Chiral n-heterocyclic carbene borane complexes: Synthesis and structural analysis

Article abstract of DOI:10.1021/om201087n

A new family of chiral N-heterocyclic carbine borane complexes was synthesized starting from their corresponding imidazolium salts. The complexes were fully characterized. X-ray crystal structures of the complexes were obtained.

Full text of DOI:10.1021/om201087n

Article
pubs.acs.org/Organometallics
Chiral N-Heterocyclic Carbene Borane Complexes: Synthesis and
Structural Analysis
Dipshikha Banerjee, Celine Besnard, and E. Peter Kundig*
́
̈
Department of Organic Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland
S
* Supporting Information
ABSTRACT: A new family of chiral N-heterocyclic carbene
borane complexes was synthesized starting from their
corresponding imidazolium salts. The complexes were fully
characterized. X-ray crystal structures of the complexes were
obtained.
Ligand 1 was shown to be the best in the Pd-catalyzed
INTRODUCTION
■
intramolecular arylation of amides to give 3,3-disubstituted
Since Arduengo’s report of the first persistent N-heterocyclic
carbene (NHC),1 NHCs have become of paramount
importance in the field of catalysis. Due to their versatility,
tunable steric and electronic properties, and ease of formation
of metal complexes, NHCs have found extensive utility in
transition metal catalyzed2,3 and organocatalytic reactions.4
Only slowly, however, do chiral NHC ligands make their way
into the realm of asymmetric catalytic reactions that provide
products with high asymmetric induction, and in a number of
routes of access, chiral ligands have to be separated by high-
performance liquid chromatography on a chiral phase.5,6
Successful reactions (product er ≥ 95:5) include olefin
metathesis,7−10 1,4-addition of arylboronic acids to enones,11
hydrosylilation of ketones,12 intramolecular α-arylation of
amides to give 3-disubstituted oxindoles,13−18 copper-catalyzed
conjugate addition to cyclic ennones,19,20 nickel-catalyzed
reductive coupling of 1,3-dienes and aldehydes with triethylsi-
lane,21 copper-catalyzed allylic arylation with aryl Grignard
reagent,22 copper-catalyzed addition of arylboronates to
isatins,23 rhodium-catalyzed hydroarylation of azabicycles,24
ruthenium-catalyzed hydrogenation of quinoxalines,25 and
palladium-catalyzed alkane activation to give fused indolines.26
The strategies that have been developed to introduce
chirality in the NHC’s can be categorized as (a) chirality
in the N-heterocyclic backbone,7,27−29 (b) chirality in the
N-substituents,30−33 and (c) a combination of the above
two.19,34,35
chiral oxindoles (eq 1).13,18 2′ was the ligand of choice for the
synthesis of chiral oxindoles having a 3-alkoxy or 3-amino
group at the stereogenic center (eq 2).14 Ligand 3 provided
excellent results in Pd-catalyzed C−H activation of unactivated
methylene groups, leading to the formation of fused indolines
with high enantioselectivity (eq 3).26
We hypothesized that minimization of allylic strain sets the
stereocontrol elements of the catalyst in place and brings about
such high stereoselectivity.18,26 This, and the high thermal
stability of the Pd-NHC system, significantly extends the range
of successful applications in catalysis. We sought to increase our
knowledge of the solid-state structure of complexes incorporat-
ing these ligands, and our choice fell on NHC−borane adducts
for this study.
Recently, NHC complexes of main group elements such as
Figure 1. Source of chirality in NHCs.
NHC−boranes are gaining importance.36−55 Experimental
Our laboratory has focused on NHC ligands with chiral N-
substituents incorporating o-substituted phenyl or 1-naphthyl
groups (Figure 2).
Received: November 5, 2011
Published: January 4, 2012
© 2012 American Chemical Society
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Figure 2. NHC ligands developed in our group.
Scheme 1. Synthesis of Imidazolium Salts
Scheme 2. Synthesis of NHC−BH3 Complexes
results along with DFT calculations on NHC−boranes have
shown recently that coupling of an NHC with BH3 changes the
electronic properties of the borane considerably.56,57 In an
NHC−BH3 complex, the B−H bond is weakened due to the π-
conjugation with the carbene ligand, and this results in a much
lower bond energy of the B−H bond (74−80 kcal/mol)
compared to that of BH3 (106 kcal/mol).56 Therefore, unlike
boranes, NHC−boranes act as radical hydrogen donors.56−58
Applications of the NHC−boranes were then followed in the
radical reduction of xanthates,56−58 ionic reduction of halides,59
Suzuki−Miyaura cross-coupling reactions,60 and reduction of
ketones.61 NHC−boranes and their derivatization and
applications have been recently reviewed.62 However, examples
of chiral NHC−borane complexes derived from chiral NHC
ligand precursors are scarce.61−63
Here we report on the synthesis and structures of sterically
demanding chiral N-heterocyclic carbene borane complexes
incorporating successful ligands for the reactions shown in eqs
1−3.
RESULTS AND DISCUSSION
■
The imidazolium salts 1−4 were employed in the synthesis of
new NHC−borane complexes. They were obtained from the
corresponding chiral amines, which in turn were prepared by
literature-known procedures (see Experimental Details for
details).13,14,26 The chiral amines were condensed with glyoxal
to provide the corresponding chiral diimines 1a−4a.13,14,26 The
diimines were then reacted with chloromethylpivalate in the
presence of silver triflate to obtain the imidazolium triflates.64
Anion metathesis to the corresponding iodides was carried out
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Figure 3. ORTEP diagram of complexes 5−8 (50% probability level for the thermal ellipsoids). Hydrogen atoms of the aromatic rings and methyl
groups have been removed for clarity.
because the iodide salts were solids and easier to purify
(Scheme 1).13,14,26
5−8 exhibit broad singlets in the range 33−37 ppm, values that
lie in between those for amine−borane and phosphine−boranes
adducts.65,66
Deprotonation of the imidazolium salts followed by the
reaction with BH3 was the route chosen to access complexes
5−8. Screening bases such as NaH, KOtBu, and sodium
hexamethyldisilazide (NaHMDS) proved NaHMDS to be the
best base. The in situ generated carbenes were then quenched
with BH3−THF. The NHC−borane complexes were isolated
by flash column chromatography in moderate to excellent
yields (55−90%) (Scheme 2).
Crystals of complexes 5−8 suitable for X-ray diffraction were
grown by vapor diffusion of hexane into a CH2Cl2 solution.
In the solid-state structures of 5−8 (Figure 3), the three-
dimensional arrangements of the substituents at the stereogenic
centers are guided by the minimization of allylic strain (A1,3-
strain). Consequently, an approximate coplanarity of the C−H
bonds at the stereogenic centers and the C(ligand)−B bond is
found in all complexes. This places the tBu groups and the aryl
groups at the two stereogenic centers in opposite hemispheres.
The ortho-substituents on the aryl (or the naphthyl ring in the
case of complex 8) are again close to coplanar with the C−H
bonds at the stereogenic centers since any rotation around the
C(aryl)−C(stereogenic center) bond would lead to an increase
in A1.3-strain except one of 180°, but then the o-aryl substituent
would enter into conflict with the tBu group.
The white, crystalline, air- and moisture-stable complexes 5−8
1
have melting points > 200 °C. They were characterized by H,
13C, and 11B NMR spectroscopy. Consistent with analogous,
previously reported complexes,62 1H NMR signals for the B−H
protons appear in the range 1.8−0.8 ppm as a quartet reflecting
the coupling with the 11B nucleus. In the 13C NMR spectra of
previously reported NHC−BH3 complexes, the carbene carbon
was observed in the range δ = 153.3−186.0 ppm.62 In case of
complexes 5−8 the 13C NMR spectra reveal the presence of a
broad multiplet between δ = 174.5 and 176.7 ppm that
corresponds to the quaternary N−C−N carbon atom. The
signal is broadened due to coupling with the quadrupolar boron
nuclei of spin 3 (10B) and 3/2 (11B), respectively. Further
analysis of 2D-NMR spectra clearly shows the correlation
between the carbene carbon (N−C−N) and the neighboring
protons (for details see the Supporting Information). In the case
of the imidazolium salts the N−CH−N carbon atoms appear in
the 13C NMR spectra below 150 ppm, whereas the change of
electronic environment around the carbene carbon atom (N−
C−N) in an NHC−BH3 complex results in a downfield shift of
the signal (above 170 ppm). The 11B NMR resonances of the
literature-known NHC−boranes appear between δ = −32 and
−38 ppm.62 The proton-decoupled 11B resonances of complexes
In comparison with the crystal structure of the carbene ligand
precursor (Figure 4), the tBu groups are lying in one
hemisphere and the aryl groups are lying in the other with
respect to the heterocycle plane. The rotational change in the
Figure 4. ORTEP diagram of imidazolium salt 4 (50% probability level
for the thermal ellipsoids). Iodine atom has been removed for clarity.
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case of the carbene complexes is as expected when considering
the minimization of A1,3-strain. The anti-arrangement of the tBu
groups is also found in the structures of an NHC palladium
complex.18,26
Further details of the structural features of the borane
complexes emerge when comparing, the pairs of molecules
having similar functional groups, namely, complexes 5 and 8
and complexes 6 and 7.
unit of 5 makes the comparison with 8 difficult. In Table 1,
entries 11−14 show the relative position of the H atom at the
stereogenic center with respect to the C(stereogenic center)−B
and C(aromatic)−CH3 bonds. It emerges that the structural
differences between 5 and 8 are not larger than that of the three
independent molecules in 5.
In 6 there are two independent molecules in the asymmetric
unit. However, contrary to what is observed in 5, the molecular
geometry of 6 is more rigid and the two independent molecules
in the asymmetric unit adopt very similar conformations
(Figure 6a).
In 5 there are three independent molecules per asymmetric
unit. As shown in Figure 5a, 5B and 5C are almost perfectly
In the superposed picture of 6 and 7 (Figure 6b), it is clear
that the common parts of 6 and 7 also adopt very similar
geometries. The main difference lies in the orientation of one of
the aryl rings.
Variation of torsion angles in the solid-state structures of 5−
8 have been displayed in Table 1. It reflects the arrangements of
different substituents at the stereogenic centers of the
molecules.
As shown in Table 1, the values of the torsion angles of the
three molecules in complex 5 are very different, which reflects
the flexible molecular geometry of 5. In contrast, in complex 6
the values of the torsion angles of the two independent
molecules are close to each other. This may result from a
stabilized geometry of the molecule due to the interaction
between the BH3 moiety and the OMe− groups in complex 6.
In the case of 7 and 8 there is only one molecule present per
asymmetric unit.
To quantify the steric bulk of the ligands, the buried volumes
(%Vbur) of the complexes 5−8 were calculated (Table 2).67,68
CONCLUSION
Figure 5. (a) Overlap of the three independent molecules present in
the unit cell of 5 (A: blue; B: orange C: green); (b) overlap of 5B
(orange) and 8 (green).
■
A series of new chiral NHC−borane complexes are reported.
Crystal structures of the complexes convincingly show the
importance of allylic strain in the arrangement of the ligands’
substituents in space.
superimposable (rms 0.183 Å for all atoms), whereas 5A is
slightly different (rms AC 0.704 Å and AB 0.788 Å for all
atoms), in particular in the arrangement of the aryl ring bearing
carbon atoms C20−C25. This is clearly reflected in the values
of the torsion angles C2−N3−C19−C20 and N3−C19−C20-
C25 (see Table 1).
In 8 there is only one molecule present per asymmetric unit.
Figure 5b represents the superimposed structure of 5B and 8.
The presence of three different molecules in one asymmetric
EXPERIMENTAL DETAILS
■
General Methods. Experiments were carried out under a nitrogen
atmosphere using standard Schlenk techniques. Solvents were dried
with a Solvtek system. For the synthesis of NHC−boranes, THF was
degassed by two pump−freeze−thaw cycles. All commercially available
compounds were used as received. Deuterated solvents were obtained
from Cambridge Isotope Laboratories. NMR spectra were recorded on
Figure 6. (a) Overlap of the two independent molecules present in the unit cell of complex 6 (A: orange; B: green); (b) overlap of 6A (orange) and
7 (green).
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a
Table 1. Comparison of Selected Torsion Angles (deg) in
Table 2. Calculation of %Vbur
Complexes 5−8
entry
complexes
%Vbur
1
2
3
4
5
6
7
8
35.4
31.8
36.6
36.4
a
Calculated using SambVca with the standard parameters: radius of
sphere 3.5 Å, distance from sphere 2.1 Å, mesh step 0.05 Å.67.
Synthesis of the Imidazolium Salts. The imidazolium salts 1,13
3,26 and 413 were synthesized following literature procedures. The
synthesis of the diimine 2a (see Scheme 1) was previously reported.14
1,3-Bis((S)-1-(4-methoxy-[1,1′-biphenyl]-3-yl)-2,2-dimethyl-
propyl)-1H-imidazol-3-ium Iodide (2). Chloromethyl pivalate (376
mg, 2.50 mmol, 1.5 equiv) was added to silver triflate (642 mg, 2.50
mmol, 1.5 equiv) in CH2Cl2 (13 mL) in a Schlenk tube, and the
mixture was stirred at rt for 45 min in the dark. The resulting
suspension was transferred via cannula equipped with a filter into a dry
Schlenk tube containing the diimine 2a (938 mg, 1.67 mmol). The
resulting mixture was stirred at 40 °C for 24 h in the dark. After
cooling to rt, the reaction mixture was dried.
entry
1
(S,S)-5
(S,S)-8
(S,S)-6
(S,S)-7
C2−N6−C7−C8
−106.3(2)
−99.4(3)
−110.0(3)
−109.9(3)
−102.5(2)
−107.5(2)
−117.7(2)
2
3
4
5
C2−N3−C19−C20 −132.6(2)
−105.4(3)
−127.9(3)
117.1(3)
98.8(4)
−113.9(2)
−115.0(2)
−108.5(2)
109.1(2)
118.7(2)
118.7(2)
−100.1(3)
C2−N6−C7−C15
121.5(2)
128.8(2)
117.2(3)
126.4(2)
121.6(2)
The crude material was redissolved in acetone (17 mL), and sodium
iodide (2.5 g, 16.7 mmol, 10 equiv) was added. The mixture was
stirred at rt overnight. The solvent was evaporated, and the residue was
taken up in a minimum amount of chloroform and filtered though a
cotton plug. After concentrating, the filtrate was submitted to a second
cycle of anion exchange. Finally flash column chromatography using
CH2Cl2/Et2O (1:1) as eluent afforded the pure imidazolium salt 2
C2−N3−C19−C27 99.9(3)
121.5(3)
114.3(2)
113.4(2)
127.6(3)
N3−C19−C20−
C25
100.1(2)
118.7(2)
132.10(19)
128.8(2)
(468 mg, 40%) as a yellowish-brown powder: mp = 157 °C; [α]20
=
130.3(2)
126.6(3)
125.2(3)
127.4(2)
121.9(3)
7
D
1
−127 (c 0.6, CH2Cl2); H NMR (400 MHz, CDCl3) δ 10.63 (s, 1H,
NCHN), 7.90 (d, J = 1.9 Hz, 2H, CHarom), 7.57−7.54 (m, 6H,
CHarom), 7.45−7.39 (m, 6H, CHarom), 7.34−7.30 (m, 2H, CHarom),
6.96 (d, J = 8.6 Hz, 2H, CHolefinic), 6.30 (s, 2H, CH), 3.86 (s, 6H,
OCH3), 1.21 (s, 18H, C(CH3)3); 13C NMR (100 MHz, CDCl3) δ
157.0 (s, Carom), 139.9 (s, Carom), 138.6 (s, NCHN), 133.5 (s, Carom),
128.9 (s, Carom), 127.8 (s, Carom), 127.1 (s, Carom), 126.8 (s, Carom),
123.8 (s, Carom), 121.5 (s, Carom), 111.9 (s, Colefinic), 66.3 (s, CH), 55.9
(s, OCH3), 36.1 (s, C(CH3)3), 27.9 (s, C(CH3)3); IR (ν, cm−1) 3773,
3715, 2962, 1486, 1255, 1142, 762; HRMS (ESI, m/z) calcd for
C39H45N2O2 ([M − I]+):573.3475, found 573.3486. Anal. Calcd for
C39H45IN2O2: C, 66.85; H, 6.47; N, 4.00. Found: C, 66.71; H, 6.77;
N, 3.76.
6
7
8
N6−C7−C8−C13
C2−N6−C7−H7
132.6(2)
140.54(19)
141.41(18)
132.6(2)
4
12
7
−4
14
3
C2−N3−C19−
H19
−20
−15
0
5
8
−1
13
9
C7−N6−C2−B1
−2.7(4)
−5.8(4)
−7.2(4)
6.2(4)
5.7(4)
8.0(4)
1
6.4(3)
−0.5(3)
4
6.4(3)
3.0(3)
0.3(5)
General Procedure for the Synthesis of the NHC−Borane
Complexes. The imidazolium salt (1 equiv, 0.5 mmol) was placed in
a flame-dried Schlenk tube. Dry, degassed THF (3 mL) was added.
The suspension was cooled to −30 °C, and NaHMDS (1.2 equiv, 0.6
mmol, 2 M solution in THF) was added. The mixture was then stirred
for 30 min at −30 °C followed by the addition of BH3−THF (3 equiv,
1.5 mmol, 1 M solution in THF) at the same temperature. The
reaction was warmed to rt over an 18 h period. Volatiles were
evaporated under vacuum, and the mixture was redissolved in CH2Cl2
(2 mL) and filtered through a short pad of Celite. The Celite was
washed with CH2Cl2, and the filtrate was concentrated. The crude
material was dissolved in a minimum amount of CH2Cl2, and silica
(same in amount compared to the crude material) was added. The
mixture was taken to dryness in a Rotavapor to get free-flowing
powder. This was loaded onto a silica gel column, and the product was
eluted using 15−20% EtOAc in cyclohexane.
10 C19−N3−C2−B1
11 B1−C2−C7−H7
−3.5(3)
1.2(3)
−1.9(5)
17
9
1
4
9
1
12 B1−C2 C19−H19 −15
−17
−2
15
0
18
13 X1−C13−C7−H7
(X1 = C14) (X1 = C36) (X1 = O14) (X1 = O14)
11
11
9
19
23
23
18
14 X2−C25−C19−
(X2 = C26) (X2 = C35) (X2 = O26) (X2 = O26)
H19
(1,3-Bis((S)-2,2-dimethyl-1-(o-tolyl)propyl-1H-imidazol-3-
ium-2-yl) Trihydroborate (5). Following the general procedure
complex 5 (121 mg, 60%) was obtained after purification by flash
chromatography on silica gel using 15% EtOAc in cyclohexane: mp =
−11
16
−6
19
16
6
15
278 °C; [α]20 = −12.02 (c 0.6, CH2Cl2); 1H NMR (500 MHz,
Bruker AMX 500 and AMX 400 spectrometers at ambient probe
temperature. IR spectra were recorded using a Perkin-Elmer Spectrum
One spectrometer. Melting points were measured using a Buchi 510
apparatus. [α]D values were recorded on a Perkin-Elmer 241
polarimeter using a quartz cell (l = 10 cm) and a Na high-pressure
lamp (λ = 589 nm).
D
CDCl3) δ 7.59−7.57 (m, 2H, CHarom), 7.32 (s, 2H, CHolefinic), 7.21−
7.18 (m, 6H, CHarom), 6.44 (s, 2H, CH), 2.62 (s, 6H, CH3), 1.01 (s,
18H, C(CH3)3), 1.62−1.01 (m, BH3); 13C NMR (125 MHz, CDCl3)
δ 175.5−174.4 (bm, NCN), 139.1(s, Carom), 136.7 (s, Carom), 131.5 (s,
Carom), 128.1 (s, Carom), 127.5 (s, Carom), 125.2 (s, Carom), 118.3 (s,
̈
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Table 3. Crystallographic Data for Complexes 5−8
5
6
7
8
chemical formula
C27H39BN2
C27H39BN2O2
434.41
C39H47BN2O2
586.60
C33H39BN2
474.47
monoclinic
20.7759(8)
7.1727(3)
19.0739(6)
90.00
formula mass
402.41
cryst syst
monoclinic
46.4335(19)
7.3273(2)
23.5143(10)
90.00
monoclinic
12.55564(15)
14.24537(18)
14.6874(2)
90.00
orthorhombic
7.4531(6)
17.8577(5)
26.0300(15)
90.00
a/Å
b/Å
c/Å
α/deg
β/deg
110.669(3)
90.00
98.7154(12)
90.00
90.00
106.583(4)
90.00
γ/deg
90.00
unit cell volume/Å3
temperature/K
space group
7485.4(5)
180.0
2596.65(6)
180.15
3464.5(3)
180.15
2724.16(18)
189.9
C2
P2
P212121
4
C2
no. of formula units per unit cell, Z
radiation type
12 (Z′=3)
Mo Kα
0.061
4 (Z′=2)
Cu Kα
4
Cu Kα
0.523
Cu Kα
0.496
−1
absorp coeff, μ/mm
0.531
no. of reflns measd
27 046
20 141
8958
5481
no. of indep reflns
7932
5393
3829
2635
Rint
0.0664
0.0392
0.0300
0.0463
0.0542
0.1336
0.0681
0.1448
1.035
final R1 values (I > 2σ(I))
final wR(F2) values (I > 2σ(I))
final R1 values (all data)
final wR(F2) values (all data)
goodness of fit on F2
0.0464
0.0440
0.0398
0.1060
0.1214
0.1129
0.0551
0.0494
0.0446
0.1098
0.1264
0.1183
1.057
1.031
0.910
Colefinic), 64.4 (s, CH), 37.6 (s, C(CH3)3), 28.1 (s, C(CH3)3), 21.5
(s, CH3); 11B NMR (CDCl3, 96.3 MHz) δ −33.16; IR (ν, cm−1) 3744,
3686, 2972, 2357, 2055, 1264, 731; HRMS (ESI, m/z) calcd for
C27H36BN2 ([M − 3H]+) 399.2966, found 399.2962. Anal. Calcd for
C27H39BN2: C, 80.58; H, 9.77; N, 6.96. Found: C, 80.16; H, 10.18; N,
6.94.
(1,3-Bis((S)-1-(2-methoxyphenyl)-2,2-dimethylpropyl)-1H-
imidazol-3-ium-2-yl) Trihydroborate (6). According to the general
procedure and after purifying by flash chromatography on silica gel
procedure and after purifying by flash chromatography on silica gel
using 15% EtOAc in cyclohexane complex 8 (130 mg, 55%) was
synthesized: mp = 225 °C; [α]20D = +80.50 (c 0.6, CH2Cl2); 1H NMR
(500 MHz, CDCl3) δ 8.72 (d, J = 8.7 Hz, 2H, CHarom), 7.85−7.82 (m,
6H, CHarom), 7.63−7.60 (m, 2H, CHarom), 7.52−7.46 (m, 4H, CHarom),
7.27 (s, 2H, merged with CDCl3 peak, CHolefinic), 7.12 (s, 2H, CH),
1.03 (s, 18H, C(CH3)3), 1.44−1.03 (m, BH3); 13C NMR (125 MHz,
CDCl3) δ 175.6−174.4 (bm, NCN), 134.6 (s, Carom), 134.1 (s, Carom),
133.0 (s, Carom), 128.8 (s, Carom), 128.5 (s, Carom), 126.7 (s, Carom),
125.8 (s, Carom), 125.3 (s, Carom), 124.2 (s, Carom), 118.4 (s, Colefinic),
63.1 (s, CH), 37.6 (s, C(CH3)3), 28.4 (s, C(CH3)3); 11B NMR
(CDCl3, 96.3 MHz) δ −32.89; IR (ν, cm−1) 3742, 2955, 2149, 1485,
1251, 732; HRMS (ESI, m/z) calcd for C33H37N2 ([M − BH2]+)
461.2951, found 461.2960.
using 20% EtOAc in cyclohexane complex 6 (195 mg, 90%) was
1
obtained: mp = 229 °C; [α]20 = +32.17 (c 0.6, CH2Cl2); H NMR
D
(500 MHz, CDCl3) δ 7.45 (dd, J = 8.1, 1.7 Hz, 2H, CHarom), 7.34 (s,
2H, CHolefinic), 7.27−7.23 (m, 2H, CHarom), 6.92−6.89 (m, 4H,
CHarom), 6.76 (s, 2H, CH), 3.86 (s, 6H, CH3), 0.93 (s, 18H,
C(CH3)3), 1.48−0.93 (m, BH3); 13C NMR (125 MHz, CDCl3) δ
176.4−175.3 (bm, NCN), 158.0 (s, Carom), 129.3 (s, Carom), 128.6 (s,
Carom), 126.9 (s, Carom), 119.7 (s, Carom), 118.1 (s, Colefinic), 111.7 (s,
Carom), 61.8 (s, CH), 55.8 (s, OCH3), 36.7 (s, C(CH3)3), 27.9 (s,
C(CH3)3); 11B NMR (CDCl3, 96.3 MHz) δ −36.16; IR (ν, cm−1)
3570, 3521, 2961, 2338, 2282, 1492, 1462, 1241, 760; HRMS
(ESI, m/z) calcd for C27H38BN2O2 ([M − H]+) 433.3020, found
433.3025. Anal. Calcd for C27H39BO2N2: C, 74.65; H, 9.05; N, 6.45.
Found: C, 74.12; H, 9.28; N, 6.43.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Figures giving H and 13C NMR spectra of molecules 3 and
5−8, 11B NMR spectra of complexes 5−8, and CIF files giving
X-ray crystal structures are available free of charge via the
Internet at http://pubs.acs.org.
(1,3-Bis((S)-1-(4-methoxy-[1,1′-biphenyl]-3-yl)-2,2-dimethyl-
propyl)-1H-imidazol-3-ium-2-yl) Trihydroborate (7). Following
the general procedure complex 7 (176 mg, 60%) was obtained after
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: peter.kundig@unige.ch.
purification by flash chromatography on silica gel using 15% EtOAc in
1
cyclohexane: mp = 294 °C; [α]20 = −79.78 (c 0.45, CH2Cl2); H
D
NMR (500 MHz, CDCl3) δ 7.66 (d, J = 4.8 Hz, 2H, CHarom), 7.52−
7.42 (m, 12H, CHarom), 7.34−7.31 (m, 2H, CHarom), 6.99 (d, J = 8.6
Hz, 2H, CHarom), 6.80 (s, 2H, CH) 3.91 (s, 6H, OCH3), 1.03 (s, 18H,
C(CH3)3), 1.59−1.03 (m, BH3); 13C NMR (125 MHz, CDCl3) δ
176.7−175.7 (bm, NCN), 157.6 (s, Carom), 141.1 (s, Carom), 132.9 (s,
Carom), 128.8 (s, Carom), 128.4 (s, Carom), 127.4 (s, Carom), 127.2 (s,
Carom), 126.8 (s, Carom), 126.7 (s, Carom), 118.3 (s, Colefinic), 112.0 (s,
Carom), 62.3 (s, CH), 56.0 (s, OCH3), 36.8 (s, C(CH3)3), 28.0 (s,
C(CH3)3); 11B NMR (CDCl3, 96.3 MHz) δ −34.31; IR (ν, cm−1)
3750, 2254, 903, 722; HRMS (ESI, m/z) calcd for C39H46BN2O2 ([M
− H]+) 585.3646, found 585.3641.
ACKNOWLEDGMENTS
■
We thank the University of Geneva and the Swiss National
Science Foundation for financial support.
REFERENCES
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