Diphosphonites as highly efficient ligands for enantioselective rhodium-catalyzed hydrogenation

Article abstract of DOI:10.1039/a805524f

Chiral ligands with achiral backbones such as ethano- or ferroceno-bridges linking two phosphonites derived from chiral diols such as binaphthol (BINOL) have been prepared; the corresponding Rh complexes are excellent catalysts in the hydrogenation of prochiral olefins such as itaconic acid dimethyl ester or 2-acetamido methyl acrylate, the ee values being 90-99.5%.

Full text of DOI:10.1039/a805524f

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Diphosphonites as highly efficient ligands for enantioselective
rhodium-catalyzed hydrogenation
Manfred T. Reetz,*† Andreas Gosberg, Richard Goddard and Suk-Hun Kyung
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim/Ruhr, Germany
Chiral ligands with achiral backbones such as ethano- or
ferroceno-bridges linking two phosphonites derived from
chiral diols such as binaphthol (BINOL) have been prepared;
the corresponding Rh complexes are excellent catalysts in
the hydrogenation of prochiral olefins such as itaconic acid
dimethyl ester or 2-acetamido methyl acrylate, the ee values
being 90–99.5%.
Although a number of chiral diphosphanes and diphosphinites
have been shown to be effective ligands in transition metal
catalyzed asymmetric reactions,1 the search for new types of
chiral auxiliaries continues.2 Surprisingly, very little is known
concerning chiral diphosphonites as ligands in these reactions.3
Perhaps this is due to the fact that in all cases reported so far the
enantioselectivity is poor (ee = 0–32%).3 We speculated that
chelating diphosphonites derived from a proper combination of
an achiral backbone and a chiral diol might constitute useful and
easily accessible ligands.4
Using ferrocene and (R)- or (S)-BINOL as cheap building
blocks,5 the diphosphonite 1 was easily assembled in three steps
(Scheme 1).6 1 is an orange–brown crystalline compound,
which in the solid state‡ shows some interesting features (Fig.
1). In spite of their different environments, the two independent
molecules in the unit cell have almost identical conformations
[P1–Cp1–Cp2–P2 29(1)°, P3–Cp3–Cp4–P4 27(1)°; Cp, cen-
troid], with the two P atoms in each molecule situated close to
one another [P1···P2 3.506(3), P3···P4 3.428(3) Å].
The ethano-bridged analog 2 was also readily synthesized
(Scheme 2).
In order to prepare hydrogenation catalysts, the ligands were
treated with Rh(cod)2BF4 under standard conditions,7 affording
the corresponding complexes (R,R)-(1)Rh(cod)BF4 or (R,R)-
(2)Rh(cod)BF4, which were characterized by NMR, ESI-MS
and IR spectroscopy. Thus far it has not been possible to obtain
crystals suitable for crystallographic investigations. Two differ-
ent types of olefins were chosen as substrates for asymmetric
P(NEt2)2
PCl2
ii
i
Fig. 1 Molecular structures of the two independent molecules of 1. Side
(upper structure, molecule 1) and top views (the toluene solvent of
crystallization has been omitted for clarity).
Fe
Fe
Fe
iii
P(NEt2)2
PCl2
(Et2N)2P
P(NEt2)2
O
O
P
i
Fe
O
O
P
O
O
P
P
1
O
O
Scheme 1 Reagents and conditions: i (a) 2.2 equiv. BuLi–TMEDA, hexane,
r.t., 12 h; (b) 2.2 equiv. ClP(NEt2)2, THF, 278 °C, 67%; ii, excess HCl,
Et2O, 278 °C, 95%; iii, 2 equiv. (R)-(+)-BINOL, toluene, heat, 36 h,
90%
2
Scheme 2 Reagents and conditions: i, 1.95 equiv. (R)-(+)-BINOL, THF,
heat, 48 h, (70–85%)
Chem. Commun., 1998
2077

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CH3
processes, metals other than rhodium constituting further
possibilities.
H2, [RhI]
CO2Me
CO2Me
MeO2C
MeO2C
MeO2C
MeO2C
*
3
5
4
Notes and References
E-mail: reetz@scmpi-muelheim.mpg.de
CH3
O
O
.
‡ Crystal data for 1: C50H32FeO4P2 C7H8, Mr = 906.7, orange–brown plate,
H2, [RhI]
crystal size 0.08 3 0.59 3 0.66 mm, a = 9.7235(3), b = 16.5610(4), c =
N
Me
N
Me
*
H
27.5239(7) Å, b = 97.765(1)°, U = 4391.6(2) Å3, T = 100 K, monoclinic,
H
space group P21 (no. 4), Z = 4, Dc = 1.37 g cm23, m = 0.47 mm21
.
6
Siemens SMART diffractometer, Mo-Ka X-radiation, l = 0.71073 Å.
39615 measured reflections, analytical absorption correction (Tmin 0.7343,
Tmax 0.9626), 15179 unique, 11532 observed [I > 2.0s(Fo2)]. The structure
was solved by direct methods (SHELXS-97) and refined by full-matrix
least-squares (SHELXL-97) on F2 for all data (C atoms of toluene solvate,
isotropic) with Chebyshev weights to R = 0.089 (obs.), wR = 0.232 (all
data), absolute stereochemistry determined [Flack parameter 0.00(3)], S =
hydrogenation, namely itaconic acid dimethyl ester 3 and
2-acetamido methyl acrylate 5, leading to the products 4 and 6,
respectively. The results of the hydrogenation experiments with
formation of the R-configurated products 4 and 6 are remark-
able in several ways (Table 1).
1.17, H atoms riding, max. shift/error 0.001, residual rmax = 1.039 e Å23
.
CCDC 182/964.
Table 1 Enantioselective hydrogenation of dimethyl itaconate (3) and
2-acetamido methyl acrylate (5)a
1 See for example: R. Noyori, Asymmetric Catalysis in Organic Synthesis,
Wiley, New York, 1994, 1st edn. Catalytic Asymmetric Synthesis, ed. I.
Ojima, VCH, New York, 1993.
Entry
Ligand
Substrate
S/Cd
Yield (%)e ee (%)e
2 Recent examples are: M. J. Burk, J. E. Feaster, W. A. Nugent and R. L.
Harlow, J. Am. Chem. Soc., 1993, 115, 10125; A. S. C. Chan, W. Hu,
C.-C. Pai and C.-P. Lau, J. Am. Chem. Soc., 1997, 119, 9570; P. J. Pye,
K. Rossen, R. A. Reamer, N. N. Tsou, R. P. Volante and P. J. Reider,
J. Am. Chem. Soc., 1997, 119, 6207; G. Zhu, P. Cao, Q. Jiang and X.
Zhang, J. Am. Chem. Soc., 1997, 119, 1799; V. Enev, C. L. J. Ewers, M.
Harre, K. Nickisch and J. T. Mohr, J. Org. Chem., 1997, 62, 7092; T. V.
RajanBabu, T. A. Ayers, G. A. Halliday, K. K. You and J. C. Calabrese,
J. Org. Chem., 1997, 62, 6012; G. Zhu and X. Zhang, J. Org. Chem.,
1998, 63, 3133; F.-Y. Zhang, C.-C. Pai and A. S. C. Chan, J. Am. Chem.
Soc., 1998, 120, 5808; Q. Jiang, Y. Jiang, D. Xiao, P. Cao and X. Zhang,
Angew. Chem., 1998, 110, 1203; Angew. Chem., Int. Ed. Engl., 1998, 37,
1100; H. Doucet and J. M. Brown, Tetrahedron: Asymmetry, 1997, 8,
3775; C. Pasquier, S. Naili, L. Pelinski, J. Brocard, A. Mortraux and J.
Agbassou, Tetrahedron: Asymmetry, 1998, 9 193.
1
1
1
1
2
2
1
2
3
3
3
3
3
5
5
1000
2000
5380
1000
2000
1000
1000
100
100
100
100
100
100
100
> 99.5
> 99.5
> 99.5
2
3b
4
5
6c
7c
97–99
97–99
99.5
90
a Hydrogenations were carried out under the following general conditions:
1.3 bar H2, dichloromethane, r.t., 20 h, c(substrate) = 0.1 mol l21, catalysts
prepared in situ with Lig/Rh = 1.1 (4 runs each). b Using preformed (R,R)-
(1)Rh(cod)BF4. c Lig/Rh = 1.0. d Substrate to catalyst ratio. e Determined
by GC analysis.
3 I. E. Nifantev, L. F. Manzhukova, M. Y. Antipin, Y. T. Struchkov and
E. E. Nifant’ev, Russ. J. Gen. Chem., 1995, 65, 682; recent examples of
chiral monophosphinates: D. Haag, J. Runsink and H. D. Scharf,
Organometallics, 1998, 17, 398; J. Sakaki, W. B. Schweizer and D.
Seebach, Helv. Chim. Acta, 1993, 76, 2654.
In the case of substrate 3 both catalysts afford essentially
enantiomerically pure product 4. However, in the hydro-
genation of 5 pronounced differences in enantioselectivity were
observed (Table 1). Thus, the ferrocene-based catalyst (R,R)-
(1)Rh(cod)BF4 leads to complete enantioselectivity for both
4 M. T. Reetz and A. Gosberg, patent applied for 1998.
5 Enantiomerically pure (R)- and (S)-BINOL are commercially available
from Kankyo Kagaku Center (Japan) at a price of about $1300 per
kilo.
6 We thank A. Meiswinkel for performing some of the experiments.
7 R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc., 1971, 93, 2397.
substrates (ee
> 99.5%). Although experiments directed
towards elucidating mechanistic and structural aspects need to
be carried out, the present study shows that catalyst (R,R)-
(1)Rh(cod)BF4 is not only readily accessible, but also highly
effective. It remains to be seen how well ligand 1 performs in
other hydrogenation reactions and in C–C bond forming
Received in Cambridge, UK, 16th July 1998; 8/05524F
2078
Chem. Commun., 1998