Chiral PEPPSI complexes: Synthesis, characterization, and application in asymmetric Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1021/om4009982

PEPPSI complexes incorporating chiral N-heterocyclic carbene (NHC) ligands based on 2,2-dimethyl-1-(o-substituted aryl)propan-1-amines were synthesized. Two complexes, with one saturated and one unsaturated NHC ligand, were structurally characterized. The chiral PEPPSI complexes were used in asymmetric Suzuki-Miyaura reactions, giving atropisomeric biaryl products in modest to good enantiomeric ratios.

Full text of DOI:10.1021/om4009982

Article
pubs.acs.org/Organometallics
Chiral PEPPSI Complexes: Synthesis, Characterization, and
Application in Asymmetric Suzuki−Miyaura Coupling Reactions
Laure Benhamou, 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: PEPPSI complexes incorporating chiral N-heterocyclic
carbene (NHC) ligands based on 2,2-dimethyl-1-(o-substituted aryl)-
propan-1-amines were synthesized. Two complexes, with one saturated
and one unsaturated NHC ligand, were structurally characterized. The
chiral PEPPSI complexes were used in asymmetric Suzuki−Miyaura
reactions, giving atropisomeric biaryl products in modest to good
enantiomeric ratios.
including PdII−bis-NHC complexes,14 donor-functionalized
INTRODUCTION
■
NHC−PdII,15 PdII−NHC−(η3-allyl) precursors,16,17 pallada-
cycles,18,19 and PEPPSI precatalysts.20,21 PEPPSI complexes are
obtained by direct palladation of an imidazolium salt in the
presence of a weak base using 3-chloropyridine as solvent. The
synthesis can be handled in the presence of air. The “user
friendly” PEPPSI complexes efficiently catalyze the coupling of
sterically demanding aryl halides with a variety of boronic acids
at room temperature. Taking advantage of their convenience of
access and handling, we herein report the synthesis of PEPPSI
complexes incorporating chiral bulky N-heterocyclic carbenes
and their application in asymmetric Suzuki−Miyaura coupling
reactions.
The synthesis of chiral biaryl compounds has received
considerable attention, because this structural motif is present
in many natural products and chiral ligands for transition-metal-
catalyzed reactions.1 Atroposelective C−C cross coupling via
asymmetric palladium-catalyzed Suzuki−Miyaura reactions
represents a powerful approach of access to this class of
compounds.2 To date, most of the protocols have employed
chiral mono- or diphosphine ligands under homogeneous or
heterogeneous conditions,3,4 bishydrazone or phosphine−
hydrazone chiral ligands,5 chiral diene ligands,6 or a
pyridylmethylamine ligand with a stereogenic center.7 The
use of chiral N-heterocyclic carbene (NHC) ligands in
asymmetric Suzuki−Miyaura coupling reactions is rare. More-
over, the reported reactions tend to lead to moderate
asymmetric induction.8
RESULTS AND DISCUSSION
■
Synthesis of Chiral PEPPSI Complexes. Chiral PEPPSI
complexes were obtained by following the procedure
established by Organ and co-workers for achiral PEPPSI
complexes.20 Chiral imidazolium salts, PdCl2 (1 equiv), and
potassium carbonate (5 equiv) were stirred in 3-chloropyridine
for 24 h. To avoid a mixture of halogen complexes, a halide
exchange was performed by using an excess of NaI in CH2Cl2 at
room temperature for 24 h. Workup (see the Experimental
Section) afforded chiral PEPPSI complexes 10−14 in good
yields (Scheme 1). These compounds are air and moisture
stable and can be stored at room temperature for months
without decomposition.
Over the past few years, our group has developed bulky chiral
NHC’s derived from chiral ortho-substituted α-alkylphenethyl-
amine (Figure 1). These ligands were successfully applied in the
Pd-catalyzed intramolecular α-arylation of anilides to yield 3,3-
disubstituted oxindoles in high yields and with excellent
enantioselectivities.9 More recently, palladium-catalyzed syn-
theses of highly enantioenriched 2,3-disubstituted and -fused
3
indolines via Csp −H/C(Ar) coupling reactions were re-
ported.10 This, and the first applications of these ligands in
gold catalysis11a and the isolation of NHC−borane adducts,11b
led us to explore the scope of such ligands in asymmetric
palladium-catalyzed Suzuki−Miyaura coupling to access
atropisomeric biaryls.
Chiral PEPPSI complexes with saturated NHC’s were more
difficult to synthesize than their unsaturated counterparts.
Complex 14 was the only example that we were able to isolate.
With dihydroimidazolium salts 6−8, Pd(3-chloropyridine)2I2
formed instead.22 It should be noted that PEPPSI complexes
Achiral palladium−NHC complexes are well-known to be
efficient in cross-coupling reactions and particularly for the
coupling of ortho mono- and disubstituted aryl halides with
hindered boronic acids.12 The Pd0−NHC active species is often
generated in situ from air- and moisture-stable PdII−NHC
complexes.13 Many such precursors have been reported,
Received: October 11, 2013
Published: December 16, 2013
© 2013 American Chemical Society
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Figure 1. Chiral imidazolium and dihydroimidazolium ligand precursors.
Scheme 1. Synthesis of Chiral PEPPSI Complexes 10−14
with a saturated NHC are rare. Precedents include SIPr-
PEPPSI (15),23 the unsymmetrical PEPPSI complexes 16 with
a chiral saturated NHC,8c and the complex 17 with triethyl-
amine in place of pyridine as the labile ligand (Figure 2).24
It remains unclear why saturated-NHC PEPPSI complexes
are more difficult to access than the unsaturated analogues. It is
unlikely to be because of an impediment to generate in situ the
saturated carbene by the action of a weak base in the presence
of a Pd precursor, as this has already been described in a
number of reports.11a,23 We have been confronted with this
problem also when trying to prepare NHC−borane adducts
with a saturated NHC formed from the reaction of
dihydroimidazolium salts with a strong base.11b
NMR Studies. NHC generation and coordination was
followed by 1H NMR analysis by monitoring the disappearance
of the imidazolium or dihydroimidazolium signal of C(2)−H
and the appearance of the characteristic M−carbene carbon
signal between 149.1 and 158.3 ppm in 13C NMR for
unsaturated NHC PEPPSI analogues 10−13 and a 13C NMR
signal at 187.9 ppm for complex 14 (Table 1). These values are
consistent with literature data for PdII−NHC complexes and
previously reported PEPPSI complexes.23,25,26
X-ray Crystallography. Yellow crystals suitable for X-ray
analysis were obtained by slow diffusion of n-pentane into
concentrated solutions of 13 and 14 in dichloromethane.
Molecular structures are depicted in Figure 3, and select
interatomic distances and bond angles are collected in Tables 2
Figure 2. Achiral and chiral PEPPSI complexes (and an analogue)
incorporating saturated NHC ligands.
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Table 1. Chemical Shift of the M−Carbene Carbon NCN in
CD2Cl2 (101 MHz)
Table 2. Select Bond Lengths (Å) for Complexes 13 and 14
bond length
13
14
complex
δNCN(PEPPSI) (ppm)
Pd−C
Pd−N
Pd−I1
Pd−I2
C−C
1.976(4)
2.108(3)
2.6022(4)
2.6027(4)
1.336(6)
1.977(2)
2.115(2)
2.5995(2)
2.6088(3)
1.509(4)
10
11
12
13
14
149.1
152.0
158.3
152.3
187.9
153.5
182.1
185.9
IPr-PEPPSI23
168c
1716
Table 3. Select Bond Angles (deg) for Complexes 13 and 14
bond angle
13
14
C−Pd−N
I1−Pd−I2
C−Pd−I2
N−C−N
177.49(15)
176.031(14)
90.96(7)
176.66(10)
176.214(10)
90.66(11)
109.9(2)
and 3. Compounds 13 and 14 both adopt square-planar
geometries with a close to linear carbon−Pd−nitrogen array
(Table 3). The Pd−NHC bonds for complexes 13 and 14 lie
within the normal range of those observed for the achiral
PEPPSI complexes that have been previously reported (IPr-
PEPPSI, 1.969 (3) Å). Also, the Pd−I bond lengths are close to
that reported for a PEPPSI analogue containing an iodide.27
The Pd−N bonds again fall within the typical range for Pd−
pyridine bonds in PEPPSI complexes (2.137 Å for IPr-
PEPPSI20 and 2.097 Å for IPent-PEPPSI).28
The main structural distortion by comparison with the
achiral PEPPSI complexes reported by Organ and co-workers is
that the pyridine lies nearly in the same plane as the N-
heterocycle (torsion angle N−CNHC−Pd−NPy−C ≈ 17° for 13
and 19° in complex 14), whereas for IPent-PEPPSI the pyridine
is twisted out of the plane with a dihedral angle of 51°.28 In
Figure 4, IPent-PEPPSI and complexes 13 and 14 are
represented by omitting the Pd−I bonds to see more clearly
the relative position of the pyridine ring in relation to the plane
of the N-heterocycle. As previously described by our group, the
smallest subsituent (H) on the stereogenic centers of 13 and 14
is approximately coplanar with the Pd−NHC bond and the o-
aryl substituent bond (in this case the second aromatic ring of
the naphthyl moiety).9−11 The stereogenic centers and the
orientations of the aryl planes are set by the minimization of
unfavorable allylic strain (A1,3). Finally, we note that the
molecular structures of complexes 13 and 14 are much alike.
The main difference between the two complexes appears in the
geometry of the N-heterocyclic core due to the saturation of
the backbone in compound 14. There is a slight difference in
the N−C−N angles (106.3(3)° for 13 and 109.9(2)° for 14),
although it is within the range of what is usually observed in the
literature.28
106.3(3)
On the basis of on their X-ray structures, the %VBur values of
the chiral NHC ligands in 13 and 14 were determined using the
Web application SambVca.29 In Table 4 are presented the
values obtained and comparison with the literature datas for
IPent and IPr carbenes.30 Appreciation of %VBur allowed us to
classify our chiral ligands as sterically hindered NHCs with
steric properties comparable to those of IPr for 13 and IPent
for 14.
Asymmetric Suzuki−Miyaura Coupling using Chiral
PEPPSI Complexes. An initial screening revealed that
complex 13 performed best in the asymmetric Suzuki−Miyaura
coupling of 1-halo-2-substituted naphthalene with hindered
boronic acids.32 Optimization of the reaction conditions
revealed a 1/1 mixture of dioxane/water associated with
potassium hydroxide as base and 5 mol % of 13 to be an
efficient combination for this reaction.33
The coupling of 1-bromo-2-methoxynaphthalene with a
number of boronic acids was investigated (Table 5, entries 1−
7). Using naphthylboronic acid as a coupling partner led to a
high yield (90%) and moderate asymmetric induction (62% ee)
(entry 1). Starting with 1.1 equiv of naphthylboronic acid
instead of 1.5 equiv resulted in a significant decrease of activity
but no degradation of enantioselectivity (entry 2). 1-Chloro-2-
methoxynaphthalene (entry 3) gave a moderate yield at room
temperature after 48 h. Coupling of 1-bromo-2-methoxynaph-
thalene with disubstituted boronic acids derived from
naphthalene (entries 4 and 5) failed altogether, and no
conversion was observed even at 40 °C, even though achiral
bulky PEPPSI catalysts are known to be efficient catalysts to
Figure 3. ORTEP representations of X-ray structures of (a) 14 and (b) 13. All H atoms, except those at the stereogenic centers and on the N-
heterocyclic core, are omitted for clarity.
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Figure 4. Comparison of the angles between pyridine and NHC planes.
access tetrasubstituted biaryl products.12 Using mono-ortho-
substituted arylboronic acids afforded products in good to
moderate yields (entries 6 and 7) but with low enantiose-
lectivity (entry 7).
We anticipated that moving to an aryl halide with a bulkier
substituent at the 2-position of the naphthalene would lead to
higher selectivities. As such, the use of 1-bromo-2-methylnaph-
thalene was investigated (Table 6). The methyl group set close
a
Table 4. Calculation of %VBur for Complexes 13 and 14
31
complex
%VBur
13
14
35.9
37.6
34.3
39.3
37.9
IPr PEPPSI
SIPr PEPPSI
IPent PEPPSI
Pd−C bond fixed at 2.00 Å to compare with literature reports.30
a
Table 6. Asymmetric Suzuki−Miyaura Coupling of 1-Halo-2-
methylnaphthalene
Table 5. Asymmetric Suzuki−Miyaura Coupling of 1-Halo-2-
methoxynaphthalene
a
b
Ratio product/1,1′-binaphthyl. Reaction time 48 h.
to the Ar−Ar′ bond generates more steric hindrance in the
transition state in comparison to the previously used methoxy
group. This hypothesis proved correct. Coupling of 1-halo-2-
methylnaphthalene with naphthylboronic acid led to improved
selectivities (77−80% ee) (entries 1 and 2). Under the
previously optimized conditions (entry 1), 85% conversion
was observed. However, a non-negligible amount of homocou-
pling product was generated during the process and was
a
b
c
1.1 equiv of naphthylboronic acid. Reaction time 48 h. Reaction
time 72 h.
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peak of a deuterated solvent as internal standard. Coupling constants J
are reported in Hz. Infrared spectra were recorded on a Perkin-Elmer
Spectrum One spectrophotometer. Flash chromatography was carried
out with silica gel 60 (40 μm). Analytical HPLC was performed using
an Agilent 1100 series chromatograph with a JASCO PU-980 pump
and Agilent 1100 Series detection system. GC analyses were
performed on a HP6890 series apparatus with an injector, and helium
was used as carrier gas.
inseparable from the desired product. The ratio of byproduct in
the final material was easily determined by H NMR of the
1
mixture by integration of the aromatic signal and was evaluated
to be 4/1 (product/1,1′-binaphthyl). With o-tolylboronic acid
we observed a loss of selectivity (57% ee) and the reaction
required more time (72 h) to reach high yield (entries 4).
Finally, with the bulky [1,1′-biphenyl]-2-ylboronic acid, no
conversion was observed after 3 days at room temperature
(entry 5). Increasing the temperature (24 h at 40 °C) allowed
us to reach a moderate isolated yield but with a dramatic loss of
enantioselectivity.
Atropisomeric 4-methyl-3-(2-methylnaphthyl)thiophene
products are configurationally stable with a rotational barrier
of ∼33.5 kcal/mol. Yamaguchi, Itami, and co-workers recently
reported a palladium-catalyzed enantioselective C−H/C−B
coupling of sterically hindered heteroarenes such as 3-
methylthiophene with boronic acids in moderate yields and
enantioselectivity.34 When 13 was used as catalyst, the coupling
reaction shown in Figure 5 gave the product in high isolated
yield but with low selectivity (29% ee). Performing the reaction
at lower temperature (15 °C) did not meet with success.35
General Procedure for the Synthesis of Chiral PEPPSI
Complexes. A Schlenk tube was charged with PdCl2 (1 equiv),
NHC·HI (1.1 equiv), K2CO3 (5 equiv), and 3-chloropyridine (to give
C ≈ 0.1−0.25 mol/L). The mixture was stirred vigorously for 24 h at
80 °C. After it was cooled to room temperature, the reaction mixture
was diluted with CH2Cl2 and passed through a short pad of Celite. NaI
(∼5 equiv) and a stirring bar were added to the filtrate, and the
solution was stirred for 24 h at room temperature. Filtration through a
small pad of Celite, addition of CH2Cl2, and removal of excess 3-
chloropyridine under reduced pressure afforded the PEPPSI complex.
(S,S)-10. The general procedure was applied starting from PdCl2
(32 mg, 0.18 mmol), (S,S)-1,3-bis(2,2-dimethyl-1-o-
methylphenylpropyl)imidazolium iodide (107 mg, 0.2 mmol),
K2CO3 (126 mg, 0.9 mmol), and 3-chloropyridine (∼1 mL). For
the anion exchange an excess of NaI (∼150 mg, 1 mmol) was used and
the product was obtained as an orange solid (138 mg, 88%). Yellow
crystals were obtained by slow diffusion of n-pentane into a
20
concentrated solution of 10 in dichloromethane: [α]D = −66.9° (c
= 5.9 mg/mL, CH2Cl2); mp 115 °C; 1H NMR (CD2Cl2, 400 MHz) δ
1.16 (s, 18H, CH3 tBu), 2.94 (s, 6H, CH3 ortho), 6.55 (s, 2H, NCH),
7.05−7.24 (m, 8H, CHAr), 7.31 (dd, JHH = 8 Hz, JHH = 5 Hz, 1H,
CHPy), 7.74 (s, 2H, CHIm‑4/5), 7.76−7.81 (m, 1H, CHPy), 9.05 (d, JHH
= 2 Hz, 1H, CHPy‑ortho), 9.14 (d, JHH = 2 Hz, 1H, CHPy‑ortho) ppm;
13C{1H} NMR (CD2Cl2, 101 MHz) δ 22.9 (CH3 ortho), 29.4 (CH3 tBu),
37.9 (CtBu), 68.5 (CHtBu), 121.5 (CHIm‑4/5), 125.1 (CHPy), 125.6
(CHAr), 127.6 (CHAr), 129.5 (CHAr), 131.5 (CHAr), 132.5 (Cq), 136.4
(Cq), 138.2 (CHPy), 149.1 (NCN), 153.1 (CHPy‑ortho), 153.9
(CHPy‑ortho) ppm; ESI-MS m/z (%) 493.5 [M − 2I − 2H − PyCl]+;
FT-IR (ATR) 2960, 2920, 2872, 1463, 1419, 1401, 1366, 1226, 1195,
1162, 1118, 1049, 1016, 905, 844, 795, 744, 719, 689 cm−1. Anal.
Calcd for C32H40ClI2N3Pd·0.5CH2Cl2: C, 43.14; H, 4.57; N, 4.64.
Found: C, 43.21; H, 4.46; N, 4.52.
Figure 5. Asymmetric Suzuki−Miyaura coupling of 3-bromo-4-
methylthiophene with boronic acids.
CONCLUSION
■
The synthesis and characterization of PEPPSI complexes
incorporating chiral NHC ligands have been reported. Their
application in asymmetric Suzuki−Miyaura coupling has shown
good yields and moderate to good enantioselectivities. The
results are highly dependent on the substrate used. Good
selectivity (80% ee) was observed for coupling of 1-bromo-2-
methylnaphthalene with naphthylboronic acid, although the
results did not reach the levels of literature precedent for this
reaction. This notwithstanding, the coupling reaction results
reported here are to date the best using chiral NHC catalysts.
(S,S)-11. The general procedure was applied starting from PdCl2
(13 mg, 0.073 mmol), the β-naphthyl-unsaturated imidazolium salt
(50 mg, 0.084 mmol), K2CO3 (60 mg, 0.43 mmol), and 3-
chloropyridine (∼0.5 mL). For the anion exchange NaI (∼55 mg,
0.36 mmol) was used and the product was obtained as an orange solid
20
(54 mg, 78%): [α]D = −258.0° (c = 5.0 mg/mL, CH2Cl2); mp 165
1
°C; H NMR (CD2Cl2, 400 MHz) δ 1.27 (s, 18H, CH3 tBu), 6.51 (s,
2H), 7.29 (dd, JHH = 5 Hz, JHH = 8 Hz, 1H, CHPy), 7.39−7.61 (m, 7H,
CHAr), 7.66−8.00 (m, 10H, 7CHAr + 2 CHIm‑4/5, 1CHPy), 8.82 (dd,
JHH = 1 Hz, JHH = 5 Hz, 1H, CHPy), 8.91 (d, JHH = 2 Hz, 1H, CHPy)
ppm; 13C{1H} NMR (CD2Cl2, 101 MHz) δ 29.2 (CH3 tBu), 36.4
(CtBu), 74.8 (CHtBu), 121.4 (CHIm‑4/5), 125.3 (CHPy), 126.4 (CHAr),
126.6 (CHAr), 127.7 (CHAr), 128.1 (CHAr), 128.2 (CHAr), 128.5
(CHAr), 130.1 (CHAr), 132.6 (Cq), 133.3 (Cq), 133.4 (Cq), 135.5 (Cq),
138.2 (CHPy), 152.0 (NCN), 152.5 (CHPy), 153.3 (CHPy) ppm; ESI-
MS m/z (%) 565.3 [M − 2I − 2H − PyCl]+; FT-IR (ATR) 3055,
2962, 2929, 2872, 1719, 1597, 1564, 1508, 1464, 1417, 1401, 1367,
1332, 1270, 1225, 1189, 1164, 1118, 1099, 1050, 1015, 959, 882, 853,
786, 763, 742, 688, 648, 625 cm−1.
EXPERIMENTAL SECTION
■
General Methods. Solvents were purified by filtration on drying
columns containing alumina. Dioxane was distilled over calcium
hydride. Reactions and manipulations involving organometallic or
moisture-sensitive compounds were carried out under nitrogen.
Glassware was dried by heating under vacuum as necessary.
Starting Materials and Reagents. 3-Chloropyridine, 1-naph-
thylboronic acid, 3-bromo-4-methylthiophene, o-tolylboronic acid, 1-
bromo-2-methylnaphthalene, and [1,1′-biphenyl]-2-ylboronic acid
were purchased from Aldrich and used as received. KOH and
anhydrous K2CO3 were stored in a desiccator. Chiral amines and chiral
imidazolium and dihydroimidazolium salts were synthesized following
literature procedures.9,36 1-Bromo-2-methoxynaphthalene37 and 1-
chloro-2-methoxynaphthalene were prepared as described previ-
ously.38
(S,S)-12. The general procedure was applied starting from PdCl2
(14.7 mg, 0.083 mmol), the o-methoxy-unsaturated imidazolium salt
(50 mg, 0.09 mmol), K2CO3 (62 mg, 0.45 mmol), and 3-
chloropyridine (∼0.5 mL). For the anion exchange NaI (∼60 mg,
0.41 mmol) was used and the product was obtained as an orange solid
20
(59 mg, 80%): [α]D = −31.2° (c = 8.0 mg/mL, CH2Cl2); mp 207
1
°C; H NMR (CD2Cl2, 400 MHz) δ 1.12 (s, 18H, CH3 tBu), 3.91 (s,
6H, OCH3), 6.86−6.98 (m, 6H, NCH + CHAr), 7.12 (br s, 2H), 7.22−
7.28 (m, 2H), 7.30 (dd, JHH = 5 Hz, JHH = 8 Hz, 1H, CHPy), 7.66 (s,
2H, CHIm‑4/5), 7.75 (ddd, JHH = 1 Hz, JHH = 2 Hz, JHH = 8 Hz, 1H,
CHPy), 8.89 (d, JHH = 5 Hz, 1H, CHortho‑Py), 9.05 (d, JHH = 2 Hz, 1H,
1
Characterizations. H and 13C{1H} NMR spectra were recorded
on Bruker AMX 400, 300, and 500 MHz spectrometers. Chemical
shifts (δ) are reported in ppm compared to TMS using the residual
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CHortho‑Py) ppm; 13C{1H} NMR (CD2Cl2, 101 MHz) δ 29.2 (CH3 tBu),
37.2 (CtBu), 55.8 (OCH3), 112.0 (CH), 119.2 (CH), 120.1(CHIm‑4/5),
124.9 (CHPy), 128.8 (CH), 130.1 (CH), 137.9 (CPy), 152.2
(CHPy‑ortho), 153.2 (CHPy‑ortho), 158.3 (NCN) ppm; ESI-MS m/z
(%) 525.3 [M − 2I − 2H − PyCl]+; FT-IR (ATR) 2953, 2923, 2855,
1728, 1601, 1589, 1565, 1492, 1460, 1422, 1403, 1368, 1332, 1288,
1243, 1198, 1185, 1164, 1108, 1051, 1031, 931, 908, 852, 842, 797,
778, 746, 729, 689, 627 cm−1. Anal. Calcd for C32H40ClI2N3O2Pd: C,
42.97; H, 4.51; N, 4.70. Found: C, 42.71; H, 4.41; N, 4.70.
(R,R)-13. The general procedure was applied starting from PdCl2
(10.1 mg, 0.056 mmol), the α-naphthyl-unsaturated imidazolium salt
(39 mg, 0.066 mmol), K2CO3 (45 mg, 0.25 mmol), and 3-
chloropyridine (∼0.5 mL). For the anion exchange NaI (∼40 mg,
0.28 mmol) was used and the product was obtained as an orange solid
(0−1%) or by PTLC with pentane (several elutions were sometimes
required). Enantiomeric ratios were checked by chiral HPLC in 2-
propanol or by SFC in Et2O.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, a table, and CIF files giving experimental details
and characterization data, 1H NMR and 13C{1H} NMR spectra
for products of catalysis, optimization of catalytic reactions,
HPLC chromatograms, and X-ray data for 13 and 14. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 957329 (for 14) and 957330 (for 13) also
contain crystallographic data.
(40 mg, 75%). Yellow crystals were obtained by slow diffusion of n-
20
pentane into a concentrated solution in dichloromethane: [α]D
=
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
+183.6° (c = 5.8 mg/mL, CH2Cl2); mp >250 °C dec; 1H NMR
(CD2Cl2, 400 MHz) δ 1.20 (s, 18H, CH3 tBu), 6.99 (dd, JHH = 8 Hz,
JHH = 5 Hz, 1H, CHPy), 7.27 (s, 2H, CHtBu), 7.30 (m, 2H, CHAr), 7.51
(m, 6H), 7.60 (ddd, JHH = 1 Hz, JHH = 2 Hz, JHH = 8 Hz, 1H, CHPy),
7.79 (dd, JHH = 1 Hz, JHH = 5 Hz, 1H, CHPy), 7.86 (d, JHH = 8 Hz, 2H,
CHAr), 7.89 (s, 2H, CHIm‑4/5), 7.93−8.00 (m, 3H, 2CHAr + 1 CHPy),
8.77 (d, JHH = 8 Hz, 2H, CHAr) ppm; 13C{1H} NMR (CD2Cl2, 101
MHz) δ 29.6 (CH3 tBu), 37.5 (CtBu), 67.9 (CHtBu), 121.5 (CHIm‑4/5),
124.4 (CHPy), 125.6 (CHAr), 125.8 (CHAr), 126.1 (CHAr), 126.7
(CHAr), 128.5 (CHAr), 128.8 (CHAr), 129.7 (CHAr), 131.8 (Cq), 134.6
(Cq), 134.8 (Cq), 135.4 (Cq), 137.9 (CHPy), 152.3 (NCN), 153.0
(CHPy), 153.7 (CHPy) ppm; ESI-MS m/z (%) 565.3 [M − 2I − 2H −
PyCl]+; FT-IR (ATR) 3050, 2965, 2924, 2872, 1663, 1593, 1510,
1475, 1437, 1397, 1368, 1317, 1263, 1233, 1117, 1049, 950, 912, 870,
778, 745, 691, 645, 625 cm−1. Anal. Calcd for C38H40ClI2N3Pd·
0.4CH2Cl2: C, 47.63; H, 4.25; N, 4.34. Found: C, 47.73; H, 4.03; N,
4.38.
■
ACKNOWLEDGMENTS
■
We thank the Swiss National Science Foundation and the
University of Geneva for financial support, Stephane Rosset and
Prof. Alexandre Alexakis (Geneva) for the use of their
supercritical fluid chromatography (SFC) equipment, and
Prof. Luigi Cavallo (Salerno and KAUST) for advice on the
calculation of %Vbur.
REFERENCES
■
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20
pentane into a concentrated solution in dichloromethane: [α]D
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2H, CHAr), 8.69 (d, JHH = 8 Hz, 2H, CHAr) ppm; 13C{1H} NMR
(CD2Cl2, 101 MHz) δ 29.3 (CH3 tBu), 36.9 (CtBu), 50.4 (CH2 Im‑4/5),
65.4 (CHtBu), 123.9 CHPy), 125.4 (CHAr), 125.5 (CHAr), 126.3
(CHAr), 126.5 (CHAr), 127.4 (CHAr), 128.5 (CHAr), 129.5 (CHAr),
131.4 (Cq), 134.6 (Cq), 134.9 (Cq), 137.6 (CHPy), 137.8 (Cq), 153.6
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N, 4.31.
́
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