New iridium and rhodium chiral di-N-heterocyclic carbene (NHC) complexes and their application in enantioselective catalysis

Article abstract of DOI:10.1039/b819524b

New iridium and rhodium complexes prepared from C₂-symmetric trans-9,10-dihydro-9,10-ethanoanthracene-11,12-bis(1-R)-benzimidazolidine-2- ylidene ligands (R = Me, ⁱPr, and diPh) have been synthesized and characterized. Their catalytic activities have been tested in enantioselective hydrogenation and hydroformylation reactions. The ee's for the reactions are low. Evidence indicates that even chelating di-N-heterocyclic carbene ligands are susceptible to reductive elimination.

Full text of DOI:10.1039/b819524b

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PAPER
www.rsc.org/dalton | Dalton Transactions
New iridium and rhodium chiral di-N-heterocyclic carbene (NHC) complexes
and their application in enantioselective catalysis†
Matthew S. Jeletic, Muhammad T. Jan, Ion Ghiviriga, Khalil A. Abboud and Adam S. Veige*
Received 3rd November 2008, Accepted 30th January 2009
First published as an Advance Article on the web 23rd February 2009
DOI: 10.1039/b819524b
New iridium and rhodium complexes prepared from C2-symmetric trans-9,10-dihydro-9,10-
ethanoanthracene-11,12-bis(1-R)-benzimidazolidine-2-ylidene ligands (R = Me, iPr, and diPh) have
been synthesized and characterized. Their catalytic activities have been tested in enantioselective
hydrogenation and hydroformylation reactions. The ee’s for the reactions are low. Evidence indicates
that even chelating di-N-heterocyclic carbene ligands are susceptible to reductive elimination.
observed major and minor products were the acylimidazolium
and methylimidazolium salts, respectively.29
Introduction
Over the next decade mounting experimental28–40 and
theoretical41–45 evidence implicates reductive elimination as a
major catalyst decomposition pathway for NHC complexes. Van
Rensburg et al. observed C–H reductive elimination of the imida-
zolium salt [IMes-H]+ from [(IMes)Co(CO)3]2 under hydroformy-
lation conditions.37,38 This decomposition pathway is not exclusive
to the late transition metals, considering Bullock et al. demon-
strated IMes reductively eliminates from [Cp(IMes)W(CO)2]-
[B(C6F5)4] as [IMes-H][B(C6F5)4] during ketone hydrogenation.40
Cavell, Yates et al. computationally explored some factors that
prevent NHC reductive elimination.31,41–45 The activation barrier
to reductive elimination increases by 5.6 kcal mol-1 when the
N-alkyl substituent changes from Cl < H < Ph < Me < Cy
< Pr < Np < Bu. Electron-donating N-substituents make the
Pd–C(NHC) bond more resilient toward reductive elimination,
and vice versa for electron-withdrawing groups.43 Changes in the
carbene twist angle show only small changes in the reductive
elimination activation energies due to a cancelling effect. As the
carbene twist angle increases from 15◦ to 90◦ the relative barriers
to reductive elimination decrease, but the ground state energies
also decrease due to steric relief.44 In a model system, calcu-
lations indicate the angle between two PMe3 spectator ligands,
opposite the NHC, is inversely proportional to the reductive
elimination activation energy. From these studies, Cavell, Yates
et al. note that using chelating NHC ligands will impede reductive
elimination.
One type of chelating ligand is the so-called mixed-NHC that
chelates through the NHC and an additional donor moiety.
These ligands are effective for hydrogenation, including highly
desirable enantioselective versions.2,46–56 In the same manner, di-
NHC ligands are expected to be resilient to reductive elimination
but there are no reports for hydrogenation of alkenes.
We report the synthesis of C1-symmetric chiral chelating di-
NHC iridium and rhodium complexes. The new iridium complexes
complement our previous synthesis57 of rhodium derivatives and
together enable a meaningful comparison of (NHC)M stability.
Fig. 1 displays the ligand architecture featured in this study.
This report will show that even chelating di-NHC ligands are
susceptible to reductive elimination under hydrogenation and
hydroformylation conditions.
N-Heterocyclic carbenes (NHCs) function as ligands in a broad
scope of metal-catalyzed organic transformations.1–10 Though
versatile ligands, much of the excitement is due to their unpar-
alleled success in Grubbs’ second-generation olefin metathesis
catalyst.2,11–13 Grubbs’ swap of a trialkylphosphine ligand for a
bulky NHC has inspired numerous additional advances in catal-
ysis. In particular, (NHC)Pd catalysts exhibit high activity and
selectivity in cross-coupling reactions.5 However, the PR3/NHC
switch is not yet a general recipe for enhanced reactivity. For
example, alkene hydrogenation with monodentate NHC com-
plexes yields modest results compared to analogous phosphorous-
based catalysts.14–27 The Achilles heel in (NHC)M-catalyzed hy-
drogenation is the tendency for NHC reductive elimination (RE)
to the imidazolium salt [NHC-H]+ (Scheme 1). Not surprisingly,
there is to date only a single example of enantioselective alkene
hydrogenation using chiral monodentate NHC complexes.19
i
t
Scheme 1 General reductive elimination pathway (RE).
The decomposition pathway is not limited to C–H bond
elimination. In 1998, Cavell et al. demonstrated [(1,3-Me-NHC)-
Pd(Me)cod]+ (where 1,3-Me-NHC = 1,3-dimethyl-imidazol-2-
ylidene) decomposes into the 1,2,3-trimethylimidazolium salt (1,3-
MeNHC-Me+), Pd0 and free 1,5-cyclooctadiene (cod).28 Two
years later, McGuinness and Cavell treated [(TMIY)Pd(Me)Cl]2
(TMIY = 1,3,4,5-tetramethyl-imidazol-2-ylidene) with AgBF4
and CO at -50 ◦C, then warmed the mixture to -20 ◦C. The
Center for Catalysis, Department of Chemistry, University of Florida, P.O.
Box 117200, Gainesville, Florida, 32611, USA. E-mail: veige@chem.ufl.edu;
Fax: +1 352-392-9844; Tel: +1 352-392-9844
† Electronic supplementary information (ESI) available: Multi-nuclear
NMR and 2D NMR spectra, IR spectra, and X-ray crystallographic data.
CCDC reference numbers 707708 and 707709. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b819524b
2764 | Dalton Trans., 2009, 2764–2776
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molecule of DME is present after standard work up procedures.
If desired, the DME can be removed by heating under vacuum for
five days, though its removal prior to metalation is not necessary.
The iridium complexes 3-diPh and 3-iPr were prepared from the
corresponding 2-R and (acac)Ir(cod) in THF. Though the reaction
is performed under an inert atmosphere, the complexes are isolated
on the bench top as air- and moisture-stable orange powders
1
in 89% (3-diPh) and 99% (3-iPr) yield. The H NMR spectrum
of 3-diPh reveals an intricate pattern of resonances, attributable
to a C1-symmetric complex. The spectrum displays two distinct
bridgehead proton resonances at 4.41 and 4.99 ppm, due to the
lowered symmetry. One of the diphenylmethine protons is located
at 8.38 ppm but the second proton is indistinguishable from
Fig. 1 Chiral di-NHC ligand precursor based on the trans-9,10-dihydro-
9,10-ethanoanthracene backbone.
1
nearby aromatic protons. The 13C{ H} NMR spectrum exhibits
Results and discussion
two prominent resonances at 189.8 and 186.9 ppm, corresponding
to the carbene carbons. The iridium-bound cod carbons resonate
at 72.3, 75.1, 81.3 and 85.8 ppm.
Synthesis and characterization of [(DEAM-diPhBY)Ir(cod)](OTf)
(3-diPh) and [(DEAM-IBY)Ir(cod)](OTf) (3-iPr)
Spectroscopic 1H NMR data for 3-iPr show a large number of
resonances, indicating the complex is again C1-symmetric. Some
of the more recognizable signals include the isopropyl methines as
septets at 5.05 and 6.23 ppm. The four corresponding isopropyl
methyls resonate as doublets at 0.91, 1.66, 1.80 and 2.02 ppm. The
In accordance with Cavell’s study we sought a large N-alkyl substi-
tuted NHC. 1-Diphenylmethanebenzimidazole (1-diPh) is synthe-
sized in 63% yield by treating benzimidazole with chlorodiphenyl-
methane in xylenes, KOH and a catalytic amount of nBu4NBr.58,59
The synthesis of 1-isopropylbenzimidazole (1-iPr) is reported
elsewhere.60 Though 1-diPh is available commercially, it is ex-
orbitantly expensive. Our spectroscopic studies and combustion
analysis confirm the identity of 1-diPh. The diagnostic diphenyl-
methine resonance on 1-diPh shifts downfield to 6.97 ppm from
1
13C{ H} NMR spectrum displays signals at 188.1 and 183.8 ppm
attributable to the carbene carbons; the four cod olefinic carbons
resonate at 69.2, 75.3, 79.1 and 85.6 ppm.
A single-crystal X-ray diffraction experiment conducted on 3-
iPr agrees with the spectroscopically determined C1 symmetry
(Fig. 2). Table 1 lists selected bond lengths, bond angles and torsion
angles.62 The asymmetric unit consists of two crystallographically
independent molecules in the Pn space group. The DEAM and
cod ligands coordinate cis to the iridium center to generate a
square-planar geometry (e.g. ∠C40–Ir1–C44 = 89.8(6)◦). The
1
6.12 ppm for chlorodiphenylmethane, and the 13C{ H} spectrum
matches the published data.59
Scheme 2 depicts the route for synthesizing the iridium com-
plexes [(DEAM-diPhBY)Ir(cod)](OTf) (3-diPh) and [(DEAM-
IBY)Ir(cod)](OTf) (3-iPr). Attaching 1-R to the trans-9,10-
dihydro-9,10-ethanoanthracene backbone by straightforward
triflate52 substitution cleanly provides (DEAM-diPhBI)(OTf)2 (2-
diPh) and (DEAM-IBI)(OTf)2 (2-iPr)61 as white powders in 75%
and 95% yield, respectively.
˚
Ir–C(NHC) bond lengths are 2.056(9) and 2.081(10) A.
Herrmann et al. report a similar complex (NHC)Ir(cod)Cl,
63
˚
with an Ir–C(NHC) bond length of 2.022(7) A. Shi et al.
report shorter Ir(III)–C(NHC) bond lengths for the BINAP–
˚
Common spectroscopic and combustion techniques elucidate
the identity and purity of 2-diPh. The 1H NMR spectrum of
2-diPh reveals a characteristic benzimidazolium salt C-2 proton
resonance at 9.27 ppm. Aromatic protons from the phenyl groups
NHC ligand (1.996(8) and 2.001(8) A), where the NHC is
a 1-methylbenzimidazolidine-2-ylidene, reflecting the difference
between IrI and IrIII.64 The two different Ir–C(NHC) bond lengths
in 3-iPr, though subtle, reflects the ~12◦ difference in the carbene
twist angles of the two heterocycles. The average torsion angle
between the hydrogen atoms (H13–H15 and H14–H16) attached
1
obscure the location of the diphenylmethine resonance. The H
1
NMR and 13C{ H} NMR spectra routinely reveal that exactly one
Scheme 2 Synthesis of [(DEAM-RBY)Ir(cod)](OTf) and [(DEAM-RBY)Rh(nbd)](OTf ) (R = diPh, iPr). Conditions: (a) M = (acac)Ir(cod), base =
◦
Cs2CO3, t = 16 h, T = 23 C, yield; R = Pr (99%) R = diPh (89%), (b) M = [Rh(nbd)2](BF4), base = KN(Si(CH3)3)2 t = 16 h, T = -35 ◦C, yield;
i
i
R = Pr (92%) R = diPh (88%).
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Table 1 Selected bond lengths, bond angles, and torsion angles for 3-iPr
iridium complexes, the rhodium versions are prepared in an inert
atmosphere, but isolated on the bench top as air- and moisture-
stable golden yellow powders in 88% (4-diPh) and 92% (4-iPr)
˚
Bond lengths/A
1
yield. The 1H NMR and 13C{ H} NMR spectra demonstrate the
Ir(1)–C(24)
Ir(1)–C(35)
Ir(1)–C(39)
Ir(1)–C(43)
Ir(1)–C(40)
Ir(1)–C(44)
N(1)–C(17)
N(1)–C(24)
N(2)–C(25)
2.056(9)
rhodium complexes are C1-symmetric in solution.
2.081(10)
2.165(10)
2.172(9)
2.228(11)
2.229(10)
1.477(11)
1.355(11)
1.503(12)
In particular, the 1H NMR spectrum of 4-iPr exhibits diagonos-
tic isopropyl methine septet resonances at 5.30 and 6.34 ppm, and
doublets for the isopropyl methyls at 1.39, 1.60, 1.86 and 1.94 ppm.
The distinct diastereotopic nbd bridge protons appear at 1.39 and
1
1.41 ppm. The 13C{ H} NMR spectrum displays doublet signals at
192.1 and 191.9 ppm attributable to the carbene carbons; the four
nbd olefinic carbons resonate as doublets at 65.1, 70.6, 74.5 and
80.3 ppm. In addition, the nbd bridge carbon resonance appears at
67.8 ppm. High-resolution mass spectrometry reveals the actual
(m/z 745.2782) and theoretical (m/z 745.2772) M+ parent ion
values match.
Bond angles/◦
C(40)–Ir(1)–C(44)
C(43)–Ir(1)–C(44)
N(1)–C(24)–N(2)
N(2)–C(25)–C(26)
N(1)–C(17)–C(16)
89.8(5)
37.4(4)
107.3(8)
107.9(8)
112.3(8)
4-diPh is characterized by multinuclear NMR and high-
1
resolution mass spectrometry. The H NMR spectrum of 4-diPh
reveals diagnostic diphenylmethine signals at 7.24 and 8.53 ppm;
the ethanoanthracene bridgehead protons resonate at 4.35 and
5.00 ppm. The nbd methylene protons are diastereotopic and
resonate at 1.19 and 1.23 ppm as doublets. The olefinic nbd
Torsion angles/◦
N(2)–C(24)–Ir(1)–C(35)
N(3)–C(35)–Ir(1)–C(24)
H(13)–C(13)–C(15)–H(15)
H(14)–C(14)–C(16)–H(16)
72
60
71
70
1
and NHC carbons in the 13C{ H} NMR spectrum are identified
by their doublet multiplicity at 67.3, 70.7, 75.4, 80.9 ppm and
194.9, 194.7 ppm, respectively. The nbd bridge (68.05 ppm) and
diphenylmethine (68.6, 68.8 ppm) resonances were identified by a
2D-gHMQC experiment. The high-resolution mass spectrometry
experiment demonstrates the actual (m/z 993.3398) and theoreti-
cal (m/z 993.3399) M+ parent ion values are nearly identical.
The dicarbonyl complex [(DEAM-MBY)Rh(CO)2](BF4)
(5-Me) is synthesized by treating 4-Me57 with 100 bar of CO at
50 ◦C for 24 h. Complex 5-Me is isolated as a yellow powder
1
in 92% yield. The H NMR spectrum clearly shows a decrease
in the number of signals, signifying the substitution of CO for
1
nbd. The 13C{ H} NMR spectrum confirms substitution of nbd
with the disappearance of the nbd olefinic resonances and the
emergence of two doublets at 179.9 and 180.0 ppm, corresponding
to two CO ligands. The carbene carbons resonate downfield as
doublets at 185.6 and 186.3 ppm. The IR spectrum of 5-Me
complements the NMR spectral studies and confirms the presence
of two CO groups. The stretching frequencies are 2025.8 and
2083.6 cm-1 and fall within the normal range for Rh(NHC)CO
complexes.66–69
Fig. 2 Molecular structure of 3-iPr. Thermal ellipsoids are shown at the
50% probability level and the OTf counter ion is removed for clarity.
Several 2D experiments, gDQCOSY, gHMQC, gHMBC and
NOESY, allow for the absolute assignment of all carbon and
proton signals. The NOESY spectrum at 25 ◦C taken with a mixing
time of 1 s displays several exchange cross peaks between protons
with similar connectivity but different stereochemistry, e.g. Ha,
Hb or Hc, Hd. Of the two methylene protons (Hc, Hd) on the
corresponding carbon, the one with two large coupling constants,
therefore anti-periplanar to Ha, exchanges with the proton (Hb)
exhibiting only one large coupling constant on the corresponding
carbon. At -15 ◦C the exchange rate is negligible and the unique
carbon and proton signals are unequivocally assigned via an NOE
difference experiment (see electronic supplementary information
(ESI) for full details†). The exchange rates at higher temperatures
imply 5-Me is fluxional.
to the bridge and bridgehead carbons is ~70◦. This is noteworthy
because the coupling constant between these protons, determined
by 1H NMR spectroscopy, is 0 Hz and is consistent with Karplus
theory.65
Synthesis and characterization of [(DEAM-diPhBY)Rh(nbd)]-
(OTf) (4-diPh), [(DEAM-IBY)Rh(nbd)](OTf) (4-iPr) and
[(DEAM-MbBI)Rh(CO)2](BF4) (5-Me)
The rhodium complexes [(DEAM-diPhBY)Rh(nbd)](OTf) (4-
diPh) and [(DEAM-IBY)Rh(nbd)](OTf) (4-iPr) were synthesized
according to Scheme 2. In the metalation step the di-NHC is
generated in situ by deprotonating ◦the dibenzimidolium triflate
salt with KN(Si(CH3)3)2 at -35 C. 4-diPh and 4-iPr were
prepared from monometallic [Rh(nbd)2](BF4) at -35 ◦C. Like the
Scheme 3 depicts the proposed ring inversion which is best
described as a degenerate isomerism. In the first step, one of the
2766 | Dalton Trans., 2009, 2764–2776
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Scheme 3 Proposed mechanism for the degenerate isomerization of 5-Me. B: Top view of the degenerate isomerization.
benzimidazole rings moves into a parallel position with respect
to the anthracene backbone. The rhodium center then moves
upwards. Finally, the rhodium moves to the opposite side of
the backbone and the benzimidazole ring swings back into a
perpendicular position. Notice that the He proton exchanges in
space relative to the backbone via the degenerate isomerization. As
well, the methylene protons (Ha and Hb) exchange stereochemical
positions. This mechanism is consistent with a NOE difference
experiment in which the exchange rates were determined at 5,
◦
25 and 45 C. The barrier to ring inversion is calculated as 16.9
1.8 kcal mol-1 (see ESI for the full details†). More importantly,
DS‡ = -7
6 cal mol-1 K-1 and is also consistent with the
proposed mechanism. At the transition state, one imidazole must
align parallel to the backbone. A small and negative entropy of
activation fits the increased order at the transition state. However,
the data used to create the Eyring plot is marred by interference by
additional NOE’s and inherent errors associated with temperature
accuracy. Thus, the DS‡ interpretation is made cautiously.
Fig. 3 Molecular structure of 5-Me. Thermal ellipsoids are shown at the
50% probability level and the BF4 counter ion is removed for clarity.
The substitution of norbornadiene for CO is further supported
by an X-ray crystallographic experiment. Fig. 3 depicts the
molecular structure of 5-Me and Table 2 lists selected bond
lengths and bond angles. The asymmetric unit consists of two
crystallographically independent molecules in the P2(1)/c space
group. The NHCs coordinate in a cis fashion along with cis car-
bonyls to complete a square-planar geometry. The Rh–C(NHC)
conclusively show the chelating di-NHC is resistant to reductive
elimination. trans-Methylstilbene is a difficult substrate to hydro-
genate and is chosen to assess the limits of the catalysts (Scheme 4).
Table 3 summarizes the results.
˚
bond lengths are 2.063(3) and 2.078(4) A and are comparable
to other Rh–C (benzimidazole-NHC) bond lengths reported by
Shi et al.64,70,71 The Rh–C(CO) bond lengths are 1.874(5) and
˚
1.897(4) A and are consistent with analogous Rh(CO)(NHC)
complexes.66,67,72,73
Scheme 4 Catalytic hydrogenation of trans-methylstilbene.
Olefin hydrogenation
3-R and 4-R were evaluated as catalysts for the hydrogenation
of trans-methylstilbene, methyl-2-acetamidoacrylate and benzene.
Recall the objective is to not only determine activity but to
Using 4-Me as the catalyst, the conversion is greater than 98%,
however a black material precipitates from solution during the
course of the reaction. Clearly, decomposition of 4-Me occurs
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Table 2 Selected bond lengths and bond angles for 5-Me
As a control experiment [Rh(nbd)2](BF4), the precursor to
4-Me, was tested for hydrogenation of trans-methylstilbene
(entry 7). Full conversion to the alkane product and a black pre-
cipitate is observed, but when Hg0 is added the conversion drops to
zero (entry 8). Using similar conditions both [Rh(nbd)2](BF4) and
4-Me generate a catalyst capable of hydrogenating benzene. RhI
precursors are well known to break down into Rh0 species that
are excellent catalysts for benzene hydrogenation.75 These results
point to a common Rh0 as the active catalyst.
As mentioned above, one route to Rh0 is NHC reductive
elimination to the corresponding imidazolium salt. The obser-
vation of black precipitates prompted a closer investigation of
the reaction mixture after hydrogenation. Indeed, a 1H NMR
spectrum of the product mixture for entry 9 reveals a resonance
at 9.75 ppm (DMSO-d6), signifying the presence of (DEAM-
MBI)(OTf)2 (2-Me).57 Changing the catalyst loading, reaction
temperature and pressure consistently produced 2-Me (entries
9–11). These results indicate even a di-NHC ligand bound to
RhI is susceptible to reductive elimination. Coupled with the
Hg0 poisoning experiments, this provides strong evidence the true
identity of the active catalyst in entries 9–10 is Rh0.
To circumvent reductive elimination, one solution is to use
a metal that provides stronger M–C(NHC) and M–H bonds.
Conveniently, iridium is a perfect candidate for a comparative
study with 4-Me. Using 3-iPr as the catalyst at 50 ◦C and 50 bar
H2 shows no conversion. However, no precipitate forms after 24 h,
and the solution remains bright orange. A 1H NMR spectrum of
the solution reveals resonances attributed to 3-iPr, though the low
catalyst loading precludes identification of all resonances. More
importantly, the spectrum shows no signals corresponding to the
imidazolium salt 2-iPr.
˚
Bond lengths/A
Rh(1)–C(36)
Rh(1)–C(35)
Rh(1)–C(34)
Rh(1)–C(25)
C(35)–O(1)
C(36)–O(2)
N(4)–C(34)
N(4)–C(33)
1.874(5)
1.897(4)
2.063(3)
2.078(4)
1.132(5)
1.122(5)
1.344(4)
1.468(4)
Bond angles/◦
Rh(1)–C(36)–O(2)
C(36)–Rh(1)–C(35)
C(25)–Rh(1)–C(35)
C(25)–Rh(1)–C(34)
N(4)–C(34)–N(3)
C(34)–Rh(1)–C(35)
175.2(6)
94.23(19)
89.34(16)
90.19(13)
106.3(3)
176.40(17)
and one possibility is the formation of Rh0. Rh0 and Ir0 species,
whether colloidal or nanoparticles, are well known to hydrogenate
alkenes, even unactivated tetrasubstituted substrates. Since these
ligands are chiral, any product enantioselectivity is evidence that
the ligand remains bound during catalysis, but using (S)-4-Me as
the catalyst (entry 2) provides no optical induction (0% ee). This
raises questions as to the true identity of the active catalyst and
the fate of the NHC ligand during the reaction.
Mercury poisoning is a common method to test for M0 species.
However, there are inherent flaws associated with Hg0 poisoning.
Mercury does not form an amalgam with Rh0 and Ir0, as with Pd0,
and therefore poisoning these species is more difficult.74 In fact, the
results from entries 3–6 demonstrate Hg0 sequestration produces
inconsistent results. For instance, compare entries 3 and 4. Though
every parameter was held constant the conversion changes from
21 to 100%. The only rational explanation is a difference in
stir rate. Though the stir settings were the same in both, the
stir bar may not stir evenly during each experiment, resulting
in uneven Hg0 distribution throughout the reaction vessel. In
another experiment, adding additional Hg0 (1.86 g) resulted in
no conversion. The key point is though it appears Rh0 may be the
active catalyst, Hg0 poisoning alone is not sufficient evidence.
Since 3-iPr is not an effective catalyst for trans-methylstilbene
hydrogenation, a rational next step is to use the activated sub-
strate methyl-2-acetamidoacrylate. Table 4 summarizes methyl-2-
acetamidoacrylate hydrogenation results. Performing the reaction
at 50 ◦C and 50 bar H2 in CDCl3 with 4-Me indicates full
conversion occurs after 3 h, concomitant with formation of a
black precipitate. These results, coupled with the observed 0%
ee, prompted another Hg0 experiment. As expected, adding Hg0
prevents catalysis. However, when the size of the R-group on the
catalyst is increased, decomposition products are not observed
Table 3 Metal catalyzed hydrogenation of trans-methylstilbene with 3 and 4
Entry
Catalyst/loading (mol%)
Solvent
H2 Pressure/bar
Temperature/◦C
Mercury/g
Salta
Conversion (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
4-Me (1)
(S)-4-Me (1)
4-Me (1)
4-Me (1)
4-Me (1)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CDCl3
CH2Cl2
CH2Cl2
CDCl3
CH2Cl2
CH2Cl2
CDCl3
CDCl3
50
50
50
50
50
50
50
50
50
50
25
50
100
40
40
40
40
44
40
50
50
50
25
50
50
97
0
0
5.03
5.16
7.434
7.011
0
N/A
N/A
N/A
N/A
N/A
Yes
N/A
N/A
Yes
Yes
Yes
No
Yes
>98
>98
21
100
0
0
>98
0
4-Me (10)
[Rh(nbd)2](BF4) (1)
[Rh(nbd)2](BF4) (1)
4-Me (10)
4-Me (1)
7.116
0
0
0
0
0
80
>98
>98
0
4-Me (1)
3-iPr (3)
3-iPr (5)
>98
a Presence of benzimidazolium salt by 1H NMR. N/A means either NHC was not part of the catalyst or the salt was not looked for. b Determined by 1H
NMR spectroscopy after 24 h.
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Table 4 Hydrogenation of methyl-2-acetamidoacrylate with 3 and 4
Entry
Catalyst/loading (mol%)
Time/h
Hg
Conversion (%)a
ee (%)b
1
2
3
4
5
6
7
8
9
(S)-4-Me (1)
(S)-4-Me (1)
(S)-4-iPr (2)
(S)-4-diPh (2)
Ir(cod)acac (3)
Ir(cod)acac (3)
(S)-3-iPr (1.5)
(S)-3-iPr (1.5)
(S)-3-diPh (3)
(S)-3-diPh (3)
3
24
24
24
3
24
24
24
36
36
No
Yes
No
No
No
Yes
No
Yes
No
Yes
>98
0
100
100
>98
0
>98
0
93
0
N/A
8 (R)
4 (S)
N/A
N/A
0
N/A
9 (R)
0
10
0
a Determined by 1H NMR (entries 1, 2, 5, 6) and GC-MS (entries 3, 4, 7–11). b Determined by GC-MS. Absolute configuration determined from literature
precedent.
Fig. 4 Left: hydrogenation of methyl-2-acetamidoacrylate catalyzed by 3-diPh (3 mol%): 50 bar H2; 50 ◦C; CH2Cl2. Right: corresponding % ee vs. time
curve.
in the 1H NMR spectrum. In addition, GC analysis of the
signal of possible decomposition products, 3-diPh was subjected
to catalytic conditions without the substrate. The1H NMR
spectrum still reveals no signals attributed to decomposition
products. This implies that only a small portion of 3-diPh
decomposes.
Fig. 4 (right) shows that the ee increases over time and then
decreases, and is at a maximum at 16 h. Typically, an intact catalyst
will start off at a given ee which will then decrease as the catalyst
decomposes. A possible scenario that fits all of the data is an Ir0
species stabilized by the NHC ligand is the active catalyst, while
the molecular species 3-diPh is inactive.
It is reasonable to assume the reaction is first-order in catalyst.
Doubling the loading of 3-diPh doubles the yield, but halves the
ee at t = 16 h. In a separate experiment, 3-diPh (6 mol%) is pre-
subjected to the reaction conditions without substrate to bypass
the induction period and start with the active catalyst. After 24 h,
the substrate is injected into the reactor and allowed to react for
16 h. The yield doubled and ee was again halved.
A few reported examples demonstrate an equilibrium between
metal-bound NHC and free NHC can exist, as in the case of
phosphine ligands.76–78 An excess of ligand was used to explore
this unlikely equilibrium. In a catalytic hydrogenation reaction in
which excess dibenzimidazolium salt and Cs2CO3 were added, the
yield decreased to 5% and the ee was 6% (S). This result suggests
the excess NHC ligand is actually serving to poison the catalyst,
as previously demonstrated by Finke et al.79
i
products show ee’s of 8% (R) and 4% (S) for R = Pr and diPh,
respectively.
Changing to the Ir catalyst, 3-iPr, shows completes hydro-
genation after 24 h (entry 7). There is no detectable precipitate
after the reaction and a bright orange solution remains. This
suggests 3-iPr survives, but a Hg0 poisoning experiment indicates
no conversion after 24 h. Using 3-diPh as the catalyst results
in an ee of 9% favoring R-N-acetylalanine methyl ester with a
conversion of 93% after 36 h. The Hg0 poisoning experiments show
conversion is halted, suggesting Ir0 species, however achievement
of enantioselectivity seems contradictory. As a control experiment,
the iridium precursor was tested for hydrogenation, and is highly
active. The mercury poisoning experiment demonstrated the
expected sequestration.
Fig. 4 (left) shows the kinetic profile for the hydrogenation
of methyl-2-acetamidoacrylate with 3-diPh as the catalyst. An
induction period is observed followed by a linear profile. The linear
portion implies zero-order kinetics in methyl-2-acetamidoacrylate.
The induction period in the kinetic profile provides prima facie
evidence for M0 species. MS analysis yielded a noteworthy peak
at m/z 1099.3. The peak at 1099.3 contains an isotopic pattern
indicative of [M]+ and is attributable to [3-diPh]+. Decomposi-
tion products, including the imidazolium salt 1-diPh were not
detected. A 1H NMR spectrum of the product mixture did
not reveal signals attributable to ligand loss. To amplify the
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Table 5 Hydrogen transfer of methyl ketones with 3-diPh
exchange rate increasing with temperature. The first step during
the catalytic cycle is most likely loss of cod from 3-diPh. Therefore,
a likely explanation for the low ee in the hydrogenation reactions
could be an ill-defined chiral pocket caused by the floppy nature
of the ligand.
Entry
Time/h
X
Y
Conversion (%)a
ee (%)b
Hydroformylation of styrene
1
2
3
4
5
1
5
5
5
5
H
H
C4H4
H
Br
H
H
H
Br
H
18
>98
>98
>98
>98
5 (R)
6 (R)
3 (R)
5 (R)
5 (R)
Catalysts bearing NHC ligands are less established in hydroformy-
lation reactions than in palladium catalyzed coupling reactions or
ruthenium olefin metathesis reactions.80 Of these reactions, few
catalysts make use of di-NHC ligands, and surprisingly there are
no reports of enantioselective hydroformylation. The rhodium
catalysts 4-R, 5-Me and 6-Me were screened for the hydroformy-
lation of styrene. 6-Me is a bimetallic complex ([m-(DEAM-
RBY)][Rh(COD)Cl]2) synthesized from [Rh(cod)Cl]2 which was
previously reported.57 Table 6 shows the results.
a Determined by H NMR. b Determined by HPLC. Absolute configura-
tion determined from literature precedent.
1
In summary, there is strong evidence to implicate M0 species as
the active catalyst, however there does not appear to be a way to
maximize both % conversion and ee.
Generally, product formation is complete after 24 h at 50 ◦C with
catalyst loadings of 0.1 mol%. Solvent effects appear to have little
effect on either conversion or branched to linear ratios. Pressures
of less than 80 bar reduced the conversion to zero. The branched
to linear ratios (b : l) are all suspiciously similar, approximately
96 : 4. In a study of common hydroformylation precatalysts, the b : l
ratios ranged from 95 : 5 to 98 : 2, implying a common catalyst.81
This prompted an investigation into the catalyst identity.
There were no black precipitates at the end of the reaction
to signal decomposition occurred. A mercury poisoning experi-
ment inhibited catalysis. Since the hydroformylation products are
volatile, decomposition products were easily isolated. A 1H NMR
spectrum clearly shows all the resonances associated with the
Me-dibenzimidazolium salt. Furthermore, the control experiment
using [Rh(nbd)2](BF4) as the hydroformylation catalyst showed a
b : l of 98 : 2. Following the hydroformylation kinetics of styrene
with 5-Me demonstrated a sigmoidal curve (Fig. 5). In summary,
the b : l ratio indicates a common species is responsible for
hydroformylation. Considering the common b : l and an observed
induction period (Fig. 5), the most likely species is Rh(CO)4H.80–82
Hydrogenation of methyl ketones
3-diPh was evaluated as a catalyst for hydrogen transfer reactions
of methyl ketones. Table 5 summarizes the results. The reaction
was carried out at 70 ◦C for 5 h in 2-propanol at 2 mol% catalyst
loading with KOH as the base. The conversion is complete after
5 h, but the % ee is low and favoring the R enantiomer in each
case. The solution at the beginning of the reaction is bright orange
and then turns dark orange by the end. Visual inspection of the
solution at the end of the reaction does not show a precipitate, and
1
the H NMR did not show any resonances connected to ligand
loss. Typically, high temperatures have an adverse effect on ee,
and as time progresses ee decreases. The ee was checked after an
hour during the hydrogenation of acetophenone, demonstrating
no change between t = 1 and t = 5 h (entries 1 and 2). Lowering
the temperature to 50 ◦C reduces the conversion to zero after 24 h.
Remember that 5-Me displays a degenerate isomerization, with the
Table 6 Hydroformylation of styrene with 4-R, 5-Me and 6-R
Entry
Catalyst/loading (mol%)
Pressure/bar
Solvent
Conversion (%)a
Branched : lineara
1
2
3
4
5
6
7
8
9
(S)-4-Me (1)
50
100
100
100
30
50
80
Chloroform
Chloroform
Chloroform
Toluene
Chloroform
Chloroform
Chloroform
Toluene
0
100
100
96
0
0
100
100
75
100
100
N/A
95 : 5
96 : 4
96 : 4
N/A
N/A
97 : 3
96 : 4
94 : 6
94 : 6
97 : 3
(S)-4-Me (1)
(S)-4-Me (0.1)
(S)-4-diPh (5)
(S)-5-Me (0.1)
(S)-5-Me (0.1)
(S)-5-Me (0.1)
(S)-5-Me (0.1)
(S)-6-Me (0.1)
(S)-6-Me (0.1)
[Rh(nbd)2](BF4) (0.1)
80
100
100
100
Chloroform
Toluene
Chloroform
10
11
a Determined by GC.
2770 | Dalton Trans., 2009, 2764–2776
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the ligand, whether on the surface of a particle or as a molecular
species. Only a 16 kcal mol-1 barrier is associated with the ring in-
version in 5-Me. Perhaps this flexibility presents too many degrees
of freedom for the incoming substrate and chirality defining step
during catalysis. Forthcoming next generation ligands will impart
a more rigid and defined chiral pocket.
Experimental
General methods
Unless specified otherwise, all manipulations were performed
under an inert atmosphere using standard Schlenk or glovebox
techniques. Glassware was oven dried before use. Tetrahydrofu-
ran (THF), 1,2 dimethoxyethane (DME) and dichloromethane
(CH2Cl2) were dried using a GlassContours drying column. C6D6
(Cambridge Isotopes) was dried over sodium–benzophenone
Fig. 5 Hydroformylation of styrene catalyzed by 5-Me (0.01 mol%):
80 bar syngas; 50 ◦C; CHCl3.
Rh(CO)4H is difficult to observe due to the stability of the
complex outside hydroformylation conditions. Recently, Garland
et al. successfully found this species by IR spectroscopic means
during the hydroformylation of alkenes with the metal precursor,
Rh4(CO)12.82
˚
ketyl, distilled, vacuum transferred and stored over 4 A
molecular sieves. CDCl3 (Cambridge Isotopes) was dried over
˚
calcium hydride, distilled, vacuum transferred and stored over 4 A
molecular sieves. (1,5-Cyclooctadiene)iridium(I)(acetylacetonate),
(cod)Ir(acac), and bis-norbornadiene-rhodium(I) tetrafluorobo-
rate, [Rh(nbd)](BF4), were purchased from Strem Chemicals
Inc. and used without further purification. Cesium carbo-
nate (Cs2CO3), trans-methylstilbene, methyl-2-acetamido-
acrylate, acetophenone, 1-(2-naphthalenyl)ethanone, 1-(4-bromo-
phenyl)ethanone, 1-(3-bromophenyl)ethanone, benzimidazole,
Conclusions
In summary, we report the synthesis of new iridium and rhodium
complexes supported by a chiral ethanoanthracene ligand bearing
two NHC moieties (DEAM-RBY). The complexes 3-iPr and 5-Me
form single crystals and permit X-ray structural analysis. Complex
5-Me undergoes a degenerate isomerism in the solution state and
multinuclear NMR spectroscopic techniques indicate the barrier
to ring inversion is 16.9 1.8 kcal mol-1 with an associated entropy
of activation that is small and negative (-7 6 cal mol-1 K-1).
Under hydrogenation conditions the catalysts decompose into
the corresponding M0 species, presumably via [NHC-H]+ reductive
elimination. Mercury poisoning experiments prevent catalytic
turnover and supports M0 as the active catalyst. In addition,
kinetic studies reveal an induction period, indicative of M0
formation. Monitoring the ee with respect to time provides further
evidence for the growth of M0 particles. As the reaction proceeds
the ee changes from 0% to a maximum of 25%, followed by
a decrease to the final ee of 9%. In the examples that show
enantioselectivity, the ligand must be associated with the metal
particle during catalysis. Only a few documented examples exist
for enantioselective catalysis by a M0 species.83–85 For example,
nano-sized M0 species are implicated in the hydrogenation of ethyl
pyruvate using Pt and Pd precatalysts. Thus, one explanation for
the change in ee is the M0 particle size must change with time.
As the particle grows, the influence of the ligand on selectivity
will change as function of surface area. A maximum is reached
once the optimum surface area/ligand coverage is achieved for
the system, but as more M0 species are initiated the selectivity
drops.
potassium
bis-trimethylsilylamide
KN(Si(CH3)3)2
and
chlorodiphenylmethane were purchased from Sigma-Aldrich
and used without further purification. Anhydrous potassium
carbonate (K2CO3), styrene, anhydrous sodium sulfate (Na2SO4),
potassium hydroxide (KOH), Celite, xylenes, ether, 2-propanol
and hexanes were purchased from Fisher Scientific and used
without further purification. Syngas, carbon monoxide and
hydrogen were purchased from Airgas. The synthetic procedure
for 2-iPr was reported previously.57 NMR spectra were obtained
on Varian Mercury Broad Band 300 MHz, Varian Mercury
300 MHz, or INOVA 500 MHz spectrometers. Chemical shifts
1
are reported in d (ppm). For 1H and 13C{ H} NMR spectra,
the residual proton solvent peak was referenced as an internal
reference. IR spectra were recorded on a Thermo Nicolet Nexus
670-FT-IR spectrometer. Spectra of solids were measured as KBr
discs. Mass spectrometry was performed at the in-house facility
of the Department of Chemistry at the University of Florida.
Chemical ionization, electrospray ionization, and MALDI-TOF
methods were used. Combustion analyses were performed by
E and R MicroAnalytical Division, Parsippany, NJ or at the
in-house facility of the Department of Chemistry at the University
of Florida. For HPLC analysis, a Shimadzu prominence system
with a LC-20AT solvent delivery module, DGU-20A3 degasser,
SPD-20A UV-vis detector (225 or 254 nm), and a CBM-20A
system controller were used. GC and GC-MS analyses were
carried out on a Thermo-Scientific Trace DSQ mass spectrometer.
The hydroformylation experiments also indicate reductive elim-
ination is a facile process for catalysts supported by di-NHCs.
The similar b : l ratios indicate a common catalyst is most likely
operating. It is well documented that RhH(CO)4 can form under
hydroformylation conditions from a variety of Rh precursors.
Under a CO atmosphere, if the NHC reductively eliminates, Rh0
deposition is prevented and RhH(CO)4 forms.
Synthesis of 1-diphenylmethanebenzimidazole (1)
To a 1 L round bottom flask was added the following
reagents in the order listed: benzimidazole (12.00 g, 0.102 mol),
500 mL xylenes, anhydrous K2CO3 (14.16 g, 0.102 mol), KOH
Though the evidence strongly supports ligand reductive elimi-
nation, it is plausible that the low ee is due to the flexible nature of
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(6.01 g, 0.107 mol) and tetra(n-butyl)ammonium bromide (1.70 g,
5.27 mmol). The flask was then attached to a condenser with
argon flowing through it. The contents of the flask were stirred
for 5 min at room temperature and then chlorodiphenylmethane
(18.0 mL, 0.124 mmol) was added through the top of the
condenser. The reaction was heated at reflux under argon. After
26 h, the reaction mixture was filtered hot through Celite. The
filtrate was dried over anhydrous Na2SO4, filtered and all volatiles
removed in vacuo to provide a beige oil. A solution of 3 : 2
ethyl acetate : hexanes (300 mL) was added to the oil inducing
precipitation. The solution was stirred with a glass rod for
20 min and then the precipitate was filtered through a course
fritted funnel. The precipitate was then washed with ether (3 ¥
15 mL) providing 1 as a white powder (18.20 g, 63%). Found:
C, 84.45%; H, 5.81%; N, 9.51%. calc. for C20H16N2: C, 84.46%;
Synthesis of iridium(I) trans-9,10-dihydro-9,10-ethanoanthracene-
11,12-bis(1-diphenylmethane-benzimidazolidine-2-ylidene 1,5-
cyclooctadiene triflate, [(DEAM-diPhBY)Ir(cod)](OTf) (3-diPh)
To a THF solution (2 mL) of 2-diPh (432 mg, 0.393 mmol)
and Cs2CO3 (269 mg, 0.823 mmol) was added a solution of
(cod)Ir(acac) (157 mg, 0.393 mmol in 2 mL THF) at 23 ◦C.
After stirring the solution overnight it was filtered and the
filtrate dropped into 20 mL of hexanes to form a precipitate.
The precipitate was filtered and dried on a high vacuum line
providing 3-diPh as a bright orange powder (422 mg, 89%). Found:
C, 65.24%; H, 4.59%; N, 4.40%. Calc. for IrC67H58N4SO3F3: C,
1
64.45%; H, 4.69%; N, 4.48%. H NMR (500 MHz, CDCl3, d):
8.38 (s, 1H, NCH), 8.04 (d, J = 10 Hz, 1H, NCCH), 7.74 (d,
J = 5 Hz, 1H, aromatic), 7.57 (d, J = 5 Hz, 1H, aromatic), 7.49–
7.04 (m, 20H, aromatic), 6.96 (s, 1H, NCH), 6.90 (dd, J = 5 Hz,
J = 5 Hz, 1H, aromatic), 6.86–6.76 (m, 6H, aromatic), 6.62 (d,
J = 5 Hz, 1H, NCCH), 6.55 (d, J = 5 Hz, 2H, aromatic), 6.36
(d, J = 10 Hz, 1H, NCCH), 6.14 (d, J = 5 Hz, 2H, aromatic),
5.32 (d, J = 15 Hz, 1H, NCHH), 4.98 (s, 1H, NCH2CHCH),
4.73–4.68 (m, 2H, IrCH, NCHH), 4.40 (s, 1H, NCH2CHCH),
4.30–4.18 (m, 2H, NCHH, NCH2CH), 3.68–3.55 (m, 2H, IrCH),
2.90 (d, J = 9 Hz, 1H, NCHH), 2.78 (d, J = 5 Hz, 1H, IrCH),
2.14–2.05 (br. s, 1H, NCH2CH), 2.02–1.85 (m, 2H, IrCHCH2),
1.67–1.53 (m, 3H, IrCHCH2), 1.30–1.16 (m, 2H, IrCHCH2), 0.57–
0.44 (m, 1H, IrCHCH2). 13C NMR (75.36 MHz, CDCl3, d):
189.9 (IrCN), 187.0 (IrCN), 145.5 (CCHC), 144.0 (CCHC), 139.1
(CCHC), 138.3 (CCHC), 137.8 (NCHC), 137.0 (NCHC), 136.8
(NCHC), 135.9 (NCHC), 135.8 (NCCH), 135.4 (NCCH), 133.9
(NCCH), 133.2 (NCCH), 129.2 (C aromatic), 128.9 (C aromatic),
128.7 (C aromatic), 128.6 (C aromatic), 128.4 (C aromatic),
127.9 (C aromatic), 127.6 (C aromatic), 127.0 (C aromatic),
126.9 (C aromatic), 126.8 (C aromatic), 126.6 (C aromatic),
126.3 (C aromatic), 125.3 (C aromatic), 124.4 (C aromatic),
124.3 (C aromatic), 123.8 (C aromatic), 123.8 (C aromatic),
123.6 (C aromatic), 122.9 (C aromatic), 115.2 (s, NCCH), 113.6
(s, NCCH), 111.8 (NCCH), 110.0 (NCCH), 85.8 (IrCH), 81.4
(IrCH), 75.1 (IrCH), 72.4 (IrCH), 68.9 (NCH), 68.2 (NCH), 54.7
(NCH2), 51.5 (NCH2), 51.2 (NCH2CH), 46.9 (NCH2CHCH),
46.8 (NCH2CHCH), 45.9 (NCH2CH), 36.8 (IrCHCH2), 35.5
(IrCHCH2), 25.8 (2 overlapping IrCHCH2).
1
H, 5.68%; N, 9.85%. H NMR (300 MHz, CDCl3, d): 8.31 (d,
J = 9 Hz, 1H, NCHN), 8.05 (d, J = 9 Hz, 1H, NCHNCCH),
7.58/7.55 (d, J = 9 Hz, 1H, NCCHCH), 7.51–7.41 (m, 6H,
C6H5), 7.32 (d, J = 9 Hz, 1H, CHNCCH), 7.22–7.15 (m, 4H,
C6H5), 6.97 (d, J = 9 Hz, 1H, CCHNC). 13C NMR (75.36 MHz,
(CD3)2SO, d): 143.6 (CHNCHNC), 142.8 (NCHN), 138.7
(NCHC), 133.9 (CHNC), 128.8 (NCHCCHCH), 128.1 (NCHC-
CHCHCH), 128.0 (NCHCCH), 122.5 (CHNCCHCH), 121.8
(NCHNCCHCH), 119.7 (NCHNCCH), 111.2 (CHNCCH), 61.9
(NCHC).
Synthesis of trans-9,10-dihydro-9,10-ethanoanthracene-11,12-
bis(1-diphenylmethane)-benzimidazolium (DEAM-diPhI)(OTf)2
(2-diPh)
To a 100 mL flask containing 9,10-dihydro-9,10-ethanoan-
thracene-11,12-diyldimethanediyl bis(trifluoromethanesulfonate)
(2.01 g, 3.79 mmol) dissolved in anhydrous DME (50 mL) was
added 1 (2.26 g, 7.95 mmol). After refluxing under argon for
75 min, the solution was reduced in vacuo to approximately
15 mL. A precipitate formed and was filtered through a course
fritted funnel. The precipitate was washed with ether (4 ¥ 15 mL)
providing 2-diPh as a mildly hygroscopic white powder (3.13
g, 75%). Found: C, 64.66%; H, 4.80%; N, 4.62%. Calc. for
(C58H48N4)(O3SCF3)2 + 1 molecule DME (C4H10O2) C, 64.62%;
H, 4.93%; N, 4.71%. 1H NMR (300 MHz, (CD3)2SO, d): 9.29 (s,
2H, NCHN), 7.97 (dd, J = 3 Hz, J = 3 Hz, 2H, NCCH), 7.85
(dd, J = 3 Hz, J = 3 Hz, 2H, NCCH), 7.74 (dd, J = 3 Hz, J =
3 Hz, 4H, CC5H5-aromatic), 7.65 (s, 2H, NCH), 7.62–7.43 (m,
20H, aromatic), 7.27 (d, J = 6 Hz, 2H, CH2CHCHCCHCH),
7.17 (dd, J = 9 Hz, J = 9 Hz, 2H, NCCHCH), 7.02 (dd, J =
9 Hz, J = 9 Hz, 2H, NCCHCH), 6.84 (d, J = 6 Hz, 2H,
CH2CHCHCCH), 4.52 (dd, J = 15 Hz, J = 3 Hz, 2H, CHH),
4.12 (s, 2H, CH2CHCH), 3.93 (dd, J = 15 Hz, J = 9 Hz, 2H,
CHH), 3.43 (s, DME), 3.24 (s, DME), 2.41 (m, 2H, CH2CHCH).
13C NMR (75.36 MHz, (CD3)2SO, d): 142.6 (s, NCN), 142.0
(CCHC), 139.2 (CCHC), 135.9 (NCHC), 135.9 (NCHC), 131.6
(NCCH), 131.4 (NCCH), 129.3 (CC5H5), 129.2 (CC5H5), 129.1
(CC5H5), 129.0 (CC5H5), 128.1 (CC5H5), 127.1 (NCCHCH), 127.1
(NCCHCH), 126.4 (CH2CHCHCCH), 126.1 (CH2CHCHCCH),
125.2 (CH2CHCHCCHCH), 124.0 (CH2CHCHCCHCH), 120.7
(q, J = 323 Hz, CF3), 114.6 (NCCH), 114.0 (NCCH), 71.0 (DME),
64.3 (NCH), 58.0 (DME), 50.1 (NCH2), 43.9 (CCHCHCH2), 42.2
(NCH2CH).
Synthesis of iridium(I) trans-9,10-dihydro-9,10-ethanoanthracene-
11,12-bis(1-isopropylbenzimidazolidine-2-ylidene
1,5-cyclooctadiene triflate, [(DEAM-IBY)Ir(cod)](OTf) (3-iPr)
To a THF solution (2 mL) of 2-iPr (428 mg, 0.503 mmol) and
Cs2CO3 (340 mg, 1.04 mmol) was added a sol◦ution of (cod)Ir(acac)
(198 mg, 0.496 mmol in 2 mL THF) at 23 C. After stirring the
solution overnight it was filtered. The filtrate was then dropped
into 20 mL of hexanes to form a precipitate. The precipitate was
filtered and dried on a high vacuum line providing 3-iPr as a bright
orange powder (490 mg, 99%). Found: C, 56.21%; H, 5.08%; N,
5.25%. Calc. for IrC47H50N4SO3F3: C, 56.43%; H, 5.05%; N, 5.60%.
1H NMR (300 MHz, CDCl3, d): 7.90 (d, J = 9 Hz, 1H, aromatic),
7.73 (dd, J = 3 Hz, J = 6 Hz, 1H, aromatic), 7.67 (dd, J = 3 Hz,
J = 6 Hz, 1H, aromatic), 7.53 (dd, J = 3 Hz, J = 6 Hz, 1H,
aromatic), 7.43–7.16 (m, 12 H, aromatic), 6.23 (septet, J = 6 Hz,
1H, CH(CH3)2), 5.13 (d, J = 12 Hz, 1H, NCHH), 5.05 (septet,
2772 | Dalton Trans., 2009, 2764–2776
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J = 6 Hz, 1H, CH(CH3)2), 4.83 (d, J = 3 Hz, 1H, CCHC), 4.67–
4.55 (m, 3H, overlapping signals; NCHH, IrCH, IrCH), 4.17 (d,
J = 3 Hz, 1H, CCHC), 3.99 (ddd, J = 6 Hz, J = 6 Hz, J = 1 Hz,
1H, NCH2CH), 3.80 (dd, J = 12 Hz, J = 12 Hz, 1H, NCHH),
3.62 (dt, J = 6 Hz, J = 6 Hz, 1H, IrCH), 2.74 (dt, J = 6 Hz,
J = 6 Hz, 1H, IrCH), 2.57–2.44 (m, 1H, IrCHCHH), 2.36 (ddd,
J = 6 Hz, J = 6 Hz, J = 1 Hz, 1H, NCH2CH), 2.28 (d, J =
15 Hz, 1H, NCHH), 2.19–2.08 (m, 1H, IrCHCHH) 2.02 (d, J =
6 Hz, 3H, CH(CH3)2), 1.80 (d, J = 6 Hz, 3H, CH(CH3)2), 1.66 (d,
J = 6 Hz, 3H, CH(CH3)2), 1.88–1.58 (m, 4H, signals overlap with
isopropyl methyls, IrCHCHH), 1.46–1.35 (m, 1H, IrCHCHH),
0.91 (d, J = 6 Hz, 3H, CH(CH3)2), 0.65–0.51 (m, 1H, IrCHCHH).
13C NMR (75.36 MHz, CDCl3, d): 188.1 (NCN), 183.8 (NCN),
145.2 (CCHC), 143.7 (CCHC), 139.0 (CCHC), 138.4 (CCHC),
136.2 (NCCH), 135.8 (NCCH), 132.1 (NCCH), 131.4 (NCCH),
127.0 (CCHCHCH), 126.9 (CCHCHCH), 126.7 (CCHCHCH),
126.4 (CCHCHCH), 126.2 (CCHCCH), 125.2 (CCHCCH),
124.2 (CCHCCH), 124.1 (NCCHCH), 123.8 (NCCHCH), 123.8
(NCCHCH), 123.6 (NCCHCH), 122.9 (CCHCCH), 121.0 (q, J =
321 Hz, CF3), 113.2 (NCCH), 112.5 (NCCH), 111.7 (NCCH),
110.2 (NCCH), 85.6 (IrCH), 79.1 (IrCH), 75.3 (IrCH), 69.2
(IrCH), 55.7 (CH(CH3)2), 54.8 (CH(CH3)2), 54.7 (NCH2), 51.7
(NCH2), 50.8 (NCH2CH), 47.2 (CCHC), 47.0 (CCHC), 46.0
(NCH2CH), 37.1 (IrCHCH2), 35.7 (IrCHCH2), 28.0 (IrCHCH2),
26.2 (IrCHCH2), 22.6 (CH(CH3)2), 21.2 (CH(CH3)2), 21.0
(CH(CH3)2), 20.9 (CH(CH3)2). MS(HR-ESI-FTICR+): Calc. for
[C46H50N4Ir]+: m/z 849.3636 M+, Found m/z 849.3572.
(CCHC), 144.0 (CCHC), 139.2 (CCHC), 138.3 (CCHC), 137.7
(NCHC), 137.6 (NCHC), 137.0 (NCHC), 136.2 (NCHC), 135.9
(2 overlapping signals, NCCH), 133.7 (NCCH), 133.1 (NCCH),
129.4 (C aromatic), 129.0 (C aromatic), 128.8 (C aromatic), 128.7
(C aromatic), 128.6 (C aromatic), 128.5 (C aromatic), 128.4 (C
aromatic), 128.3 (C aromatic), 127.8 (C aromatic), 127.7 (C aro-
matic), 127.3 (C aromatic), 127.2 (C aromatic), 126.9 (C aromatic),
126.8 (C aromatic), 126.7 (C aromatic), 126.6 (C aromatic), 126.3
(C aromatic), 125.3 (C aromatic), 124.5 (q, J = 321, CF3),
124.1 (C aromatic), 124.0 (C aromatic), 123.9 (C aromatic),
123.5 (C aromatic), 123.3 (C aromatic), 122.8 (C aromatic), 114.4
(NCCH), 113.0 (NCCH), 111.4 (NCCH), 109.9 (NCCH), 80.9
(d, J = 7.5, RhCH), 75.4 (d, J = 7.5, RhCH), 70.7 (d, J =
7.5, RhCH), 68.8 (NCH), 68.6 (NCH), 68.05 (RhCHCHCH2),
67.3 (d, J = 7.5, RhCH), 54.6 (NCH2), 52.5 (RhCHCH), 52.3
(NCH2CH), 51.5 (RhCHCH), 50.5 (NCH2), 46.9 (NCH2CHCH),
46.7 (NCH2CHCH), 45.2 (NCH2CH). MS(HR-ESI-FTICR+):
Calc. for [C65H54N4Rh]+: m/z 993.3398 M+, Found m/z 993.3399.
Synthesis of rhodium(I) trans-9,10-dihydro-9,10-ethanoanthracene-
11,12-bis(1-isopropylbenzimidazolidine-2-ylidene norbornadiene
triflate, [(DEAM-IBY)Rh(nbd)](OTf) (4-iPr)
To a THF (2 mL) solution of 2-iPr (108 mg, 0.127 mmol was
slowly added a solution of KN(Si(CH3)3)2 (56 mg, 0.281 mmol
in 3 mL THF) at -35 ◦C in a glovebox. After stirring this
solution for 10 min at room temperature, it was cooled to -35 ◦C.
To this solution was then added a solution of [Rh(nbd)2](BF4)
(54 mg◦, 0.144 mmol in 4 mL) and the final solution was kept
at -35 C overnight. On the bench top the solution was filtered
through a 0.2 mm nylon filter into a stirring solution of hexanes.
After 10 min the yellow precipitate that forms was collected on
a fine fritted funnel and washed with ether (3 ¥ 15 mL). The
precipitate was dried providing 4-iPr as a golden yellow powder
(110 mg, 92%). Found: C, 61.71%; H, 5.20%; N, 6.23%. Calc.
Synthesis of rhodium(I) trans-9,10-dihydro-9,10-ethanoanthracene-
11,12-bis(1-diphenylmethane-benzimidazolidine-2-ylidene
norbornadiene triflate, [(DEAM-diPhBY)Rh(nbd)](OTf) (4-diPh)
To a THF (2 mL) solution of 2-diPh (294 mg, 0.268 mmol) was
slowly added a solution of KN(Si(CH3)3)2 (112 mg, 0.563 mmol
in 3 mL THF) at -35 ◦C in a glovebox. After stirring this
solution for 10 min at room temperature, it was cooled to -35 ◦C.
To this solution was then added a solution of [Rh(nbd)2](BF4)
(101 mg, 0.270 mmol in 4 mL) and the final solution was kept
1
for RhC46H46N4SO3F3: C, 61.73%; H, 5.19%; N, 6.26. H NMR
(300 MHz, CDCl3, d): 7.77 (dd, J = 9 Hz, J = 9 Hz, 2H, aromatic),
7.66 (dd, J = 3 Hz, J = 6 Hz, 2H, aromatic), 7.55 (dd, J = 3 Hz,
J = 6 Hz, 2H, aromatic), 7.38–7.14 (m, 10 H, aromatic), 6.38
(septet, J = 6 Hz, 1H, CH(CH3)2), 5.34 (septet, J = 6 Hz, 1H,
CH(CH3)2), 5.22 (dd, J = 6 Hz, J = 9 Hz, 1H, NCH2CH), 5.09 (d,
J = 12 Hz, 1H, NCH2), 4.82 (s, 1H, NCH2CHCH), 4.71–4.62 (m,
4H, RhCH (3H) and NCH2), 4.19 (s, 1H, NCH2CHCH), 4.10 (br
s, 1H, RhCHCH), 3.81 (m, 1H, RhCH), 3.70 (dd, J = 9 Hz, J =
12 Hz, 1H, NCH2), 2.96 (br s, 1H, RhCHCH), 2.34 (s, 1H, NCH2),
2.03 (br s, 1H, NCH2CH), 1.94 (d, J = 6 Hz, 3H, CH(CH3)2), 1.86
(d, J = 6 Hz, 3H, CH(CH3)2), 1.60 (d, J = 6 Hz, 3H, CH(CH3)2),
1.41 (dd, J = 9 Hz, J = 9 Hz, 1H, RhCHCHCHH), 1.39 (dd,
J = 9 Hz, J = 9 Hz, 1H, RhCHCHCHH), 1.04 (d, J = 6 Hz,
3H, CH(CH3)2). 13C NMR (75.36 MHz, CDCl3, d): 192.1 (d,
RhCN, J = 57), 191.9 (d, RhCN, J = 57), 144.6 (CCHC), 143.9
(CCHC), 139.1 (CCHC), 138.5 (CCHC), 136.5 (NCCH), 136.3
(NCCH), 132.0 (NCCH), 131.3 (NCCH), 129.0 (CHCCHCH),
128.2 (CHCCHCH), 126.9 (CHCCHCH), 126.5 (CHCCHCH),
126.4 (NCCHCH), 125.3 (NCCHCH), 124.0 (NCCHCH), 123.6
(CHCCH), 123.5 (CHCCH), 123.3 (CHCCH), 123.1 (CHCCH),
122.9 (NCCHCH), 112.9 (NCCH), 112.0 (NCCH), 111.0
(NCCH), 109.7 (NCCH), 80.3 (d, RhCH, J = 7.5), 74.5 (d,
RhCH, J = 7.5), 70.6 (d, RhCH, J = 7.5), 67.8 (RhCHCHCH2),
◦
at -35 C overnight. On the bench top the solution was filtered
through a 0.2 mm nylon filter into a stirring solution of hexanes.
After 10 min the yellow precipitate that forms was collected on
a fine fritted funnel and washed with ether (3 ¥ 15 mL). The
precipitate was dried providing 4-diPh as a golden yellow powder
(269 mg, 88%). Found: C, 68.77%; H, 4.98%; N, 4.64%. Calc.
1
for RhC66H54N4SO3F3: C, 69.35%; H, 4.76%; N, 4.90. H NMR
(300 MHz, CDCl3, d): 8.53 (s, 1H, NCH), 8.02 (d, J = 6 Hz,
1H, NCCH), 7.86 (d, J = 6 Hz, 1H, aromatic), 7.59 (d, J =
9 Hz, 1H, aromatic), 7.45–6.75 (m, 30H, aromatic), 7.24 (s, 1H,
NCH), 6.62 (d, J = 9 Hz, 1H, NCCH), 6.22 (d, J = 9 Hz,
2H, NCCH and aromatic), 5.37 (d, J = 12 Hz, 1H, NCHH),
5.17 (dd, J = 9 Hz, J = 9 Hz, 1H, NCH2CH), 5.00 (s, 1H,
NCH2CHCH), 4.80–4.72 (m, 2H, RhCH and NCHH), 4.42 (br
s, 1H, RhCH), 4.35 (s, 1H, NCH2CHCH), 4.21 (dd, J = 15,
J = 12, 1H, NCHH) 3.88 (br s, 1H, RhCH), 3.46 (br s, 1H,
RhCHCH), 3.33 (br s, 1H, RhCH), 3.03-2.99 (m, 2H, NCHH,
RhCHCH), 2.18 (dd, J = 9 Hz, J = 9 Hz, 1H, NCH2CH), 1.23
(dd, J = 9 Hz, J = 9 Hz, 1H, RhCHCHCHH), 1.19 (dd, J = 9 Hz,
J = 9 Hz, 1H, RhCHCHCHH). 13C NMR (75.36 MHz, CDCl3,
d): 194.9 (d, J = 57, RhCN), 194.7 (d, J = 57, RhCN), 144.8
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65.1 (d, RhCH, J = 7.5), 55.8 (CHCH3), 55.2 (CHCH3), 54.5
(NCH2), 52.4 (RhCHCH), 52.0 (RhCHCH), 51.7 (NCH2CH),
49.8 (NCH2), 47.2 (NCH2CHCH), 46.6 (NCH2CHCH), 45.2
(NCH2CH), 22.5 (CHCH3), 21.5 (CHCH3), 21.3 (CHCH3), 21.1
(CHCH3). MS(HR-ESI-FTICR+): Calc. for [C45H46N4Rh]+: m/z
745.2772 M+, Found m/z 745.2782.
three times with hydrogen before it was finally pressurized with
hydrogen (25–100 bar). The autoclave was heated to the desired
temperature. After 24 h, the hydrogen was released. The solution
was then filtered. The filtrate was removed in vacuo and a 1H NMR
spectrum was obtained in CDCl3. Any precipitate that formed was
extracted into DMSO-d6 and a 1H NMR spectrum obtained. The
autoclave was cleaned with nitric acid between reactions to avoid
contamination by any Rh0 and Ir0 that may have formed during
the previous catalysis. Enantioselectivity was evaluated by HPLC
analysis.
Synthesis of the rhodium(I) trans-9,10-dihydro-9,10-
ethanoanthracene-11,12-bis(1-methylbenzimidazolidine-2-ylidene
bis-carbon monoxide tetrafluoroborate,
[(DEAM-MbBI)Rh(CO)2](BF4) (5-Me)
Catalytic hydrogenation of methyl-2-acetamidoacrylate87
To a Parr Instruments 45 mL capacity autoclave was added a stir
bar, [(DEAM-MBY)Rh(nbd)](OTf) (600 mg, 0.716 mmol) and
12 mL chloroform. The autoclave was then charged with 100 bar of
carbon monoxide and stirred at 50 ◦C for 24 h. The autoclave was
then discharged, and the chloroform was washed with deionized
water (3 ¥ 10 mL). The chloroform was removed in vacuo to provide
5-Me as a yellow powder (584 mg, 92%). IR (KBr, cm-1): 2025.8
s (CO), 2083.6 s (CO). Found C, 47.08%; H, 3.26%; N, 6.10%.
Calc. for RhC38H32N4O2BF4Cl6: C, 46.61%; H, 3.49%; N, 5.71%.
1H NMR (500 MHz, CDCl3, d): 7.71 (d, J = 10 Hz, (M/N(1H)),
7.69 (d, J = 5 Hz, K/L(1H)), 7.55–7.40 (m, overlapping signals,
S/T(1H), Q/R(1H), O/P(2H), W/X(2H), K/L(1H)), 7.22–7.36
(m, overlapping signals, Y/Z(2H), S/T(1H), U/V(2H), Q/R(1H),
7.19 (d, J = 5 Hz, M/N(1H)), 4.93 (d, J = 15 Hz, 1H, –CH2–), 4.66
(t, J = 7.5 Hz, 1H, CH2CH), 4.61 (d, J = 0 Hz, 1H, CCHC), 4.44 (s,
3H, CH3), 4.11 (d, J = 0 Hz, 1H, CCHC), 3.93 (dd, 1H, J = 10 Hz,
J = 15 Hz, 1H, –CH2–), 3.84 (s, 3H, CH3), 3.80 (dd, J = 10 Hz,
J = 15 Hz, 1H, –CH2–), 2.24 (d, J = 15 Hz, –CH2–), 1.97 (t, J =
The catalyst, 3-R or 4-R, and methyl-2-acetamidoacrylate
(200 mg, 1.40 mmol) were dissolved in 5 mL of solvent. The
resulting solution was transferred to a stainless steel Parr In-
struments autoclave with a gauge block attachment in an inert
atmosphere. The autoclave was filled and flushed three times with
hydrogen before it was finally pressurized to 50 bar of hydrogen.
The autoclave was heated at 50 ◦C for the allotted time. After
the allotted time, the hydrogen was released. The sample was
then submitted to GC-MS to determine % conversion and %
enantiomeric excess. The autoclave was cleaned with nitric acid
between reactions to avoid contamination of Rh0 and Ir0 that may
have formed during the previous catalysis. GC conditions: Alltech
Chirasil-VAL (25 m ¥ 0.25 mm ¥ 16◦mm), 40 ◦C isothermal for
◦
1 min, followed by an increase of 10 C min-1 to 160 C, t(R) =
9.3 min and t(S) = 9.6 min.
Catalytic hydrogenation transfer of methyl ketones66
1
10 Hz, 1H, CH2CH). 13C{ H} NMR (75 MHz, CDCl3, d): 186.32
The catalyst, 3-diPh (2 mol%), KOH (5 mg, 0.089 mmol) and the
methyl ketone (0.500 mmol) were dissolved in 2 mL of 2-propanol.
The resulting solution was stirred at 70 ◦C. After 5 h the 2-
propanol was removed in vacuo. The % conversion was determined
(d, J = 34 Hz, A/B), 185.57 (d, J = 34 Hz, A/B), 179.96 (d, J =
45 Hz, C/D), 179.93 (d, J = 45 Hz, C/D), 144.12 (s, O/P), 143.52
(s, O/P), 138.17 (s, Q/R), 137.97 (s, Q/R), 135.03 (s, i/j), 134.70
(s, i/j), 134.59 (s, k/l), 133.87 (s, k/l), 127.15 (s, Y/Z), 127.12 (s,
Y/Z), 126.77 (s, W/X), 126.71 (s, W/X), 125.75 (s, c/d), 125.40
(s, e/f), 125.03 (s, g/h), 124.93 (s, e/f), 124.82 (s, g/h), 124.73
(s, c/d), 123.98 (s, a/b), 122.52 (s, a/b), 111.92 (s, U/V), 111.72
(s, U/V), 110.65 (s, S/T), 109.91 (s, S/T), 54.88 (s, K/L/M/N),
51.93 (s, K/L/M/N), 49.71 (s, I/J), 47.12 (s, G/H), 46.77 (s,
G/H), 45.13 (s, I/J), 37.70 (s, E/F), 36.17 (s, E/F). For a legend
to the NMR assignment see ESI.† MS(HR-ESI-FTICR+): Calc.
for [C36H30N4RhO2]+: m/z 653.1418 M+, Found m/z 653.1348.
1
by H NMR spectroscopy and the ee by HPLC analysis. HPLC
conditions: Diacel chiralpak IB column (250 ¥ 4.6 mm id); 98 : 2
hexanes : 2-propanol; 212 nm; 0.5 mL min-1; 1-phenylethanol:
t(S) = 10.7 min and t(R) = 12.2 min; 1-(naphthalene-2-yl)ethanol:
t(R) = 46.5 min and t(S) = 48.8 min; 1-(4-bromophenyl)ethanol:
t(R) = 27.3 min and t(S) = 28.6; 1-(3-bromophenyl)ethanol: t(R) =
25.9 min and t(S) = 27.3 min.
Catalytic hydroformylation of styrene
Catalytic hydrogenation of benzene
The catalyst, 4-R or 5-Me and styrene (50 mg, 0.480 mmol)
were dissolved in 1.5 mL of solvent. The resulting solution was
transferred to a stainless steel Parr Instruments autoclave with
a gauge block attachment in an inert atmosphere. The autoclave
was filled and flushed three times with syngas (1 : 1 CO : H2) before
it was finally pressurized to the desired pressure. The autoclave
was heated at 50 ◦C. After 24 h, the pressure was released. The
reaction mixture was analysed without further purification by GC.
The autoclave was cleaned with nitric acid between reactions to
avoid contamination of Rh0 that may have formed during the
previous catalysis. GC conditions: Supleco Beta Dex 225 column,
100 ◦C isothermal for 5 min, followed by an increase of 4 ◦C
min-1 to 160 ◦C. Styrene, t = 9 min; 2-phenylpropioaldehyde
(branched isomer), t(S) = 18 min and t(R) = 18.2 min;
3-phenylpropioaldehyde (linear isomer), t = 22 min.
In an inert atmosphere, a spatula tip of catalyst (~5 mg) was added
to a J-Young tube followed by approximately 1 mL of deuterated
benzene. A freeze–pump–thaw method was then used to remove
the nitrogen. Three cycles were performed before 1 atm of hydrogen
was added via a Schlenk line. The reaction vessel was then placed
in an oil bath at 50 ◦C for 24 h.
Catalytic hydrogenation of trans-methylstilbene86
The catalysts, 3-R, 4-R or [Rh(nbd)2](BF4), and trans-
methylstilbene (200 mg, 1.03 mmol) were dissolved in 5 mL
of solvent. The resulting solution was transferred to a stainless
steel Parr Instruments autoclave with a gauge block attachment
in an inert atmosphere. The autoclave was filled and flushed
2774 | Dalton Trans., 2009, 2764–2776
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X-Ray crystallographic study†
31 K. J. Cavell and D. S. McGuiness, Coord. Chem. Rev., 2004, 248,
671.
32 A. A. Danopoulos, N. Tsoureas, J. C. Green and M. B. Hursthouse,
Chem. Commun., 2003, 756.
Data were collected at 173 K on a Siemens SMART PLATFORM
equipped with a CCD area detector and a graphite monochroma-
33 L. C. Campeau, P. Thansandote and K. Fagnou, Org. Lett., 2005, 7,
˚
tor utilizing Mo Ka radiation (l = 0.71073 A). The structure
1857.
was solved by the Direct Methods in SHELXTL6, and refined
using full-matrix least squares. The non-H atoms were treated
anisotropically, whereas the hydrogen atoms were calculated in
ideal positions and were riding on their respective carbon atoms.
34 W. J. Marshall and V. V. Grushin, Organometallics, 2003, 22, 1591.
35 S. Caddick, F. G. N. Cloke, P. B. Hitchcock, J. Leonard, A. K. Lewis,
D. McKerrecher and L. R. Titcomb, Organometallics, 2002, 21, 4318.
36 D. S. McGuinness and K. J. Cavell, Organometallics, 1999, 18, 1596.
37 H. van Rensburg, R. P. Tooze, D. F. Foster and A. M. Z. Slawin, Inorg.
Chem., 2004, 43, 2468.
38 H. van Rensburg, R. P. Tooze, D. F. Foster and S. Otto, Inorg. Chem.,
2007, 46, 1963.
39 A. M. Magill, B. F. Yates, K. J. Cavell, B. W. Skelton and A. H. White,
Dalton Trans., 2007, 3398.
40 F. Wu, V. K. Dioumaev, D. J. Szalda, J. Hanson and R. M. Bullock,
Organometallics, 2007, 26, 5079.
41 D. S. McGuinness, N. Saendig, B. F. Yates and K. J. Cavell, J. Am.
Chem. Soc., 2001, 123, 4029.
Acknowledgements
The authors thank the Camille and Henry Dreyfus Foundation
and K. A. A. thanks the NSF and UF for funding the purchase
of X-ray equipment. Research quantities of the DEAM-type
ligands presented herein are now available for purchase from Strem
Chemicals Inc.
42 D. S. McGuinness, K. J. Cavell and B. F. Yates, Chem. Commun., 2001,
355.
43 D. C. Graham, K. J. Cavell and B. F. Yates, Dalton Trans., 2006, 1768.
44 D. C. Graham, K. J. Cavell and B. F. Yates, Dalton Trans., 2005, 1093.
45 D. C. Graham, K. J. Cavell and B. F. Yates, Dalton Trans., 2007, 4650.
46 X. Cui and K. Burgess, Chem. Rev., 2005, 105, 3272.
47 K. Ka¨llstro¨m and P. G. Andersson, Tetrahedron Lett., 2006, 47, 7477.
48 S. Nanchen and A. Pfaltz, Helv. Chim. Acta, 2006, 89, 1559.
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51 M. C. Perry, X. Cui, M. T. Powell, D. R. Hou, J. H. Reibenspies and
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