Convenient in situ generation of a chiral bis-N-heterocyclic carbene palladium catalyst and its application in enantioselective synthesis

Article abstract of DOI:10.1016/j.poly.2012.07.046

To simplify catalytic reactions employing chelating bisNHC-metal complexes, studies were conducted to elucidate conditions for the in situ generation of a chiral chelating bisNHC-palladium catalyst from the corresponding diimidazolium salt. The method provides a convenient entry to catalytic reactions and eliminates catalyst preparation steps. In addition to the in situ prepared catalyst, for comparative purposes, isolable species were synthesized and characterized by NMR spectroscopy, combustion analysis, and single-crystal X-ray diffraction. Employing C₂-symmetric ligands derived from trans-9,10-Dihydro-9,10-EthanoAnthracene-11,12-diyl (DEA) and trans-9,10-Dihydro-9,10-EthanoAnthracene-11,12-diylMethanediyl (DEAM), diNHC-Pd complexes were synthesized and tested for activity in enantioselective arylboronic acid addition to cyclic enones.

Full text of DOI:10.1016/j.poly.2012.07.046

Polyhedron 52 (2013) 810–819
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Convenient in situ generation of a chiral bis-N-heterocyclic carbene palladium
catalyst and its application in enantioselective synthesis
⇑
Amrita B. Mullick, Matthew S. Jeletic, Andrew R. Powers, Ion Ghiviriga, Khalil A. Abboud, Adam S. Veige
University of Florida, Center for Catalysis, P.O. Box 117200, Gainesville, FL 32611, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 9 August 2012
To simplify catalytic reactions employing chelating bisNHC–metal complexes, studies were conducted to
elucidate conditions for the in situ generation of a chiral chelating bisNHC–palladium catalyst from the
corresponding diimidazolium salt. The method provides a convenient entry to catalytic reactions and
eliminates catalyst preparation steps. In addition to the in situ prepared catalyst, for comparative pur-
poses, isolable species were synthesized and characterized by NMR spectroscopy, combustion analysis,
and single-crystal X-ray diffraction. Employing C2-symmetric ligands derived from trans-9,10-Dihydro-
9,10-EthanoAnthracene-11,12-diyl (DEA) and trans-9,10-Dihydro-9,10-EthanoAnthracene-11,12-diyl-
Methanediyl (DEAM), diNHC–Pd complexes were synthesized and tested for activity in enantioselective
arylboronic acid addition to cyclic enones.
Dedicated to Alfred Werner on the 100th
Anniversary of his Nobel Prize in Chemistry
in 1913.
Keywords:
Carbene
Catalyst
In situ
Chiral
Published by Elsevier Ltd.
Enantioselective
Asymmetric
1. Introduction
term storage, and (5) solves the potential problem that active cata-
lysts can be difficult to isolate.
One of the most exciting advances in homogeneous metal-med-
iated catalysis over the past decade is the emergence of N-heterocy-
clic carbene (NHC) ligands [1–11]. The rapid rise in application of
NHC ligands is due in part to numerous fundamental studies that
probe important NHC–metal properties including: size/volume
A few examples of in situ generated mono-NHC metal catalysts
have been reported. Nolan and co-workers demonstrate the utility
of in situ generated NHC–metal catalysts in Suzuki, Kumada, and
Sonogashira couplings [72–75]. Employing a chelating diphosphor-
ous ligand, Minnaard and co-workers demonstrate the competence
of in situ generated catalysts in the asymmetric conjugate addition
of arylboronic acids to enones [76], a reaction important for creating
chiral centers [77–80], which is primarily catalyzed by copper- and
rhodium-based catalysts [81–87]. The catalyst can be generated in
one pot containing the ligand, a base, and a metal precursor. Upon
addition of reagents, catalysis proceeds yielding enantio-enriched
product.
Mono- and diNHC supported palladium complexes catalyze
Michael addition reactions [57]. Our group [44,57,65,88–91] and
several others have developed diNHC ligands and employed them
in metal-mediated asymmetric catalysis [49–70,92–103]. Specifi-
cally, our focus centers on exploiting chiral C2-symmetric ligands
derived from trans-9,10-Dihydro-9,10-EthanoAnthracene-11,12-
diyl (DEA) and trans-9,10-Dihydro-9,10-EthanoAnthracene-11,12-
diylMethanediyl (DEAM) (Chart 1). Upon metalation the diNHC
ligands provide isolable and thermally stable C1-symmetric palla-
dium, iridium and rhodium catalysts. Unprecedented however, are
in situ generated catalysts derived from di-N-heterocyclic carbene
ligands (diNHC). Herein, we present the first transient formation
of a chiral diNHC-supported palladium species capable of efficiently
[12–16],
p-acidity [17–27], r-donating strength [14–16,21,22,
28–43], and flexibility [44–47]. Exploiting this wealth of data per-
mits the design of highly refined ligands and, in the area of metal-
catalyzed asymmetric catalysis [48], ligands capable of producing
products in high enantiomeric excess (% e.e.) [44,49–70]. Creating
a useful chiral catalyst involves more than simply achieving high
enantioselectivity; a catalyst must be easy to use and inexpensive
to prepare. One disadvantage to employing a NHC ligand is that cat-
alysts are typically preformed and stored prior to use. Many ligands,
including the class of C2-symmetric ‘‘privileged ligands’’ [71], can
simply be added to a metal catalyst precursor to generate the active
species in situ. This method offers several distinct advantages over
the use of preformed catalysts; for example it: (1) facilitates bench-
top chemistry, (2) boasts the ability to rapidly screen a wide scope
of metal ion precursors, (3) avoids additional catalyst synthetic
steps, (4) prevents potential loss of catalyst integrity from long-
⇑
Corresponding author. Tel.: +1 352 392 9844; fax: +1 352 392 3255.
E-mail address: veige@chem.ufl.edu (A.S. Veige).
0277-5387/$ - see front matter Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2012.07.046

A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
811
Mercury 300 MHz, Varian VXR 300 MHz, Gemini 300 MHz and Var-
ian Inova 500 MHz spectrometers. HPLC analysis was done using a
Shimadzu prominence instrument with a LC-20AT solvent delivery
module, DGU-20A3 degasser, SPD-20A UV–Vis detector (254 nm,
215 nm) and a CBM-20A system controller.
2.1.1. Synthesis of 3
To an oven dried 100 mL flask containing a stir bar and trans-
9,10-dihydro-9,10-ethanoanthracene-11,12-diyldimethanediyl-
bis(trifluoromethanesulfonate) [104] (2.52 g, 4.75 mmol) in dry
DME (30 mL) was added 1-(2-methylbenzyl)-1H-benzimidazole
(2.11 g, 9.49 mmol). After refluxing under argon for 1 h, all
volatiles were removed in vacuo to provide 3 as a hygroscopic
off-white flocculent powder (4.29 g, 93%). 1H NMR (300 MHz, ace-
tone-d6): 9.65 (s, 2H, NCHN), 8.02 (dd, J = 6 Hz, J = 3 Hz, 2H,
NCCHCH), 7.96 (dd, J = 9 Hz, J = 3 Hz, 2H, NCCHCH), 7.78 (dd,
J = 6 Hz, J = 3 Hz, 4H, NCCHCH), 7.36–7.21 (m, 12H, aromatic,
CHCCHCH and CHCCHCH, signals overlap), 7.15 (ddd, J = 15 Hz,
J = 15 Hz, J = 6 Hz, 4H, CHCCHCH), 5.91/5.90 (4H, NCH2C), 4.76
(dd, J = 15 Hz, J = 3 Hz, 2H, NCH2CH), 4.42 (s, 2H, CCHC), 4.28 (dd,
J = 15 Hz, J = 9 Hz, NCH2CH, 2H), 2.77 (dd, J = 3 Hz, J = 3 Hz, NCH2-
CH, 2H), 2.45 (s, CH3, 6H). 13C{1H} NMR (75 MHz, acetone-d6):
143.3 (NCHN), 143.2 (s, NCCH), 140.5 (s, NCCH), 137.8 (quaternary
C), 132. 7 (quaternary C), 132.6 (quaternary C), 132.2 (aromatic),
131.9 (quaternary C), 130.0 (aromatic), 129.7 (aromatic), 128.22
(aromatic), 128.19 (aromatic), 127.54 (aromatic), 127.50 (aro-
matic), 127.20 (aromatic), 126.5 (aromatic), 125.4 (aromatic),
122.1 (q, J = 300 Hz, CF3), 115.1 (NCCH), 114.9 (NCCH), 51.3 (NCH2-
CH), 50.1 (NCH2C), 45.9 (CCHC), 44.1 (NCH2CH), 19.3 (CH3).
HR-ESI-FTICR-MS: Calc. for C49H43N4SO3F3: m/z 825.3081 [M+SO3-
CF3]+. Found: m/z 825.3020. Anal. Calc. for C50H44N4S2O6F6: C,
61.60; H, 4.56; N, 5.75. Found: C, 61.76; H, 4.66; N, 5.41%.
2.1.2. Synthesis of 8exo/8endo
Two solutions are prepared in
a glovebox: (A) 500 mg
(0.455 mmol) 2 and 310 mg (0.954 mmol) Cs2CO3 in 2 mL THF,
(B) 86 mg (0.236 mmol) [Pd(C3H5)Cl]2 in 3 mL THF. Solution A is
stirred for 2 min and B is added at room temperature. Solution
AB is then stirred for 16 h at room temperature and then removed
from the glovebox. The solution is filtered through a medium frit-
ted funnel and the filtrate is dropped into 35 mL of hexanes. The
precipitate is filtered leaving 8exo/8endo as a white solid (485 mg,
92% yield). Complexes 8exo/8endo are isolated as a mixture of endo
and exo isomers, thus not all resonances were assigned. MS(HR-
Chart 1. DEAM and DEA ligand precursors.
catalyzing the enantioselective conjugate addition of arylboronic
acids to cyclic enones.
ESI-FTICR+): Calc. for [C61H51N4Pd]+: m/z 945.3164 M+. Found: m/
20.85
2. Experimental
z 945.3172. (S,S) enantiomer [
a]
ꢀ20.593 (c 0.10, CH2Cl2),
D
20.97
(R,R) enantiomer [
a
]
D
+24.083 (c 0.10, CH2Cl2).
2.1. Materials and methods
2.1.3. Synthesis of 9exo/9endo
Two solutions are prepared in
Unless specified otherwise, all manipulations were performed
under an inert atmosphere using standard glove box techniques.
Glassware was oven dried before use. Tetrahydrofuran (THF) was
dried using a GlassContours drying column and was degassed
using three freeze–pump–thaw cycles before use. CDCl3 was pur-
chased from Cambridge Isotopes dried with calcium hydride and
stored over 4 Å molecular sieves. Chloro(allyl)palladium(II) dimer
and palladium acetate were purchased from Strem Chemicals Co.
and used without further purification. Cesium carbonate, 2-cycloh-
exen-1-one and 2-cyclopenten-1-one were purchased from Sigma–
Aldrich. Boronic acids were purchased from Combi-Blocks Inc. and
used without further purification. Diimidazolium ligand precursors
2 [65] and 6 [90] were prepared according to literature methods.
Anhydrous potassium carbonate, ether, hexanes, ethyl acetate,
THF, dioxane and methanol were purchased from Fischer Scientific
and used without further purification. 1H and 13C{1H} NMR spectra
were recorded on Varian Mercury broad Band 300 MHz, Varian
a glovebox: (A) 500 mg
(0.512 mmol) 3 and 351 mg (1.08 mmol) Cs2CO3 in 2 mL THF, (B)
94 mg (0.256 mmol) [Pd(C3H5)Cl]2 in 3 mL THF. Solution A is stirred
for 2 min and B is added at room temperature. Solution AB is then
stirred for 16 h at room temperature and then removed from the
glovebox. The solution is filtered through a medium fritted funnel
and the filtrate is dropped into 35 mL of hexanes. The precipitate
is filtered leaving 9exo/9endo as a white solid (446 mg, 89% yield).
Complexes 9exo/9endo are isolated as a mixture of endo and exo iso-
mers, thus not all resonances were assigned. Partial assignment of
the 1H and 13C chemical shifts were made in acetone-d6 at ꢀ60 °C,
and confirm two isomers (59:31) of a molecule having the two
NHC moieties bound to the metal at the C2 position (see Fig. 2 for de-
tails). Anal. Calc. for C52H47N4SPdO3F3: C, 64.29; H, 4.88; N, 5.77.
Found: C, 64.25; H, 4.90; N, 5.69%. MS(HR-ESI-FTICR+): Calc. for [C51-
H47N4Pd]+: m/z 821.2848. Found: m/z 821.2867. [
a]
D
ꢀ5.799 (c
20.42
0.10, CH2Cl2).

812
A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
2.1.4. Synthesis of 10
136.6 (C aromatic), 135.9 (C aromatic), 135.7 (C aromatic), 135.1 (C
aromatic), 131.7 (C aromatic), 128.5 (C aromatic), 128.2 (C aro-
matic), 128.0 (C aromatic), 127.7 (C aromatic), 127.4 (C aromatic),
127.3 (C aromatic), 124.9 (C aromatic), 124.3 (C aromatic), 124.2 (C
aromatic), 124.1 (C aromatic), 123.9 (C aromatic), 122.33 (C aro-
matic), 112.3 (CNCCNCH3), 111.6 (CNCCNCH3), 111.5 (CNCCNCH3),
111.2 (CNCCNCH3), 68.2 (THF), 64.3 (s, NCHCH), 62.9 (s, NCHCH),
49.1 (s, NCHCH), 47.4 (s, NCHCH), 39.6 (s, NCH3), 36.0 (s, NCH3),
26.0 (THF). Anal. Calc. for C32H26I2N4Pd plus one molecule THF: C,
48.10; H, 3.81; N, 6.23. Found: C, 48.19; H, 3.77; N, 6.19%.
Two solutions are prepared in a glovebox: (A) 500 mg (0.455
mmol) 2 and 310 mg (0.954 mmol) Cs2CO3 in 2 mL THF, (B)
139 mg (0.456 mmol) Pd(acac)2 in 3 mL THF. Solution A is stirred
for 2 min and B is added at room temperature. Solution AB is then
stirred for 16 h at room temperature and then removed from the
glovebox. The solution is filtered through a medium fritted funnel
and the filtrate is dropped into 35 mL of hexanes. The precipitate is
filtered providing 10 as a pale yellow solid (399 mg, 76% yield). 1H
NMR (300 MHz, CDCl3, d): 8.24 (dd, J = 3 Hz, J = 6 Hz, 1H, aromatic),
8.22 (1H, NCH), 7.66 (dd, J = 3 Hz, J = 6 Hz, 2H, aromatic), 7.46–7.13
(m, 21H, NCH + aromatic), 7.02–6.90 (m, 5H, aromatic), 6.84–6.75
(m, 3H, aromatic), 6.64 (d, J = 6 Hz, 2H, aromatic), 6.47 (d,
J = 9 Hz, 1H, aromatic), 6.14 (dd, J = 6 Hz, J = 6 Hz, 1H, NCH2CH),
5.98 (d, J = 9 Hz, 2H, aromatic), 5.32 (d, J = 15 Hz, 1H, CH2), 5.23
(s, 1H, OCCHCO), 5.05 (s, 1H, NCH2CHCH), 4.33 (dd, J = 9 Hz,
J = 12 Hz, 1H, CH2), 4.17 (s, 1H, NCH2CHCH), 4.13 (dd, J = 9 Hz,
J = 12 Hz, 1H, CH2), 2.64 (d, J = 15 Hz, 1H, CH2), 2.17 (dd, J = 6 Hz,
J = 6 Hz, 1H, NCH2CH), 1.84 (s, 3H, CH3), 1.71 (s, 3H, CH3). 13C{1H}
NMR (75 MHz, CDCl3, d): 187.7 (NCN), 186.1 (NCN), 167.0 (CO),
166.1 (CO), 145.0 (NCH2CHCHC), 143.9 (NCH2CHCHC), 138.2
(NCH2CHCHC), 137.7 (NCH2CHCHC), 137.5 (NCHC), 136.0 (NCHC),
135.5 (NCHC), 135.3 (NCHC), 135.2 (NCCH), 134.8 (NCCH), 133.1
(NCCH), 132.3 (NCCH), 129.1 (aromatic), 128.6 (aromatic), 128.5
(aromatic), 128.3 (aromatic), 127.9 (aromatic), 127.2 (aromatic),
127.1 (aromatic) 126.4 (aromatic), 126.1 (aromatic), 125.2 (aro-
matic), 125.1 (aromatic), 124.7 (aromatic), 124.3 (aromatic),
121.7 (aromatic), 115.1 (NCCH), 113.8 (NCCH), 112.6 (NCCH),
109.9 (NCCH), 101.0 (OCCHCO), 68.3 (NCH), 67.9 (NCH), 53.2
(NCH2), 51.3 (NCH2CH), 50.2 (NCH2), 46.9 (NCH2CHCH), 46.6
(NCH2CHCH), 45.8 (NCH2CH), 26.5 (CH3), 26.3 (CH3). MS(HR-ESI-
2.1.6. Catalysis procedure with in-situ catalyst generation
Under an inert atmosphere, to a 20 mL glass vial was added
5 mol% of the ligand, 12 mol% of base, 2.5 mol% of palladium pre-
cursor and THF (1 mL). The reaction was allowed to stir at room
temperature for the appropriate time, whereupon 1.5 equiv. of
arylboronic acid, base [KOH (33 mol%) or K2CO3 (50 mol%)] was
added to the vial. Finally, 2-cyclohexen-1-one or 2-cyclopenten-
1-one (1 mmol) and degassed water were added to the reaction
mixture and the reaction was heated to 68 °C with stirring. One
milliliter of saturated sodium bicarbonate was then added and
the product was extracted with 10 ꢂ 2 mL of diethyl ether. The or-
ganic layer was concentrated and the product was purified via col-
umn chromatography using 10% EtOAc/hexanes. The yield was
measured using 1H NMR spectroscopy and the % e.e. was deter-
mined using chiral HPLC.
2.1.7. Catalysis procedure with isolable catalyst
On the bench-top, to a 20 mL glass vial was added 1.5 equiv. of
aryl boronic acid, 5 mol% of the catalyst and base (KOH, 33 mol%
and K2CO3, 50 mol%). Degassed 1 mL of THF/water (10:1) or diox-
ane/MeOH (4:1) was then added. Finally, 2-cyclohexen-1-one
(1 mmol) was added to the reaction mixture and the reaction
was heated to 68 °C with stirring. One milliliter of saturated
sodium bicarbonate was then added and extracted with
10 ꢂ 2 mL of diethyl ether. The organic layer was concentrated
and the product was purified via column chromatography using
10% EtOAc/hexanes. The yield was measured using 1H NMR spec-
troscopy and the % e.e. was determined using chiral HPLC.
FTICR+): Calc. for [C63H53N4PdO2]+: m/z 1003.3219 M+. Found: m/
20.23
z 1003.3252. [
a]
ꢀ95.827 (c 0.10, CH2Cl2).
D
2.1.5. Synthesis of 11
To a stirring 10 mL THF suspension of the DEA precursor 1,10-
(9,10-dihydro-9,10-ethanoanthracene-11,12-diyl)di(3-methyl-1H-
imidazol-3-ium) diiodide (6) (150 mg, 0.21 mmol) was added
palladium acetate (50 mg, 0.22 mmol). The reaction was allowed
to stir at room temperature under argon for 5 h, at which point
the temperature of the system was increased incrementally in
the following manner, for the following durations: 2 h at 40 °C,
2 h at 60 °C, 3 h at 90 °C, and 1 h at 120 °C. The reaction mixture
was allowed to cool overnight, with stirring and then solvent
was removed in vacuo. On a bench top, the residue was taken up
in fresh THF and filtered. The solid remaining on the frit was
washed with warm THF until the filtrate was colorless. The volume
of the filtrate was reduced to induce crystallization. The product
was isolated via filtration as yellow crystals of 11 (70 mg, 37%
yield). An alternative procedure for synthesizing 11 involves a sim-
ilar tiered heating scheme as above, but utilizing DMSO as the sol-
vent. Single crystals amenable to X-ray diffraction were grown
from material synthesized in this manner. To a vial containing a
very concentrated solution of 11 in DMSO was layered hexanes,
and filled with CH2Cl2. After 2 days at room temperature, large yel-
low single crystals were obtained. Anal. Calc. for C32H26I2N4PdꢁTHF:
C, 48.10; H, 3.81; N, 6.23. Found: C, 48.19; H, 3.77; N, 6.19%. 1H
NMR (499.8 MHz, CD2Cl2) d ppm: 8.77 (d, J = 10.1 Hz, 1H, bridge),
7.82 (d, J = 7.4 Hz, 1H, aromatic), 7.72 (d, J = 7.3 Hz, 1H, aromatic),
7.63–7.59 (m, 2H, aromatic), 7.51 (t, J = 7.5 Hz, 1H, aromatic), 7.40–
7.46 (m, 4H, aromatic), 7.30–7.35 (m, 3H, aromatic), 7.18–7.29 (m,
3H, aromatic), 7.13 (m, 1H, aromatic), 5.24 (s, 1H, bridgehead), 5.20
(s, 1H, bridgehead), 4.49 (d, J = 10.1 Hz, 1H, bridge), 4.22 (s, 3H,
NCH3), 3.95 (s, 3H, NCH3), 3.68 (m, 2H, THF), 1.82 (m, 2H, THF).
13C{1H} NMR (125.69 MHz, CD2Cl2) d (ppm): 177.1 (Pd–C), 174.4
(Pd–C), 146.7 (C aromatic), 144.8 (C aromatic), 139.0 (C aromatic),
2.2. X-ray crystallography
2.2.1. X-ray experimental details for complexes 8exo/8endo
Data are collected at 173 K on a Siemens SMART PLATFORM
equipped with a CCD area detector and a graphite monochromator
utilizing Mo K
fined using up to 8192 reflections. A full sphere of data (1850
frames) was collected using the -scan method (0.3° frame width).
a radiation (k = 0.71073 Å). Cell parameters are re-
x
The first 50 frames were measured again at the end of data collec-
tion to monitor instrument and crystal stability (maximum correc-
tion on I is <1%). Absorption corrections by integration are applied
based on measured indexed crystal faces. The structure is solved by
the Direct Methods in SHELXTL6, and refined using full-matrix least
squares. The non-H atoms are treated anisotropically, whereas
the hydrogen atoms are calculated in ideal positions and are riding
on their respective carbon atoms. The asymmetric unit consists of
the Pd complex cation, a triflate anion and a THF molecule. The lat-
ter is disordered and could not be modeled properly, thus program
SQUEEZE, a part of the PLATON package of crystallographic software, is
used to calculate the solvent disorder area and remove its contri-
bution to the overall intensity data. The C19–C21 group is disor-
dered with the middle CH unit occupying two positions. The
disordered parts are refined with dependent occupation factors.
All atoms of the triflate anion are disordered except the S atom.
The final model refines two parts with their site occupation factors
fixed at 50%. A total of 650 parameters are refined in the final cycle

A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
813
of refinement using 8485 reflections with I > 2
r
(I) to yield R1 and
wR2 of 5.52% and 14.20%, respectively. Refinement is done using F2.
2.2.2. X-ray experimental details for complex 11
X-Ray intensity data were collected at 100 K on a Bruker DUO
diffractometer using Mo Ka radiation (k = 0.71073 Å) and an APEX-
II CCD area detector. Raw data frames were read by program SAINT
and integrated using 3D profiling algorithms. The resulting data
were reduced to produce hkl reflections and their intensities and
estimated standard deviations. The data were corrected for Lorentz
and polarization effects and numerical absorption corrections were
applied based on indexed and measured faces.
The structure was solved and refined in SHELXTL6.1, using full-
matrix least-squares refinement. The non-H atoms were refined
with anisotropic thermal parameters and all of the H atoms were
calculated in idealized positions and refined riding on their parent
atoms. The asymmetric unit consists of the Pd complex, a DMSO
and 1.5 dichloromethane (DCM) solvent molecules. All solvent
molecules were disordered and could not be modeled properly,
thus program SQUEEZE, a part of the PLATON package of crystallo-
graphic software, was used to calculate the solvent disorder area
and remove its contribution to the overall intensity data. The
DMSO molecule is disordered in a general position while the
DCM molecules are disordered in channels along the b-axis. In
the final cycle of refinement, 8742 reflections (of which 6698 are
Chart 2. List of new DEAM–Pd and DEA–Pd preformed catalysts employed in this
study.
observed with I > 2r(I)) were used to refine 354 parameters and
the resulting R1, wR2 and S (goodness of fit) were 2.46%, 4.47%
and 0.861, respectively. The refinement was carried out by mini-
mizing the wR2 function using F2 rather than F values. R1 is calcu-
lated to provide a reference to the conventional R value but its
function is not minimized.
3. Results and discussion
3.1. Synthesis and characterization of isolable catalysts
The purpose of this study is to develop a protocol to generate
diNHC–metal catalysts in situ from corresponding diimidazolium
salt ligand precursors. To be meaningful, it is important to compare
the in situ generated catalyst with isolable versions and compare
the difference, if any, on product enantiomeric excess. To accom-
plish this task, we first synthesized a series of DEAM and DEA diN-
HC–Pd–L (L = allyl, acetylacetonate, iodide) derivatives (Chart 2).
Treating 0.6 equiv. of [Pd(C3H5)Cl]2 with 2 or 3 and 2.2 equiv.
Cs2CO3 at 25 °C provides 8exo/8endo and 9exo/9endo as colorless
microcrystalline powders in 92% and 87% yield, respectively. 1H
NMR spectra of 8exo/8endo and 9exo/9endo reveal signals attributable
to two conformers in which the allyl orients endo and exo with re-
spect to the NHC N-substituents. Employing ligand
5 and
[Pd(allyl)Cl]2 in a previous study resulted in a similar mixture of
isomers [57] (Scheme 1).
Scheme 1. Synthesis of 8endo/8exo and 9endo/9exo
.
A single crystal X-ray diffraction experiment, performed on a
crystal comprising a racemic mixture of the compound, confirms
the identity of 8exo/8endo. Complexes 8exo/8endo contain a C1-sym-
metric distorted square planar Pd(II) ion coordinated by the diNHC
(acetone-d6 at ꢀ60 °C) are possible and confirm the presence of
two isomers (59:31). The nOe’s observed in a NOESY spectrum
did not permit distinction between the exo and endo conformers.
Drawing on the structural data of 8 (Fig. 1) and the published data
employing ligand 6, the major isomer is tentatively assigned to
9exo. Fig. 2 depicts the NMR data that are conclusively assignable
for 9exo/9endo. For example, within the ethanoanthracene unit,
the four bridgehead protons for the two conformers (exo: 4.83,
4.39, and endo: 4.89, 4.53 ppm) are identifiable by their long-range
coupling with two aromatic carbons at 125 ppm and with two or
more quaternary aromatic carbons between 140 and 145 ppm
(see ESI). Cross-peaks in a gDQCOSY spectrum enable assignment
and one g
3-C3H5 ligand. The solid state structure reveals a disorder
in the central allyl carbon with site occupancies refined to give an
exo to endo ratio of 0.78(1):0.22(1) (Fig. 1, endo depicted; see
Table 5 for refinement data). Complexes 8exo/8endo feature a ligand
environment comprised entirely of Pd–C bonds. Appropriately, the
Pd–NHC bonds are shorter (Pd1–C1 = 2.053(3) and Pd1–C18
=2.086(3) Å) than the Pd–g
3-allyl bonds (Pd1–C19 = 2.162(4),
Pd1–C20 = 2.195(4), and Pd–C21 = 2.195(4) Å).
Single crystals of 9exo/9endo were not obtained; however, partial
absolute assignment of the 1H and 13C{1H} chemical shifts

814
A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
Scheme 2. Synthesis of 10.
cross-peaks with proximate methylene protons enable their abso-
lute stereochemical assignments. Other cross-peaks to the carbene
carbons permit assignment of the benzyl methylene protons (exo:
4.79/5.70, 5.97/5.77, and endo: 5.45/4.82, 5.60/5.60 ppm). Finally,
a gDQCOSY experiment identifies the individual allyl protons.
Generating catalysts in situ from various metal precursors will
undoubtedly result in the formation of complexes that feature dif-
ferent non-NHC ancillary ligands. Acetylacetonate (acac) is a com-
mon ligand found in many metal precursor reagents. Accessing the
acac complex 10 as a pale-yellow solid in 79% yield involves treat-
ing 2 with 1 equiv. of Pd(acac)2 and 2.1 equiv. of Cs2CO3 (Scheme 2)
followed by stirring the solution for 12 h, filtration, and precipita-
tion from hexanes. The 1H and 13C{1H} NMR spectra of 10 are
indicative of a C1-symmetric complex. Several unique 1H NMR res-
onances support the structure assignment of 10. The acac methyl
proton signals resonate at 1.71 and 1.84 ppm, and the acac
methine proton resonates at 5.23 ppm. The ethanoanthracene
bridgehead protons resonate at 2.17 and 6.14 ppm with the large
4 ppm difference a consequence of the distinct chiral environment
of 10. One of the diphenylmethine proton signals resonates at
8.22 ppm, while the other proton resonates in a region with several
aromatic protons that obscures its exact location. The 13C{1H} NMR
spectrum also exhibits several diagnostic signals. Notably, the car-
bene C2 carbons resonate at 187.7 and 186.1 ppm. Signals at 26.3
and 26.5 ppm are attributable to the acac methyl carbons and
Fig. 1. Molecular structure of DEAM–palladium catalyst 8exo/8endo (8exo depicted).
Thermal ellipsoids are drawn at the 50% probability level and the OTfꢀ counter ion
and disordered allyl C200 are removed for clarity. Bond lengths (Å): Pd1–C1
2.053(3), Pd1–C18 2.086(3), Pd1–C20 2.149(6), Pd1–C200 2.135(13) (not shown),
Pd1–C19 2.162(4), Pd1–C21 2.195(4). Bond angles (°): C1–Pd1–C18 97.14(12), C1–
Pd1–C19 95.22(15), C19–Pd–C21 67.28(16), C18–Pd1–C21 100.32(15), C1–Pd1–C21
162.18(15), C18–Pd1–C19 167.60(14).
of the bridge (exo: 4.53, 2.14, and endo: 4.79, 2.28 ppm) and
adjacent methylene protons (exo: 3.98/2.93, 5.23/3.58, and endo
4.08/3.07, 5.37/3.81 ppm) for both conformers. A gHMQC experi-
ment allows for the assignment of the corresponding carbons to
the above protons. Most importantly, the carbene carbons bound
to Pd(II) are uniquely identifiable by their downfield chemical
shifts (exo: 186.3, 189.9 ppm, and endo: 186.1, 189.2 ppm), and
Fig. 2. Partial assignment of the 1H and 13C{1H} NMR data for 9exo/9endo
.

A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
815
the corresponding acac methine resonates at 101.0 ppm. The acac
ketone carbons appear in the appropriate region at 167.0 and
166.1 ppm, and the diphenylmethine carbons resonate at 68.3
and 67.9 ppm.
bridge proton H17 (Fig. 3) is attributable to the location of the pro-
ton directly beneath the palladium center. Analogous Rh–DEA
derivatives exhibit a similar downfield shift [88,90]. Dihedral an-
gles of approximately 78.51° and 80.92° between the two sets of
bridge and bridgehead protons results in no observable coupling
between the protons, consistent with the vicinal Karplus correla-
tion [108,109].
Efforts to synthesize the corresponding DEA–Pd–allyl or Pd–
acac derivatives were unsuccessful. Instead, employing a progres-
sive heating method of the ligand precursor 2 and Pd(OAc)2 in
THF provides the DEA–PdI2 complex 11 in 37% yield (Scheme 3).
Notably, Hahn et al. [105] and later Özdemir et al. [106] employed
this method of progressive heating with Pd(OAc)2 to synthesize
other diNHC–palladium complexes. Attempts to simplify the syn-
thesis by heating at 90 or 120 °C alone resulted in either poor yield
or no yield. The method seems to be ligand dependant because
simply stirring 1,10-methylene-bis(N-methylbenzimidazolyl) diio-
dide with Pd(OAc)2 in acetonitrile overnight provides the corre-
spond Pd–diiodide biscarbene [107]. A 1H NMR spectrum of 11
indicates the complex is C1-symmetric, yielding distinct singlet
resonances for the methyl substituent protons at 4.22 ppm and
3.95 ppm. The bridgehead protons appear as singlets at 5.24 ppm
and 5.20 ppm and the bridge protons appear as doublets at
8.77 ppm and 4.49 ppm. A large downfield shift of the aliphatic
Single crystals grow by slowly diffusing CH2Cl2 into a concen-
trated solution of a racemic mixture of 11 in DMSO/hexanes.
Fig. 3 depicts the structure and Table 5 lists refinement data from
a single crystal X-ray diffraction experiment and confirms the
molecular structure of 11. The neutral complex comprises a
slightly distorted square-planar Pd(II) center chelated by the diN-
HC ligand and cis-iodides. Evidence for the DEA ligand providing
a more constrained coordination environment than the related
DEAM counterparts is the much smaller NHC–Pd–NHC angle of
86.75(10)° compared to 97.14(12)° found in the DEAM–Pd–
complex 8exo/8endo. Despite the strain, 11 contains shorter Pd–NHC
distances (Pd1–C1 1.979(3) and Pd1–C9 2.013(3) Å) than in 8exo
8endo, a consequence perhaps of the much weaker trans-influence
of an iodide versus an
3-allyl ligand.
g
3-allyl
/
g
3.2. Catalytic arylboronic acid addition to enones with isolated diNHC–
Pd complexes
Isolated catalysts 8exo/8endo and the reaction of 2-cyclohexen-1-
one with phenylboronic acid were chosen to standardize reaction
conditions. Solvent and base screening experiments indicate that
dioxane/methanol and K2CO3 yield optimal results in terms of yield
and enantioselectivity (Table 1, entry 4). The solvent combination
of THF/H2O provides slightly lower % e.e. (Table 1, entries 5–7).
Employing KOH as the base provided either no enantiomeric
enrichment or in one case 34% (Table 1, entry 3). The conclusion
from this study is that no single set of conditions were found that
significantly enhanced the % e.e.; as a consequence both bases and
solvent systems were employed in subsequent screening studies.
Employing the isolable catalysts in Chart 1 provides two impor-
tant results. Notably, the more constrained DEA ligand in complex
11 does not turnover the reaction. This is surprising since the re-
lated Rh(I) version with cyclooctadiene as counter ligand provides
82% e.e. [44,88]. Employing either 8exo/8endo or 9exo/9endo did not
significantly improve the % e.e.; for example, complex 9exo/9endo
yields 44% e.e. (Table 2, entry 2). Moreover, the DEAM–Pd–acac
complex 10 is not active at 35 °C and must be heated to 80 °C for
Scheme 3. Synthesis of 11.
Table 1
Optimization of base and solvent.
O
O
8
exo/8endo (5 mol%)
PhB(OH)2 (1.5 eq)
+
*
base,
Ph
dioxane/MeOH (4:1)
25oC , 24 h
Entry
Base (mol%)
Solvent combination
% Yielda
% e.e.b
1
2
3
4
5
6
None
dioxane:MeOH(4:1)
dioxane:MeOH(4:1)
dioxane:MeOH(4:1)
dioxane:MeOH(4:1)
THF:H2O(10:1)
THF:H2O(10:1)
THF:H2O(10:1)
THF:H2O(10:1)
dioxane:MeOH(4:1)
0
0
KOH(25)
KOH(50)
K2CO3(50)
KOH(50)
KOH(40)
K2CO3(40)
KOH(40)
K2CO3(50)
73
>98
>98
>98
>98
>98
0
34(R)
0
46(R)
20(R)
29(R)
38(R)
0
7
8c
9c
0
0
Fig. 3. Molecular structure of 11. Thermal ellipsoids are shown at the 50%
probability level. Bond lengths (Å): Pd1–C1 1.979(3), Pd1–C9 2.013(3), Pd1–I1
2.6528(5), Pd1–I2 2.6410(5). Bond angles (°): C1–Pd1–C9 86.75(10), C1–Pd1–I2
86.85(7), C9–Pd1–I1 91.48(7), I1–Pd1–I2 94.813(11).
a
b
c
NMR yield.
Enantiomer in parenthesis, determined by HPLC.
T = 0 °C.

816
A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
Table 4
Table 2
Asymmetric conjugate addition of boronic acids to cyclic enones using in-situ
generated catalyst 12.
Catalyst optimization using pre-formed catalysts..
O
O
O
O
catalyst (5 mol%)
12
(5 mol %)
PhB(OH)2 (1.5 eq)
+
*
Ar B(OH)2
base, solvent
25 oC, 24 h
Ph
n
K2CO3
68oC
THF:H2O(10:1)
n
Ar
Entry
Base
K2CO3(50)
Catalyst
Time (h)
% Yielda
% e.e.b
1
2
(S,S)-9
(S,S)-8
(S,S)-8
(S,S)-8
(S,S)-10
(S,S)-10
18
24
24
24
18
18
>98
>98
95
95
0
11(R)
46(R)
44(R)
40–60(R)
0
0
Entry
n
Ar
% Yielda
% e.e.b
K2CO3(50)
K2CO3(50)
K2CO3(50)
KOH(40)
3c
4d
5e
6f
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
C6H5
2-CH3C6H4
87
36
35
30
24
16
0
0
0
98
62
48
0
c
50(R)
51(R)
35(R)
30(R)
d
–
–
–
51(R)
33(R)
30(R)
–
–
–
2-CH3OC6H4
4-CH3OC6H4
1-naphthyl
2-CF3C6H4
2-FC6H4
KOH(40)
50
a
b
c
d
e
f
NMR yield.
Enantiomer in parenthesis, determined by HPLC.
4-Fluorophenylboronic acid.
2-Naphthylboronic acid, e.e. estimated.
T = 50 °C, THF:H2O(10:1).
4-FC6H4
2,6-diCH3C6H3
C6H5
T = 80 °C, THF:H2O(10:1).
2-CH3C6H4
1-naphthyl
2-CF3C6H4
2-FC6H4
4-FC6H4
2,6-diFC6H3
0
0
0
–
Table 3
Palladium precursor screening with ligands 2, 3, and 6.
a
b
c
NMR yield.
HPLC.
Entry
Ligand
Metal precursor
Solvent
Base
% Yielda
Enantiomers could not be separated by HPLC.
Product could not be separated from reaction mixture.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2
2
2
2
2
2
3
3
3
3
3
3
6
6
6
6
6
6
Pd(BzCN)2Cl2
[Pd(allyl)Cl]2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
THF:H2O
THF:H2O
THF:H2O
dioxane:MeOH
THF:H2O
THF:H2O
THF:H2O
THF:H2O
THF:H2O
dioxane:MeOH
THF:H2O
THF:H2O
THF:H2O
THF:H2O
THF:H2O
dioxane:MeOH
THF:H2O
Cs2CO3
–
–
>98
–
–
–
–
–
–
–
–
–
–
10
–
–
–
–
d
Cs2CO3
Cs2CO3
Cs2CO3
KHMDS
KOtBu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KHMDS
KOtBu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
KHMDS
KOtBu
3.3. Catalytic arylboronic acid addition to enones with in situ
generated diNHC–Pd complexes
Pd(OAc)2
Pd(BzCN)2Cl2
[Pd(allyl)Cl]2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Employing the ligand precursor 2, reactions were screened to
find the ideal conditions to generate a catalyst in situ. The method
involves heating the ligand precursor diimidazolium salt with a Pd
source followed by substrate addition. Palladium acetate (Pd(OAc)2)
proved to be the best metal precursor. Other palladium reagents
including palladium bis(benzonitrile)dichloride and palla-
dium(allyl)chloride dimer do not generate active catalysts (Table 3,
entries 1 and 2). Other than cesium carbonate, bases such as potas-
sium hexamethyldisilazide and potassium tert-butoxide were
screened, but the catalytic results were poor (Table 3, entries 5
and 6) (Scheme 4).
Pd(OAc)2
Pd(BzCN)2Cl2
[Pd(allyl)Cl]2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
THF:H2O
a
NMR yield, allyl =
g
3-BzCN = benzonitrile, OAc = acetate.
Optimal conditions require palladium acetate, ligand precursor
2, and cesium carbonate at 25 °C (Table 3, entry 3). In fact, the yield
is >98% and the % e.e. is essentially identical to the corresponding
product formation to be observed, but the % e.e. = 0 (Table 2, entry
9).
2OTf
Ph
Ph
Ph
Ph
N
N
2.1 eq. base
0.5 eq. Pd precursor
2 h, solvent
N
N
in-situ generated catalyst
diNHC-Pd
12
2
in-situ generated catalyst
O
diPh-DEAM
O
12
PhB(OH)2 (1.5 eq.)
*
H2O or MeOH, 68 o
K2CO3
C
Ph
Scheme 4. Method for generating an active diNHC–Pd–L complex in situ and subsequent catalysis.

A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
817
Table 5
Crystallographic data for 8 and 11.
Complex
8
11
Empirical formula
Formula weight
T (K)
C
66H59N4PdF3O3S
C35.50H35Cl3I2N4OPdS
1032.29
100(2)
1151.63
173(2)
k (Å)
0.71073
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P2(1)/c
11.2878(8)
26.8378(19)
18.4977(13)
90
96.4820(10)
90
5567.9(7)
4
1.374
0.19 x 0.19 x 0.08
0.432
2384
triclinic
P1
ꢀ
12.1289(19)
13.523(2)
14.568(2)
115.711(2)
97.645(3)
109.958(2)
1908.7(5)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (mg mmꢀ3
)
1.796
0.09 x 0.07 x 0.02
2.402
Crystal size (mm3)
Absorption coefficient (mmꢀ1
F(000)
)
1006
h range for data collection (°)
Limiting indices
Number of reflections collected
Number of independent reflections (Rint
Completeness to h = 27.50° (%)
Absorption correction
1.48–27.50
1.64–27.50
ꢀ14 6 h 6 14, ꢀ31 6 k 6 34, ꢀ24 6 l 6 24
ꢀ15 6 h 6 15, ꢀ17 6 k 6 17, ꢀ18 6 l 6 18
37347
23289
)
12743 (0.9224)
99.7
integration
8742(0.0344)
99.8
numerical
Refinement method
Data/restraints/parameters
Full-matrix least-squares on F2
12743/60/650
0.0552, 0.1420
0.0853, 0.1522
0.984
Full-matrix least-squares on F2
8742/0/354
b
R1,a wR2 [I > 2
r(I)]
0.0246, 0.0447 [6698]
0.0356, 0.0461
0.861
0.667 and ꢀ0.699
R1,a wR2 (all data)
b
GOFc on F2
Largest difference in peak and hole (e Åꢀ3
)
1.101 and ꢀ1.002
a
b
c
R1
wR2 = (
GOF = (
=
R
||Fo ꢀ |Fc||/
R|Fo|.
2
R
(w(Fo ꢀ Fc2)2)/
R .
(w(Fo2)2))1/2
R
w(Fo ꢀ Fc2)2/(n ꢀ p))1/2. w = 1/[
r
2(Fo2)+(m * p)2 + n * p], p = [max(Fo2,0) + 2 * Fc2]/3, m and n are constants.
2
isolated catalyst 8exo/8endo. To maximize efficiency, the induction
time to generate the active catalyst was also examined. The reac-
tion mixture was allowed to stir for different durations (0.5, 2, 6,
and 8 h) prior to substrate addition. The ideal time for catalyst gen-
eration is 2 h. Two additional diimidazolium ligand precursors (3
and 6) were screened but the results indicate no active catalysts
form (Table 3, entries 7–18). The only combination that results in
product formation (10% yield) is ligand 6 and [Pd(allyl)Cl]2.
Next, various substrates were screened to elucidate the func-
tional group tolerance of the in situ catalyst (Table 4). From the re-
sults listed in Table 4 it is apparent the substrates can be separated
into two classes: boronic acids containing electron-donating aryl
groups, and boronic acids containing electron-withdrawing aryl
groups. Electronically donating aryl groups on the boronic acid
form product in low yields and modest to low % e.e., while elec-
tron-withdrawing fluoro-substituted arenes provide no yield, ex-
cept in one case 16% yield was obtained for 2-CF3C6H4-B(OH)2.
Introduction of a single electron-withdrawing substituent retards
the reaction. Considering the arylboronic acid acts as a nucleophile
in the mechanism, the poor performance of fluorinated substrates
is understandable. The maximum % e.e. of ꢃ50% was obtained for
Ar-B(OH)2, where Ar = 2-CH3C6H4 (n = 1), 2-CH3OC6H4 (n = 1), and
C6H5 (n = 2).
products. In the case of NHC ligands, due to the common technique
of employing transmetalating reagents, such as Ag+, an in situ ap-
proach is particularly inviting. However, methods are not well
developed for creating catalysts in situ from diimidazolium salt
precursors. A major conclusion from this study is that establishing
a general set of conditions, even for a very specific class of ligand,
would be difficult. Results within Table 3 indicate only one set of
conditions results in an active catalyst, despite the fact ligand pre-
cursors 2 and 3 only differ in their N-substituent group (2 = diph-
enylmethine, and 3 = o-methylbenzyl). Clearly, small changes in
ligand structure dictate whether a catalyst forms in situ and is ac-
tive. Moreover, the conditions are rather specific. Employing ligand
2, a catalysts forms only in the presence of Pd(OAc)2, THF/H2O and
Cs2CO3 in conjunction with a carefully regulated heating regimen.
Other variations do not result in an active catalyst. Thus, it seems
likely the perfect conditions could be overlooked. Despite the fact
a clear trend was not elucidated in this study, an active catalyst
was generated in situ from diimidazolium salt 2 and the inexpen-
sive metal precursor Pd(OAc)2. The different anionic ancillary li-
gands bound to the Pd(II) ion may account for the difference in
activity between the catalysts generated in situ and the isolable
versions. For the in situ catalyst acetate is likely the ancillary li-
gand, whereas for 8, 10, and 11, they are iodide, acetylacetonate,
and allyl, respectively.
Though the enantioselectivities obtained for the boronic acid
addition to enones were low, this is consistent with previous re-
sults employing isolable catalysts, and was not unexpected
[65,88]. The positive outcome from this study is that the discovery
of a method to generate a catalyst in situ will allow more rapid
screening of other enantioselective reaction types to find a class
of reactions that better suites the inherent DEAM and DEA ligand
chiral environment.
4. Conclusions
Straightforward, efficient, and cost effective access to chiral cat-
alysts is important for rapid screening of catalyst activity and
enantioselectivity. Simple addition of a chiral auxiliary (ligand) to
a metal ion precursor to generate an active catalyst in situ
eliminates costly catalyst preparation steps and associated waste

818
A.B. Mullick et al. / Polyhedron 52 (2013) 810–819
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