed a boosted catalyst activity coupled with
an enhanced enantioselectivity in the rhodium(I)-
catalyzed hydrogenation of a wide-range of repre-
sentative olefinic substrates (dehydro-a-amido
acids, itaconates, acrylates, enamides, enol acetates,
a,a-diarylethylenes, etc). The rhodium(I)-(i-Pr-
SMS-Phos) catalytic profile was investigated reveal-
ing its structural attributes and robustness, and in
contrast to the usual trend, 31P NMR analysis re-
vealed that its methyl (Z)-a-acetamidocinnamate
(MAC) adduct consisted of a reversed diastereo-
meric ratio of 1.4:1 in favour of the most reactive
diastereomer.
P-stereogenic 1,2-bis[(o-R-phenyl)ACTHNUTRGNE(NUG phenyl)phos- phi-
no]ethane ligands.[4] Curiously enough, their ultimate
conceptual hypothesis was that the o-alkoxy groups of
their
optimum
1,2-bis[(o-alkoxyphenyl)ACTHNGUTERNNU(G phenyl)-
phosphino]ethane model ligand, had to be “normal al-
koxys” in order to “provide no significant interfer-
ence with the steric requirements around the phos-
phorus atom”.[1c,d,f] Noteworthy, Imamoto et al.
prepared a set of 1,2-bis[(o-alkylphenyl)ACTHNUTRGENUG(N phenyl)-
phosphino]ethanes, however the overall performance
of the new ligands in the Rh(I)-catalyzed hydrogena-
tion of a-acetamidoacrylic acids was not better than
that of DiPAMP.[5] Since the work of Knowles, the (o-
Keywords: asymmetric catalysis; hydrogenation; P
ligands; rhodium
anisyl)ACTHNUTRGENUG(N phenyl)phosphino moiety has been systemati-
cally appended to new ligand frameworks in order to
assess the efficiency of the new P-stereogenic de-
The genesis of stereogenic phosphine ligand design signs.[6]
for asymmetric catalysis can be traced back to Nobel
co-laureate Knowles and co-workers.[1] The success the incorporation of substituents onto DiPAMPꢁs P
story of their DiPAMP {DiPAMP=1,2-bis
[(o-anisyl)- o-anisyl rings has been shown.[3c] Herein, we report
ACHTUNGTRENNUNG(phenyl)phosphino]ethane} ligand application in the that the alteration of DiPAMPꢁs methyl groups by
The beneficial effect of our alteration as regards to
À
AHCTUNGTRENNUNG
l-DOPA (l-DOPA=l-3,4-dihydroxyphenylalanine) switching to a higher branched homologue, for exam-
process was the catalyst for the design of C2-symmet- ple, i-Pr, has even a greater impact on the Rh(I)-cata-
rical diphosphines with a backbone shifted chirality lyzed asymmetric hydrogenation of olefins.
which presented an easier challenge than P-chirality.[2]
We have prepared[3a] the crystalline 1,2-bis[(o-
Nevertheless, despite major leaps achieved in the area hydroxyphenyl)
ACHTUNGTRENN(UNG phenyl)phosphino-P-borane]ethane
over the last four decades, a universal solution is lack- (1) (Scheme 1) in 57% overall yield by resorting to an
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Michel Stephan et al.
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Scheme 1. Conditions: a) i-PrI, K2CO3, acetone, reflux; b) Et2NH, 608C.
adapted Jugꢀ–Stephan expedient strategy towards catalyst. However, (Z)- and (E)-dehydro-b-methyl-b-
phosphines[6c,7]. Their asymmetric route relies upon amido acids (S11 and S12) were reduced with 82.1%
the sequential displacement of either an (+)- or an and 90.4% ee, respectively. Interestingly, hydrogena-
(À)-ephedrine auxiliary from the enantiomerically tion of MAC under 5 bar H2 at room temperature
pure 1,3,2-oxazaphospholidine-2-borane complex (ox- with an S/C=10000 led to 98.5% ee with full conver-
azaPB) with organolithium reagents. Following a sion in 3 h; also, operating at 0.5M MAC concentra-
straightforward and high-yielding (94%) functionali- tion did not affect the reaction rate or ee.
zation-decomplexation sequence, this key intermedi-
ate led to a very large variety of new enantiopure 1,2- attained with 2-methylidene-succinamic acid (S14),
bis[(o-RO-phenyl)(phenyl)phosphino]ethane ligands itaconic acid (S13: IA) and its dimethyl ester (S15:
High enantioselectivities (97.4–99.0% ee) were also
AHCTUNGTRENNUNG
(2) which we have dubbed “R-SMS-Phos”, and DMI). Noteworthy, for itaconic acid, a 98.7% ee with
wherein R=branched or substituted/functionalized 100% conversion was attained in 4 min.
chain.[3a] Thus, the crystalline (R,R)-1,2-bis[(o-i-PrO-
a-Substituted styrenes and trifluoromethylethylenes
[(R,R)-i-Pr-SMS- are interesting candidates for asymmetric hydrogena-
Phos] ligand was obtained in 54% overall yield from tion and present a stiff challenge. i-Pr-SMS-Phos dis-
played exceptionally high enantioselectivities and re-
phenyl)ACHTUNGTRENNUNG(phenyl)phosphino]ethane
(À)-oxazaPB.
The assessment of the efficiency of i-Pr-SMS-Phos action rates in the Rh(I)-catalyzed hydrogenation of
in the Rh(I)-catalyzed hydrogenation was first per- a-carboxy- (S16), a-acetamido- (S17), and a-acetoxy-
formed on the standard probe substrate methyl (Z)-a-
ACHTUNGTRENsNUNG tyrenes (S18). Namely, atropic acid (S16) was re-
acetamidocinnamate (S6: MAC). The modification of duced under 10 bar of H2 with a respectable 89.1% ee
DiPAMPꢁs MeO groups revealed valuable changes in in the presence of dicyclohexylamine (79.3% ee in its
enantioselectivity and reactivity, as a 99.8% ee was absence). Also, by analogy to atropic acid, hydrogena-
obtained in 4 min vs. a 94.9% ee in 18 min (S/C=100, tion of a-trifluoromethylacrylic acid (S21) furnished a
1 bar of H2, room temperature in MeOH, 100% con- high ee (94.5%) with the same sense of stereoselec-
version). Hence, we explored further the potential of tion.[10]
i-Pr-SMS-Phos on a selected broad spectrum of con-
Interestingly enough, new a,a-diarylethylene sub-
ventional, more challenging benchmark and new ole- strates capable of a three-pronged binding such as a-
finic substrates (Table 1; results correspond to unopti- (2-pyridyl)- (S19) and a-(2-pyridyl-N-oxide)-styrenes
mized conditions).[8,9] The outcome of screening re- (S20), were hydrogenated with 84.6 and 97.4% ee, re-
vealed the excellent efficiency and versatility of the spectively, with a protracted reaction time in the case
Rh(I)-(i-Pr-SMS-Phos) catalyst under extremely mild of S19. Hence, the incidence on hydrogenation of the
conditions.
Since its conception, DiPAMP performed well in strength of the heteroatoms involved are manifested.
hydrogenation of dehydro-a-amido acids. i-Pr-SMS- Moreover, enol acetates S22 and S23 were hydro-
Rh···substrate chelate ring size and the coordination
Phos led to an identical sense of stereoselection, how- genated with exceptionally high enantioselectivities as
ever with 2–5-fold higher reaction rates and superior 94.5 and 99.6% ees were obtained, respectively.
enantioselectivities for the preparation of a variety of
X-ray of the catalyst precursor {Rh
ACHTUNGTREN[NUNG (R,R)-i-Pr-
b-substituted- or unsubstituted alanines (hydrogena- SMS-Phos)AHCTUNGRT(EUNNNG NBD)]}BF4: The X-ray diffraction analy-
tion products of S1–S10). Notably, N-Boc-alanine was sis of this complex revealed a pronounced dissymme-
obtained with >99.9% ee, and a same magnitude of try of this C2-symmetrical ligand (Figure 1).[11] It
reaction rate coupled with a high enantioselectivity showed the 5-membered chelate ring in an envelope
was maintained for substrates containing coordinating l-conformation where one backbone CH2 is at the
groups such as 3-pyridyl (S7) or 4-cyanophenyl (S8). flap of the envelope, and the other CH2 is situated
À
Also, b,b-disubstituted dehydro-a-amido acids S9 and nearly in plane. Also, the bulky P o-i-PrOPh and the
S10, which present a difficult challenge, were similarly P Ph at the fold of the envelope occupy an equatorial
À
reduced under 3 bar of H2 in 5 min with 98.7% and (face/in) and an axial (edge) position, respectively,
99.2% ee, respectively, expanding the prospects of this while the other P o-i-PrOPh is in a face/out orienta-
À
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Table 1. [Rh(i-Pr-SMS-Phos)ACHTNUGRTNEUNG
(MeOH)2]BF4 catalyzed hydrogenation of miscellaneous olefinic model substrates S1–S23.[a]
Olefin
t [min]
ee [%]
Olefin
t [min]
ee [%]
S1
5
98.6
S13 (IA)
4
98.7
S14
3
6
99.0
98.4
S14[d]
S2 (MAA)
S3
8
5
3
5
99.4
>99.9
98.8
S15 (DMI)
S15[b]
5
97.4
97.8
79.3
S4
13
3 h
S5 (AAC)
99.6
S16
S6 (MAC)
S6[b]
4,
15
3 h
99.7
98.9
98.5
S16[e]
S17
3 h
3
15
89.1
97.8
97.7
S6[c]
S17[b]
S7
5
97.3
S18
45
92.8
S8
S9
3
5
99.1
98.7
S19
S20
16 h
10
84.6
97.4
S10
5
99.2
99.1
82.1
90.4
S21[e]
S22
3 h
15
94.5
96.1
99.6
S10[b]
30
20
90
S11 (Z-MAB)
S12 (E-MAB)
S23
16 h
[a]
The catalyst was prepared in situ from [RhACHTNUTRGNE(UNG NBD)2]BF4. Runs were carried out under 1 bar of H2 (3 bar for S9, S10 and
S19, and 10 bar for S16 and S21) at room temperature (508C for S20) in MeOH (0.5 mmol of substrate in 7.5 mL MeOH)
with an S/C=100 (S/C=500 for S1, and 1000 for S2) for the time indicated (100% conversion) if not stated otherwise.
Typical isolated yields were >90%. The ee values were determined by: chiral GC for S1–S18 (prior to analysis carboxylic
groups of hydrogenation products of S1, S5, S7, S13, S14, and S16 were esterified with TMSCHN2); chiral HPLC for S19,
S20, and S23 (hydrogenation product of S19 was analyzed as its N-oxide); 19F NMR [in the presence of (+)-ephedrine]
1
and H NMR [in the presence of (+)-Pr
N
SMS-Phos, S-configured products were obtained except with S13, S14, S15, and S21.
S/C=500.
S/C=10000, under 5 bar of H2 and using 2 mmol of S6, (S)-N-acetylphenylalanine methyl ester isolated in 97% yield.
S/C=1000.
Cy2NH (1.1 equiv.) was added. With Et3N, 87.8% ee was obtained for S16.
[b]
[c]
[d]
[e]
tion.[12a] The aryls are in an alternating face-edge dis- aryls in {Rh
position in accordance with the averaged torsion half chair d-conformation) are in an almost symmetri-
ACHUTNGTREN[UNG (S,S)-DiPAMP]HCATUNGTRNEN(UGN NBD)}BF4 (which has a
angles g1-4 [Rh P Cipso Cortho]
[12b]. By contrast, the cal orientation with the P Ph and the P o-MeOPh
À À
À
À
À
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Figure 1. ORTEP drawing of [RhACHTNUGTRENUN(NG (R,R)-i-Pr-SMS-Phos)ACHTGNUTRENNUNG
Coordinated NBD (NBD=2,5-norbornadiene) and counter-anion (BF4 ) are omitted for clarity. Relevant bond lengths or
À
À
À
interatomic distances [ꢃ] and angles [deg]: Rh P(1) 2.2758(6), Rh P(2) 2.3218(6), Rh···O(1) 3.667(2), Rh···O(2) 3.888(2),
À
À
P(1) Rh P(2) 83.17(2) (bite angle), g1 =65.0, g2 =5.5, g3 =À73.9, g4 =2.0 (for aryls going from top left to top right).
Table 2. Solvent dependence.[a]
Solvent
t [min]
ee [%]
MeOH
EtOH
3–4
4
5
120
4
3–4
6
6
8
15
99.7
99.5
98.6
86.7
99.2
99.7
97.6
98.7
99.4
97.9
i-PrOH
MeCN
EtOAc
Acetone
THF
CH2Cl2
CHCl3
Toluene
[a]
Crystalline [Rh(i-Pr-SMS-Phos)ACTHNUTRGNEG(NU NBD)]BF4 catalyst pre-
cursor was used.
Figure 2. Temperature effect.
groups in a respective alternating edge-face/out dispo- In toluene, the reaction time was increased by a
sition.[13] factor of 3 to allow full conversion.[14] Performing the
Establishment of the catalytic profile of Rh[(R,R)- hydrogenation with i-Pr-SMS-Phos at temperatures in
AHCTUNGTRENNUNG
i-Pr-SMS-Phos]: We have investigated the suscepti- the range of À20 to 508C demonstrated an increase in
bility of the [Rh(i-Pr-SMS-Phos)]+-catalyzed hydroge- the ee with increasing temperature. Thus, a 99.8% ee
nation of MAC (S6) to reaction parameters (Table 2 with full conversion within 2 min was reached at 508C
and Figure 2 and Figure 3 ; for the remaining reaction compared to 99.2% ee in 60 min at À208C.[15] In gen-
parameters, see Table 1). A high ee in the range of eral, the H2 pressure markedly affects the hydrogena-
97.6–99.7% was maintained in various reaction media. tion results of MAC. However, with i-Pr-SMS-Phos a
However, the binding solvent acetonitrile led to a 98.8% ee was maintained up to 10 bar of H2, and it
13% decrease in ee and to a protracted reaction time. only dropped to 94.6% at 40 bar of H2.
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AHCTUNGRTENN(GUN MeOH)2]BF4 solvate complex. Upon admixing this
solution and MAC (8 equiv.) in MeOH at room tem-
perature, an identical pattern of signals corresponding
to the formation of two [Rh(i-Pr-SMS-Phos)-
AHCTUNGTREGNUN(N MAC)]BF4 diastereomers was obtained, reminiscent
of the observed one[16] with [Rh (MAC)]+
ACTHNUTRGENNU(G DiPAMP)ACHTUGNTRENNUGN
(Figure 4). The 31P NMR signals of the minor diaste-
reomer resonate at d=+53.0 [dd, J(103Rh,31P)=
152 Hz, J
(31P, 31P)=37 Hz], +72.7 [dd, J(103Rh,31P)=
ACHTUNGTRENNUNG
162 Hz, J
AHCTUNGTRENNUNG
of the major diastereomer resonate at d=+61.2 [dd,
J(103Rh,31P)=158 Hz, J
J(103Rh,31P)=158 Hz,
JACTHNUTRGNEUNG
ACHTUNGTRENNUNG
two major differences are noticeable: (1) the signals
corresponding to the Ptrans to the amide carbonyl of
the major diastereomer are broadened, and (2) the
ratio of diastereomers is 1.4 in favour of the most re-
Figure 3. Pressure effect.
31P NMR study of the {Rh
ACTHUNTRGENNG[U (R,R)-i-Pr-SMS-Phos]- active one towards H2. This was confirmed by equili-
ACHTUNGTRENNUNG
(NBD)]BF4 throughout ted at À40 to À508C in the NMR tube and 31P NMR
the various stages of its transformation was carried spectrum was recorded at À508C. The latter experi-
out by 31P NMR. Thus, the hydrogenation of NBD in ment unambiguously demonstrated the disappearance
[Rh(i-Pr-SMS-Phos)ACHTNUGTRENUNG(NBD)]BF4 in MeOH at room of the “major diastereomer” which otherwise corre-
temperature led to the disappearance of the doublet sponds at the placement in 31P NMR of the “minor
at d=+52.4 ppm (d, J=158 Hz) and the appearance diastereomer” with DiPAMP as ligand. In the present
of a doublet at d=+81.0 ppm (d, J=206 Hz) corre- case using i-Pr-SMS-Phos as ligand, a reversed ratio
sponding to the formation of the [Rh(i-Pr-SMS-Phos)- of diastereomers is obtained to the traditional one.
Figure 4. Plots of 31P NMR spectra showing (a) equilibrated [Rh
ACTHNUGTRNENUG[(R,R)-i-Pr-SMS-Phos)ACHTUNGTRENN(UGN MAC)]BF4 diastereomers: minor (at
+53.0 and +72.7 ppm) and major (at +61.2 and +69.4 ppm) at room temperature, (b) at À508C, and (c) subsequent partial
hydrogenation at À40 to À508C and recording the spectrum at À508C.
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This is an unprecedented event in Rh(I)-catalyzed
ACHTUGTNRENG(NU RP,RP)-1,2-Bis[(2-isopropoxyphenyl)ACHTUNGTERN(NUGN phenyl)phos-
asymmetric hydrogenation with diphosphines where phino]ethane [(RP,RP)-i-Pr-SMS-Phos]
typically the most reactive diastereomer (dominating
ACHUTNRGEN(NUG RP,RP)-1,2-Bis[(2-isopropoxyphenyl)ACHUTNGTREN(NGUN phenyl)phosphino-P-
the selectivity) is present in a considerably lower
amount than the less reactive diastereomer.[17] This af-
fects the enantioselectivity which is regulated by the
relative reactivities and concentration ratio of the dia-
stereomeric substrate···Rh complex intermediates.[18]
In summary, we have prepared with high overall
yield (94%) enantiopure P-stereogenic i-Pr-SMS-Phos
borane]ethane (0.41 g, 0.75 mmol) in Et2NH (7.5 mL) was
heated at 55–608C for 3 h under an inert atmosphere. After
concentration, purification on silica gel eluting with toluene,
concentration and recrystallization from hexane/CH2Cl2
under inert atmosphere, iPr-SMS-Phos was obtained as col-
ourless crystals; yield: 375 mg (97%): mp 72–748C; Rf 0.58
(toluene); [a]2D5: À57.8 degcm3 gÀ1dmÀ1 (c 1.0 gdcLÀ1 in
CHCl3); 1H NMR: d=1.03 and 1.17 (2d, J=6.0 Hz, 12H,
4Me), 1.95 and 2.31 (2 m, 4H, 2CH2), 4.47 (sept, J=6.0 Hz,
2H, 2CH), 6.72–6.85 (m, 4H, Ar-H), 7.04 (m, 2H, Ar-H),
7.15–7.38 (m, 12H, Ar-H); 13C NMR: d=21.5, 21.7, 22.0 (d,
in 2-steps from 1,2-bis[(o-hydroxyphenyl)ACTHNUTRGENUG(N phenyl)-
phosphino-P-borane]ethane. The seemingly trivial
and dꢀjꢄ vu alteration of the DiPAMPꢁs MeO groups
carried out by Knowles and co-workers, turned out
still to offer interesting perspectives. The present al-
teration culminated in an efficient ligand which boost-
ed reaction rate by several times and increased enan-
tioselectivities in the Rh(I)-catalyzed asymmetric hy-
drogenation of a wide-range of dehydro-a-amido
acids, itaconates, acrylates, enamides, enol acetates,
J
C,P =4 Hz), 69.7, 111.9, 120.2, 127.7 (m), 128.1 (m), 128.4,
129.6, 132.4 (m), 133.4 (m), 137.8 (m), 159.1 (m); 31P NMR:
d=À19.1 (s); MS (FAB): m/z=515 [M+ +H] (100%);
HRMS (EI): m/z=514.220, calcd. for C32H36O2P2 [M+]:
514.219; anal. calcd. for C32H36O2P2: C 74.69, H 7.05; found:
C 74.82, H 7.19.
and a,a-diarylethylenes. Furthermore, the study out- {Rh
ACHTUGTNRENG[NU (R,R)-i-Pr-SMS-Phos]ACHTUNTGERN(NUGN NBD)}BF4 Complex
come of the catalytic profile of Rh(I)-(i-Pr-SMS-
To a solution of (R,R)-i-Pr-SMS-Phos (257 mg, 0.5 mmol) in
MeOH (2 mL) was added in one portion [RhCl(NBD)]2
Phos) revealed the structural attributes and robust-
ness of this catalyst system. Ongoing studies to gain
insight into the hydrogenation mechanism pathway
will be forthcoming.
AHCTUNGTRENNUNG
(115 mg, 0.25 mmol) at room temperature. The resulting
mixture was left to stir for 1 h affording a deep red solution.
Subsequently, a solution of NaBF4 (82 mg, 0.75 mmol) in
H2O (1 mL) was slowly added. The precipitated fine orange
powder was filtered, rinsed with H2O (5 mL), and dried in
vacuum; yield: 360 mg (90%). The complex was recrystal-
lized from EtOH (5 mL) affording red crystals; yield:
Experimental Section
1
320 mg (80%); mp 227–2298C; H NMR: d=0.87 and 0.91
General Remarks
(2d, J=6.1 Hz, 12H, 4 Me), 1.78 (s, 2H, CHCH2CH), 2.21–
2.73 (m, 4H, CH2CH2), 4.17 (m, 2H, CHCH2), 4.44 (sept,
J=6.1 Hz, 2H, 2 CH), 5.19 (m, 2H, CH=CH), 5.31 (m, 2H,
CH=CH), 6.88 (m, 2H, Ar-H), 7.15 (m, 2H, Ar-H), 7.34–
7.57 (m, 12H, Ar-H), 7.86 (m, 2H, Ar-H); 13C NMR: d=
21.0, 21.4, 25.1 (d, JC,P =2 Hz), 25.4, 25.7 (d, JC,P =2 Hz),
55.3, 70.2, 71.0, 87.9 (m), 91.4 (m), 112.7, 117.0 (m), 120.9
(m), 129.2 (m), 130.7, 130.8 (m), 131.5 (m), 134.1, 136.4 (m),
All operations were conducted under N2 or Ar atmosphere
1
using anhydrous and degassed solvents. H (300 MHz, inter-
nal Me4Si), 13C (75 MHz, internal CDCl3), and 31P NMR
(120 MHz, external 85% H3PO4) were recorded for solu-
tions in CDCl3.
A
ACHTUNGTREN(NUNG phenyl)phos-
158.3; 31P NMR: d=+52.4 [d, J
ACHTNUTRGNEUNG(P,Rh)=158 Hz].
A
mixture of (RP,RP)-1,2-bis[(2-hydroxyphenyl)ACTHNGUETRNNU(G phenyl)-
In situ Preparation of [Rh
ACHTUNGTREN(NUGN (R,R)-i-Pr-SMS-
phosphino-P-borane]ethane (1)[3a] (458 mg, 1 mmol), K2CO3
(553 mg, 4 mmol) and 2-iodopropane (0.68 g, 0.40 mL,
4.0 mmol) in acetone (2 mL) was refluxed overnight. The
mixture was concentrated, taken with CH2Cl2, filtered
through a pad of silica gel, concentrated and recrystallized
from hexane/CH2Cl2 affording colourless crystals; yield:
525 mg (97%): mp 159–1618C; Rf =0.62 (toluene/EtOAc
9:1); [a]D25: +102.0 degcm3 gÀ1 dmÀ1 (c 1.0 gdcLÀ1 in CHCl3);
1H NMR: d=0.30–1.75 (br m, 6H, 2 BH3), 0.91 and 1.16
(2d, J=6.1 Hz, 12H, 4 Me), 2.65 (m, 4H, 2 CH2), 4.44 (sept,
J=6.0 Hz, 2H, 2 CH), 6.75 (m, 2H, Ar-H), 6.97 (m, 2H, Ar-
H), 7.39 (m, 8H, Ar-H), 7.64 (m, 4H, Ar-H), 7.90 (m, 2H,
Ar-H); 13C NMR: d=17.4 (m), 20.8, 21.4, 69.7, 111.6, 115.7
(d, JC,P =53 Hz), 120.1 (m), 128.2 (m), 129.7 (d, JC,P =59 Hz),
130.4, 131.7 (m), 133.6, 136.60 (m), 159.2; 31P NMR: d=+
18.3 (br m); MS (FAB): m/z=541 [M+ÀH] (19%); anal.
calcd for C32H42B2O2P2: C 70.88, H 7.81; found: C 71.10, H
8.10.
Phos)(MeOH)2]BF4 catalyst
AHCTUNGTRENNUNG
To a solution of [RhACHTNUTRGNE(UGN NBD)2]BF4 (2.4 mg, 6.3 mmol) in
MeOH (0.5 mL), a solution of (R,R)-i-Pr-SMS-Phos (2.6 mg,
5.0 mmol) in MeOH (0.5 mL) was added dropwise at room
temperature. The resulting solution was hydrogenated under
1 bar of H2 for ca. 15 min. Elimination of metallic rhodium
by filtration through a No. 3 sintered-glass filter afforded a
clear brown solution of [Rh
T
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Hydrogenation Procedure for the Substrates in
Table 1
To a solution of the olefin (0.5 mmol) in MeOH (7 mL),
three freeze-pump-thaw cycles were applied and the system
was filled with Ar. Then to this solution was added under
argon a solution of the preformed [Rh(i-Pr-SMS-Phos)-
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DiPAMPꢁs Big Brother “i-Pr-SMS-Phos” Exhibits Exceptional Features
COMMUNICATIONS
A
For literature overview on P-stereogenic phosphines,
see: h) W. Tang, X. Zhang, Chem. Rev. 2003, 103,
3029–3069; i) A. Grabulosa, J. Granell, G. Muller,
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gands in Asymmetric Catalysis, Vol. 1–3, (Ed.: A.
Bçrner), Wiley-VCH, Weinheim, 2008.
see Table 1) in MeOH (prepared in situ as above). A
vacuum was applied to the system then it was backfilled
with H2. The mixture was stirred at room temperature under
1–10 bar of H2. The progress of the hydrogenation was
monitored by the diminution of the volume of the closed re-
action system at 1 bar (until H2 uptake ceased) and the
colour change of the solution. The ee values were deter-
mined by: chiral GC for S1–S18 (prior to analysis carboxylic
groups of hydrogenation products of S1, S5, S7, S13, S14,
and S16 were esterified with TMSCHN2); chiral HPLC for
S19, S20, and S23 (hydrogenation product of S19 was ana-
lyzed as its N-oxide); 19F NMR [in the presence of (+)-
[3] a) M. Stephan, B. Mohar, (PhosPhoenix SARL; Nat.
Inst. of Chem. of Slovenia), French Patent 2,887,253,
2005; WO Patent 2006136695, 2006 (http://www.cas.org/
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ˇ
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2007, 72, 8010–8018; c) B. Zupancic, B. Mohar, M. Ste-
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ephedrine] and 1H NMR [in the presence of (+)-Pr
ACTHNUTRGNEN(UG hfc)3]
ˇ
Sterk, PhD thesis, University of Ljubljana, FKKT
for hydrogenation products of S21 and S22, respectively.
Absolute configurations were assigned by comparison of the
optical rotations of the isolated products with the literature
data.
ˇ ˇ
(SLO), 2008; e) B. Zupancic, B. Mohar, M. Stephan,
Tetrahedron Lett. 2009, 50, 7382–7384.
[4] Knowles et al. have reported their results of study for
the Rh(I)-catalyzed hydrogenation of (Z)-a-acetamido-
(AAC) or (E/Z)-benzamidocinnamic (BAC) acids with
various
{RhACTHNGUTREN[UNG (R,R)-1,2-bis[(o-R-phenyl)CAHTUNGTNER(NUNG phenyl)phos-
AHCTUNGTREGpNNNU hino]ethane)]} catalysts (conditions: S/C=1000 under
Acknowledgements
3 atm of H2 in i-PrOH (88%) at 508C for 0.8–1 h): 96%
ee (S) with o-R-Ph=o-MeO-Ph (DiPAMP), 84% ee
with o-HO-Ph, 63% ee with o-AcO-Ph, 73% ee with o-
MeSO2-Ph, 60% ee with o-F-Ph, while no reaction oc-
curred with o-MeS-Ph.[1d].
This work was supported by the Ministry of Higher Educa-
tion, Science, and Technology of the Republic of Slovenia
(grants P1-242 and J1-9806), and by the Ministry of National
Education, Research, and Technology of France (grant
00P141) to PhosPhoenix SARL. We thank Dr. Barbara
Modec, University of Ljubljana, for the X-ray crystal struc-
ture analysis.
[5] Y. Wada, T. Imamoto, H. Tsuruta, K. Yamaguchi, I. D.
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[8] For convenient evaluation of the catalyst activity, the
induction period was eliminated by preforming the cat-
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of S9 and S10, the reaction mixture was only analyzed
after 5 min, with S16 and S21 after 3 h, and with S19
after 16 h. Reported results correspond to the average
of duplicate independent runs.
[15] This constitutes a very small fluctuation in ee compared
to results obtained using DiPAMP. In the case of
DiPAMP, a dramatic decay of ee was already observed
at elevated pressures (70–80% ee at ~10 atm H2). It is
known that an increase in temperature leads to an in-
crease in ee, and an increase in pressure leads to a de-
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[9] For our hydrogenation results of substrates in Table 1
using [RhACHTUNGTRENNUNG(DiPAMP)AHCUTNGTREN(NUGN MeOH)2]BF4 and literature data
with representative diphosphine ligands, see Supporting
Information.
(MAC)]+ a ca. 10:1 ratio of “less
[16] With [RhACTHNUGRTNENUG(DiPAMP)ACHTUNGTRENNUGN
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a) ref.[15]; b) J. M. Brown, P. A. Chaloner, G. A. Morris,
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[11] CCDC 728657 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif
or from the Cambridge Crystallographic Data Centre
(12, Union Road, Cambridge CB2 1EZ, UK; fax: (+
44)-1223-336-033, or deposit@ccdc.cam.ac.uk.
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2786
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Adv. Synth. Catal. 2009, 351, 2779 – 2786