398
Published on the web March 13, 2010
Syntheses of Novel C2-Symmetric Chiral Bidentate Bis(N-heterocyclic carbene) Ligands
and Their Molybdenum Complexes
Satoshi Anezaki, Yoshitaka Yamaguchi, and Masatoshi Asami*
Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501
(Received January 27, 2010; CL-100084; E-mail: m-asami@ynu.ac.jp)
Novel chiral bis(N-heterocyclic carbene) was prepared
O
O
excess
imidazole
reflux
N
N
starting from (S,S)- and (R,R)-1,2-bis(1-hydroxypropyl)benzene.
The chiral bis(N-heterocyclic carbene) (NHC) ligand formed C2-
symmetric chiral complex of molybdenum formulated as cis-
[Mo(CO)4{bis(NHC)}].
2.6 equiv CDI
OH
OH
O
O
N
N
( )-3
toluene
65 °C
10 min
( )-1
( )-2
85%
Chiral phosphines are widely applicable as ligands in
various metal-catalyzed enantioselective organic transforma-
tions. In particular, C2-symmetric bidentate diphosphine ligands
are used successfully in many metal-catalyzed enantioselective
reactions.1 N-Heterocyclic carbenes (NHCs), first reported by
Öfele2 and Wanzlick3 and later isolated in the free form by
Arduengo,4 have emerged as an extremely useful class of
ligands for transition-metal catalysis.5 Although the rapid
development of so-called chiral “hybrid” ligands including one
NHC and one pendant donating group for chelation6 has been
made, only a limited number of chiral C2-symmetric bidentate
bis(NHC) ligands have been reported to date.7 The first chelated
chiral bis(NHC) complexes of Pd and Ni derived from 2,2¤-
di(bromomethyl)-1,1¤-binaphthyl were prepared by RajanBabu
and his co-workers, whereas in these complexes the two NHCs
are located in trans-positions (complex I in Figure 1).8 Recently,
a novel family of axially chiral bis(NHC) metal complexes with
1,1¤-binaphthyl-2,2¤-diamine (BINAM) or H8-BINAM frame-
work have been developed by Shi and his co-workers (complex
II in Figure 1).9 They also reported chiral bis(NHC) with
biphenyl framework.10 In contrast to the complex I featuring a
chelate ring containing 10 atoms (two NHCs are orientated trans
to each other), the bis(NHC) ligand in the complex II consists of
an 8 atom chelating ring and thus two NHCs are cis-orientation
(Figure 1). These results suggest that in order to construct a C2-
symmetric chiral environment around the metal center, the
backbone skeletal structure for constructing bis(NHC) is
important.
Scheme 1.
diastereoselective reactions.12 In this communication, we wish to
report the syntheses of novel C2-symmetric chiral bis(NHC)
ligands and their Mo complexes derived from enantiomerically
pure 1,4-diol, 1,2-bis(1-hydroxypropyl)benzene (1).
First, the conversion of two hydroxy groups in 1 to 1-
imidazolyl groups was examined. As such a conversion was
reported using carbonyldiimidazole (CDI) under thermal con-
ditions,13 we examined the reaction of («)-1 with 2.6 equiv-
alents of CDI in toluene at 65 °C. The starting material 1
disappeared in 10 min, however not bis(imidazole) («)-3 but
bis(urethane) («)-2 was obtained in 85% yield (Scheme 1). The
resulting («)-2 was not converted into the corresponding («)-3
via decarboxylation, even under higher reaction temperature.
Then, the reaction was examined in the presence of excess
imidazole and the desired («)-3 was obtained. As («)-3 was
obtained from («)-1, the reaction sequence was applied to the
synthesis of (R,R)-3. As a result, the chiral bis(imidazole)
compound (R,R)-3 was obtained in 45% yield by the reaction of
(S,S)-1 with CDI (2.6 equivalents) in the presence of 2.6
equivalents of imidazole in refluxing toluene. In the course of
the formation of bis(imidazole), two chiral centers in (S,S)-1
were inverted without loss of diastereo- and enantiomeric
purities. The absolute configuration of the chiral centers were
determined by the X-ray analysis (vide infra). Quaternization of
the imidazole rings of (R,R)-3 with an excess of 2-iodopropane
gave the imidazolium salt (R,R)-4a in quantitative yield. In a
similar manner, bis(imidazolium) salts (R,R)-4b and (S,S)-4a
were prepared (Scheme 2).14
As the precursors of the chiral NHC ligands were obtained,
formation of molybdenum complex was examined. Treatment of
bis(imidazolium) salt (R,R)-4a with [Mo(CO)6] using two
equivalents of KO-t-Bu as a base in refluxing 1,2-dimethoxy-
ethane (DME) afforded a carbene complex of molybdenum
(R,R)-5a as a yellow solid in 52%. Complexes (R,R)-5b and
(S,S)-5a were also obtained as yellow solids in 68% and
52% yields, respectively. The formations of cis-[Mo(CO)4-
{bis(NHC)}] were fully confirmed by elemental analyses, IR, 1H
and 13C NMR spectra.14 In order to obtain further information
about stereochemistry of bis(NHC) ligand in the coordination
sphere and the chirality of two carbon centers of the benzylic
positions, the X-ray analyses of (R,R)-5a, (R,R)-5b, and (S,S)-5a
Previously, we reported a convenient method for the
preparation of C2-symmetric chiral 1,4-diol, 1,2-bis-(1-hydroxy-
alkyl)bezene,11 and an application of the chiral diol to highly
R
N
N
N
N
MLn
N
R
R
N
N
MLn
N
R
I
II
(cis orientation)
(trans orientation)
Figure 1. Chiral C2-symmetric bis(NHC) complexes.
Chem. Lett. 2010, 39, 398-399
© 2010 The Chemical Society of Japan
Anezaki, Satoshi
