Me-AnilaPhos: a new chiral phosphine-phosphoramidite ligand for a highly efficient Rh-catalyzed asymmetric olefin hydrogenation

Article abstract of DOI:10.1016/j.tetlet.2006.08.136

A cationic rhodium(I) complex with a novel chiral phosphine-phosphoramidite ligand based on 2-diphenylphosphino-N-methylaniline and R-BINOL moieties has been synthesized. The complex provided remarkably high activity and enantioselectivity in the asymmetr

Full text of DOI:10.1016/j.tetlet.2006.08.136

Tetrahedron Letters 47 (2006) 7947–7950
Me-AnilaPhos: a new chiral phosphine–phosphoramidite ligand
for a highly efficient Rh-catalyzed asymmetric olefin hydrogenation
Kalliopi A. Vallianatou,a Ioannis D. Kostas,a,* Jens Holzb and Armin Bo¨rnerb,c
aNational Hellenic Research Foundation, Institute of Organic and Pharmaceutical Chemistry, Vas. Constantinou 48,
116 35 Athens, Greece
bLeibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock, A.-Einstein-Str. 29a, 18059 Rostock, Germany
cInstitut fu¨r Chemie der Universita¨t Rostock, A.-Einstein-Str. 3a, 18059 Rostock, Germany
Received 10 July 2006; revised 24 August 2006; accepted 31 August 2006
Available online 25 September 2006
Abstract—A cationic rhodium(I) complex with a novel chiral phosphine–phosphoramidite ligand based on 2-diphenylphosphino-N-
methylaniline and R-BINOL moieties has been synthesized. The complex provided remarkably high activity and enantioselectivity
in the asymmetric hydrogenation of methyl (Z)-a-acetamidocinnamate (100% conversion after 10 min, 98% ee) and dimethyl itac-
onate (100% conversion after 26 min, 96% ee) under ambient conditions (1 bar hydrogen pressure, room temperature) using 1 mol %
of the catalyst in dichloromethane as solvent. On the other hand, when hydrogenation was performed in methanol, both conversion
and enantioselectivity were significantly diminished, due to the partial decomposition of the rhodium/phosphine–phosphoramidite
complex.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Transition-metal asymmetric hydrogenation is a highly
attractive strategy for the synthesis of optically active
organic molecules with enormous academic and indus-
trial interest, and thus the development of new chiral
ligands which provide high activity and enantioselectivity
remains a challenge of high importance.1 Among several
classes of trivalent phosphorus ligands, chiral phospho-
ramidites have been largely overlooked in asymmetric
hydrogenation to date, and some monodentate phos-
phoramidites2 (e.g., MonoPhos, SiPhos, DpenPhos,
PipPhos, MorfPhos, and PegPhos) as well as diphosphor-
amidites2a,3 have displayed high enantioselectivities.
Most of these ligands are BINOL-based phosphorami-
dites, and possess chirality close to the phosphorus atom
and also a rigid structure imposed by the binaphthyl
group. Remarkable enantioselectivities have also been
reported by the use of chiral phosphine–phosphorami-
dite ligands in transition-metal catalysis. Up to now,
only two families of this type of chiral ligands have been
applied in asymmetric hydrogenation: (a) 1,2-dihydro-
quinoline-based phosphine–phosphoramidites (Quina-
Phos),4 and (b) ferrocene-based phosphine–phosphor-
amidites.5
We have previously been engaged in preparing several
new chiral phosphorus ligands6 as well as non-chiral
P,N (or S)-ligands7 for transition-metal homogeneous
catalysis. In our ongoing research on the development
of new chiral ligands for asymmetric catalysis, we now
introduce a new chiral phosphine–phosphoramidite
ligand (Me-AnilaPhos, 2)8 based on 2-diphenylphos-
phino-N-methylaniline and R-BINOL moieties. The
ligand, possessing two different phosphorus donor sites,
is readily available from simple starting materials, and it
was found to be a highly efficient ligand for the rho-
dium-catalyzed asymmetric hydrogenation of prochiral
olefins.
The chiral phosphine–phosphoramidite ligand 2 (Me-
AnilaPhos) was synthesized by hydrogen abstraction
of 2-diphenylphosphino-N-methylaniline 1 (easily pre-
pared from N-methylaniline)9 using n-BuLi and subse-
quent reaction of the resulting lithium amide with
1 equiv of [(R)-(1,10-binaphthalene-2,20-diyl)]chloro-
phosphite (Scheme 1).10 The phosphine and the phos-
phoramidite phosphorus atoms display two distinct
doublets in the 31P NMR spectrum of 2 at d ꢀ14.9
and 141.8, respectively, with a P–P coupling constant of
Keywords: Chiral ligand; Phosphine; Phosphoramidite; BINOL; Asym-
metric hydrogenation; Homogeneous catalysis.
*
Corresponding author. Tel.: +30 210 7273878; fax: +30 210
7273831; e-mail: ikostas@eie.gr
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.08.136

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K. A. Vallianatou et al. / Tetrahedron Letters 47 (2006) 7947–7950
Table 1. Enantioselective hydrogenation of methyl (Z)-a-acetamido-
cinnamate (Z-AMe) and dimethyl itaconate (ItMe2) catalyzed by the
rhodium precatalyst 3
Entry Substrate Solvent Time (min) Conv. (%) ee (%)
(Conf.)
1
2
3
4
5
Z-AMe
Z-AMe
Z-AMe
ItMe2
CH2Cl2
THF
MeOH 1380
CH2Cl2
MeOH
10
6
100
100
85
100
100
97.9 (S)
96.1 (S)
19.5 (S)
96.2 (R)
7.3 (R)
26
390
ItMe2
Reaction conditions: 0.5 mmol of prochiral olefin and 0.005 mmol of
complex 3 in 7.5 mL of solvent at 25 ꢁC, 1 bar H2 pressure over the
solution.
meric excess was obtained in CH2Cl2. In a recent paper
reported jointly by four research groups (Minnaard,
Reetz, de Vries, Feringa),2m twenty highly active
monodentate phosphoramidite ligands were applied in
the rhodium-catalyzed asymmetric hydrogenation under
identical reaction conditions,2m although most were pre-
viously evaluated in hydrogenation under different con-
ditions.2 For the hydrogenation of Z-AMe and ItMe2,
only the following six ligands displayed similar or higher
enantiomeric excesses to those observed for Me-Anila-
Phos: PipPhos, the H8-analogue of PipPhos, MorfPhos,
the H8-analogue of MorfPhos, the H8-analogue of N-
phenylpiperazinePhos and tetrahydroisoquinolinePhos.
However, harsher conditions were required for a
complete conversion (5 bar hydrogen pressure, 4 h,
2 mol % Rh) using these ligands compared to the condi-
tions used with Me-AnilaPhos (1 bar hydrogen pressure,
10 or 26 min, 1 mol % Rh). In some cases, other chiral
phosphine–phosphoramidite ligands provided similar
or higher enantiomeric excesses to those observed with
Me-AnilaPhos, but these ligands also required a hydro-
gen pressure of 10–30 bar and longer reaction times for
the complete conversion of the substrate.4,5
Scheme 1. Synthesis of the ligand Me-AnilaPhos 2 and the corre-
sponding Rh-precatalyst 3.
42.0 Hz. The treatment of [Rh(COD)2]BF4 in dichloro-
methane with 1 equiv of ligand 2 yielded the cationic
rhodium complex 3,11 in which the rhodium is involved
in a six-membered chelate ring according to the NMR
data. The 31P NMR spectrum of 3 showed a doublets
of doublets at d 24.2 for the phosphine phosphorus with
a Rh–P coupling constant of 139.2 Hz and a P–P cou-
pling constant of 53.4 Hz, and another doublets of dou-
blets at d 134.8 for the phosphoramidite phosphorus
with a Rh–P coupling constant of 255.6 Hz and a P–P
coupling constant of 53.4 Hz.
Hydrogenation with complex 3 was also performed in
methanol, and it was found that both conversion and
enantioselectivity were significantly diminished (entries
3 and 5). This probably resulted from the partial decom-
position of the rhodium/phosphine–phosphoramidite
complex in the protic solvent. In order to confirm this,
a sample of complex 3 dissolved in MeOH-d4 was
allowed to stand overnight in an NMR tube, under argon.
The resulting 31P NMR spectrum of 3 showed signifi-
cant differences compared to the spectrum of the com-
plex in CD2Cl2; in addition to the two sets of doublets
of doublets assigned to the two phosphorus atoms in
3, other peaks of a higher intensity were also observed.
Although the decomposition products were not identi-
fied, it was obvious that this complex was not stable in
protic solvents. Other phosphoramidites such as
H8-MonoPhos were also decomposed in MeOH,2c in
contrast to MonoPhos which was stable over 48 h in
MeOH at room temperature even in the presence of
[Rh(COD)2]BF4.2e A lower activity and enantioselectiv-
ity during the Rh/phosphoramidite-catalyzed asymmet-
ric olefin hydrogenation in MeOH compared to catalysis
in non-protic solvents was also observed by other
authors.2a–c,e,5a However, it was noted that the effect
Complex 3 was tested in the asymmetric hydrogenation
of the benchmark alkenes methyl (Z)-a-acetamidocinna-
mate (Z-AMe) and dimethyl itaconate (ItMe2) (Scheme
2, Table 1).12 Catalysis was performed under ambient
conditions (1 bar hydrogen pressure, room temperature)
with a substrate to a precatalyst molar ratio of 100:1. In
non-protic solvents (CH2Cl2, THF), conversions were
quantitative in only a few minutes, with enantioselectiv-
ities reaching 98% ee (entries 1, 2 and 4), indicating a
remarkably high catalytic activity for the complex and
also its potential for a high asymmetric induction. It is
worth noting that the high enantioselectivity observed
for complex 3 (Rh:ligand = 1:1) occurred without the
addition of an excess of the ligand. A higher enantio-
Scheme 2. Asymmetric hydrogenation of prochiral olefins.

K. A. Vallianatou et al. / Tetrahedron Letters 47 (2006) 7947–7950
7949
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of the solvent on both enantioselectivity and conversion
was not always consistent.2i
In conclusion, we have developed a cationic rhodium
complex with the new chiral phosphine–phosphorami-
dite ligand Me-Anilaphos. The precatalyst was utilized
for the asymmetric hydrogenation of methyl (Z)-a-acet-
amidocinnamate and dimethyl itaconate under ambient
conditions (1 bar hydrogen pressure, 25 ꢁC), and led to
excellent results (quantitative conversion in 10 or
26 min, ee of up to 98%), comparable with those ob-
served for other highly efficient systems for asymmetric
olefin hydrogenation. Taking into consideration (a) the
ease and the relatively economic synthesis of the ligand,
(b) the remarkably high catalytic activity and (c) the
high enantioselectivity towards asymmetric hydrogena-
tion, we consider Me-Anilaphos to be a very promising
ligand for the asymmetric induction. The development
of analogous chiral ligands for hydrogenation as well
as other transition-metal-catalyzed asymmetric reac-
tions is currently in progress.
3. Reetz, M. T.; Mehler, G.; Bondarev, O. Chem. Commun.
2006, 2292–2294.
`
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Lu, G.; Yeung, C.-H.; Chan, A. S. C. Tetrahedron:
Asymmetry 2004, 15, 2273–2278; (b) Hu, X.-P.; Zheng, Z.
Org. Lett. 2004, 6, 3585–3588; (c) Zeng, Q.-H.; Hu, X.-P.;
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Z. Org. Lett. 2005, 7, 419–422.
6. Recent selected papers: (a) Korostylev, A.; Selent, D.;
Monsees, A.; Borgmann, C.; Bo¨rner, A. Tetrahedron:
Asymmetry 2003, 14, 1905–1909; (b) Almena, J.; Monsees,
A.; Kadyrov, R.; Riermeier, T. H.; Gotov, B.; Holz, J.;
Bo¨rner, A. Adv. Synth. Catal. 2004, 346, 1263–1266; (c)
Korostylev, A.; Monsees, A.; Fischer, C.; Bo¨rner, A.
Tetrahedron: Asymmetry 2004, 15, 1001–1005; (d) Dubro-
vina, N. V.; Bo¨rner, A. Angew. Chem., Int. Ed. 2004, 43,
5883–5886; (e) Kostas, I. D.; Vallianatou, K. A.; Holz, J.;
Bo¨rner, A. Appl. Organomet. Chem. 2005, 19, 1090–1095;
(f) Dubrovina, N. V.; Tararov, V. I.; Monsees, A.;
Spannenberg, A.; Kostas, I. D.; Bo¨rner, A. Tetrahedron:
Asymmetry 2005, 16, 3640–3649; (g) Holz, J.; Zayas, O.;
Jiao, H.; Baumann, W.; Spannenberg, A.; Monsees, A.;
Riermeier, T. H.; Almena, J.; Kadyrov, R.; Bo¨rner, A.
Chem. Eur. J. 2006, 12, 5001–5013.
Acknowledgement
This work was supported by the General Secretariat
of Research and Technology of Greece and Deutsches
Zentrum fur Luft-und Raumfahrt e.V.
¨
References and notes
1. Recent selected monographs and reviews: (a) Brown, J. M.
In Comprehensive Asymmetric Catalysis; Jacobsen, E. N.,
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Noyori, R. In Catalytic Asymmetric Synthesis; Ojima, I.,
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H.-U.; Schmidt, E. Asymmetric Catalysis on Industrial
Scale; Wiley-VCH: Weinheim, 2004; (d) Tang, W.; Zhang,
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X. Chem. Rev. 2003, 103, 3029–3069; (e) Dieguez, M.;
7. Recent selected papers: (a) Kostas, I. D. J. Organomet.
Chem. 2001, 626, 221–226; (b) Kostas, I. D. J. Organomet.
Chem. 2001, 634, 90–98; (c) Kostas, I. D. Inorg. Chim.
Acta 2003, 355, 424–427; (d) Kostas, I. D.; Steele, B. R.;
Terzis, A.; Amosova, S. V. Tetrahedron 2003, 59, 3467–
3473; (e) Kostas, I. D.; Steele, B. R.; Andreadaki, F. J.;
Potapov, V. A. Inorg. Chim. Acta 2004, 357, 2850–2854;
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Kostas, I. D. Appl. Organomet. Chem. 2006, 20, 335–
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Yueng, T. T.-L.; Chan, A. S. C. Coord. Chem. Rev. 2004,
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2. Recent selected papers: (a) van den Berg, M.; Minnaard,
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Vries, J. G.; Feringa, B. L. J. Am. Chem. Soc. 2000, 122,
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Chan, A. S. C. Tetrahedron Lett. 2002, 43, 5541–5544; (c)
Zeng, Q.; Liu, H.; Mi, A.; Jiang, Y. H.; Li, X. S.; Choi, M.
C. K.; Chan, A. S. C. Tetrahedron 2002, 58, 8799–8803; (d)
Jia, X.; Li, X. S.; Xu, L. J.; Shi, Q.; Yao, X. S.; Chan, A. S.
C. J. Org. Chem. 2003, 68, 4539–4541; (e) van den Berg,
M.; Minnaard, A. J.; Haak, R. M.; Leeman, M.; Schudde,
E. P.; Meetsma, A.; Feringa, B. L.; de Vries, A. H. M.;
Maljaars, C. E. P.; Willans, C. E.; Hyett, D.; Boogers, J.
A. F.; Henderickx, H. J. W.; de Vries, J. G. Adv. Synth.
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metry 2003, 14, 3219–3224; (g) Wu, S. L.; Zhang, W. C.;
Zhang, Z. G.; Zhang, X. M. Org. Lett. 2004, 6, 3565–3567;
8. We labelled this ligand Me-AnilaPhos as an Me-Aniline-
functionalized Phosphoramidite, in accordance with the
acronyms previously given for analogous ligands, such as
QuinaPhos.4a
9. Phosphine
1 was prepared according to a known
procedure, in which N-methylaniline was converted into
the lithium N-methyl-N-phenylcarbamate, o-lithiated
with t-BuLi and treated with Ph2PCl: van Oort, A. B.;
Budzelaar, P. H. M.; Frijns, J. H. G.; Orpen, A. G.
J. Organomet. Chem. 1990, 396, 33–47. We report here
the NMR data for 1, which has not been reported
1
previously. H NMR (CDCl3): d 7.68–7.34 (m, 11H, Ar),
6.79–6.63 (m, 3H, Ar), 5.10 (br s, 1H, NH), 2.83 (s, 3H,
NCH3); 13C{1H} NMR (CDCl3): d 152.0–109.9 (Ar),
31.0 (NCH3); 31P{1H} NMR (CDCl3): d ꢀ22.2 (s).

7950
K. A. Vallianatou et al. / Tetrahedron Letters 47 (2006) 7947–7950
10. Ligand 2 was prepared as follows: n-BuLi (1.70 M in
methylcyclohexane, 0.96 mL, 1.63 mmol) was added drop-
wise to a solution of 1 (0.435 g, 1.49 mmol) in THF (3 mL)
at ꢀ70 ꢁC and the reaction mixture was stirred at this
temperature for 4 h. Then the temperature was allowed to
increase to ꢀ50 ꢁC, and a solution of [(R)-(1,10-binaph-
thalene-2,20-diyl)]chlorophosphite (0.519 g, 1.48 mmol) in
THF (5 mL) was added dropwise. The reaction mixture
was warmed slowly to room temperature and stirred
overnight. The volatiles were evaporated under reduced
pressure, dichloromethane (10 mL) was added, and the
mixture was filtered through Celite. The solvent was
evaporated under reduced pressure and the remaining
solid was washed with hexane (10 mL) and dried at 80 ꢁC
under vacuum, yielding 2 as a white solid (0.747 g, 83%),
(0.094 g, 80%), mp (dec.) 230 ꢁC. 1H NMR (CD2Cl2): d
8.24 (d, 3J = 8.5 Hz, 1H, Ar), 8.14 (d, 3J = 9.2 Hz, 1H,
Ar), 8.08–8.00 (m, 2H, Ar), 7.81–7.24 (m, 22H, Ar), 5.76
(br m, 2H, COD–CH), 4.62 (br m, 1H, COD–CH), 3.97
(br m, 1H, COD–CH), 2.83–2.75 (m, 2H, COD–CH2),
3
2.58 (m, 2H, COD–CH2), 2.21 (d, JH,P = 5.5 Hz, 3H,
NCH3), 2.07–1.96 (m, 4H, COD–CH2); 13C{1H} NMR
(CD2Cl2): d 149.3–120.2 (Ar), 108.7–108.5 (m, CH–COD),
108.4–108.2 (m, CH–COD), 107.3–107.1 (m, CH–COD),
100.3–100.1 (m, COD–CH), 37.4 (NCH3), 33.5, 28.1, 27.0
and 26.0 (COD–CH2); 31P{1H} NMR (CD2Cl2): d 134.8
(dd, JRh,P = 255.6 Hz, JP,P = 53.4 Hz, PN), 24.2 (dd,
JRh,P = 139.2 Hz, JP,P = 53.4 Hz, PPh2). Anal. Calcd for
C47H41NO2BF4P2Rh: C, 62.48; H, 4.57; N, 1.55%. Found:
C, 61.91; H, 4.95; N, 1.97%.
25
mp 119–127 ꢁC. ½aꢁD ꢀ103.9 (c 1.0, CH2Cl2). 1H NMR
12. General experimental procedure for the asymmetric hydro-
genation: The prochiral olefin (0.5 mmol) and the
(CDCl3): d 7.94–7.88 (m, 4H, Ar), 7.59–7.20 (m, 21H, Ar),
3
6.97–6.94 (m, 1H, Ar), 2.53 (d, JH,P = 2.4 Hz, 3H,
rhodium precatalyst 3 (4.52 mg, 0.005 mmol) were
NCH3); 13C{1H} NMR (CDCl3): d 151.1–122.1 (Ar),
transferred into the hydrogenation device (a standard
device for hydrogenation under 1 bar hydrogen pressure).
Under a hydrogen atmosphere, the solvent (7.5 mL) was
added and the hydrogenation was followed by measure-
ment of the gas-consumption under isothermic (25 ꢁC)
and isobaric (1 bar) conditions with an automatically
registering gas measuring device. When no further gas
consumption occurred the reaction was finished. The
conversions were determined by GC/HPLC during anal-
ysis of the ee values; the integrals/areas of starting
materials and products corresponded nearly exactly with
the composition of the mixture. In some cases, conver-
36.5 (NCH3); 31P{1H} NMR (CDCl3):
d 141.8 (d,
JP,P = 42.0 Hz, PN), ꢀ14.9 (d, JP,P = 42.0 Hz, PPh2).
Anal. Calcd for C39H29NO2P2: C, 77.35; H, 4.83; N,
2.31%. Found: C, 76.94; H, 5.15; N, 2.38%.
11. Rhodium complex 3 was prepared as follows: A solution
of ligand 2 (0.078 g, 0.13 mmol) in dichloromethane
(5 mL) was added dropwise to a dark-red solution of
[Rh(COD)2]BF4 (0.051 g, 0.13 mmol) in dichloromethane
(3 mL) at ꢀ50 ꢁC, and stirred at this temperature for 1 h.
The reaction mixture was warmed to room temperature
and stirred for an additional 2 h. The resulting orange
solution was evaporated under reduced pressure, and the
remaining solid was washed with ether (2 · 10 mL) and
dried at 80 ꢁC under vacuum, yielding 3 as an orange solid
1
sions were also determined by H NMR. Configurations
were assigned by comparison with the retention time of a
known enantiomer.