Helical Chirality Control of Palladium Complexes That Bear a Tetrakis(phosphanyl)terphenyl Ligand: Application as Asymmetric Lewis Acid Catalysts

Article abstract of DOI:10.1002/anie.200352300

Both axial and helical chirality are generated by complexation of the novel tetrakis(phosphanyl)terphenyl (tetraphos) ligand to a metal center (see picture). The tropos nature of the ligand was confirmed, and enantiopure Pd complexes controlled by diamines were applied as asymmetric Lewis acid catalysts in carbonyl-ene reactions.

Full text of DOI:10.1002/anie.200352300

Communications
Atropisomerism and Helicity
Helical Chirality Control of Palladium Complexes
That Bear a Tetrakis(phosphanyl)terphenyl
Ligand: Application as Asymmetric Lewis Acid
Catalysts**
Kohsuke Aikawa and Koichi Mikami*
Helical structures (e.g. classical helicenes[1]) have attracted
much attention for quite some time. The helical chirality
stems from the nonplanar structure and, therefore, we
designed the novel 2,4’,6’,2’’-tetrakis(diphenylphosphanyl)-
[1,1’;3’,1’’]terphenyl (tetraphos) ligand model, which exhibits
nonplanar helicity when complexed to metal centers
(Scheme 1).
[*] Prof. Dr. K. Mikami, 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
[**] We are grateful to Dr. Kenji Yoza and Mr. Kazuyoshi Kitajima in the
Nippon Bruker Co. for X-ray crystallographic analysis. K. Aikawa is
grateful to the Japan Society for the Promotion of Science for Young
Scientists for a research fellowship.
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DOI: 10.1002/anie.200352300
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Scheme 1. Tropos nature of the tetraphos–M complex.
Many chiral phosphane ligands have been reported to
induce high enantioselectivity in catalytic asymmetric reac-
tions,[2] and much attention has been devoted to their
syntheses. However, the synthesis and resolution of these
chiral P ligands are difficult because of racemization at higher
temperature.[3] We have reported the chirally flexible (tropos,
meaning turn in Greek) biphep ligand, which has a biphenyl
backbone[4] whose axial chirality can be controlled by chiral
controllers without resolution. Herein we report a further
advanced strategy that employs the tropos nature of the
designed tetraphos ligand to generate helicity upon complex-
ation with a metal and its application as an asymmetric Lewis
acid catalyst.
The tetraphos–Pd complex was quantitatively obtained
from the tetraphos ligand and 2 equivalents of [PdCl2(cod)]
(cod = cycloocta-1,5-diene) in CH2Cl2. Recrystallization from
CH2Cl2/MeOH gave single crystals, which allowed the 3D
structure of this complex to be clarified by X-ray crystallo-
graphic analysis (Figure 1a).[5] The complex has two
biphep–Pd units in which the phenyl groups adopt either
axial or equatorial orientations (Figure 1b). The biphep
portions of the tetraphos–Pd complex are quite similar not
only to the biphep–Pd complex but also to many binap–metal
complexes.[4d,6] In Figure 1, the axial chirality is S,S and the
helical chirality is P (plus), that is, clockwise around the helix
axis. The meso diastereomer complex, which would have S,R
axial chirality, was not obtained, apparently as a consequence
of the sterically demanding diphenylphosphane groups. Once
the axial chirality (S or R) is determined in one biphenyl unit,
the axial and helical chirality in the other unit is inevitably
fixed as S,P or R,M, respectively.
Figure 1. a) X-ray crystal structure of the tetraphos–Pd complex.
Selected bond lengths [Š]: Pd-P1 2.241(2), Pd-P2 2.261(2), Pd-Cl1
2.369(3), Pd-Cl2 2.347(2), selected bond angles [8]: P1-Pd-P2 93.67(3),
Cl1-Pd-Cl2 91.42(4). b) tetraphos–Pd complex with two biphep units
c) Helical chirality of the tetraphos–Pd complex.
808C within a shorter period of time (Scheme 3).[4d] It was
therefore clear that tetraphos–Pd complex 1 changes its
conformation much less readily than biphep–Pd complex 3.
Next, complexation of (Æ )-1 and 2 equivalents of (S)-
dabn also gave a diastereomeric mixture in a nonselective
manner ((P,S,S)-5/(M,R,R)-5 = 1:1) (Scheme 4).[9] However,
the diastereomeric mixture was converted into the single
diastereomer, even at room temperature for 120 h (or 808C
for 2 h). While the configuration of 1 was retained at room
temperature as shown in Scheme 2, the dabn complex 5
isomerized under the same conditions. Therefore, the flexi-
bility of tetraphos–Pd complex 1 can be increased by
association of the chiral controller dabn.
3,3’-Dimethyl-2,2’-diamino-1,1’-binaphthyl
(dm-dabn)
In the combination of biphep–Pd complex (Æ )-3 and
1 equivalent of (S)-dabn, isomerization of (R)-6 took place at
808C for 8 h to give the single S diastereomer, but not at room
temperature (Scheme 5).[4e] It can therefore be seen that
tetraphos–Pd complex 1 is more rigid than biphep–Pd
complex 3 by itself, but more flexible with a chiral diamine
controller.
These enantiopure tetraphos–Pd complexes obtained
through isomerization by diamines were employed as asym-
metric catalysts for carbon–carbon bond-forming reactions.
As a probe reaction, the carbonyl-ene reaction of methyl-
enecyclohexane (7) and ethyl glyoxylate (8) was examined
(Table 1).[2a,10] Enantiopure [(S)-binap–Pd–(S)-dabn] resulted
in moderate enantioselectivity and yield (Table 1, entry 1).
The [biphep–Pd–dabn] complex led to a decrease in enantio-
with sterically demanding methyl substituents in the 3,3’-
positions of 2,2’-diamino-1,1’-binaphthyl (dabn)[4b,7] was used
for the enantiomeric resolution and isomerization of the
tetraphos–Pd complex (Æ )-1 complex (Scheme 2). Complex
(Æ )-1 was treated with 2 equivalents of (S)-dm-dabn to give
the single diastereomer (P,S,S)-2, along with remaining
(M,R,R)-1 and (S)-dm-dabn. Importantly, no isomerization
of the remaining (M,R,R)-1 to (P,S,S)-1 was observed to
complex with (S)-dm-dabn at room temperature for more
than 120 h. After heating at 808C for 12 h, however, only
(P,S,S)-2 was obtained through isomerization of (M,R,R)-1.[8]
Under the same conditions, the combination of biphep–Pd
complex (Æ )-3 and 1 equivalent of (S)-dm-dabn gave the
single diastereomer (S)-4 through isomerization of (R)-3 at
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Scheme 2. Chirally stable tetraphos–Pd complex at room temperature and its isomerization to the diastereopure form at 808C.
Scheme 5. Isomerization of the biphep–Pd complex by (S)-dabn.
Scheme 3. Isomerization of the biphep–Pd complex by (S)-dm-dabn.
selectivity (Table 1, entry 2). However, a higher yield and
enantioselectivity was observed with [tetraphos–Pd–dabn]
((P,S,S)-5) than with [biphep–Pd–dabn] ((S)-6) and [(S)-
binap–Pd–(S)-dabn] (Table 1, entry 3). These results unam-
biguously prove the efficiency of tropos but sterically
demanding and hence relatively rigid helical tetraphos
ligands. The complex (P,S,S)-2, which bears dm-dabn, gave a
slightly lower enantioselectivity (Table 1, entry 4).
In summary, we have succeeded in control over not only
axial but also helical chirality of Pd complexes that bear the
tropos tetraphos ligand. Significantly, the flexibility of the
tetraphos–Pd complex can be increased by association of the
chiral controller dabn. The tetraphos–Pd complexes, through
isomerization into single enantiomers, lead to a higher
enantioselectivity and yield, as integrated asymmetric cata-
lysts for the carbon–carbon bond-forming reaction.
Scheme 4. Isomerization of the [tetraphos–Pd–dabn] complex in diastereopure form at room temperature.
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Chemie
284– 288; d) K. Mikami, K. Aikawa, Y. Yusa, M. Hatano, Org.
Table 1: Asymmetric carbonyl-ene reaction by Pd catalysts with chirally
flexible phosphane ligands.
Lett. 2002, 4, 91 – 94; e) K. Mikami, K. Aikawa, Y. Yusa, Org.
Lett. 2002, 4, 95 – 97; f) K. Mikami, K. Aikawa, Y. Yusa, J. J.
Jodry, M. Yamanaka, Synlett 2002, 10, 1561 – 1578; for similar
work on the biphep ligand, see: g) M. D. Tudor, J. J. Becker, P. S.
White, M. R. Gagne, Organometallics 2000, 19, 4376 – 4484;
h) J. J. Becker, P. S. White, M. R. Gagne, J.Am.Chem.Soc. 2001,
123, 9478 – 9479.
Entry PdII catalyst[a]
[mol%] ee [%][b] Yield [%][c]
1
2
3
4
[(S)-binap–Pd–(S)-dabn]
5.0
5.0
2.5
2.5
78
69
81
76
80
78
86
85
[5] X-ray crystallographic analysis was performed with a Bruker
SMART 1000 diffractometer (graphite monochromator, MoKa
radiation, l = 0.71073 Š) at 299 K. Crystal data for [Pd2Cl4(te-
traphos)]·2MeOH (C66H58O2P4Cl4Pd2): monoclinic, C2/C, a =
18.796(19) Š, b = 12.631(13) Š, c = 27.10(3) Š, a = 908, b =
[(S)-biphep–Pd–(S)-dabn]
[(P,S,S)-tetraphos–Pd–(S)-dabn]
[(P,S,S)-tetraphos–Pd–(S)-dm-
dabn]
105.748(18)8, g = 908, V= 4249.9(3) Š3, Z = 4, 1calcd
=
[a] All reactions were examined through isomerization by enantiopure
diamines except for (S)-binap. [b] Enantiopurity was determined by chiral
GC analysis on a CP-Cyclodextrin-b-2,3,6-M-19 column. [c] Yield of
isolated product.
1.486 gcmÀ3, crystal dimensions 0.37 ” 0.11 ” 0.05 mm3, range
for data collection 2qmax = 54.968, reflections collected 18277,
independent reflections 6783 (Rint = 0.0382). The structures were
solved by direct methods (SHELXL-97); the final cycle of full-
matrix least-squares on F2 was based on 6783 observed
reflections (I > 2s(I)) and 390 variable parameters, and con-
verged to R = 0.038, Rw = 0.080, and goodness of fit = 1.001.
CCDC-213825 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk). 1H NMR (300 MHz, CD2Cl2): d = 6.24–6.28 (m,
3H), 6.77 (dd, J = 8.1, 11.7 Hz, 2H), 6.92 (t, J = 8.1 Hz, 2H),
7.13–7.18 (m, 2H), 7.23–7.62 (m, 35H), 7.77–7.83 (m, 2H),
7.89 ppm (dd, J = 7.5, 12.6 Hz, 4H); 31P NMR (162 MHz,
CD2Cl2): d = 25.0 (s, 2P), 27.8 ppm (s, 2P).
Experimental Section
Dichloroethane (2.0 mL) was added to a mixture of (Æ )-1 (22.9 mg,
0.01 mmol) and (S)-dabn (6.0 mg, 0.021 mmol) in a 10-mL Schlenk
tube under an argon atmosphere, and the reaction mixture was stirred
at 808C for 2 h. After concentration under reduced pressure, the flask
was replenished with argon, and dichloromethane (2.0 mL), ethyl
glyoxylate (8) (30.6 mg, 0.6 mmol), and methylenecyclohexane (7)
(48 mL, 0.4mmol) was added sequentially to the solution. The
reaction mixture was stirred at room temperature for 24h, directly
loaded onto a silica-gel column, and eluted with hexane/EtOAc (3:1)
to afford (R)-9 in 86% yield as a colorless oil. The enantiomeric
excess was determined by chiral GC analysis; GC (column: CP-
Cyclodextrin-b-2,3,6-M-19, i.d. 0.25 mm ” 25 m, CHROMPACK; car-
rier gas: nitrogen, 75 kPa; column temperature: 1308C; injection and
detection temperature: 1608C; split ratio: 100:1), tR (R isomer):
31.3 min, tR (S isomer): 32.4min; 1H NMR (300 MHz, CDCl3): d =
1.26 (t, J = 6.9 Hz, 3H), 1.48–1.63 (m, 4H), 1.93–1.99 (m, 4H), 2.24
(dd, J = 7.8, 14.1 Hz, 1H), 2.40 (dd, J = 4.5, 14.1 Hz, 1H), 2.60 (d, J =
6.3 Hz, 1H), 4.19 (q, J = 6.9 Hz, 2H), 5.50 ppm (m, 1H); 13C NMR
(75 MHz, CDCl3): d = 14.4, 22.4, 23.0, 25.5, 28.7, 43.5, 61.8, 69.6, 126.0,
133.7, 175.9 ppm.
[6] a) M. Ogasawara, K. Yoshida, T. Hayashi, Organometallics 2000,
19, 1567 – 1571; b) F. Ozawa, A. Kubo, Y. Matsumoto, T.
Hayashi, Organometallics 1993, 12, 4188 – 4196.
[7] a) K. Mikami, T. Korenaga, T. Ohkuma, R. Noyori, Angew.
Chem. 2000, 112, 3854– 3857; Angew.Chem.Int.Ed. 2000, 39,
3707 – 3710; b) K. Mikami, Y. Yusa, T. Korenaga, Org.Lett. 2002,
4, 1643 – 1645.
[8] (Æ )-1: 31P NMR (162 MHz, CD2Cl2): d = 25.9 (s, 2P), 29.3 ppm
(s, 2P). (P,S,S)-2: 31P NMR (162 MHz, CD2Cl2): d = 22.0 (d,
J
P-P = 25.9 Hz, 2P), 25.7 ppm (d, JP-P = 25.9 Hz, 2P).
[9] (P,S,S)-5: 31P NMR (162 MHz, CD2Cl2): d = 24.8 (d, JP-P
=
Received: July 3, 2003 [Z52300]
19.9 Hz, 2P), 26.3 ppm (d,
JP-P = 19.9 Hz, 2P). (M,R,R)-5:
31P NMR (162 MHz, CD2Cl2): d = 22.7 (d, JP-P = 19.1 Hz, 2P),
28.2 ppm (d, JP-P = 19.1 Hz, 2P).
Keywords: chirality · ene reaction · helical structures ·
.
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Paras, T. Vojkovsky, J.Am.Chem.Soc. 2000, 122, 7936 – 7943.
isomerization · palladium
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