Chiral bidentate N-heterocyclic carbene complexes of Rh and Pd

Article abstract of DOI:10.1139/v04-165

The synthesis of a new chiral, bidentate oxazoline/imidazolidene carbene precursor is described. This species is reacted with various metal salts in the presence of a base to generate rhodium and palladium complexes, which are characterized spectroscopica

Full text of DOI:10.1139/v04-165

1781
Chiral bidentate N-heterocyclic carbene
complexes of Rh and Pd
Li Ren, Austin C. Chen, Andreas Decken, and Cathleen M. Crudden
Abstract: The synthesis of a new chiral, bidentate oxazoline/imidazolidene carbene precursor is described. This species
is reacted with various metal salts in the presence of a base to generate rhodium and palladium complexes, which are
characterized spectroscopically and crystallographically.
Key words: chiral N-heterocyclic carbene, rhodium, palladium, oxazolidine, asymmetric catalysis.
Résumé : On décrit la synthèse d’un nouveau précurseur chiral et bidentate du carbène oxazoline/imidazolidène. On a
fait réagir cette espèce avec divers sels métalliques, en présence de base, pour générer des complexes du rhodium et du
palladium qu’on a caractérisés par des méthodes spectroscopiques et cristallographiques.
Mots clés : carbène chiral N-hétérocyclique, rhodium, palladium, oxazolidine, catalyse.
[Traduit par la Rédaction] Ren et al. 1787
Introduction
([RhCl(CO)(PPh3)2]) under identical conditions. Further-
more, the carbene complex was more selective for the
branched isomer than the bis-phosphine complex (96:4 vs.
88:12). With the regioselectivity and reactivity of these com-
plexes established for the important hydroformylation reac-
tion, we began the more challenging task of preparing chiral
versions of these carbene complexes.
Several examples of chiral N-heterocyclic carbenes and their
corresponding metal complexes have been reported in the lit-
erature (7–13) and an increasing number are being tested in
catalytic reactions (14–20). In 1997, Herrmann reported the
hydrosilylation of acetophenone with chiral monodentate
carbene complex 3, which proceeded in 32% enantiomeric
excess (ee) (eq. [2]) (14b). The hydrosilylation of cyclo-
hexylmethyl ketone was also examined with chiral
The use of N-heterocyclic carbenes as ligands for transi-
tion metals has experienced a renaissance of late (1). These
ligands display intriguing properties relative to the more
commonly employed phosphine ligands. Most importantly,
the binding constants of carbene ligands with catalytically
active transition metals such as Ru are approximately double
that of the related phosphine ligands (2, 3). Carbene ligands
have proven to be extremely useful in important reactions
such as Ru–catalyzed metathesis reactions (4) and Pd–cata-
lyzed coupling reactions (5).
We have reported that mixed rhodium complexes 1 and 2,
which contain both phosphine and N-heterocyclic carbene
ligands, are effective catalysts for the hydroformylation of
vinyl arenes, giving the branched isomer with up to 98:2 se-
lectivity (eq. [1]) (6). The activity of the carbene/phosphine
complex (2) was twice that of the related bis-phosphine
Received 4 February 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 9 February 2005.
L. Ren, A.C. Chen, A. Decken,1 and C.M. Crudden.2,3 Department of Chemistry, University of New Brunswick, P.O. Box 45222,
Fredericton, NB E3B 6E2, Canada.
1Author to whom questions concerning crystallography should be addressed.
2Current address: Department of Chemistry, Queen’s University, Kingston ON K7L 3N6, Canada.
3Corresponding author (e-mail: Cruddenc@chem.queensu.ca).
Can. J. Chem. 82: 1781–1787 (2004)
doi: 10.1139/V04-165
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Can. J. Chem. Vol. 82, 2004
Chart 1. Successful chiral N-heterocyclic carbene complexes and carbene precursors.
triazolium complexes and proceeded with up to 44% ee
(14c, d).
Scheme 1. Synthesis of Rh complex 16.
Grubbs (15), Hoveyda (16), Hartwig (17), and Burgess
(18) have reported carbene ligands that give high enantio-
selectivity for metathesis, arylation of ketones, and hydroge-
nation reactions, respectively (Chart 1). Recent reports from
the groups of Shi (19), Douthwaite (14h), and Andrus (20)
have described more complex carbenes that provide >90%
ee in hydrosilylation, allylic alkylation, and enone arylation,
respectively. Although high selectivities have been obtained
using these catalysts for specific reactions, substrate general-
ity still needs improvement.
Inspired by the successes of Pfaltz (21) and Brunner (22)
with chiral phosphine/oxazoline ligands, we have prepared
carbene ligands that have a chiral oxazoline as the second
point of ligation. Related oxazoline/carbene complexes have
been prepared by Herrmann (14), and complex 7 was intro-
duced by Burgess during this study (18). With the aim of in-
creasing the transfer of chiral information by increasing the
rigidity between the carbene and the oxazoline, we em-
barked on the synthesis of carbene precursor, 14.
181 ppm, corresponding to the ligated carbene carbon, with
a distinctive rhodium-carbon coupling constant of 50 Hz.
X-ray quality crystals4 of 16 were obtained by recry-
stallizing the air-stable, yellow rhodium complex from di-
chloromethane/hexane. The crystal structure (see Fig. 1)
revealed that the rhodium adopts a distorted square planar
geometry with combined angles of 359.2°. The bite angle of
the carbene-imidazoline ligand in complex 16 is 84.2°. As
Results and Discussion
Beginning with commercially available 2-fluorobenzonitrile
(11), nucleophilic aromatic displacement of the fluoride with
imidazole proceeded quantitatively yielding 12 (Scheme 1).
Subsequent condensation with optically pure valinol (23)
followed by methylation with methyl iodide furnished
carbene precursor 14.
Although attempts to prepare and isolate the free carbene
were unsuccessful, rhodium carbene complex 16 could be
obtained in two steps from [Rh(η4-1,5-COD)(µ-Cl)]2 and 14.
Thus, reaction of crude 14 and the rhodium complex in the
presence of potassium t-butoxide followed by chlorine ab-
straction with silver hexafluorophosphate generated 16. The
13C NMR spectrum of this material contained a signal at
expected, the carbene–rhodium bond ((Rh—C(2)
=
2.0150(16) Å) is much shorter than the oxazoline nitrogen–
rhodium bond ((Rh—N(15) = 2.0954(14) Å). A stronger
trans influence of the carbene ligand could be inferred from
the longer distances separating the rhodium and the olefinic
carbons trans to the carbene ((Rh—C(22) = 2.2085(16) Å,
Rh—C(23) = 2.2097(17) Å) relative to those trans to the
4 Crystal data for 16: C24H31F6N3OPRh, crystal size 0.125 mm3 × 0.150 mm3 × 0.275 mm3, orthorhombic, P212121, a = 11.6588(4) Å, b =
13.3143(5) Å, c = 16.6038(6) Å, α = β = γ = 90°, V = 2577.39(16) Å3, Z = 4, 2θ (max) = 32.50°, ω and φ scans, 28 697 reflections scanned,
8910 unique reflections (Rint = 0.0179), T = 173(2) K, full-matrix least-squares on F2, wR2 = 0.0547, R1 = 0.0231. Full details of the crystal-
lographic analysis of 13 are described in the Supporting information.6
© 2004 NRC Canada

Ren et al.
1783
Fig. 1. View of the cation of 16. Thermal ellipsoids are at the
30% probability level and hydrogen atoms have been removed for
clarity.
Scheme 2. Synthesis of Pd complex 17.
Fig. 2. View of the cation of 17. Thermal ellipsoids are at the
30% probability level and hydrogen atoms have been removed for
clarity.
oxazoline nitrogen ((Rh—C(26) = 2.1206(17) Å, Rh—
C(27) = 2.1509(18) Å). However, the carbon–carbon double
bond trans to the oxazoline ((C(26)—C(27) = 1.390(3) Å) is
slightly longer than the C=C bond trans to the carbene
((C(22)—C(23) = 1.374(3)Å).
complex 16 was evaluated in hydroboration and hydrosilyl-
ation reactions. Despite the chelating, monomeric form of
the catalysts, the enantioselectivity of the described reac-
tions never exceeded 10% ee. It is especially noteworthy
that the bidentate ligand proved to be inferior to the
monodentate ligand 3 in the hydrosilylation reaction under
very similar conditions. One key difference between com-
plex 16 and Herrmann’s catalyst 3 is the C2-symmetry of the
latter species. This cannot be the sole reason for the poor
performance of this species, as Pfaltz and co-workers (21)
and Brunner (22) have shown that C2-symmetry is not a nec-
essary parameter for asymmetric induction. Regardless, it is
clear that Herrmann’s catalyst creates a chiral environment
more effectively than the carbene derived from 14.
Our synthetic method requires that the second group intro-
duced onto the imidazole ring be a reactive electrophile. In
this case methyl iodide was employed, with the consequence
that the coordination site cis to the carbene is relatively un-
hindered. This may be the reason for the poor asymmetric
induction observed in the reactions of complex 16.
The corresponding palladium complex 17 could be pre-
pared from either palladium acetate or [Pd(η4-1,5-COD)Cl2]
(Scheme 2). Specifically, treatment of imidazolium salt 14
with [Pd(η4–1,5–COD)Cl2] in the presence of potassium
tert-butoxide at room temperature, or simply heating 14 in
DMSO with palladium acetate gave the same palladium-
carbene iodide complex 17 (24). This structure was unam-
biguously demonstrated by X-ray crystallography5 (see
Fig. 2). The palladium, with the sum of the angles equal to
359.98°, is in a nearly perfect square-planar environment.
Like its rhodium analogue, the palladium–iodide bond trans
to the carbene (Pd—I(2) = 2.6571(3) Å) is longer than that
trans to the oxazoline nitrogen (Pd—I(1) = 2.5657(3) Å). It
is also noteworthy that within each asymmetric unit are two
independent molecules with almost identical metric parame-
ters. The main difference between them lies in the disposi-
tion of the two methyl residues of the isopropyl group
(θ (C(21)—C(19)—C(20) = 112.0(4)° and C(41)—C(39)—
C(40) = 115.3(4)°).
The catalytic activity of the new chiral rhodium-carbene
5 Crystal data for 17: C32H38I4N3OPd, crystal size 0.175 mm3 × 0.250 mm3 × 0.325 mm3, monoclinic, P2(1), a = 8.8335(4) Å, b = 14.3771(7)
Å, c = 15.7653(7) Å, α = γ = 90°, β = 99.630(1)°, V = 1973.98(16) Å3, Z = 2, 2θ (max) = 30.00°, ω and φ scans, 19 249 reflections scanned,
10 005 unique reflections (Rint = 0.0168), T = 296(1) K, full-matrix least-squares on F2, wR2 = 0.0450, R1 = 0.0206. Full details of the crys-
tallographic analysis of 14 are described in the Supporting information.6
© 2004 NRC Canada

1784
Can. J. Chem. Vol. 82, 2004
(60 mmol) in 30 mL of DMSO. The reaction mixture was
stirred at 120 °C under argon for 36 h. The reaction mixture,
after cooling to room temperature, was poured into 50 mL of
water to precipitate 12 as a white powder. The resulting sus-
pension was filtered, washed, and dried in vacuo to give
4.894 g (97% yield) of the desired product. 1H NMR
(400 MHz, CDCl3) δ: 7.88 (s, 1H), 7.85 (dd, 1H, J = 1.6,
7.6 Hz), 7.77 (dt, 1H, J = 1.6, 7.6 Hz), 7.56 (dt, 1H, J = 1.2,
7.6 Hz), 7.49 (dd, 1H, J = 1.2, 8.4 Hz), 7.38 (t, 1H, J =
1.2 Hz), and 7.30 (t, 1H, J = 1.2 Hz). 13C NMR (100 MHz,
CDCl3) δ: 139.4, 136.7, 134.5, 134.3, 130.8, 128.4, 125.6,
119.6, 115.8, and 108.1
Conclusions
In conclusion, a chiral, bidentate oxazoline/carbene pre-
cursor can be prepared in three simple steps from commer-
cially available starting materials. Herrmann’s in situ
deprotonation method obviates the need for the synthesis of
the free carbene. The Rh catalyst 16 was examined in the
hydrosilylation of acetophenone and the hydroboration of
styrene. Although the catalyst was active, it gave less than
10% ee under all experimental conditions examined.
Experimental Section
General methods and materials
2-[(4S)-4-Isopropyloxazolyl]phenyl imidazole (13)
All solvents were dried according to standard procedures
immediately prior to use and, where necessary, were purged
using repeated freeze–pump–thaw cycles. Acetophenone was
distilled under argon from phosphorous pentoxide. Styrene
was fractionally distilled under a partial vacuum and perco-
lated through a column of dry, neutral alumina immediately
prior to use. Catecholborane was distilled from a partial
pressure of argon to remove B2(catechol)3 and stored under
argon in a Schlenk flask at –25 °C. All other reagents were
used as received.
Zinc chloride (0.331 g, 2.43 mmol) was fused in a
250 mL flame-dried, round-bottomed flask. Then 4.083 g of
(2-cyano-phenyl)imidazole (12, 24.3 mmol), 3.756 g of L-
valinol (36.4 mmol), and 80 mL of chlorobenzene were
added sequentially. The reaction mixture was left stirring at
140 °C under argon for 60 h. Following removal of the
volatiles in vacuo, the pinkish residue was taken up in
100 mL of dichloromethane and washed with water
(30 mL × 3). The aqueous wash was back-extracted with an-
other 100 mL of dichloromethane. The organic layers were
combined and dried over magnesium sulfate. Filtration and
concentration in vacuo afforded a viscous oil that was identi-
fied as a mixture of unreacted starting material and desired
product. Due to its very low solubility in ether, the starting
material was removed by trituration with diethyl ether. Fil-
tration, followed by concentration of the filtrate in vacuo af-
Instrumentation
Proton and 13C NMR spectra were recorded on a Varian
400 MHz instrument using the solvent resonance as the in-
ternal standard. Gas chromatography (He carrier, 12.5 psi of
head pressure (1 psi = 6.894 757 kPa), 1.14 mL/min flow)
was performed with flame ionization (FI) detection using a
split/splitless injector (split ratio 50) and hexane solutions.
Retention times are given in min. All analyses were carried
out using a 2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl-β-
cyclodextrin column (30 m, 0.25 mm diameter, 0.25 µm
thickness).
1
forded 3.276 g of the desired product (53% yield). H NMR
(400 MHz, CDCl3) δ: 7.87 (dd, 1H, J = 1.2, 7.6 Hz), 7.62 (d,
1H, J = 0.8 Hz), 7.56 (dt, 1H, J = 2.0, 7.6 Hz), 7.48 (dt, 1H,
J = 1.2, 7.6 Hz), 7.34 (dd, 1H, J = 1.2, 8.0 Hz), 7.14 (d, 1H,
J = 1.2 Hz), 7.10 (t, 1H, J = 1.2 Hz), 4.22 (dd, 1H, J = 7.6,
8.8 Hz), 4.01–3.89 (m, 2H), 1.80–1.70 (m, 1H), 0.94 (d, 3H,
J = 6.8 Hz), and 0.88 (d, 3H, J = 6.8 Hz). 13C NMR
(100 MHz, CDCl3) δ: 161.8, 137.6, 131.5, 131.1, 129.2,
129.1, 128.5, 126.8, 120.6, 120.5, 72.7, 70.7, 32.7, 18.7, and
18.3.
Crystallography
Single crystals of 16 were coated with Paratone-N oil,
mounted using a glass fibre and frozen in the cold nitrogen
stream of the goniometer. Single crystals of 17 were coated
with Paratone-N oil and mounted on a glass fibre with epoxy
glue. A hemisphere of data was collected on a Bruker AXS
P4/SMART 1000 diffractometer using ω and θ scans with a
scan width of 0.3° and 30 s (for 16) or 10 s (for 17) expo-
sure times. The detector distance was 4 cm. The data were
reduced (SAINT) (25) and corrected for absorption
(SADABS) (26). The structure was solved by direct methods
and refined by full-matrix least squares on F2(SHELXTL)
(27). All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included in calculated positions and
refined using a riding model for 17. For 16, hydrogen atoms
were found in Fourier difference maps and refined isotro-
pically. Thermal ellipsoid plots are at the 30% probability
level. In some plots, hydrogen atoms have been omitted for
clarity.
1-(2-[(4S)-4-Isopropyloxazolyl]phenyl)-3-methyl
imidazolium iodide (14)
255 mg of 2-[(4S)-4-isopropyloxazolyl]phenyl imidazole
(13, 1.0 mmol) was dissolved in 20 mL of chloroform in a
50 mL round-bottomed flask. Excess methyl iodide (852 mg,
6.0 mmol) was then added dropwise to this mixture. After
stirring for 16 h in the dark at room temperature, the
volatiles were removed in vacuo to afford 395 mg of the de-
sired product, which could not be purified by column chro-
matography or recrystallization. The crude material thus
obtained was spectroscopically pure and was used in the
1
next step without further purification. H NMR (400 MHz,
CDCl3) δ: 9.69 (s, 1H), 8.09–8.06 (m, 1H), 7.86–7.83 (m,
1H), 7.73–7.66 (m, 2H), 7.52–7.48 (brs, 1H), 7.25 (s, 1H),
4.37 (dd, 1H, J = 8.0, 9.2 Hz), 4.27 (s, 3H), 4.02–3.89 (m,
2H), 1.68–1.60 (m, 1H), 0.86 (d, 3H, J = 2.8 Hz), and 0.84
(d, 3H, J = 2.8 Hz). 13C NMR (100 MHz, CDCl3) δ: 158.9,
138.3, 133.0, 132.5, 131.4, 131.1, 128.3, 124.2, 123.6,
123.0, 73.4, 70.3, 37.6, 32.9, and 18.7 (2).
(2-Cyanophenyl)imidazole (12)
A 100 mL flame-dried, round-bottomed flask was charged
with 3.634 g of 2-fluorobenzonitrile (11, 30 mmol), 3.064 g
of imidazole (45 mmol), and 8.293 g of potassium carbonate
© 2004 NRC Canada

Ren et al.
1785
(␩4-1,5-Cyclooctadiene)(1-(2-[(4S)-4-
isopropyloxazolyl]phenyl)-3-methylimidaz-olin-2-
ylidene)rhodium-(I)-hexafluorophosphate (16)
of 1-(2-[(4S)-4-isopropyloxazolyl]phenyl)-3-methyl imidazolium
iodide (11, 1.12 mmol) and 0.251 g of palladium acetate
(1.12 mmol) was dissolved in 10 mL of DMSO. The reac-
tion mixture was stirred under argon at 50 °C for 2 h and
then at 110 °C for another 48 h. The solvent was then re-
moved and the residue purified by column chromatography,
eluting with pure dichloromethane. The desired product was
then crystallized from dichloromethane/hexane yielding dark
orange crystals (102 mg, 16%). 1H NMR (400 MHz, CDCl3)
δ: 7.88 (dd, 1H, J = 1.6, 7.6 Hz), 7.80 (dt, 1H, J = 1.6,
8.0 Hz), 7.66 (dt, 1H, J = 1.2, 8.0 Hz), 7.55 (dd, 1H, J = 1.2,
8.0 Hz), 7.23 (d, 1H, J = 2.0 Hz), 7.19 (d, 1H, J = 2.0 Hz),
5.24 (ddd, 1H, J = 3.2, 3.6, 3.6 Hz, 1H), 4.64 (dd, 1H, J =
9.2, 10.4 Hz), 4.34 (dd, 1H, J = 6.8, 9.2 Hz), 4.16 (s, 3H),
2.40 (m, 1H), 0.88 (d, 3H, J = 6.8 Hz), and 0.60 (d, 3H, J =
6.8 Hz). 13C NMR (100 MHz, CDCl3) δ: 162.4, 136.9,
134.1, 131.0, 129.5, 126.7, 124.3, 123.8, 122.2, 72.6, 70.4,
40.7, 30.4, 17.9, and 14.8.
In a 100 mL flame-dried, round-bottomed flask, 0.572 g
of [Rh(COD)Cl]2 (1.16 mmol) was dissolved in 15 mL of
THF. Under vigorous stirring and an argon purge, 325 mg of
potassium tert-butoxide (2.90 mmol, 1.25 equiv.) was then
added. The resulting suspension was stirred at room temper-
ature for 10 min and then transferred via cannula into an-
other 100 mL flame-dried, round-bottomed flask containing
920 mg of 1-(2-[(4S)-4-isopropyloxazolyl]phenyl)-3-methyl
imidazolium iodide (14, 2.32 mmol) dissolved in 5.0 mL of
THF. After stirring under argon at room temperature for
40 h, the solvent was removed and the product was purified
by column chromatography, eluting with solvent systems of
increasing polarity (pure dichloromethane to 10% methanol
in dichloromethane). The neutral rhodium(I) complex 12
was isolated as an orange foam in 69% yield (821.2 mg).
This was then redissolved in 20 mL of THF and 402.7 mg of
silver hexafluorophosphate (1.59 mmol) was added. A white
precipitate formed immediately. After stirring for 1 h, the
suspension was filtered through a sintered glass funnel and
the filtrate concentrated in vacuo. The resulting residue was
then purified by column chromatography, eluting with sol-
vent systems of increasing polarity (pure dichloromethane to
2% methanol in dichloromethane). Crystallization from di-
chloromethane/hexane furnished the desired product as yel-
low crystals (500 mg, 35% yield). 1H NMR (400 MHz,
CDCl3) δ: 7.89 (dd, 1H, J = 1.6, 8.0 Hz), 7.79 (dt, 1H, J =
1.6, 7.6 Hz), 7.65 (dt, 1H, J = 1.2, 8.0 Hz), 7.48 (dd, 1H, J =
1.2, 8.0 Hz), 7.28–7.24 (m, 2H), 4.88–4.80 (m, 1H), 4.73
(dd, 1H J = 9.2, 10.4 Hz), 4.49–4.41 (m, 1H), 4.27 (dd, 1H,
J = 7.2, 9.2Hz), 4.10–4.03 (m, 4H), 3.84–3.77 (m, 1H),
3.71–3.64 (m, 1H), 2.52–2.34 (m, 2H), 2.13–1.63 (m, 6H),
0.88 (d, 3H, J = 6.8 Hz), and 0.79 (d, 3H, J = 6.8 Hz). 13C
NMR (100 MHz, CDCl3) δ: 181.1 (d, JRh-C = 50.2 Hz),
162.9, 137.7, 133.6, 130.6, 129.1, 126.3, 124.6, 123.73,
123.69, 98.8 (d, JRh-C = 7.2 Hz), 95.7 (d, JRh-C = 7.2 Hz),
Hydroboration of styrene
In a 10 mL flame-dried, round-bottomed flask, 12.5 mg of
rhodium complex 9 (0.01 mmol) was suspended in 2 mL of
dried, deoxygenated THF. Freshly purified styrene (0.11 mL,
0.96 mmol) was then added to the reaction vessel and the
mixture was stirred at room temperature for 10 min. Upon
cooling to the desired temperature (0 °C or –30 °C) catech-
olborane (0.16 mL, 1.5 mmol) was added dropwise as a so-
lution in 1 mL of THF over 20 min. Care was taken to
minimize the variation in internal reaction temperature dur-
ing this addition. After 16 h, 2 mL of methanol was added.
The temperature was brought to 0 °C and the boronate ester
was treated, under an atmosphere of nitrogen, with 1 mL of
a 2 mol/L aqueous sodium hydroxide solution and 0.2 mL of
hydrogen peroxide (30% (w/v)). The resulting black reaction
mixture was then diluted with ether and the organic layer
was separated. The aqueous layer was extracted further with
ether (5 mL × 3). The organic extracts were combined,
washed with brine (5 mL), and dried over magnesium sul-
fate. Filtration through a sintered glass funnel, followed by
concentration of the resulting mixture in vacuo yielded the
crude alcohols. The linear-to-branched ratio was elucidated
79.3 (d, JRh-C = 13.6 Hz), 72.1, 71.4, 71.2 (d, JRh–C
5.6 Hz), 37.7, 32.6, 31.9, 30.8, 29.6, 27.9, 17.2, and 17.1.
=
1
from H NMR of the crude product mixture with a 30 s de-
(1-(2-[(4S)-4-Isopropyloxazolyl]phenyl)-3-
lay inserted between each acquisition. Spectroscopically
pure 1-phenylethanol was obtained by flash chromatography
with 4:1 hexane/ether as eluent and its enantiomeric purity
was determined by chiral GC (temperature protocol, 80 °C,
3 min, then increase 1 °C per min to 130 °C; retention times
(R) = 33.5 min, (S) = 35.4 min). The following results were
obtained at 0 °C: 58% combined yield, branched:linear =
55:45, ee = 7%.
methylimidazolin-2-ylidene) palladium-(II)-diiodide (17)
In a 100 mL round-bottomed flask, 348.3 mg of
Pd(COD)Cl2 (1.22 mmol) was dissolved in 15 mL of THF.
To this was added 161.6 mg of potassium tert-butoxide
(1.44 mmol). An immediate colour change from yellow to
black was observed. After stirring at room temperature for
10 min, the reaction mixture was transferred via cannula into
another 100 mL flame-dried, round-bottomed flask contain-
ing 474 mg of 1-(2-[(4S)-4-isopropyloxazolyl]-phenyl)-3-
methyl imidazolium iodide (14, 1.2 mmol) in 5.0 mL of
THF. The resulting mixture was then stirred for another
24 h. The solvent was then removed and the residue purified
by column chromatography, eluting with solvent systems of
increasing polarity (6:1 hexane/dichlormethane to pure di-
chloromethane). The desired product was then crystallized
from dichloromethane/hexane yielding dark orange crystals
(100 mg, 19% yield).
Hydrosilylation of acetophenone
In a 25 mL flame-dried, round-bottomed flask was dis-
solved 27 mg of rhodium complex 9 (0.04 mmol) in 2 mL of
deoxygenated, dried THF. Then 0.5 mL of freshly purified
acetophenone (4.3 mmol) was added to the reaction mixture
and the resulting pale yellow solution was cooled to –25 °C.
Next 0.8 mL of diphenylsilane (4.3 mmol) was added neat,
dropwise, over a period of 15 min and the reaction mixture
was stirred under argon at –25 °C for 41 h. Hydrolysis of the
silyl ether was accomplished through the addition of 1 mL
Alternatively, the same complex can be prepared as fol-
lows: In a 100 mL flame-dried, round-bottomed flask, 0.444 g
© 2004 NRC Canada

1786
Can. J. Chem. Vol. 82, 2004
C.W. Lehmann, R. Mynott, F. Stelzer, and O.R. Thiel. Chem.
Eur. J. 7, 3236 (2001).
of methanol and a few crystals of p-toluenesulfonic acid.
Complete hydrolysis was achieved after 10 h at room
temperature. Spectroscopically pure 1-phenylethanol was
obtained by flash chromatography with 4:1 hexane/ether as
eluent (141 mg, 27% yield, 34% conversion by NMR).
Enantiomeric excess (2%) was determined by chiral GC
(temperature protocol, 80 °C, 3 min, then increase 1 °C
per min to 130 °C; R = 33.5 min, S = 35.4 min).
5. For applications of the Suzuki–Miyaura reaction see: (a) G.
Altenhoff, R. Goddard, C.W. Lehmann, and F. Glorius. Angew.
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Supplementary material
Supporting information available lists details of the crystal
structure determination for compounds 13 and 14.6
Acknowledgements
The Natural Sciences and Engineering Research Council
of Canada (NSERC) is gratefully acknowledged for support
of this research in terms of equipment and operating grants
to CMC, and a postgraduate scholarship to ACC. The Can-
ada Foundation for Innovation is also thanked for funding
this research. The University of New Brunswick and BioChem
Pharma are thanked for scholarships to LR and ACC.
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6 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A OS2, Canada (http://www/nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). CCDC
253890 and 253891 contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 lEZ,
U.K.; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
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