Chiral diphosphine ddppm-iridium complexes: Effective asymmetric imine hydrogenations at ambient pressures

Article abstract of DOI:10.1002/adsc.200505276

Complexes of the type [Ir(ddppm)(COD)]X were prepared and tested in the asymmetric hydrogenation of a range of imine substrates. Contrary to known iridium catalysts, the ddppm complexes formed efficient catalysts under an atmospheric hydrogen pressure, wh

Full text of DOI:10.1002/adsc.200505276

FULL PAPERS
DOI: 10.1002/adsc.200505276
Chiral Diphosphine ddppm-Iridium Complexes: Effective
Asymmetric Imine Hydrogenations at Ambient Pressures
Athanasia Dervisi,* Cristina Carcedo, Li-ling Ooi
School of Chemistry, Cardiff University, Main Building, ParkPlace, Cardiff, CF10 3AT, UK
Phone: (þ44)-29-2087-4081, fax: (þ44)-29-2087-4030, e-mail: dervisia@cf.ac.uk
Received: July 14, 2005; Accepted: October 20, 2005
Abstract: Complexes of the type [Ir(ddppm)(COD)]X chloride did not form active Ir-ddppm hydrogenation
were prepared and tested in the asymmetric hydroge- catalysts. The cationic Ir-ddppm hydrogenation sys-
nation of a range of imine substrates. Contrary to tem performed well in chlorinated solvents, whereas
known iridium catalysts, the ddppm complexes coordinating solvents deactivated the system. Dimeric
formed efficient catalysts under an atmospheric hy- and trimeric Ir(III) polyhydride complexes were
drogen pressure, whereas at higher pressures the cat- formed from the reaction of [Ir(ddppm)(COD)]PF6
alytic activity of the system was drastically reduced. with molecular hydrogen at atmospheric pressure
Depending upon the reaction conditions, N-aryl- and were found to inhibit catalytic activity.
¼
imines, Ar’N CMeAr, were hydrogenated to the cor-
responding secondary amines in high yields and enan-
tioselectivities (80–94% ee). In contrast to the [BF4]À Keywords: amines; asymmetric hydrogenation; di-
and [PF6]À complexes, coordinating anions such as phosphane ligands; imines; iridium; iridium hydrides
Introduction
Zhang et al. with very high enantioselectivities.[1j] The
Ir-ferrocenyl-diphosphine system, developed by Blaser
The synthesis of non-racemic chiral amines is of central et al., achieved high reaction rates (TOF~2800 hÀ1
)
importance in many aspects of chemistry and biology. with good enantioselectivity (79% ee).[5,6] This catalyst
The preparation of amines often necessitates the tedious is used industrially for the synthesis of a key intermedi-
resolution of racemic amines, which is both laborious ate to the herbicide (S)-Metolachlor. Pfaltz et al. have
and atom inefficient and thus there is a general need used a phosphino-oxazoline system obtaining up to
for efficient alternative routes to chiral amines that uti- 89% ee.[1m] More recently, de Vriesꢀ group reported
lise readily available precursors. Chiral aromatic amines the development of a chiral monodentate phosphinite-
are particularly sought after due to applications in the Ir catalyst;[1g] using pyridine as an additive raised the
pharmaceutical, agrochemical and fine chemical indus- ee to 83% at 25 bar. Phosphite- and phosphonite-based
tries. In this respect, metal-catalysed asymmetric reduc- iridium catalysts however, did not induce enantioselec-
tions of imines have attracted much interest in the last tivity in imine hydrogenations.[1k]
decade and have been the subject of several studies.[1]
Despite the above advances, iridium imine hydroge-
Ruthenium- and rhodium-based catalytic systems are nations are still underdeveloped when compared to
excellent in hydrogenating functionalised olefins and the state of the art in alkene hydrogenation chemistry.
ketonesbut are much less efficientwith imine substrates, The main obstacles that need to be overcome are the
with the notable exception of imine transfer hydrogena- normally high operating hydrogen pressures, the moder-
tions.[2] Iridium hydrogenation catalysts, on the other ate enantioselectivities usually obtained, the limited
hand, will reduce substrates which are difficult to hydro- range of imines hydrogenated and the catalyst stability.
genate, such as imines[1,3] and sterically crowded, non-
Recently, we reported in a communication the syn-
functionalised olefins.[4] Several iridium catalysts have thesis of the endo-diphosphine ddppm (3) and prelimi-
been reported that afford good enantioselectivities in nary results from the hydrogenation of alkenes with
imine reduction, but in order to achieve reasonable hy- Rh and Ru ddppm-based catalysts.[7] As shown in
drogenation rates high pressures of H2 are required, typ- Scheme 1, ddppm is prepared from the commercially
ically 25–100 bar. An iridium-diphosphine catalyst available dehydrohexitol (1) after short syntheses.
based on a binaphthyl backbone has been reported by Here, we give an account of our studies on the hydroge-
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Athanasia Dervisi et al.
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Scheme 1. (a) 2 equivs. of Br2/PPh3/imidazole, CH3CN, reflux;
(b) 2 equivs. of LiPPh2, Et2O.
nation of prochiral imines with cationic iridium-ddppm
catalytic systemand Discussion
Synthesis and Structure of the Cationic Iridium-ddppm
Complexes 4a and b
Iridium-ddppm complexes of the type [Ir(ddppm)-
(COD)]X, where X¼BF4 or PF6, were prepared accord-
ing to Scheme 2. Attempts in isolating an Ir-ddppm
chloride complex from the reaction of [Ir(COD)Cl]2
with ddppm did not afford a distinct complex.
Figure 1. ORTEP-3[8] representations of complex 4a. The BF4
anion and part of the hydrogen atoms on the aromatic rings
and COD have been omitted for clarity. In the second draw-
ing, only the Ir(ddppm) fragment of 4a is shown.
Scheme 2. Synthesis of the iridium complexes 4a and 4b.
Reaction of [Ir(COD)2]BF4 with one equivalent of
ddppm under an inert atmosphere formed the cationic [Rh(ddppm)(COD)]X, which show moderate stability
complex [Ir(ddppm)(COD)]BF4 (4a) in quantitative under aerobic conditions.[7]
yield. The 31P{1H} NMR spectrum of 4a reveals a singlet
Needle-shaped red crystals of the iridium complex
at d¼15.2. In the 1H NMR spectrum complex 4a shows [Ir(ddppm)(COD)]BF4 grown from slow diffusion of
four unresolved multiplets in 1:1:1:1 ratio for the ether into a dichloromethane solution were used in the
ddppm protons at d¼3.98, 4.12, 4.24 and 5.38 ppm. crystal structure determination of 4a. Structural repre-
1
The most characteristic H NMR shifts for ddppm sentations of 4a are given in Figure 1, showing the
upon coordination are those of the CH protons. The res- ddppm ligand chelating to the metal centre. Of note is
onance for the bridgehead CH protons shifts from d¼ the nearly eclipsed conformation of the two PPh2 phenyl
2.94 ppm in the free ligand to d¼3.98 in the iridium groups, shown more clearly in the lower drawing of Fig-
complex. Similarly, the methine CHP protons shift ure 1. This orientation of the aromatic rings in 4a lifts the
from d¼4.52 to 5.38. Two signals were observed for pseudo-C2 symmetry, of the M-ddppm fragment, usually
the olefinic protons of the 1,5-COD ligand, consistent observed in the solid state for the ddppm complexes.[7,9]
with the C2 symmetric environment, at d¼3.53 and 4.32. Such an arrangement although sterically more demand-
The hexafluorophosphate derivative [Ir(ddppm)- ing is not unusual in the solid state for Ir(I)(diphos-
(COD)]PF6 (4b) was prepared from the reaction of phine)-COD complexes, and it is attributed to crystal
ddppm with [Ir(COD)Cl]2 and NH4PF6 following a pre- packing effects.[10,11] The ligand bite angle at 94.50(10)8
viously reported protocol.[1m] NMR spectroÀscopic data is the smallest observed so far within the range of biden-
were identical with the ones for the BF4 complex. tate complexes with ddppm.[12] The related
The Ir(I) complexes 4a and 4b are air-stable and thus [Rh(ddppm)(CH3CN)5]BF4 complex has abidentate an-
easier to handle than their rhodium analogues, gle of 98.61(3)8.[7]
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Table 1. Effect of the H2 pressure in the asymmetric hydroge-
nation of imine 5a.[a]
Hydrogenation of Imines with Ir-ddppm Catalysts
The new iridium complexes were tested in the asymmet-
ric hydrogenation of a range of aromatic imines. Under
the given reaction conditions the cationic iridium com-
plexes 4a and 4b afforded N-arylamines in high yields
and with enantioselectivities at the top of the reported
range for this class of compounds. The effect of the hy-
drogen pressure, solvent and other variables in the yield
and selectivity of the reaction were studied and results
are presented in the following paragraphs.
Entry
P(H2) bar
Yield [%]
ee [%]
1
2
1
4
10
45
4
68
72
80
73
–
–
–
3[b]
4[b]
5[b]
<5
14
5[a]
Hydrogen Pressure Effect
Reaction conditions: 0.01 mmol [Ir(COD)(ddppm)]BF4,
1 mmol substrate in 5 mL of CH2Cl2, at room temperature,
24 h.
N-(1-Phenylethylidene)aniline (5a) was used as a repre-
sentative aromatic imine in order to establish the opti-
mum reaction conditions. Table 1 shows the effect of hy-
drogen pressure in the hydrogenation of imine 5a, using
[Ir(COD)(ddppm)][BF4] as the catalyst precursor. In
contrast to previous observations,[1] increasing the hy-
[a]
Catalyst solution saturated with hydrogen before addition
of imine 5a.
The ee (%) was not determined.
[b]
drogen pressure led to deactivation of the system, with hydrogenation reactions using conventional Schlenk
best results obtained under an atmospheric pressure of techniques without the need for high-pressure appara-
hydrogen gas. Increasing the hydrogen pressure to tus. Table 2 shows the effect of different solvents and
4 bar resulted in a small increase in imine formation catalyst precursors in the hydrogenation of imine 5a.
from 68 to 72%. However, this was followed by an ap- In 1,2-dichloroethane (entry 1) [Ir(COD)(ddppm)][PF6]
preciable drop in enantioselectivity from 80 to 73% ee afforded amine 5b in quantitative yield with 84% enan-
(entries 1 and 2). A positive effect on enantioselectivity tioselectivity compared to 62% yield and 82% ee with
À
at lower hydrogen pressures has been observed previ- the BF4 precursor 4a (entry 2). In CH2Cl2 comparable
ously, although within a broader range of hydrogen pres- yields and enantioselectivities were obtained with 4a
sures (20–50 bar).[1c,4a] Further increase of the hydrogen (entry 3). Other weakly co-ordinating solvents such as
pressureto10 bar resulted invirtuallycomplete deactiva- toluene were much less effective affording both very
tionofthecatalyst. Finally, at45 bar ofhydrogen pressure low yields and selectivities. In co-ordinating solvents
only a slight increase in the yield was observed, at 14%.
such as methanol a low yield was obtained (20%) but
Iridium hydrogenation reactions usually take place with higher enantioselectivity at 89% ee. In THF the cat-
within the range of 20–100 bar due to the adverse effect alyst was almost inactive and less than 5% of conversion
of low H2 pressure on the reaction rate which, in many to the secondary amine was obtained. A similar solvent
cases, leads to complete catalyst deactivation.[1c, h,13] In- dependence has been reported for the cationic Ir(I)-
deed, kinetic studies with the [Ir(COD)(PPh3)2][PF6] COD systems with triphenylphosphine[3] and phosphi-
precatalyst show a linear increase in the reaction rate no-oxazoline ligands.[1e, m,14] A catalytic run with the
with increasing hydrogen pressure.[3] The loss of activity [Ir(COD)Cl]2/ddppm system in dichloromethane
of the Ir-ddppm system at higher hydrogen pressures (entry 7) showed negligible activity in imine reduction.
may be attributed to the predominance of inactive iridi- This is in contrast to other diphosphine-based Ir cata-
um-polyhydride clusters at elevated pressures. Forma- lysts which perform better with neutral precursors
tion of unreactive Ir H complexes is also supported by ([Ir(COD)Cl]2/diphosphine).[1a, j,h, m,5,6,15] The above re-
À
the results of entry 5 (Table 1), where saturation of the sults show that the activity of the Ir-ddppm system is in-
catalyst solution with hydrogen gas before addition of creased dramatically in non-co-ordinating solvents and
the substrate produced a practically inactive catalyst af- with cationic precursors bearing weakly co-ordinating
fording the corresponding amine in very low yield (5%). anions (in increasing order: ClÀ, BF4À, PF6À). Pfaltz
The Ir(III) hydride species formed have been character- et al. have reported a similar counterion dependence
ised and their properties are discussed in a later section. with iridium catalysts, where the more bulky and weakly
co-ordinating anions formed better catalysts.[4a] Contra-
ry to literature reports, hydrogenation reactions with ad-
Solvent/Catalyst Precursor Effects
ditives such as molecular sieves, trifluoroacetic acid,
methanol or tetrabutylammonium iodide gave low
The ability of the Ir-ddppm system to operate at atmos- yields and significant amounts of hydrolysis pro-
pheric H2 pressures allowed us to carry out the following ducts.[6,13a,15]
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Athanasia Dervisi et al.
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Table 2. Effect of the solvent and catalyst precursor in the asymmetric hydrogenation of imine 5a.
Entry
Solvent
Catalyst
Yield [%][a]
ee (R) [%]
1
2
3
4
(CH2Cl)2
(CH2Cl)2
CH2Cl2
CH3OH
THF
4b
4a
4a
4a
4a
4a
100
62
68
20
<5
7
84
82
80
89
–
5[b]
6[b]
7[b]
PhCH3
CH2Cl2
–
–
[Ir(COD)Cl]2/ddppm
<5
Reaction conditions: 0.01 mmol of catalyst precursor, 1 mmol substrate in 5 mL of solvent, atmospheric H2 pressure, at room
temperature, 24 h. % ee values were measured by chiral HPLC using a Chiralcel OD column and hexane/IPA as the eluent.
[a]
Determined by NMR.
The ee (%) was not determined.
[b]
After determining the optimum reaction conditions
with N-phenylimine (5a) a number of imine substrates
were tested in order to establish the range of imines hy-
drogenated by the Ir-ddppm system. The imines shown
in Table 3 were hydrogenated in dichloroethane under
ð1Þ
an atmospheric pressure of hydrogen gas with the
[PF6]À catalyst precursor 4b. N-Arylimines were hydro-
genated quantitatively to the corresponding amine with Since iridium hydrogenation catalysts do not appear to
enantioselectivities between 80 and 94%. A clear corre- hydrogenate ketonic substrates we hydrogenated direct-
lation between enantioselectivity and the electronic ly a mixture of acetophenone and aniline [Eq. (1)].[3a,5a]
properties of para-substituents was not observed. The NMR analysis after termination of the reaction showed
N-anisylimine 8a gave the best enantioselectivity within formation of the secondary amine at 15% yield and 80%
the series. Hydrogenation of 8a was of interest due to the ee together with equimolar amounts of acetophenone,
ease of conversion into the primary amine.[1i] Lower cat- aniline and the corresponding imine. The low reaction
alyst loadings were also tested with the hydrogenation of yields observed in this case may be attributed to the
8a. At 0.5 mol % of the catalyst close to quantitative competition of acetophenone and aniline with the imine
yields were obtained with 93% ee (entry 5); further low- substrate for coordination to the active site of the metal.
ering of the catalyst resulted in incomplete reaction. Re-
actions were normally run for 24 hours without monitor-
ing. When the hydrogenation of imine 8a was followed it Iridium(III)-ddppm Hydride Complexes
was found that after 6 hours the amine was formed in
70% yield and 90% enantioselectivity; the reaction Iridium complexes of the type [Ir(COD)L2]X, where L
reached completion after 9 hours with 94% ee (entry is a P or N donor ligand, react with hydrogen to form di-
À
6). Researchers have reported previously the facile rac- meric and trimeric Ir H clusters analogous to structures
emisation of N-arylamine-iridium complexes via C H 11 and 12, respectively (Scheme 3).[17] In the case of
À
activation.[16] In order to establish whether a similar phosphino-oxazoline systems such hydride species
process takes place with the Ir-ddppm catalyst the hy- were found to be inactive in hydrogenation reactions.[17a]
drogenation of imine 8a was followed over the course
of several days. A slow drop in enantioselectivity from ddppm catalyst an immediate colour change was ob-
94 to 88% was observed during the course of 3 days. served when H2 gas was introduced, from bright red to
During hydrogenation experiments with the Ir-
N-Benzylimine 9a was also hydrogenated under the pale yellow attributed to the formation of Ir-ddppm hy-
standard conditions and gave the corresponding amine dride species. In addition, catalytic activity was inhibited
in low yield (20%) and enantioselectivity (5%). Trime- when the catalyst solution was treated with hydrogen
thylindolenine imine, 10a, remained unreacted under gas prior to addition of the imine substrate (Table 1, en-
the same reaction conditions. During hydrogenation of try 5). These observations prompted us to study further
10a a characteristic pale yellow solution, which indicates the reactivity of the Ir-ddppm system with molecular hy-
[15]
À
formation of the Ir H catalyst, was not observed.
drogen. In a typical experiment, a methanol solution of
Thus leading us to attribute the absence of any activity complex 4b containing a twenty-fold excess of KPF6 was
with imine 10a to the formation of a stable Ir-imine com- stirred for one day under an atmosphere of hydrogen.
plex.
During that period a microcrystalline solid precipitated.
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Table 3. Asymmetric hydrogenation of imines catalysed with
the iridium complex 4b.
Entry
1
Substrate
Yield [%]
99
ee [%][a]
5a
6a
7a
8a
84% (R)
Scheme 3. Hydride complexes obtained from the reaction of
[Ir(COD)ddppm]PF6 with H2 (11/12¼3:2).
NMR analysis of the isolated solid showed the forma-
2
3
99
80 (À)
81 (þ)
tion of the iridium clusters: [Ir2(ddppm)2(m2-H)3
(H)2]PF6
(11)
and
[Ir3(ddppm)3(m3-H)(m2-H)3
(H)3][PF6]2 (12), in a 3/2 ratio (Scheme 3). Although at-
tempts to separate the two complexes did not meet with
success, by using a variety of one- and two-dimensional
NMR techniques, including 1H-31P correlation measure-
ments, their identities were confirmed (Table 4).
100
The 31P NMR signals of 11 at 17.6 and 9.9 ppm are part
À
of an AX system showing a small but unresolved cis P P
1
coupling. Figure 2 shows the 31P, H correlation spec-
4
100
98
99
94 (þ)
93 (þ)
94 (þ)
trum for complexes 11 and 12. Dimer 11 displays four
cross-peaks in the hydride region arising from 2JP, H inter-
actions. The doublet at d¼ À6.54 (2JP, H ¼83.6 Hz) arises
from thetwoequivalentbridging hydrides (Hb) trans to a
single phosphorus atom. The remaining bridging hy-
dride (Hb’) resonates as a triplet, due to two trans phos-
phorus atoms, at d¼ À8.48 (2JP, H ¼67.1 Hz). The ob-
served coupling constants for the bridging iridium hy-
drides are smaller than the ones reported for terminal
hydrides trans to phosphorus (120–160 Hz) due to their
reduced bond order.[18,19] The two equivalent terminal
hydrides resonate at a significantly lower frequency at
d¼ À24.42 showing a multiplet with a small coupling
constant, 2JP, H ~25 Hz, as a result of coupling to two cis
phosphorus atoms. The 1H NMR data for dimer 11, in-
cluding coupling constants, are in good agreement with
those reported for the analogous PPh3 and dppp[20] com-
plexes.[17d, f]
5[b]
6[c]
9a
7
8
20
0
5 (R)
10a
–
Reaction conditions: 0.01 mmol [Ir(COD)(ddppm)]PF6,
1 mmol substrate in 5 mL of (CH2Cl)2, at room temperature,
run for 24 h unless otherwise stated.
[a]
Determined by chiral HPLC; absolute configurations (or
the sign of optical rotation) assigned by comparison of re-
tention times with literature values.
[b]
0.5% mmol of 4b.
Run for 9 hours.
[c]
The 31P NMR spectrum of trimer 12 reveals two sig-
À
nals at 20.3 and 9.9 ppm showing unresolved cis P P
coupling. The correlated hydride resonances appear at
Table 4. NMR data for the iridium-hydride complexes 11 and 12.[a]
Complex
d(31P)[b]
Bridging H (2JP,H, Hz)
Terminal H (2JP,H, Hz)
11
12
17.6; 9.9
20.3; 9.9
À6.54 (d, 83.6, 2Hb); À8.48 (t, 67.1, 1Hb’)
À6.88 (d, 92.4, 3Hb); À7.12 (qt, 34.7, 1Hb’)
À24.42 (m, ~25, 2Ht)
À23.71 (m, ~20, 3Ht)
[a]
In dichloromethane-d2.
Unresolved multiplets.
[b]
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Athanasia Dervisi et al.
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effect that different solvents had on the catalystꢀs activ-
ity. Co-ordinating solvents such as THF and methanol
effectively deactivated the catalyst, whereas in 1,2-di-
chloroethane quantitative yields of the secondary
amine were obtained. Our results indicate a considera-
ble potential for this class of catalysts especially for hy-
drogenations under atmospheric pressure, which merit
further investigation. Based on our current under-
standing of the system, the negative effect of hydrogen
pressure on the activity and selectivity of the catalyst
has been attributed to the formation of inactive iridiu-
m(III) hydride clusters. Further workis in progress to
address this point.
Figure 2. Section of the 1H,31P correlation for 11 and 12 show-
ing the three hydride resonances associated with the signals
at 17.6 and 9.9 (dP) for 11 and the hydride cross-peaks owing
to complex 12 (correlating with the signals at dP ¼9.9 and
20.3 ppm).
Experimental Section
General Remarks
2
2
À6.88 (d, JP, H ¼92.4 Hz), À7.12 (qt, JP, H ¼34.7 Hz)
and À23.71 (m, 2JP, H ~20 Hz) ppm, in a 3/1/3 ratio; the
two bridging hydride signals at dH ¼6.88 and 7.12 over-
lap with each other. The magnitudes of the 2JP, H coupling
constants are consistent with their assignment and close
All manipulations were performed using standard Schlenk
techniques under an argon atmosphere, except where other-
wise noted. Complexes 4a and 4b after their formation were
treated under aerobic conditions. Solvents of analytical grade
and deuterated solvents for NMR measurements were distilled
from the appropriate drying agents under N2 immediately prior
to use following standard literature methods.[21] Literature
to those of other reported Ir H trimers.[17a, d] Complexes
11 and 12 were also the only phosphorus species identi-
fied after NMR analysis of a typical Ir-ddppm hydroge-
nation reaction.
À
[22]
methods were employed for the synthesis of [Ir(COD)Cl]2
and imines 5a,[1m] 6a,[1i] 7a,[1i] 8a[1i] and 9a.[1m] All other reagents
were used as received.
The above findings show that the main path of catalyst
deactivation for the Ir-ddppm system is via formation of
unreactive Ir(III) hydride clusters. During the catalytic
reactions imine coordination prevents formation of
such clusters but as the reaction progresses imine con-
centration diminishes and formation of the complexes
11 and 12 commences. At higher than atmospheric hy-
drogen pressures (and consequently higher molecular
H2 concentrations) the imine substrate competes less fa-
vourably with molecular hydrogen for binding to the
metal leading to rapid catalyst deactivation. The nar-
rower range of H2 pressures that the Ir-ddppm system
operates may be attributed to the weakbinding of N-ar-
ylimine substrates to the Ir-ddppm catalyst.
Microanalyses were obtained from Warwickanalytical serv-
ice Ltd. Where reproducible microanalyses could not be ob-
tained the NMR spectra of the samples suggested their purity
was greater than 95%. NMR spectra were obtained on Bruker
Avance AMX 400, 500 or Jeol Eclipse 300 spectrometers and
referenced to external TMS. HPLC analyses were performed
on an Agilent 1100 series instrument. Mass spectra were ob-
tained in the ES (electrospray) mode unless otherwise report-
ed from the EPSRC Mass Spectrometry Service, Swansea Uni-
versity. High-resolution mass spectra (ES) were recorded in
house on a Waters Q-Tof micromass spectrometer.
Preparation of 1,4:3,6-Dianhydro-2,5-dideoxy-2,5-
dibromo-l-iditol (2)
A 500-mL flaskwas charged with triphenylphosphine (23 g,
86 mmol) under argon followed by the addition of dry acetoni-
trile (150 mL). To the resulting solution bromine (90 mmol)
Conclusion
We have shown that the cationic iridium-ddppm com- was added dropwise at 08C. Subsequently an acetonitrile solu-
tion of imidazole (5.6 g, 82 mmol) and isomannide (6 g,
41 mmol) were transferred to the flask. The reaction mixture
was refluxed for 2 days. The mixture was cooled in an ice
bath and any precipitate formed was filtered and the filtrate
concentrated to ca. 50 mL. The filtrate was then absorbed
onto a silica column (15Â6 cm) and eluted with a petroleum
ether/ethyl acetate (9:1) mixture. Pooling of fractions and sol-
vent evaporation afforded compound 2 (Rf ¼0.30). 1,4:3,6-Di-
anhydro-2,5-dideoxy-2,5-dibromo-l-iditolwas obtained as col-
ourless crystals after recrystallisation from either hot ethanol
plexes 4a and b form efficient imine hydrogenation cat-
alysts displaying good to high enantioselectivities with-
out the need to operate at high hydrogen pressures for
effective hydrogenation. The catalyst precursors are
readily prepared, easily handled and air-stable. In con-
trast to the cationic complexes 4a and b, the neutral
[Ir(COD)Cl]2-ddppm system was inactive in the hydro-
genation of imines suggesting that cationic, unsaturat-
ed intermediates are preferred with the Ir-ddppm cata-
lyst. This was further supported from the pronounced or petroleum ether; yield: 9.15 g (83%). The above is an im-
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Effective Asymmetric Imine Hydrogenations at Ambient Pressures
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proved procedure for compound 2; spectroscopic data are
CCOD), 45.15 (m, 2C, CH2), 74.99 (m, 2C, CHP), 83.71 (m, 2C,
CHP), 84.33 (s, HCCOD), 88.18 (s, HCCOD), 128.77 (m, 4C,
Ar), 129.53 (m, 4C, Ar), 131.66 (s, 2C, Ar), 132.03 (s, 2C, Ar),
133.94 (m, 4C, Ar), 134.17 (m, 4C, Ar); 31P NMR (121 MHz,
CDCl3): d¼15.3; MS (accurate mass, ESþ): m/z (%), calculated
mass for [MÀBF4]þ: 783.2127; measured: 783.2130; 675.2 (25)
[MÀBF4 ÀCOD]þ; elemental analysis calcd. (%) for
C38H40BF4IrO2P2 (869.7): C 52.48, H 4.64; found: C 52.53, H
4.59.
available in the literature.[23]
1,4:3,6-Dianhydro-2,5-bis(diphenylphosphino)-d-
mannitol (ddppm)
Diphenylphosphine (3.3 mL, 19.1 mmol) was syringed into
ether (30 mL) at 08C and subsequently n-BuLi (1.71 M,
11.2 mL) was added. After 30 minutes an ether solution of 2
(2.6 g, 9.6 mmol) was added dropwise to the stirred reaction
mixture. The yellow solution was allowed to slowly warm to
room temperature and stirred until the solution became col-
ourless with the precipitation of a heavy white precipitate. De-
gassed water (15 mL) was added and the ether layer separated.
The water layer successively was washed with ether (2Â
20 mL) and the ether extracts collected. Evaporation of the sol-
vent under vacuum afforded ddppm as an air-stable white mi-
crocrystalline solid. Recrystallisation from hot ethanol afford-
ed the title compound as colourless needles; yield: 0.67 g
(52%). 1H NMR (400 MHz, CDCl3): d¼2.94 (m, 2H, CHPPh2),
[Ir(COD)(ddppm)]PF6 (4b)
Ddppm (96 mg, 0.2 mmol) and [Ir(COD)Cl]2 (67 mg,
0.1 mmol) were stirred for two hours in CH2Cl2 (5 mL) [dP
(CH2Cl2, 121 MHz): À21.0 (broad), 18.0 (m)]. The resulting
orange solution was washed with an aqueous solution of NH4
PF6 (0.4 M, 5 mL). Subsequently the bright red dichlorome-
thane layer was collected, washed with water and dried over
MgSO4. Filtration of the MgSO4 and evaporation of the vola-
tiles afforded a red solid. Complex 4b was isolated in 88% yield
(163 mg) after column chromatography using CH2Cl2/MeOH
(99:1) as the eluent (Rf ¼0.2). 31P NMR (121 MHz, CDCl3):
d¼15.3 (s, ddppm), À143.8 (septet, PF6). All other NMR
data were identical to those of complex 4a.
3.88 (m, 4H, CH2), 4.52 (m, 2H, CCH), 7.2–7.4 (m, 20H); 31
P
NMR (75 MHz , CDCl3): d¼ À21.4 (s); 13C NMR (100 MHz,
CDCl3): d¼45.78 (d, JC,P ¼11.5 Hz, 2C), 73.04 (d, JC,P
¼
26.5 Hz, 2C), 86.12 (m, 2C), 128.63 (s, 2C, Ar), 128.65 (m, 2C,
Ar), 129.03 (m, 2C, Ar), 133.04 (d, JC,P ¼19.6 Hz, 2C, Ar),
133.51 (d, JC,P ¼21.4 Hz, 2C, Ar), 136.69 (d, JC,P ¼13.3 Hz, 1C,
Ar), 137.44 (d, JC,P ¼15.0 Hz, 1C, Ar); MS (accurate mass,
ESþ): m/z (%), calculated mass for [MþH]þ: 483.1637, meas-
ured: 483.1636; elemental analysis calcd. (%) for C30H28O2P2
(482.5): C 74.68, H 5.85; found: C 73.82, H 5.57.
Reaction of 4b with H2
A solution of [Ir(COD)(ddppm)]PF6 (45 mg, 0.05 mmol) and
NH4PF6 (163 mg, 1.00 mmol) in 2 mL of dry methanol was stir-
red under an atmospheric pressure of hydrogen for one hour.
Subsequently the vessel was isolated and left stirring further
for one day. During the course of the reaction a yellow micro-
crystalline powder precipitated. The formed solid was collect-
ed via filtration, washed with Et2O (2Â5 mL) and dried under
vacuum. NMR analyses showed the formation of complexes 11
and 12.
[Ir(COD)2]BF4
To a dichloromethane solution (2 mL) of [Ir(COD)Cl]2
(0.135 g, 0.2 mmol) and 1,5-cyclooctadiene (0.5 mL,
4.07 mmol, previously purified by passing through a short silica
column) AgBF4 (92.5 mg, 0.475 mmol) was added. The result-
ing deep red slurry was stirred in the darkfor 1.5 hours, subse-
quently filtered through celite and washed with CH2Cl2 (2 mL).
The solution was concentrated and anhydrous Et2O (15 mL)
was added to precipitate the complex. The red solid formed
was filtered, washed with cold Et2O (3Â10 mL) and dried;
Data for complex 11: 1H NMR (500 MHz, CDCl3): d¼
2
2
À24.42 (m, JP, H ~25 Hz, 2H, Ht), À8.48 (t, JP,H ¼67.1 Hz,
2
1H, Hb’), À6.54 (d, JP, H ¼83.6 Hz, 2H, Hb), 3.0–4.1 (m, 6H),
5.24 (m, 2H, CCH), 6.4–8.5 (m, 20H, ArH);? 13C NMR
(125 MHz, CDCl3): d¼43.7 (m, 2C, CH2), 73.8 (m, 2C,
CHP), 84.2 (m, 2C, CHP), 128.5 (m, 4C, Ar), 131.1 (m, 2C,
Ar), 133.4–134.5 (m, 6C, Ar); 31P NMR (121 MHz, CDCl3):
1
yield: 0.18, (91%). H NMR (CDCl3, 250 MHz): d¼2.43 (m,
8H, CH2), 5.18 (m, 4H, CH); 13C NMR (100 MHz, CDCl3):
À
d¼9.9 (m), 17.6 (m); IR (KBr): n¼2215 br, m (Ir H), 1966,
þ
1889, 1810 cmÀ1 br, w (Ir H); MS (ES ): m/z (%), [MÀ
À
d¼30.55 (s, CH2), 100.09 (s, CH).
PF6]þ ¼1353.58 (100.0%); MS (accurate mass, ESþ) m/z (%),
calculated mass for [MÀ(HþPF6)]2þ: 676.1339; measured:
676.1325.
[Ir(COD)(ddppm)]BF4 (4a)
Data for complex 12: 1H NMR (500 MHz, CDCl3): d¼
2
2
To a solution of [Ir(COD)2]BF4 (103 mg, 0.207 mmol) in di-
chloromethane ddppm (100 mg, 0.207 mmol) was added. The
reaction mixture was left stirring for 1 h after which the solvent
was evaporated under vacuum. The resulting bright red solid
was washed twice with ether (20 mL) and dried; yield:
154 mg (86%). Deep red crystals of 4a were obtained after
À23.71 (m, JP, H ~20 Hz, 3H, Ht), À7.12 (qt, JP,H ¼35 Hz,
2
1H, Hb’), À6.88 (d, JP, H ¼92.4 Hz, 3H, Hb), 3.0–4.1 (m, 6H),
5.24 (m, 2H, CCH), 6.4–8.5 (m, 20H, ArH); 31P NMR
(121 MHz, CDCl3): d¼9.9 (m), 20.3 (m).
1
slow diffusion of ether into a chloroform solution. H NMR
Typical Hydrogenation Protocol
(400 MHz, CDCl3): d¼1.67 (m, 2H, H2CCOD), 2.07 (m, 4H,
H2CCOD), 2.32 (m, 2H, H2CCOD) 3.53 (m, 2H, HCCOD), 3.98
(m, 2H, CHPPh2), 4.12 (m, 2H, CH2), 4.24 (m, 2H, CH2), 4.32
(m, 2H, HCCOD), 5.38 (m, 2H, CCH), 7.2–7.5 (m, 20H); 13C
NMR (100 MHz, CDCl3): d¼28.48 (s, H2CCOD), 34.70 (s, H2
A Schlenk(50 mL) was charged with N-(1-phenylethylide-
ne)aniline 5a (195 mg, 1 mmol), [Ir(ddppm)(COD)]BF4
(9 mg, 0.01 mmol) and (CH2Cl)2 (5 mL) under an N2 atmos-
phere. The resulting red solution was placed under partial vac-
Adv. Synth. Catal. 2006, 348, 175 – 183
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
181

Athanasia Dervisi et al.
FULL PAPERS
uum. Subsequently, hydrogen gas was introduced to the vessel,
causing an immediate colour change of the solution to pale yel-
low. The reaction mixture was stirred under an atmospheric
pressure of hydrogen for 24 h, at room temperature. Solvents
were removed under vacuum and the % conversion was deter-
mined by NMR. The crude product was dissolved in hexane/
ether (1:1) and passed through a short plug of silica. Pure N-
(1-phenylethyl)aniline 5b can be obtained from a silica column
(hexane/2-propanol, 9:1, Rf ¼0.38). % ees were calculated
based on chiral HPLC analysis (Daicel Chiralcel OD, 250Â
4.6 mm, ldetector ¼220 nm, flow rate¼0.5 mL/min hexane/2-
propanol (9:1, v/v); tR ¼12.1 min (S, minor), tR ¼14.2 min (R,
major).
Uiso set at 1.2 or 1.5 times the Ueq of the parent atom. In the
final cycles of refinement, a weighting scheme that gave a rel-
atively flat analysis of variance was introduced and refinement
continued until convergence was reached.
Crystal data for complex 4a: C39H37Cl3Ir1O2P2, B1F4, M¼
984.99, orthorhombic, space group P212121, a¼10.4744(4),
b¼18.1714(7), c¼19.9436(7) Š, V¼3796.0(2) Š3, Z¼4, D¼
1.724 g cmÀ3, m(Mo-Ka)¼3.868 mmÀ1, F(000)¼1944, T¼
150(2) K, red needles, crystal size 0.05Â0.05Â0.25 mm; 6545
independent measured reflections, F2 refinement, R1 ¼0.0572
wR2 ¼0.1183, 5617 independent observed absorption correct-
ed reflections [jFo j >2s(jFoj), 2qmaz ¼50.22o], 464 param-
eters, 0 restraints.
(À)-N-[1-(4-Chlorophenyl)ethyl]benzenamine (6b): HPLC
analysis: Chiralcel OD, 1.0 mL/min, n-hexane/2-propanol
(99:1, v/v), tR ¼16.8 min (minor), 18.8 min (major).
CCDC-268684 contains the supplementary crystallographic
data for complex 4a. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (þ44)-1223-336-033; or e-
mail: deposit@ccdc.cam.ac.uk).
(þ)-N-[1-(4-methoxyphenyl)ethyl]benzenamine
(7b):
HPLC analysis: Chiralcel OJ, 1.0 mL/min, n-hexane/2-propa-
nol (97:3, v/v), tR ¼45.6 min (major), 52.0 min (minor).
(þ)-4-Methoxy-N-(1-phenylethyl)benzenamine (8b): the
product was passed through a column using CH2Cl2 as eluent
prior to HPLC analysis due to overlap of the signals of the pa-
rent imine with the first eluted isomer of 8b. HPLC analysis:
Chiralcel OD, 0.75 mL/min, n-hexane/2-propanol (99:1, v/v),
tR ¼21.8 min (major), 24.3 min (minor).
Acknowledgements
We thank the EPSRC National Centre in Swansea for MS meas-
urements; CITER for the MS facility in Cardiff; Synetix for
funding C. C.ꢀs Ph.D. studentship and Johnson Matthey for a
generous loan of IrCl3. Dr. Ian A. Fallis (Cardiff University) is
also greatfully acknowledged for useful discussions during the
preparation of this manuscript and Mr. Robert L. Jenkins for as-
sistance with the NMR measurements.
(R)-N-Benzyl-1-phenylethanamine (9b): the product was
passed through a column using CH2Cl2 as eluent prior to
HPLC analysis. HPLC analysis: Chiralcel OJ, 0.75 mL/min,
n-hexane/2-propanol (99:1, v/v), tR ¼21.2 min (minor),
23.3 min (major).
Physical and spectroscopic data for the amines 5b–9b were
in agreement with values reported in the literature. Absolute
configurations or the sign of optical rotation were assigned
by comparison of the retention times with literature
values.[1e, i,m,15b]
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