Achiral benzophenone ligand-rhodium complex with chiral diamine activator for high enantiocontrol in asymmetric transfer hydrogenation

Article abstract of DOI:10.1039/b517585b

The chirality of an achiral benzophenone-based rhodium complex can be controlled by chiral diamines to afford significantly high enantioselectivity in the catalytic asymmetric transfer hydrogenation of ketones (up to 99% ee, 99% yield). The Royal Society

Full text of DOI:10.1039/b517585b

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Achiral benzophenone ligand–rhodium complex with chiral diamine
activator for high enantiocontrol in asymmetric transfer hydrogenation{
Koichi Mikami,* Kazuki Wakabayashi, Yukinori Yusa and Kohsuke Aikawa
Received (in Cambridge, UK) 13th December 2005, Accepted 18th April 2006
First published as an Advance Article on the web 3rd May 2006
DOI: 10.1039/b517585b
The chirality of an achiral benzophenone-based rhodium
complex can be controlled by chiral diamines to afford
significantly high enantioselectivity in the catalytic asymmetric
transfer hydrogenation of ketones (up to 99% ee, 99% yield).
The control of the chirality of DPBP–Rh complex 1 by the
addition of (S,S)-DPEN led to the formation of diastereomerically
6
pure complex [Rh(dpbp){(S,S)-dpen}]SbF 3 (Eq. 3). This
isomerization was found to be significantly fast. Upon addition
of (S,S)-DPEN at room temperature, Rh(dpbp)[(S,S)-dpen] 3 was
10
Asymmetric catalysis is one of the most important subjects in
1
modern science and technology. Asymmetric catalysts are
instantaneously obtained in diastereomerically pure form.
generally metal complexes with enantiopure ligands. We report,
however, the development of novel achiral benzophenone ligands
2
for rhodium catalysts in transfer hydrogenation of ketones with a
ð3Þ
significantly high level of enantioselectivity and yields (up to 99%
ee, 99% yield). Rhodium catalyzed asymmetric transfer hydro-
3
genation has never been described in an efficient format.
Two possible mechanisms can be envisaged for isomerization of
DPBP–Rh complex 3. Path (a) involves internal rotation between
the two phenyl rings, and path (b) needs dissociation of one
phosphine–metal (P–M) bond followed by rotation back to the
enantiomeric DPBP–metal complex (Eq. 4). In the case of
BIPHEP–metal complex, the single diastereomer was obtained at
Metal complexes are generally employed as asymmetric catalysts
4
bearing chiral and atropisomeric ligands (originating from atropos
5
6
in Greek) such as BINAP in enantiopure forms. However,
achiral and tropos ligands, such as benzophenone-derived dipho-
sphine DPBP (2,29-bis(diphenylphosphino)benzophenone), could
7,8
11
be controlled into a single enantiomeric conformation and hence
9
replace the enantiopure and atropos ligands (Eq. 1).
80 uC over 5 h via path (b). In sharp contrast, DPBP–metal
complex was instantaneously obtained in a diastereomerically pure
12
form at room temperature via path (a).
ð1Þ
(
4)
Upon addition of DPBP to [Rh(cod)
was obtained in a pure form (Eq. 2). Not only [Rh(cod)
but also [Rh(nbd) ]SbF gave DPBP–Rh complexes in pure forms.
2
]SbF
6
, DPBP–Rh complex
1
2
]SbF
6
2
6
ð2Þ
The advantage of ‘‘achiral’’ but tropos benzophenone ligand
over the enantiopure atropos BINAP for asymmetric catalysis can
be seen in transfer hydrogenation of 19-acetonaphthone (Table 2).§
At the outset, the reaction conditions were optimized using the
Unfortunately, a single crystal of [Rh(dpbp)(cod)]SbF6 or
Rh(dpbp)(nbd)]SbF could not be obtained, but PtCl (dpbp) 2
was obtained. Indeed, X-ray analysis of a single crystal of
PtCl (dpbp) 2{ clearly shows the chiral conformation of the
[
6
2
13
BINAP catalysts. BINAP–Rh complexes were examined by
changing the chirality and aliphatic or aromatic nature of diamines
complexed to give single diastereomers in all cases (Table 1).
Among diamines examined, 1,2-diphenylethylenediamine
2
benzophenone skeleton of the DPBP ligand, which adopts a helical
propeller conformation (Fig. 1).
(DPEN) afforded the highest enantioselectivity, though at a
Department of Applied Chemistry, Tokyo Institute of Technology,
Tokyo, 152-8552, Japan. E-mail: kmikami@o.cc.titech.ac.jp;
Fax: +81 3 5734 2776; Tel: +81 3 5734 2142
moderate level (71% ee, 96%). It should be noted here that
the matched enantiomer of DPEN for acetophenone is changed
1
4
from RuCl [(R)-binap][(R,R)-dpen] to [Rh{(R)-binap}{(S,S)-
2
{
Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b517585b
6
dpen}]SbF .
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Chem. Commun., 2006, 2365–2367 | 2365

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Table 2 Transfer hydrogenation catalyzed by DPBP–Rh complex
Product
Entry
Substrate
Diphosphine
conv. (%)
ee (%)
a
1
(R)-BINAP
DPBP
98
. 99
72
99
2
a
3
(R)-BINAP
DPBP
61
99
57
99
4
2
Fig. 1 3D X-ray structure of PtCl (dpbp) 2.
With the best diamine for [Rh{(R)-binap}{(S,S)-dpen}]SbF
the reaction temperature and co-solvent were examined for
Rh(dpbp){(S,S)-dpen}]SbF6. 1,2-Dichloroethane rather than
6
,
a
5
(R)-BINAP
DPBP
97
95
68
91
b
6
[
dichloromethane was found to be the best co-solvent and room
temperature was preferable to give the highest enantioselectivity
(
see Table S1 in supporting information).
a
7
(R)-BINAP
DPBP
96
97
71
89
c
8
Virtually complete (99% ee) enantioselectivities were thus
obtained for 19-acetonaphthone and o-substituted acetophenone
under the optimized reaction conditions (Table 2). The benzophe-
none catalyst 3 gave (R)-1-(19-naphthyl)ethanol and (R)-o-methyl-
phenethyl alcohol with 99% ee in 99% yield, which is significantly
higher than the 72% ee and 57% ee respectively obtained with
a
b
(few drops). For 48 h. With (CH
c
At 60 uC, with CH Cl
2
2-propanol : (CH Cl) = 18 : 1).
2
2
2
Cl)
2
(
2
15
6
.
enantiopure
[Rh{(R)-binap}{(S,S)-dpen}]SbF
[Rh{(R)-
16
biphep}{(S,S)-dpen}]SbF catalyst also gave (R)-1-phenyletha-
6
We are grateful to Dr K. Yoza of Nippon Bruker AXS K.K. for
nol with only 59% ee in 12% yield.
X-ray analysis.
In summary, we have succeeded in the development of achiral
benzophenone ligands for rhodium catalysts in transfer hydro-
genation of ketones with virtually perfect enantiocontrol. The
enantiopure benzophenone complex affords much higher
enantioselectivity than those attained by the enantiopure BINAP
counterpart.
Notes and references
{ Crystal data for PtCl
one of these dichloromethanes is disordered over two sites, Triclinic, space
˚
group P-1(#2), a = 11.1806(4) A, b = 11.2602(4) A, c = 16.3715(5) A, a =
2 2 2 2 2
(dpbp) (2): formula C37H28Cl OP Pt?2(CH Cl ),
˚
˚
3
˚
8
3.78(10)u, b = 85.79(10)u, c = 67.10(10)u, V = 1886.46(11) A , Z = 2 and
3
D = 1.736 Mg/m . X-ray diffraction data were collected on a Bruker
˚
APEX2 with graphite-monochromated Mo-Ka (l = 0.7107 A) at 100 K
Table 1 Transfer hydrogenation catalyzed by BINAP–Rh/diamines
and the structure was solved by direct methods (SHELXL-97). The final
cycle of full-matrix least-squares on F2 was based on 8833 observed
reflections and converged to R = 0.0204 and Rw = 0.0513. Crystallographic
data (excluding structure factors) for the structure reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 293556. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b517585b).
§
Typical procedure for Rh-catalyzed hydrogenation of ketones. Table 2,
entry 3: To a solution of [Rh(dpbp){(S,S)-dpen}]SbF (11.0 mg, 0.01 mmol)
in 2-propanol (3.6 ml) was added 0.1 M of BuOK/2-propanol (0.6 ml,
.06 mmol) at room temperature under argon atmosphere in a Schlenk
6
t
0
tube. After being stirred for 20 min at room temperature, the reaction
mixture was treated with o-methylacetophenone (43 ml, 0.33 mmol) and
stirred for 24 h at room temperature under Ar atmosphere. After the
reaction mixture was concentrated under reduced pressure, the residue was
filtered through a short column of silica gel (hexane/ethyl acetate = 1/3) to
give alcohol products.
Product
Entry
Diamine
conv. (%)
ee (%)
1
2
3
4
5
6
(R)-DABN
(S)-DABN
(R,R)-DACH
(S,S)-DACH
(R,R)-DPEN
(S,S)-DPEN
58
73
37
81
93
96
. 99
12 (S)
0
10 (S)
57 (R)
0
1 (a) E. N. Jacobsen, A. Pfaltz and H. Yamamoto, Comprehensive
Asymmetric Catalysis, Vol. 1–3, Springer, Berlin, 1999; (b) Transition
Metals for Organic Synthesis, Eds. M. Beller and C. Bolm, VCH,
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Synthesis, Wiley, New York, 1994; (d) H. Brunner and W. Zettlmeier,
Handbook of Enantioselective Catalysis, VCH, Weinheim, 1993; (e)
71 (R)
80 (S)
a
cf.
a
RuCl
2
[(R)-tol-binap][(R,R)-dpen]
Ref. 14.
2
366 | Chem. Commun., 2006, 2365–2367
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Catalytic Asymmetric Synthesis, Vol. I and II, Ed. I. Ojima, VCH,
New York, 1993, p. 2000.
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H. Kumobayashi, T. Taketomi, H. Takaya, A. Miyashita, R. Noyori
and S. Otsuka, J. Am. Chem. Soc., 1984, 106, 5208; (b) DIPAMP–Rh
catalyst: C. Landis and J. Halpern, J. Am. Chem. Soc., 1987,
8 Enantiocontrol of NUPHOS: (a) S. Doherty, C. R. Newman,
R. K. Rath, H.-K. Luo, M. Nieuwenhuyzen and J. G. Knight, Org.
Lett., 2003, 5, 3863; (b) S. Doherty, C. R. Newman, R. K. Rath,
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2
3
1
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998, 1199; (b) K. Murata and T. Ikariya, J. Org. Chem., 1999, 64, 4447;
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1
(
T. Ikariya, Chirality, 2000, 12, 383; (d) X. Wu, D. Vinci, T. Ikariya and
J. Xiao, Chem. Commun., 2005, 4447.
4
5
R. Kuhn, Molekulare Stereochemie, Ed. K. Freudenberg, Franz &
Deutike, Leipzig–Wien, 1933, pp. 803–824.
The word atropos consists of ‘‘a’’ meaning ‘‘not’’ and ‘‘tropos’’ meaning
10 Rh–DPBP complex was formed in diastereomerically pure form by the
addition of NOBIN (29-amino-1,19-binaphth-2-ol).
11 (a) T. Korenaga, K. Aikawa, M. Terada, S. Kawauchi and K. Mikami,
Adv. Synth. Catal., 2001, 343, 284; (b) M. Yamanaka and K. Mikami,
Organometallics, 2002, 21, 5847.
‘‘turn’’ in Greek. Therefore, the chirally rigid or flexible nature of a
ligand can be called atropos or tropos, respectively. K. Mikami,
K. Aikawa, Y. Yusa, J. J. Jodry and M. Yamanaka, Synlett, 2002, 1561.
(a) BINAP: R. Noyori and H. Takaya, Acc. Chem. Res., 1990, 23, 345;
12 DFT calculations on the isomerization process of DPBP–metal
complexes will be reported elsewhere.
6
7
(b) DET (diethyl tartrate): T. Katsuki and K. B. Sharpless, J. Am.
13 Noyori reported Ru–binap/diamine systems for asymmetric hydrogena-
tion of ketones. However, Rh–binap/diamine systems have never been
reported for asymmetric transfer hydrogenation of ketones.
14 R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 2001, 40, 40.
15 In this asymmetric transfer hydrogenation, chiral phosphine parts quite
affect the yields and enantiomeric excesses of the products. In transfer
Chem. Soc., 1980, 102, 5974; (c) DIOP: D. Sinou and H. B. Kagan,
J. Organomet. Chem., 1976, 114, 325.
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T. Ohkuma, T. Pham and R. Noyori, Angew. Chem., Int. Ed., 1999, 38,
495; (b) K. Mikami, K. Aikawa, Y. Yusa and M. Hatano, Org. Lett.,
2
002, 4, 91; (c) M. D. Tudor, J. J. Becker, P. S. White and M. R. Gagn e´ ,
hydrogenation, [Rh(cod){(S,S)-dpen}]SbF
ethanol with only 43% ee in 7% yield.
16 [Rh{(R)-BIPHEP}{(S,S)-dpen}]SbF
6
gave (R)-1-(19-naphthyl)-
Organometallics, 2000, 19, 4376; (d) J. J. Becker, P. S. White and
M. R. Gagn e´ , J. Am. Chem. Soc., 2001, 123, 9478; (e) K. Mikami,
K. Aikawa and Y. Yusa, Org. Lett., 2002, 4, 95; (f) K. Mikami,
S. Kataoka, Y. Yusa and K. Aikawa, Org. Lett., 2004, 6, 3699.
6
was obtained by the addition of
(S,S)-DPEN to the diastereomerically pure [Rh{(R)-BIPHEP}-{(R)-
dabn}]SbF complex in methanol.
6
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Chem. Commun., 2006, 2365–2367 | 2367