Enantiodiscrimination and enantiocontrol of neutral and cationic Pt II complexes bearing the Tropos biphep ligand: Application to asymmetric lewis acid catalysis

Article abstract of DOI:10.1002/anie.200502682

(Chemical Equation Presented) The stereochemically stable, enantiopure biphep-Pt complexes can be employed as an atropos asymmetric catalyst at 50°C or below. Both enantiomeric forms of this complex can be obtained by using either the chiral diamine (R)-dabn or its diamide (R)-dabnTf followed by chirally controlled formation of the single biphep-Pt enantiomer at higher (> 60°C) temperatures (see scheme; Tf=trifluoromethanesulfonate).

Full text of DOI:10.1002/anie.200502682

Angewandte
Chemie
Asymmetric Catalysts
DOI: 10.1002/anie.200502682
Enantiodiscrimination and Enantiocontrol of
Neutral and Cationic PtII Complexes Bearing the
Tropos Biphep Ligand: Application to
Asymmetric Lewis Acid Catalysis**
Koichi Mikami,* Hitomi Kakuno, and Kohsuke Aikawa
The development of asymmetric catalysts for organic reac-
tions is one of the most challenging subjects in modern science
and technology.[1] These catalysts are generally metal com-
plexes that bear chiral and atropisomeric ligands, which
require the enantioresolution and synthetic transformation of
a chiral pool (atropisomeric is from the Greek atropos, a
meaning not and tropos meaning turn).[2] However, we have
already demonstrated that chirally flexible (tropos) 2,2’-
bis(diphenylphosphanyl)biphenyl (biphep)[3] can act as a
chiral ligand for Ru,[4] Rh,[5] and Pd[6] complexes through
axial chirality control by chiral diamines. Therefore, the
biphep complexes of Ru, Rh, and Pd can provide high levels
of enantioselectivity in asymmetric hydrogenation, ene-type
cyclization, and hetero-Diels–Alder reactions, respectively.
Similarly, in the neutral biphep–Pt complex the chirality
[*] Prof. Dr. K. Mikami, H. Kakuno, Dr. K. Aikawa
Department of Applied Chemistry
Graduate School of Science and Engineering
Tokyo Institute of Technology
Ookayama, Meguro-ku, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-2776
E-mail: kmikami@o.cc.titech.ac.jp
[**] Biphep is 2,2’-bis(diphenylphosphanyl)biphenyl.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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control of the biphep moiety by chiral 1,1ꢀ-binaphthalene-
2,2ꢀ-diol (binol) was reported not high enough (95:5) and
hence recrystallization was essential to obtain the single
biphep–Pt diastereomer and enantiomer.[7a,b]
Herein, we report the complete chirality control of both
neutral and cationic biphep–Pt complexes without the need
for recrystallization and the application of this complex to
asymmetric Lewis acid catalysis (Scheme 1). Interestingly,
both enantiopure (S)- and (R)-biphep–Pt complexes can be
obtained quantitatively through enantiodiscrimination by
using
(R)-2,2’-bis(trifluoromethanesulfonylamino)-1,1’-
binaphthyl (dabnTf) and (R)-2,2’-diamino-1,1’-binapthyl
(dabn) with the same absolute configuration.
The highly effective enantiodiscriminating agent dabnTf
was first used for enantiomer-selective complexation[8] of the
neutral racemic biphep-PtCO3 complex 1 (Figure 1)[7a] with
complete enantiomer discrimination. Complexation of race-
mic complex 1 and (R)-dabnTf at a specified temperature was
monitored by NMR spectroscopy until product (S)/(R)-2 was
precipitated. A combination of racemic complex 1 with
1.0 equivalent of (R)-dabnTf in [D8]toluene gave the single
diastereomer (S,R)-2 along with the remaining (R)-1 and (R)-
dabnTf (0.5 equiv), which form a hydrogen bond between the
proton of the diamide and oxygen atom of the carbonate
complex (R)-1. In fact, treatment of [{(S)-binap}PtCO3] with
(R)-dabnTf (1.0 equiv) gave the (S,R) diastereomer complex,
but the combination of [{(R)-binap}PtCO3] with (R)-dabnTf
Figure 1. Temperature dependence of the isomerization of complex 1.
complex (S,R)-2 provided the enantiopure (S)-3 (>
99% ee) quantitatively (Scheme 2).[7b] Pure dabnTf
could also be recovered quantitatively. The enan-
tiomeric excess (% ee) of (S)-3 was determined by
1H and 31P NMR spectroscopies after complexation
with enantiopure (S,S)-diphenylethylenediamine
(dpen) and upon addition of 2.0 equivalents of
AgSbF6. In sharp contrast to the Pt complex, the Pd
complex was protonated with concentrated HCl in
dichloromethane even at 08C and the biphep
moiety was epimerized (24% ee) as biphep–Pd is
stereochemically less stable through decomplexa-
tion of the sterically demanding dabnTf. These
results show that the Pt center coordinates to the
biphep diphosphine ligand more strongly than the
Pd center.
Scheme 1. Complete chirality control of both neutral and cationic biphep–Pt complexes.
We also clarified the atropos nature of cationic
biphep–Pt complexes at room temperature. Chiral diamine
(1.0 equiv) did not.[8] There was no isomerization of the
remaining (R)-1 enantiomer, which did not form a complex
with (R)-dabnTf even at 508C after 120 h (Figure 1). There-
fore, these results imply that complex 1 is atropos below 508C.
At higher temperatures, isomerization of complex (R)-1
followed by complexation with remaining (R)-dabnTf was
observed. These results indicate that complex 1 is tropos at
higher (> 608C) temperatures.
After proving the atropos nature of biphep–Pt species
even at 508C, we tried to isolate the enantiopure biphep–
PtCl2 complex 3 without epimerization. Upon addition of
concentrated HCl in dichloromethane at room temperature,
Scheme 2. Quantitative isolation of enantiopure (S)-3.
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dabn was used for nonselective complexation and axial
chirality control of the racemic cationic complex 4 (Sche-
me 3).[6b] With 1.0 equivalent of (R)-dabn, complexation of
both enantiomers took place to afford a 1:1 ratio of a
Figure 2. Enantiomer discrimination of (R)-dabnTf and (R)-dabn.
Scheme 3. Complete chirality control of cationic biphep–Pt complex 4.
ization of the opposite enantiomer (R)-1. In the cationic
(S,R)-5 complex, there is strong repulsion between equatorial
phenyl group of biphep and hydrogen atom of chiral diamine
dabn. In contrast, there is no steric repulsion in the complex
(R,R)-5 (Figure 2b).
diastereomer mixture of (R,R)-5 and (S,R)-5 complexes. The
complex 5 bearing (R)-dabn did not isomerize at room
temperature over 3 days but did isomerize at 808C after 30 h
to afford exclusively the favorable (R,R)-5 (> 99% de). In the
Pd complex, the favorable (R,R) complex is obtained
exclusively after an even shorter period.[6b]
The enantiopure complex
3 thus obtained can be
employed as an atropos asymmetric catalyst (Scheme 5).[5–7]
The complex (R,R)-5 provided enantiopure (R)-3
(> 99% ee) quantitatively upon addition of aqueous HCl at
room temperature (Scheme 4); dabn could also be recovered
Scheme 4. Quantitative isolation of the enantiopure (R)-3.
Scheme 5. Enantiopure 3 as an atropos asymmetric Lewis acid catalyst.
quantitatively. The enantiomer excess of (R)-3 was deter-
mined by addition of enantiopure dpen under the same
conditions as those shown in Scheme 2. The opposite axial
chirality to that controlled by its amide (R)-dabnTf can be
controlled by (R)-dabn with the same absolute configuration.
The structure of racemic complex 3 was determined by X-
ray crystal structure analysis of a single crystal obtained from
dichloromethane/hexane solution.[9] Indeed, the orientations
of axial and equatorial phenyl groups in biphep portion were
quite similar to those of biphep–Pd complexes.[6,10] In the
enantiodiscrimination of the neutral (R,R)-2 complex, there is
strong steric repulsion between equatorial phenyl group of
biphep and the trifluoromethanesulfonyl substituent (Tf) of
chiral amide dabnTf (Figure 2a). In contrast, there is no steric
repulsion in complex (S,R)-2. As a result, (R)-dabnTf could
complex only with the single enantiomer (S)-1 after isomer-
Indeed, the hetero-Diels–Alder reaction of 1,3-cyclohexa-
diene with glyoxylate[11] was catalyzed by enantiopure (S)-3
(5 mol%) and AgSbF6 (11 mol%) as a highly efficient Lewis
acid catalyst. The hetero-Diels–Alder product was obtained
with high enantioselectivity (96% ee, endo:exo = 99:1) even
at room temperature (Scheme 5a). Use of enantiopure (R)-3
gave the hetero-Diels–Alder product with opposite absolute
configuration in equally high selectivity. The complex (S)-3
was obtained with (R)-dabnTf, and (R)-3 was obtained with
(R)-dabn. It was confirmed by addition of enantiopure dpen
that racemization of the biphep moiety did not occur during
the course of reaction. Additionally, enantiopure dicationic
complex 3 gave high chemical yields and high levels of
enantio- and E-selectivity in carbonyl-ene reactions with
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7259

Communications
[10] M. Ogasawara, K. Yoshida, T. Hayashi, Organometallics 2000,
19, 1567.
[11] a) M. Johannsen, K. A. Jørgensen, J. Org. Chem. 1995, 60, 5757;
b) M. Johannsen, K. A. Jørgensen, Tetrahedron 1996, 52, 7321;
trifluoropyruvate and with less reactive monosubstituted
olefins (Scheme 5b).[12]
In summary, we have proven that the racemic Pt
complexes even with tropos biphep ligands can be resolved
as the atropos complex even at 508C but converted at higher
temperatures into either enantiopure complex just by
exchanging (R)-dabn and (R)-dabnTf with identical absolute
configuration. In carbon–carbon bond-forming reactions, the
enantiopure biphep–Pt complexes thus obtained can be used
as the atropos catalysts to give high enantioselectivity.
c) S. Oi, K. Kashiwagi, E. Terada, K. Ohuchi,
Y Inoue,
Tetrahedron Lett. 1996, 37, 6351; d) S. Oi, E. Terada, K.
Ohuchi, T. Kato, Y. Tachibana, Y. Inoue, J. Org. Chem. 1999,
64, 8660.
[12] a) K. Aikawa, S. Kainuma, M. Hatano, K. Mikami, Tetrahedron
Lett. 2004, 45, 183; b) K. Mikami, K. Aikawa, S. Kainuma, Y.
Kawakami, T. Saito, N. Sayo, H. Kumobayashi, Tetrahedron:
Asymmetry 2004, 15, 3885.
Received: July 30, 2005
Published online: October 18, 2005
Keywords: chirality · Diels–Alder reaction · enantioselectivity ·
.
ene reaction · platinum
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[2] K. Mikami, K. Aikawa, Y. Yusa, J. J. Jodry, M. Yamanaka,
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[7] For biphep-Pt complex: a) M. D. Tudor, J. J. Becker, P. S. White,
M. R. GagnØ, Organometallics 2000, 19, 4376; b) J. J. Becker,
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[9] Crystal data for 3: formula C36H28Cl2P2Pt·CH2Cl2, monoclinic,
space group P21/n, a = 10.406(4), b = 17.887(6), c = 18.695(7) Š,
b = 94.096(5)8, V= 3470.8(21) Š3, Z = 4, D = 1.672 gcmÀ3, and
m = 44.52 cmÀ1. All measurements were made on a Rigaku
Saturn CCD area detector with graphite monochromated MoKa
radiation (l = 0.71070 Š) at 193 K and the structure was solved
by direct methods (SIR92). Of the 32323 reflections that were
collected, 9997 were unique (Rint = 0.082). R = 0.080, Rw = 0.171,
goodness of fit = 0.993, and shift/error= 0.008. CCDC 277173
contains the supplementary crystallographic 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.
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