m-carborane-based chiral NBN pincer-metal complexes: Synthesis, structure, and application in asymmetric catalysis

Article abstract of DOI:10.1021/ic2002095

We have succeeded in synthesizing m-carborane-based chiral NBN-pincer ligands, 1,7-bis(oxazolinyl)-1,7-dicarba-closo-dodecaborane (Carbox) (7-9). The combination of bis(hydroxyamides) and 3 equiv of diethylaminosulfur trifluoride (DAST) is a key step for

Full text of DOI:10.1021/ic2002095

ARTICLE
pubs.acs.org/IC
m-Carborane-Based Chiral NBN Pincer-Metal Complexes: Synthesis,
Structure, and Application in Asymmetric Catalysis
Mohamed E. El-Zaria,† Hidekazu Arii, and Hiroyuki Nakamura*
Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan
S Supporting Information
b
ABSTRACT: We have succeeded in synthesizing m-carborane-
based chiral NBN-pincer ligands, 1,7-bis(oxazolinyl)-1,7-dicar-
ba-closo-dodecaborane (Carbox) (7ꢀ9). The combination of
bis(hydroxyamides) and 3 equiv of diethylaminosulfur trifluor-
ide (DAST) is a key step for cyclization to form oxazoline rings
in excellent yields. X-ray crystal structures of these ligands
confirmed three donor sites, one central B and two flanking
N atoms in fixed positions. The electrophilic halogenation of
the Carbox pincer ligands with iodine and a catalytic amount of Lewis acid led to ring-opening of the oxazolines and afforded
bis(haloamides) (13 and 14). The air- and moisture-stable Carbox pincer complexes of rhodium(III), nickel(II), and palladium(II)
were synthesized by the oxidative addition of RhCl3 3H2O, Ni(COD)2, and Pd(CH3CN)4[BF4]2 to the Carbox pincer ligands
3
(7ꢀ9), respectively. The catalytic activity of the rhodium(III) complexes (18ꢀ20) was examined for the asymmetric conjugate
reduction of R,β-unsaturated esters and reductive aldol reaction. Among these catalysts, [(S,S)-Carbox-iPr]Rh(OAc)2 H2O (18)
3
showed the highest enantioselective catalytic ability for both asymmetric conjugate reduction and reductive aldol reaction.
’ INTRODUCTION
bond formation was observed as an analogue of the CꢀPd bond
(Figure 1).18 Yamashita and Nozaki reported a similar BꢀIr bond
formation observed in PBP pincer iridium complexes.19 Although
both findings showed a new aspect of boron-based synthons
in organometallic and coordination chemistry,22 their practical
application in organic reactions has not been examined. In this
paper, we report the first carborane-based chiral NBN pincers,
1,7-bis(oxazolinyl)-1,7-dicarba-closo-dodecaborane (Carbox) and
their pincer complexes with rhodium(III), palladium(II),
and nickel(II). An interesting feature of the optically active
C2-symmetric NCN pincers (Phebox) is the ease with which a
number of chiral analogues can be synthesized from readily
available homochiral amino alcohols.13,14 These pincer ligands
can coordinate a variety of transition metals, including Ni,23ꢀ25
Pd,26ꢀ29 Pt,28,30,31 and Rh,14,32 to form complexes with C2-
symmetry. Pincer complexes are multipurpose, often air-stable
compounds that have attracted interest not only in asymmetric
catalysis but also in molecular electronics33 and medicine.34 We
demonstrate the Carbox-rhodium complexes as suitable catalysts
for asymmetric conjugate reduction of R,β-unsaturated esters
and the asymmetric reductive aldol reaction of tert-butyl acrylate
and benzaldehyde using diethoxymethylsilane as a reductant.14c
An icosahedral carborane framework involves delocalized
electron-deficient three-center two-electron bonding, forming
trigonal faces and hyperconjugation.1,2 The carboranes consist
of 2 carbon atoms and 10 boron atoms, which result in three
isomers: 1,2-, 1,7-, and 1,12-dicarba-closo-dodecaboranes (o-, m-,
and p-carboranes) and thus highly tailorable structures.3,4 Car-
borane units can be attached to other molecules either via their C
or B atoms. While substitution at the carbon atom in carborane
derivatives has been extensively studied,5 alternative routes
involving reactions at the boron center are still a challenge.6
Transition-metal catalysis is one of the most important subjects
in organic synthesis.7 Especially, the proper choice of ligands for
transition-metal-catalyzed reactions is crucial for expanding syn-
thetic applications. Among the various ligand systems that can
be found in the literature, pincer-type ligands and their metal
complexes have attracted long-lasting interest because of their
high stability and activity as catalysts.8ꢀ14 Pincers are a class of the
rigid tridentate ligands that are connected to the metals via at least
one-metal carbon σ bond. Beside these carbon-based platforms,
pincers having amido,15 silyl,16 and phosphido17 units have also
been studied extensively.
Recently, the boryl-based pincers were reported by two
research groups.18,19 In general, boryl species are very strong
σ donors and exhibit a strong trans effect.20 However, π back-
donation is considered to be rather weak in most metal com-
plexes; therefore, the boryl ligands have been considered as
reactive ligands.21 Mirkin and co-workers reported the carbor-
ane-based tridentate metal SeBSe and SBS pincers and complexes
of these ligands metalated with palladium(II), where a BꢀPd
’ RESULTS AND DISCUSSION
The construction of ligands for transition metal complexes
is very important, especially in the field of molecular catalysis
and organic synthesis, because ligands can provide appropriate
Received: January 31, 2011
Published: March 25, 2011
r
2011 American Chemical Society
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|

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ARTICLE
stereochemical and electronic environments around active metal
centers, thereby controlling catalysis. This fact prompted us to
develop new tridentate chiral pincers, 1,7-bis(oxazolinyl)-1,7-
dicarba-closo-dodecaboranes (Carbox). We adopted a hetero-
cyclic oxazoline skeleton, 4,5-dihydro-1,3-oxazole, as the mod-
ular unit of these multidentate ligands because the oxazoline
skeleton could supply sufficient substituent diversity, including
chiral centers at 4- and 5-positions, using a variety of β-amino
alcohols (Scheme 1).35
Synthesis of NBN-Pincer (Carbox) Ligands. At the start
of our investigations, we synthesized chiral ligands (7ꢀ9) in a
similar manner to Phebox ligands via the combination of meth-
odologies described in the literature,14c,23 as shown in Scheme 1.
m-Carborane dicarboxylic acid (2) was obtained from commer-
cially available m-carborane (1) using the reported protocol36
and transformed to the acyl chloride (3) with SOCl2. Condensa-
tion with 2 equiv of amino alcohols, such as L-valinol, L-pheny-
lalaninol, and D-phenylglycinol, afforded trace amounts of
bis(oxazolinyl)-m-carboranes (7ꢀ9), and the corresponding un-
cyclized bis(hydroxyamide)-m-carboranes (4ꢀ6) were obtained
in 95ꢀ98% yields. Treatment of 4ꢀ6 with MeSO2Cl/Et3N led to
the formation of 1-(oxazolinyl)-7-(hydroxyamide)-1,7-dicarba-
closo-dodecaboranes (10ꢀ12) in high yields after 8 h. These
incompletely cyclized intermediates were obtained as major
products even when the compounds (4ꢀ6) were treated with
an excess of MeSO2Cl/Et3N reagents for 2 days. We also noted
that BF3 Et2O and PPh3/CCl423 were not tolerated during the
3
cyclizations of 4ꢀ6. Therefore, we examined the possibility of
modifications of these methods with respect to the availability of
starting materials, literature knowledge, and ease of synthesis.
By carefully studying the reaction, we discovered that diethy-
laminosulfur trifluoride (DAST)37 is an effective reagent for this
cyclization process. Bis(hydroxyamide)-m-carborane 4 under-
went the double cyclization when treated with 3 equiv of DAST
to afford [(S,S)-Carbox-iPr]H (7) in a 96% yield. Pincer ligands
[(R,R)-Carbox-Ph]H (8) and [(S,S)-Carbox-Bn]H (9) were also
synthesized from bis(hydroxyamide)-m-carboranes 5 and 6
using the similar procedure in 92% or 94% yields, respectively.
Reaction with Halogens. Introduction of heavy atoms such
as iodine and bromine is necessary to confirm the absolute con-
figuration of chiral compounds by X-ray single crystal analysis.
Therefore, we performed the reaction of bis(oxazolinyl)-m-car-
boranes (7 and 9) with halogens by refluxing carbon tetrachloride
in the presence of either ultraviolet radiation (chlorination) or
aluminum trichloride (chlorination, bromination, and iodination).
Molecular iodine and a Lewis acid catalyst are the reagents most
commonly used to achieve BꢀI formation from the BꢀH of
icosahedral carboranes.38 However, the halogenation of bis-
(oxazolinyl)-m-carboranes 7 and 9 using the methodology of
Andrews et al.38a led to the ring-opening of the oxazolines,
affording the undesired bis(haloamides) 13 and 14, respectively
(Scheme 2). The type of halogenation depends on the Lewis acid
used in the catalysis. Treatment of 7 and 9 with iodine and AlBr3
Scheme 2. Reaction of 7 and 9 with Halogensa
a (i) CH2Cl2, I2, 7, AlBr3, reflux, 16.5 h, 13 98%, or CH2Cl2, I2, 9, AlCl3,
reflux, 16.5 h, 14 98%.
Figure 1. Strucutures of boryl-based pincers.
Scheme 1. Synthesis of Chiral NBN-Pincer Ligands 4ꢀ9a
a (i) n-BuLi, ether, CO2, HCl 95%; (ii) SOCl2, reflux, 24 h, quant.; (iii) amino alcohol, CH2Cl2, Et3N, r.t, 24 h, 4 98%, 5 97%, 6 95%; (iv) CH2Cl2, amino
alcohol, Et3N, MeSO2Cl, r.t, 8 h, 10 72%, 11 75%, 12 69%; (v) 3 equiv DAST, CH2Cl2, ꢀ20 °C, 12 h, 7 96%, 8 92%, 9 94%.
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Scheme 3. Synthetic Pathways of NBN-Pincer Complexesa
Figure 2. X-ray crystal structure of 4. Thermal probability ellipsoids are
drawn at the 50% probability level (hydrogen atoms are omitted for
clarity).
treatment with 2 equiv of (n-Bu)4NCl, which furnished [(S,S)-
Carbox-iPr]PdCl (24), [(R,R)-Carbox-Ph]PdCl (25), and
[(S,S)-Carbox-Bn]PdCl (26) in 76%, 74%, and 80% yields,
respectively. All resulting complexes were stable and could be
stored in air at room temperature for several months without
decomposition.
X-ray Structural Analysis. We analyzed the molecular struc-
tures of 4, 7, 9, 10, 13, and 14 by X-ray crystallography to
determine their C2v symmetrical forms (Figures 2ꢀ5). Table 1
presents the crystallographic data. Single crystals of compound 4
were grown at room temperature from methanol, and the struc-
ture is shown in Figure 2. Compound 4 crystallized in the acen-
tric space group P212121 retains an S-configuration on the
R-carbon originated the amino alcohol as a starting material. The
m-carborane cluster has a slightly distorted icosahedron with
typical BꢀB bond lengths of 1.779(5) Å (average), ranging
between 1.767(5) and 1.791(5) Å. The average CꢀB bond
length for 4 was 1.715(5) Å, ranging between 1.698(5) and
1.731(8) Å. The amide carbon C(2) has the longer CꢀO bond
length of 1.267(5) Å and the shorter CꢀN bond of 1.300(5) Å
than that of the other amide carbon (C(9)ꢀO(3) 1.226 Å and
C(9)ꢀN(2) 1.341 Å, respectively). The difference arises from an
intermolecular hydrogen bond between the carbonyl oxygen of
the amide group and the hydroxyl groups of two solvated
methanol of which the distances were observed at about 2.76 Å.
Single crystals suitable for X-ray crystallography of pincers 7
and 9 were obtained by slow evaporation from hexane and
dichloromethane/hexane solution, respectively. As shown in
Figure 3, the carborane clusters were distorted icosahedrons in
which the BꢀB bond lengths (average distance: 1.781(3) Å,
range: 1.769(2)ꢀ1.795(2) Å) and CꢀB bond lengths (average
distance: 1.726(2) Å, range: 1.701(2)ꢀ1.730 (2) Å) were within
the expected values. We identified the carbon atoms in the cage
by noting the shorter CꢀB bond lengths relative to the elongated
BꢀB bond distances. The CꢀN and CꢀO bond lengths of
2-position carbon on the oxazoline rings in 7 and 9 were
unexceptional at about 1.26 and 1.35 Å, respectively.
a (i) MeOH, RhCl3 3H2O, NaHCO3, H2O, 60 °C, 6 h, 15: 79%, 16:
3
76%, 17: 82%; (ii) Pd(CH3CN)4[BF4]2, CH3CN, reflux, 24 h, then (n-
Bu)4NCl (2 equiv), 21: 73%, 22: 86%, 23: 82% or Ni(COD)2, CH3CN,
reflux, 24 h, then (n-Bu)4NCl (2 equiv), 24: 76%, 25: 74%, 26: 80%; (iii)
AgOAc, CH2CH2, r.t, 24 h, quant.
or AlCl3 in CH2Cl2 gave bis(bromoamide) 13 and bis(chloroamide)
14 in nearly quantitative yields, respectively. Furthermore, the
electrophilic halogenations of bis(oxazolinyl)-m-carboranes (7 and
8) with iodine or LDA/TMEDA, followed by iodine, using the
methodology of Harris et al.39 did not proceed at all. According
to the X-ray single crystal analysis of bis(bromoamide) 13, the
absolute configuration was a (S,S) isomer, and this result in-
dicates that the current cyclization conditions did not affect the
configuration of the starting amino alcohols.
Synthesis of Metal-Carbox Complexes via the ortho-Me-
tallative BꢀH Bond Insertion of Rh(III), Ni(0), and Pd(II). A
method similar to that reported by Nishiyama14c was used to
synthesize Rh(III) pincer complexes of Carbox ligands. Heating a
mixture of the pincerligands (7ꢀ9), RhCl3 3H2O, and NaHCO3
3
in a cosolvent of methanol/water (10:1) gave the corresponding
chloride complexes [(S,S)-Carbox-iPr]RhCl2 H2O (15, 79%),
3
[(R,R)-Carbox-Ph]RhCl2 H2O (16, 76%), and [(S,S)-Carbox-
3
Bn]RhCl2 H2O (17, 82%), respectively (Scheme 3). The chlor-
3
ide complexes could, in turn, be converted to the corresponding
acetate complexes 18ꢀ20 in quantitative yields by treatment with
an excess of silver acetate in dichloromethane at room tempera-
ture for 24 h.
We next examined the synthesis of nickel complexes, which
required a different strategy, as BꢀNi derivatives of m-carborane
are unknown. The reaction of pincer ligands, [(S,S)-Carbox-
iPr]H, [(R,R)-Carbox-Ph]H, and [(S,S)-Carbox-Bn]H, with Ni-
(COD)2 in acetonitrile at reflux temperature resulted in a slow
change of the solution color from yellow to orange over 24 h. The
treatment of this solution with 2 equiv of (n-Bu)4NCl provided
the nickel complexes [(S,S)-Carbox-iPr]NiCl (21), [(R,R)-Car-
box-Ph]NiCl (22), and [(S,S)-Carbox-Bn]NiCl (23) in 73%,
86%, and 82% yields, respectively.
A similar protocol was employed for the synthesis of palla-
dium complexes. A mixture of the pincer ligands, [(S,S)-Carbox-
iPr]H, [(R,R)-Carbox-Ph]H, and [(S,S)-Carbox-Bn]H, and Pd-
(CH3CN)4[BF4]2 was refluxed in acetonitrile, followed by
Single crystals of compound 10 were grown at room tempera-
ture from a mixture of dichloromethane/hexane. As shown in
Figure 4, compound 10 was also a slightly distorted icosahedron
having typical BꢀB bond lengths (average distance: 1.686(4) Å,
range: 1.761(4)ꢀ1.795(4) Å). The average CꢀB bond length
for compound 10 was 1.711(4) Å, ranging between 1.694(3) and
1.730(4) Å. The CꢀN and CꢀO bond lengths around C(2)
were 1.270(4) and 1.349(3) Å, respectively, similar to their of the
pincer ligands 7 and 9.
The crystal structures of 13 and 14 obtained from a mixture
of dichloromethane/hexane locate in the acentric space group P1
of triclinic system and P43212 of tetragonal system, respectively
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Figure 3. X-ray crystal structures of 7 and 9. Thermal probability ellipsoids are drawn at the 50% probability level (hydrogen atoms are omitted for
clarity).
chemical shifts of the ꢀCH2O and ꢀCHN protons in 8 and 9 were
also slightly shifted to a higher field (∼0.2 ppm) than those of (S,S)-
PheBox-Ph and (S,S)-PheBox-Bn, respectively.14c The CdN carbon
signals of Carbox ligands 7ꢀ9 also shifted to a higher field (158.29ꢀ
159.88 ppm) than those of the corresponding PheBox ligands
(162.97ꢀ163.9 ppm).
1
The H and 13C NMR spectra of the Carbox ligand rhodium
complexes, such as [(S,S)-Carbox]RhCl2 H2O (15ꢀ17) and [(S,
3
S)-Carbox]Rh(OAc)2 H2O (18ꢀ20), displayed a similar profile
3
to that of the metal-free Carbox ligands (7ꢀ9), except for the
additional peaks of the OAc, which appeared at 1.69ꢀ1.75 ppm in
1
the H NMR and at 175.11ꢀ175.42 (CdO) and 23.72ꢀ23.76
ppm (CH3) in the 13C NMR for the complexes 18ꢀ20. Others
Figure 4. X-ray crystal structure of 10. Thermal probability ellipsoids
are drawn at the 50% probability level (hydrogen atoms are omitted for
clarity).
have reported that the oxazoline protons (ꢀCH2O and ꢀCHN
groups) of the PheBox ligands shifted to a lower field after
1
complexation with rhodium in the range 0.12ꢀ0.39 ppm in H
(Table 1). The structures, as shown in Figure 5, confirmed that
the oxazoline rings of 7 and 9 were opened and brominated or
chlorinated by the presence of AlBr3 or AlCl3 as the reactant,
respectively. The R-carbon in 13 including heavier element
bromine exhibited a (S)-configuration coincident with that of
the starting amino alcohol, revealing an evidence of the retention
of absolute configuration on the R-carbon under the synthetic
condition of pincer ligands. The BꢀC (1.708(5)∼1.742(5) Å)
and BꢀB bond lengths (1.759(5)∼1.806(5) Å) in 14 did not
differ significantly from the corresponding lengths in 13 (BꢀC,
1.691(3)∼1.734(3); BꢀB, 1.762(3)∼1.807(4)); hence, the
same cage distortions were observed. The average CꢀBr bond
lengths for 13 and the CꢀCl bond lengths for 14 were 1.907 Å,
ranging between 1.847(4) and 1.964(5) Å, and 1.974(2) Å,
ranging between 1.970(2) and 1.978(2), respectively.
Characterization of Carbox-Metal Complexes. Although we
tried to confirm the structures of the Carbox-metal complexes by
X-ray crystallographic analysis, we did not obtain crystals suitable for
X-ray structural analysis. Therefore, we characterized the complexes
by spectroscopic analysis. In the 1H NMR spectra of (S,S)-Carbox-
iPr (7), the chemical shifts of the ꢀCH2O and ꢀCHN groups
appeared at 4.26 (2H), 4.06 (2H), and 3.98 (2H) ppm, respectively.
In contrast, the chemical shifts of the ꢀCH2O and ꢀCHN protons
in (S,S)-PheBox-iPr appeared at 4.09ꢀ4.19 (4H) and 4.38ꢀ4.48
(2H) ppm,13 the CHNH proton of 7 shifted at a higher field. The
NMR and 8.3ꢀ9.2 ppm in 13C NMR because of the conforma-
tional change of the PheBox ligands by the coordination of the
oxazoline nitrogen to the metal. Because the chemical shifts of the
Carbox ligands did not change significantly, the conformation of
the Carbox ligands might be fixed by the unique structure of the m-
carborane framework (Figure 2).
The 11B NMR resonances of 4ꢀ14 exhibited a distinctly
different pattern from those of the corresponding metalated
pincer complexes 15ꢀ26 (Figures 6 and 7). Although the
chemical shifts of the signals due to the boron atoms coupled
with hydrogens basically appeared in the same field among the m-
carboranes (4ꢀ26), the metal-substituted boron atoms were
deshielded and produced the most downfield resonances, a
typical feature of a boronꢀmetal bond formation, and similar
downfield shifts have been reported by Mirkin and co-workers.18
The metalation of the Carbox ligands 7ꢀ9 broke the C2v
symmetry of its parent m-carborane cage, and we observed a
larger number of 11B resonances as compared with the spectra of
ligands 7ꢀ9. The 11B{1H} spectra of 15ꢀ26 consisted of four
overlapping singlet signals (from approximately ꢀ10 to ꢀ19
ppm) and an isolated signal at approximately þ0.92 ppm.
Integration of the area under these signals showed a 9:1 ratio.
A broad multiplet of the 1H-coupled 11B NMR spectra of
complexes 15ꢀ26 was observed in the ꢀ10 to ꢀ19 ppm region
of the spectra, but the resonance around þ0.92 ppm remained a
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Figure 5. X-ray crystal structures of 13 and 14. Thermal probability ellipsoids are drawn at the 50% probability level (hydrogen atoms are omitted for
clarity).
Table 1. Details of Crystallographic Data Collection for 4, 7, 9, 10, 13, and 14
4
7
9
10
13
14
formula
Fw
C15H38B10N2O5 C14H30B10N2O2 C22H30B10N2O2 C14H32B10N2O3 C14H32B10Br2N2O2 C22H32B10Cl2N2O2
434.57
366.50
462.58
monoclinic
P2(1)
384.52
monoclinic
C2
528.34
535.50
crystal system
space group
a/Å
orthorhombic
P2(1)2(1)2(1)
6.7826(10)
17.136(3)
21.403(3)
90.00
orthorhombic
P2(1)2(1)2(1)
6.6356(8)
15.8615(19)
19.740(2)
90.00
tetragonal
P4(3)2(1)2
21.6668(17)
21.6668(17)
10.4424(8)
90.00
triclinic
P1
12.2405(10)
6.6058(5)
31.330(3)
90.00
33.866(14)
6.684(3)
9.784(4)
90.00
9.7591(10)
10.1277(10)
15.0464(15)
94.2120(10)
102.3010(10)
94.3040(10)
1.4428(3)
2
b/Å
c/Å
R/deg
β/deg
γ/deg
V/nm3
Z
90.00
90.00
99.5400(10)
90.00
92.669(5)
90.00
90.00
90.00
90.00
90.00
2.4876(6)
4
2.0777(4)
4
2.4983(3)
4
2.2124(16)
4
4.9022(7)
8
Dc/g cmꢀ3
F(000)
μ/mmꢀ1
λ/Å
1.118
1.172
1.230
1.154
1.432
1.233
896
776
968
816
2128
556
0.069
0.067
0.071
0.069
3.321
0.249
0.71073
0.71073
1.65ꢀ27.53
1.04
0.71073
2.64ꢀ27.47
0.986
0.71073
2.36ꢀ27.33
1.025
0.71073
2.16ꢀ26.59
1.057
0.71073
2.29ꢀ27.47
1.042
range (2θ) for data collection/deg 1.52ꢀ27.48
GOF
1.155
123
T/K
123
123
123
123
123
R1a (I > 2σ(I))
WR2b (I > 2σ(I))
R1a (all data)
WR2b (all data)
0.0787
0.2159
0.0900
0.2239
0.0398
0.0402
0.0879
0.0504
0.0934
0.0539
0.1472
0.0630
0.1537
0.0259
0.0601
0.0926
0.0592
0.1692
0.0484
0.0320
0.0635
0.0966
0.0608
0.1736
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|. b wR2 = [∑w(Fo ꢀ Fc2)2/∑w(Fo ) ]
.
2
2 2 1/2
singlet, consistent with its assignment as the B(2) atom bound
to the metals.
found only slight differences among the compounds, indicating
that the substitution of the icosahedron did not perturb the
intracluster bonding.
The carborane ligands (7ꢀ9) and the metal complexes
(15ꢀ26) had characteristic stretching modes that were suitable
for study by IR spectroscopy. The vibrational frequencies of the
BꢀH bond (ν(BꢀH)) and the BꢀB bond (ν(BꢀB)) were not
sensitive to the cyclization reactions. The BꢀH stretching
We characterized all synthesized boronated compounds by
electrospray ionization mass spectrometry (ESI-MS). The nega-
tive-ion ESI-MS of compounds 4ꢀ12 and 15ꢀ26 showed only
the signal of a singly charged ion, whose mass and typical isotopic
pattern of boron isotopes (10B and 11B) suggested a molecular
formula at m/z = [M]ꢀ and [M þ Cl]ꢀ; however, compounds 13
and 14 gave molecular ion peaks corresponding to [M þ Br]ꢀ
and [M þ I]ꢀ, respectively. The results of the microanalysis for
the Rh(III), Pd(II), and Ni(II) complexes with Carbox ligands
(7ꢀ9) were in good agreement with those calculated for the
suggested formulas and confirmed the formation of 1:1 (metal:
ligand) complexes (for details see the Experimental Section).
frequencies of 7ꢀ9 were broad (ν(BꢀH) 2596ꢀ2623 cmꢀ1
;
ν(BꢀB) 1049ꢀ1057 cmꢀ1) and within the expected range for
related m-carborane derivatives, including 1,7-dihydroxycarbo-
nyl-1,7-dicarba-closo-dodecaborane reported by Kahl et al.36 For
the pincer complexes 15ꢀ26, ν(BꢀH) was observed in the
2622ꢀ2625 cmꢀ1 region, whereas ν(BꢀB) varied from 1065 to
1052 cmꢀ1. Among the frequencies of the C2B10H10 moiety
(ν(BꢀH) 2619ꢀ2626 cmꢀ1; ν(BꢀB) 1073ꢀ1057 cmꢀ1),40 we
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Figure 6. 11B and 11B{1H} Spectra of [(S,S)-Carbox-iPr]H (7) in CDCl3.
Figure 7. 11B and 11B{1H} spectra of [(S,S)-Carbox-iPr]Rh(OAc)2 H2O (18) in CDCl3.
3
Application to Asymmetric Catalysis. The structural rigidity
of the carborane cage and its unique spatial and chemical features
might be beneficial in chemical catalysis. To apply NBN-pincer
complexes in a more general sense and to find the optimal
catalyst for a respective transformation, we were interested in
systematic variations of the steric and electronic parameters of
the corresponding ligands. Therefore, we demonstrated the
following two types of reactions: (i) an asymmetric conjugate
reduction of R,β-unsaturated esters and (ii) an asymmetric
reductive aldol reaction, using [(S,S)-Carbox-iPr]Rh(OAc)2 H2O
3
(18) as a chiral catalyst.14c
(i). Catalytic Asymmetric Conjugate Reduction of R,
β-Unsaturated Esters. The enantioselective transfer of hydrogen
to prochiral carbonꢀcarbon double bonds is one of the most
important catalytic asymmetric processes to date. In this context,
the conjugate reduction of R,β-unsaturated carbonyl compounds
offers a valuable method for the construction of building blocks
with a β-stereogenic center found in many natural products.41
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Scheme 4. Asymmetric Conjugate Reduction of R,
β-Unsaturated Esters 27ꢀ29 with [(S,S)-Carbox-iPr]
of benzaldehyde, tert-butyl acrylate, and (EtO)2MeSiH with
Phebox-Rh complex gave 33 with a 87:13 ratio of anti:syn, and
the ee of the major product 33 (anti) was 83%. 4-NO2-sub-
stituted Phebox-Rh complex exhibited higher enantioselectivity
(87% ee), and use of 4-MeO-substituted Phebox-Rh complex
decreased the enantioselectivity to 77%. These results suggested
that electron deficient Phebox ligands were preferable in the
catalytic asymmetric reductive aldol reaction.14a In this regard,
the current Carbox-Rh complex resulted in the production of 33
(anti) with higher enentioselectivity presumably because of the
electron deficient property of an icosahedral dicarba-closo-dode-
caborane framework.
Rh(OAc)2 H2O (18) Catalyst
3
Scheme 5. Asymmetric Reductive Aldol Reaction of tert-
Butyl Acrylate and Benzaldehyde with [(S,S)-Carbox-
’ CONCLUSIONS
We have succeeded in synthesizing Carbox ligands 7ꢀ9 as the
first chiral NBN pincer ligands. The combination of bis(hydro-
xyamides) and 3 equivofDASTisa key stepfor cyclizationto form
oxazoline rings in excellent yields. We clarified the structures of the
Carbox ligands (7 and9) aswellasthe intermediates(4 and 10) by
X-ray analysis. The electrophilic halogenation of the Carbox pincer
ligands with iodine and a catalytic amount of Lewis acid led to the
ring-opening of the oxazolines and afforded bis(haloamides) (13
and 14). The Carbox pincer complexes of rhodium(III), nickel-
(II), and palladium(II) were synthesized by the oxidative addition
iPr]Rh(OAc)2 H2O (18) Catalyst
3
of RhCl3 3H2O, Ni(COD)2, and Pd(CH3CN)4[BF4]2 to the
3
Carbox pincer ligands (7ꢀ9), respectively. The Carbox-Rh com-
plexes (15ꢀ17) were transformed into their acetate complexes
(18ꢀ20) by acetate abstraction using AgOAc. These rhodium
complexes 18ꢀ20 were examined as chiral catalysts for the
asymmetric conjugate reduction of R,β-unsaturated esters and
the reductivealdol reaction. Among these catalysts, [(S,S)-Carbox-
There are few catalysts that can reduce carbonꢀcarbon double
bonds to generate products with β-stereocenters from carbonyls
with high enantiomeric excess (ee). We used [(S,S)-Carbox-
iPr]Rh(OAc)2 H2O (18) as a chiral pincer catalyst for the
3
iPr]Rh(OAc)2 H2O (18) showed the highest enantioselective
3
asymmetric conjugate reduction of R,β-unsaturated esters (27ꢀ
29). The reaction of 27 with (EtO)2MeSiH proceeded smoothly
at 60 °C in the presence of 18 (1 mol %) in toluene, giving the
corresponding conjugated reduction products 30 in a 92%
yield with 94% ee (Scheme 4).42ꢀ44 When we reduced the
amount of the catalyst to 0.5 mol %, a longer reaction time was
needed to complete the reaction, but the ee of the product
was not affected. Moreover, a lower temperature resulted in
substantial decreases in both yield and selectivity. The chlor-
ide form 15 was also effective for the asymmetric conjugate
reduction, and the corresponding conjugated reduction pro-
duct 30 was obtained with 93% ee. However, the other Carbox
catalysts (19 and 20) decreased the ee to 27% and 82%,
respectively. A bulky substituent at the β-position of the
R,β-unsaturated esters, such as an ethyl (28) or an iso-propyl
(29) group, resulted in higher enantioselectivity, and the corre-
sponding reduction products 31 and 32 were obtained in 89%
and 96% yields with 97% and 98% ee, respectively. These enan-
tioselectivities are similar to those obtained from the use of
Phebox ligands.14c
catalytic ability for both asymmetric conjugate reduction and
reductive aldol reaction. The versatile coordination ability of the
Carbox structures to various transition metals and their rigidity as
coplanar NBN-tridentate ligands make them an excellent chiral
induction environment for the transition metals. We previously
found that the electron-deficient feature of carboranes is effective
in a certain catalytic reaction;45 therefore, we believe that the chiral
Carbox ligands have a great potential for development of transition
metal-catalyzed asymmetric synthesis.
’ EXPERIMENTAL SECTION
Materials and Methods. Most chemicals were of analytical grade
and used without further purification. The substrates 2, 28, and 29
were prepared according to
a previously reported literature
procedure.24,31 1H NMR and 13C NMR spectra were measured on a
JEOL JNM-AL 300 (300 MHz) and VARIAN UNITYꢀINOVA 400
1
and 500 (400 and 500 MHz) spectrometers. Chemical shifts of H
NMR and 13C NMR were expressed in parts per million (ppm, δ
units), and coupling constant (J) values were expressed in units of hertz
(Hz). 11B NMR spectra were recorded on a JEOL JNMꢀAL 300
spectrometer (96.3 MHz), and the chemical shifts were reported in δ
(ii). Catalytic Asymmetric Reductive Aldol Reaction. We also
examined the asymmetric reductive aldol reaction of benzalde-
hyde, tert-butyl acrylate, and (EtO)2MeSiH with Carbox-Rh
complexes 18, 19, and 20 (Scheme 5). The reaction proceeded
units relative to external BF3 Et2O in CDCl3. IR (cmꢀ1) spectra were
3
determined as KBr disk on a Shimadzu FTIRꢀ8600PC spectrometer.
Electron spray ionization (ESI) mass spectra were recorded on a
Shimadzu LCMSꢀ2010 eV spectrometer or Bruker Daltonics micro
TOFꢀ15 focus. Elemental analyses were performed by a CE instru-
ment EA1110 CHNSꢀO automatic elemental analyzer. All com-
pounds gave elemental analysis within (0.4% of the theoretical
values. Analytical thin layer chromatography (TLC) was performed
in the presence of [(S,S)-Carbox-iPr]Rh(OAc)2 H2O (18)
3
(1 mol %) and (EtO)2MeSiH (1.5 equiv) in toluene at 60 °C to
give the corresponding reductive aldol product 3325 in a 98%
yield with a 9:1 ratio of anti:syn. The ee of the major product 33
(anti) was 91%. It was reported that the reductive aldol reaction
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on a glass plates of silica gel 60 GF254 (Merck). Visualization was
accompanied by UV light (254 nm), I2, KMnO4, or PdCl2. Column
chromatography was conducted on silica gel (Merck Kieselgel 70ꢀ
230 mesh).
aromatic CH), 73.02 (2C, CHNH), 67.56 (2C, clusterꢀC), 54.63 (2C,
CH2OH), 40.5 (2C, BnꢀCH2). 11B NMR (96.3 MHz; CDCl3):
δ ꢀ9.84 to ꢀ18.38 (bm, 10B). IR (KBr, cmꢀ1): ν(OH) 3421 (S), ν(NH)
3294 (S), ν(CH) 3058 (m), 3024 (m), ν(BH) 2607 (S), ν(CdO) 1654
(S), ν(NH) 1539 (S), ν(CH) 1473 (m), ν(CꢀO) 1288 (m), ν(BꢀB)
1041 (S), ν(CH) 835 (m), 744 (m). MS (ESI, negative): m/z 534.4
(MþCl)ꢀ. Elemental analysis calcd for C22H34B10N2O4: C, 52.99; H, 6.87;
N, 5.62%. Found: C, 52.92; H, 6.85; N, 5.59%.
General Synthesis of NBN-Pincer Ligands (4ꢀ6). m-Carbor-
ane dicarboxylic acid (2) (1.0 g, 4.3 mmol) and SOCl2 (50 mL) were
placed under nitrogen atmosphere into a three neck 100 mL round-
bottom flask equipped with a condenser, and the mixture was refluxed
for 24 h. Excess SOCl2 was then removed under reduced pressure to give
the diacid chloride (3) as a colorless oil in quantitative yield. A solution
of the acid dichloride (3) (1.15 g, 4.27 mmol) in dichloromethane
(20 mL) was slowly added via cannula to a solution of amino alcohol (9.4
mmol) in dichloromethane (20 mL) cooled to 0 °C under nitrogen.
Then a solution of triethylamine (4.5 mL, 3.24 g, 32.0 mmol) in
dichloromethane (20 mL) was added slowly, and the mixture was stirred
at room temperature for 24 h. Formation of the intermediate diamide/
dialcohol was monitored by TLC. After completion, the mixture was
washed with NH4Cl and dried with sodium sulfate. The solution was
concentrated under reduced pressure to give the corresponding crude
bis(amide), which was purified by column chromatography (EtOAc:
hexane, 2:1), affording a white solid of analytically pure bis(amide)-m-
carboranes.
General Synthesis of Carboxes (7ꢀ9). DAST (396 μL, 3.0
mmol) was slowly added to a solution of bis(hydroxyamide)-m-carbor-
ane (1.0 mmol) in CH2Cl2 (5 mL) at ꢀ20 °C under nitrogen atmo-
sphere. After 12 h stirring at ꢀ20 °C, ammonium hydroxide solution
(1 mL) was added slowly. Subsequently the cooling bath was removed
and water (20 mL) was added. When the reaction mixture had reached
roomtemperature, twolayers were separated and theaqueous layerswere
extracted with CH2Cl2 (20 mL ꢁ 2). The combined organic layer was
dried over MgSO4 and concentrated under reduced pressure. The crude
product was purified by column chromatography (EtOAc/hexane, 1:3)
to give bis(oxazolines)-m-carboranes as analytically pure white solids.
[(S,S)-Carbox-iPr]H (7). This compound was prepared from 4 (402
mg, 1.0 mmol) to give 7 (353 mg, 96%) as a white solid. Colorless needle
crystals suitable for X-ray analysis were obtained by recrystallization
from a hexane solution. [R]25D = ꢀ1.14 (c 1.0 in CHCl3), Rf = 0.38, Mp:
43ꢀ45 °C. 1H NMR (400 MHz, CDCl3): δ 4.26 (dd, J = 9.2, 8.4 Hz, 2H,
oxazolineꢀCH), 4.06 (t, J = 8.4 Hz, 2H, oxazolineꢀCH2), 3.98 (m, 2H,
oxazolineꢀCH2), 3.25ꢀ1.22 (bm, 10H, BꢀH), 1.79 (m, 2H, iPrꢀCH),
0.88 (d, J = 6.8 Hz, 6H, iPrꢀCH3), 0.82 (d, J = 6.8 Hz, 6H, iPrꢀCH3).
13C NMR (100 MHz, CDCl3): δ 158.29 (2C, CdN), 72.4 (2C, CHN),
71.52 (2C, CH2ꢀO), 66.87 (2C, clusterꢀC), 32.1 (2C, iPrꢀCH), 18.1,
17.32 (4C, iPrꢀCH3). 11B NMR (96.3 MHz; CDCl3): δ ꢀ9.33
to ꢀ19.37 (bm, 10B). IR (KBr, cmꢀ1): ν(CH) 2962 (m), 2927 (m),
ν(BH) 2623 (S), ν(CdN) 1666 (S), ν(CH) 1465 (m), ν(CꢀO) 1284
(m), ν(CꢀN) 1107 (m), ν(BꢀB) 1055 (S), ν(CH) 806 (m), 736 (m).
MS (ESI, negative): m/z 368.3 (M)ꢀ. Elemental analysis calcd for
C14H30B10N2O2: C, 45.88; H, 8.25; N, 7.64%. Found: C, 45.72; H, 8.19;
N, 7.58%.
[(R,R)-Carbox-Ph]H (8). This compound was prepared from 5 (472
mg, 1.0 mmol) to give 8 (399 mg, 92%) as a white solid. [R]25D = þ1.64
(c 1.04 in CHCl3), Rf = 0.38, Mp: 118ꢀ120 °C. 1H NMR (400 MHz,
CDCl3): δ 7.34ꢀ7.12 (m, 10H, aromaticꢀCH), 5.19 (dd, J = 10.4, 8.0
Hz, 2H, oxazolineꢀCH), 4.65 (dd, J = 10.4, 8.4 Hz, 2H,
oxazolineꢀCH2), 4.14 (t, J = 8.4 Hz, 2H, oxazolineꢀCH2),
3.95ꢀ1.56 (bm, 10H, BꢀH). 13C NMR (100 MHz, CDCl3): δ
159.88 (2C, CdN), 140.99, 128.77, 127.84, 126.26 (10C,
aromaticꢀCH), 76.67 (2C, CHN), 70.1 (2C, CH2ꢀO), 66.66 (2C,
clusterꢀC). 11B NMR (96.3 MHz; CDCl3): δ ꢀ10.51 to ꢀ19.97 (bm,
10B). IR (KBr, cmꢀ1): ν(CH) 2973 (m), 2932 (m), ν(BH) 2620
(S), ν(CdN) 1657 (S), ν(CH) 1486 (m), ν(CꢀO) 1282 (m),
ν(CꢀN) 1115 (m), ν(BꢀB) 1052 (S), ν(CH) 968 (m), 745 (m).
MS (ESI, negative): m/z 436.2 (M)ꢀ. Elemental analysis calcd for
C20H26B10N2O2: C, 55.28; H, 6.03; N, 6.45%. Found: C, 55.21; H,
5.98; N, 6.39%.
(S,S)-1,7-Bis(1-hydroxy-3-methylbutanamide-2-yl)-1,7-dicarba-
closo-dodecaborane (4). This compound was prepared from L-valinol
(969 mg, 9.4 mmol) to give 4 (1.68 g, 98%) as a white solid. A sample
suitable for X-ray analysis was prepared by slow evaporation of a
methanol solution in air to give colorless needle crystals. [R]25D = ꢀ3.43
1
(c 1 in CHCl3), Rf = 0.55, Mp: 120ꢀ122 °C. H NMR (400 MHz,
CDCl3): δ 6.07 (d, JCH = 10.0 Hz, 2H, NH), 3.75ꢀ1.52 (bm, 10H,
BꢀH), 3.65 (m, 4H, CH2OH), 3.61 (m, 2H, CHNH), 1.87 (hept, 2H,
iPrꢀCH), 0.95, 0.87 (dd, 12H, J = 8.8 Hz, J = 10.0 Hz, iPrꢀCH3). 13
C
NMR (100 MHz, CDCl3): δ 158.37 (2C, CONH), 72.4 (2C, CHNH),
71.52 (2C, CH2OH), 66.75 (2C, clusterꢀC), 32.11 (2C, iPrꢀCH),
18.17, 17.29 (4C, iPrꢀCH3). 11B NMR (96.3 MHz; CDCl3): δ ꢀ9.84
to ꢀ18.56 (bm, 10B). IR (KBr, cmꢀ1): ν(OH) 3413 (S), ν(NH) 3286
(S), ν(CH) 2962 (S), 2896 (W), ν(BH) 2596 (S), ν(CdO) 1658 (S),
ν(NH) 1531 (S), ν(CH) 1470 (m), ν(CꢀO) 1288 (m), ν(BꢀB) 1056
(m), ν(CH) 829 (m), 732 (m). MS (ESI, negative): m/z 402.4 (Mꢀ).
Elemental analysis calcd for C14H34B10N2O4: C, 51.05; H, 6.43; N,
5.95%. Found: C, 50.92; H, 6.31; N, 5.86%.
(R,R)-1,7-Bis(1-hydroxy-2-phenylacetamide-2-yl)-1,7-dicarba-clo-
so-dodecaborane (5). This compound was prepared from D-phenylgy-
cinol (1.28 g, 9.4 mmol) to give 5 (1.95 g, 97%) as a white solid. [R]25
=
D
1
þ2.68 (c 1.04 in CHCl3), Rf = 0.59, Mp: 71ꢀ73 °C. H NMR (400
MHz, CDCl3): δ 7.38ꢀ7.18 (m, 10H, aromaticꢀCH), 6.67 (d, J = 6.4
Hz, 2H, NH), 3.82ꢀ1.55 (bm, 10H, BꢀH), 4.9 (m, 2H, CHNH), 3.84
(m, 4H, CH2OH). 13C NMR (100 MHz, CDCl3): δ 159.68 (2C,
CONH), 137.93, 128.66, 127.77, 126.05 (10C, aromatic CH), 72.82
(2C, CHNH), 65.68 (2C, clusterꢀC), 56.14 (2C, CH2OH). 11B NMR
(96.3 MHz; CDCl3): δ ꢀ9.92 to ꢀ18.54 (bm, 10B). IR (KBr, cmꢀ1):
ν(OH) 3417 (S), ν(NH) 3400 (S), ν(CH) 2935 (W), 3031 (W),
ν(BH) 2603 (S), ν(CdO) 1678 (S), ν(NH) 1556 (S), ν(CH) 1454
(m), ν(CꢀO) 1280 (m), ν(BꢀB) 1068 (S), ν(CH) 837 (m), 759 (m).
MS (ESI, negative): m/z 472.4 (Mꢀ). Elemental analysis calcd for
C20H30B10N2O4: C, 51.05; H, 6.43; N, 5.95%. Found: C, 50.98; H, 6.39;
N, 5.87%.
[(S,S)-Carbox-Bn]H (9). This compound was prepared from 6 (499
mg, 1.0 mmol) to give 9 (434 mg, 94%) as a white solid. A sample
suitable for X-ray analysis was prepared by slow evaporation of a mixture
of dichloromethane/hexane solution in air to give colorless needle
(S,S)-1,7-Bis(1-hydroxy-3-phenylpropanamide-2-yl)-1,7-dicarba-
closo-dodecaborane (6). This compound was prepared from L-pheny-
lalaninol (1.42 g, 9.4 mmol) to give 6 (2.02 g, 95%) as a white solid.
[R]25D = ꢀ2.48 (c 1.3 in CHCl3), Rf = 0.53, Mp: 72ꢀ74 °C. 1H NMR
(400 MHz, CDCl3): δ 7.31ꢀ7.14 (m, 10H, aromaticꢀCH), 6.13 (d, J =
7.6 Hz, 2H, NH), 4.12 (m, 2H, CHNH), 3.75ꢀ1.62 (bm, 10H, BꢀH),
3.6 (m, 4H, CH2OH), 2.86 (m, 4H, BnꢀCH2). 13C NMR (100 MHz,
CDCl3): δ 159.34 (2C, CONH), 136.48, 129.43, 128.46, 126.64 (10C,
crystals. [R]25 = ꢀ2.85 (c 1.1 in CHCl3), Rf = 0.42, Mp:
D
1
108ꢀ110 °C. H NMR (400 MHz, CDCl3): δ 7.31ꢀ7.13 (m, 10H,
aromaticꢀCH), 4.40 (m, 2H, oxazolineꢀCH), 4.20 (dd, J = 9.6, 8.8 Hz,
2H, oxazolineꢀCH2), 4.03 (dd, J = 8.8, 6.8 Hz, 2H, oxazolineꢀCH2),
3.65ꢀ1.62 (bm, 10H, BꢀH), 2.98 (dd, J = 14.0, 4.8 Hz, 2H, BnꢀCH2),
2.70 (dd, J = 14.0, 8.0 Hz, 2H, BnꢀCH2). 13C NMR (100 MHz,
CDCl3): δ 159.34 (2C, CdN), 136.48, 129.43, 128.46, 126.64 (10C,
aromaticꢀCH), 73.02 (2C, CHN), 67.56 (2C, clusterꢀC), 54.62 (2C,
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CH2ꢀO), 40.5 (2C, BnꢀCH2). 11B NMR (96.3 MHz; CDCl3):
δ ꢀ9.76 to ꢀ20.03 (bm, 10B). IR (KBr, cmꢀ1): ν(CH) 2986 (m), 2908
(m), ν(BH) 2619 (S), ν(CdN) 1658 (S), ν(CH) 1492 (m), ν(CꢀO)
1261 (m), ν(CꢀN) 1114 (m), ν(BꢀB) 1055 (S), ν(CH) 950 (m), 748
(m). MS (ESI, negative): m/z 464.4 (M)ꢀ. Elemental analysis calcd
for C22H30B10N2O2: C, 57.12; H, 6.54; N, 6.06%. Found: C, 57.09; H,
6.51; N, 6.02%.
General Synthesis of 10ꢀ12. A solution of 3 (1.15 g, 4.27 mmol)
in dichloromethane (20 mL) was slowly added via cannula to a sol-
ution of amino alcohols (9.39 mmol) and triethylamine (4.5 mL, 3.24 g,
32.0 mmol) in dichloromethane (40 mL) at 0 °C. The mixture was
stirred at room temperature for 24 h. Formation of the intermediate
diamide/dialcohol was monitored by TLC. Then, methanesulfonyl
chloride (1.07 g, 9.39 mmol) was added at 0 °C, and the mixture was
stirred at room temperature for 8 h. Formation of the product (10ꢀ12)
was monitored by TLC. At 0 °C, aqueous potassium carbonate (1 N, ca.
30 mL) was added, and the mixture was extracted with ethyl acetate. The
organic layer was washed with saturated brine, dried over magnesium
sulfate, and concentrated. The crude product was purified by column
chromatography using EtOAc:hexane (1:1) as eluent.
(1.41 g, 69%) as a white solid. [R]25D = ꢀ1.95 (c 1.0 in CHCl3), Rf = 0.53,
1
Mp: 103ꢀ105 °C. H NMR (500 MHz, CDCl3): δ 7.31ꢀ7.13 (m,
10H, aromaticꢀCH), 6.13 (d, J = 9.5 Hz, 1H, NH), 4.41 (m, 1H,
oxazolineꢀCH), 4.20 (t, J = 12 Hz, 2H, oxazolineꢀCH2), 4.05 (m, 1H,
CHNH), 3.93 (m, 2H, CH2ꢀOH), 3.75ꢀ1.59 (bm, 10H, BꢀH), 3.58
(m, 2H, CH2ꢀOH), 3.0 (m, 2H, BnꢀCH2), 2.71 (m, 2H, BnꢀCH2). 13
C
NMR (125 MHz, CDCl3): δ 159.34 (1C, CONH), 157.62 (1C, CdN),
139.76, 136.48, 129.62, 128.5, 126.68 (10C, aromaticꢀCH), 73.03 (2C,
CHN), 67.59 (2C, clusterꢀC), 54.66 (2C, CH2ꢀO), 40.51 (2C,
BnꢀCH2). 11B NMR (96.3 MHz; CDCl3): δ ꢀ10.55 to ꢀ18.72 (bm,
10B). IR (KBr, cmꢀ1):ν(OH) 3434, ν(NH) 3291, ν(CH) 2961 (S), 2935
(m), ν(BH) 2615 (S), ν(CdN) 1667 (S), ν(CdN) 1629 (S), ν(CH)
1469 (m), ν(CꢀO) 1287 (m), ν(CꢀN) 11025 (m), ν(BꢀB) 1054 (S),
ν(CH) 968 (m), 827(m), 752 (m). MS (ESI, negative): m/z 480.3 (M)ꢀ.
Elemental analysis calcd for C22H32B10N2O3: C, 54.98; H, 6.71; N, 5.83%.
Found: C, 54.83; H, 6.59; N, 5.72%.
Synthesis of (S,S)-1,7-Bis(1-bromo-3-methylbutanamide-2-yl)-1,7-
dicarba-closo-dodecaborane (13). A mixture of 7 (500 mg, 1.36 mmol),
iodine (344 mg, 2.72 mmol), and AlBr3 (100 mg, 0.37 mmol) in 10 mL of
dry CH2Cl2 was refluxed for 16.5 h under nitrogen. The reaction mixture
was poured into 100 mL of ice-cold distilled water, and the organic phase
was separated from the mixture. The aqueous layer was then extracted
with diethyl ether (20 mL). The combined organic phase was washed
with dilute Na2S2O3 solution followed by water and dried over MgSO4.
After removal of the solvent, the residue was purified by recrystallization
from hexane to yield 13 (703 mg, 98%) as colorless needle crystals.
[R]25D = ꢀ63.4 (c 1.06 in CHCl3), Mp: 138ꢀ140 °C. 1H NMR (400
MHz, CDCl3): δ 5.89 (d, J = 8.8 Hz, 2H, NH), 3.75ꢀ1.58 (bm, 10H,
BꢀH), 3.66 (m, 2H, CHNH), 3.45 (m, 2H, CH2Br), 1.81 (m, 2H,
iPrꢀCH), 0.88 (d, 6H, J = 6.8 Hz, iPrꢀCH3), 0.83 (d, 6H, J = 6.8 Hz,
iPrꢀCH3). 13C NMR (100 MHz, CDCl3): δ 159.73 (2C, CONH),
75.05 (1C, clusterꢀC), 55.91 (2C, CHNH), 55.75 (1C, clusterꢀC),
35.5 (2C, CH2Br), 30.41 (2C, iPrꢀCH), 19.08, 18.52 (4C, iPrꢀCH3).
11B NMR (96.3 MHz; CDCl3): δ ꢀ11.29 to ꢀ19.12 (bm, 10B). IR
(KBr, cmꢀ1): ν(NH) 3344 (S), ν(CH) 2966 (m), 2873 (W), ν(BH)
2605 (S), ν(CdO) 1670 (S), ν(NH) 1535 (S), ν(CH) 1470 (m),
ν(CꢀO) 1296 (m), ν(BꢀB) 1060 (m), ν(CH) 745 (m). MS (ESI,
negative): m/z 608.1 (MþBr)ꢀ. Elemental analysis calcd for
C14H32B10Br2N2O2: C, 31.83; H, 6.1; N, 5.3%. Found: C, 31.8; H,
6.07; N, 5.28%.
(S,S)-1-(4-Isopropyl-4,5-dihydrooxazol-2-yl)-7-(1-hydroxy-3-methyl-
butanamide-2-yl)-1,7-dicarba-closo-dodecaborane (10). This com-
pound was prepared from L-valinol (969 mg, 9.4 mmol) to give 10
(1.18 g, 72%) as a white solid. Colorless needle crystals suitable for X-ray
analysis were obtained by recrystallization from a dichloromethane/
hexane solution. [R]25 = þ1.08 (c 1.2 in CHCl3), Rf = 0.47, Mp:
D
100ꢀ102 °C. 1H NMR (400 MHz, CDCl3): δ 6.16 (d, J = 8.4 Hz, 1H,
NH), 4.27 (t, J = 9.2 Hz, 1H, oxazolineꢀCH), 4.08 (t, J = 7.8 Hz, 2H,
oxazolineꢀCH2), 3.99 (m, 1H, CHNH), 3.62 (m, 2H, CH2OH),
3.25ꢀ1.22 (bm, 10H, BꢀH), 1.87 (m, 1H, iPrꢀCH), 1.78 (m, 1H,
iPrꢀCH), 0.93 (d, J = 7.2 Hz, 3H, iPrꢀCH3), 0.88 (d, J = 6.8 Hz, 3H,
iPrꢀCH3), 0.87 (d, J = 6.8 Hz, 3H, iPrꢀCH3), 0.83 (d, J = 7.2 Hz, 3H,
iPrꢀCH3). 13C NMR (100 MHz, CDCl3): δ 160.15 (1C, CONH),
158.37 (1C, CdN), 75.19 (1C, clusterꢀC), 72.31 (1C, CHN), 71.59
(1C, CH2ꢀO), 67.04 (1C, clusterꢀC), 62.35 (1C, CH2ꢀO), 57.85
(1C, CHN), 32.06, 28.92 (2C, iPrꢀCH), 19.43, 18.5, 18.05, 17.26 (4C,
iPrꢀCH3). 11B NMR (96.3 MHz; CDCl3): δ ꢀ10.75 to ꢀ18.54 (bm,
10B). IR (KBr, cmꢀ1): ν(OH) 3417, ν(NH) 3282, ν(CH) 2962 (S),
2931 (m), ν(BH) 2600 (S), ν(CdN) 1665 (S), ν(CdN) 1625 (S),
ν(CH) 1469 (m), ν(CꢀO) 1288 (m), ν(CꢀN) 11026 (m), ν(BꢀB)
1052 (S), ν(CH) 960 (m), 825(m), 759 (m). MS (ESI, negative): m/z
384.4 (M)ꢀ. Elemental analysis calcd for C14H32B10N2O3: C, 43.73; H,
8.39; N, 7.29%. Found: C, 43.62; H, 8.25; N, 7.05%.
(S,S)-1,7-Bis(1-chloro-3-phenylpropanamide-2-yl)-1,7-dicarba-clo-
so-dodecaborane (14). This compound was prepared from 9 (500 mg,
1.08 mmol), I2 (274 mg, 2.16 mmol), and AlCl3 (100 mg, 0.75 mmol) by
the procedure described for 13 to yield 14 (567 mg, 98%) as colorless
needle crystals. [R]25D = ꢀ47.4 (c 1.03 in CHCl3), Mp: 147ꢀ149 °C.
1H NMR (400 MHz, CDCl3): δ 7.34ꢀ7.16 (m, 10H, aromaticꢀCH),
6.0 (d, J = 8.0 Hz, 2H, NH), 4.3 (m, 2H, CHNH), 3.85ꢀ1.61 (bm, 10H,
BꢀH), 3.61 (dd, J = 11.6, 4.0 Hz, 2H, CH2Cl), 3.48 (dd, J = 11.6, 4.0 Hz,
2H, CH2Cl), 2.89 (d, J = 7.6 Hz, 4H, BnꢀCH2). 13C NMR (100 MHz,
CDCl3): δ 159.14 (2C, CONH), 135.89, 129.13, 129.1, 128.87 (10C,
aromatic CH), 74.83 (1C, clusterꢀC), 52.11 (2C, CHNH), 51.22 (1C,
clusterꢀC), 45.81 (2C, BnꢀCH2), 37.06 (2C, CH2Cl). 11B NMR (96.3
MHz; CDCl3): δ ꢀ11.29 to ꢀ19.12 (bm, 10B). IR (KBr, cmꢀ1):
ν(NH) 3348 (S), ν(CH) 3062 (W), 3028 (W), ν(BH) 2619 (S),
ν(CdO) 1666 (S), ν(NH) 1523 (S), ν(CH) 1472 (m), ν(CꢀO) 1303
(m), ν(BꢀB) 1087 (S), ν(CH) 767 (m), 740 (m). MS (ESI, negative):
m/z 662.3 (MþI)ꢀ. Elemental analysis calcd for C22H32B10Cl2N2O2:
C, 49.34; H, 6.02; N, 5.23%. Found: C, 49.29; H, 5.99; N, 5.21%.
Synthesis of [(S,S)-Carbox-iPr]RhCl2 H2O (15). A mixture of RhCl3
(R,R)-1-(4-phenyl-4,5-dihydrooxazol-2-yl)-7-(1-hydroxy-2-phenyla-
cetamide-2-yl)-1,7-dicarba-closo-dodecaborane (11). This com-
pound was prepared from D-phenylglycinol (1.28 g, 9.4 mmol) to give
11 (1.45 g, 75%) as a white solid. [R]25D = þ3.15 (c 1.0 in CHCl3), Rf =
1
0.5, Mp: 85ꢀ87 °C. H NMR (500 MHz, CDCl3): δ 7.32ꢀ7.11 (m,
10H, aromaticꢀCH), 6.71 (d, J = 5.0 Hz, 1H, NH), 5.19 (t, J = 10.0 Hz,
1H), 4.92 (m, 1H), 4.65 (t, J = 13.0 Hz, 1H), 4.17 (t, J = 11.5 Hz, 1H),
3.84 (m, 2H), 3.75ꢀ1.52 (bm, 10H, BꢀH). 13C NMR (125 MHz,
CDCl3): δ 159.89 (1C, CONH), 158.06 (1C, CdN), 140.99, 137.94,
128.78, 127.84, 126.27 (10C, aromaticꢀCH), 76.56(1C, CHN), 75.83
(1C, clusterꢀCHN), 70.13 (1C, CH2O), 66.66 (2C, clusterꢀC), 56.32
(1C, CH2ꢀO). 11B NMR (96.3 MHz; CDCl3): δ ꢀ10.58 to ꢀ18.12
(bm, 10B). IR (KBr, cmꢀ1): ν(OH) 3421, ν(NH) 3288, ν(CH) 2965
(S), 2945 (m), ν(BH) 2612 (S), ν(CdN) 1667 (S), ν(CdN) 1631 (S),
ν(CH) 1471 (m), ν(CꢀO) 1287 (m), ν(CꢀN) 11025 (m), ν(BꢀB)
1054 (S), ν(CH) 827(m), 747 (m). MS (ESI, negative): m/z 452.5
(M)ꢀ. Elemental analysis calcd for C20H28B10N2O3: C, 53.08; H, 6.24;
N, 6.19%. Found: C, 52.91; H, 6.05; N, 6.01%.
3
3
3H2O (289 mg, 1.1 mmol), 7 (368 mg, 1.0 mmol), and sodium
bicarbonate (84 mg, 1.0 mmol) were placed in a 30 mL flask. After
addition of methanol (10 mL) and water (1 mL), the mixture was heated
at 60 °C for 5 h. The concentrated residue was purified by column
chromatography on silica gel with ethyl acetate/hexane (1:1) as eluent
(S,S)-1-(4-Benzyl-4,5-dihydrooxazol-2-yl)-7-(1-hydroxy-3-phenyl-
propanamide-2-yl)-1,7-dicarba-closo-dodecaborane (12). This com-
pound was prepared from L-phenylalaninol (1.42 g, 9.4 mmol) to give 12
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to give 15 in 79% yield (440 mg, 0.79 mmol) as a yellow solid. Rf = 0.34,
Mp: 252ꢀ254 °C (decomp). 1H NMR (500 MHz, CDCl3): δ 4.29 (m,
2H, oxazolineꢀCH), 4.1 (t, J = 9 Hz, 2H, oxazolineꢀCH2), 3.99 (m,
2H, oxazolineꢀCH2), 3.73ꢀ1.52 (bm, 9H, BꢀH), 3.46 (bs, 2H, H2O),
1.81 (m, 2H, iPrꢀCH), 0.88 (d, J = 5.5 Hz, 6H, iPrꢀCH3), 0.83 (d, J =
8.5 Hz, 6H, iPrꢀCH3). 13C NMR (125 MHz, CDCl3): δ 160.21 (2C,
CdN), 74.00 (2C, CHN), 71.80 (2C, CH2ꢀO), 67.57 (2C,
clusterꢀC), 33.24 (2C, iPrꢀCH), 19.96, 19.09 (2C, iPrꢀCH3). 11B-
{1H} NMR (96.3 MHz; CDCl3): δ 0.77 (s, 1B), ꢀ9.56 to ꢀ18.61 (bm,
9B). IR (KBr, cmꢀ1): ν(H2O) 3421 (m), ν(CH) 3031 (m), 2927 (m),
ν(BH) 2607 (S), ν(CdN) 1624 (S), ν(CH) 1470 (m), ν(CꢀO) 1288
(m), ν(CꢀN) 1105 (m), ν(BꢀB) 1052 (S), ν(CH) 806 (m), 736 (m).
MS (ESI, negative): m/z 592.8 (MþCl)ꢀ. Elemental analysis calcd for
C14H31B10Cl2N2O3Rh: C, 30.17; H, 5.61; N, 5.03%. Found: C, 30.04;
H, 5.52; N, 4.93%.
(100 MHz, CDCl3): δ 175.11 (2C, CO), 160.11 (2C, CdN), 72.41 (2C,
CHN), 71.52 (2C, CH2ꢀO), 66.85 (2C, clusterꢀC), 32.22 (2C,
iPrꢀCH), 23.72 (2C, acetateꢀCH3), 18.15, 17.39 (4C, iPrꢀCH3).
11B{1H} NMR (96.3 MHz; CDCl3): δ 0.92 (s, 1B), ꢀ9.49 to ꢀ18.54
(bm, 9B). IR (KBr, cmꢀ1): ν(H2O) 3422 (m), ν(CH) 3032 (m), 2925
(m), ν(BH) 2611 (S), ν(CdO) 1747 (S), ν(CdN) 1625 (S), ν(CH)
1472 (m), ν(CꢀO) 1287 (m), ν(CꢀN) 1105 (m), ν(BꢀB) 1049 (S),
ν(CH) 748(m). MS(ESI, negative):m/z604.4 (M)ꢀ. Elementalanalysis
calcd for C18H37B10N2O7Rh: C, 35.76; H, 6.17; N, 4.63%. Found: C,
35.68; H, 6.07; N, 4.51%.
[(R,R)-Carbox-Ph]Rh(OAc)2 H2O (19). This compound was pre-
3
pared from 16 (312 mg, 0.5 mmol) by the procedure described for 18
to give 19 (336 mg, 0.5 mmol, >99%) as an orange solid. Rf = 0.49, Mp:
233ꢀ235 °C (decomp). 1H NMR (500 MHz, CDCl3): δ 7.35ꢀ7.12 (m,
10H, aromaticꢀCH), 5.19 (dd, J = 10.0, 9.5 Hz, 2H, oxazolineꢀCH),
4.99 (bs, 2H, H2O), 4.63 (dd, J = 15.0, 10.0 Hz, 2H, oxazolineꢀCH2),
4.14 (t, J = 10.0 Hz, 2H, oxazolineꢀCH2), 3.79ꢀ1.58 (bm, 9H, BꢀH),
1.67 (s, 6H, acetateꢀCH3).13C NMR (125 MHz, CDCl3): δ 176.69
(2C, CO), 160.00 (2C, CdN), 140.49, 129.14, 126.65, 125.23 (10C,
aromaticꢀCH), 75.56 (2C, CHN), 69.56 (2C, CH2ꢀO), 67.53 (2C,
clusterꢀC), 23.53 (2C, acetateꢀCH3). 11B NMR (96.3 MHz; CDCl3):
δ 0.86 (s, 1B), ꢀ9.69 to ꢀ19.24 (bm, 9B). IR (KBr, cmꢀ1): ν(H2O)
3425 (m), ν(CH) 2974 (m), 2930 (m), ν(BH) 2622 (S), ν(CdO) 1745
(S), ν(CdN) 1657 (S), ν(CH) 1482 (m), ν(CꢀO) 1286 (m), ν-
(CꢀN) 1111 (m), ν(BꢀB) 1045 (S), ν(CH) 968 (m), 743 (m). MS
(ESI, negative): m/z 672.5 (M)ꢀ. Elemental analysis calcd for
C24H33B10N2O7Rh: C, 42.86; H, 4.95; N, 4.17%. Found: C, 42.62; H,
4.76; N, 4.01%.
[(R,R)-Carbox-Ph]RhCl2 H2O (16). This compound was prepared
3
from 8 (434 mg, 1.0 mmol) by the procedure described for 15 to give 16
in 76% yield (475 mg, 0.76 mmol) as a yellow solid. Rf = 0.34, Mp:
240ꢀ242 °C (decomp). 1H NMR (500 MHz, CDCl3): δ 7.29ꢀ7.14 (m,
10H, aromaticꢀCH), 5.26 (dd, J = 12.0, 10.2 Hz, 2H, oxazolineꢀCH),
4.69 (dd, J = 15.0, 11.5 Hz, 2H, oxazolineꢀCH2), 4.18 (dd, J = 16, 12.6
Hz, 2H, oxazolineꢀCH2), 3.79ꢀ1.58 (bm, 9H, BꢀH), 3.58 (bs, 2H,
H2O), 13C NMR (125 MHz, CDCl3): δ 159.67 (2C, CdN), 141.02,
129.68, 129.03, 126.08 (10C, aromatic CH), 76.68 (2C, CHN), 70.13
(2C, CH2ꢀO), 66.32 (2C, clusterꢀC). 11B NMR (96.3 MHz; CDCl3):
δ 0.82 (s, 1B), ꢀ9.65 to ꢀ19.35 (bm, 9B). IR (KBr, cmꢀ1): ν(H2O)
3421 (m), ν(CH) 2972 (m), 2935 (m), ν(BH) 2624 (S), ν(CdN) 1658
(S), ν(CH) 1485 (m), ν(CꢀO) 1288 (m), ν(CꢀN) 1112 (m),
ν(BꢀB) 1049 (S), ν(CH) 967 (m), 745 (m). MS (ESI, negative): m/
z 660.8 (MþCl)ꢀ. Elemental analysis calcd for C20H27B10Cl2N2O3Rh:
C, 38.41; H, 4.35; N, 4.48%. Found: C, 38.32; H, 4.27; N, 4.33%.
[(S,S)-Carbox-Bn]Rh(OAc)2 H2O (20). A procedure analogous to the
3
synthesis of 18 was employed using 17 (326 mg, 0.5 mmol) to give 20 as
a yellowishꢀorange solid (35 mg, 0.5 mmol) in quantitative yield. Rf =
1
0.44, Mp: 246ꢀ248 °C (decomp). H NMR (500 MHz, CDCl3): δ
[(S,S)-Carbox-Bn]RhCl2 H2O (17). A procedure analogous to the
3
7.28ꢀ7.14 (m, 10H, aromaticꢀCH), 5.05 (bs, 2H, H2O), 4.41 (m, 2H,
oxazolineꢀCH), 4.23 (m, 2H, oxazolineꢀCH2), 4.05 (m, 2H,
oxazolineꢀCH2), 3.71ꢀ1.55 (bm, 9H, BꢀH), 3.01 (dd, JCH = 5.0 Hz,
JCH = 10.0 Hz, 2H, BnꢀCH2), 2.73 (dd, JCH = 10.0 Hz, JCH = 10.0 Hz,
2H, BnꢀCH2), 1.67 (s, 6H, acetateꢀCH3).13C NMR (125 MHz,
CDCl3): δ 174.84 (2C, CO), 159.89 (2C, CdN), 138.07, 130.09,
128.88, 127.23 (10C, aromaticꢀCH), 73.09 (2C, CHN), 67.56 (2C,
CH2ꢀO), 54.62 (2C, clusterꢀC), 40.51 (2C, BnꢀCH2), 23.06 (2C,
acetateꢀCH3). 11B NMR (96.3 MHz; CDCl3): δ 0.88 (s, 1B), ꢀ9.53
to ꢀ18.85 (bm, 9B). IR (KBr, cmꢀ1): ν(H2O) 3427 (m), ν(CH) 2990
(m), 2902 (m), ν(BH) 2625 (S), ν(CdO) 1751 (S), ν(CdN) 1663
(S), ν(CH) 1492 (m), ν(CꢀO) 1286 (m), ν(CꢀN) 1113 (m),
ν(BꢀB) 1052 (S), ν(CH) 968 (m), 745 (m). MS (ESI, negative): m/
z 700.5 (M)ꢀ. Elemental analysis calcd for C26H37B10N2O7Rh: C,
44.57; H, 5.32; N, 4.0%. Found: C, 44.39; H, 5.26; N, 3.84%.
synthesis of 15 was employed using 9 (464 mg, 1.0 mmol) to give 17 as a
yellow solid in 82% yield (553 mg, 0.82 mmol). Rf = 0.32, Mp:
1
215ꢀ217 °C (decomp). H NMR (500 MHz, CDCl3): δ 7.32ꢀ7.14
(m, 10H, aromaticꢀCH), 4.45 (m, 2H, oxazolineꢀCH), 4.21 (t, JCH
=
12.5 Hz, 2H, oxazolineꢀCH2), 4.06 (t, JCH = 9.5 Hz, 2H,
oxazolineꢀCH2), 3.65ꢀ1.55 (bm, 9H, BꢀH), 3.58, (bs, 2H, H2O),
3.04, 2.98 (dd, JCH = 11.5 Hz, JCH = 11.5 Hz, 2H, BnꢀCH2), 2.75, 2.67
(dd, JCH 9.5 Hz, JCH = 13.0 Hz, 2H, BnꢀCH2). 13C NMR (125 MHz,
CDCl3): δ 160.15 (2C, CdN), 135.89, 129.99, 129.37, 126.23 (10C,
aromatic CH), 71.99 (2C, CHN), 66.68 (2C, CH2ꢀO), 54.68 (2C,
clusterꢀC), 40.81 (2C, BnꢀCH2). 11B NMR (96.3 MHz; CDCl3):
δ 0.89 (s, 1B), ꢀ9.55 to ꢀ18.87 (bm, 9B). IR (KBr, cmꢀ1): ν(H2O)
3429 (m), ν(CH) 2989 (m), 2905 (m), ν(BH) 2622 (S), ν(CdN) 1657
(S), ν(CH) 1495 (m), ν(CꢀO) 1266 (m), ν(CꢀN) 1112 (m),
ν(BꢀB) 1057 (S), ν(CH) 955 (m), 745 (m). MS (ESI, negative): m/
z 687.9 (MþCl)ꢀ. Elemental analysis calcd for C22H31B10Cl2N2O3Rh:
C, 40.44; H, 4.78; N, 4.24%. Found: C, 40.3; H, 4.59; N, 3.99%.
Synthesis of [(S,S)-Carbox-iPr]NiCl (21). A mixture of 7 (368 mg, 1.0
mmol) and Ni(COD)2 (275 mg, 1.0 mmol) in 3 mL of dry CH3CN was
refluxed under nitrogen for 24 h, by which time the reaction was deemed
complete by TLC (EtOAc/methanol 10:1). The resulting yellowish
reaction mixture was cooled to room temperature, and 2 equiv of
(n-Bu)4NCl dissolved in 2 mL of dry CH3CN was added to this mixture
via syringe. After 1 h, the solution was filtered through a layer of Celite,
and the organic filtrate was evaporated in vacuo to yield crude product.
Recrystallization from the dichloromethane/pentane mixture yielded
analytically pure 21 as a yellow solid in 73% yield (335 mg, 0.73 mmol).
Mp: 203ꢀ205 °C (decomp). 1H NMR (500 MHz, CDCl3): δ 4.28 (m,
2H, oxazolineꢀCH), 4.05 (m, 2H, oxazolineꢀCH2), 3.96 (m, 4H,
oxazolineꢀCH2), 3.73ꢀ1.55 (bm, 9H, BꢀH), 1.78 (m, 2H, iPrꢀCH),
0.89, 0.85 (dd, JCH = 7.0 Hz, JCH = 7.0 Hz, 12H, iPrꢀCH3). 13C NMR
(125 MHz, CDCl3): δ 159.77 (2C, CdN), 72.88 (2C, CHN), 70.74
(2C, CH2ꢀO), 65.96 (2C, clusterꢀC), 32.99 (2C, iPrꢀCH), 18.84,
17.27 (4C, iPrꢀCH3). 11B{1H} NMR (96.3 MHz; CDCl3): δ 0.93 (s,
Synthesis of [(S,S)-Carbox-iPr]Rh(OAc)2 H2O (18). A solution of 15
3
(278 mg, 0.50 mmol) and silver acetate (166 mg, 2.0 mmol) in dry
dichloromethane (10 mL) was stirred for 24 h at room temperature
under nitrogen and wrapped in aluminum foil to protect the reaction
mixture from light, and the reaction was monitored by TLC (EtOAc/
methanol, 10:1). The reaction mixture was filtered through Celite
eluting with dichloromethane. The solvent was removed under reduced
pressure, and the concentrated residue was purified by column chroma-
tography on silica gel with ethyl acetate/methanol (10:1) to give 18 as an
orange solid (301 mg, 0.5 mmol, >99%). Rf = 0.47, Mp: 241ꢀ243 °C
(decomp). 1H NMR (500 MHz, CDCl3): δ 4.88 (bs, 2H, H2O),
4.26 (t, J = 9.0 Hz, 2H, oxazolineꢀCH), 4.09 (t, J = 7.8 Hz, 2H,
oxazolineꢀCH2), 3.99 (m, 2H, oxazolineꢀCH2), 3.76ꢀ1.55 (bm, 9H,
BꢀH), 1.82 (m, 2H, iPrꢀCH), 1.69 (s, 6H, acetateꢀCH3), 0.87 (d, J =
8.5 Hz, 6H, iPrꢀCH3), 0.82 (d, J = 8.5 Hz, 6H, iPrꢀCH3). 13C NMR
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1B), ꢀ9.32 to ꢀ18.55 (bm, 9B). IR (KBr, cmꢀ1): ν(CH) 3035 (m),
2930 (m), ν(BH) 2619 (S), ν(CdN) 1652 (S), ν(CH) 1471 (m),
ν(CꢀO) 1287 (m), ν(CꢀN) 1104 (m), ν(BꢀB) 1055 (S), ν(CH) 862
(m), 745 (m). MS (ESI, negative): m/z 494.6 (MþCl)ꢀ. Elemental
analysis calcd for C14H29B10ClN2NiO2: C, 36.58; H, 6.36; N, 6.09%.
Found: C, 36.42; H, 6.29; N, 5.94%.
as a yellow solid in 74% yield (425 mg, 0.74 mmol). Mp: 229ꢀ231 °C
(decomp). 1H NMR (500 MHz, CDCl3): δ 7.36ꢀ7.18 (m, 10H,
aromaticꢀCH), 5.27 (t, JCH = 15.5 Hz, 2H, oxazolineꢀCH), 4.65 (t,
JCH = 10.5 Hz, 2H, oxazolineꢀCH2), 4.16 (t, JCH = 11.0 Hz, 2H,
oxazolineꢀCH2), 3.82ꢀ1.57 (bm, 9H, BꢀH). 13C NMR (125 MHz,
CDCl3): δ 160.27 (2C, CdN), 141.14, 128.89, 128.37, 126.27 (10C,
aromaticꢀCH), 76.68 (2C, CHN), 70.04 (2C, CH2ꢀO), 66.27 (2C,
clusterꢀC). 11B NMR (96.3 MHz; CDCl3): δ 0.87 (s, 1B), ꢀ9.52
to ꢀ19.24 (bm, 9B). IR (KBr, cmꢀ1): ν(CH) 2974 (m), 2936 (m),
ν(BH) 2623 (S), ν(CdN) 1662 (S), ν(CH) 1487 (m), ν(CꢀO) 1287
(m), ν(CꢀN) 1115 (m), ν(BꢀB) 1049 (S), ν(CH) 968 (m), 749 (m).
MS (ESI, negative): m/z 610.9 (MþCl)ꢀ. Elemental analysis calcd for
C20H25B10ClN2O2Pd: C, 41.75; H, 4.38; N, 4.87%. Found: C, 41.66; H,
4.21; N, 4.72%.
[(S,S)-Carbox-Bn]PdCl (26). A procedure analogous to the synthesis
of 24 was employed using 9 (464 mg, 1.0 mmol) to give 26 as a yellow
solid in 80% yield (482 mg, 0.8 mmol). Mp: 242ꢀ244 °C (decomp). 1H
NMR (500 MHz, CDCl3): δ 7.32ꢀ7.10 (m, 10H, aromaticꢀCH), 4.45
(m, 2H, oxazolineꢀCH), 4.21 (t, JCH = 15.5 Hz, 2H, oxazolineꢀCH2),
4.05 (t, JCH = 9.5 Hz, 2H, oxazolineꢀCH2), 3.79ꢀ1.56 (bm, 9H, BꢀH),
3.07, 2.97 (dd, JCH = 10.5 Hz, JCH = 9.5 Hz, 2H, BnꢀCH2), 2.73, 2.69
(dd, JCH = 8.5 Hz, JCH = 10.0 Hz, 2H, BnꢀCH2). 13C NMR (125 MHz,
CDCl3): δ 159.17 (2C, CdN), 136.56, 130.00, 129.10, 126.32 (10C,
aromaticꢀCH), 71.98 (2C, CHN), 67.68 (2C, CH2ꢀO), 45.32 (2C,
clusterꢀC), 40.68 (2C, BnꢀCH2). IR (KBr, cmꢀ1): ν(CH) 2993 (m),
2907 (m), ν(BH) 2620 (S), ν(CdN) 1659 (S), ν(CH) 1492 (m),
ν(CꢀO) 1284 (m), ν(CꢀN) 1114 (m), ν(BꢀB) 1053 (S), ν(CH) 962
(m), 749 (m). MS (ESI, negative): m/z 638.9 (MþCl)ꢀ. Elemental
analysis calcd for C22H29B10ClN2O2Pd: C, 43.79; H, 4.84; N, 4.64%.
Found: C, 43.65; H, 4.71; N, 4.52%.
[(R,R)-Carbox-Ph]NiCl (22). A procedure analogous to the synthesis
of 21 was employed using pincer ligand 8 (434 mg, 1.0 mmol) to give 22
as a yellow solid in 86% yield (453 mg, 0.86 mmol). Mp: 196ꢀ198 °C
(decomp). 1H NMR (500 MHz, CDCl3): δ 7.35ꢀ7.11 (m, 10H,
aromaticꢀCH), 5.21 (t, JCH = 10 Hz, 2H, oxazolineꢀCH), 4.68 (t,
JCH = 10.5 Hz, 2H, oxazolineꢀCH2), 4.16 (t, JCH = 5.5 Hz, 2H,
oxazolineꢀCH2), 3.85ꢀ1.58 (bm, 9H, BꢀH). 13C NMR (125 MHz,
CDCl3): δ 159.89 (2C, CdN), 140.99, 128.78, 127.85, 126.27 (10C,
aromaticꢀCH), 75.06 (2C, CHN), 70.74 (2C, CH2ꢀO), 65.96 (2C,
clusterꢀC). 11B NMR (96.3 MHz; CDCl3): δ 0.72 (s, 1B), ꢀ9.52
to ꢀ19.16 (bm, 9B). IR (KBr, cmꢀ1): ν(CH) 2975 (m), 2939 (m),
ν(BH) 2621 (S), ν(CdN) 1662 (S), ν(CH) 1487 (m), ν(CꢀO) 1285
(m), ν(CꢀN) 1114 (m), ν(BꢀB) 1045 (S), ν(CH) 962 (m), 745 (m).
MS (ESI, negative): m/z 563.18 (MþCl)ꢀ. Elemental analysis calcd for
C20H25B10ClN2NiO2: C, 45.52; H, 4.78; N, 5.31%. Found: C, 45.39; H,
4.61; N, 5.22%.
[(S,S)-Carbox-Bn]NiCl (23). A procedure analogous to the synthesis
of 21 was employed using 9 (464 mg, 1.0 mmol) to give 23 as a yellow
solid in 82% yield (455 mg, 0.82 mmol). Mp: 214ꢀ216 °C (decomp).
1H NMR (500 MHz, CDCl3): δ 7.30ꢀ7.13 (m, 10H, aromaticꢀCH),
4.61 (m, 2H, oxazolineꢀCH), 4.23 (t, JCH = 8.5 Hz, 2H, oxazolineꢀCH2),
4.03 (t, JCH = 8.0 Hz, 2H, oxazolineꢀCH2), 3.85ꢀ1.56 (bm, 9H, BꢀH),
3.02, 2.98 (dd, JCH = 7.5 Hz, JCH = 8.0 Hz, 2H, BnꢀCH2), 2.75, 2.61 (dd,
JCH = 7.5 Hz, JCH = 7.5 Hz, 2H, BnꢀCH2). 13C NMR (125 MHz, CDCl3):
δ 160.03 (2C, CdN), 137.86, 129.69, 128.56, 126.25 (10C, aromaticꢀCH),
74.00 (2C, CHN), 67.44 (2C, CH2ꢀO), 54.32 (2C, clusterꢀC), 40.65
(2C, BnꢀCH2). 11B NMR (96.3 MHz; CDCl3): δ 0.73 (s, 1B), ꢀ9.57
to ꢀ18.85 (bm, 9B). IR (KBr, cmꢀ1): ν(CH) 2969 (m), 2907 (m),
ν(BH) 2623 (S), ν(CdN) 1659 (S), ν(CH) 1494 (m), ν(CꢀO) 1286
(m), ν(CꢀN) 1115 (m), ν(BꢀB) 1052 (S), ν(CH) 968 (m), 747 (m).
MS (ESI, negative): m/z 591.2 (Mþ Cl)ꢀ. Elemental analysis calcd for
C22H29B10ClN2NiO2: C, 47.55; H, 5.26; N, 6.38%. Found: C, 47.34; H,
5.17; N, 6.26%.
Synthesis of [(S,S)-Carbox-iPr]PdCl (24). A mixture of 7 (368 mg, 1.0
mmol) and Pd(CH3CN)4[BF4]2 (444 mg, 1.0 mmol) in 3 mL of dry
CH3CN was refluxed under nitrogen for 24 h, by which time the reaction
was deemed complete by TLC (EtOAc/methanol 10:1). The resulting
yellowish compound was cooled to room temperature, and 2 equiv of (n-
Bu)4NCl in 2 mL of dry CH3CN was added via syringe. After 1 h, the
solution was filtered through a layer of Celite, and the organic filtrate was
evaporated in vacuo to yield a crude product. Recrystallization from the
dichloromethane/pentane mixture yielded analytically pure 24 as a
yellow solid in 76% yield (385 mg, 0.76 mmol). Mp: 224ꢀ226 °C
(decomp). 1H NMR (500 MHz, CDCl3): δ 4.28 (t, JCH = 8.0 Hz, 2H,
oxazolineꢀCH), 4.08 (m, 2H, oxazolineꢀCH2), 3.98 (m, 4H,
oxazolineꢀCH2), 3.76ꢀ1.55 (bm, 9H, BꢀH), 1.77 (m, 2H, iPrꢀCH),
0.88, 0.82 (dd, JCH = 6.5 Hz, JCH = 8.5 Hz, 12H, iPrꢀCH3). 13C NMR (125
MHz, CDCl3): δ 159.26 (2C, CdN), 73.08 (2C, CHN), 70.13 (2C,
CH2ꢀO), 66.02 (2C, clusterꢀC), 32.03 (2C, iPrꢀCH), 19.07, 18.52 (4C,
iPrꢀCH3). 11B{1H} NMR (96.3 MHz; CDCl3): δ 0.92 (s, 1B), ꢀ9.59
to ꢀ18.57 (bm, 9B). IR (KBr, cmꢀ1): ν(CH) 3035 (m), 2929 (m), ν(BH)
2615 (S), ν(CdN) 1625 (S), ν(CH) 1471 (m), ν(CꢀO) 1287 (m),
ν(CꢀN) 1104 (m), ν(BꢀB) 1050 (S), ν(CH) 845 (m), 742 (m). MS
(ESI, negative): m/z 542.8 (MþCl)ꢀ. Elemental analysis calcd for
C14H29B10ClN2O2Pd: C, 33.14; H, 5.76; N, 5.52%. Found: C, 33.07; H,
5.62; N, 5.45%.
Conjugate Reduction of r,β-Unsaturated Esters. Typical
Procedure for Synthesis of Ethyl (R)-3-Phenylbutyrate (30). Methyl-
diethoxysilane (201 mg, 1.5 mmol) was slowly added to a mixture of
ethyl trans-β-methylcinnamate 27 (190 mg, 1.0 mmol) and the catalyst
18 (6.04 mg, 0.01 mmol) in toluene (1.0 mL) at 60 °C. The mixture was
stirred for 1.5 h and then treated with hydrochloric acid (1 N, 1 mL).
After extraction with ethyl acetate, the solvents removed under reduced
pressure, and the residue was purified by column chromatography on
silica gel with diethyl ether/hexane (1:20) to give 30 (176 mg, 0.92
mmol, 92%) as a colorless oil. [R]25D = ꢀ17.9 (c 1.1 in CHCl3); lit.35
[R]26D = þ19.0 (c 1.1 in CHCl3), 94% ee for R. 1H NMR (400 MHz,
CDCl3): δ 7.33ꢀ7.14 (m, 5H), 4.07 (q, J = 7.6 Hz, 2H), 3.28 (m, 1H),
2.61 (dd, J = 15.0, 8.2 Hz, 1H), 2.53 (dd, J = 15.0, 8.0 Hz, 1H), 1.30 (d, J
1
= 7.6 Hz, 3H), 1.18 (t, J = 7.2 Hz, 3H). The H NMR spectra were
consistent with previously reported literature data.25
Ethyl (R)-3-Phenylpentanoate (31). Following the procedure for
synthesis of 30, compound 28 (204 mg, 1.0 mmol) was converted to 31
as a colorless oil in 89% yield (183 mg, 0.89 mmol). [R]25D = ꢀ17.5 (c
1.1 in CHCl3); lit.31 [R]25D = þ18 (c 1.1 in CHCl3). 97% ee for R. 1H
NMR (400 MHz, CDCl3): δ 7.35ꢀ7.15 (m, 5H), 4.03 (q, J = 8.0 Hz,
2H), 3.02 (m, 1H), 2.57 (dd, J = 15.2, 7.6 Hz, 2H), 1.72ꢀ1.53 (m, 2H),
1.15 (t, J = 7.5 Hz, 3H), 0.76 (t, J = 6.8 Hz, 3H). The 1H NMR spectra
were consistent with previously reported literature data.31
Ethyl (S)-4-Methyl-3-phenylpentanoate (32). A procedure analo-
gous to the synthesis of 30 was employed for 1 h using substrate 29 (218
mg, 1.0 mmol) to give 32 as a colorless oil in 96% yield (211 mg, 0.96
mmol), [R]25D = ꢀ25 (c 1.1 in CHCl3); lit.14c [R]26D = ꢀ25.4 (c 1.0 in
CHCl3). 98% ee for S. 1H NMR (400 MHz, CDCl3): δ 7.34ꢀ7.16 (m,
5H), 4.03 (q, J = 8.2 Hz, 2H), 3.05 (m, 1H), 2.51 (m, 1H), 1.75ꢀ1.55
(m, 2H), 1.16 (t, J = 7.6 Hz, 3H), 1.02 (d, J = 7.0 Hz, 6H). The 1H NMR
spectra were consistent with previously reported literature data.14c
Reductive Aldol Reaction. Synthesis of tert-Butyl-3-hydroxy-2-
methyl-3-phenylpropanoate (33). To a mixture of the rhodium com-
plex 18 (6.04 mg, 0.01 mmol) and benzaldehyde (106 mg, 1.0 mmol) in
[(R,R)-Carbox-Ph]PdCl (25). A procedure analogous to the synthesis
of 24 was employed using pincer ligand 8 (434 mg, 1.0 mmol) to give 25
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toluene (3.0 mL), tert-butyl acrylate (192 mg, 1.5 mmol) was added at
50 °C. Methyldiethoxysilane (215 mg, 1.6 mmol) was slowly added, and
the mixture was stirred at 50 °C for 0.5 h. The reaction was quenched by
addition of aqueous HCl (4 N, 1 mL), MeOH (1 mL), and tetrahy-
drofuran (THF, 1 mL) at 0 °C. After stirring for 30 min, the mixture was
extracted with ethyl acetate (3 ꢁ 5 mL). The combined organic layer was
washed with aqueous NaHCO3 (2 ꢁ 10 mL), dried over MgSO4, and
concentrated. The residue was purified by column chromatography on
silica gel with hexaneꢀethyl acetate (20:1) as eluent to give a mixture of
the desired products 33anti and 33syn (224 mg, 0.95 mmol, 95%). The
anti/syn ratio was determined by 1H NMR to be 90:10. 33anti (majior):
[R]25D = ꢀ33.5 (c 1.0 in EtOH), 91% ee; lit.25 [R]26D = 36.7 (c 1.0 in
EtOH); 1H NMR (400 MHz, CDCl3): δ 7.22 (m, 5H), 5.12 (d, J = 6.2
Hz, 1H), 3.02 (m, 1H), 1.45 (m, 9H), 1.2 (m, 3H). The 1H NMR spectra
were consistent with previously reported literature data.25
Crystal Structure Determination of 4, 7, 9, 10, 13, and 14. Each
crystal was mounted on a glass fiber, and the diffraction data of all the
complexes were collected on a BrukerꢀAXS APEX II CCD detector
using graphite monochromated Mo KR radiation at 123 K. The crystal
data and experimental details are listed in Table 1. All the structures were
solved by the combination of the direct method and Fourier techniques,
and all the non-hydrogen atoms were anisotropically refined by full-matrix
least-squares calculations. The atomic scattering factors and anomalous
dispersion terms were obtained from the International Tables for X-ray
Crystallography IV.46 The hydrogen atoms of the boron cluster were
obtained from difference Fourier maps.
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S
Supporting Information. X-ray crystallographic files for
b
4, 7, 9, 10, 13, and 14 in CIF format. 1H and 13C NMR spectra of
compounds 4ꢀ26 and 11B NMR spectra of compounds 4, 7, and
18. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ81-3-5992-1029. E-mail: hiroyuki.nakamura@gakushuin.
ac.jp.
(17) Mankad, N. P.; Rivard, E.; Harkins, S. B.; Peters, J. C. J. Am.
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Present Addresses
†Department of Chemistry, Faculty of Science, University of
Tanta, 31527-Tanta, Egypt.
’ ACKNOWLEDGMENT
We thank Professor Yukihiro Motoyama, Kyushu University,
for useful suggestion of assignment of Carbox ligands. M.E.E.-Z.
thanks the Japan Society for the Promotion of Science (JSPS, ID
No. P08363) for the financial support.
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