Dual chirality control of palladium(II) complexes bearing tropos biphenyl diamine ligands

Article abstract of DOI:10.1039/b510910h

Axial and center chirality of Pd complexes with tropos biphenyl secondary diamine ligands is shown to be controlled by chiral amide (R)-DABNTf, which can efficiently discriminate between two enantiomeric Pd complexes. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510910h

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Dual chirality control of palladium(II) complexes bearing tropos
biphenyl diamine ligands{
Kohsuke Aikawa and Koichi Mikami*
Received (in Cambridge, UK) 2nd August 2005, Accepted 3rd October 2005
First published as an Advance Article on the web 20th October 2005
DOI: 10.1039/b510910h
the complexes 2a–d indicated the single diastereomer [R/R,R and
S/S,S or R/S,S and S/R,R (axial chirality/center chirality,
respectively)]. The combination of PtCl2(CH3CN)2 complex and
1.0 equiv. of diamine 1a in dichloroethane at 80 uC resulted in the
desired diamine Pt complex, which is very similar to the Pd
Axial and center chirality of Pd complexes with tropos biphenyl
secondary diamine ligands is shown to be controlled by chiral
amide (R)-DABNTf, which can efficiently discriminate between
two enantiomeric Pd complexes.
1
The development of asymmetric catalysts for organic reactions is
one of the most challenging subjects in modern science and
technology.1 These catalysts are generally metal complexes bearing
chiral and atropisomeric ligands such as BINAP. Through
enantio-resolution and synthetic transformation, many enantio-
pure atropisomeric (atropos in Greek; a 5 not, tropos 5 turn)2
ligands are synthesized and used in catalytic asymmetric reactions.
By contrast, we have already reported that chirally flexible (tropos)
bis(phosphanyl)biphenyl (BIPHEP) ligands,2,3 of which the axial
chirality can be controlled by a chiral diamine as a chiral activator,
effectively act like atropisomeric ligands for Ru, Rh, and Pd
complexes.2,4,5 On the other hand, when a biphenyl diamine,
instead of a biphenyl phosphine such as BIPHEP, coordinates to a
metal, two centers of chirality in the diamine are generated in
addition to the chiral axis (Eq. 1). In this paper, we report dual
control of N-center chirality6 and axial chirality7 in Pd complexes
with the tropos diamines bearing the biphenyl backbone like
BIPHEP ligands (Eq. 1).
complex 2a in H NMR.
(2)
ð2Þ
In the case of the racemic complexes 2a and 2c, the relative
configuration of the single diastereomers was determined by X-ray
analyses of a single crystal obtained from dichloromethane–hexane
solution (Fig. 1).{ It was clarified that the coordination of diamine
ligands provides the axial chirality of the biphenyl backbone and
N-chirality on secondary amines by coordinating to the Pd center.
The chirality of the complex 2a is described as S/S,S and no
symmetry in the solid. On the other hand, the chirality of complex
2c is also described as S/S,S and C2-symmetry in the solid state.
Interestingly, the N-substituents adopt the axial orientations. In
sharp contrast, it has been reported that the N-substituents occupy
the equatorial orientations in diamine ligands bearing an ethylene
(1)
ð1Þ
The various tropos diamine ligands (1a–d) were prepared in
short steps. Treatment of 2,29-dinitrobiphenyl with sodium
borohydride over 10% palladium on carbon in MeOH/H2O
afforded 2,29-diaminobiphenyl (DABP) in 91% yield.8 Upon
treatment of DABP with 2.5 equiv. of aldehydes under toluene
reflux, the DABP imines were obtained. The imines were reduced
with sodium borohydride in toluene/MeOH to give the secondary
diamines 1a–d respectively, in good yields.9 Complexation of the
diamines 1a–d with PdCl2(cod) failed. However, complexation of
PdCl2(CH3CN)2 and 1.0 equiv. of 1a–d in dichloromethane at
room temperature was successful, giving diamine Pd complexes
2a–d in good yields (81–97%) (Eq. 2). All the 1H NMR spectra of
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
{ Electronic supplementary information (ESI) available: experimental
section. See DOI: 10.1039/b510910h
Fig. 1 ORTEP drawings of complexes 2a and 2c.
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Fig. 2 Axial orientations of the N-substituents of complex 2a.
backbone.6,10 It was indicated that there were CH–p interactions
between benzyl protons and the benzene ring of the biphenyl
backbone. We thus propose that the axial orientations of the
N-substituents stem from the CH–p interactions (Fig. 2).
The racemic complex 2a in dichloromethane at room tempera-
ture did not complex with a soluble chiral source (R)-M2BINOL
(M 5 Na, K) in THF obtained by in situ deprotonation of
(R)-BINOL with MOtBu.6 When (R)-DABNTf bearing higher
acidity due to a trifluoromethanesulfonyl substituent was used
instead of BINOL under similar conditions, the (R)-DABNTf
complex 3a was obtained after recrystallization from dichloro-
methane–acetone–hexane solution in 81% yield (Eq. 3). It was
confirmed by 1H and 19F NMR analyses that this isolated complex
3a was a single diastereomer. The other diastereomers were not
observed by NMR analyses of the crude mixture before
recrystallization. Using the complexes 2b and 2c with a mesityl
and 1-naphthyl substituent respectively, (R)-DABNTf complexes
3b and 3c were not obtained due to steric hindrance. The use of
complex 2d led to the complex with (R)-K2DABNTf, to give the
single diastereomer 3d in 75% yield.
Fig. 3 ORTEP drawing of complex 3a.
Fig. 4 Enantiomer discrimination of (R)-DABNTf.
We are grateful to Dr K. Yoza and Mr K. Kitajima in Nippon
Brucker Co. for X-ray analysis of complex 3a.
(3)
Notes and references
{ Crystal data for 2a in X-ray analysis: formula C26H24Cl2N2Pd?CH2Cl2,
˚
˚
monoclinic, space group P21/n, a 5 14.333(5) A, b 5 10.700(4) A,
ð3Þ
3
˚
˚
c 5 17.692(6) A, b 5 91.002(5)u, V 5 2712.8(16) A , Z 5 4, D 5
1.534 g cm23, m 5 10.97 cm21, F000 5 1264.0. All measurements were made
on a Rigaku Saturn CCD area detector with graphite monochromated
˚
Mo-Ka (l 5 0.71070 A) radiation at 193 K and the structure was solved by
direct methods (SIR92). Of the 24204 reflections that were collected, 7849
were unique (Rint 5 0.054). R 5 0.094, Rw 5 0.144, goodness of fit 5 1.000,
shift/error 5 0.000. CCDC reference number 277174. Crystal data for 2c in
X-ray analysis: formula C34H28Cl2N2Pd, monoclinic, space group P21/n,
Recrystallization of 3a from dichloromethane–methanol gave
crystals suitable for X-ray diffraction. The X-ray analysis revealed
that complex 3a was the single diastereomer (R/R,R/R: diamine
axial chirality/diamine center chirality/DABNTf axial chirality)
(Fig. 3).{ The N-benzyl substituents in complex 3a also occupy the
axial orientations.
˚
˚
˚
a 5 8.376(3) A, b 5 14.977(6) A, c 5 22.767(9) A, b 5 93.923(5)u,
V 5 2849.4(19) A , Z 5 4, D 5 1.496 g cm23, m 5 8.65 cm21, F000 5 1304.0.
3
˚
All measurements were made on a Rigaku Saturn CCD area detector with
˚
graphite monochromated Mo-Ka (l 5 0.71070 A) radiation at 193 K and
the structure was solved by direct methods (SIR92). Of the 25322
reflections that were collected, 8328 were unique (Rint 5 0.042). R 5 0.070,
Rw 5 0.112, goodness of fit 5 0.770, shift/error 5 0.000. CCDC reference
number 277175. Crystal data for 3a in X-ray analysis: formula
In the S/S,S/R-3a complex, there is strong steric repulsion
between the equatorial benzyl group of the diamine and the
trifluoromethanesulfonyl substituent (Tf) of the chiral amide
DABNTf (Fig. 4).11 In sharp contrast, there is no steric repulsion
in the complex R/R,R/R-3a. As a result, (R)-DABNTf could
complex only with the single enantiomer R/R,R-2a after
isomerization of the opposite enantiomer S/S,S-2a (Fig. 3).
In summary, we have succeeded in the dual chirality (axial
chirality and center chirality) control of Pd complexes bearing
tropos secondary amines by chiral (R)-DABNTf rather than
(R)-BINOL. Interestingly, it is clarified by X-ray analyses that the
N-substituents of the diamine moiety adopt axial orientations in
both the dichloride and DABNTf complexes.
C
25H19Cl1.5F3N2O3Pd0.5S, tetragonal, space group P43212, a 5
3
˚
˚
˚
˚
12.3252(5) A, b 5 12.3252(5) A, c 5 33.675(3) A, V 5 5115.6(5) A ,
Z 5 8, D 5 1.534 g cm23, m 5 6.76 cm21, F000 5 2388. All measurements
were made on a SMART APEX diffractometer with CCD detector using
˚
Mo-Ka (l 5 0.71073 A) radiation at 90 K and the structure was solved by
direct methods (SHELXL97). Of the 51634 reflections that were collected,
4678 were unique (Rint 5 0.0628). R 5 0.0745, Rw 5 0.1860, goodness of
fit 5 1.179, shift/error 5 0.003, the Flack parameter 5 0.06(7). CCDC
277176. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b510910h
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5800 | Chem. Commun., 2005, 5799–5801
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3 The activation barrier to axial torsion in selectively deuterated BIPHEP
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M. A. Khan and M. T. Ashby, Organometallics, 1999, 18, 5112; (b)
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J. Am. Chem. Soc., 2002, 124, 6929.
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Chem. Soc., 1989, 111, 9243.
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T. Ohkuma, T. Pham and R. Noyori, Angew. Chem., Int. Ed., 1999, 38,
495. For the BIPHEP–Pt complex; (b) M. D. Tudor, J. J. Becker,
P. S. White and M. R. Gagne´, Organometallics, 2000, 19, 4376; (c)
J. J. Becker, P. S. White and M. R. Gagne´, J. Am. Chem. Soc., 2001,
123, 9478. For the BIPHEP–Pd complex; (d) K. Mikami, K. Aikawa,
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K. Aikawa and Y. Yusa, Org. Lett., 2002, 4, 95. For the BIPHEP–Rh
complex; (f) K. Mikami, S. Kataoka, Y. Yusa and K. Aikawa, Org.
Lett., 2004, 6, 3699.
5 Use of different chirally flexible (tropos) NUPHOS ligands: (a)
S. Doherty, C. R. Newman, R. K. Rath, H. Luo, M. Nieuwenhuyzen
and J. G. Knight, Org. Lett., 2003, 5, 3863; (b) S. Doherty, J. K. Knight,
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M. Nieuwenhuyzen, Organometallics, 2004, 23, 6127.
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105.
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