Synthesis of rhodium(I) and iridium(I) complexes of chiral N-heterocyclic carbenes and their application to asymmetric transfer hydrogenation

Article abstract of DOI:10.1039/b909290k

Rhodium and iridium complexes of chiral NHC-phenolimine and NHC-amine ligands have been prepared and studied for asymmetric transfer hydrogenation. X-ray and NMR spectroscopy show that for NHC-phenolimine complexes abstraction of chloride results in a change in ligand coordination from NHC only to chelating NHC-imine. Complexes of NHC-amines are inactive for transfer hydrogenation, whereas complexes of NHC-phenolimines are active at room temperature for a range of aryl containing ketones. Enantioselectivity is very sensitive to the NHC N-substituent resulting in a switch in the predominant enantiomer.

Full text of DOI:10.1039/b909290k

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N-Heterocyclic Carbenes
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Westfälische Wilhelms-Universität, Münster, Germany
Published in issue 35, 2009 of Dalton Transactions
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www.rsc.org/dalton | Dalton Transactions
Synthesis of rhodium(I) and iridium(I) complexes of chiral N-heterocyclic
carbenes and their application to asymmetric transfer hydrogenation†
Gavin Dyson, Jean-Ce´dric Frison, Adrian C. Whitwood and Richard E. Douthwaite*
Received 11th May 2009, Accepted 24th June 2009
First published as an Advance Article on the web 22nd July 2009
DOI: 10.1039/b909290k
Rhodium and iridium complexes of chiral NHC–phenolimine and NHC–amine ligands have been
prepared and studied for asymmetric transfer hydrogenation. X-ray and NMR spectroscopy show that
for NHC–phenolimine complexes abstraction of chloride results in a change in ligand coordination
from NHC only to chelating NHC–imine. Complexes of NHC–amines are inactive for transfer
hydrogenation, whereas complexes of NHC–phenolimines are active at room temperature for a range of
aryl containing ketones. Enantioselectivity is very sensitive to the NHC N-substituent resulting in a
switch in the predominant enantiomer.
a transition metal hydride or dihydride intermediate and may
Introduction
incorporate a coordinated NH functionality, which mediates
protonation in a formal heterolytic H+/H- transfer from the donor
to a substrate.48,49 However, non-NH containing catalysts can also
exhibit excellent activity and enantioselectivity.50
The chemistry of N-heterocyclic carbenes (NHCs) has developed
significantly over recent years encompassing their synthesis,
reactivity, coordination chemistry and application.1–7 Arguably,
the most significant stimulus for continued interest is the catalytic
application to organo- and metal-mediated catalysis and there
is now ample evidence demonstrating that NHC systems can
lead to new reactivity and improved catalytic rate, lifetime and
selectivity. Of the many reactions that have been investigated
using NHC containing catalysts, hydrogenation and transfer
hydrogenation have featured prominently, which is in part due
to their wide ranging use in synthetic chemistry and continued
industrial importance.
Previously we have reported the synthesis of imidazolium
salt precursors to chiral NHC ligands derived from 1,2-
diaminocyclohexane (Fig. 1) and subsequently prepared examples
of transition metal complexes.51–53 Palladium complexes incor-
porating an NHC–imine ligand gave a maximum enantiomeric
excess (ee) of up to 92% for asymmetric allylic substitution52 and
iridium complexes of the NHC–phosphine and NHC–phosphite
ligands were shown to be less successful for the asymmetric transfer
hydrogenation of acetophenone, with a maximum ee of 38%.29
Whilst direct hydrogenation of unsaturated molecules is more
widely applied, transfer hydrogenation is an attractive alternative
because the use of a potentially dangerous high pressure of
hydrogen is avoided, the protocols are simple and mild, and
the reaction can be highly chemo- and enantioselective. In
NHC chemistry transfer hydrogenation has been described for
ruthenium,8–20 rhodium,21–26 iridium,23,24,27–38 osmium39 nickel40 and
palladium41 precatalysts for the reduction of carbonyl, imine
and nitro functionalities. NHC based ligands which have been
investigated include mono-NHC, and chelating di-NHC and
hybrid examples. Whilst excellent rates have been reported for
NHC catalysts, a notable underdeveloped area is asymmetric
transfer hydrogenation. There are very few reports describing
catalytic asymmetric transfer hydrogenation using NHC catalysts
and the enantioselectivities obtained to date are poor.17,26,29,37,42
This is in contrast to hydrogenation with dihydrogen where chiral
NHC containing catalysts can exhibit excellent enantioselectivities
even for challenging substrates including unfunctionalised tri-
substituted alkenes.43–47
Fig. 1 Chiral imidazolium salt ligands derived from 1,2-diamino-
cyclohexane.
Here we report the synthesis of iridium NHC–phenoxyimine
and rhodium and iridium NHC–amine complexes and investigate
their catalytic application to enantioselective transfer hydrogena-
tion. One motivation was to examine if the presence of a proximal
alcohol or amine group in the NHC–hybrid complexes has a
positive effect on enantioselectivity by mediating proton transfer
to a substrate.
Results and discussion
Established asymmetric transfer hydrogenation catalysts are
predominantly complexes of Ru, Rh or Ir that operate via
Ligand and complex synthesis
The synthesis of two classes of hybrid NHC ligands and their metal
complex derivatives are presented. Scheme 1 shows the synthetic
route to chiral NHC–phenolimine ligands containing a Schiff base
moiety analogous to the Salen class of ligands successfully used in
several classes of enantioselective reactions.54–59 We have recently
Department of Chemistry, University of York, Heslington, York, UK YO10
5DD. E-mail: red4@york.ac.uk; Fax: +44 (0)1904 434183
† CCDC reference numbers 731703–731706. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b909290k
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The Royal Society of Chemistry 2009
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©

Scheme
2
Preparation of ligand precursors and rhodium and
iridium complexes of NHC–amines. Reaction conditions (i)
1-(isocyano-phenyl-methanesulfonyl)-4-methylbenzene, K2CO3, MeCN,
60 ◦C; (ii) (CH3)2C(H) I, reflux; (iii) NaBH4, MeOH, 0–25 ◦C; (iv)
PhCH2Br, K2CO3, MeCN, 80 ◦C; (v) 13 and 15 [Rh(COD)Cl]2, KOtBu,
THF, Na[B{3,5-(CF3)2C6H3}4], -78 to 25 ◦C. 14 [Ir(COD)Cl]2, Ag2O,
THF, Na[B{3,5-(CF3)2C6H3}4], 25 ◦C.
Scheme 1 Preparation of ligand precursors and silver(I) and irid-
ium(I) complexes of NHC–phenolimines. Reaction conditions (i) 2-hy-
droxy-3,5-t-butyl benzaldehyde, EtOH, 80 ◦C; (ii) BrC(H)Ph2, MeCN,
80 ◦C; (iii) Ag2O, CH2Cl2, 25 ◦C; (iv) [Ir(COD)Cl]2, CH2Cl2,
25 ◦C; (v) Na[B{3,5-(CF3)2C6H3}4], CH2Cl2, 25 ◦C; (vi) [Ir(COD)Cl]2,
Na[B{3,5-(CF3)2C6H3}4], CH2Cl2, 25 ◦C.
was synthesised from 2 via initial formation of the imine bond to
give 3 followed by subsequent imidazolium salt synthesis. It was
initially suspected that 1a–d may exhibit intramolecular hydrogen
bonding to account for the modified amine reactivity, however a
single crystal diffraction study of a [BPh4]- derivative of 1b does
not indicate any unusual features.‡ Unusually for imidazolium salt
derivatives, compounds 4a–d are very soluble in both polar and
non-polar solvents with significant insolubility exhibited only in
saturated hydrocarbons. All spectroscopic data is consistent with
the formulations of 4a–d and all exhibit a signal at ca. 13 ppm in the
1H NMR spectrum characteristic of hydrogen bonding between
the phenol and imine moiety.69,70 Iridium(I) complex derivatives
of ligand precursors 4a–d are most readily accessed via silver(I)
complexes 5a–d that are prepared straightforwardly from reaction
between 4a–d and Ag2O. Attempts to prepare free carbenes or
imidazolium-phenoxide compounds via deprotonation using a
variety of bases results in intramolecular cyclisation between NHC
and imine.71
The 1H NMR spectra of 5a–d all exhibit a signal at ca. 13 ppm
indicating that the phenol OH hydrogen atom is retained. Heating
solutions of 5a–d did not result in a loss of hydrogen halide
to give a clean product as judged by 1H NMR and addition
of bases NEt3, NaOH and BuLi led to insoluble products or
decomposition. Reaction between 5b and [Ir(COD)Cl]2 (where
COD = cyclo-1,4-diene) gives 6b in high yield that undergoes
chloride abstraction on addition of Na[[B{3,5-(CF3)2C6H3}4] to
reported one example of a rhodium(I) complex derivative53 of this
class of ligand and here expand the synthetic chemistry to access
new derivatives and a series of iridium(I) complexes. Scheme 2
shows the synthesis of a new class of ligand comprising an NHC
and either a secondary or tertiary amine moiety. Imidazolium
precursors have been prepared that do not contain C–H bonds
in the C4 or C5 positions, in part, to avoid metal activation at
these positions60–66 and to possibly modify the conformation of
the N-substituent in the resulting metal complex catalysts.
NHC–phenolimines
Imidazolium-amine (1a–c) and imidazole-amine (2) (Scheme 1)
can serve as precursors to a range of NHC containing lig-
ands via modification at the amine and imidazole moieties
respectively,29,51–53,67,68 and the synthesis of compounds 3 and
4 occurs in high yield. However, the close proximity of an
imidazolium moiety appears to significantly attenuate the amine
reactivity. For example compound 4d could not be prepared
from the CHPh2 N-substituted analogue 1d (not shown in
Scheme 1) using the analogous method for 1a–c. However, 4d
7142 | Dalton Trans., 2009, 7141–7151
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The Royal Society of Chemistry 2009
©

◦
˚
give 7b (Scheme 1). Alternatively 7a–d can be prepared directly
from 5a–d, [Ir(COD)Cl]2 and Na[[B{3,5-(CF3)2C6H3}4] in a single-
pot reaction. Complex 6b is an air and water stable solid, soluble
in organic and chlorinated solvents and exhibits similar 1H NMR
spectroscopic signals as 5b including a signal at 13.88 ppm for the
OH proton. A characteristic signal at 181.2 ppm is observed in
the 13C NMR spectrum for the carbene carbon atom.
The molecular structure of 6b was confirmed by a single crystal
X-ray diffraction study and is shown in Fig. 2 with select data
given in Table 1.‡§
Table 1 Selected bond lengths (A) and angles ( ) for complexes 7b
and 13
Complex 7b
Ir(1)–C(1)
2.059(2)
2.3635(5)
2.087(2)
2.120(2)
2.169(2)
2.203(2)
1.413(3)
1.391(4)
C(1)–Ir(1)–Cl(1)
C(1)–Ir(1)–C(34)
C(1)–Ir(1)–C(39)
Cl(5)–Ir(1)–C(34)
Cl(5)–Ir(1)–C(39)
92.76(5)
91.06(8)
165.85(9)
152.13(7)
89.04(7)
Ir(1)–Cl(5)
Ir(1)–C(34)
Ir(1)–C(35)
Ir(1)–C(38)
Ir(1)–C(39)
C(34)–C(35)
C(38)–C(39)
Unfortunately single crystals of 7a–d could not be grown,
however NMR spectroscopy clearly indicates coordination of the
1
imine moiety as exemplified by 7b. In the H NMR spectrum
Complex 13
the OH hydrogen atom is observed at 6.32 ppm for 7b that is
typical for phenolic OH not participating in hydrogen bonding.
In addition, the cyclohexyl hydrogen atom adjacent to the NHC
moiety c-hexCHNHC at 5.82 ppm is upfield by ca. 1.7 ppm with
respect to 4–6b.53 This observation is consistent with previous
work where metallocycle formation causes the c-hexCHNHC proton
Rh(1)–C(11)
Rh(1)–N(3)
Rh(1)–C(1)
Rh(1)–C(2)
Rh(1)–C(5)
Rh(1)–C(6)
C(1)–C(2)
2.025(8)
2.206(7)
2.143(9)
2.072(11)
2.244(9)
2.168(9)
1.397(16)
1.345(16)
C(11)–Rh(1)–N(3)
C(1)–Rh(1)–C(l1)
C(11)–Rh(1)–C(6)
C(1)–Rh(1)–N(3)
C(6)–Rh(1)–N(3)
C(17)–N(3)–C(18)
C(17)–N(3)–Rh(1)
C(18)–N(3)–Rh(1)
81.6(3)
99.9(4)
162.9(4)
176.5(4)
96.5(4)
113.0(7)
111.6(6)
113.5(5)
C(5)–C(6)
‡ Crystallographic data for 1b: Colourless crystals, C102H128B2N6, dimen-
sions 0.30 ¥ 0.20 ¥ 0.20 mm; Mr = 1459.72; Monoclinic P21, a =
◦
˚
˚
19.5255(14), b = 10.9535(8), c = 21.0834(15) A, b = 114.9760(10) ,
V = 4087.5(5) A , Z = 2, l(MoKa) = 0.71073 A, rcalc = 1.186 g cm-3,
T = 110(2) K, F(000) = 1584, q range for data collection 2.19 to
27.98◦, limiting indices -26 ≤ h ≤ 25, -14 ≤ k ≤ 14, -28 ≤ l ≤
28, 42956/16572 collected/unique reflections (R(int) = 0.0244), absolute
3
˚
structure parameter 1(3), goodness of fit on F2 = 1.015, Drmax/min
=
-3
˚
1.480/-0.714 e A , final R indices (I > 2s(I)) R1 = 0.0882, wR2 = 0.2570.
The structure was solved using SHELXS-97 and refined using SHELXL-
97.103 Crystallographic data for 6b: Yellow crystals, C41H57ClN3OIr,
dimensions 0.26 ¥ 0.09 ¥ 0.09 mm; Mr = 835.55; Orthorhombic P212121,
3
˚
˚
a = 12.7958(7), b = 16.4449(9), c = 18.4264(10) A, V = 3877.4(4) A , Z =
4, l(MoKa) = 0.71073 A, rcalc = 1.431 g cm-3, T = 110(2) K, F(000) =
˚
1704, q range for data collection 2.30 to 29.92◦, limiting indices -18 ≤ h ≤
17, -23 ≤ k ≤ 22, -25 ≤ l ≤ 25, 58607/10675 collected/unique reflections
(R(int) = 0.0317), absolute structure parameter -0.009(3), goodness of
fit on F2 = 0.973, Drmax/min = 1.163/-0.299 e A , final R indices (I >
-3
Fig. 2 Molecular structure of 6b. Displacement ellipsoids are set at 50%
probability. Hydrogen atoms except for H(1) have been omitted for clarity.
˚
2s(I)) R1 = 0.0186, wR2 = 0.0374. The structure was solved using
SHELXS-97 and refined using SHELXL-97.103 Crystallographic data for
9: Colourless crystals, C31H34IN3, dimensions 0.28 ¥ 0.22 ¥ 0.06 mm;
to be projected into the vicinity of the metal atom resulting in a
significant upfield shift.52
Mr = 575.51; Orthorhombic P212121, a = 9.3749(6), b = 11.8875(8), c =
3
˚
˚
˚
51.257(4) A, V = 5712.3(7) A , Z = 8, l(MoKa) = 0.71073 A, rcalc
=
1.338 g cm-3, T = 110(2) K, F(000) = 2352, q range for data collection
2.31 to 24.94◦, limiting indices -11 ≤ h ≤ 11, -13 ≤ k ≤ 14, -60 ≤ l ≤
60, 44449/9414 collected/unique reflections (R(int) = 0.0440), absolute
NHC–amines
structure parameter 0.001(18), goodness of fit on F2 = 1.244, Drmax/min
=
Although hybrid NHC–amine ligands are an obvious target for
ligand design there are relatively few structurally characterised
examples of their complexes,22,51,67,80–86 and there are reports in the
literature to suggest NHC–amines are prone to decomposition.86–89
It has also been shown that NHC ligands derived from imi-
dazolium salts can result in metal–C bond formation via C–H
activation at the C4 or C5 position giving abnormal carbenes.60–66
To limit metal–C bond formation to C2, and potentially increase
the kinetic stability of NHC–amine ligands compound 8 was
prepared as a precursor to a new class of NHC–amine ligands.
Reaction between phenylsubstituted tosylmethylisocyanide (Ph-
TosMIC) and the dimine (1R,2R)-N,N’-bis-[1-phenyl-meth-(E)-
ylidene]-cyclohexane-1,2-diamine gave compound 8 in good yield
(Scheme 2).
-3
˚
0.866/-1.397 e A , final R indices (I > 2s(I)) R1 = 0.0444, wR2 = 0.0884.
The structure was solved using SHELXS-97 and refined using SHELXL-
97.103 Crystallographic data for 13: Yellow crystals, C152.5H128B2F48N6Rh2,
dimensions 0.38 ¥ 0.18 ¥ 0.10 mm; Mr = 3186.07; Orthorhombic P212121,
3
˚
˚
a = 13.297(3), b = 13.558(3), c = 80.440(16) A, V = 14501(5) A , Z = 4,
l(MoKa) = 0.71073 A, rcalc = 1.459 g cm-3, T = 110(2) K, F(000) = 6476,
˚
q range for data collection 1.81 to 22.55◦, limiting indices -14 ≤ h ≤ 14,
-14 ≤ k ≤ 14, -86 ≤ l ≤ 86, 104979/19029 collected/unique reflections
(R(int) = 0.0495), absolute structure parameter 0.06(3), goodness of fit on
-3
F2 = 1.205, Drmax/min = 0.975/-0.989 e A , final R indices (I > 2s(I))
˚
R1 = 0.0696, wR2 = 0.1611. The structure was solved using SHELXS-97
and refined using SHELXL-97.103
§ A distorted square planar geometry is observed at the iridium atom, if
the COD ligand is considered as occupying two coordination sites, with
˚
the Ir–CNHC distance (C(1)–Ir(1) 2.059(2) A) similar to related iridium
NHC complexes.27,35,72–78 It is clear from the ligand conformation that the
phenolimine moiety does not interact with the iridium atom and that the
hydrogen bonding is retained between the phenol and imine group. The
hydrogen atom was found in a difference map and bond lengths and angles
clearly suggest a phenolimine and not the potential amine-one tautomer.79
Substitution at both C4 and C5 retards imidazolium salt
formation and reactions between 8 and hydrocarbyl chlorides
and bromides did not yield the corresponding imidazolium
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©

salts in acceptable yield. Addition to the imidazole moiety of
8 can occur using neat isopropyl iodide giving compound 9,
however subsequent reduction chemistry of the imine group was
problematic. NHC secondary/tertiary amine ligand precursors
are most conveniently prepared via reduction of an imidazole
imine and subsequent selective addition of a hydrocarbyl iodide
or bromide. Borohydride reduction of 8 gives compound 10 that
undergoes addition of isopropyl iodide exclusively at the imidazole
moiety giving 11. Reaction between 11 and benzyl bromide gives
the tertiary amine 12 in 92% yield with no indication of quaternary
ammonium salt formation. NMR spectroscopy of compounds
10 and 11 are consistent with a either a single diastereomer or
rapid inversion at the secondary amine nitrogen atom. Variable
˚
(Rh(1)–N(3) = 2.206(7) A) similar to the only other rhodium
complex containing an NHC–amine ligand.22 The metallocycle ex-
hibits a boat-like conformation, with the NHC phenyl substituents
orientated perpendicular to the NHC plane, and the isopropyl
substituent appears to be orientated to minimise non-covalent
interactions with the COD co-ligand.
The iridium complex 14 that is the analogue of 13 again exhibits
in the H and 13C NMR spectra the number of signals expected
1
for a single diastereoisomer with chemical shifts similar to 13.
For example in the 13C NMR spectrum a single carbene signal at
173.7 ppm is observed and in the 1H NMR spectrum two doublets
for the diastereotopic isopropyl methyl groups, which have pre-
viously been shown to be sensitive to the stereochemistry in this
class of ligand.71 It is therefore assumed that the stereochemistry
of 14 is analogous to 13 with an RRS configuration.
1
temperature H NMR spectroscopy of 11 in CD2Cl2 down to
160 K did not show any significant changes in chemical shift or
the number of signals.
Complex 15 exhibits a doublet carbene signal in the 13C spec-
trum at 173.5 ppm in addition to four other doublets attributable
to the carbon atoms of the COD ligand. The most notable feature
of the 1H and 13C signals are those due to the benzyl CH2 group that
are observed as a broad singlet at 3.47 and 51.2 ppm respectively. A
signal attributable to c-hexCHNHC is also observed at 3.89 ppm which
is in contrast to complexes 13 and 14 where the corresponding
signal is observed ca. 1.5 ppm further upfield. Collectively these
data suggest that in CDCl3 solution the amine group is not fully
coordinated to the rhodium atom and is labile. Elemental analysis
is consistent with the proposed formulation shown in Scheme 2
indicating that there is no additional ligand, but a single crystal
structure has yet to be obtained.
The preparation of iridium(I) and rhodium(I) transition metal
complexes derived from 11 and 12 is shown in Scheme 2. Rhodium
complexes 13 and 15 were synthesised in a one pot reaction from
successive addition of [Rh(COD)Cl]2, KOtBu, 11(or 12) in THF,
followed by Na[B{3,5-(CF3)2C6H3}4] and work-up with column
chromatography to give 13 and 15 in 76 and 68% yield respectively.
Iridium complex 14 was prepared via a putative intermediate
silver(I) NHC complex salt in a similar procedure to 13 and 15 to
give complex 14 in 54% yield. Frustratingly, the iridium analogue
of 15 resisted attempts at purification.
NMR spectroscopy of complex 13 shows the number of signals
consistent with a single diastereoisomer including a doublet at
176.5 ppm in the 13C NMR spectrum assigned to the rhodium-
coupled carbene atom and four other rhodium-coupled doublets
assigned to the coordinating COD carbon atoms. The amine
NH atom could not be unambiguously assigned but fortunately
a single crystal structure could be obtained. The molecular
structure is shown in Fig. 3 and select data are provided in
Table 1.ठThe amine nitrogen atom has an S-configuration
that can be understood on the basis of minimising non-covalent
interactions between the benzyl and cyclohexylamine substituents.
A pseudo square planar geometry is observed at the rhodium
atom with Rh–C bond lengths similar to other cationic Rh–NHC
complexes containing a COD co-ligand72,90–97 and a Rh–N bond
Catalytic transfer hydrogenation
Initial test reactions using Rh and Ir NHC–amines 13–15 precat-
alysts for the transfer hydrogenation between acetophenone and
2-propanol at 20 ◦C and at reflux (80 ◦C) gave no enantioselectivity
and very slow rates. These findings are analogous to those found
for the only other related study using Rh NHC–imines and are
consistent with the formation of colloidal rhodium metal.22 It was
also found that a rhodium analogue of 7b exhibits significantly
lower activity and enantioselectivity and therefore the current
study was restricted to iridium complexes. Slower catalytic rates
of transfer hydrogenation for NHC–rhodium in comparison to
NHC–iridium catalysts has been noted previously.28
Complexes 7a–d are precatalysts for transfer hydrogenation
as shown in Tables 2 and 3. Reaction at 20 ◦C allowed differ-
entiation of the activity between the precatalysts, whereas with
acetophenone, reactions conducted at 80 ◦C gave products in
> 95% yield within 1 h (Table 2, entries 5, 7, 9 and 11). A ca. 5%
reduction in enantioselectivity was observed for reactions at 80 ◦C
in comparison to those conducted at 20 ◦C. A previous study using
chiral NHC–phosphine ligands has shown a similar reduction in
enantioselectivity at 80 ◦C when comparing reaction times of 1
and 5 h respectively.29
Table 2 shows data for the asymmetric transfer hydrogenation
of acetophenone derivatives. Catalysis usually requires a base and
hydroxide or alkoxide are the most commonly used. Entries 1–4
show that alkoxide, KOtBu, gives the greatest yield and enan-
tioselectivity with a small drop in enantioselectivity at extended
reaction times. In the absence of a base or in the presence of
HCOOH/NEt3 the reaction rate is very low. Entries 3–11 show
Fig. 3 Molecular structure of 13. Displacement ellipsoids are set at 50%
probability. Hydrogen atoms have been omitted for clarity.
7144 | Dalton Trans., 2009, 7141–7151
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Table 2 Asymmetric transfer hydrogenation of acetophenone and
to 7a–c. Subsequent reactions were conducted using 7b because
7b exhibited the best overall performance.
derivativesa
Electron donating (entries 12–15) and withdrawing (entries
16–18) substituents on the phenyl ring modify yield and enan-
tioselectivity. Groups ortho to the carbonyl (entries 12 and 16)
gave significantly lower yield than substitution at the 3- or 4-
position. All reactions resulted in predominant formation of the
S-enantiomer where the 3-Cl substituent gives the highest ee of
56%. Although this is certainly modest with respect to what can
be achieved using well developed transfer hydrogenation catalysts,
this represents the highest ee to date for a transfer hydrogenation
catalyst incorporating a chiral NHC ligand.
Table 3 shows catalytic transfer hydrogenation of a range of aryl
containing ketones. It is clear from entries 3–4 that purely alkyl
substituents possessing b-hydrogens are not amenable to transfer
hydrogenation, whereas the presence of an aryl substituent (entry
2) allows reaction, although the enantioselectivity is significantly
poorer than for aryl ketones (Table 2). Acetonaphthone (entry 1)
gives the same ee as acetophenone although the yield is lower,
presumably due to increased substrate bulk. Entries 6–7 show
that biaryl ketones will undergo transfer hydrogenation and in
contrast to the acetophenone derivatives (Table 2, entries 12–
14) the 2-chloro derivative gives a comparable yield and the
highest ee.
Entry
Precat.
R
Base
Yield (%)b
ee (%)c
1
2d
3
7b
7b
7b
7b
7b
7a
7a
7c
7c
7d
7d
7b
7b
7b
7b
7b
7b
7b
H
H
H
H
H
H
H
H
H
H
H
2-Me
3-Me
4-Me
3-OMe
2-Cl
3-Cl
4-Cl
KOH
KOH
38
72
72
94
99
64
97
83
99
21
95
18
83
39
57
18
31
47
32(S)
28(S)
43(S)
41(S)
41(S)
0
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
KOtBu
4d
5e
6
7e
0
8
33(S)
27(S)
41(R)
36(R)
10(S)
36(S)
31(S)
47(S)
12(S)
56(S)
22(S)
9e
10
11e
12
13
14
15
16
17
18
◦
a 0.5 mol% 7, 2 mol% base, 2 mL 2-propanol, 20 C, 20 h; b Determined
by 1H NMR; c Determined by chiral HPLC; d 30 h; e 1 h, 80 ◦C.
Table 3 Asymmetric transfer hydrogenation of aryl ketonesa
Entry
1
Substrate
Product
Yield(%)b
64
ee(%)c
Conclusions
41(S)
Asymmetric transfer hydrogenation using chiral NHC complexes
remains a challenge. The mechanism of transfer hydrogenation
is complex48,49 and therefore it is difficult to correlate metal
precatalyst structure with activity and enantioselectivity, however
some broad features can be extracted. Clearly the development of
NHC–amines for transfer hydrogenation does not appear to be
promising given the results shown here and reported elsewhere.22
Iridium NHC complexes are superior catalysts to their rhodium
analogues for transfer hydrogenation and the enantioselectivity is
very sensitive to the NHC N-substituent. At least a secondary sub-
stituent is required as evidenced by the lack of enantioselectivity
for the ethyl substituted complex 7a (Table 1 entry 5). The switch
in the predominant isomer on introduction of a second aryl group
(compare entries 6 and 7 in Table 1) would suggest aryl–substrate
or aryl–alkoxide interactions are important. Noyori has shown for
ruthenium based catalysts the importance of CH–p interactions
for inducing high levels of enantioselectivity98 that may also be
operative here.
2
3
4
61
0
4(S)
0
0
0
5
6
7
8
0
42
53
68
0
23(S)
6(R)
3(S)
Experimental
a 0.5 mol% 7b, 2 mol% KOtBu, 2 mL 2-propanol, 20 ◦C, 20 h; b Determined
General procedures
by 1H NMR; c Determined by chiral HPLC.
All manipulations were performed under argon using standard
Schlenk techniques unless stated otherwise. All solvents were dried
over the appropriate drying agent and distilled under dinitrogen
according to literature methods.99 Reagents were purchased from
Aldrich, Acros or Lancaster and used as supplied. [M(COD)Cl]2
(where M = Rh100 and Ir101) and Na[B{3,5-(CF3)2C6H3}4]102 were
prepared using literature procedures. The synthesis of precursors
1b52 229 and 4b53 has been reported previously.
the effect of modifying the NHC N-substituent indicating that
the rate and particularly the enantioselectivity are very sensitive
to changes at this position. Particularly noteworthy are the lack
of significant enantioselectivity for 7a (entries 6 and 7) and the
inversion of stereochemistry observed for 7d (entries 10 and 11).
Complex 7d also shows significantly lower activity in comparison
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7141–7151 | 7145
©

NMR spectra were recorded at probe temperature on a Jeol
EX 270 (1H, 270 MHz; 13C, 67.9 MHz), Bru¨ker AMX-400 (1H,
400 MHz; 13C, 100 MHz) and Bru¨ker AV-500 (1H, 500 MHz;
13C, 126 MHz); respectively. Chemical shifts are described in
parts per million downfield shifted from SiMe4 and are reported
consecutively as position (dH or dC), relative integral, multiplicity
(s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet,
dd = doublet of doublet, br = broad), coupling constant (J/Hz)
and assignment. Proton NMR spectra were referenced to the
chemical shift of residual proton signals (CHCl3 d 7.27, and
C6D5H d 7.16). Carbon spectra were referenced to a 13C resonance
of the solvent (CDCl3 d 77.16 and C6D6 d 128.06). 13C HSQC,
PENDANT and Gradient HMBC experiments were performed
using standard Bru¨ker pulse sequences.
drop wise to 40–60 petroleum ether (100 mL) to give a pale yellow
precipitate. The supernatant was decanted and the solid residue
washed with cold 40–60 petroleum ether (2 ¥ 20 mL) to give 4a as
1
a pale yellow solid. Yield = 789 mg, 90%. H NMR (270 MHz,
CDCl3, 25 ◦C): d = 1.26 (9 H, s, C(CH3)3), 1.41 (9 H, s, C(CH3)3),
1.60 (3 H, t, 3JH-H = 3.4, NCH2CH3), 1.75, 1.95, 2.12, 2.41 (8 H,
m, c-hexCH2), 4.13 (1 H, m, c-hexCHimine), 4.36 (3 H, m, c-hexCHimid
+
NCH2CH3), 6.90 (1 H, s, NCHCPh), 7.10–7.58 (7 H, m, PhCH),
8.55 (1 H, s, imineCH), 11.55 (1 H, s, NCHN), 13.01 (1 H, s, OH); 13C
NMR (68 MHz, CDCl3, 25 ◦C): d = 15.6 (NCH2CH3), 23.1 (CH2),
25.1 (CH2), 29.3 (C(CH3)3), 31.3 (C(CH3)3), 33.2 (CH2), 34.1
(C(CH3)3), 34.3 (CH2), 34.9 (C(CH3)3), 45.4 (NCH2CH3), 62.7
(
c-hexCHimid), 70.8 (c-hexCHimine), 117.5 (NCHCPh), 117.7 (PhCipso),
124.6 (NCHCPh), 126.6, 127.4, 126.3, 130.1, 130.8 (PhCH), 136.0,
136.4 (PhCipso), 136.7 (NCHN), 140.7 (PhCipso), 157.5 (COH), 167.7
Mass spectra were recorded on VG 70-250E or Kratos MS-50
spectrometers. Electrospray (ES) was recorded using methanol or
acetonitrile as the mobile phase. Major fragments were given as
percentages of the base peak intensity (100%). Elemental analyses
were performed at the University of North London.
(
imineCH); MS (ESI), m/z 486 ([M - Cl]+), 100%; HRMS calc. for
C32H47N3O 486.3484, found 486.3479.
[BnC(H)NCHAr][Br] (4c). Prepared and purified by an analogous
method to 4a using an ethanol solution (20 mL) of 3-((1R,2R)-
2-amino-cyclohexyl)-1-benzyl-4-phenyl-3H-imidazol-1-ium bro-
mide (1c) (135 mg, 0.33 mmol) and 3,5-di-tert-butyl 2-
2,4-Di-tert-butyl-6-{[(E)-(1R,2R)-2-(5-phenyl-imidazol-1-yl)-
cyclohexylimino]-methyl}-phenol [C(H)NCHAr] (3)
˚
hydroxybenzaldehyde (116 mg, 0.50 mmol) containing 4 A molec-
An ethanol solution (50 mL) of (1R,2R)-2-(5-phenyl-imidazol-
1-yl)-cyclohexylamine (1.500 g, 6.22 mmol) and 3,5-di-tert-butyl
2-hydroxybenzaldehyde (2.18 g, 9.33 mmol) containing 4 A
ular sieves (250 mg). Compound 4a was isolated as a dark yellow
solid. Yield = 191 mg, 92%. 1H NMR (270 MHz, CDCl3, 25 ◦C),
d = 1.24 (9 H, s, C(CH3)3), 1.35 (9 H, s, C(CH3)3), 1.71, 1.91, 2.10,
2.35 (8 H, m, c-hexCH2), 4.14 (1 H, m, c-hexCHimine), 4.38 (1 H, m,
c-hexCHimid), 5.51 (1 H, s, NCH2Ph), 6.72 (1 H, s, NCHCPh), 7.09–
7.51 (12 H, m, PhCH), 8.59 (1 H, s, imineCH), 11.72 (1 H, s, NCHN),
12.93 (1 H, s, OH); 13C NMR (68 MHz, CDCl3, 25 ◦C), d = 23.2
˚
molecular sieves (1.00 g) was heated at reflux for 16 h in a thick
walled ampoule sealed with a Teflon stopcock. On cooling to
25 ◦C the mixture was filtered and the volatiles removed from the
filtrate under reduced pressure and the resulting solid purified by
flash chromatography on silica gel. The 1st band (3,5-di-tert-butyl
2-hydroxybenzaldehyde) was eluted with dichloromethane, the 2nd
band (3) was eluted with 50:50 dichloromethane:ethyl acetate
and the volatiles removed from the second band under reduced
(
(
c-hexCH2), 25.1 (c-hexCH2), 29.4 (C(CH3)3), 31.4 (C(CH3)3), 33.4
c-hexCH2), 34.2 (C(CH3)3), 34.5 (C(CH3)3), 34.9 (c-hexCH2), 53.5
(NCH2Ph), 62.9 (c-hexCHimid), 71.0 (c-hexCHimine), 117.5 (NCHCPh),
117.6 (PhCipso), 124.5 (NCHCPh), 126.8, 127.6, 128.7, 129.5,
130.1, 130.9, 132.8 (PhCH), 136.2, 136.4 (PhCipso), 137.3 (NCHN)
140.6 (PhCipso), 157.7 (COH), 168.1 (imineCH); MS (ESI), m/z
548 ([M - Br]+); HRMS calc. for C37H49N3O 548.3641, found
548.3635.
1
pressure to give 3 as an orange solid. Yield = 1.68 g, 60%. H
NMR (270 MHz, CDCl3, 25 ◦C) d = 1.25 (9 H, s, C(CH3)3),
1.41 (9 H, s, C(CH3)3), 1.87–2.23 (8 H, m, c-hexCH2), 3.36 (1 H,
m, c-hexCHimine), 4.08 (1 H, m, c-hexCHimid), 6.81 (1 H, s, NCHCPh),
6.87–7.39 (7 H, m, PhCH), 7.71 (NCHN), 7.91 (1 H, s, imineCH). 13
C
NMR (68 MHz, CDCl3, 25 ◦C) d = 24.0 (c-hexCH2), 25.2 (c-hexCH2),
29.4 (C(CH3)3), 31.4 (C(CH3)3), 33.7 (c-hexCH2), 34.1 (c-hexCH2),
34.1 (C(CH3)3), 34.0 (C(CH3)3), 58.5 (c-hexCHimid), 74.1 (c-hexCHimine),
117.4 (NCHCPh), 126.2 (NCHCPh), 127.2, 128.1, 128.5, 129.7
(PhCH), 134.0, 134.3 (PhCipso), 136.3 (NCHN), 140.1 (PhCipso), 157.6
(COH), 165.7 (imineCH); MS (ESI), m/z 458.3 ([M + H]+) 100%;
HRMS calc. for C30H40N3O, 458.3171, found 458.3166.
[
Ph2CHC(H)NCHAr][Br] (4d). An acetonitrile solution (50 mL) of
compound 3 (625 mg, 1.46 mmol) and bromodiphenylmethane
˚
(433 mg, 1.75 mmol) containing 4 A molecular sieves (250 mg) was
heated at reflux for 16 h in a thick-walled ampoule. On cooling to
25 ◦C the mixture was filtered and the volatiles removed from the
filtrate under reduced pressure. The work up was analogous to 4a
giving 4d as a yellow solid. Yield = 971 mg, 94%. 1H NMR (CDCl3,
270 MHz): d = 1.25 (9 H, s, C(CH3)3) 1.39 (9 H, s, C(CH3)3),
1.71, 1.91, 2.20, 2.43 (8 H, m, c-hexCH2), 4.21 (2 H, m, c-hexCH2imine
,
3-((1R,2R)-2-{[1-(3,5-Di-tert-butyl-2-hydroxy-phenyl)-meth-(E)-
ylidene]-amino}-cyclohexyl)-1-ethyl-4-phenyl-3H-imidazol-1-ium
chloride [EtC(H)NCHAr][Cl] (4a)
and c-hexCH2imid), 6.68–7.49 (17 H, m, PhCH + NCHCPh), 7.61 (1
H, s, NCHPh2), 8.52 (1 H, s, imineCH), 11.52 (1 H, s, NCHN),
12.88 (1 H, s, OH); 13C NMR (68 MHz, CDCl3, 25 ◦C): d = 23.2
(CH2), 25.0 (CH2), 29.3 (C(CH3)3), 31.4(C(CH3)3), 32.8 (CH2),
34.1 (C(CH3)3), 34.3 (CH2), 34.9 (C(CH3)3), 63.0 (c-hexCHimid), 66.3
(NCHPh2), 71.1 (c-hexCHimine), 117.0 (NCHCPh), 117.3(PhCipso),
124.5 (NCHCPh), 126.7, 126.9, 127.5, 128.3, 128.8, 129.9, 129.1,
129.3, 130.0, 130.8 (PhCH), 136.1, 136.3, 136.4, 136.8 (PhCHipso),
137.4 (NCHN), 140.5 (PhCHipso), 157.5 (COH), 168.2 (imineCH); MS
(ESI), m/z 624 ([M - Br]+), 100%; HRMS calc. for C43H53N3O,
624.3954, found 624.3948.
An ethanol solution (40 mL) of 3-((1R,2R)-2-amino-cyclohexyl)-
1-ethyl-4-phenyl-3H-imidazol-1-ium chloride (1a) (509 mg,
1.67 mmol) and 3,5-di-tert-butyl 2-hydroxybenzaldehyde (578 mg,
˚
2.50 mmol) containing 4 A molecular sieves (1.00 g) was heated
at reflux for 16 h in a thick-walled ampoule sealed with a Teflon
stopcock. On cooling to 25 ◦C the mixture was filtered and the
solvent removed from the filtrate under reduced pressure. The
resulting solid was dissolved in dichloromethane (2 mL) and added
7146 | Dalton Trans., 2009, 7141–7151
This journal is
The Royal Society of Chemistry 2009
©

[EtC(AgCl)N] (5a). Silver(I) oxide (59 mg, 0.25 mmol) and
4 A molecular sieves (200 mg) are added to a dichloromethane
0.15 mmol) to immediately give a white precipitate. The mixture
was stirred in the dark for 1 h, filtered and the solvent removed
from the filtrate under reduced pressure to give an orange/yellow
solid. Purification by flash chromatography on silica eluted with
dichloromethane gave three yellow/orange bands. The third band
was collected and the volatiles removed under reduced pressure
to give 6b as yellow/orange solid. Yield = 146 mg, 58%. 1H
˚
solution (20 mL) of 4a (200 mg, 0.39 mmol) and the mixture
stirred for 16 h at 25 ◦C in the absence of light. The mixture was
filtered and the volatiles removed from the filtrate under reduced
pressure to give 5a as a light brown solid. Yield 240 mg, 98%. 1H
NMR (270 MHz, CDCl3, 25 ◦C), d = 1.22 (9 H, s, C(CH3)3), 1.38
(12 H, s, C(CH3)3 + NCH2CH3), 1.72, 1.84, 2.36, 2.47 (8 H, m,
c-hexCH2), 3.98 (1 H, m, c-hexCHimine), 4.08 (2 H, m, NCH2CH3),
4.22 (1 H, m, c-hexCHNHC), 6.68 (1 H, s, NCHCPh), 6.93–7.42
(7H, m, PhCH), 8.14 (1 H, s, imineCH), 12.96 (1 H, s, OH); 13C
NMR (68 MHz, CDCl3, 25 ◦C), d = 17.0 (NCH2CH3), 23.8
(CH2), 25.4 (CH2), 29.3 (C(CH3)3), 31.5 (C(CH3)3), 34.2 (CH2),
34.4 (C(CH3), 34.7 (CH2), 35.0 (C(CH3)3), 48.1 (NCH2CH3),
61.0 (c-hexCHNHC), 74.0 (c-hexCHimine), 117.2 (NCHCPh), 117.5
(PhCipso), 126.3, 127.2 (PhCH), 127.5 (NCHCPh), 128.9, 129.6,
130.3 (PhCH), 136.4, 137.0, 140.4 (PhCipso), 157.2 (COH), 166.2
◦
3
NMR (500 MHz, C6D6, 25 C): d = 1.08 (3 H, d, JH-H = 5.9,
3
CH(CH3)2), 1.27 (9 H, s, C(CH3)3), 1.35 (3 H, d, JH-H = 5.9,
CH(CH3)2), 1.70 (9 H, s, C(CH3)3), 1.87, 2.12, 2.19 (8 H, m,
c-hexCH2), 2.36 (4 H, m, CODCH2), 2.79 (1 H, m, CODCH, 2.99 (1 H,
m, CODCH), 4.21 (1 H, m, c-hexCHNHC), 4.43 (1 H, m, CODCH2), 4.93
(2H, m, CODCH + c-hexCHimine), 5.26 (1 H, m, CODCH), 5.91 (1 H, m,
NCH(CH3)2), 6.12 (1 H, s, NCHCPh), 7.12–7.55 (7 H, m, PhCH),
9.21 (1 H, s, imineCH), 13.88 (1 H, s, OH); 13C NMR (126 MHz,
C6D6, 25 ◦C): d = 22.1 (CH(CH3)2), 24.3 (CH2), 24.7 (C(CH3)2),
26.3 (CH2), 28.0 (CH2) 29.7 (CH2), 29.9 (C(CH3)3), 31.6 (CH2),
31.7 (C(CH3)3), 32.0 (CH2), 34.0 (CH2), 34.2 (C(CH3)3), 35.3
(C(CH3)3), 35.6 (CH2), 51.6 (CODCH), 54.6 (NCH(CH3)2), 54.7
(
imineCH); MS (ESI), m/z 1079 ([2M - Ag - 2Cl]+), 100%; Anal.
[Found (calc.)] C32H42AgClN3O, C 60.99 (61.10), H 6.89 (6.74), N
6.68 (6.81).
(
(
CODCH), 63.4 (c-hexCHNHC), 70.0 (c-hexCHimine), 84.0 (CODCH), 85.1
CODCH) 113.6 (NCHCPh) 119.1 (NCHCPh), 126.6, 127.3, 128.3,
[BnC(AgBr)NCHAr] (5c). Prepared by an analogous method to
5a using dichloromethane (10 mL), 4c (85 mg, 0.14 mmol), silver(I)
128.9 129.9 (PhCH), 132.5, 136.4, 139.3, 139.8 (PhCipso), 158.5
(COH), 168.1 (imineCH), 181.2 (IrC); MS (ESI), m/z 800 ([M -
Cl]+, 100%); Anal. [Found (calc.)] for C41H58ClIrN3O. C 58.97
(58.86); H 6.93 (6.99); N 4.98 (5.02).
˚
oxide (21 mg, 0.09 mmol) and 4 A molecular sieves (100 mg)
giving 5c as a light brown solid. Yield = 102 mg, 99%. 1H NMR
(270 MHz, CDCl3, 25 ◦C): d = 1.30 (9 H, s, C(CH3)3), 1.42 (9
H, s, C(CH3)3), 1.81, 1.93, 2.17, 2.50 (8 H, m, c-hexCH2), 4.14 (1 H,
m, c-hexCHimine), 4.33 (1 H, m, c-hexCHNHC), 5.28 (2 H, s, NCH2Ph),
6.68 (1 H, s, NCHCPh), 6.98–7.47 (12 H, m, PhCH), 8.28 (1 H, s,
imineCH), 13.00 (1 H, s, OH); 13C NMR (68 MHz, CDCl3, 25 ◦C):
d = 23.9 (CH2), 25.4 (CH2), 29.3 (C(CH3)3), 31.5 (C(CH3)3),
34.2 (C(CH3)3), 34.6 (CH2), 34.7 (CH2), 34.9 (C(CH3)3), 56.6
(NCH2Ph), 61.2 (c-hexCHNHC), 73.9 (c-hexCHimine), 117.5 (NCHCPh),
118.0 (PhCipso), 126.3, 127.2 (PhCH), 127.3 (NCHCPh), 128.3,
128.7, 129.0, 129.3, 129.7, 130.2 (PhCH) 135.3, 136.5, 137.5, 140.4
(PhCipso), 157.6 (COH), 166.5 (imineCH); MS (ESI), m/z 1204 ([2M -
Ag - 2Br]+), 100%; Anal. [Found (calc.)] C37H46AgBrN3O: C 60.53
(60.42), H 6.26 (6.17), N 5.56 (5.71).
[Ir(j2-EtCNCHAr)(COD)][B{3, 5-(CF3)2C6H3}4] (7a). A dichlo-
romethane solution (10 mL) of [Ir(COD)Cl]2 (29 mg, 0.04 mmol)
was added drop wise to a dichloromethane solution (10 mL) of 5a
(54 mg, 0.09 mmol), to immediately give a white precipitate. The
mixture was stirred in the dark for 1 h, filtered and the volatiles
were removed from the filtrate under reduced pressure to give an
orange solid. Purification by flash chromatography on silica eluted
with dichloromethane gave three yellow/orange bands. The third
coloured band was collected and the solvent removed to give a
red/yellow orange solid (38 mg, 0.05 mmol) that was dissolved in
dichloromethane (10 mL) and Na[B{3,5-(CF3)2C6H3}4] (62 mg,
0.07 mmol) added. After stirring for 15 min the mixture was
washed with water (2 ¥ 10 mL) and the organic layer separated,
dried over magnesium sulfate and the volatiles removed under
reduced pressure to give 7a as a red/brown solid. Yield = 42 mg,
[
Ph2CHC(AgBr)NCHAr] (5d). Prepared by an analogous method
to 5a using dichloromethane (40 mL), 4d (300 mg, 0.43 mmol),
˚
silver(I) oxide (100 mg, 0.43 mmol) and 4 A molecular sieves
63%. H NMR (500 MHz, CDCl3, 25 ◦C): d = 1.47 (9 H, s,
1
(250 mg) giving 5d as a light brown solid. Yield = 335 mg, 96%.
1H NMR (270 MHz, CDCl3, 25 ◦C): d = 1.24 (9 H, s, C(CH3)3),
1.45 (9 H, s, C(CH3)3), 1.84, 1.95, 2.23, 2.45 (8 H, m, c-hexCH2), 4.22
(1 H, m, c-hexCHimine), 4.30 (1 H, m, c-hexCHNHC), 6.61–7.47 (19 H,
m, PhCH + NCHCPh + NCH(Ph)2), 8.24 (1 H, s, CHimine), 12.99
(1 H, s, OH); 13C NMR (68 MHz, CDCl3, 25 ◦C): d = 23.9 (CH2),
25.5 (CH2), 29.4 (C(CH3)3), 31.5 (C(CH3)3), 34.2 (C(CH3)3),
34.5 (CH2), 34.6 (CH2), 35.0 (C(CH3)3), 61.4 (c-hexCHNHC), 70.2
(NC(Ph)2), 74.1 (c-hexCHimine), 117.4 (NCHCPh), 126.5 (NCHCPh),
127.0, 127.2, 127.4, 128.0, 128.9, 128.8, 128.9, 129.0, 129.1, 130.2
(PhCH), 136.9, 137.3 138.8, 138.7, 140.4 (PhCipso), 157.7 (COH),
166.7 (imineCH); MS (ESI), m/z 1356 ([2M - Ag - 2Cl]+), 100%;
Anal. [Found (calc.)] C43H49AgBrN3O, C 63.54 (63.63), H 6.19
(6.09), 5.15 (5.18).
C(CH3)3), 1.49 (9 H, s, C(CH3)3), 1.60–2.40 (16 H, m, c-hexCH2 +
CODCH2), 3.54 (1 H, m, c-hexCHimine), 4.00 (2 H, m, NCH2CH3),
4.02, 4.15, 4.28, 4.39 (1 H, m, CODCH), 5.77 (1 H, m, c-hexCHNHC),
6.05 (1 H, s, OH), 6.75 (1 H, s, NCHCPh), 7.41–8.09 (19 H◦, m,
PhCH), 9.22 (1 H, s, imineCH); 13C NMR (126 MHz, CDCl3, 25 C):
d = 16.7 (NCH2CH3), 24.4 (CH2), 25.5 (CH2), 28.1 (CH2), 29.7
(CH2), 30.2 (C(CH3)3), 31.3 (CH2), 31.6 (C(CH3)3), 31.9 (CH2),
33.5 (C(CH3)3), 34.1 (CH2), 34.6 (CH2), 35.6 (C(C(CH3)3), 45.3
(NCH2CH3), 58.3 (CODCH), 65.3 (CODCH), 67.2 (c-hexCHNHC), 74.7
(
c-hexCHimine), 83.2 (CODCH), 87.1 (CODCH), 117.5 (BArFCHpara), 119.6
1
(NCHCPh), 120.6 (NCHCPh), 124.2 (PhCH), 124.5 (q, JC-F
=
273, BArFCF3), 127.8, 128.9 (m, BArFF3CCipso), 128.8, 130.2, 130.5,
130.5 (PhCH), 134.8 (BArFCHortho), 136.0, 143.6 (PhCipso), 153.5 (CO),
161.7 (q, JB-C = 51, BArFBCipso), 168.4 (imineCH), 171.8 (IrC);
1
[Ir(j1-iPrCNCHAr)Cl(COD)] (6b). To
a
dichloromethane
MS (FAB), m/z 786 ([M - BArF ]+), 100%, 674 ([M - BArF
-
4
4
(10 mL) solution of 5b (195 mg, 0.30 mmol) was added a
dichloromethane (5 mL) solution of [Ir(COD)Cl]2 (102 mg,
COD]+), 95%; HRMS calc. for C40H55N3OIr 786.3974, found
786.3969.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7141–7151 | 7147
©

CODCH), 4.35 (1 H, m, CODCH), 5.94 (1 H, m, c-hexCHNHC), 6.41
(1 H, s, NCHCPh), 6.91–8.43 (29 H, m, PhCH + NCH(Ph)2),
9.30 (imineCH), OH not observed; 13C NMR (126 MHz, CDCl3,
25 ◦C): d = 22.6 (CH2), 24.4 (CH2), 25.5 (CH2), 27.9 (CH2), 29.7
(C(CH3)3), 30.6 (CH2), 31.2 (C(CH3)3), 31.7 (CH2), 33.4 (CH2),
34.3 (C(CH3)3), 34.7 (C(CH3)3), 35.7 (CH2), 66.0 (CODCH), 67.2
[Ir(j2-iPrCNCHAr)(COD)][B{3, 5-(CF3)2C6H3}4] (7b). To
a
dichloromethane (10 mL) solution of 6 (80 mg, 0.01 mmol) was
added Na[B{3,5-(CF3)2C6H3}4] (126 mg, 0.14 mmol) and the
mixture stirred for 15 min. The mixture was washed with water
(2 ¥ 10 mL) and the organic layer separated, dried over magnesium
sulfate and the volatiles removed under reduced pressure to give 7b
as a red/brown solid. Yield = 156 mg, 98%. 1H NMR (500 MHz,
(
(
CODCH), 67.5 (c-hexCHNHC), 75.3 (c-hexCHimine), 83.3 (CODCH), 87.6
CODCH), 117.5 (BArFCHpara), 120.0 (NCHCPh), 120.6(NCHCPh),
◦
3
CDCl3, 25 C): d = 1.23 (6 H, d, JH-H = 6.5, CH(CH3)2), 1.30
(2 H, m, CODCH2), 1.46 (9 H, s, C(CH3)3), 1.49 (9 H, s, C(CH3)3),
1.88, 1.89 (8 H, m, c-hexCH2), 2.14, 2.18, 2.41 (6 H, s, CODCH2),
3.57 (1 H, m, c-hexCHimine), 4.03 (2 H, m, CODCH), 4.15 (1 H, m,
CODCH), 4.29 (1 H, m, CODCH), 4.97 (1 H, m, NCH(CH3)2),
5.82 (1 H, m, (c-hexCHNHC), 6.32 (1 H, s, OH), 6.79 (1 H, s,
NCHCPh), 7.19–8.07 (19 H, m,◦ PhCH), 9.19 (1 H, s, imineCH);
13C NMR (126 MHz, CDCl3, 25 C): d = 23.3 (CH(CH3)2), 24.4
(CH2), 25.2 (C(CH3)2), 25.3 (CH2), 25.5 (CH2), 28.0 (CH2), 29.3
(CH2), 30.2 (C(CH3)3), 31.4 (C(CH3)3), 31.8 (CH2), 33.6 (CH2),
34.2 (C(CH3)3), 34.6 (C(CH3)3), 35.6 (CH2), 52.1 (NCH(CH3)2),
58.9 (CODCH), 64.6 (CODCH), 67.1 (c-hexCHNHC), 74.8 (c-hexCHimine),
83.1 (CODCH), 87.0 (CODCH), 116.6 (NCHCPh), 117.5 (BArFCHpara),
124.3 (PhCH), 124.5 (q, 1JC-F = 273, BArFCF3), 126.2, 128.6 (PhCH)
128.9 (m, BArFF3CCipso), 129.0, 129.1, 129.2. 129.3, 129.5 (PhCH),
130.1 (PhCipso), 130.3, 130.8, 131.3 (PhCH), 134.6 (PhCipso), 134.8
(
BArFCHortho), 136.6, 138.3, 138.9, 143.8 (PhCipso), 153.8 (COH),
1
161.7 (q, JB-C = 51, BArFBCipso), 168.4 (imineCH), 173.4 (IrC); MS
(FAB), m/z 925 ([M - BArF ]+), 100%; Anal. [Found (calc.)] for
4
C83H73BF24IrN3O, C 55.82 (55.77), H 4.42 (4.12), N 2.37 (2.35).
[(1R,2R)-2-(5-Phenyl-2H-imidazol-1-yl)-cyclohexyl]-[1-phenyl-
meth-(E)-ylidene]-amine [C(H)N(CHPh)] (8). A 3-necked flask
fitted with a reflux condenser, an addition funnel and a thermome-
˚
ter was charged with 4 A molecular sieves (5.00 g), potassium
carbonate (15.00 g) and (1R,2R)-N,N¢-bis-[1-phenyl-meth-(E)-
ylidene]-cyclohexane-1,2-diamine (10.00 g, 34 mmol) and dried
under dynamic vacuum at 60 ◦C for 12 h. Acetonitrile (50 mL)
was added and the mixture maintained at 60 ◦C whilst an acetoni-
trile solution (50 mL) of 1-(isocyano-phenyl-methanesulfonyl)-
4-methylbenzene (Ph-TosMIC) (12.00 g, 44 mmol) was added
dropwise. The mixture was stirred at 60 ◦C for ca. 36 h until
complete disappearance of the starting diimine as judged by TLC
1
120.5 (NCHCPh), 124.2 (PhCH), 124.5 (q, J = 273, BArFCF3),
127.5, 128.5 (PhCH) 128.9 (m, BArFF3CCipso), 130.2, 130.6 (PhCH),
130.7 (PhCipso), 134.8 (BArFCHortho), 135.0, 136.4, 143.7 (PhCipso), 153.8
1
(COH), 161.7 (q, JB-C = 51, BArFBCipso), 168.5 (imineCH), 170.8
(IrC); MS (FAB), m/z 800 ([M - BArF ]+), 100%; Anal. [Found
4
(calc.)] for C73H69BF24IrN3O, C 52.60 (52.71), H 4.07 (4.18), N
2.57 (2.53).
◦
[Ir(j2-BnCNCHAr)(COD)][B{3, 5-(CF3)2C6H3}4] (7c). Prepared
by an analogous method to 7a using [Ir(COD)Cl]2 (25 mg,
0.04 mmol) 5c (54 mg, 0.07 mmol), and Na[B{3,5-(CF3)2C6H3}4]
(30 mg, 0.03 mmol) to give 7c as a red/brown solid. Yield = 47 mg,
and cooled to 25 C. On removal of the volatiles under reduced
pressure the solid residue was extracted with diethyl ether (3 ¥
200 mL) and the filtrates combined and cooled at 4 ◦C for
24 h to precipitate impurities. After filtration, the solution was
concentrated to ca. 150 mL and dropped into hexane, resulting in
the formation of an oily precipitate that was decanted and washed
with hexane to give 8 as an off white waxy solid. Yield = 10.74 g,
78% 1H (500 MHz, CDCl3, 25 ◦C) d = 1.25–2.20 (m, 8H, c-hexCH2),
3.39 (m, 1H, c-hexCHimine), 3.92 (m, 1H, c-hexCHimid), 6.90–7.60 (m,
15H, PhCH), 7.64 (s, 1H, NCHN), 7.92 (s, 1H, imineCH); 13C NMR
68%. H NMR (500 MHz, CDCl3, 25 ◦C): d = 1.27 (2 H, m,
1
CODCH2), 1.39 (9 H, s, C(CH3)3), 1.44 (9 H, s, C(CH3)3), 1.72–2.31
(10 H, m, c-hexCH2 + CODCH2), 3.54 (1 H, m, c-hexCHimine), 3.86, 3.99,
4.12, 4.26 (1 H, m, CODCH), 5.18 (1 H, d, 2J = 15 Hz, NCH2Ph),
5.66 (1 H, d, 2JH-H = 15, NCH2Ph), 5.81 (1 H, m, c-hexCHNHC), 5.95
(1 H, s, OH), 6.55 (1 H, s, NCHCPh), 7.05–8.12 (24 H, m, PhCH),
9.23 (1 H, s, imineCH); 13C NMR (126 MHz, CDCl3, 25 ◦C): d = 24.4
(CH2), 25.5 (CH2), 28.9 (CH2), 29.5 (CH2), 30.2 (C(CH3)3), 31.2
(CH2), 31.5 (C(CH3)3), 32.2 (CH2), 33.3 (CH2), 34.1 (C(CH3)3),
34.7 (C(CH3)3), 35.8 (CH2), 54.1 (NCH2Ph), 58.1 (CODCH), 65.6
◦
(125 MHz, CDCl3, 25 C): d = 22.7 (c-hexCH2), 25.4 (c-hexCH2),
33.8 (c-hexCH2), 33.9 (c-hexCH2), 58.5 (c-hexCHimid), 75.1 (c-hexCHimine),
125.9, 126.6, 127.9, 128.2, 128.4, 128.8, 129.4, 129.9, 130.7, 130.9,
131.5, 133.5, 134.7, 135.8, 137.0, 160.7 (imineCH). M.S.: m/z = 405
([M]+), 100%; Anal. [Found (calc.)] for C28H27N3, C 82.08 (82.93),
H 7.04 (6.91), N 10.82 (10.36).
(
(
CODCH), 67.4 (c-hexCHNHC), 74.9 (c-hexCHimine), 83.8 (CODCH), 87.7
CODCH), 117.5 (BArFCHpara), 120.5 (NCHCPh), 121.3 (NCHCPh),
124.1 (PhCH), 124.5 (q, 1JC-F = 273, BArFCF3), 126.7 127.8, 128.5,
128.8 (PhCH), 128.9 (m, BArFF3CCipso), 129.5 (PhCipso), 129.6, 130.3,
130.6 (PhCH), 130.7 (PhCipso), 134.8 (BArFCHortho), 135.1, 136.0, 143.6
[
iPrC(H)N(CHPh)][I] (9). Compound 8 (4.05 g, 10 mmol) was
refluxed in 2-iodopropane (17.3 g, 100 mmol) for 24 h in a thick-
walled ampoule sealed with a Teflon stopcock. On cooling to
1
(PhCipso), 161.7 (q, JB-C = 51, BArFBCipso), 153.6 (COH),), 168.5
◦
25 C the volatiles were removed under reduced pressure to give
(
imineCH), 173.1 (IrC); MS (FAB), m/z 848 ([M - BArF ]+), 100%;
4
a white foam that was dissolved in methanol (10 mL) and added
dropwise into diethyl ether (100 mL) giving a yellow precipitate.
The precipitate was isolated by filtration washed with diethyl ether
(3 ¥ 50 mL) and dried under reduced pressure at 60 ◦C for 24 h to
give 9 as a white solid. Yield = 5.46 g, 95%. 1H NMR (400 MHz,
CD3OD, 25 ◦C): d = 1.40–2.50 (m, 8H, c-hexCH2), 1.47 (d, J =
6.7 Hz, 3H, CH(CH3)3), 1.60 (d, J = 6.7 Hz, 3H, CH(CH3)3), 3.92
(m, 1H, c-hexCHimine), 4.40–4.25 (m, 2H, c-hexCHimid + CH(CH3)3),
7.01 (d, J = 7.0 Hz, 2H, PhCH), 7.10–7.70 (m, 13H, PhCH), 8.28 (s,
1H,imineCH), 9.78 (s, 1H, NCHN); 13C NMR (100 MHz, CD3OD,
25 ◦C): d = 23.0, 23.5, 24.1, 25.4, 32.3, 33.8, 51.9, 63.5, 72.7,
Anal. [Found (calc.)] for C77H70BF24IrN3O, C 53.93 (54.04), H 3.95
(4.06), N 2.37 (2.46).
[Ir(j2-Ph2CHCNCHAr)(COD)][B{3, 5-(CF3)2C6H3}4] (7d). Pre-
pared by an analogous method to 7a using [Ir(COD)Cl]2 (27 mg,
0.04 mmol) 5d (66 mg, 0.008 mmol), and Na[B{3,5-(CF3)2C6H3}4]
(40 mg, 0.05 mmol) to give 7d as a red/brown solid. Yield =
47 mg, 66%. 1H NMR (500 MHz, CDCl3. 25 ◦C): d = 0.90
(2 H, m, CODCH2), 1.24 (6 H, m, CODCH2), 1.39 (9 H, s,
C(CH3)3), 1.50 (9 H, s, C(CH3)3), 1.72–2.29 (8 H, m, c-hexCH2),
3.64 (1 H, m, c-hexCHimine), 3.82 (2 H, m, CODCH), 4.16 (1 H, m,
7148 | Dalton Trans., 2009, 7141–7151
This journal is
The Royal Society of Chemistry 2009
©

124.9, 125.0, 128.0, 128.6, 129.0, 129.1, 130.1, 130.3, 130.5, 130.9,
131.0, 132.6, 134.4, 135.7, 162.0 (imineCH); MS (ESI): m/z = 1023.7
([2M + I]+), 100%; Anal. [Found (calc.)] for C31H34N3I, C 64.82
(64.69), H 5.96 (5.95), N 7.20 (7.30).
129.9, 130.0, 130.1, 130.2, 131.7, 131.8, 132.1, 132.7, 133.0, 134.1,
140.2; M.S. (ESI): m/z = 540 ([M - I]+), 100%; HRMS: Calc. for
C38H42N3 540.3373, found 540.3369.
[Rh(j2-iPrCN(Bn)(H))(COD)][B{3,5-(CF3)2C6H3}4] (13). At
-78 ◦C, tetrahydrofuran (10 mL) was added to a mixture
of [Rh(COD)Cl]2 (62 mg, 0.125 mmol) and potassium tert-
[C(H)N(Bn)(H)] (10). To a stirring methanol (50 mL) solution
of 8 (4.05 g, 10 mmol) cooled to 0 ◦C was added sodium
borohydride (760 mg, 20 mmol) continually over 15 min. The
solution was allowed to warm to 25 ◦C and stirred for 30 min. With
stirring 1M sodium hydroxide (100 mL) was cautiously added and
the majority of the methanol was removed under reduced pressure.
The remaining aqueous phase was extracted with ethyl acetate (3 ¥
50 mL) and the combined organic layers dried over magnesium
sulfate. On filtration through a short pad of neutral aluminium
oxide the volatiles were removed to give 10 as a transparent glassy
solid that is extremely hygroscopic. Yield = 3.30 g, 81%. 1H NMR
(270 MHz, CDCl3, 25 ◦C): d = 1.18–1.31 (m, 4H, c-hexCH2), 1.68
(m, 2H, c-hexCH2), 1.88 (m, 1H, c-hexCH2), 2.09 (m, 1H, c-hexCH2),
2.72 (m, 1H, c-hexCHimine), 3.44 (d, J = 13.7 Hz, 1H, PhCH2), 3.56
(m, 1H, c-hexCHimid), 3.66 (d, J = 13.7 Hz, 1H, PhCH2), 6.95–
7.49 (m, 16H); 13C NMR (100 MHz, CDCl3, 25 ◦C): d = 24.3,
25.3, 26.9, 31.8, 34.6, 50.6, 60.0, 126.0, 126.8, 127.2, 127.8, 127.9,
128.0, 128.1, 128.2, 129.6, 130.9, 131.2, 133.8, 134.8, 137.6, 140.0;
MS (EI): m/z = 408 ([M + H]+), 100%; Anal. [Found (calc.)] for
C28H29N3, C 82.52 (82.52), H 7.14 (7.17), N 10.23 (10.31).
◦
butoxide (70 mg, 0.625 mmol) at -78 C and stirred for 15 min.
Subsequently, a tetrahydrofuran (5 ml) solution of 11 (144 mg,
0.25 mmol) was added dropwise via cannula and the mixture
allowed to warm to 25 ◦C and stirred for 12 h. On filtration,
a tetrahydrofuran (5 mL) solution of Na[B{3,5-(CF3)2C6H3}4]
(332 mg, 0.375 mmol) was added to the filtrate and stirred for
1 h. The volatiles were removed under reduced pressure and the
residue purified by column chromatography on silica gel with 1:1
dichloromethane:hexane to give 13 as a yellow solid. Yield =
290 mg, 76%. H (400 MHz, CDCl3, 25 ◦C): d = 1.21 (d, J =
1
7.0 Hz, 3H, CH(CH3)3), 1.41 (d, J = 7.0 Hz, 3H, CH(CH3)3),
1.45–1.70 (m, 6 H, c-hexCH2
+
CODCH), 1.76 (m, 1H, c-hexCH2),
1.95–2.25 (m, 8H, c-hexCH2 + CODCH), 2.38 (m, 1H, c-hexCH2), 2.52
(m, 1H, c-hexCHimine), 2.84 (m, 1H, PhCH2), 3.22 (m, 1H,CODCH),
3.43 (m, 1H, PhCH2), 3.83 (m, 1H, CODCH), 4.0 (m, 1H, CODCH),
4.51 (m, 1H, CODCH), 5.25 (sep, J = 6.9 Hz, 1H, CH(CH3)3), 5.49
(m, 1H, c-hexCHimid), 6.96 (d, J = 7.6 Hz, 2H, PhCH), 7.00–7.35 (m,
9H, PhCH), 7.40–7.50 (m, 8H, PhCH), 7.64 (brs, 8H, PhCH); 13C
NMR (100 MHz, CDCl3, 25 ◦C): d = 22.5, 24.5, 24.9, 25.5, 26.3,
[
iPrC(H)N(Bn)(H)][I] (11). Was prepared by an analogous
28.9, 29.9, 31.5, 33.4, 35.1, 54.7, 60.5, 65.3, 69.8, 70.6 (d, 1JRh-C
=
procedure to 9 using compound 10 (4.07 g, 10 mmol) and 2-
iodopropane (17.3 g, 100 mmol) to give 11 as a white solid. Yield =
13.5 Hz, CODCH), 77.8 (d, JRh-C = 13.5 Hz, CODCH), 95.4 (d,
1JRh-C = 13.5 Hz,CODCH), 95.6 (d, 1JRh-C = 13.5 Hz,CODCH) 117.4,
121.4, 125.9, 128.3, 128.5, 128.7, 129.0, 129.6, 130.9, 131.8, 133.4,
133.5, 134.8, 160.9, 161.4, 161.9, 162.4, 176.5 (d, 1JRh-C = 51.1 Hz,
RhNHCC); MS (FAB): m/z = 661 ([M - B{3,5-(CF3)2C6H3}4]+),
100%; Anal. [Found (calc.)] for C71H60BF24N3Rh, C 56.07 (55.92),
H 3.91 (3.97), N 2.69 (2.76).
1
5.48 g, 95%. H NMR (400 MHz, CD3OD, 25 ◦C): d = 1.31
1
(m, 1H, c-hexCH2), 1.49 (d, J = 6.7 Hz, 3H, CH(CH3)3), 1.50 (m,
3H, c-hexCH2), 1.76 (d, J = 6.7 Hz, 3H, CH(CH3)3), 1.87 (m, 1H,
c-hexCH2), 2.04 (m, 1H, c-hexCH2), 2.48 (m, 1H, c-hexCH2), 2.55 (m,
1H, c-hexCH2), 4.08 (m, 1H, c-hexCHimine), 4.27 (m, 1H, c-hexCHimid),
4.37 (d, J = 13.4 Hz, 1H, PhCH2), 4.45 (d, J = 13.4 Hz, 1H,
PhCH2), 4.48 (sep, J = 6.7 Hz, 1H, CH(CH3)3), 7.35–7.52 (m,
9H, PhCH), 7.65–7.80 (m, 6H, PhCH), 9.60 (s, 1H, NCHN); 13C
NMR (100 MHz, CD3OD, 25 ◦C): d = 23.5, 23.8, 24.0, 25.1, 28.7,
36.5, 47.9, 53.7, 58.2, 60.1, 126.0, 126.4, 129.9, 130.2, 130.5, 130.7
131.6, 131.8, 132.1, 132.4, 132.6, 132.7, 135.1; MS (ESI): m/z =
450 ([M]+), 100%.
[Ir(j2-iPrCN(Bn)(H))(COD)][B{3,5-(CF3)2C6H3}4]
(14).
A
tetrahydrofuran (10 mL) solution of 11 (150 mg, 0.26 mmol)
and Ag2O (46 mg, 0.2 mmol) and were stirred at 25 ◦C for
16 h. The resulting green mixture was filtered via cannula into
a tetrahydrofuran (10 mL) solution of [Ir(COD)Cl]2 (87 mg,
0.13 mmol) immediately giving an off white precipitate and the
mixture stirred for 24 h. Subsequently, the mixture was filtered
and Na[B{3,5-(CF3)2C6H3}4] (310 mg, 0.35 mmol) added to
the solution and stirred for 1 h. The volatiles were removed
under reduced pressure and the residue purified by column
chromatography on silica gel with dichloromethane to give 14
[
iPrC(H)NBn2][I] (12). A mixture of compound 11 (1.15 g,
2 mmol), benzyl bromide (400 mg, 2.25 mmol), potassium
carbonate (417 mg, 3 mmol) and acetonitrile (50 mL) were heated
at 80 ◦C for 16 h in a thick-walled ampoule sealed with a Teflon
stopcock. On cooling to 25 ◦C the mixture was filtered into diethyl
ether (100 mL) to give a white solid precipitate that was isolated
by filtration and washed with diethyl ether (3 ¥ 20 mL). The solid
was dried under dynamic vacuum to give 12 as a white◦powder.
1
as a yellow solid. Yield = 227 mg, 54%. H (400 MHz, CDCl3,
◦
25 C): d = 0.90–2.10 (m, 16H, c-hexCH2 + CODCH), 1.18 (d, J =
6.4 Hz, 3H, CH(CH3)3), 1.45 (d, J = 6.4 Hz, 3H, CH(CH3)3),
2.74 (m, 1H, c-hexCHimine), 3.10 (m, 3H, CODCH + PhCH2), 3.47 (m,
1H, CODCH), 3.56 (m, 1H, CODCH), 3.64 (m, 1H, PhCH2), 4.26 (m,
1H, CODCH), 5.24 (m, 1H, c-hexCHimid), 5.31 (sep, J = 6.9 Hz, 1H
CH(CH3)3), 6.95 (d, J = 8.9 Hz, 2H, PhCH), 7.00–7.35 (m, 13H,
PhCH), 7.44 (brs, 4H, ◦PhCH), 7.63 (brs, 8H, PhCH); 13C NMR
(100 MHz, CDCl3, 25 C): d = 22.5, 24.3, 25.2, 26.8, 29.7, 29.8,
31.6, 32.2, 33.1, 35.8, 54.8, 56.7, 62.0, 63.7, 65.8, 68.3, 81.1, 81.2,
117.4, 122.3, 126.2, 128.0, 128.5, 128.7, 129.6, 129.8, 130.2, 131.5,
131.6, 131.8, 133.8, 134.2, 134.8, 160.9, 161.4, 161.9, 162.4, 173.7
(IrNHCC); MS (FAB): m/z = 751 ([M - B{3,5-(CF3)2C6H3}4]+)
1
Yield = 1.23 g, 92%. H NMR (400 MHz, CD3OD, 25 C): d =
1.31 (m, 2H, c-hexCH2), 1.47 (d, J = 6.7 Hz, 3H, CH(CH3)3), 1.50
(m, 1H, c-hexCH2), 1.67 (d, J = 6.7 Hz, 3H, CH(CH3)3), 1.82 (m,
2H, c-hexCH2), 1.98 (m, 1H, c-hexCH2), 2.25 (m, 1H, c-hexCH2), 2.48
(m, 1H, c-hexCH2), 3.38 (m, 1H, c-hexCHimine), 3.47 (dd, J = 10.3,
25.5 Hz, 4H, PhCH2), 4.15 (m, 1H, c-hexCHimid), 4.61 (sep, J =
6.7 Hz, 1H, CH(CH3)3), 6.83 (d, J = 8.6 Hz, 2H, PhCH), 7.08–7.20
(m, 5H,PhCH), 7.30–7.55 (m, 13H, PhCH), 8.91 (s, 1H, NCHC);
13C NMR (100 MHz, CD3OD, 25 ◦C): d = 23.0, 24.9, 25.7, 26.2,
32.7, 36.1, 52.2, 54.0 (br, PhCH2), 60.7, 62.5, 126.0, 126.4, 128.5,
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 7141–7151 | 7149
©

100%; Anal. [Found (calc.)] for C71H60BF24N3Ir, C 53.01 (52.86),
H 3.87 (3.69), N 2.61 (2.60).
1-ol (R); 2-chlorophenyl benzylmethanol. 1.0 mL/min (90%
hexane, 10% 2-propanol), 20 ◦C, Chiralcel OD column, de-
tected at 210 nm. 9.74 min (3-chlorophenyl)(phenyl)methanol
(S), 10.40 min (3-chlorophenyl)(phenyl)methanol (R); 3-
chlorophenyl benzylmethanol. 1.0 mL/min (90% hexane, 10%
2-propanol), 20 ◦C, Chiralcel OD column, detected at
210 nm. 9.74 min (3-chlorophenyl)(phenyl)methanol (S),
10.40 min (3-chlorophenyl)(phenyl)methanol (R).; 4-chlorophenyl
benzylmethanol. 1.0 mL/min (98% hexane, 2% 2-propanol),
20 ◦C, Chiralcel OD column, detected at 210 nm.
31.00 min (4-chlorophenyl)(phenyl)methanol (R), 32.20 min
(4-chlorophenyl)(phenyl)methanol (S).
[Rh(j2-iPrCNBn2)(COD)][B{3, 5-(CF3)2C6H3}4] (15). Was pre-
pared by an analogous method to 13 using [Rh(COD)Cl]2 (62 mg,
0.125 mmol), potassium tert-butoxide (70 mg, 0.625 mmol) and 12
(167 mg, 0.25 mmol). Purification by column chromatography on
silica gel with 1:2 dichloromethane:hexane to give 15 as◦a yellow
1
solid. Yield = 275 mg, 68%. H (400 MHz, CDCl3, 25 C): d =
1.25–2.25 (m, 14H, c-hexCH2), 1.28 (d, J = 6.8 Hz, 3H, CH(CH3)3),
1.36 (d, J = 6.8 Hz, 3H, CH(CH3)3), 2.40–2.65 (m, 3H, c-hexCH2 +
c-hexCHimine), 3.47 (br, 4H, PhCH2), 3.89 (m, 1H, c-hexCHimid), 4.21
(m, 1H, CODCH), 4.30 (m, 1H, CODCH), 4.74 (m, 1H, CODCH), 5.50
(m, 1H, CODCH), 6.00 (sep, J = 6.9 Hz, 1H, CH(CH3)3), 6.86 (d,
J = 7.5 Hz, 2H, PhCH), 6.90–7.35 (m, 18H, PhCH), 7.44 (br,◦4H,
PhCH), 7.63 (br, 8H, PhCH); 13C NMR (100 MHz, CDCl3, 25 C):
d = 22.6, 23.2, 25.4, 26.5, 27.0, 29.7, 30.2, 31.4, 34.6, 38.1, 52.1 (br,
PhCH2), 55.8, 64.4, 66.3, 75.6 (d, 1JRh-C = 13 Hz, CODCH), 82.4 (d,
1JRh-C = 14 Hz,CODCH), 96.2 (d, 1JRh-C = 14 Hz, CODCH), 97.0 (d,
1JRh-C = 14 Hz, CODCH), 117.4, 123.2, 125.9, 129.0, 128.7, 128.5,
128.3, 129.6, 129.7, 130.9, 131.8, 133.4, 133.5, 134.8, 139.4, 160.9,
161.4, 161.9, 162.4, 173.5 (d, 1JRh-C = 51 Hz, RhNHCC); MS (FAB):
m/z = 752 ([M - B{3,5-(CF3)2C6H3}4]+), 100%; Anal. [Found
(calc.)] for C78H66BF24N3Rh, C 57.93 (58.01), H 4.05 (4.12), N
2.48 (2.60).
Acknowledgements
The authors thank EPRSC and The University of York for
financial support and Johnson Matthey for loan of rhodium and
iridium salts.
Notes and references
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General procedure for catalytic asymmetric hydrogenation. To
a reaction vial was added 2-propanol (2 mL), precatalyst complex
(0.5 ¥ 10-5 mol) from a 2-propanol stock solution, the substrate
(1 ¥ 10-3 mol) and potassium t-butoxide (2.5 mg, 2 ¥ 10-5 mol).
◦
After 20 h stirring at 20 C an aliquot (100 mL) was then taken
and diluted with 40–60 petroleum ether (1 mL), passed through a
silica plug and analysed by 1H NMR and chiral HPLC.
Chiral separation conditions: Phenyl ethanol. 1.0 mL/min
(95% hexane, 5% 2-propanol), 20 ◦C, Chiralcel OD column.
13.05 min phenylethan-1-ol (R), 18.39 min phenylethan-1-ol (S); 2-
methylphenyl ethanol. 1.0 mL/min (99% hexane, 1% 2-propanol),
◦
20 C, Chiralcel OD column. 17.32 min 2-methylphenylethan-1-
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ethanol. 1.0 mL/min (98% hexane, 2% 2-propanol) 20 ◦C,
Chiralcel OD column. 11.93 min 3-methylphenylethan-1-ol (R),
18.41 min 3-methylphenylethanol (S); 4-methylphenyl ethanol.
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column. 14.85 min 4-methylphenylethan-1-ol (R), 16.04 min 4-
methylphenylethan-1-ol (S); 2-Chlorophenyl ethanol. 1.0 mL/min
(99% hexane, 1% 2-propanol), 20 ◦C, Chiralcel OD col-
umn. 14.04 min 2-chlorophenylethan-1-ol (R), 15.60 min 2-
chlorophenylethan-1-ol (S); 3-Chlorophenyl ethanol. 0.5 mL/min
(96% hexane, 4% 2-propanol), 20 ◦C, Chiralcel OD col-
umn. 18.06 min 3-chlorophenylethan-1-ol (R), 18.89 min 3-
chlorophenylethan-1-ol (S); 4-Chlorophenyl ethanol. 1.0 mL/min
(99% hexane, 1% 2-propanol), 20 ◦C, Chiralcel OD col-
umn. 20.07 min 4-chlorophenylethan-1-ol (S), 21.63 min 4-
chlorophenylethan-1-ol (R); 2-naphthylalcohol (Table 3 entry 4).
1.0 mL/min (98% hexane, 2% 2-propanol), 20 ◦C, Chiral-
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8.75 min 4-phenyl butan-1-ol (S), 12.55 min 4-phenyl butan-
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©