Published on the web May 19, 2010
643
Axially Chiral Anilido-Aldimine Aluminum Complexes with a Pseudobinaphthyl Skeleton
Kazuhiro Hayashi, Yumiko Nakajima, Fumiyuki Ozawa, and Takeo Kawabata*
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011
(Received April 8, 2010; CL-100342; E-mail: kawabata@scl.kyoto-u.ac.jp)
Axially chiral anilido-aldimine aluminum complexes with a
R
R
O2N
O2N
∆G ≈ 28.2 kcal mol−1
t1/2 (20 °C) ≈ two years
N
H
N
H
pseudobinaphthyl skeleton 3a and 3b have been prepared. X-ray
analysis indicates that complex 3b adopts a conformation similar
to that of 1,1¤-binaphthyls. The racemization barrier of 3a was
determined to be 23.0 kcal mol¹1, which was higher by ca.
N
N
O2N
O2N
¹1
: R = CHMe
: R = CHPh
4
5
4 kcal mol than the corresponding diarylamine precursor with
2
(a
R
)
(a
S
)
2
an N-H-N hydrogen bond.
Figure 2. Axially chiral binaphthyl surrogates with long half-
lives of racemization.
Chiral binaphthyls have been extensively used in asym-
metric synthesis. In particular, metal complexes of 2,2¤-disubsti-
tuted-1,1¤-binaphthyls 1 have been shown to be extremely
effective catalysts for a variety of asymmetric transformations
(Figure 1).1-3 While the catalytically active metal center (M) in 1
is located far from the chiral axis (C(1)-C(1¤)) by three bonds, it
is quite effective for asymmetric induction in many cases. The
ultimate structure to minimize the distance between the catalyti-
cally active metal center and the chiral axis is shown as 2, where
the central metal is directly connected to the chiral C(1¤)-X axis.
Here we report the first example of the preparation and structural
elucidation of such organometallic complexes as exemplified by
anilido-aldimine aluminum complexes 3.
Crucial questions about 2 and 3 are whether (1) they adopt
the conformation like that of 1,1¤-binaphthyls, (2) enantiomers
of 2 and 3 with axial chirality could exist without rapid
racemization at ambient temperature. We have investigated
possible precursors for 2, and found that axially chiral diaryl-
amines with an intramolecular N-H-N hydrogen bond are
tolerant against racemization at ambient temperature. For
example, the racemization barrier of 4 was found to be
28.2 kcal mol¹1, which corresponds to a half-life of racemization
of 24 months at 20 °C (Figure 2). The diarylamine 5 was shown
to adopt quite similar conformation to that of 1,1¤-binaphthyls by
X-ray structural analysis.4 Here we describe the structure and
racemization barrier of anilido-aldimine aluminum complexes 3.
Preparation of anilido-aldimine aluminum complexes 3a
and 3b is shown in Scheme 1. Aldehyde 6 was prepared via
Buchwald cross coupling reaction5 between 2-methylnaphtha-
lene-1-amine and aryl bromide as reported previously.4 Con-
densation of 6 with isopropylamine gave 7a in 83% yield.
Anilido-aldimine aluminum complex 3a was obtained by
i)
CH(OEt)2
R
CHO
NH
N
H
Br
N
NH2
Pd(OAc)2, Binap
CsCO3
RNH2
AlMe3
3a, 3b
toluene
ii) 5% HCl
7a : R = CHMe2
7b : R = 3,5-di-tert-butylphenyl
6
Scheme 1. Preparation of anilido-aldimine aluminum com-
plexes 3a and 3b.
treatment of 7a with 1.0 equiv of trimethylaluminum in dry
toluene at 0 °C to room temperature in quantitative yield.
Disappearance of the NH signal of 7a (11.47 ppm) and
appearance of the resonance of AlMe2 protons at high field
1
(¹0.48 and ¹0.52 ppm) in the H NMR in benzene-d6 indicates
the formation of the aluminum complex 3a. The formation of 3a
was further confirmed by the observation of a molecular ion
peak of 3a in the mass spectrum (m/z 343). The observation of
1
the CH imine proton at 8.24 ppm in the H NMR in benzene-d6
shifted to higher field at 7.66 ppm as well as the observation that
the resonance of imine carbon at 162.1 ppm in 13C NMR shifted
to lower field at 167.7 ppm are consistent with the reported
formation of anilido-aldimine aluminum complexes from the
corresponding anilido-aldimine ligands.6a Anilido-aldimine
aluminum complex 3b was also prepared from 6 via condensa-
tion with 3,5-di-tert-butylaniline followed by treatment with
trimethylaluminum in dry toluene in 54% overall yield.
1
Formation of 3b was confirmed by H NMR, 13C NMR, MS,
elemental analysis, and X-ray structural analysis.7,8
A single crystal of 3b for X-ray analysis was obtained by
recrystallization of 3b from hexane at ¹30 °C. The crystal
structure of 3b is shown in Figure 3 along with selected bond
lengths and angles.8 Anilido-aldimine aluminum complex 3b
adopts a distorted tetrahedral geometry around the aluminum
center chelated by the imine and amido nitrogen atoms. The
Al-N (imine) distance (1.958 ¡) and Al-N (amido) distance
(1.893 ¡) are similar to the reported bond lengths of the related
anilido-aldimine aluminum complexes.6,9 In complex 3b, the
six-membered chelate ring is nearly planar with the aluminum
atom lying 0.114 ¡ out of the plane,10 and is almost coplanar
with the adjacent phenyl ring (side view). Thus, the ring
R
R
N
N
L
2
M
Al
Me
Me
L
X
1'
X
Y
N
1'
1
L
M
1'
L
2'
X, Y=O, NR, PR2, S
L=ligand, solvent
1
2
3
Figure 1. Metal complexes of 1,1¤-binaphthyls 1 and organo-
metallic complexes with a pseudobinaphthyl skeleton 2 and 3.
Chem. Lett. 2010, 39, 643-645
© 2010 The Chemical Society of Japan
Hayashi, Kazuhiro
