Iridium complexes with new 1,2-dithioether chiral ligands containing a rigid cyclic backbone. Application in homogeneous catalytic asymmetric hydrogenation

Article abstract of DOI:10.1039/a803626h

New chiral dithioether compounds (-)-1-benzyl-3,4-bis(methylsulfanyl)pyrrolidine (-)-degusme, (-)-1-benzyl-3,4-bis(isopropylsulfanyl)pyrrolidine (-)-degusprⁱ and (+)-1-benzyl-3,4-bis(phenylsulfanyl)pyrrolidine (+)-degusph were prepared from (+)-L-tartaric acid. The addition of the dithioether compounds to a dichloromethane solution of [Ir(cod)₂]BF₄ afforded the chiral cationic complexes [Ir(cod){(-)-degusme}]BF₄ 1 [Ir(cod){(-)-degusprⁱ}]BF₄·CH₂Cl ₂ 2 and [Ir(cod){(+)-degusph}]BF₄ 3. The dithioether ligands were replaced by PPh₃ in complexes 1, 2 and 3 providing the [Ir(cod)(PPh₃)₂]BF₄ complex. The addition of H₂ to 1, 2 and 3 at -70°C gave cis-dihydridoiridium(III) complexes [IrH₂(cod)L]BF₄ [L = (-)-degusme 4, (-)-degusprⁱ 5 or (+)-degusph 6]. The relative stability of possible isomers for complexes 1-6 was studied by molecular mechanics calculations. Complexes 1-3 were active precursors in the asymmetric hydrogenation of different prochiral dehydroamino acid derivatives and itaconic acid, at room temperature under atmospheric pressure of H₂, and the highest enantiomeric excess obtained was 68%.

Full text of DOI:10.1039/a803626h

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Iridium complexes with new 1,2-dithioether chiral ligands
containing a rigid cyclic backbone. Application in homogeneous
catalytic asymmetric hydrogenation†
Montserrat Diéguez,a Aurora Ruiz,*a Carmen Claver,a Maria M. Pereirab and
António M. d’A. Rocha Gonsalvesb
a Departament de Química Física i Inorgànica, Facultat de Química, Universitat Rovira i Virgili,
Pl. Imperial Tarraco 1, 43005 Tarragona, Spain. E-mail: aruiz@quimica.urv.es
b Departamento de Quimica, Universidade de Coimbra, Rua Larga, 3049 Coimbra, Portugal
Received 14th May 1998, Accepted 4th August 1998
New chiral dithioether compounds (Ϫ)-1-benzyl-3,4-bis(methylsulfanyl)pyrrolidine (Ϫ)-degusme, (Ϫ)-1-benzyl-3,4-
bis(isopropylsulfanyl)pyrrolidine (Ϫ)-deguspri and (ϩ)-1-benzyl-3,4-bis(phenylsulfanyl)pyrrolidine (ϩ)-degusph
were prepared from (ϩ)--tartaric acid. The addition of the dithioether compounds to a dichloromethane solution of
[Ir(cod)2]BF4 afforded the chiral cationic complexes [Ir(cod){(Ϫ)-degusme}]BF4 1 [Ir(cod){(Ϫ)-deguspri}]BF4ؒCH2Cl2
2 and [Ir(cod){(ϩ)-degusph}]BF4 3. The dithioether ligands were replaced by PPh3 in complexes 1, 2 and 3 providing
the [Ir(cod)(PPh3)2]BF4 complex. The addition of H2 to 1, 2 and 3 at Ϫ70 ЊC gave cis-dihydridoiridium() complexes
[IrH2(cod)L]BF4 [L = (Ϫ)-degusme 4, (Ϫ)-deguspri 5 or (ϩ)-degusph 6]. The relative stability of possible isomers for
complexes 1–6 was studied by molecular mechanics calculations. Complexes 1–3 were active precursors in the
asymmetric hydrogenation of different prochiral dehydroamino acid derivatives and itaconic acid, at room
temperature under atmospheric pressure of H2, and the highest enantiomeric excess obtained was 68%.
closer to the metal, considerably improved the reaction stereo-
Introduction
selectivity (the e.e. was as high as 90%9 ) in comparison with
The development of homogeneous catalysis has made con-
DIOP derivatives which form seven-membered chelates.
siderable progress with the design and synthesis of new chiral
phosphines.1 New complexes of RhI, IrI and RuII and a better
understanding of their properties have played a fundamental
role in revealing the mechanisms of the catalytic process (e.g.
the hydrogenation of prochiral olefins such as enamides2 and
α,β-unsaturated acids2f,3).
H
H
O
O
SR
SR
SR
SR
SMe
SMe
The synthesis and catalytic activity of organometallic com-
plexes containing sulfur ligands has not been much exploited in
spite of having advantages such as lower cost, toxicity and oxid-
ation potential.4 The synthesis, characterisation and reactivity
of rhodium and iridium complexes with new chiral dithioether
ligands, that form seven-membered metal chelate rings,
BIPHESR2 R=Me, Pri
diosR2 R= Me, Pri, Ph
BINASMe2
6
7
BINASMe2,5 BIPHESMe2 and diosR2 (R = Me, Pri or Ph)
have recently been described. The hydroformylation of styrene
with rhodium complexes of the dithioether BINASMe2,5a,6
H
H
PPh2
PPh2
O
O
PPh2
PPh2
Ph
N
6
7b
BIPHESR2 (R = Me or Pri), diosR2 (R = Me or Pri) takes
place with high catalytic activity, but the enantiomeric excesses
(e.e.s) obtained are quite low. The iridium complexes of the
diosR2 ligands are also able efficiently to catalyse the hydrogen-
ation of acrylic acids. However the enantiomeric excesses
obtained are always lower than 47%.7a The interconversion
through conformational equilibration of the chelate ring
achieved with the dithioether ligands that form seven-
membered chelate rings could be one of the reasons for the low
enantiomeric excess.5b Thus, the structure of the ligand back-
bone and the size of the metal chelate ring proved to be
determining factors in the efficiency and stereoselectivity of the
catalytic transfer hydrogenation reactions of acrylic acids.8
Chiral diphosphines that form a five-membered metal chelate
ring, e.g. Deguphos with a rigid backbone and the chiral centre
H
H
DIOP
Deguphos
In this paper we describe the synthesis of three new chiral
dithioethers with rigid cyclic backbone, (Ϫ)-degusme,
a
(Ϫ)-deguspri and (ϩ)-degusph. The new iridium complexes in
which the chiral ligands form a five-membered ring with the
metal center have been isolated and their structure and reactiv-
ity are discussed. They have proved to be active in the asym-
metric hydrogenation of prochiral acrylic acids, giving 68% e.e.
in the reduction of itaconic acid (methylenebutanedioic acid).
To the best of our knowledge this is the highest e.e. reported
using metal chiral dithioether complexes.
Results and discussion
Synthesis of the dithioether
The new compounds (Ϫ)-degusme, (Ϫ)-deguspri and (ϩ)-
† Supplementary data available: minimised structures. For direct elec-
tronic access see http://www.rsc.org/suppdata/dt/1998/3517/, otherwise
available from BLDSC (No. SUP 57426, 3 pp.) or the RSC Library. See
Instructions for Authors, 1998, Issue 1 (http://www.rsc.org/dalton).
J. Chem. Soc., Dalton Trans., 1998, 3517–3522
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Table 1 The NMR spectroscopic data for the dithioethersa
Compound
CH2Ph
CH2N
CH
SMe
SCH
Me
—
Ph
1H
(Ϫ)-degusme
3.55 (d)b
3.65 (d)
3.60 (d)c
3.62 (d)
3.65e
2.55 (m)
3.12 (m)
2.55 (m)
3.02 (m)
2.71 (m)
3.21 (m)
3.05 (m)
2.15 (s)
—
7.30 (m)
(Ϫ)-deguspri
(ϩ)-degusph
3.20 (m)
3.65 (m)
—
—
3.01 (m)
—
1.26 (d)d
1.27 (d)d
—
7.30 (m)
7.10–7.30 (m)
3.77 (d)
13C
(Ϫ)-degusme
59.6
59.6
59.5
60.1
61.3
59.3
50.4
14.9
—
—
127.0, 128.2
128.5, 138.4
126.9, 128.2
128.6, 138.8
126.8, 127.0
128.2, 128.5
128.9, 131.1
135.2, 138.3
(Ϫ)-deguspri
(ϩ)-degusph
48.5
51.7
35.5
23.7
23.8
—
a In CDCl3. Chemical shifts δ in ppm with SiMe4 as internal standard, coupling constants in Hz; room temperature. Abbreviations: s, singlet; m,
b 2
c 2
d 3
e 2
multiplet; d, doublet.
J
= 15 Hz.
J
= 11.5 Hz.
J
= 7.2 Hz.
J = 13.1 Hz.
HH
HH
HH
HH
O
H
C
III. The ditriflate was isolated as a white solid and characterised
by elemental analysis and 1H and 13C NMR spectroscopy. The
13C NMR spectrum shows a quadruplet at δ 118.2 (1JCF = 321
Hz) which confirms the presence of a triflate group. Compound
IV is stable in air at low temperature. Treatment of compound
IV with NaH and methanethiol, propane-2-thiol or benz-
enethiol in tetrahydrofuran (thf) affords (Ϫ)-degusme, (Ϫ)-
deguspri and (ϩ)-degusph, respectively. The (Ϫ)-degusme and
(Ϫ)-deguspri were isolated as colourless liquids and they were
not stable in air at room temperature but at low temperature
were stable for several days. The (ϩ)-degusph was isolated as a
white solid stable in air at room temperature.
OH
OH
HO
HO
C
O
H
I
Ref. 9
O
O
H
OH
OH
Ph
Ph
Ph
N
H
II
The dithioethers were characterised by elemental analysis
and 1H, 13C NMR spectroscopy. The NMR signals for
(Ϫ)-deguspri and (ϩ)-degusph were assigned using correlation
spectroscopy (COSY) and heteronuclear correlation spec-
troscopy (HETCOR). The chemical shifts and coupling con-
stants are listed in Table 1. The diastereotopic methylenic
protons CH2Ph appear as two doublets in all cases. The four
methylenic protons CH2N appear as two multiplet signals and
in 13C NMR spectra only one signal is observed for these
Ref. 9
H
OH
OH
N
H
III
1
secondary carbon atoms for all compounds. The H and 13C
(CF3SO2)2O
NMR spectra also show only one signal for the methinic
protons CH and tertiary carbon atoms for the three com-
pounds. The two Me, Pri and Ph groups bonded to each sulfur
atom are equivalent. A C2 symmetry seems to be observed in
all cases. This suggests that the CH2Ph group rotates freely, as
predicted by a molecular mechanics calculation.
H
O3SCF3
O3SCF3
N
H
IV (75%)
MeSH
PhSH
Synthesis of the dithioether complexes [Ir(cod){(؊)-degusme}]-
BF4 1, [Ir(cod){(؊)-deguspri}]BF4ؒCH2Cl2 2 and [Ir(cod){(؉)-
degusph}]BF4 3
PriSH
The reaction of the corresponding chiral dithioether degusR
(R = Me, Pri or Ph) with [Ir(cod)2]BF4 in dichloromethane solu-
tion proceeded with the displacement of one cycloocta-1,5-
diene ligand to afford the cationic complexes 1, 2 and 3
(Scheme 2). The complexes were isolated by adding diethyl
ether as yellow (1 and 3) and orange (2) moderately air-stable
powders. The elemental analysis of C, H and S matches the
stoichiometry [{Ir(cod)(degusR)}]n[BF4]n for 1 and 3 and
[{Ir(cod)(degusR)}]n[BF4]nؒnCH2Cl2 for 2. The FAB mass spec-
tra show the ions at m/z 554 1, 610 2 and 678 3 which corre-
H
H
H
SiPr
SiPr
H
SCH3
SCH3
SPh
Ph
N
Ph
N
Ph
N
SPh
H
(+)-degusph
(76%)
H
(-)-deguspri
(74%)
(-)- degusme
(75%)
Scheme 1 Synthetic procedures for the preparation of (Ϫ)-degusme,
(Ϫ)-deguspri and (ϩ)-degusph.
Ϫ
degusph were prepared from (ϩ)--tartaric acid I (Scheme 1).
Compounds II and III have been described in the literature.9
The diol III was converted into the ditriflate IV by adding
pyridine and triflic anhydride to a dichloromethane solution of
spond to the loss of the BF4 anion in the molecular species.
For complex 2, CH2Cl2 is also lost. These peaks are accom-
panied by peaks at ϩ16 (m/z 570 1, 626 2, 694 3) and ϩ32 mass
units (m/z 586 1, 642 2, 710 3). It is probable that all these peaks
3518
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R
Ir
H
H
R
R
Ir
+
R
Ir
S
S
H
N
H
H
R
S
S
S
S
S
H
H
H
H
H
H
R
H
H
R
H
H
H
H
Ir
Ph
H
H
H
syn
C1
anti
S
R
H
H
H
C2
(RRRR)
C2
H
(RRRS or
RRSR)
(RRSS)
Fig. 1 Molecular symmetry of the cationic complexes [Ir(cod)-
(degusR)]ϩ.
Fig. 2 The three possible diastereomers expected for complexes
[Ir(cod)(degusR)]ϩ.
+
[Ir(cod)2]BF4
degusR
1,3-dioxolane; or (ϩ)-diosph, (ϩ)-2,2-dimethyl-4,5-bis(phenyl-
sulfanylmethyl)-1,3-dioxolane] and [Rh(cod)L]ClO4 7b [L = (Ϫ)-
diosme or (Ϫ)-diospri].
CH2Cl2
In order to determine the relative stability of the three
possible diastereomers, molecular mechanics calculations were
carried out. From the relative strain energies obtained for the
three possible diastereomers for complexes 1, 2 and 3 it can be
concluded that for 2 and 3 the most stable diastereomer is the
anti with the configuration RRSS. These results correlate well
with experimental NMR data and suggest that no interconver-
sion between anti diastereomers takes place on the NMR time-
scale at room temperature. For complex 1 no significant energy
differences between the anti RRSS and syn diastereomers have
been observed. The molecular mechanics calculation also indi-
cates that the CH2Ph group freely rotates which is in accord
with the C2 symmetry observed by 1H NMR. Minimised struc-
tures of the three possible diastereoisomers of complex 2 are
available as supplementary material (SUP 57426).
R
S
H
H
Ph
N
BF4
Ir
S
R
1 R = Me
2 R = Pri
3 R = Ph
Scheme 2
correspond to the addition of one and two oxygen atoms from
the matrix to the mononuclear complexes.10 The IR spectra
show a strong band between 1090 and 1050 cmϪ1 and a medium
band around 450 cmϪ1 for all complexes which are character-
Reactivity of olefinic complexes
Ϫ
istic of the non-co-ordinated BF4 anion in cationic com-
plexes.11 Complex 1 is unstable in solution even under a
With PPh3. The reaction of the diene complexes [Ir(cod)-
(degusR)]BF4 (R = Me 1, Pri 2 or Ph 3) with PPh3 in a com-
plex:PPh3 molar ratio of 1:2 results in displacement of the
dithioether ligands and provides the previously prepared
complex [Ir(cod)(PPh3)2]BF4.11
nitrogen atmosphere and it could only be characterised in the
solid state. The H NMR data for complexes 2 and 3 were
1
assigned using COSY spectra.
When the chiral dithioethers (Ϫ)deguspri and (ϩ)-degusph
were co-ordinated to iridium–cyclooctadiene fragments a C2
symmetry seemed to be observed in cationic complexes 2 and 3
With H2. When H2 is bubbled for 30 min at Ϫ70 ЊC and at
atmospheric pressure through CD2Cl2 solutions of olefinic irid-
ium complexes 1, 2 and 3 the cis-dihydrido olefin complexes 4,
5 and 6 are formed in solution (Scheme 3). In the high-field
1
(Fig. 1). In this way the H NMR spectra show the olefinic
proton signals of the co-ordinated cyclooctadiene ligand as two
multiplets. For the endo- and exo-methylenic protons of
cyclooctadiene four signals were observed for complex 3 and
three for 2. For the signals from the dithioether ligand, the four
diastereotopic methylenic protons CH2N appear as an ABX
system showing two multiplets and the two methinic protons
CH as one multiplet. In complex 2 the two PriS groups are
equivalent, so diastereotopic methyl protons appear as two
doublets at δ 1.50 and 1.60 (3JHH = 6.3 Hz) and the methinic
protons as one multiplet. For complex 3 the aromatic protons
of the PhS and CH2Ph groups are very close together and they
could not be separated sufficiently to be reliably determined.
A feature of dithioether ligands is that upon co-ordination
they can give rise to different diastereomeric complexes, since
the two sulfur donors become stereogenic centers. Since the
chiral dithioether ligands degusR have a (R,R) configuration,
there are three possible diastereomers with the configurations
RRSS, RRRR and RRRS or RRSR (Fig. 2). The diastereomers
RRSS and RRRR correspond to the anti invertomer and the
RRRS or RRSR to the syn.12 If the CH2Ph group rotates freely
the diastereomers RRSS and RRRR present a C2-geometry. All
the NMR data indicate that only one of the two possible
anti diastereomers can be distinguished for complexes 2 and 3
or the interconversion among them is fast on the NMR
timescale at room temperature. When the temperature was
changed from Ϫ70 to 75 ЊC no other signals were observed in
the 1H NMR spectra. Similar behaviour has been observed
1
region of the H NMR spectrum of the CD2Cl2 solution of all
complexes at Ϫ70 ЊC four signals of two different intensities
can be observed at δ Ϫ12.51, Ϫ13.43 and Ϫ12.94 and Ϫ13.74
(relation 2:1) for 4, at δ Ϫ13.10, Ϫ13.67 and Ϫ12.54 and
Ϫ14.19 (relation 9:1) for 5 and at δ Ϫ9.85, Ϫ13.30 and Ϫ10.69
and Ϫ13.79 (relation 10:1) for 6. These signals may be from
two diastereomeric dihydridoiridium complexes in each case. In
a CD2Cl2 solution of the related complex cis-[IrH2(cod){(Ϫ)-
7a
diospri}]BF4
two possible diastereomers were also dis-
1
tinguished. In the low-field region of the H NMR spectra for
the CD2Cl2 solutions of 4, 5 and 6 there are signals correspond-
ing to co-ordinated cyclooctadiene and dithioether, together
with starting material. The Fourier-transform IR spectrum in
CD2Cl2 solution at Ϫ70 ЊC of 4, 5 and 6 shows two asymmetric
absorption bands, ν(Ir–H) between 2000 and 2094 cmϪ1, which
are expected for cis-dihydridoiridium() compounds.13 Only
the major diastereomer is observed in all the cases.
When the temperature is increased to Ϫ40 ЊC hydrogen is lost
and the parent complexes are recovered. This indicates that the
equilibrium of the dihydridoiridium complexes in solution with
the parent complexes depends on the temperature (Scheme 3).
The related olefinic dihydrido complexes cis-[IrH2(cod)L]BF4
[L = (Ϫ)-diosme, (Ϫ)-diospri or (ϩ)-diosph]7a and cis-[IrH2-
(cod)L2]PF6 (L = PMePh2, PPh3, ¹dppe, PBun or ¹DIOP) also
3
¯
²
¯
²
behave similarly14 but hydrogen is lost on warming to 25 ЊC.
The hydride resonances have T1 values around 400 ms for all
complexes, in CD2Cl2 at Ϫ70 ЊC and 300 MHz. These values are
consistent with classical hydride.15
7a
for the related complexes [Ir(cod)L]BF4 [L = (Ϫ)-diosme,
(Ϫ)-2,2-dimethyl-4,5-bis(methylsulfanylmethyl)-1,3-dioxolane;
(Ϫ)-diospri, (Ϫ)-4,5-bis(isopropylsulfanylmethyl)-2,2-dimethyl-
J. Chem. Soc., Dalton Trans., 1998, 3517–3522
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Table 2 Hydrogenation results with catalytic systems 1–3 and 7a
+
+
R
S
H
Ir
R
S
H
Ir
H
Conversion
(%)
e.e.
(%)
*
*
Entry
Precursor
Substrate
t/h
*
*
S
R
S
R
H
1
2
3
4
5
6
7
8
9
1
2
3
2
3
2
3
7
7
7
V
16
3
3
2
2
2
2.5
6
6
6
60
100
100
96
100
100
100
96
2(S)
V
2(S)
20(S)
10(R)
27(R)
35(R)b
68(R)b
—
a
a
RRRR
RRSS
b
b
RRRR
RRSS
V
VI
VI
VII
VII
V
Fig. 3 The four possible anti isomers of complexes cis-[IrH2(cod)-
(degusR)]ϩ.
R
H
S
VI
VII
100
98
—
—
10
Ph
N
BF4
Ir
a At 20 ЊC and 1 atm H2. Solvent 6 ml, CH2Cl2. Substrate:precur-
S
R
H
sor = 40:1. b Determined by polarimetry.
H
CO2H
CO2H
CO2H
C
C
CH2
C
H2C
C
-40 o
C
-70 oC
H ,
2
Ph
NHCOCH3
NHCOCH3
CH2CO2H
V
VI
VII
Table 2. It can be seen, that the iridium complexes 1, 2 and 3 all
lead to active hydrogenation systems. However 2 and 3 are more
efficient (entries 2, 3) than 1 (entry 1). The hydrogenation of V
with the catalytic systems 2 and 3 is completed in 3 h although
the e.e. is low (entries 2 and 3). For hydrogenation of com-
pounds VI (entries 4, 5) and VII (entries 6, 7) the activity of
catalytic systems 2 and 3 is similar, but the e.e. is higher for
precursor 3. Thus the e.e. is highest when compound VII is
hydrogenated with precursor 3 (68% R).
For comparative purposes, experiments using [Ir(cod)2]BF4 7
without a chiral dithioether ligand were carried out, giving
lower activities (entries 8, 9 and 10). This, together with the
enantioselectivity obtained in all cases, indicates that the active
species for precursors 1, 2 and 3 are modified by the chiral
dithioether ligand.
In general, although catalytic systems 1, 2 and 3 are active
under ambient conditions of pressure and temperature, the e.e.s
are low. The higher e.e. obtained with catalytic system 3 could
be due to the phenyl ring bonded to sulfur facilitating the
transmission of chirality from the dithioether ligand.
The catalytic sytems 1, 2 and 3 in which the chiral
ligands form a five-membered ring with the metal center are
more active and 3 more enantioselective than the related
seven-membered ring iridium–chiral dithioether (Ϫ)-diosme,
(Ϫ)-diospri and (ϩ)-diosph systems previously reported7a for
the asymmetric hydrogenation of the prochiral olefins cited. It
should be noted that the absolute configuration obtained with
systems 1, 2 and 3 is opposite to the one obtained with the
related diosR2 (R = Me, Pri or Ph) for the hydrogenation of
compounds V–VII.7a
HA
Ir
HB
R
BF
S
H
S
R
H
N
Ph
4 R = Me
5 R = Pri
6 R = Ph
Scheme 3
When diethyl ether is added to a CH2Cl2 solution of
cis-[IrH2(cod){(ϩ)-degusph}]BF4 6 at Ϫ70 ЊC a light yellow
powder is obtained. The Fourier-transform IR spectrum in KBr
shows a very broad signal at 2003 cmϪ1 which may include the
two asymmetric absorption bands, ν(Ir–H), expected. The situ-
ation is similar for the related complexes cis-[IrH2(cod)-
7a
{(Ϫ)-diosme}]BF4 and cis-[IrH2(cod)(dth)]ClO4 (dth = 2,6-
dithiaheptane).16
Molecular mechanics calculations were carried out for di-
hydride complexes 4, 5 and 6. The dihydride complexes can be
present in eight different possible isomers, however this study
has shown that due to the steric hindrance the number is
reduced to the four anti isomers (Fig. 3). The relative strain
energies for these indicate that the two most stable isomers are
a. For complexes 5 and 6 the most stable isomer in solution has
the anti RRSS configuration of the dithioether. In the case of 4
there is no significant energy difference between the two iso-
mers a. In all cases, the difference obtained in the calculated
strain energies between the two isomers a correlates well with
the relative abundance of these isomers observed by 1H NMR.
Minimised structures for compound 4 are available as supple-
mentary material (SUP 57426).
Experimental
General comments
Elemental analyses were carried out with a Carlo-Erba micro-
analyzer. Infrared spectra were recorded on a Midac Grams/
Catalytic activity
1
386 spectrophotometer, H and 13C NMR spectra on a Varian
Asymmetric hydrogenation of prochiral olefins. It has been
previously reported that cationic complexes containing chiral
dithioether ligands behave as catalyst precursors in asymmetric
hydrogenation7a and hydroformylation5a,6,7b without the add-
ition of phosphorus ligands. In this work the mononuclear
cationic iridium complexes 1–3 were initially used as catalyst
precursors in the asymmetric hydrogenation of prochiral olefins
Z-α-(acetamido)cinnamic acid V, α-(acetamido)acrylic acid VI
and itaconic acid (methylenebutanedioic acid) VII at room
temperature under atmospheric pressure of H2.
Gemini 300 MHz spectrometer. Proton T1 studies were per-
formed using the standard inversion recovery 180Њ–τ–90Њ pulse
sequence method.15a The FAB mass spectrometry was per-
formed on a VG autospect in a 3-nitrobenzyl alcohol matrix.
Gas chromatography analyses were performed in a Hewlett-
Packard 5890A instrument (fused silica capillary column 25
m × 0.25 mm permabond L-Chirasil-Val). Optical rotations
were determined with a Perkin-Elmer 241 MC polarimeter.‡ All
‡ Specific rotations (at the temperature indicated) are given in Њ cm3 gϪ1
dmϪ1 with the concentration of the solution expressed in 10Ϫ2 g cmϪ3
.
The conversion and enantioselectivity results are shown in
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iridium complexes and ligands were synthesized under nitrogen
using standard Schlenk techniques. Solvents were distilled and
deoxygenated before use. The iridium compound [Ir(cod)2]-
BF4 11 was prepared by the general procedures described.
34.15; H, 2.91; N, 3.12; S, 13.96. Calc. for C13H13F6NO6S2: C,
34.14; H, 2.86; N, 3.06; S, 14.02%). δH(300 MHz, CDCl3, SiMe4)
2.85 (2 H, m, CH2N), 3.20 (2 H, m, CH2N), 3.70 (2 H, m,
CH2Ph), 5.35 (2 H, m, CH) and 7.30 (5 H, m, Ph). δC(74.5
MHz, CDCl3) 57.2 (CH2N), 58.6 (CH2Ph), 87.8 (CH2N), 118.2
(q, CF3,JCF = 320.2 Hz), 127.9, 128.5 and 128.7 (CH, Ph) and
136.1 (C, Ph).
Computational details
The molecular mechanics calculations were made using the
program CERIUS 217 with the force field UFF developed by
Rappe and co-workers.18 Electrostatic interations were taken
into account from atomic charges generated by the Qeq
method.19
Synthesis of the complexes
[Ir(cod){(؊)-degusme}]BF4 1. The compound was prepared
by adding an excess of (Ϫ)-degusme (20 mg, 0.1 mmol) to a
dichloromethane solution of [Ir(cod)2]BF4 (40 mg, 0.08 mmol)
which produced an immediate colour change from brown-red
to orange. Subsequent addition of diethyl ether precipitated
the desired complex, which was filtered off, washed with cold
ether, and vacuum dried. Yield 46.5 mg, 90% (Found: C, 38.94;
H, 5.03; N, 2.22; S, 9.82. Calc. for C21H3BF4IrNS2: C, 39.37;
H, 4.87; N, 2.19; S, 10.00%). m/z 554 (Mϩ).
Ligand synthesis
(؊)-1-Benzyl-3,4-bis(methylsulfanyl)pyrrolidine,
(؊)-
degusme. A suspension of NaH (0.83 g, 34 mmol) in paraffin,
cleaned twice in hexane, in thf (16 ml) was cooled to Ϫ78 ЊC
and methanethiol (0.5 ml, 9 mmol) at Ϫ78 ЊC added. The
resulting solution was stirred and the temperature increased to
0 ЊC. After 5 min the solution was cooled and compound IV
(0.5 g, 1.1 mmol) in thf (5 ml) added. After 45 min the solvent
was evaporated and water (100 ml) added to the residue
and extracted with dichloromethane (3 × 50 ml). The extract
was then dried and concentrated. The residue was purified by
column chromatography (hexane–ethyl acetate, 5:1) and
degusme was obtained (0.2 g, 75%) as a colourless liquid:
[α]D23 = Ϫ19.9 (c 0.55 in CHCl3) (Found: C, 61.94; H, 7.93;
N, 5.66; S, 25.33. Calc. for C13H19NS2: C, 61.61; H, 7.56; N, 5.53;
S, 25.30%).
[Ir(cod){(؊)-deguspri}]BF4ؒCH2Cl2 2. The compound was
prepared by adding an excess of (Ϫ)-deguspri (25 mg, 0.1
mmol) to a dichloromethane solution of [Ir(cod)2]BF4 (40 mg,
0.08 mmol) which immediately caused a colour change from
brown-red to orange. Subsequent addition of ether precipitated
the desired complex 2, which was filtered off, washed with cold
ether and vacuum dried (46.0 mg, 81%) (Found: C, 39.19; H,
5.78; N, 1.80; S, 8.16. Calc. for C25H3BF4IrNS2: C, 39.95; H,
5.20; N, 1.79; S, 8.20%). m/z 610 (Mϩ) δH(300 MHz, CDCl3,
SiMe4) 1.50 (6 H, d, JHH = 6.3, Me) and 1.60 (6 H, d, JHH = 6.3,
Me), 2.15 (2 H, m, CH2, cod), 2.25 (2 H, m, CH2, cod), 2.45 (4
H, m, CH2, cod), 3.15 (2 H, m CH2N), 3.35 (2 H, m CH2N),
3.35 (2 H, m, CH), 3.85 (1 H, m, CH2Ph), 4.05 (2 H, m, SCH),
4.10 (1 H, m, CH2Ph), 4.40 (2 H, m, CH, cod), 4.60 (2 H, m,
CH, cod), 5.32 (2 H, s, CH2Cl2) and 7.40 (5 H, m, Ph).
(؊)-1-Benzyl-3,4-bis(isopropylsulfanyl)pyrrolidine,
(؊)-
deguspri. A solution of propane-2-thiol (0.52 ml, 5.6 mmol) in
thf (8 ml) was added to a suspension of NaH (0.56 g, 23.3
mmol) in paraffin, cleaned twice in hexane, in thf (1 ml). The
resulting solution was stirred for 20 min. Then compound IV
(0.9 g, 2.0 mmol) in thf (9 ml) was added. After 45 min the
solvent was evaporated and 50 ml of water were added to the
residue and extracted with dichloromethane (3 × 50 ml). The
extract was then dried and concentrated. The residue was puri-
fied by column chromatography (hexane–ethyl acetate, 20:1)
and deguspri was obtained (0.4 g, 74%) as a colourless liquid:
[α]D23 = Ϫ24.5 (c 0.55 in CHCl3) (Found: C, 65.38; H, 8.96;
N, 4.65; S, 20.32. Calc. for C17H27NS2: C, 65.97; H, 8.79; N, 4.65;
S, 20.72%).
[Ir(cod){(؉)-degusph}]BF4 3. The compound was prepared
by adding an excess of (ϩ)-degusph (30 mg, 0.1 mmol) to a
dichloromethane solution of [Ir(cod)2]BF4 (40 mg, 0.08 mmol)
which caused an immediate colour change from brown-red to
yellow. Subsequent addition of ether precipitated the desired
complex 3, which was filtered off, washed with cold ether, and
vacuum dried (61.0 mg, 98%) (Found: C, 48.72; H, 5.19; N,
1.72; S, 8.16. Calc. for C31H35BF4IrNS2: C, 48.69; H, 4.61; N,
1.83; S, 8.38%). m/z 678 (Mϩ) δH(300 MHz, CDCl3, SiMe4) 1.50
(2 H, m, CH2, cod), 2.10 (2 H, m, CH2, cod), 2.20 (2 H, m, CH2,
cod), 2.60 (2 H, m, CH2, cod), 2.80 (2 H, m, CH2N), 2.85 (1 H,
m, CH2Ph), 3.30 (2 H, m, CH2N), 3.80 (2 H, m, CH), 4.00 (1 H,
m, CH2Ph), 4.50 (2 H, m, CH, cod) and 4.80 (2 H, m, CH, cod)
and 7.30–7.90 (15 H, m, Ph).
(؉)-1-Benzyl-1,4-bis(phenylsulfanyl)pyrrolidine, (؉)-degusph.
A solution of benzenethiol (0.50 ml, 4.9 mmol) in thf (6 ml)
was added to a suspension of NaH (0.62 g, 25.8 mmol) in
paraffin, cleaned twice in hexane, in thf (1 ml). The resulting
solution was stirred for 20 min. Then compound IV (0.8 g,
1.76 mmol) in thf (9 ml) was added. After 45 min the solvent
was evaporated and 50 ml of water were added to the residue
and extracted with dichloromethane (3 × 50 ml). The extract
was then dried and concentrated. The residue was purified by
column chromatography (hexane–ethyl acetate, 20:1) and
[IrH2(cod){(؊)-degusme}]BF4 4. Hydrogen was bubbled
through a yellow solution of [Ir(cod){(Ϫ)-degusme}]BF4 (40
mg, 0.062 mmol) in CD2Cl2 (0.4 ml) at Ϫ70 ЊC for 30 min. The
compound was then transferred to an NMR spectrometer tube
1
1
and the H spectrum recorded (see text for H NMR data and
characterisation).
degusph was obtained (0.5 g, 76%) as
a white solid:
[α]D23 = ϩ52.7 (c 0.55 in CHCl3) (Found: C, 73.53; H, 6.23;
N, 3.69; S, 16.99. Calc.for C23H23NS2: C, 73.17; H, 6.14; N, 3.71;
S, 16.98%).
[IrH2(cod){(؊)-deguspri}]BF4 5. Hydrogen was bubbled
through a brown-orange solution of [Ir(cod){(Ϫ)-deguspri}]-
BF4 (40 mg, 0.057 mmol) in CD2Cl2 (0.4 ml) at Ϫ70 ЊC for 30
minutes. The compound was then transferred to an NMR
(S,S)-1-Benzyl-3,4-bis(trifluoromethylsulfanyloxy)pyrrolidine
IV. Pyridine (0.56 ml, 7.0 mmol) was added to a solution of
compound III (0.5 g, 2.6 mmol) in dichloromethane (18.5 ml).
The resulting solution was stirred for 10 min. Then it was
cooled to Ϫ20 ЊC and triflic anhydride (1 ml, 6.1 mmol) slowly
added. TLC Monitoring of the reaction showed that it was
complete after 25 min (hexane–ethyl acetate, 3:2). The solvents
were evaporated under vacuum and the residue was purified by
column chromatography (hexane–ethyl acetate, 5:1). White
solid (0.9 g, 75%), [α]D23 = Ϫ42.0 (c 3.2 in CHCl3) (Found: C,
1
spectrometer tube and the H spectrum recorded (see text for
1H NMR data and characterisation).
[IrH2(cod){(؉)-degusph}]BF4 6. Hydrogen was bubbled
through a yellow solution of [Ir(cod){(Ϫ)-deguspri}]BF4 (40
mg, 0.052 mmol) in CD2Cl2 (0.4 ml) at Ϫ70 ЊC for 30 min. By
adding diethyl ether at Ϫ70 ЊC a yellow powder 6 was obtained
(Found: C, 48.01; H, 5.00; N, 1.82; S, 8.44. Calc. for C31H38-
J. Chem. Soc., Dalton Trans., 1998, 3517–3522
3521

View Article Online
5952; (c) J. Halpern, Science, 1982, 217, 401; (d) C. R. Landis and
BF4IrNS2: C, 48.00; H, 4.90; N, 1.82; S, 8.46%); see text for 1H
NMR data and characterisation: νmax/cmϪ1 (Ir–H) 2003 (br).
J. Halpern, J. Am. Chem. Soc., 1987, 109, 1746; (e) J. M. Brown,
P. S. Chaloner and G. A. Morris, J. Chem. Soc., Perkin Trans. 2, 1987,
2, 1583; ( f ) P. A. Chaloner, M. A. Esteruelas, F. Joó and L. A. Oro,
in Homogeneous Hydrogenation, Kluwer, Dordrecht, 1994; (g) J. M.
Brown, J. A. Ramsden and J. M. Clavidge, J. Chem. Soc., Chem.
Commun., 1995, 2469; (h) B. R. Bender, M. Koller, D. Nanz and
W. von Phillipsborn, J. Am. Chem. Soc., 1993, 15, 5889; (i) C. R.
Landis and T. W. Brauch, Inorg. Chim. Acta, 1998, 270, 285.
3 M. T. Ashby and J. Halpern, J. Am. Chem. Soc., 1991, 113, 589;
A. S. C. Chan, C. C. Chen, T. K. Yang, J. H. Huang and Y. C. Lin,
Inorg. Chim. Acta, 1995, 234, 95.
Catalytic hydrogenations
The reactions under 1 atm of H2 were performed in a previously
described hydrogen-vacuum line.20 In a typical run, substrate
(100 mg) and catalyst precursor (2.5 mg), dissolved in dichloro-
methane (6 ml), were shaken under H2 (1 atm) at 293 K. After
the required time (see Table 2), the solvent was removed. The
conversion was measured by 1H NMR.
4 F. Fache, P. Gamez, F. Nour and M. Lemaire, J. Mol. Catal., 1993,
85, 131.
Work-up of the hydrogenation product. The following pro-
cedures were used to isolate the hydrogenation product. A, For
N-acetylalanine, the residue was dissolved in water and separ-
ated from the insoluble catalyst by filtration; evaporation to
dryness afforded the product. B, For methylsuccinic acid and
N-acetylphenylalanine, the residue was dissolved in 0.5 mol
dmϪ3 NaOH and separated from the insoluble catalyst by
filtration. The filtrate was acidified with dilute HCl, extracted
with ether, and washed with a little water. The ether phase
was dried over sodium sulfate and evaporated to dryness. C,
For N-acetylphenylalanine and N-acetylalanine, gas chrom-
atography analyses were performed in a Hewlett-Packard
5890A instrument (fused silica capillary column 25 m × 0.25
mm permabond L-Chirasil-Val) before treating the sample as
described.21
5 (a) C. Claver, S. Castillón, N. Ruiz, G. Delogu, D. Fabbri and
S. Gladiali, J. Chem. Soc., Chem. Commun., 1993, 1833; (b)
S. Gladiali, D. Fabbri, L. Kollàr, C. Claver, N. Ruiz, A. Alvarez-
Larena and J. F. Piniella, Eur. J. Inorg. Chem, 1998, 113.
6 N. Ruiz, A. Aaliti, J. Forniés, A. Ruiz, C. Claver, C. J. Cardin,
D. Fabbri and S. Gladiali, J. Organomet. Chem., 1997, 79, 545.
7 (a) M. Diéguez, A. Orejón, A. M. Masdeu-Bultó, R. Echarri,
S. Castillón, C. Claver and A. Ruiz, J. Chem. Soc., Dalton Trans.,
1997, 4611; (b) A. Orejón, A. M. Masdeu-Bultó, R. Echarri,
M. Diéguez, J. Forniés-Cámer, C. Claver and C. J. Cardin,
J. Organomet. Chem., in the press.
8 A. M. d’A. Rocha Gonsalves, J. C. Bayón, M. M. Pereira, M. E. S.
Serra and J. P. R. Pereira, J. Organomet. Chem., 1997, 7577.
9 U. Nagel, E. Kinzel, J. Andrade and G. Prescher, Chem. Ber.,
1986, 119, 3326.
10 P. Didier, L. Jacquet and A. Kirsch, Inorg. Chem., 1992, 31, 4803.
11 M. Green, T. A. Kuc and S. H. Taylor, J. Chem. Soc. A, 1971, 2334.
12 E. W. Abel, S. K. Bhargava and K. G. Orrell, Prog. Inorg. Chem.,
1984, 32, 1.
13 K. Nakamoto, in Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley, New York, 1978.
14 R. H. Crabtree, H. Felkin, T. Fillebeen-Khan and G. E. Morris,
J. Organomet. Chem., 1979, 168, 183.
15 (a) D. G. Hamilton and R. H. Crabtree, J. Am. Chem. Soc., 1988,
110, 4126; (b) J. C. Lee, jun., W. Yao and R. H. Crabtree, Inorg.
Chem., 1996, 35, 695.
16 J. C. Rodriguez, C. Claver and A. Ruiz, J. Organomet. Chem., 1985,
293, 115.
Acknowledgements
We thank the Ministerio de Educación y Ciencia and the
Generalitat de Catalunya for financial support (QFN-95-
4725-C03-2; CICYT-CIRIT) and Fundação para a Ciência e
Tecnologia (Praxis XXI contract no. 2/2.1/QUI/390/94). We are
also very much indebted to Mr Oscar Pàmies for the molecular
mechanics calculations.
17 CERIUS 2, version 3.5, Molecular Simulations Inc., 1997.
18 L. A. Castonguay and A. K. Rappe, J. Am. Chem. Soc. 1992, 114,
583; A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard-III
and W.M. Skiff, J. Am. Chem. Soc. , 1992, 114, 10024; A. K. Rappe
and K. S. Colwell, Inorg. Chem., 1993, 32, 3438.
19 A. K. Rappe, C. J. Casewit and W. A. Goddard-III, J. Phys. Chem.,
1991, 95, 3358.
20 C. Cativiela, J. Fernandez, J. A. Mayoral, E. Melendez, R. Usón,
L. A. Oro and M. J. Fernandez, J. Mol. Catal., 1992, 16, 19.
21 H. Frank, C. J. Nickolson and E. Bayer, J. Chromatogr. Sci., 1977,
15, 174.
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J. Chem. Soc., Dalton Trans., 1998, 3517–3522